NJDOT ITS DESIGN STANDARDS - Njdot design manual
It can be configured as either a bioretention basin or a bioretention swale. Category One Waters Those waters designated in the tables in N. These waters received special protection under the Surface Water Quality Standards because of their clarity, color, scenic setting or other characteristics of aesthetic value, exceptional ecological significance, exceptional recreational significance, exceptional water supply significance or exceptional fisheries resource s.
Channel - A perceptible natural or artificial waterway which periodically or continuously contains moving water.
It has a definite bed and banks which confine the water. A roadside ditch, therefore, would be considered a channel. Culvert A hydraulic structure that is typically used to convey surface waters through embankments. A culvert is typically designed to take advantage of submergence at the inlet to increase hydraulic capacity. It is a structure, as distinguished from a bridge, which is usually covered with embankment and is composed of structural material around the entire perimeter, although some are supported on spread footings with the stream bed serving as the bottom of the culvert.
Culverts are further differentiated from bridges as having spans typically less than 25 feet. Dam - Any artificial dike, levy or other barrier together with appurtenant works, which impounds water on a permanent or temporary basis, that raises the water level 5 feet or more above its usual mean low water height when measured from the downstream toe-of-dam to the emergency spillway crest or, in the absence of an emergency spillway, to the top of dam.
Design Flow - The flow rate at a selected recurrence interval. Fluvial Flood - A flood which is caused entirely by runoff from rainfall in the upstream drainage area and is not influenced by the tide or tidal surge. Floodplain - The area described by the perimeter of the Design Flood. That portion of a river valley which has been covered with water when the river overflowed its banks at flood stage.
An area designated by a governmental agency as a floodplain. Pipe - A conduit that conveys stormwater which is intercepted by the inlets, to an outfall where the stormwater is discharged to the receiving waters. The drainage system consists of differing lengths and sizes of pipe connected by drainage structures. Recurrence Interval - The average interval between floods of a given magnitude.
Regulatory Flood For delineated streams i. For non-delineated streams, it is the year peak discharge, based on fully developed conditions within the watershed.
Scour Erosion of stream bed or bank material due to flowing water; often considered as being localized. Stream Encroachment - Any manmade alteration, construction, development, or other activity within a floodplain.
Time of Concentration Tc Time required for water to flow from the most hydraulically distant but hydraulically significant point of a watershed, to the outlet. Total Suspended Solids TSS - Solids in water that can be trapped by a filter, which include a wide variety of material, such as silt, decaying plant and animal matter, industrial wastes, and sewage.
Detailed information regarding drainage design is included in the remainder of this Manual. Preliminary investigation will be performed using available record data, including reports, studies, plans, topographic maps, etc. Information should be obtained for the project area and for adjacent stormwater management projects that may affect the highway drainage. Site Analysis: At each site where a drainage structure s will be constructed, the following items should be evaluated as appropriate from information given by the preliminary investigation: 1.
Drainage Area. Land Use. Allowable Headwater. Effects of Adjacent Structures upstream and downstream. Existing Streams and Discharge Points. Stream Slope and Alignment. Stream Capacity. Soil Erodibility. Environmental permit concerns and constraints. Coordination with representatives of the various environmental disciplines is encouraged. Recurrence Interval: Select a recurrence interval in accordance with the design policy set forth in Section 2. Hydrologic Analysis: Compute the design flow utilizing the appropriate hydrologic method outlined in Section 3.
Hydraulic Analysis: Select a drainage system to accommodate the design flow utilizing the procedures outlined in the following parts: 1. Channel Design Section 4. Drainage of Highway Pavements Section 5. Storm Drains - Section 6. Median Drainage Section 7. Culverts - Section 8. Environmental Considerations: Environmental impact of the proposed drainage system and appropriate methods to avoid or mitigate adverse impacts should be evaluated.
Items to be considered include: 1. Stormwater Management 2. Water Quality 3. Soil Erosion and Sediment Control 4. Special Stormwater Collection Procedures 5.
Special Stormwater Disposal Procedures These elements should be considered during the design process and incorporated into the design as it progresses. Drainage Review: The design engineer should inspect the drainage system sites to check topography and the validity of the design. Items to check include: 1. Drainage Area a. Size b. Land Use c. Improvements 2. Effects of Allowable Computed Headwater 3.
Performance of Existing or Adjacent Structures a. Evidence of High Water 4. Channel Condition a. Erosion b. Vegetation c. Alignment of Proposed Facilities with Channel 5. Impacts on Environmentally Sensitive Areas. In many instances, stormwater problems signal either misuse of a resource or unwise land activity.
Poor management of stormwater increases total flow, flow rate, flow velocity and depth of water in downstream channels. In addition to stormwater peak discharge and volume impacts, roadway construction or modification usually increases non-point source pollution primarily due to the increased impervious area. Section 2. An assessment of the impacts the project will have on existing peak flows and watercourses shall be made by the design engineer during the initial phase.
Stormwater management, whether structural or non-structural, on or off site, must fit into the natural environment, and be functional, safe, and aesthetically acceptable. Several alternatives to manage stormwater and provide water quality may be possible for any location. Careful design and planning by the engineer, hydrologist, biologist, environmentalist, and landscape architect can produce optimum results.
The design engineer should be guided by this and include measures in design plans that are compatible with the site specific surroundings. Revegetation with native, non-invasive grasses, shrubs and possibly trees may be required to achieve compatibility with the surrounding environment. Disposal of roadway runoff to available waterways that either cross the roadway or are adjacent to it spaced at large distances, requires installation of long conveyance systems.
Vertical design constraints may make it impossible to drain a pipe or swale system to existing waterways. Discharging the runoff to the groundwater with a series of leaching or seepage basins sometimes called a Dry Well may be an appropriate alternative if groundwater levels and non-contaminated, permeable soil conditions allow a properly designed system to function as designed.
The decision to select a seepage facility design must consider geotechnical, maintenance, and possibly right-of-way ROW impacts and will only be allowed if no alternative exists. The seepage facilities must be designed to store the entire runoff volume for a design storm compatible with the storm frequency used for design of the roadway drainage facilities or as directed by the Department.
As a minimum, the seepage facilities shall be designed to store the increase in runoff volume from new impervious surfaces as long as adequate overflow conveyance paths are available to safely carry the larger flows to a stable discharge point.
Installation of seepage facilities can also satisfy runoff volume control and water quality concerns which may be required by an environmental permit. The proposed drainage structure should cause a ponding level, hydraulic grade line elevation, or backwater elevation no greater than the AWS when the design flow is imposed on the facility. An AWS that exceeds a reasonable limit may require concurrence of the affected property owner. Army Corps of Engineers, or other recognized agencies will be available at some sites.
The elevations provided in the approved study will be used in the hydraulic model. Also, pipes along the mainline of a freeway or interstate highway that convey runoff from a roadway low point to the disposal point, a waterway, or a stormwater maintenance facility.
Also pipes along mainline of a land service highway that convey runoff from a roadway low point to the disposal point, a waterway, or a stormwater maintenance facility. Depending on the project location, these agencies could include, but are not limited to, the US Army Corps of Engineers, U. The Stormwater Management Rule governs all projects that provide for ultimately disturbing one 1 or more acres of land or increasing impervious surface by 0.
The following design and performance standards need to be addressed for any project governed by the Stormwater Management Rule: Nonstructural Stormwater Management Strategies, N. If the design engineer determines that it is not feasible for engineering, environmental or safety reason to utilize nonstructural stormwater BMPs, structural BMPs may be utilized.
Groundwater Recharge, N. NJDEP has provided an Excel Spreadsheet to determine the project sites annual groundwater recharge amounts in both pre- and post-development site conditions.
Stormwater Quantity, N. The post-construction hydrograph for the 2-year, year, and year storm events do not exceed, at any point in time, the pre-construction runoff hydrographs for the same storm events. There shall be no increase, as compared to the pre-construction condition, in peak runoff rates of stormwater leaving the project site for the 2-year, year, and year storm events and that the increased volume or change in timing of stormwater runoff will not increase flood damage at or downstream of the site.
This analysis shall include the analysis of impacts of existing land uses and projected land uses assuming full development under existing zoning and land use ordinances in the drainage area. The percentages apply only to the post-construction stormwater runoff that is attributed to the portion of the site on which the proposed development or project is to be constructed. Tidal flooding is the result of higher than normal tides which in turn inundate low lying coastal areas.
Tidal areas are not only activities in tidal waters, but also the area adjacent to the water, including fluvial rivers and streams, extending from the mean high water line to the first paved public road, railroad or surveyable property line.
At a minimum, the zone extends at least feet but no more than feet inland from the tidal water body. Stormwater Quality, N.
Section Stormwater Maintenance Plan, N. At a minimum the maintenance plan shall include specific preventative maintenance tasks and schedules.
For projects located within the Pinelands or Highlands areas of the State, the design engineer should consult with the NJDEP to determine what additional Pinelands and Highlands stormwater management requirements may apply to the project.
Specific requirements for bridges and culverts are contained in N. In cases where the regulatory flood causes the water surface to overflow the roadway, the design engineer shall, by raising the profile of the roadway, by increasing the size of the opening or a combination of both, limit the water surface to an elevation equal to the elevation of the outside edge of shoulder.
The design engineer is cautioned, however, to critically assess the potential hydrologic and hydraulic effects upstream and downstream of the project, which may result from impeding flow by raising the roadway profile, or from decreasing upstream storage and allowing additional flow downstream by increasing existing culvert openings.
The design engineer shall determine what effect the resulting reduction of storage will have on peak flows and the downstream properties in accordance with the Flood Hazard Area Control Act Rules. Stormwater management facilities may be required to satisfy these requirements. Hydraulic evaluation of existing roadway stream crossings may reveal that the water surface elevation for this discharge overtops the roadway.
Compliance with both the bridge and culvert requirements presented in N. In addition to the regulations listed above, the bridge and culvert design will be in compliance with the NJDEPs Technical Manual for Land Use Regulation Program, Bureaus of Inland and Coastal Regulations, Stream Encroachment Permits, which includes the following: Structures will pass the regulatory flood without increasing the upstream elevation of the flood profile by more than 0.
For new structures that result in lowering the downstream water surface elevation by 2 or 3 feet, the engineer must perform a routing analysis to verify that there are no adverse impacts further downstream.
A title block identifying the project by name, the applicant, and the name of the quadrangle. The limits of the project and point of encroachment shown in contrasting colors on the map. In the case of a new or replacement structure or other type encroachment, the regulatory floodwater surface elevation as required for the review and analysis of the project impacts and permit requirements.
The design engineer is also required to determine whether a particular watercourse involved in the project is classified by the State as a Category One waterbody, and if so, shall design the project in accordance with the provisions at N.
Projects involving a Category One waterbody shall be designed such that a foot special water resource protection area is provided on each side of the waterbody. Encroachment within this foot buffer is prohibited except in instances where preexisting disturbance exists. The Soil Erosion and Sediment Control Report shall include calculations and plans that address both temporary and permanent items for the engineering and vegetative standards.
Calculations shall be shown for items that require specific sizing e. For the purpose of this section, hydrology will deal with estimating flood magnitudes as the result of precipitation.
In the design of highway drainage structures, floods are usually considered in terms of peak runoff or discharge in cubic feet per second cfs and hydrographs as discharge per time. For drainage facilities which are designed to control volume of runoff, like detention facilities, or where flood routing through culverts is used, then the entire discharge hydrograph will be of interest. The analysis of the peak rate of runoff, volume of runoff, and time distribution of flow is fundamental to the design of drainage facilities.
Errors in the estimates will result in a structure that is either undersized and causes more drainage problems or oversized and costs more than necessary. In the hydrologic analysis for a drainage facility, it must be recognized that many variable factors affect floods. Some of the factors which need to be recognized and considered on an individual site by site basis include: rainfall amount and storm distribution, drainage area size, shape and orientation, ground cover, type of soil, slopes of terrain and stream s , antecedent moisture condition, storage potential overbank, ponds, wetlands, reservoirs, channel, etc.
The type and source of information available for hydrologic analysis will vary from site to site. It is the responsibility of the design engineer to determine the information required for a particular analysis. This subsection contains hydrologic methods by which peak flows and hydrographs may be determined for the hydraulic evaluation of drainage systems of culverts, channels and median drains.
These hydrologic models are not limited by the size of the drainage area. They are instead limited by uniform curve number, travel time, etc. Most of these limitations can be overcome by subdividing the drainage areas into smaller areas.
See the appropriate users manual for a complete list of limitations for each hydrologic model. Many hydrologic models exist beyond those that are listed here. If a model is not included, then the design engineer should ensure that the model is appropriate and that approvals are obtained from the Department.
The peak flow from a drainage basin is a function of the basins physiographic properties such as size, shape, slope, soil type, land use, as well as climatological factors such as mean annual rainfall and selected rainfall intensities. The methods presented in the guideline should give acceptable predictions for the indicated ranges of drainage area sizes and basin characteristics. Other hydrologic methods may be used only with the approval of the Department.
NOTE: If a watercourse has had a NJDEP adopted study prepared for the particular reach where the project is located, that study should be used for the runoff and water surface profiles. The design engineer should ensure that the hydrologic model used takes into account the NJDEP requirement that the entire upstream drainage area is to be considered fully developed. Computation of peak discharge must consider the condition that yields the largest rate.
Proper hydrograph combination is essential. It may be necessary to evaluate several different hydrograph combinations to determine the peak discharge for basins containing hydrographs with significantly different times for the peak discharge. For example, the peak discharge for a basin with a large undeveloped area contributing toward the roadway may result from either the runoff at the time when the total area reaches the roadway or the runoff from the roadway area at its peak time plus the runoff from the portion of the overland area contributing at the same time.
Basic Assumptions 1. The peak rate of runoff Q at any point is a direct function of the average rainfall intensity I for the Time of Concentration Tc to that point.
The recurrence interval of the peak discharge is the same as the recurrence interval of the average rainfall intensity. The Time of Concentration is the time required for the runoff to become established and flow from the most distant point of the drainage area to the point of discharge. A reason to limit use of the rational method to small watersheds pertains to the assumption that rainfall is constant throughout the entire watershed.
Severe storms, say of a year return period, generally cover a very small area. Applying the high intensity corresponding to a year storm to the entire watershed could produce greatly exaggerated flows, as only a fraction of the area may be experiencing such an intensity at any given time.
The variability of the runoff coefficient also favors the application of the rational method to small, developed watersheds. Although the coefficient is assumed to remain constant, it actually changes during a storm event. The greatest fluctuations take place on unpaved surfaces as in rural settings. In addition, runoff coefficient values are much more difficult to determine and may not be as accurate for surfaces that are not smooth, uniform and impervious.
To summarize, the rational method provides the most reliable results when applied to small, developed watersheds and particularly to roadway drainage design.
The validity of each assumption should be verified for the site before proceeding. Procedure 1. Obtain the following information for each site: 1. Drainage area 2. Soil types highly permeable or impermeable soils 4. Distance from the farthest point of the drainage area to the point of discharge 5. Difference in elevation from the farthest point of the drainage area to the point of discharge 2. Determine the Time of Concentration Tc. Section 3. Minimum Tc is 10 minutes.
Determine the rainfall intensity rate I for the selected recurrence intervals. Select the appropriate C value. The runoff coefficient C accounts for the effects of infiltration, detention storage, evapotranspiration, surface retention, flow routing and interception. The product of C and the average rainfall intensity I is the rainfall excess of runoff per acre.
The runoff coefficient should be weighted to reflect the different conditions that exist within a watershed. Refer to Section 3. Determine the value for rainfall intensity for the selected recurrence interval with a duration equal to the Time of Concentration from Figure North, Figure South, or Figure East. The curves provide for variation in rainfall intensity according to location, storm frequency, and Time of Concentration. Select the curve of a particular region Figure where the site in question is located.
For project that fall on the line or span more than one boundary, the higher intensity should be used for the entire project. Appendix A of HEC contains an example of the development of rainfall intensity curves and. The NRCS approach, however, is more sophisticated in that it considers also the time distribution of the rainfall, the initial rainfall losses to interception and depression storage, and an infiltration rate that decreases during the course of a storm.
With the NRCS method, the direct runoff can be calculated for any storm, either real or fabricated, by subtracting infiltration and other losses from the rainfall to obtain the precipitation excess. Two types of hydrographs are used in the NRCS procedure, unit hydrographs and dimensionless hydrographs.
A unit hydrograph represents the time distribution of flow resulting from 1 inch of direct runoff occurring over the watershed in a specified time. A dimensionless hydrograph represents the composite of many unit hydrographs. The dimensionless unit hydrograph is plotted in non dimensional units of time versus time to peak and discharge at any time versus peak discharge.
Characteristics of the dimensionless hydrograph vary with the size, shape, and slope of the tributary drainage area. The most significant characteristics affecting the dimensionless hydrograph shape are the basin lag and the peak discharge for a specific rainfall. Basin lag is the time from the center of mass of rainfall excess to the hydrograph peak. Steep slopes, compact shape, and an efficient drainage network tend to make lag time short and peaks high; flat slopes, elongated shape, and an inefficient drainage network tend to make lag time long and peaks low.
The NRCS method is based on a hour storm event which has a certain storm distribution. To use this distribution it is necessary for the user to obtain the hour rainfall value for the frequency of the design storm desired.
Central to the NRCS methodology is the concept of the Curve Number CN which relates to the runoff depth and is itself characteristic of the soil type and the surface cover. CNs in Table a to d of the TR Manual June represent average antecedent runoff condition for urban, cultivated agricultural, other agricultural, and arid and semiarid rangeland uses. Infiltration rates of soils vary widely and are affected by subsurface permeability as well as surface intake rates.
Appendix A of the TR Manual defines the four groups and provides a list of most of the soils in the United States and their group classification. The soils in the area of interest may be identified from a soil survey report, which can be obtained from the local Soil Conservation District offices.
Several techniques have been developed and are currently available to engineers for the estimation of runoff volume and peak discharge using the NRCS methodology. Some of the more commonly used of these methods are summarized below: A. This methodology is particularly useful for the comparison of pre- and post-development runoff rates and consequently for the design of control structures.
There are basically two variations of this technique: the Tabular Hydrograph method and the Graphical Peak Discharge method. The procedure divides the watershed into subareas, completes an outflow hydrograph for each subarea and then combines and routes these hydrographs to the watershed outlet. This method is particularly useful for measuring the effects of changed land use in a part of the watershed. However, this method is sufficient to estimate the effects of urbanization on peak rates of discharge for most heterogeneous watersheds.
This method calculates peak discharge using an assumed hydrograph and a thorough and rapid evaluation of the soils, slope and surface cover characteristics of the watershed. The Graphical method provides a determination of peak discharge only. If a hydrograph is required or subdivision is needed, the Tabular Hydrograph method should be used.
This method should not be used if the weighted CN is less than For a more detailed account of these methods and their limitations the design engineer is referred to the NRCS TR document. The model may be used to simulate runoff in a simple single basin watershed or in a highly complex basin with a virtually unlimited number of sub-basins and for routing interconnecting reaches. It can also be used to analyze the impact of changes in land use and detention basins on the downstream reaches.
It can serve as a useful tool in comprehensive river basin planning and in the development of area-wide watershed management plans.
Other synthetic unit hydrograph methods available in HEC-1 can be used with the approval of the Department. Refer to the available Program Documentation Manual for additional information. The NRCS TR Model: This computer program is a rainfall-runoff simulation model which uses a storm hydrograph, runoff curve number and channel features to determine runoff volumes as well as unit hydrographs to estimate peak rates of discharge. The dimensionless unit hydrographs from sub-basins within the watershed can be routed through stream reaches and impoundments.
The TR method may be used to analyze the impact of development and detention basins on downstream areas. The parameters needed in this method include total rainfall, rainfall distribution, curve numbers, Time of Concentration, travel time and drainage area. It may take a few computations at different locations. Tc is computed by summing all the travel times for consecutive components of the drainage conveyance system.
Tc influences the shape and peak of the runoff hydrograph. Development usually decreases the Tc, thereby increasing the peak discharge, but Tc can be increased as a result of a ponding behind small or inadequate drainage systems, including storm drain inlets and road culverts, or b reduction of land slope through grading.
Surface Roughness: One of the most significant effects of development on flow velocity is less retardance of flow. That is, undeveloped areas with very slow and shallow overland flow through vegetation become modified by development; the flow is then delivered to streets, gutters, and storm sewers that transport runoff downstream more rapidly. Travel time through the watershed is generally decreased.
Channel Shape and Flow Patterns: In small watersheds, much of the travel time results from overland flow in upstream areas. Typically, development reduces overland flow lengths by conveying storm runoff into a channel as soon as possible.
Since channel designs have efficient hydraulic characteristics, runoff flow velocity increases and travel time decreases. Slope: Slopes may be increased or decreased by development, depending on the extent of site grading or the extent to which storm sewers and street ditches are used in the design of the stormwater management system.
Slope will tend to increase when channels are straightened and decrease when overland flow is directed through storm sewers, street gutters, and diversions. Sheet flow is sometimes commonly referred to as overland flow. The type of flow that occurs is a function of the conveyance system and is best determined by field inspection, review of topographic mapping and subsurface drainage plans.
A brief overview of methods to compute travel time for the components of the conveyance system is presented below. Rational Method: Travel time for each flow regime shall be calculated as described below: a.
Sheet Flow: Using the slope and land cover type, determine the velocity from Figure Gutter Flow: The gutter flow component of Time of Concentration can be computed using the velocity obtained from the Manning equation for the triangular gutter of a configuration and longitudinal slope as indicated by roadway geometry. Pipe Flow: Travel time in a storm sewer can be computed using full flow velocities for the reach as appropriate.
Open Channel Flow: Travel time in an open channel such as a natural stream, swale, man-made ditch, etc. The maximum length of sheet flow to be used is feet. The open channel portion may be a natural channel, man-made ditch, or gutter flow along the roadway.
The open channel portion time is determined by using the Mannings equation or other acceptable procedure for open channel flow such as HEC Refer to TR, Chapter 3 for detailed information on the procedures. The minimum Time of Concentration used shall be 10 minutes. This type of design may result in major drainage and flooding problems downstream. Under favorable conditions, the temporary storage of some of the storm runoff can decrease downstream flows and often the cost of the downstream conveyance system.
Flood routing shall be used to document the required storage volume to achieve the desired runoff control. A hydrograph is required to accomplish the flood routing. A hydrograph represents a plot of the flow, with respect to time.
The predicted peak flow occurs at the time, Tc. The area under the hydrograph represents the total volume of runoff from the storm. A hydrograph can be computed using either the Modified Rational Method for drainage areas up to 20 acres or the Soil Conservation Service hour storm methodology described in previous sections.
Storage may be concentrated in large basin-wide regional facilities or distributed throughout the watershed. Storage may be developed in roadway interchanges, parks and other recreation areas, small lakes, ponds and depressions. The utility of any storage facility depends on the amount of storage, its location within the system, and its operational characteristics. An analysis of such storage facilities should consist of comparing the design flow at a point or points downstream of the proposed storage site with and without storage.
In addition to the design flow, other flows in excess of the design flow that might be expected to pass through the storage facility should be included in the analysis. The design criteria for storage facilities should include: release rate, storage and volume, grading and depth requirements, outlet works, and location.
Control structure release rates shall be in accordance with criteria outlined in Drainage Policy. Multi-stage control structures may be required to control runoff from different frequency events. Storage volume shall be adequate to meet the criteria outlined in Stormwater Management and Non-Point Source Pollution Control, to attenuate the post-development peak discharge rates or to meet the Allowable Water Surface Elevation.
Outlet works selected for storage facilities typically include a principal spillway and an emergency overflow, and must be able to accomplish the design functions of the facility.
Outlet works can take the form of combinations of drop inlets, pipes, weirs, and orifices. The total stage-discharge curve shall take into account the discharge characteristics of all outlet works.
Detailed information on outlet hydraulics can be found in the "Handbook of Hydraulics", by Brater and King. Stormwater storage facilities are often referred to as either detention or retention facilities. For the purposes of this section, detention facilities are those that are designed to reduce the peak discharge and detain the quantity of runoff required to achieve this objective for a relatively short period of time.
These facilities are designed to completely drain after the design storm has passed. Retention facilities are designed to contain a permanent pool of water. Since most of the design procedures are the same for detention and retention facilities, the term storage facilities will be used in this chapter to include detention and retention facilities.
Routing calculations needed to design storage facilities, although not extremely complex, are time consuming and repetitive. Many reservoir routing computer programs, such as HEC-1, TR and Pond-2, are available to expedite these calculations.
Use of programs to perform routings is encouraged. Natural channels are crossed at highway sites and often need to be modified to accommodate the construction of a modern highway. Channels in the form of roadside ditches are added to the natural drainage pattern. This part contains design methods and criteria to aid the design engineer in preparing designs incorporating these factors.
Other open channel analysis methods and erosion protection information is also included. Either grassed channels or non-erodible channels are typically used. The features of each are presented in the following narrative. Grassed Channels: The grassed channel is protected from erosion by a turf cover. It is used in highway construction for roadside ditches, medians, and for channel changes of small watercourses.
A grassed channel has the advantage of being compatible with the natural environment. This type of channel should be selected for use whenever possible. Non-erodible Channel: A non-erodible channel has a lining that is highly resistant to erosion.
This type of channel is expensive to construct, although it should have a very low maintenance cost if properly designed. Non-erodible lining should be used when stability cannot be achieved with a grass channel. Typical lining materials are discussed in the following narrative. Concrete Ditch Lining: Concrete ditch lining is extremely resistant to erosion. Its principal disadvantages are high initial cost, susceptibility to failure if undermined by scour and the tendency for scour to occur downstream due to an acceleration of the flow velocity on a steep slope or in critical locations where erosion would cause extensive damage.
Aggregate Ditch Lining: This lining is very effective on mild slopes. It is constructed by dumping crushed aggregate into a prepared channel and grading to the desired shape. The advantages are low construction cost and self-healing characteristics. It has limited application on steep slopes where the flow will tend to displace the lining material. Alternative Linings: Other types of channel lining such as gabion, or an articulated block system may be approved by the Department on a case-by-case basis, especially for steep sloped high velocity applications.
Road Ditches: Road ditches are channels adjacent to the roadway used to intercept runoff and groundwater occurring from areas within and adjacent to the right-of-way and to carry this flow to drainage structures or to natural waterways. Road ditches should be grassed channels except where non-erodible lining is warranted. A minimum desirable slope of 0. Interceptor Ditch: Interceptor ditches are located on the natural ground near the top edge of a cut slope or along the edge of the right-of-way to intercept runoff from a hillside before it reaches the backslope.
Interceptor ditches should be built back from the top of the cut slope, and generally at a minimum slope of 0. In potential slide areas, stormwater should be removed as rapidly as practicable and the ditch lined if the natural soil is permeable.
Channel Changes: Realignment or changes to natural channels should be held to a minimum. The following examples illustrate conditions that warrant channel changes: 1. The natural channel crosses the roadway at an extreme skew. The embankment encroaches on the channel. The natural channel has inadequate capacity.
The location of the natural channel endangers the highway embankment or adjacent property. Grade Control Structure: A grade control structure allows a channel to be carried at a mild grade with a drop occurring through the structure check dam. Methods to design grass-lined and non-erodible channels are presented in the following narrative. Grassed Channel: A grassed channel shall have a capacity designated in Section 2.
A non-erodible channel should be used in locations where the design flow would cause a grassed channel to erode. Non-Erodible Channels: Non-erodible channels shall have a capacity as designated in Section 2. The unlined portion of the channel banks should have a good stand of grass established so large flows may be sustained without significant damage.
The minimum design requirements of non-erodible channels shall be in accordance with the NJDOT Soil Erosion and Sediment Control Standards Manual where appropriate unless otherwise stated in this section.
Capacity: The required size of the channel can be determined by use of the Mannings equation for uniform flow. Mannings formula gives reliable results if the channel cross section, roughness, and slope are fairly constant over a sufficient distance to establish uniform flow. The Mannings equation is as follows: 1. Formed, no finish 2. Trowel finish 3. Float finish 4. Float finish, some gravel on bottom 5. Gunite, good section 6. For non-uniform flow, a computer program, such as HEC-2, should be used to design the channel.
Height of Lining: The height of the lined channel should be equal to the normal depth of flow D based on the design flow rate, plus 1 foot for freeboard if possible. Horizontal Alignment: Water tends to superelevate and cross waves are formed at a bend in a channel. If the flow is supercritical as it will usually be for concrete-lined channels , this may cause the flow to erode the unlined portion of the channel on the outside edge of the bend.
This problem may be alleviated either by superelevating the channel bed, adding freeboard to the outside edge, or by choosing a larger radius of curvature. Friction Factor Range 0. Freeboard in feet ft. Radius of curvature in feet ft. Additional Design Requirements: a. The minimum d50 stone size shall be 6 inches. The filter layer shall be filter fabric wherever possible. A 3 feet wide by 3 feet deep cutoff wall extending a minimum of 3 feet below the channel bed shall be provided at the upstream and downstream limits of the non-erodible channel lining.
Additional design requirements may be required for permit conditions or as directed by the Department. Gradation of Aggregate Lining: The American Society of Civil Engineers Subcommittee recommends the following rules as to the gradation of the stone: 1 Stone equal to or larger than the theoretical d50, with a few larger stones, up to about twice the weight of the theoretical size tolerated for reasons of economy in the utilization of the quarried rock, should make up 50 percent of the rock by weight.
This requires tolerance of about 30 percent of the thickness of the stone. Detailed requirements regarding water quality control is included in Section Water on the pavement slows traffic and contributes to accidents from hydroplaning and loss of visibility from splash and spray.
Free-standing puddles which engage only one side of a vehicle are perhaps the most hazardous because of the dangerous torque levels exerted on the vehicle. Thus, the design of the surface drainage system is particularly important at locations where ponding can occur.
Runoff collection and conveyance for a curbed roadway is typically provided by a system of inlets and pipe, respectively. Runoff from an uncurbed roadway, typically referred to as an umbrella section, proceeds overland away from the roadway in fill sections or to roadside swales or ditches in roadway cut sections. Conveyance of surface runoff over grassed overland areas or swales and ditches allows an opportunity for the removal of contaminants.
The ability of the grass to prevent erosion is a major consideration in the design of grass-covered facilities. Use of an umbrella roadway section may require additional ROW. Areas with substantial development adjacent to the roadway, particularly in urbanized areas, typically are not appropriate for use of a roadway umbrella section. The decision to use an umbrella section requires careful consideration of the potential problems.
Benefits associated with umbrella sections include cost savings and eliminating the possibility of vehicle vaulting. Umbrella sections used on roadways with higher longitudinal slopes have been found to be prone to berm washouts. Debris build-up along the edge of the roadway creates a curb effect that prevents sheet flow and directs the water along the edge of the roadway.
This flow usually continues along the edge until a breach is created, often resulting in substantial erosion. Some situations may also warrant installing inlets along the edge of an umbrella section to pick up water which may become trapped by berm buildup or when snow is plowed to the side of the roadway and creates a barrier that will prevent sheet flow from occurring.
Bermed sections are designed with a small earth berm at the edge of the shoulder to form a gutter for the. Care should be taken to avoid earth berms on steep slopes that would cause erosive velocities yielding berm erosion.
An umbrella section should be used where practical. However, low points at umbrella sections should have inlets and discharge pipes to convey the runoff safely to the toe of slope.
A Type E inlet and minimum 15 inch diameter pipe shall be used to drain the low point. Snow inlets Section 5. Umbrella sections should be avoided on land service roadways where there are abutting properties and driveways. Slope treatment shall be provided at all low points of umbrella sections and all freeway and interstate projects to provide erosion protection see NJDOT Standard Details.
Combination Inlets 2. A special inlet shall be designed, with the appropriate detail provided in the construction plans, and the item shall be designated "Special Inlet", when the pipe size requires a structure larger than a Type B2, B2 Modified or E2.
A special inlet shall also be designed, with the appropriate detail provided in the construction plans, and the item shall be designated "Special Inlet", when the transverse pipe size requires a structure larger than the standard inlet types. Drainage structure layout should minimize irregularities in the pavement surface.
Manholes should be avoided where practicable in the traveled way and shoulder. An example is a widening project where inlets containing a single pipe should be demolished and the pipe extended to the proposed inlet, as opposed to placing a slab with a standard manhole cover or square frame with round cover on the existing inlet and extending the pipe to the new inlet.
The typical curbed gutter section is a right triangular shape with the curb forming the vertical leg of the triangle. Design shall be based on the following frequencies: Recurrence Interval Year Year Facility Description Freeway or interstate highway Land service highway. The Manning equation has been modified to allow its use in the calculation of curbed gutter capacity for a triangular shaped gutter.
Concrete gutter troweled finish 0. Asphalt pavement 1 Smooth texture 0. Concrete gutter with asphalt pavement 1 Smooth 0. Concrete pavement 1 Float finish 0. Brick 0. As spread from the curb increases, the risks of traffic accidents and delays and the nuisance and possible hazard to pedestrian traffic increase. The following shall be used to determine the allowable spread. Width of inside and outside shoulder along interstate and freeway mainline 2.
Each standard is described below. These standards help prevent certain solids and floatables e. For new roadway projects and reconstruction of existing highway, storm drain inlets must be selected to meet the following design requirements: A. The first option especially for storm drain inlets along roads is simply to use the Departments bicycle safe grate.
The other option is to use a different grate, as long as each clear space in the grate each individual opening is: No larger than seven 7. Curb-Opening Inlets If the storm drain inlet has a curb opening, the clear space in that curb opening or each individual clear space, if the curb opening has two or more clear spaces must be: No larger than two 2.
Storm Drain inlets that are located at rest areas, service areas, maintenance facilities, and along streets with sidewalks operated by the Department are required to have a label placed on or adjacent to the inlet.
The label must contain a cautionary message about dumping pollutants. No Waste Here. Although a stand-alone graphic is permissible, the Department strongly recommends that a short phrase accompany the graphic. The hydraulic capacity of an inlet depends on its geometry and gutter flow characteristics. Inlets on grade demonstrate different hydraulic operation than inlets in a sump. The design procedures for inlets on grade are presented in Section 5.
The design procedures for inlets in a sump are presented in Section 5. Proper hydraulic design in accordance with the design criteria maximizes inlet capture efficiency and spacing. An alternative procedure, that yields results reasonably close to those obtained by using the runoff collection capacity equation presented above, is to compute the collection capacity in accordance with the procedures presented in Federal Highway Administration, Hydraulic Engineering Circular No.
Use of computer programs is encouraged to perform the tedious hydraulic capacity calculations. HEC contains useful charts and tables. The HEC procedure is also incorporated in a number of computer software programs.
Procedures to compute the collection capacity for each condition are presented separately below. The weir flow coefficient is 3. A, B Mod. The equations must be modified for use with inlets that do not have a curb opening to account for reduced interception capacity resulting from debris collecting on the grate. The perimeter around the open area of the grate P used in the weir equation should be divided in half for inlets without a curb opening. The orifice flow coefficient is 0.
The equations must be modified for use with inlets that do not have a curb opening to account for reduced interception capacity Inlet Type.
The clear opening area of the grate Ao used in the orifice equation should be divided in half for inlets without a curb opening. Inlets should be located primarily as required by spread computations. See Section 5. Additional items to be considered when locating inlets include: A. Low points in gutter grade. Adjust grades to the maximum extent possible to ensure that low point do not occur at driveways, handicap accessible areas, critical access points, etc.
At intersections and ramp entrances and exits to limit the flow of water across roadways. Upgrade of all bridges and downgrade of bridges in fill section before the end of curb where the curb is not continuous. Along mainline and ramps as necessary to limit spread of runoff onto roadway in accordance with Section 5.
Maximum distance between inlets is feet. The procedure for spacing of inlets is as follows: 1. Calculate flow and spread in the gutter. Tributary area is from high point to location of first inlet. This location is selected by the design engineer. Overland areas that flow toward the roadway are included. Place the first inlet at the location where spread approaches the limit listed in Section 5.
Calculate the amount of water intercepted by the inlet, check the grate efficiency. The water that bypasses the first inlet should be included in the flow and spread calculation for the next inlet. This procedure is repeated to the end of the system.
Sample calculations are presented in Section Also, the downstream transition out of the depression causes backwater which further increases the amount of water captured. Locations of Depressed Inlets 1. All inlets in shoulders greater than 4 feet wide. All inlets in one-lane, low speed ramps. Inlets will not be depressed next to a riding lane, acceleration lane, deceleration lane, two-lane ramps, and direct connection ramps or within the confines of a bridge approach and transition slab.
Limits of Depression 1. Begin depression a distance of 4 feet upgrade of inlet. End depression a distance of 2 feet downgrade of inlet. Begin depression 4 feet out from gutter line. Depth of depression, 2 inches below projected gutter grade. Spacing of Depressed Inlets Use the same procedure as described in Section 5. This method will give a conservative distance between inlets; however, this will provide an added safety factor and reduce the number of times that water will flow on the highway riding lanes when the design storm is exceeded.
Collection of snow melt runoff is important on the. A discussion of each situation and the design approach is outlined below. Snowmelt Collection on High Side of Superelevation Collection of snow melt on the high side of a superelevated section from roadway and berm areas before it crosses the roadway prevents icing during the freeze-thaw process. The snow melt inlets should be placed along the outer curbline at the upstream side of all intersections and at convenient cross drain locations.
The snow melt inlets should be connected to the drainage system with a 15 inch diameter pipe to the trunk storm sewer. The small shoulder and snow inlets will not be designed to control stormwater runoff but shall be designed to handle only the small amount of expected flow from the snowmelt. Snowmelt Collection at Low Points Collection of snowmelt is important at low points where the pile-up of snow over existing inlets prevents draining of snowmelt and runoff off the edge of road.
The addition of inlets placed away from the edge of curb and beyond anticipated snow piles provides a means to drain snowmelt. Snow inlets are required at all roadway profile low points. All snow inlets shall be Type "E".
Snow inlets shall not be depressed. Snow inlets shall be provided in the shoulder immediately adjacent to the travel lane without encroaching on the travel lane. Snow inlets shall not be installed in shoulders where the width is so narrow that placement of a snow inlet will encroach upon the inlet at the curb.
Compliance with the established spread criteria for roadways with flat grades typically requires many inlets, usually installed at close intervals.
Use of alternative collection systems such as trench drains may be appropriate to reduce the number of inlets required to satisfy the spread criteria. Therefore, use of trench drains for runoff collection on roads with flat grades may be warranted. The trench drain should be located upstream of the inlet to which it connects. The length of trench drain should provide the capture capacity that together with the inlet limits bypass at the inlet to zero.
Trench drain capture computations require consideration of both frontal and side flow capture. Frontal flow captured by the narrow trench drain is small and is, therefore, disregarded.
Side flow into the trench drain is similar to flow into a curb opening inlet. The trench drain must be long enough to intercept the bypass after frontal flow plus the additional runoff contributed by the roadway for the length of the trench drain. The process includes the following steps: A. Compute the total runoff to the inlet. Compute the frontal flow captured by inlet with no bypass allowed for the spread limited to the width of the grate.
The runoff to be intercepted by the trench drain is the total runoff minus the runoff captured by the inlet. The computed length shall be multiplied by two to reflect inefficiencies due to clogging. Maintenance requirements for trench drains should also be considered in the evaluation of trench drains.
Use of a trench drain system should be discussed with the Department early in the design process with recommendations submitted prior to completion of the Initial Submission. This section. Minimum pipe size is 15 inches. Minimum pipe size is 18 inches downstream of mainline lowpoints. Storm sewer pipe materials for proposed systems typically include concrete, corrugated metal, aluminum alloy and Smooth interior High Density Polyethylene HDPE.
Manning's roughness coefficient "n" for concrete and HDPE pipe is 0. Manning's roughness coefficient values for corrugated metal and aluminum alloy pipe are presented in Table Manning's roughness coefficients for other materials occasionally encountered are indicated below: Table Wood forms, rough 0. Wood forms, smooth 0. Steel forms 0. Concrete floor and top 0. Natural floor 0. Design to flow full, based on uniform flow.
Minimum self-cleaning velocity of 2. Spiral flow will not occur when the following conditions exist, in which case the "n" value for annular corrugations is to be used:. Partly full flow Non-circular pipes, such as pipe arches When helical C. Pipe arches have the same roughness characteristics as their equivalent round pipes F. Aluminum Alloy Pipes as recommended by manufacturer 4. Target operating speeds cannot be determined arbitrarily, but must be consistent with conditions along the roadway and subject to reasonable enforcement.
The designer may proactively alter the existing geometry and roadway environment in an attempt to decrease the operating speed and enhance the safety of pedestrians and bicyclists, or the viability of downtown or residential areas, in balance, not competition, with the safety of motorists.
Roadway design should lead the driver to adopt a driving behavior appropriate to local conditions. The designer thus should carefully consider the appropriate target speed for a roadway section based upon land use conditions, building densities, the environment and the disparate needs of users of the facility. It should be recognized that streets do not only serve transportation related functions but are also places of commercial and social encounter.
Therefore, a designer should also consider the non-vehicular uses of a roadway and seek consistency between all aspects of the roadway, its environment, and the chosen Design Speed. What does this mean? If a physical, environmental or other impediment posed an obstacle to a project, the Design Speed established the limit below which it would be difficult to compromise, in effect, the maximum safe speed.
If, however, no such obstacles exist on one or more stretches of road, the design would be to optimum standards, potentially yielding an infinite Design Speed. This could lead to inconsistencies between the Design Speed, posted speed, and desirable vehicle operating speed, and result in drivers making inappropriate decisions.
The NJDOT's Design Philosophy takes into account functional classification, existing or intended land use, and the context of the project, and then uses an appropriately selected Design Speed as the basis for all of the design elements.
If there are no physical or environmental impediments to alter the geometry of a roadway, the designer may consider introducing design elements that reinforce and encourage the intended operating speed, which should be based on the needs of all road users. There is a wide range of options available to the designer to do so, including some that fall under the umbrella "traffic calming.
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Hydraulic Analysis: Select a drainage system to accommodate the design flow utilizing the procedures outlined in the following parts: 1. Channel Design Section 4. Drainage of Highway Pavements Section 5. Storm Drains - Section 6. Median Drainage Section 7. Culverts - Section 8. Environmental Considerations: Environmental impact of the proposed drainage system and appropriate methods to avoid or mitigate adverse impacts should be evaluated.
Items to be considered include: 1. Stormwater Management 2. Water Quality 3. Soil Erosion and Sediment Control 4. Special Stormwater Collection Procedures 5. Special Stormwater Disposal Procedures These elements should be considered during the design process and incorporated into the design as it progresses. Drainage Review: The design engineer should inspect the drainage system sites to check topography and the validity of the design.
Items to check include: 1. Drainage Area a. Size b. Land Use c. Improvements 2. Effects of Allowable Computed Headwater 3. Performance of Existing or Adjacent Structures a.
Evidence of High Water 4. Channel Condition a. Erosion b. Vegetation c. Alignment of Proposed Facilities with Channel 5. Impacts on Environmentally Sensitive Areas. In many instances, stormwater problems signal either misuse of a resource or unwise land activity. Poor management of stormwater increases total flow, flow rate, flow velocity and depth of water in downstream channels. In addition to stormwater peak discharge and volume impacts, roadway construction or modification usually increases non-point source pollution primarily due to the increased impervious area.
Section 2. An assessment of the impacts the project will have on existing peak flows and watercourses shall be made by the design engineer during the initial phase. Stormwater management, whether structural or non-structural, on or off site, must fit into the natural environment, and be functional, safe, and aesthetically acceptable. Several alternatives to manage stormwater and provide water quality may be possible for any location. Careful design and planning by the engineer, hydrologist, biologist, environmentalist, and landscape architect can produce optimum results.
The design engineer should be guided by this and include measures in design plans that are compatible with the site specific surroundings. Revegetation with native, non-invasive grasses, shrubs and possibly trees may be required to achieve compatibility with the surrounding environment.
Disposal of roadway runoff to available waterways that either cross the roadway or are adjacent to it spaced at large distances, requires installation of long conveyance systems. Vertical design constraints may make it impossible to drain a pipe or swale system to existing waterways. Discharging the runoff to the groundwater with a series of leaching or seepage basins sometimes called a Dry Well may be an appropriate alternative if groundwater levels and non-contaminated, permeable soil conditions allow a properly designed system to function as designed.
The decision to select a seepage facility design must consider geotechnical, maintenance, and possibly right-of-way ROW impacts and will only be allowed if no alternative exists. The seepage facilities must be designed to store the entire runoff volume for a design storm compatible with the storm frequency used for design of the roadway drainage facilities or as directed by the Department.
As a minimum, the seepage facilities shall be designed to store the increase in runoff volume from new impervious surfaces as long as adequate overflow conveyance paths are available to safely carry the larger flows to a stable discharge point. Installation of seepage facilities can also satisfy runoff volume control and water quality concerns which may be required by an environmental permit.
The proposed drainage structure should cause a ponding level, hydraulic grade line elevation, or backwater elevation no greater than the AWS when the design flow is imposed on the facility. An AWS that exceeds a reasonable limit may require concurrence of the affected property owner.
Army Corps of Engineers, or other recognized agencies will be available at some sites. The elevations provided in the approved study will be used in the hydraulic model. Also, pipes along the mainline of a freeway or interstate highway that convey runoff from a roadway low point to the disposal point, a waterway, or a stormwater maintenance facility.
Also pipes along mainline of a land service highway that convey runoff from a roadway low point to the disposal point, a waterway, or a stormwater maintenance facility. Depending on the project location, these agencies could include, but are not limited to, the US Army Corps of Engineers, U.
The Stormwater Management Rule governs all projects that provide for ultimately disturbing one 1 or more acres of land or increasing impervious surface by 0.
The following design and performance standards need to be addressed for any project governed by the Stormwater Management Rule: Nonstructural Stormwater Management Strategies, N. If the design engineer determines that it is not feasible for engineering, environmental or safety reason to utilize nonstructural stormwater BMPs, structural BMPs may be utilized.
Groundwater Recharge, N. NJDEP has provided an Excel Spreadsheet to determine the project sites annual groundwater recharge amounts in both pre- and post-development site conditions. Stormwater Quantity, N. The post-construction hydrograph for the 2-year, year, and year storm events do not exceed, at any point in time, the pre-construction runoff hydrographs for the same storm events.
There shall be no increase, as compared to the pre-construction condition, in peak runoff rates of stormwater leaving the project site for the 2-year, year, and year storm events and that the increased volume or change in timing of stormwater runoff will not increase flood damage at or downstream of the site.
This analysis shall include the analysis of impacts of existing land uses and projected land uses assuming full development under existing zoning and land use ordinances in the drainage area. The percentages apply only to the post-construction stormwater runoff that is attributed to the portion of the site on which the proposed development or project is to be constructed.
Tidal flooding is the result of higher than normal tides which in turn inundate low lying coastal areas. Tidal areas are not only activities in tidal waters, but also the area adjacent to the water, including fluvial rivers and streams, extending from the mean high water line to the first paved public road, railroad or surveyable property line. At a minimum, the zone extends at least feet but no more than feet inland from the tidal water body.
Stormwater Quality, N. Section Stormwater Maintenance Plan, N. At a minimum the maintenance plan shall include specific preventative maintenance tasks and schedules.
For projects located within the Pinelands or Highlands areas of the State, the design engineer should consult with the NJDEP to determine what additional Pinelands and Highlands stormwater management requirements may apply to the project.
Specific requirements for bridges and culverts are contained in N. In cases where the regulatory flood causes the water surface to overflow the roadway, the design engineer shall, by raising the profile of the roadway, by increasing the size of the opening or a combination of both, limit the water surface to an elevation equal to the elevation of the outside edge of shoulder.
The design engineer is cautioned, however, to critically assess the potential hydrologic and hydraulic effects upstream and downstream of the project, which may result from impeding flow by raising the roadway profile, or from decreasing upstream storage and allowing additional flow downstream by increasing existing culvert openings. The design engineer shall determine what effect the resulting reduction of storage will have on peak flows and the downstream properties in accordance with the Flood Hazard Area Control Act Rules.
Stormwater management facilities may be required to satisfy these requirements. Hydraulic evaluation of existing roadway stream crossings may reveal that the water surface elevation for this discharge overtops the roadway. Compliance with both the bridge and culvert requirements presented in N. In addition to the regulations listed above, the bridge and culvert design will be in compliance with the NJDEPs Technical Manual for Land Use Regulation Program, Bureaus of Inland and Coastal Regulations, Stream Encroachment Permits, which includes the following: Structures will pass the regulatory flood without increasing the upstream elevation of the flood profile by more than 0.
For new structures that result in lowering the downstream water surface elevation by 2 or 3 feet, the engineer must perform a routing analysis to verify that there are no adverse impacts further downstream.
A title block identifying the project by name, the applicant, and the name of the quadrangle. The limits of the project and point of encroachment shown in contrasting colors on the map.
In the case of a new or replacement structure or other type encroachment, the regulatory floodwater surface elevation as required for the review and analysis of the project impacts and permit requirements. The design engineer is also required to determine whether a particular watercourse involved in the project is classified by the State as a Category One waterbody, and if so, shall design the project in accordance with the provisions at N.
Projects involving a Category One waterbody shall be designed such that a foot special water resource protection area is provided on each side of the waterbody. Encroachment within this foot buffer is prohibited except in instances where preexisting disturbance exists. The Soil Erosion and Sediment Control Report shall include calculations and plans that address both temporary and permanent items for the engineering and vegetative standards. Calculations shall be shown for items that require specific sizing e.
For the purpose of this section, hydrology will deal with estimating flood magnitudes as the result of precipitation. In the design of highway drainage structures, floods are usually considered in terms of peak runoff or discharge in cubic feet per second cfs and hydrographs as discharge per time.
For drainage facilities which are designed to control volume of runoff, like detention facilities, or where flood routing through culverts is used, then the entire discharge hydrograph will be of interest. The analysis of the peak rate of runoff, volume of runoff, and time distribution of flow is fundamental to the design of drainage facilities. Errors in the estimates will result in a structure that is either undersized and causes more drainage problems or oversized and costs more than necessary.
In the hydrologic analysis for a drainage facility, it must be recognized that many variable factors affect floods. Some of the factors which need to be recognized and considered on an individual site by site basis include: rainfall amount and storm distribution, drainage area size, shape and orientation, ground cover, type of soil, slopes of terrain and stream s , antecedent moisture condition, storage potential overbank, ponds, wetlands, reservoirs, channel, etc.
The type and source of information available for hydrologic analysis will vary from site to site. It is the responsibility of the design engineer to determine the information required for a particular analysis. This subsection contains hydrologic methods by which peak flows and hydrographs may be determined for the hydraulic evaluation of drainage systems of culverts, channels and median drains.
These hydrologic models are not limited by the size of the drainage area. They are instead limited by uniform curve number, travel time, etc. Most of these limitations can be overcome by subdividing the drainage areas into smaller areas. See the appropriate users manual for a complete list of limitations for each hydrologic model.
Many hydrologic models exist beyond those that are listed here. If a model is not included, then the design engineer should ensure that the model is appropriate and that approvals are obtained from the Department. The peak flow from a drainage basin is a function of the basins physiographic properties such as size, shape, slope, soil type, land use, as well as climatological factors such as mean annual rainfall and selected rainfall intensities. The methods presented in the guideline should give acceptable predictions for the indicated ranges of drainage area sizes and basin characteristics.
Other hydrologic methods may be used only with the approval of the Department. NOTE: If a watercourse has had a NJDEP adopted study prepared for the particular reach where the project is located, that study should be used for the runoff and water surface profiles.
The design engineer should ensure that the hydrologic model used takes into account the NJDEP requirement that the entire upstream drainage area is to be considered fully developed. Computation of peak discharge must consider the condition that yields the largest rate. Proper hydrograph combination is essential. It may be necessary to evaluate several different hydrograph combinations to determine the peak discharge for basins containing hydrographs with significantly different times for the peak discharge.
For example, the peak discharge for a basin with a large undeveloped area contributing toward the roadway may result from either the runoff at the time when the total area reaches the roadway or the runoff from the roadway area at its peak time plus the runoff from the portion of the overland area contributing at the same time.
Basic Assumptions 1. The peak rate of runoff Q at any point is a direct function of the average rainfall intensity I for the Time of Concentration Tc to that point. The recurrence interval of the peak discharge is the same as the recurrence interval of the average rainfall intensity. The Time of Concentration is the time required for the runoff to become established and flow from the most distant point of the drainage area to the point of discharge.
A reason to limit use of the rational method to small watersheds pertains to the assumption that rainfall is constant throughout the entire watershed. Severe storms, say of a year return period, generally cover a very small area. Applying the high intensity corresponding to a year storm to the entire watershed could produce greatly exaggerated flows, as only a fraction of the area may be experiencing such an intensity at any given time. The variability of the runoff coefficient also favors the application of the rational method to small, developed watersheds.
Although the coefficient is assumed to remain constant, it actually changes during a storm event. The greatest fluctuations take place on unpaved surfaces as in rural settings. In addition, runoff coefficient values are much more difficult to determine and may not be as accurate for surfaces that are not smooth, uniform and impervious. To summarize, the rational method provides the most reliable results when applied to small, developed watersheds and particularly to roadway drainage design.
The validity of each assumption should be verified for the site before proceeding. Procedure 1. Obtain the following information for each site: 1.
Drainage area 2. Soil types highly permeable or impermeable soils 4. Distance from the farthest point of the drainage area to the point of discharge 5. Difference in elevation from the farthest point of the drainage area to the point of discharge 2.
Determine the Time of Concentration Tc. Section 3. Minimum Tc is 10 minutes. Determine the rainfall intensity rate I for the selected recurrence intervals. Select the appropriate C value. The runoff coefficient C accounts for the effects of infiltration, detention storage, evapotranspiration, surface retention, flow routing and interception.
The product of C and the average rainfall intensity I is the rainfall excess of runoff per acre. The runoff coefficient should be weighted to reflect the different conditions that exist within a watershed. Refer to Section 3. Determine the value for rainfall intensity for the selected recurrence interval with a duration equal to the Time of Concentration from Figure North, Figure South, or Figure East. The curves provide for variation in rainfall intensity according to location, storm frequency, and Time of Concentration.
Select the curve of a particular region Figure where the site in question is located. For project that fall on the line or span more than one boundary, the higher intensity should be used for the entire project. Appendix A of HEC contains an example of the development of rainfall intensity curves and. The NRCS approach, however, is more sophisticated in that it considers also the time distribution of the rainfall, the initial rainfall losses to interception and depression storage, and an infiltration rate that decreases during the course of a storm.
With the NRCS method, the direct runoff can be calculated for any storm, either real or fabricated, by subtracting infiltration and other losses from the rainfall to obtain the precipitation excess.
Two types of hydrographs are used in the NRCS procedure, unit hydrographs and dimensionless hydrographs. A unit hydrograph represents the time distribution of flow resulting from 1 inch of direct runoff occurring over the watershed in a specified time.
A dimensionless hydrograph represents the composite of many unit hydrographs. The dimensionless unit hydrograph is plotted in non dimensional units of time versus time to peak and discharge at any time versus peak discharge. Characteristics of the dimensionless hydrograph vary with the size, shape, and slope of the tributary drainage area.
The most significant characteristics affecting the dimensionless hydrograph shape are the basin lag and the peak discharge for a specific rainfall. Basin lag is the time from the center of mass of rainfall excess to the hydrograph peak. Steep slopes, compact shape, and an efficient drainage network tend to make lag time short and peaks high; flat slopes, elongated shape, and an inefficient drainage network tend to make lag time long and peaks low.
The NRCS method is based on a hour storm event which has a certain storm distribution. To use this distribution it is necessary for the user to obtain the hour rainfall value for the frequency of the design storm desired.
Central to the NRCS methodology is the concept of the Curve Number CN which relates to the runoff depth and is itself characteristic of the soil type and the surface cover.
CNs in Table a to d of the TR Manual June represent average antecedent runoff condition for urban, cultivated agricultural, other agricultural, and arid and semiarid rangeland uses. Infiltration rates of soils vary widely and are affected by subsurface permeability as well as surface intake rates.
Appendix A of the TR Manual defines the four groups and provides a list of most of the soils in the United States and their group classification. The soils in the area of interest may be identified from a soil survey report, which can be obtained from the local Soil Conservation District offices. Several techniques have been developed and are currently available to engineers for the estimation of runoff volume and peak discharge using the NRCS methodology. Some of the more commonly used of these methods are summarized below: A.
This methodology is particularly useful for the comparison of pre- and post-development runoff rates and consequently for the design of control structures. There are basically two variations of this technique: the Tabular Hydrograph method and the Graphical Peak Discharge method.
The procedure divides the watershed into subareas, completes an outflow hydrograph for each subarea and then combines and routes these hydrographs to the watershed outlet.
This method is particularly useful for measuring the effects of changed land use in a part of the watershed. However, this method is sufficient to estimate the effects of urbanization on peak rates of discharge for most heterogeneous watersheds.
This method calculates peak discharge using an assumed hydrograph and a thorough and rapid evaluation of the soils, slope and surface cover characteristics of the watershed. The Graphical method provides a determination of peak discharge only. If a hydrograph is required or subdivision is needed, the Tabular Hydrograph method should be used.
This method should not be used if the weighted CN is less than For a more detailed account of these methods and their limitations the design engineer is referred to the NRCS TR document. The model may be used to simulate runoff in a simple single basin watershed or in a highly complex basin with a virtually unlimited number of sub-basins and for routing interconnecting reaches. It can also be used to analyze the impact of changes in land use and detention basins on the downstream reaches.
It can serve as a useful tool in comprehensive river basin planning and in the development of area-wide watershed management plans.
Other synthetic unit hydrograph methods available in HEC-1 can be used with the approval of the Department. Refer to the available Program Documentation Manual for additional information. The NRCS TR Model: This computer program is a rainfall-runoff simulation model which uses a storm hydrograph, runoff curve number and channel features to determine runoff volumes as well as unit hydrographs to estimate peak rates of discharge.
The dimensionless unit hydrographs from sub-basins within the watershed can be routed through stream reaches and impoundments. The TR method may be used to analyze the impact of development and detention basins on downstream areas. The parameters needed in this method include total rainfall, rainfall distribution, curve numbers, Time of Concentration, travel time and drainage area.
It may take a few computations at different locations. Tc is computed by summing all the travel times for consecutive components of the drainage conveyance system. Tc influences the shape and peak of the runoff hydrograph.
Development usually decreases the Tc, thereby increasing the peak discharge, but Tc can be increased as a result of a ponding behind small or inadequate drainage systems, including storm drain inlets and road culverts, or b reduction of land slope through grading. Surface Roughness: One of the most significant effects of development on flow velocity is less retardance of flow. That is, undeveloped areas with very slow and shallow overland flow through vegetation become modified by development; the flow is then delivered to streets, gutters, and storm sewers that transport runoff downstream more rapidly.
Travel time through the watershed is generally decreased. Channel Shape and Flow Patterns: In small watersheds, much of the travel time results from overland flow in upstream areas. Typically, development reduces overland flow lengths by conveying storm runoff into a channel as soon as possible. Since channel designs have efficient hydraulic characteristics, runoff flow velocity increases and travel time decreases. Slope: Slopes may be increased or decreased by development, depending on the extent of site grading or the extent to which storm sewers and street ditches are used in the design of the stormwater management system.
Slope will tend to increase when channels are straightened and decrease when overland flow is directed through storm sewers, street gutters, and diversions. Sheet flow is sometimes commonly referred to as overland flow. The type of flow that occurs is a function of the conveyance system and is best determined by field inspection, review of topographic mapping and subsurface drainage plans.
A brief overview of methods to compute travel time for the components of the conveyance system is presented below. Rational Method: Travel time for each flow regime shall be calculated as described below: a.
Sheet Flow: Using the slope and land cover type, determine the velocity from Figure Gutter Flow: The gutter flow component of Time of Concentration can be computed using the velocity obtained from the Manning equation for the triangular gutter of a configuration and longitudinal slope as indicated by roadway geometry.
Pipe Flow: Travel time in a storm sewer can be computed using full flow velocities for the reach as appropriate. Open Channel Flow: Travel time in an open channel such as a natural stream, swale, man-made ditch, etc. The maximum length of sheet flow to be used is feet.
The open channel portion may be a natural channel, man-made ditch, or gutter flow along the roadway. The open channel portion time is determined by using the Mannings equation or other acceptable procedure for open channel flow such as HEC Refer to TR, Chapter 3 for detailed information on the procedures. The minimum Time of Concentration used shall be 10 minutes. This type of design may result in major drainage and flooding problems downstream. Under favorable conditions, the temporary storage of some of the storm runoff can decrease downstream flows and often the cost of the downstream conveyance system.
Flood routing shall be used to document the required storage volume to achieve the desired runoff control. A hydrograph is required to accomplish the flood routing. A hydrograph represents a plot of the flow, with respect to time. The predicted peak flow occurs at the time, Tc. The area under the hydrograph represents the total volume of runoff from the storm.
A hydrograph can be computed using either the Modified Rational Method for drainage areas up to 20 acres or the Soil Conservation Service hour storm methodology described in previous sections. Storage may be concentrated in large basin-wide regional facilities or distributed throughout the watershed. Storage may be developed in roadway interchanges, parks and other recreation areas, small lakes, ponds and depressions.
The utility of any storage facility depends on the amount of storage, its location within the system, and its operational characteristics. An analysis of such storage facilities should consist of comparing the design flow at a point or points downstream of the proposed storage site with and without storage.
In addition to the design flow, other flows in excess of the design flow that might be expected to pass through the storage facility should be included in the analysis. The design criteria for storage facilities should include: release rate, storage and volume, grading and depth requirements, outlet works, and location.
Control structure release rates shall be in accordance with criteria outlined in Drainage Policy. Multi-stage control structures may be required to control runoff from different frequency events.
Storage volume shall be adequate to meet the criteria outlined in Stormwater Management and Non-Point Source Pollution Control, to attenuate the post-development peak discharge rates or to meet the Allowable Water Surface Elevation. Outlet works selected for storage facilities typically include a principal spillway and an emergency overflow, and must be able to accomplish the design functions of the facility. Outlet works can take the form of combinations of drop inlets, pipes, weirs, and orifices.
The total stage-discharge curve shall take into account the discharge characteristics of all outlet works. Detailed information on outlet hydraulics can be found in the "Handbook of Hydraulics", by Brater and King. Stormwater storage facilities are often referred to as either detention or retention facilities. For the purposes of this section, detention facilities are those that are designed to reduce the peak discharge and detain the quantity of runoff required to achieve this objective for a relatively short period of time.
These facilities are designed to completely drain after the design storm has passed. Retention facilities are designed to contain a permanent pool of water. Since most of the design procedures are the same for detention and retention facilities, the term storage facilities will be used in this chapter to include detention and retention facilities. Routing calculations needed to design storage facilities, although not extremely complex, are time consuming and repetitive. Many reservoir routing computer programs, such as HEC-1, TR and Pond-2, are available to expedite these calculations.
Use of programs to perform routings is encouraged. Natural channels are crossed at highway sites and often need to be modified to accommodate the construction of a modern highway.
Channels in the form of roadside ditches are added to the natural drainage pattern. This part contains design methods and criteria to aid the design engineer in preparing designs incorporating these factors.
Other open channel analysis methods and erosion protection information is also included. Either grassed channels or non-erodible channels are typically used. The features of each are presented in the following narrative. Grassed Channels: The grassed channel is protected from erosion by a turf cover. It is used in highway construction for roadside ditches, medians, and for channel changes of small watercourses. A grassed channel has the advantage of being compatible with the natural environment.
This type of channel should be selected for use whenever possible. Non-erodible Channel: A non-erodible channel has a lining that is highly resistant to erosion.
This type of channel is expensive to construct, although it should have a very low maintenance cost if properly designed. Non-erodible lining should be used when stability cannot be achieved with a grass channel. Typical lining materials are discussed in the following narrative. Concrete Ditch Lining: Concrete ditch lining is extremely resistant to erosion. Its principal disadvantages are high initial cost, susceptibility to failure if undermined by scour and the tendency for scour to occur downstream due to an acceleration of the flow velocity on a steep slope or in critical locations where erosion would cause extensive damage.
Aggregate Ditch Lining: This lining is very effective on mild slopes. It is constructed by dumping crushed aggregate into a prepared channel and grading to the desired shape. The advantages are low construction cost and self-healing characteristics. It has limited application on steep slopes where the flow will tend to displace the lining material.
Alternative Linings: Other types of channel lining such as gabion, or an articulated block system may be approved by the Department on a case-by-case basis, especially for steep sloped high velocity applications. Road Ditches: Road ditches are channels adjacent to the roadway used to intercept runoff and groundwater occurring from areas within and adjacent to the right-of-way and to carry this flow to drainage structures or to natural waterways. Road ditches should be grassed channels except where non-erodible lining is warranted.
A minimum desirable slope of 0. Interceptor Ditch: Interceptor ditches are located on the natural ground near the top edge of a cut slope or along the edge of the right-of-way to intercept runoff from a hillside before it reaches the backslope. Interceptor ditches should be built back from the top of the cut slope, and generally at a minimum slope of 0. In potential slide areas, stormwater should be removed as rapidly as practicable and the ditch lined if the natural soil is permeable.
Channel Changes: Realignment or changes to natural channels should be held to a minimum. The following examples illustrate conditions that warrant channel changes: 1.
The natural channel crosses the roadway at an extreme skew. The embankment encroaches on the channel. The natural channel has inadequate capacity. The location of the natural channel endangers the highway embankment or adjacent property. Grade Control Structure: A grade control structure allows a channel to be carried at a mild grade with a drop occurring through the structure check dam.
Methods to design grass-lined and non-erodible channels are presented in the following narrative. Grassed Channel: A grassed channel shall have a capacity designated in Section 2. A non-erodible channel should be used in locations where the design flow would cause a grassed channel to erode. Non-Erodible Channels: Non-erodible channels shall have a capacity as designated in Section 2. The unlined portion of the channel banks should have a good stand of grass established so large flows may be sustained without significant damage.
The minimum design requirements of non-erodible channels shall be in accordance with the NJDOT Soil Erosion and Sediment Control Standards Manual where appropriate unless otherwise stated in this section. Capacity: The required size of the channel can be determined by use of the Mannings equation for uniform flow. Mannings formula gives reliable results if the channel cross section, roughness, and slope are fairly constant over a sufficient distance to establish uniform flow.
The Mannings equation is as follows: 1. Formed, no finish 2. Trowel finish 3. Float finish 4. Float finish, some gravel on bottom 5. Gunite, good section 6. For non-uniform flow, a computer program, such as HEC-2, should be used to design the channel. Height of Lining: The height of the lined channel should be equal to the normal depth of flow D based on the design flow rate, plus 1 foot for freeboard if possible. Horizontal Alignment: Water tends to superelevate and cross waves are formed at a bend in a channel.
If the flow is supercritical as it will usually be for concrete-lined channels , this may cause the flow to erode the unlined portion of the channel on the outside edge of the bend. This problem may be alleviated either by superelevating the channel bed, adding freeboard to the outside edge, or by choosing a larger radius of curvature.
Friction Factor Range 0. Freeboard in feet ft. Radius of curvature in feet ft. Additional Design Requirements: a. The minimum d50 stone size shall be 6 inches. The filter layer shall be filter fabric wherever possible. A 3 feet wide by 3 feet deep cutoff wall extending a minimum of 3 feet below the channel bed shall be provided at the upstream and downstream limits of the non-erodible channel lining.
Additional design requirements may be required for permit conditions or as directed by the Department. Gradation of Aggregate Lining: The American Society of Civil Engineers Subcommittee recommends the following rules as to the gradation of the stone: 1 Stone equal to or larger than the theoretical d50, with a few larger stones, up to about twice the weight of the theoretical size tolerated for reasons of economy in the utilization of the quarried rock, should make up 50 percent of the rock by weight.
This requires tolerance of about 30 percent of the thickness of the stone. Detailed requirements regarding water quality control is included in Section Water on the pavement slows traffic and contributes to accidents from hydroplaning and loss of visibility from splash and spray.
Free-standing puddles which engage only one side of a vehicle are perhaps the most hazardous because of the dangerous torque levels exerted on the vehicle. Thus, the design of the surface drainage system is particularly important at locations where ponding can occur.
Runoff collection and conveyance for a curbed roadway is typically provided by a system of inlets and pipe, respectively. Runoff from an uncurbed roadway, typically referred to as an umbrella section, proceeds overland away from the roadway in fill sections or to roadside swales or ditches in roadway cut sections. Conveyance of surface runoff over grassed overland areas or swales and ditches allows an opportunity for the removal of contaminants.
The ability of the grass to prevent erosion is a major consideration in the design of grass-covered facilities. Use of an umbrella roadway section may require additional ROW. Areas with substantial development adjacent to the roadway, particularly in urbanized areas, typically are not appropriate for use of a roadway umbrella section.
The decision to use an umbrella section requires careful consideration of the potential problems. Benefits associated with umbrella sections include cost savings and eliminating the possibility of vehicle vaulting. Umbrella sections used on roadways with higher longitudinal slopes have been found to be prone to berm washouts.
Debris build-up along the edge of the roadway creates a curb effect that prevents sheet flow and directs the water along the edge of the roadway. This flow usually continues along the edge until a breach is created, often resulting in substantial erosion. Some situations may also warrant installing inlets along the edge of an umbrella section to pick up water which may become trapped by berm buildup or when snow is plowed to the side of the roadway and creates a barrier that will prevent sheet flow from occurring.
Bermed sections are designed with a small earth berm at the edge of the shoulder to form a gutter for the. Care should be taken to avoid earth berms on steep slopes that would cause erosive velocities yielding berm erosion. An umbrella section should be used where practical. However, low points at umbrella sections should have inlets and discharge pipes to convey the runoff safely to the toe of slope.
A Type E inlet and minimum 15 inch diameter pipe shall be used to drain the low point. Snow inlets Section 5. Umbrella sections should be avoided on land service roadways where there are abutting properties and driveways. Slope treatment shall be provided at all low points of umbrella sections and all freeway and interstate projects to provide erosion protection see NJDOT Standard Details. Combination Inlets 2. A special inlet shall be designed, with the appropriate detail provided in the construction plans, and the item shall be designated "Special Inlet", when the pipe size requires a structure larger than a Type B2, B2 Modified or E2.
A special inlet shall also be designed, with the appropriate detail provided in the construction plans, and the item shall be designated "Special Inlet", when the transverse pipe size requires a structure larger than the standard inlet types. Drainage structure layout should minimize irregularities in the pavement surface. Manholes should be avoided where practicable in the traveled way and shoulder.
An example is a widening project where inlets containing a single pipe should be demolished and the pipe extended to the proposed inlet, as opposed to placing a slab with a standard manhole cover or square frame with round cover on the existing inlet and extending the pipe to the new inlet. The typical curbed gutter section is a right triangular shape with the curb forming the vertical leg of the triangle.
Design shall be based on the following frequencies: Recurrence Interval Year Year Facility Description Freeway or interstate highway Land service highway.
The Manning equation has been modified to allow its use in the calculation of curbed gutter capacity for a triangular shaped gutter. Concrete gutter troweled finish 0. Asphalt pavement 1 Smooth texture 0. Concrete gutter with asphalt pavement 1 Smooth 0. Concrete pavement 1 Float finish 0.
Brick 0. As spread from the curb increases, the risks of traffic accidents and delays and the nuisance and possible hazard to pedestrian traffic increase. The following shall be used to determine the allowable spread. Width of inside and outside shoulder along interstate and freeway mainline 2. Each standard is described below. These standards help prevent certain solids and floatables e.
For new roadway projects and reconstruction of existing highway, storm drain inlets must be selected to meet the following design requirements: A. The first option especially for storm drain inlets along roads is simply to use the Departments bicycle safe grate. The other option is to use a different grate, as long as each clear space in the grate each individual opening is: No larger than seven 7. Curb-Opening Inlets If the storm drain inlet has a curb opening, the clear space in that curb opening or each individual clear space, if the curb opening has two or more clear spaces must be: No larger than two 2.
Storm Drain inlets that are located at rest areas, service areas, maintenance facilities, and along streets with sidewalks operated by the Department are required to have a label placed on or adjacent to the inlet. The label must contain a cautionary message about dumping pollutants. No Waste Here. Although a stand-alone graphic is permissible, the Department strongly recommends that a short phrase accompany the graphic. The hydraulic capacity of an inlet depends on its geometry and gutter flow characteristics.
Inlets on grade demonstrate different hydraulic operation than inlets in a sump. The design procedures for inlets on grade are presented in Section 5. The design procedures for inlets in a sump are presented in Section 5. Proper hydraulic design in accordance with the design criteria maximizes inlet capture efficiency and spacing.
An alternative procedure, that yields results reasonably close to those obtained by using the runoff collection capacity equation presented above, is to compute the collection capacity in accordance with the procedures presented in Federal Highway Administration, Hydraulic Engineering Circular No.
Use of computer programs is encouraged to perform the tedious hydraulic capacity calculations. HEC contains useful charts and tables. The HEC procedure is also incorporated in a number of computer software programs.
Procedures to compute the collection capacity for each condition are presented separately below. The weir flow coefficient is 3. A, B Mod. The equations must be modified for use with inlets that do not have a curb opening to account for reduced interception capacity resulting from debris collecting on the grate.
The perimeter around the open area of the grate P used in the weir equation should be divided in half for inlets without a curb opening. The orifice flow coefficient is 0. The equations must be modified for use with inlets that do not have a curb opening to account for reduced interception capacity Inlet Type. The clear opening area of the grate Ao used in the orifice equation should be divided in half for inlets without a curb opening.
Inlets should be located primarily as required by spread computations. See Section 5. Additional items to be considered when locating inlets include: A. Low points in gutter grade.
Adjust grades to the maximum extent possible to ensure that low point do not occur at driveways, handicap accessible areas, critical access points, etc. At intersections and ramp entrances and exits to limit the flow of water across roadways. Upgrade of all bridges and downgrade of bridges in fill section before the end of curb where the curb is not continuous. Along mainline and ramps as necessary to limit spread of runoff onto roadway in accordance with Section 5.
Maximum distance between inlets is feet. The procedure for spacing of inlets is as follows: 1. Calculate flow and spread in the gutter. Tributary area is from high point to location of first inlet. This location is selected by the design engineer. Overland areas that flow toward the roadway are included.
Place the first inlet at the location where spread approaches the limit listed in Section 5. Calculate the amount of water intercepted by the inlet, check the grate efficiency. The water that bypasses the first inlet should be included in the flow and spread calculation for the next inlet. This procedure is repeated to the end of the system. Sample calculations are presented in Section Also, the downstream transition out of the depression causes backwater which further increases the amount of water captured.
Locations of Depressed Inlets 1. All inlets in shoulders greater than 4 feet wide. All inlets in one-lane, low speed ramps. Inlets will not be depressed next to a riding lane, acceleration lane, deceleration lane, two-lane ramps, and direct connection ramps or within the confines of a bridge approach and transition slab. Limits of Depression 1. Begin depression a distance of 4 feet upgrade of inlet. End depression a distance of 2 feet downgrade of inlet. Begin depression 4 feet out from gutter line.
Depth of depression, 2 inches below projected gutter grade. Spacing of Depressed Inlets Use the same procedure as described in Section 5. This method will give a conservative distance between inlets; however, this will provide an added safety factor and reduce the number of times that water will flow on the highway riding lanes when the design storm is exceeded.
Collection of snow melt runoff is important on the. A discussion of each situation and the design approach is outlined below. Snowmelt Collection on High Side of Superelevation Collection of snow melt on the high side of a superelevated section from roadway and berm areas before it crosses the roadway prevents icing during the freeze-thaw process.
The snow melt inlets should be placed along the outer curbline at the upstream side of all intersections and at convenient cross drain locations. The snow melt inlets should be connected to the drainage system with a 15 inch diameter pipe to the trunk storm sewer. The small shoulder and snow inlets will not be designed to control stormwater runoff but shall be designed to handle only the small amount of expected flow from the snowmelt.
Snowmelt Collection at Low Points Collection of snowmelt is important at low points where the pile-up of snow over existing inlets prevents draining of snowmelt and runoff off the edge of road.
The addition of inlets placed away from the edge of curb and beyond anticipated snow piles provides a means to drain snowmelt. Snow inlets are required at all roadway profile low points. All snow inlets shall be Type "E". Snow inlets shall not be depressed. Snow inlets shall be provided in the shoulder immediately adjacent to the travel lane without encroaching on the travel lane.
Snow inlets shall not be installed in shoulders where the width is so narrow that placement of a snow inlet will encroach upon the inlet at the curb.
Compliance with the established spread criteria for roadways with flat grades typically requires many inlets, usually installed at close intervals. Use of alternative collection systems such as trench drains may be appropriate to reduce the number of inlets required to satisfy the spread criteria.
Therefore, use of trench drains for runoff collection on roads with flat grades may be warranted. The trench drain should be located upstream of the inlet to which it connects. The length of trench drain should provide the capture capacity that together with the inlet limits bypass at the inlet to zero.
Trench drain capture computations require consideration of both frontal and side flow capture. Frontal flow captured by the narrow trench drain is small and is, therefore, disregarded. Side flow into the trench drain is similar to flow into a curb opening inlet. The trench drain must be long enough to intercept the bypass after frontal flow plus the additional runoff contributed by the roadway for the length of the trench drain.
The process includes the following steps: A. Compute the total runoff to the inlet. Compute the frontal flow captured by inlet with no bypass allowed for the spread limited to the width of the grate.
The runoff to be intercepted by the trench drain is the total runoff minus the runoff captured by the inlet. The computed length shall be multiplied by two to reflect inefficiencies due to clogging. Maintenance requirements for trench drains should also be considered in the evaluation of trench drains. Use of a trench drain system should be discussed with the Department early in the design process with recommendations submitted prior to completion of the Initial Submission.
This section. Minimum pipe size is 15 inches. Minimum pipe size is 18 inches downstream of mainline lowpoints. Storm sewer pipe materials for proposed systems typically include concrete, corrugated metal, aluminum alloy and Smooth interior High Density Polyethylene HDPE.
Manning's roughness coefficient "n" for concrete and HDPE pipe is 0. Manning's roughness coefficient values for corrugated metal and aluminum alloy pipe are presented in Table Manning's roughness coefficients for other materials occasionally encountered are indicated below: Table Wood forms, rough 0.
Wood forms, smooth 0. Steel forms 0. Concrete floor and top 0. Natural floor 0. Design to flow full, based on uniform flow. Minimum self-cleaning velocity of 2. Spiral flow will not occur when the following conditions exist, in which case the "n" value for annular corrugations is to be used:. Partly full flow Non-circular pipes, such as pipe arches When helical C.
Pipe arches have the same roughness characteristics as their equivalent round pipes F. Aluminum Alloy Pipes as recommended by manufacturer 4. HDPE, shall be used for pipe lengths outside of the roadbed only. HDPE pipe is not allowed for lateral pipes or for the outlet pipe to a receiving watercourse or water body.
The density of polyethylene pipe is less than water, therefore when wet conditions are expected, polyethylene pipe will float and should not be specified. End sections for HDPE shall be concrete. Flared end-sections should be used whenever and wherever possible, for concrete and metal pipe. Pipe sizes should not decrease in the downstream direction even though an increase in slope would allow a smaller size.
Pipe slopes should conform to the original ground slope so far as possible to minimize excavation. For durability, the minimum thickness for steel pipe is 14 gauge and for aluminum alloy pipe is 16 gauge. In extremely corrosive areas and where high abrasion can be expected the design engineer shall determine whether heavier gauges should be used. Material types: Figure Alternate Items: a. Alternate pipe materials include corrugated metal, aluminum alloy, and HDPE. Some materials may be eliminated as alternate items due to unstable support, high impact, concentrated loading, limited clearance, steep gradients, etc.
The drainage layout should attempt to avoid conflicts with existing underground utilities and such items as utility poles, signal pole foundations, guide rail posts, etc. Implementation of the following design approaches may be necessary.
Use of pipe material with the lowest friction factor to minimize pipe size b. Use of elliptical or arch pipe to minimize vertical dimension of pipe. Test pits should be obtained early in the design process to obtain horizontal and vertical information for existing utilities.
If the suggested design approaches do not avoid conflict, use of special drainage structures may be used to avoid the utility. The reason for exclusive use of a pipe material must be explained in the Drainage Report. Round corrugated metal pipe shall have helical corrugations, except that annular corrugated pipe may be used where velocity reduction is desired.
Drainage structures must accommodate all pipe materials used including concrete, corrugated metal, aluminum alloy, and HDPE. Aluminum alloy pipe shall not be used as a section or extension of a steel pipe. Precast manholes or inlets shall not be used for pipes 54 inches or larger diameter or when three or more pipes tie in and at least two of them are connected at some angles.
When these conditions exist, cast-in-place inlets or manholes are more practical. Cleaning existing drainage pipes and structures shall be incorporated on all projects when the existing drainage system has substantial accumulation of sediments.
The cleaning shall extend to the first structure beyond the project limits. On projects where contaminated areas have been identified, the drainage system should be designed to avoid these locations, if possible.
If avoidance is not feasible, a completely watertight conveyance system, including structures such as manholes, inlets, and junction chambers, should be designed to prevent. Retrofitting existing pipes to make them watertight may require installation of an appropriate internal liner.
The design engineer shall provide recommendations prior to proceeding with the final design. The soffits overts between the inflow and outflow pipes at a drainage structure shall be matched where possible. A minimum 1 inch drop between inverts within the structure shall be provided, if feasible. Existing drainage facilities that are not to be incorporated into the proposed drainage system are to be completely removed if they are in conflict with any element of the proposed construction.
Existing drainage facilities that are not to be incorporated into the proposed drainage system that do not conflict with any element of the proposed construction are to be abandoned. Abandonment of existing drainage facilities requires the following: 1. Plugging the ends of the concrete pipes to remain.
Metal pipes shall be either removed or filled. Filling abandoned pipes in accordance with geotechnical recommendations. Removing the top of the drainage structure to 1 foot below the bottom of the pavement box, breaking the floor of the structure, and filling the structure with either granular material or concrete in accordance with geotechnical recommendations.
A concrete collar, as shown in the standard detail CD In conceiving, scoping and designing projects, the NJDOT will consider the needs of all road users and neighbors. This includes pedestrians, bicyclists, and neighbors, such as residents, and businesses, as well as drivers. One of the key steps in accomplishing this is to carefully and systematically decide on the appropriate functional classification of the roadway, and the appropriate target operating speed a.
Properly selected design criteria should result in motorists driving freeways like freeways, arterials like arterials, collectors like collectors, and local streets like local streets. In deciding the appropriate functional classification and target operating speed for an existing roadway, one of the most important considerations will be the consistency of the existing geometry and surrounding context of the roadway, and how they relate to the existing operating speed and the posted speed limit.
Target operating speeds cannot be determined arbitrarily, but must be consistent with conditions along the roadway and subject to reasonable enforcement. The designer may proactively alter the existing geometry and roadway environment in an attempt to decrease the operating speed and enhance the safety of pedestrians and bicyclists, or the viability of downtown or residential areas, in balance, not competition, with the safety of motorists.
Roadway design should lead the driver to adopt a driving behavior appropriate to local conditions. The designer thus should carefully consider the appropriate target speed for a roadway section based upon land use conditions, building densities, the environment and the disparate needs of users of the facility. It should be recognized that streets do not only serve transportation related functions but are also places of commercial and social encounter.
Therefore, a designer should also consider the non-vehicular uses of a roadway and seek consistency between all aspects of the roadway, its environment, and the chosen Design Speed. What does this mean? If a physical, environmental or other impediment posed an obstacle to a project, the Design Speed established the limit below which it would be difficult to compromise, in effect, the maximum safe speed.
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