DelDOT Road Design Manual

Chapter Six Drainage and Stormwater Management

6.1 INTRODUCTION Federal, state, county and local regulations, laws, and ordinances that may impact the Adequate drainage is essential in the design design of storm drain systems. of highways since it affects the highway’s serviceability and usable life, including the Many federal laws have implications that pavement’s structural strength. If ponding on affect drainage design. These include laws the traveled way occurs, hydroplaning concerning: becomes an important safety concern. • Flood insurance and construction in flood Drainage design involves providing facilities hazard areas, that collect, transport and remove stormwater from the highway. The design must also • Navigation and construction in navigable consider the stormwater reaching the roadway waters, embankment through natural stream flow or • Water pollution control, manmade ditches. • Environmental protection, This chapter deals with drainage policies, • Protection of fish and wildlife, and procedures and guidance to be followed in • Coastal zone management. achieving cost-effective design and construction within DelDOT's Highway Federal agencies formulate and promulgate System. The information contained herein is rules and regulations to implement these laws. compiled from various federal and national Highway hydraulic engineers should keep publications, textbooks, and drainage manuals. informed regarding proposed and final The information provided is of a general regulations. nature with the inclusion of methods, criteria Some of the more significant federal laws and references specifically applicable to affecting highway drainage are: DelDOT projects. • The Department of Transportation Act Source documents are listed in the established the Department of introduction to each section. It is presumed Transportation and sets forth its powers, that the designer is familiar with the basic duties, and responsibilities to establish, theory and methods of analysis and design in coordinate, and maintain an effective both hydrology and hydraulics. The administration of the transportation information provided herein will have to be programs of the Federal Government. supplemented with hard copies or on-line access to the referenced documents. • Federal-Aid Highway Acts provide for the administration of the Federal-Aid The regulatory environment related to Highway Program. Proposed Federal-aid drainage design is ever changing and projects must meet existing and probable continues to grow in complexity. Engineers future traffic needs and conditions in a responsible for the planning and design of manner conducive to safety, durability, drainage facilities must be familiar with and economy of maintenance, and must be

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designed and constructed according to guidance in FHWA Technical Advisory standards best suited to accomplish this T5140.23. and to conform to the needs of each • FHWA Policy 23CFR650 Subpart locality. H⎯Navigational Clearances for • Federal-Aid Highway Act of 1970 Bridges requires coordination with the provides for the establishment of general United States Coast Guard (USCG) and guidelines to ensure that possible adverse United States Army Corps of Engineers economic, social and environmental (USACE) in providing adequate vertical effects relating to any proposed Federal- and horizontal clearance for navigation on aid project have been fully considered in navigable waterways. developing the project. In compliance with Other federal laws affecting hydraulic the Act, the Federal Highway tasks, analyses, design, or construction include Administration (FHWA) issued process those formulated under the following guidelines for the development of legislative acts: environmental action plans as contained in the Federal-Aid Highway Program • The Coastal Zone Management Act of Manual (FHPM) and in 23 CFR 795 et 1972 seq. • The Federal Water Pollution Control Act • Federal-Aid Highway Act of 1966 (Clean Water Act) amended by the Act of 1970 requires the • The Fish and Wildlife Act of 1956 issuance of guidelines for minimizing • The Fish and Wildlife Coordination Act possible soil erosion from highway construction. In compliance with these • The Flood Disaster Protection Act of 1973 requirements, the FHWA issued • The National Environmental Policy Act guidelines that are applicable to all (NEPA) of 1969 Federal-aid highway projects. These • The Rivers and Harbors Act (1899) guidelines are included in the FHPM. • The Safe Drinking Water Act of 1974 Regulatory material is found in 23 CFR • Water Quality Act of 1987 650.201. • Wild and Scenic Rivers Act All Federal-aid projects shall conform to FHWA guidelines. FHWA policy includes the 6.2 DESIGN RESPONSIBILITIES following: Responsibilities for drainage design are • FHWA Policy 23CFR635 Subpart divided between the Bridge Design and D⎯General Material Requirements Project Development sections based primarily • FHWA Policy 23CFR650 Subpart on the size of the drainage basin. Bridge A⎯Location and Hydraulic Design of Design is responsible for all watersheds over Encroachments on Floodplains 300 ac and locations where the design 2 establishes policy affecting any project discharge capacity opening(s) exceeds 20 ft . that includes an encroachment on a base Project Development is responsible for: floodplain. • All locations requiring pipe culverts not • FHWA Policy 23CFR650 Subpart exceeding 20 ft2 of waterway opening; C⎯National Bridge Inspection • Closed drainage systems involving storm Standards defines the national standards drains; for the proper safety inspection and • Roadside drainage including swales and evaluation of all highway bridges ditches; including the evaluation of bridges for scour susceptibility in accordance with the • Erosion and sediment control for transportation projects;

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• Stormwater quantity and quality Step 1. Data collection (Section 6.5) management. includes: The section responsible for design of the a. Establish agency requirements and drainage structure shall also design the related define design criteria and parameters. stream bank stabilization and erosion control. b. Access the applicable online drainage The scope of this chapter is limited to the design resources (hard copies, if necessary), drainage design responsibilities of the Project other design references, environmental Development Section. Refer to Chapter Three controls, drainage criteria, etc. of the Bridge Design Manual for information c. Prepare a drainage area map using USGS on drainage design for structures. Structural maps, topographic maps and aerial designs are coordinated through the Bridge photography. Design Section. d. Determine soil characteristics using the 6.3 DESIGN CRITERIA county soil survey map from the US Natural Resource and Conservation Service (NRCS). The Department has established basic design criteria for design storm frequency e. Obtain As-Built or record plans from depending upon roadway type, and the design previous projects in the area. of open channel flow, pipe flow, pavement f. Contact the area maintenance office for drainage, and storm drains (closed drainage existing flooding problems. systems). These criteria are summarized in Figures 6-1 through 6-3. g. Request soil borings to establish the typical water table elevation. Saturated soil 6.4 DESIGN PROCEDURES conditions can adversely impact the drainage system design; require expensive dewatering The goal of drainage design is to to build; and require permits. Therefore this economically design systems that limit the condition should be identified early in the exposure of adjoining upstream and process. downstream properties, the newly constructed roadway and the traveling public to an h. Obtain existing and future land use acceptable flood risk during high flows. information. Attachment B has several example problems i. Visit the anticipated drainage area site to that illustrate the design procedure for the verify collected data, locate existing structures most commonly used drainage facilities used and visually observe any problems with their on a roadway project. AASHTO’s Model past performance, signs of past high water Drainage Manual is one of the best resources levels, stream erosion problems, currently for preparing a project’s drainage design. In developed areas, existing stormwater addition, DelDOT has an Internet Web Site management facilities, etc. (http://www.deldot.gov/) that has additional drainage references. j. Request a video inspection of any drainage facility to remain in service. Below is a brief description of drainage design steps normally followed when k. Obtain existing and proposed designing roadway facilities. Each of these is underground utility locations. expanded in sections referenced in the steps.

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Figure 6-1 Design Criteria ─ Frequency (Return Period in Years) Type of Drainage Installation 1 Functional Classification Roadside Pipe Culverts Storm Drains Median Drains Ditches

Interstate, Freeways and 50 10 2 50 50 Expressways

Arterials 50 10 2 25 25 2

Collectors 50 3 10 2 25 4 10 2

Local Roads and Streets including 25 10 5 10 10 5 Subdivision Streets

1 For Stormwater Management see Section 6.10. 2 Use a 50-yr frequency at sag points, i.e. underpasses or depressed roadways, where ponded water can be removed only through the storm drain system. 3 Use a 25-yr frequency for rural collectors. 4 Use a 10-yr frequency for rural collectors. 5 Use a 25-yr frequency at underpasses or depressed roadways where ponded water can be removed only through the storm drain system.

Figure 6-2 Design Criteria ─ Allowable Spread on the Pavement Cross Section Functional Classification Allowable Water Spread Interstate, Freeways, Expressways Full shoulder width

Arterials with full shoulder or parking lane Full shoulder or parking lane Arterials with less than full shoulder or parking lane One half of adjacent driving lane

Collectors Design speed < 45 mph One half of driving lane Design speed ≥ 45 mph Full shoulder width

Local Roads and Streets including Subdivision One half of driving lane Streets

Sag Points (all facilities) Full shoulder width

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Figure 6-3 Design Criteria ─ Miscellaneous Ditches Ditch flow line below edge of shoulder ≥2.5 ft preferred Ditch water surface elevation below edge 0.5 ft minimum; 1 foot preferred of shoulder Minimum ditch grade 0.003 ft/ft (preferred 0.005) Pipes Minimum size - cross road pipe / culvert 18 in Minimum size - storm drain 15 in Minimum full flow velocity 3.0 ft/s Based on scour, erosion, risk potential of discharge Maximum outlet velocity (determined by channel, and mitigating measures such as energy design parameters) dissipators Maximum continuous distance between 300 ft storm drain structures (without clean-out access) Reinforced Concrete Pipe Bedding Class C, unless specified otherwise Personnel Grate for Pipe Inlet All pipes 12 in and larger with an open inlet that is not a straight run to the outlet without full daylight visible when looking through the pipe to the other end Minimum Pipe Cover 3 ft preferred at profile grade line, 1 ft at the shoulder, depending upon the structural requirements of the pipe material, bedding type and fill height at the installation location; see supplemental figures on DelDOT’s web site and Table 12.6.6.3-1 of the AASHTO LRFD Bridge Design Specifications General Criteria - Storm Drain System Hydraulic Grade Line (HGL) ≥ 1 ft below top elevation of all manhole covers or top of any inlet

Required manhole or inlet locations Intersection of two or more storm drains Pipe size change Alignment change Grade changes and sag points in curbed sections Outfall pipe invert elevation ≥ 0.2 ft below lowest incoming pipe elevation (preferred) Discharge pipe Invert higher than the outfall elevation Inlet clogging factor of safety 1.5 with curb and 2.0 without curb 1.0 for curb opening inlet

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Step 2. Hydrology (Section 6.6) Data generated by TR-20 has been used to develop a more user-friendly method for The principles of a hydrologic study for a analysis of small watersheds (1 to 300 acres) watershed are common to most design referred to as TR-55 and WINTR-55. Except methods. The watershed is defined and for pavement drainage, these are commonly divided into subareas contributing to proposed used when analyzing stormwater flow for stormwater outlets. Water flows through the watersheds and their subareas under both pre- watershed as sheet flow, shallow concentrated and post-development conditions. TR-55 can flow, swales, open channels, pavement be applied to designing most of a project’s drainage, roof drains, and storm drains The drainage design features, including stormwater existing and future surface runoff management facilities. characteristics of these subareas are determined and a coefficient or runoff curve USGS has developed regression equations number is assigned. The flow parameters are for Delaware based upon years of data from than determined including the hydraulic length stream gauging stations. This method is most of the reaches, the types of flow, and average applicable for estimating the magnitude and slope. Using this data flow velocities are frequency of flood-peak discharges and flood determined and the flow times of each type of hydrographs for large streams, channels and flow is determined and combined into a time culverts. These equations relate peak of concentration (tc) for the watershed. A discharge to independent variables describing design rainfall frequency and intensity is the physical and climatic basin characteristics. selected. Using this data the peak discharge is Refer to the Bridge Design Manual for more determined. details. Available methods of hydrological analysis Step 3. Hydraulics include the Rational Method; TR-20 Project Using the hydrologic data and other Formulation Hydrology Program System; TR- acquired information, lay out an initial 55 Urban Hydrology for Small Watersheds drainage system to accommodate this flow. and its Windows version WINTR-55; and the Drainage system may include various types U.S. Geological Survey (USGS) Method and shapes of open channels (i.e., ditches, which has developed regression equations for swales, and gutters), storm drains, median Delaware. Each of these procedures has its drains, and cross road pipe culverts. The own methodology, assumptions, limitations, system profile should begin at the farthest and applications. discharge point downstream, i.e., a natural or The Rational Method is most frequently artificially created channel, or an existing used for estimating small, homogenous or drainage system. To minimize the expense of highly impervious drainage areas which are pipe excavation, pipe slopes should conform smaller than 50 acres, including gutter flow, to the surface slope wherever possible. drainage inlets, stormwater systems, small Roadside ditches and culverts pose a ditches, swales and culverts. This method is roadside hazard and should be designed not recommended for storage design, i.e., considering safety as a design parameter. Use stormwater management. the principles found in AASHTO’s Roadside TR-20 is a computer hydrological model Design Guide to minimize their potential as simulating runoff hydrographs using natural roadside hazards. and synthetic rainfall events over a large a. Open Channel Design (Section 6.7): complex watershed up to 2000 acres with There are several types of open channels used multiple subareas, channel reaches, and varied in drainage design including roadside ditches, confluence points that are routed through toe-of-slope ditches, intercepting ditches, downstream reaches and structures. chutes (stone-lined or pipes) used on step

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embankments, and gutters used in curbed upgrading existing pipe culverts. In this sections. instance see Step 4. i. For design of roadside channels, i. The hydrologic analysis should be hydraulic conditions usually are assumed to be consistent with Step 2. The rational method is steady and uniform. Flow under these used to calculate the peak discharge to be conditions has an energy line approximately carried by the gutter and storm drains. equal to the average ditch grade, and a flow ii. Gutters are designed using the depth that changes gradually over time. This principles for open channel flow by allows for determining normal flow values establishing the rate of flow or discharge, the where velocity, depth of flow, surface depth of flow, the velocity, the surface roughness and slope is constant over the roughness and the slope using a modified form design section.. The capacity or discharge of of Manning’s equation. this open channel is determined by using the modified Manning’s equation (which iii. Inlets can be grate inlets, curb-opening combines Manning’s and the continuity inlets, (a combination inlet involving a curb equations). Resources are referenced in opening combined with a grate inlet), or Section 6.7 that detail open channel design occasionally slotted inlets. Refer to DelDOT’s procedures. Standard Construction Details for the detailed description of their shape and dimensions. See ii. For non-uniform flow where either the Figure 6-2 and 6-3 for the recommended depth of flow, velocity, or cross section allowable spread on the pavement and changes, the principles and equations for clogging capacity (efficiency). Inlet design unsteady, non-uniform flow are used, the consists of determining the interception computer program HEC-RAS is capacity of the type of inlet under recommended. consideration and its location on a continuous iii. Filter strips, biofiltration swales, slope or in a sag point. bioretention areas and infiltration ditches are The locations of inlets are a function of the designed using DURMM, DNREC’s green roadway geometrics and cross section, in technology computer model based on a addition to hydraulic considerations. The combination of computer routines from TR-22 underground connecting pipes have to be and TR-55. accessible for maintenance. The physical b. Pavement (Storm) Drainage System constraints from using available inspection Design (Section 6.8): There are several and cleanout equipment are a limiting factor in reasons to provide adequate pavement the maximum distance between inlets. The drainage design, including maintaining the volume of inlet bypass flow combined with service level during stormy conditions; the normal flow calculated to reach each protecting the user from ponding water that downstream inlet affect the inlet spacing. could cause loss of vehicle control; protecting Other factors include features such as curb adjacent natural resources, development and ramps, crosswalks, driveways, intersecting property; and maintaining the structural streets, points of superelevation transition, and integrity of the pavement section. A typical roadway profile low points or sag vertical pavement/storm drainage system consists of curves. Refer to Section 6.8 and Example 5, curb, gutter, inlets, drainage structures (i.e., Attachment B, for a description of manually junction boxes and manholes), storm drains determining inlet spacing. It is important to (pipes), and an outfall structure into an note that inlet design in sag locations (both for acceptable discharge point. vertical curves and superelevated sections) is critical due to ponding that might encroach on The design of a project’s drainage system may the roadway. require installing new pipe culverts, or

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iv. Storm drains (pipes) are used in areas slope and size needed for the design discharge. of limited right-of-way and in curbed sections, Losses in the system at the various drainage normally in combination with inlets, junction structures, bends, etc. are calculated. The final boxes, manholes and outfall structures. step is determining the system HGL to ensure Sections of smaller pipe are referred to as open channel flow exists throughout the segments or runs. The shorter runs usually system. The design should be evaluated and discharge into a larger main storm drain called adjusted to avoid pressure flow conditions. a trunk line discharging into an outfall. See Section 6.8 for a more detailed discussion. Locating storm drains, particularly manholes Median drainage is an important or other drainage appurtenances, outside of the consideration on divided roadways, especially roadway (especially the traveled way) should with high-speed lanes. Ponding water and its be considered for safer maintenance spread onto the travel lane or shoulder need to operations and minimizing traffic disruption. be minimized and the pavement’s structural In addition, if there is pavement settlement integrity protected from saturation leading to due to compaction problems or erosion of the failure. This is difficult since the shape and subgrade from joint separation, the repairs can width of the median is primarily controlled by be made with a reduced effect on the flow of safety considerations and not by the preferred traffic. hydraulic characteristics. Therefore medians Pipe criteria includes the minimum are usually wide, shallow, and depressed with allowable pipe size, the material type based on a swale with relatively flat side slopes and the allowable service life for the project and drainage inlets to control the flow. location being designed, and any special The principles of open channel flow are installation or structural requirements used for locating inlets to limit the depth of specified by the Standard Specifications or water to an elevation of 0.5 ft minimum, one ft manufacturer. Refer to DelDOT Design preferred, below the outside edge of shoulder. Guidance Memorandum Number 1-20R. Inlets in sags may function either as a weir or Placement with respect to underground orifice depending upon the type of grate and and above ground utilities is to be considered depth of water, so sag locations must be given throughout the system design. Utility special consideration. The preferred method of relocations and/or their protection are ensuring adequate drainage in a sag location is expensive. The cost may be fully paid by the to install inlets (flanker basins) upstream from project or shared with the utility provider and the low point. their customers. Drainage structures include manholes, Storm drain capacity design is based on junction boxes and outlet structures. Each of the principles of open channel flow, i.e., these has a particular function in the drainage steady, uniform flow with a constant average system and a role in the efficiency, capacity velocity within each pipe run. This is and future maintenance of the system. Refer to accomplished by assuring the HGL is below the Standard Construction Details for the size, the top of pipe. To complete this analysis the shape, etc. of these structures. The designer designer needs to have a working plan should check the structure dimensions versus showing the layout and profile of the storm the storm drain size to ensure they are drainage system with the location of storm compatible. Of primary concern is allowing drain runs and trunk lines, direction of flow, for the losses in flow capacity these features location and numbering of drainage structures introduce into design of the system. HDS-5 and manholes, and location of utilities. and HEC-22 discuss this subject. Starting with the upstream storm drain run, the Designing a storm drainage system for the designer performs the hydrological roadway section is normally an iterative calculations for the system. Storm drains are process with several data inputs; the use of sized using Manning’s equation, varying the

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available drainage software is recommended. Step 6. Stormwater Management (Sections Available references include HEC-22 Urban 6-11 and 6-12) Drainage Design Manual; HEC-12 Drainage Managing highway runoff involves of Highway Pavements; The State-of–the Art, designing systems to address stormwater Storm Drain Systems (Volume IX of the quantity and quality. The design of these AASHTO Highway Drainage Guidelines); facilities is covered in more detail in Sections and the AASHTO Model Drainage Manual. 6.11 and 6.12; the ES2M Design Guide; and Step 4. Culverts (Pipe Culverts Section the Delaware Erosion & Sediment Control 6-9) Handbook; and DNREC’s Green Technology: The Delaware Urban Runoff Management Culverts are closed conduits (pipes) placed Approach DURMM: The Delaware Urban through an embankment as a part of the Runoff Management Manual. drainage system to convey runoff collected by roadside channels or natural watercourses such The goal of runoff quantity management is as streams. to minimize excessive downstream flooding and property damage. The basic principle is Frequently a roadway project will involve that any proposed land development installation of a new cross pipe culvert and/or (including roadways) should not increase the upgrading an existing culvert. For culverts downstream flow above what existed prior to requiring an area of 20 square feet or more, the changes. Two methods used to control this refer the design to the Bridge Design Section. flow are: Hydraulic design considerations include 1. Retention facilities are used to control establishing the design storm frequency, the stormwater runoff and dissipate it through allowable headwater, the allowable tailwater, evaporation and infiltration. They are the minimum allowable pipe diameter, and the commonly used when there is no nearby type of pipe material (considering allowable natural watercourse or body of water for service life and location with respect to the release of the stored water. location under the pavement or the embankment, and the anticipated design load). 2. Detention facilities are used to reduce Headwater and tailwater levels depend upon the peak discharge and slowly release this the roadway typical section and profile, runoff. Normally they are designed to acceptable upstream and downstream release all of the stored water after the flooding, stormwater management interests storm has passed. Frequently detention and and other environmental concerns. retention facilities are combined to: manage pollution and sediment; control Refer to the resources listed in Section 6.9 stormwater quantity discharge; and as an for detailed design procedures for culverts. aquifer recharge basin. Culvert placement and end treatments affect roadside safety. A second goal of stormwater management is to maintain water quality. Runoff from Step 5. Erosion and Sediment Control developed areas can degrade water quality. (Section 6-10) Designing for water quality is referred to as Erosion and Sediment Control is an Non-Point Source Pollution Control. integral part of designing and constructing a Legislation referred to as the National drainage system. The erosion and sediment Pollutant Discharge Elimination System plan has to be prepared concurrently with the (NPDES) requires that the pollutants in runoff drainage plan. The requirements of this plan from storm systems be reduced. Pollutions are are in DelDOT’s ES2M Design Guide and the reduced using Green Technology BMP’s Delaware Sediment and Stormwater including biofiltration grass swales, Regulations. biorentention areas, infiltration ditches and area filter strips, forebays at pipe outfalls

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and/or retention ponds with slow release drainage plan and, most importantly, the outlets. proposed design criteria and methodology to be used in performing the detailed drainage Step 7. Drainage Report: analysis. The final report would contain this The final product of the drainage design is data along with the final drainage calculations the H&H (Hydrology and Hydraulic) Report and final drainage plan. Refer to DelDOT’s (Stormwater Management Report). This report ES2M Design Guide for the required contents should clearly compare the analysis results to of a stormwater management report. the approved design criteria including whether Additional guidance and samples are available or not the proposed design meets the from the Quality Section. appropriate criteria. 6.5 DESIGN PROCESS This report should include the following: The following describes the process for • Executive Summary with an overview of performing an acceptable drainage design. the project intent, drainage methodology and stormwater management approach. 6.5.1 LEGAL REQUIREMENTS AND AGENCY COORDINATION • Project Description with detailed information of project scope and proposed Highway construction encroaching flood construction. plains, subaqueous lands, wetlands, lakes, ponds, and natural streams shall conform to • Hydrological and Hydraulic Criteria prevailing governmental regulations. Strict including methodology used for the attention shall be given at the planning stage drainage design of open channels, of the project to avoid potential legal pavement drainage and culverts. problems. Design activities shall be closely • Stormwater Management Approach coordinated with DelDOT's Environmental with geographical description of project Studies Section and other agencies for and design criteria for stormwater quality obtaining necessary permits complying with and quantity management. the regulations and the project schedule. • Watershed Descriptions discussing the Sediment and stormwater regulations were various flow characteristics of drainage enacted into State Law in 1990 and 1991. The areas noting existing drainage facilities and designer should be familiar with the assessment of offsite drainage (beyond provisions and ensure that the design survey data collection). Each area is given conforms to the requirements. an identification code as shown on the drainage plans. 6.5.2 DATA COLLECTION • Conclusions describing the proposed 6.5.2.1 INITIAL PHASE drainage for each area with a summary comparing the pre- and post-project The initial phase of the design is to acquire conditions. the relevant data and information. This information is available from various sources • Special Conditions requiring any special including federal, state, and county agencies. design measures to meet permit See Attachment A for resource references. requirements, water quality standards, etc. • Backup Data plans, maps, quadrangles, Site investigations and field surveys are assumptions, hand calculations, computer absolutely necessary and should include: output, etc. • Watershed characteristics, soil surveys, The report is normally prepared in two U.S.G.S. map topography, and land use; phases. The preliminary report would include • Stream course data—profile and cross- an initial hydrologic analysis, a preliminary sections within the right-of-way and in the

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vicinity. Note high water marks when • Fill areas with beginning and end stations possible. Obtain FEMA maps indicating depths of fill; (http://msc.fema.gov), if available, for • Stations and elevations of high points; 100-year flood plain location and elevation; • Stations and elevations of low points; • Existing drainage facilities with design • Limits of relatively flat sections (grades details, if available, and field data; flatter than 0.5%); • Current aerial photographs; • Drainage areas and subareas indicating basin divides from cross sections and • Wetlands locations from field survey data topographic sheets; and DelDOT’s Environmental Studies Section; and • Preliminary locations of proposed drainage facilities; • Any affected resource protection areas from available maps. • Limits of steep profiles where erosion protection is anticipated; When evaluating existing facilities within • Locations of existing utilities which may the project area, the designer should: impact highway drainage; • Video and investigate the need for • Potential drainage storage and outfall replacing existing drainage facilities due locations; and to inadequate strength or functional • Sediment and stormwater management deficiency; project review and design checklist items. • Check utilities for possible impact and constraints on the drainage design; and 6.5.3.2 FINAL DRAINAGE PLAN • Verify the ditch and stream bank The preliminary drainage plan and elevations. previously gathered information are reviewed The designer should plan on removing and in the field in developing the final drainage replacing any existing damaged pipe. It is also plans. important that all pipes are relatively free of In fill areas the designer should: silt and debris in order to properly inspect and ensure the bottom of the pipe is intact. • Investigate the need for roadside ditches to carry surface runoff and indicate their 6.5.3 DRAINAGE PLANS approximate locations on the working drawings; The drainage plans are prepared in two phases⎯-preliminary and final. The • Estimate the extent of possible erosion preliminary drainage plans are included in the qualitatively and indicate where preliminary construction review package. The preventive measures may be needed. final drainage plans are included in the semi- In cut areas, the designer should: final construction plan review package. Based • Indicate where benching may be upon the comments from the two reviews, the necessary, final H&H Report is prepared and submitted. • Delineate approximate ditch locations 6.5.3.1 PRELIMINARY DRAINAGE PLAN and/or inlets and storm drains; and The preliminary drainage plans include the • Evaluate potential erosion problems and following information related to drainage: indicate what measures should be taken to minimize them. • Cut areas with beginning and end stations In flat sections, the designer should: indicating depths of cut; • Indicate where the runoff can be disposed; and

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• Evaluate the impact of ditch or storm producing an acceptable preliminary plan drain slopes. package. Obtaining written approval from the Project Manager of this report and proposed • At intersecting roads and superelevation methodology is required. transitions the designer should: For the semi-final plan submittal, a final • Establish and verify inlet locations; and H&H Report is prepared which also includes • Note changes in cross slopes, warping and the final analysis for drainage and stormwater crowning of roadways, and the means of management with verification that the effective interception of surface runoff. approved criteria have been met or exceeded • Field reviews should include gathering along with construction detail. The final information required by the sediment and drainage report describes the drainage stormwater management checklist. characteristics within the project area, the various design criteria, data collected, data In all cases, check the boundaries of sources and references, anticipated computer drainage areas, describe the topography, software for preliminary analysis, a proposed determine the ground cover, and verify soil analysis format, etc. This report may need types against the published soil survey maps minor revisions after its final review. Refer to from the county conservation district, to aid in DelDOT’s ES2M Design Guide for the determining the coefficient of runoff, runoff required contents of a stormwater management curve number, and time of concentration. report. Additional guidance and samples are available from the Quality Section. 6.5.4 DRAINAGE REPORT Drainage design and therefore the report is 6.6 HYDROLOGY divided into several topic areas including, Hydrology studies the effect of rainfall hydrology; roadside ditches; open channels; intensity, duration, and runoff at prescribed pavement and storm drains; cross road frequencies on a watershed. The design of culverts; erosion and sedimentation control; most drainage facilities requires the peak rate stormwater quantity management; and of flow at a specified frequency; others, such stormwater quality management. as drainage systems involving detention Each of these topic areas has multiple storage, depend on runoff hydrographs that resource publications, Internet sites, and estimate the pre- and post-development peak software for design as well as design aides discharge. such as charts, nomographs, tables, and figures. In addition, some topic areas have 6.6.1 REFERENCES several acceptable methods for performing the Publications design calculations. In order for a reviewer to check the report, it is necessary that all the The following references are needed in resources or applicable portions of each be order to prepare a hydrological study. Because included or identified. of the availability of computer software to perform the calculations, updated versions of The analysis performed during design is these publications frequently delete the basic documented in a comprehensive report of the reasoning and understanding of principles hydrological characteristics of the drainage used to develop the software. Therefore, some areas and hydraulic calculations relating to references also include the original drainage and stormwater management. The publications. basics of the drainage report are developed after defining federal, state and local • Urban Hydrology for Small Watersheds, requirements, data collection, a preliminary Technical Release No. 55 (TR-55), NRCS, design and a comprehensive field review. A Revised 2003 preliminary H&H Report is necessary for

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• Hydraulic Design of Highway Culverts, • Hydrologic Soil Groups for Delaware Highway Design Series 2 (HDS-2), o Figure 6-10 FHWA, 2002 o Exhibit A, TR-55 • Introduction to Highway Hydraulics, • Delaware’s 24-Hour Rainfall Depths Hydraulic Design Series No. 4 (HDS-4), o Figure 6-11 FHWA, 1983 and 2001 • Runoff Curve Numbers • Hydrology, Hydraulic Engineering o Table 5.4, HDS-2 Circular 19 (HEC-19), FHWA, 1984 o Table 2-2a thru d, TR-55 • Urban Drainage Design Manual, • Manning’s Roughness Coefficients (n) Hydraulic Engineering Circular 22 (HEC- o Table 2.1, HDS-2 22), FHWA, 2001 o Table 3-4, HEC-22 • Magnitude and Frequency of Floods on o Table 3-1, TR-55 Nontidal Streams in Delaware, U.S. • Unit Peak Discharge (qu) for NRCS Type Geological Survey Scientific II Rainfall Distribution Investigations Report 2006-5146, o Exhibit 4-II, TR-55 (http://pubs.usgs.gov/sir/2006/5146/) - • Adjustment Factor (Fp) for Pond & regression equations Swamp Areas Spread Throughout the Tables and Figures Watershed The following tables and figures are o Table 5.6, HDS-2 required to perform a hydrological study of a o Table 3-9, HEC-22 watershed: o Table 4-2, TR-55 Computer Software • DelDOT’s Criteria for Design Frequency • TR-20 o Figure 6-1 • TR-55 • Frequency Factor, Cf • WINTR-55 o Section 3.2.2.1, HEC-22 • PondPack • Intercept Coefficient (k) for Velocity vs. • WMS (Watershed Modeling System) Slope Relationship • HEC-HMS o Figure 6-4 o Table 3-3, HEC-22 6.6.2 DESIGN FREQUENCY • Delaware’s Rainfall Intensity Estimate Since it is not economically feasible to o Figures 6-5 to 6-7 design drainage facilities for the maximum • Runoff Coefficients for Rational Method potential runoff, a limit on the runoff for the o Figure 6-8 respective design frequency must be specified. o Table 5.7, HDS-2 The frequency at which the corresponding o Appendix B, Tables 11 and 12, flood can be expected to occur is the HDS-4 reciprocal of the probability that the expected o Table 3-1, HEC-22 flood will be equaled or exceeded in a given • Nomograph for Determining Velocities year. The frequency period is also known as Using the Upland Method of Estimating the recurrence interval or return period. The T and T risk of flooding and economy in construction t1 t2 are the two factors that govern the design o Figures 37 and 52, HEC-19 storm frequency to be used for designing any o Figure 3-1, TR-55 drainage facility. See Figure 6-1 for design • Soil Group Descriptions for TR-55 frequency criteria. Method o Figure 6-9 o Appendix A, TR-55

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6.6.3 PEAK DISCHARGE A = Drainage area (ac) Peak discharge is the maximum rate of This equation is most reliable for surfaces flow that results from a storm corresponding that are smooth, uniform and impervious such to the selected design frequency for a as pavements. The equation’s reliability is particular drainage location being analyzed or subject to three basic assumptions: (1) the designed. There are numerous methods peak runoff at any point is a direct function of available for determining peak discharge. For the average rainfall intensity for the time of roadway drainage DelDOT uses the Rational concentration to that point; (2) the recurrence Method for homogenous drainage areas such interval of the peak discharge is the same as as pavement and stormwater drain system the recurrence interval of the average rainfall design. This method is most accurate for intensity; and (3) the time of concentration is watersheds smaller than 50 acres, but can be the time required for the runoff to become used for watersheds up to 200 acres. The established and flow from the most distant NRCS TR-55 Method is used for complex point of the drainage area to the point of watersheds up to 300 acres having several discharge. subareas that may or may not be homogenous. For a drainage area consisting of different This method also can be used for stormwater subareas with varying soil characteristics and management analysis such as detention, cover, different runoff coefficients are retention and control structures. Areas larger assigned and a weighted average is developed than this normally require drainage facilities using the equation: that fall within the responsibility of the Bridge Design section. Both of these methods are ∑ AC xx briefly discussed in following sections. C = (6.2) Example 1, Attachment B, illustrates the use Atotal of the rational method; Example 2 uses TR-55. For more detailed information about the 6.6.3.1 THE RATIONAL METHOD rational formula refer to HDS-2 and HEC-22.

The rational method employs an empirical PROCEDURE equation relating runoff to rainfall intensity. It estimates the peak rate of runoff using: the Step 1. Obtain the following information for drainage area; a weighted runoff coefficient each design segment: that considers existing and proposed land use a. Drainage area; and types of soils and surfaces; and the rainfall b. Land use (percentage of impermeable intensity. area such as pavement, sidewalks, roofs, The Rational Method’s equation is etc.); expressed as: c. Soil types (highly impermeable or

Q = Cf CIA (6.1) impermeable soils); where: d. The hydraulically most distant point of the drainage area to the point of Q = Peak flow (ft3/s) discharge with significant contribution;

Cf = Dimensionless frequency factor e. Difference in elevation from the farthest for design frequencies greater than point of the drainage area to the point of 10-yr discharge. C = Dimensionless runoff coefficient, Step 2. Determine time of concentration (tc) a function of the watershed’s ground cover The time of concentration (tc) is the I = Design storm rainfall intensity maximum time required for the runoff to flow (in/hr) from the most remote point in a drainage area

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or subarea to the proposed point of discharge. Overland (Sheet Flow) takes place on Among a number of alternative paths that plane surfaces, nonconverging irregular runoff could take from far distant points of a surfaces, usually at the uppermost areas of drainage area, the time of concentration watersheds, and is less than 1.5 inches deep. represents the longest possible travel time The hydraulic length for sheet flow is (which is not necessarily the longest distance). normally assumed to be a maximum 100 feet The path of runoff that governs the time of for unpaved surfaces and 150 feet for paved concentration is the longest hydraulic length surfaces. The equation for sheet flow is a of the drainage area. It may be necessary to version of the kinematic wave equation and is: check several watercourses to determine the 6.0 longest flow path with significant 93.0 ⎛ nL ⎞ T = ⎜ 1 ⎟ (6.3) contribution. t1 4.0 ⎜ ⎟ I ⎝ s ⎠ The type and length of flow vary over the where: hydraulic length. The characteristics of these flow types divide them into three flow Tt1 = Travel time (min) patterns: overland (sheet) flow (T ), shallow t1 n = Roughness coefficient from Table 3-1, concentrated flow (T ), and open channel flow t2 TR-55 or Table 3-2, HEC-22 (Tt3). The time of concentration is the summation of the travel times for each type of L1 = Length of flow, (ft) flow over each of their hydraulic lengths. The I = Average rainfall excess intensity for a minimum value of t is 6 minutes (0.1 hr) and c storm duration = t (in/hr) the maximum is 600 minutes (10 hr). c s = Surface slope (ft/ft) The factors affecting the time of concentration and the time of flow are: I depends on tc and is initially assumed from Figures 6-5, 6-6 or 6-7. t is calculated 1. Surface roughness: Development in the c from equation 6.4 and checked against the watershed changes the flow velocity initial assumed value. This is repeated until retardance from very slow shallow the two successive values are the same. overland flow through vegetation, redirecting the flow to impervious areas NRCS (TR-20), has also developed a such streets and gutters, to storm drains method for determining Tt1 for overland flow that transport the runoff more rapidly. based on a 24-hr Type II rainfall distribution, Therefore, the travel time is usually eliminating the need for trial and error decreased. calculations until Tt equals storm duration. 2. Channel shape and flow pattern: Travel The equation is: time in small watersheds is influenced by nL )(42.0 8.0 the resulting upstream overland flow. In T = 1 (6.4) t1 5.0 4.0 developing or developed areas, the runoff 24 sP is directed to defined channels as soon as possible. This results in increased flow where: velocity and decreased travel time. Tt1 = Travel time (min) 3. Slope: Drainage designs have a variety of n = Roughness coefficient from Table 3-1, site grading solutions to manage water TR-55, Table 3-2, HEC-22 or Table 2- flow. If there is extensive use of swales, 1, HDS-2. ditching or storm drains, the slopes within the watershed can be modified L1 = Length of flow, (ft) dramatically. Generally, swales and P24 = 2-yr Type II 24-hr rainfall (in) ditches increase slopes, and storm drains decrease slopes. s = Surface slope (ft/ft

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The Upland (Velocity) Method is used to 100 feet for unpaved areas and 150 feet for determine the travel time for shallow paved areas. The equation for determining the concentrated and open channel flow segments average velocity for shallow concentrated flow by dividing their respective hydraulic lengths is: by each segment’s average flow velocity: = 8.32 ksV 5.0 (6.6) L T t = (6.5) where: V 60 V V = Average velocity (ft/s) where: k = velocity intercept coefficient based Tt = Segment travel time (min) on slope and surface type, Figure L = Segment length (ft) 6-4 V = Segment velocity (ft/s) s = average slope of the watercourse (ft/ft) Each segment’s velocity depends upon the watercourse’s average slope and surface V can be obtained graphically from Figures roughness. 37 or 52, HEC-19; Figure 4b, HDS-4; or Figure 3-1, TR-55. The travel time (T ) is Shallow Concentrated Flow takes over t2 determined by using equation 6.5. from sheet (overland) flow as very shallow channels or gutters along the hydraulic length, 1.5 to 6.0 inches in depth. Limit sheet flow to

Figure 6-4 Intercept Coefficient (k) for Velocity vs. Slope Relationship for Rational Method Land Cover and (Type of Flow) k

Forest with heavy ground litter; hay meadow (overland flow) 0.076

Trash fallow or minimum tillage cultivation, contour or strip 0.152 cropped; woodland (overland flow)

Short grass pasture (overland flow) 0.213

Cultivated straight row (overland flow) 0.274

Nearly bare and untilled (overland flow) 0.305

Grassed waterway (shallow concentrated flow) 0.457

Unpaved (shallow concentrated flow) 0.491

Paved area (shallow concentrated flow); small upland gullies 0.619

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Open Channel Flow includes natural The time of concentration, tc, is equal to the streams, ditches, pipes and gutters in the lower sum of the travel times derived for all three stretch of the watershed along the hydraulic types of flow: length to the point of termination at an outfall. Determining when channel flow begins is = + + TTTt tttc 321 (6.8) based on several indicators, including survey Step 3. Determine the rainfall intensity (I) data defining the channel, visible signs on aerial photography of the watershed, on area Rainfall intensity is the amount of rainfall topographic maps, and USGS quadrangle in inches per hour based on a duration that sheets. equals the time of concentration and the design storm frequency. The rainfall intensity Determine the velocities by using can be obtained from Figures 6-5 to 6-7 Manning's equation: provided that the time of concentration sR )()(49.1 2/13/2 (duration) is known. V = (6.7) n Step 4. Select the appropriate runoff coefficient (C) where: The rational method uses a runoff V = Average velocity (ft/s) coefficient that is representative of the R = Hydraulic radius (ft) = A/Pw (6.7a) drainage area being studied. This coefficient A = Cross sectional flow area (ft2) averages the area ambient moisture content, P = Wetted perimeter (ft) infiltration/ evaporation potential, average w slope, existing and future ground cover, and s = Energy grade line (channel or pipe flow developed surfaces. line) (ft/ft), and n = Manning’s roughness coefficient for The drainage area is divided into its various open channel flow pre- and post-developed subareas. Using Figure 6-8, a runoff coefficient is assigned and The travel time for open channel flow, Tt3, a weighted coefficient is calculated. Higher is obtained from equation 6.5. In developed values of C are used for steeply sloped areas areas there may be a variety of open channel and longer return periods because infiltration facilities including storm drains, swales, and other losses have a reduced effect on the ditches, gutters. The travel time for each one runoff. of these is computed by using Manning's Step 5. Compute the design peak flow (Q) formula and then combined to provide Tt3 for the complete open channel flow in the Determine the peak discharge using the watershed. Rational Method equation, Q = Cf CIA.

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Figure 6-5 Rainfall Intensity Estimates (in/hr) for Rational Method - New Castle County Duration (min) Frequency (yr) 5 10 15 30 60 120 180 360 720 1440 (1 hr) (2 hr) (3 hr) (6 hr) (12 hr) (24 hr) 2 4.97 3.97 3.33 2.30 1.44 0.87 0.62 0.38 0.23 0.13 5 5.83 4.67 3.94 2.80 1.79 1.08 0.78 0.48 0.29 0.17 10 6.42 5.13 4.33 3.13 2.04 1.24 0.89 0.55 0.34 0.20 25 7.13 5.68 4.80 3.55 2.37 1.45 1.05 0.66 0.41 0.25 50 7.60 6.05 5.10 3.84 2.60 1.61 1.18 0.74 0.47 0.29 100 8.06 6.40 5.40 4.13 2.85 1.77 1.30 0.83 0.53 0.34 200 8.44 6.69 5.63 4.38 3.07 1.93 1.43 0.92 0.60 0.39 500 8.88 7.02 5.89 4.69 3.36 2.14 1.60 1.05 0.70 0.46

Figure 6-6 Rainfall Intensity Estimates (in/hr) for Rational Method - Kent County Duration (min) Frequency (yr) 5 10 15 30 60 120 180 360 720 1440 (1 hr) (2 hr) (3 hr) (6 hr) (12 hr) (24 hr) 2 5.06 4.05 3.40 2.34 1.47 0.90 0.65 0.40 0.24 0.14 5 6.01 4.81 4.06 2.88 1.85 1.13 0.82 0.50 0.30 0.18 10 6.68 5.35 4.51 3.27 2.13 1.31 0.95 0.59 0.36 0.21 25 7.54 6.01 5.08 3.76 2.50 1.56 1.14 0.71 0.44 0.27 50 8.15 6.49 5.48 4.13 2.79 1.76 1.29 0.81 0.51 0.32 100 8.76 6.96 5.86 4.49 3.09 1.96 1.45 0.92 0.59 0.37 200 9.92 7.39 6.22 4.84 3.39 2.17 1.62 1.04 0.67 0.43 500 10.02 7.93 6.65 5.29 3.80 2.45 1.85 1.21 0.80 0.52

Figure 6-7 Rainfall Intensity Estimates (in/hr) for Rational Method - Sussex County Duration (min) Frequency (yr) 5 10 15 30 60 120 180 360 720 1440 (1 hr) (2 hr) (3 hr) (6 hr) (12 (24 hr) hr) 2 5.06 4.04 3.39 2.34 1.47 0.91 0.66 0.40 0.24 0.14 5 6.02 4.83 4.07 2.89 1.85 1.16 0.84 0.52 0.30 0.19 10 6.76 5.40 4.56 3.30 2.15 1.35 0.99 0.61 0.36 0.22 25 7.67 6.11 5.15 3.82 2.54 1.61 1.19 0.74 0.45 0.28 50 8.32 6.62 5.59 4.21 2.85 1.83 1.35 0.85 0.52 0.33 100 8.96 7.12 6.00 4.59 3.16 2.05 1.53 0.97 0.61 0.38 200 9.60 7.61 6.40 4.98 3.49 2.28 1.71 1.10 0.70 0.45 500 10.38 8.21 6.88 5.48 3.93 2.59 1.97 1.28 0.84 0.54

Note: Interpolation shall be used for rainfall for intermediate durations.

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Figure 6-8 Recommended Runoff Coefficients (C) for Rational Method

Type of Drainage Area Surface Runoff Coefficient, C Sand, from uniform grain Bare 0.15 size with no fines to well Light Vegetation 0.10 graded with clay or silt Heavy Vegetation 0.05 Loam, from sandy or Bare 0.20 gravelly to clayey Light Vegetation 0.10 Dense Vegetation 0.05 Earth Surfaces Gravel, from clean gravel & Bare 0.25 gravel sand mixtures with no silt or clay to high clay Light Vegetation 0.15 or silt contents Dense Vegetation 0.10

Clay, from coarse sandy to Bare 0.40 pure colloidal clays Light Vegetation 0.30 Dense Vegetation 0.25 Sandy soil Slope, Flat to 2% 0.05 Average Slope 2% to 7% 0.10 Lawns Steep Slope Over 7% 0.15 Clayey soil Slope, Flat to 2% 0.13 Average Slope 2% to 7% 0.18 Steep Slope Over 7% 0.25 Asphalt 0.95 Pavements Concrete 0.95 Compacted graded aggregate or gravel 0.70 Railroad yard areas 0.30 Parks, golf courses and cemeteries 0.15 Playgrounds 0.25 Unimproved Areas 0.15 Cultivated areas 0.30 Swamp or marsh areas 0.08 Roofs 0.90 Drives and walkways 0.90 City business areas 0.85 City with dense residential areas and varying soil and vegetation conditions 0.70 Residential areas, single family units 0.40 Residential areas, duplexes and twins 0.50 Residential areas, multi-units 0.70 Suburban residential areas (lots 1/2 ac or more) 0.35 Apartment complexes 0.60 Light industrial areas 0.70 Heavy industrial areas 0.80

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6.6.3.2 THE NRCS METHOD CN represents the hydrologic characteristics of the drainage area including the average slope, NRCS Technical Release 55 (TR-55, 1986; soil group type, plant cover, amount of WINTR-55, 2003), Urban Hydrology for impervious surfaces, interception rates, and Small Watersheds is one of the methods surface storage. Using the runoff curve commonly used to determine a watershed’s number and the NRCS 24-hour rainfall data, peak discharge. TR-55 is a computer the design storm(s) runoff depth for the application based on data and routines watershed(s) is determined as follows: developed using TR-20. This method is particularly useful for determining pre- and Runoff Depth (qd) post-development runoff rates and the The runoff depth equation is: subsequent design of any required stormwater 2 control structures. ( 24 − SP )2.0 ) qd = (6.9) The two most commonly used design P24 + 0.8S applications for roadway drainage in TR-55 are: (1) the Graphical Peak Discharge method where and (2) the Tabular Hydrograph method. The qd = Runoff depth for a specified return Graphical method is limited to a single period (in) homogenous watershed where land use, soil, P = 24-hour rainfall using Figure 6-11 for and cover are uniformly distributed throughout 24 the corresponding return period (in) the watershed. The Tabular method uses hydrographs for analyzing a larger more S = Potential maximum retention after complex watershed having: multiple runoff begins, inches homogenous subareas with several converging S is related to the soil and cover conditions reaches; changing land uses within any of the through the runoff curve number for the existing homogenous watershed; and storage various subareas and defined by the equation: and control structures. Worksheets for each method are provided in TR-55. 1000 S −= 10 (6.10) The two methods use the same approach to CN determine the basic design data necessary to for calculating the watershed(’s) peak where discharge: the runoff depth, time of CN = Weighted average runoff curve concentration and travel time within the number using a ratio of assigned subarea watershed(s) and an initial abstraction factor CN’s to their area as related to the entire (Ia) based on the CN to determine the unit watershed area. CN must 40 or greater. peak discharge per square mile per inch of runoff. Equations convert the runoff depth, the The hydrologic soil group(s) in each unit peak discharge, the drainage area, and a subarea is based on the soil classification storage adjustment factor to calculate the peak (denoted as A, B, C and D) based on their watershed discharge or generate a pre- and rainfall infiltration rates. A represents soils post-hydrograph of the peak discharge. with the maximum rate of rainfall infiltration and D the soils with the lowest rate of The study begins by defining the watershed infiltration. and dividing it into homogenous subareas using existing and proposed land uses. Land Refer to Table 2-2a-d, TR-55, for the use determines the percentage of pervious and runoff curve numbers for urban and rural impervious surfaces. The drainage runoff areas. These tables were developed assuming characteristics are also based upon the an Initial abstraction (Ia) of 20% of the hydrological soil classification. A runoff curve potential maximum retention after runoff number (CN) is assigned to these subareas. begins (S) in the drainage area. Ia represents

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losses prior to runoff and includes initial n = Manning’s roughness coefficient for infiltration storage in surface depressions, sheet flow (Table 3-1, TR-55) evapotranspiration, leaf and vegetation L = Portion of hydraulic length on retention, and other factors. Cover types 1 which sheet flow takes place (ft) represented by vegetation, bare soil, and impervious surfaces are usually determined s= Average slope of the hydraulic length from field reconnaissance, aerial photographs, L1 (ft/ft) land use maps, and soil survey maps. The maximum for L1 is 100 feet for unpaved Hydrologic condition indicates infiltration and surfaces and 150 feet for paved surfaces. runoff; it is estimated from the density of plant and residue cover in rural areas. Shallow Concentrated Flow A soil survey for each county is available For shallow concentrated flow, the time of from the United States Department of concentration (T ) is computed using equation Agriculture. These publications contain aerial t2 6.5. V2 is obtained from Table 3-1, TR-55, or photographic maps of Delaware with detailed the equations: soils information. Figure 6-10 lists the hydrologic soil groupings commonly found in = sV )(10.16 5.0 Unpaved surfaces (6.12)

Delaware. Soil survey maps for Kent and 5.0 Sussex counties are available online at the web = sV )(99.14 Grassed waterway (6.12a) site http://www.udel.edu/FREC/spatlab for the = sV )(30.20 5.0 Paved surfaces (6.13) University of Delaware Spatial Analysis Lab. After determining CN and the 24-hour where rainfall for the watershed, the runoff depth (qd) V = Average velocity (ft/s), and is calculated using Worksheet 2, figure 2-1 s = Slope of the hydraulic gradeline and table 2-1 in TR-55 or equations 6.9 and (watercourse slope) (ft/ft) 6.10. Next, the travel time for each type of flow Open Channel Flow and the ultimate time of concentration to the The travel time for open channel flow (T ) point of interest must be determined. t3 is determined by using Manning’s equation, Time of Concentration and Travel Time (tc) and equations 6.3 and 6.5 When using equation 6.5 use 3600 instead of 60 to The flow times for the three types of flow calculate hours instead of minutes. within each watershed (as described in Section 6.4) are calculated as follows. The time of concentration for the drainage basin (t ) is the sum of the travel time for each Sheet (Overland) Flow c flow segment. The minimum value for this The travel time when using TR-55 is method is 6 minutes (0.1 hr) and a maximum obtained from the modified Manning's- of 10 hours. See Worksheet 3, TR-55, for the kinematic solution and is determined in hours, procedure. modified from equation 6.4: At this point the designer must decide on nL )(007.0 8.0 the scope of the watershed analysis. If the = = 1 (6.11) T t1 5.0 4.0 watershed is hydrologically homogenous with 2 sP land use, soils, and cover uniformly where distributed with one main watercourse, then the Graphical Peak Discharge Method can be Tt1 = Travel time for overland flow (hr) used. If the watershed has nonhomogeneous P2 = 2-year, 24-hour rainfall depth (in) areas that can be divided into homogenous areas, land use changes are proposed for a

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portion of the watershed and stormwater TABULAR HYDROGRAPH METHOD management facilities requiring hydrographs First, use worksheet 5a to develop a are necessary, then the Tabular Hydrograph summary of basic watershed data by subarea. Method is used. The total tc for each subarea is the total travel There are other regulatory controls that time for the subarea reach. Find the influence the selected method. State sediment hydrograph coordinates for selected tc‘s using and stormwater policy is that all hydrologic exhibit 5. The flow time is found using the computations must use the NRCS Type II-24 equation: hour rainfall event and for all projects south of the Chesapeake and Delaware Canal, the = mt QAqq (6.15) Delmarva Unit Hydrograph will be where: incorporated into the design process. q = Hydrograph coordinate (cfs) at GRAPHICAL PEAK DISCHARGE hydrograph time (t) METHOD qt = Tabular hydrograph unit discharge The peak discharge at the site under from exhibit 5 (csm/in) consideration is obtained from the equation: Am = Individual subarea drainage area 2 Q = qui Am qd F p (6.14) (mi ) where, Q = Runoff (in) Qi = Peak discharge for the i'th year frequency (ft3/s) The data needed to model the present and 3 future conditions of the watershed using qu = Unit peak discharge (ft /s /sq mi/in) hydrographs for each subarea and routing these hydrographs to the watershed control A = Drainage area (sq mi) m structures and outfall are: qd = Runoff depth (in) (equation 6.9) 1. Drainage area for each homogenous F = Pond and adjustment factor p subarea. (mi2) (Table 4-2, TR-55) 2. Time of concentration for each subarea The only unknown variable in this equation is in hours. See the previous discussion q The steps to find the value of q are: u. u and Chapter 3, Worksheet 3, TR-55 for 1. Using the weighted CN determine the the procedure. initial abstraction (I ) from Table 4-1, a 3. Time of travel for each routing reach in TR-55. hours. See the previous discussion and 2. For the design frequency, select the 24- Chapter 3, Worksheet 3, TR-55 for the hour rainfall (P) from Figure 6-11. procedure

3. Calculate the ratio of Ia/P. 4. The weighted CN for each subarea. See Table 2-2 and Worksheet 2, TR-55 for 4. Peak discharge per square mile per the procedure. inch of runoff (qu) is obtained from exhibit 4-II for Type II Rainfall 5. 24-hour rainfall for the design Distribution, the previously defined tc frequency using the Delmarva Unit and the Ia/P ratio. Hydrograph Worksheet 4, Appendix D, TR-55, provides 6. Runoff, in inches, for each subarea. See an easy guide for calculating Qi. Also, the the previous discussion and Chapter 2, equations used to generate the figures and Worksheet 2, TR-55 for the procedure. tables in TR-55 can be found in Appendix F, 7. Initial abstraction (I ) from Table 5-1, TR-55. a TR-55 .

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8. Calculate the ratio of Ia/P for each Chapter 5 in TR-55 describes the procedure subarea. for developing a watershed’s composite flood hydrograph with the limitations that may 9. Develop a composite hydrograph by apply. summing prerouted individual subarea

hydrographs using Worksheet 5b, TR- 55. Figure 6-9 Soil Group Descriptions for TR-55 Method Soil Hydrologic Description Soil Type Group High infiltration and low runoff, very well drained sands Sand, loamy sand or sandy A or gravels loam

B Moderately well drained soils with fine to moderately Silt loam or loam coarse textures C Low infiltration rate, moderately fine to fine texture Sandy clay loam Very low infiltration rate, high runoff potential, Clay loam, silty clay loam, D predominately clayey soil sandy clay, silty clay or clay

Figure 6-10 Hydrologic Soils Descriptions for TR-55 Method

Aldino C Elsinboro B Keyport C Pepperbox B Assawoman D Evesboro A Kinkora D Plummer D Klej B Pocomoke D Bayboro D Fort Mott A Portsmouth D Berryland D Fallsington D Mulica D Pone D Butlertown C Manor B Greenwich B Metapeake B Rumford B Calvert D Gleneig B Matawan C Rutlege D Chester B Mattapex C Rosedale A Codorus C Hammonton C Montalto C Collington B Hatboro D Sassafras B Comus B Hurlock D Nanticoke D Neshaminy B Talleyville B Delanco C Johnston D Osier D Watchung D Elioak C Kalmia B Othello D Woodstown C Elkton D Kenansville A

Other common hydrologic soils Borrow Pits Extremely variable Ingleside A Coastal Beaches A Swamp D Made land C Broadkill, Westbrook D Mixed alluvial C-D Urban Land C-D Mixed silts and clay C

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Figure 6-11 24-Hour Rainfall Depths for Delaware for TR-55 Graphical Method

NRCS Type II, 24-Hour Duration County Storm Event New Castle Kent Sussex 1-yr 2.7 2.7 2.8 2-yr 3.2 3.3 3.4 5-yr 4.1 4.3 4.4 10-yr 4.8 5.2 5.3 25-yr 6.0 6.5 6.7 50-yr 6.9 7.6 7.9 100-yr 8.0 8.9 9.2 500-yr 10.9 12.6 13.0

6.7 OPEN CHANNEL FLOW • Longitudinally as intercepting channels at the top of a cut section; Open channels are the most commonly used component of a drainage system. They • In cut sections of roadways to remove may occur naturally within the drainage basin stormwater; or be artificially introduced in the design • In medians to covey roadway runoff to process. Open channels include inlets: natural/manmade ditches, streams, median • At the bottom and toe of embankment swales and gutters. They may be unlined or slopes to convey stormwater to a lined with artificial or natural materials to discharge point. protect against erosion. In drainage design, the term “open 6.7.1 REFERENCES channel” is frequently used to describe the Publications: design and flow characteristics of all drainage The primary references for understanding systems (ditches, storm drains or culverts) that the principles and analysis of open channel have their flow controlled by atmospheric flow are: pressure and gravity. The primary design control is the difference in elevation between • Design Charts For Open-Channel Flow, the various open channel sections. However, Hydraulic Design Series No. 3, (HDS-3) the open channel’s shape, surface type, FHWA, 1973 coefficient of friction, and alignment are • Introduction to Highway Hydraulics, considered in determining its flow. Hydraulic Design Series No. 4, (HDS-4) FHWA, 1983 In open channel design, the final geometric parameters are selected to ensure that the flow • Design of Riprap Revetment, Hydraulic does not exceed critical depth or critical Engineering Circular 11 (HEC-11), velocity. This is an iterative process to reach FHWA, 1989 the appropriate design. • Hydraulic Design of Energy Dissipaters for Culverts and Channels, Hydraulic Open channels are used:

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Engineering Circular 14 (HEC-14), o Figure 6-14 FHWA, 1975 • Maximum Allowable Velocities for Open • Design of Roadside Channels with Channels Flexible Lining, Hydraulic Engineering o Figure 6-15 Circular 15 (HEC-15), FHWA, 1986 and • Channel Lining Design Procedure 2005 o Figure 6-17 • Urban Drainage Design Manual, • Channel Lining Design Computation Hydraulic Engineering Circular 22 (HEC- Chart 22), FHWA, 2001 o Figure 6-18 • Open-Channel Hydraulics, Ven Te Chow, • DelDOT’s Roadside Ditch Design Form 1959 o Example 4, Attachment B

Tables and Charts: • Kb Factor for Maximum Stress on Channel Access to the following tables and charts is Bends needed for analyzing open channel flow: o Chart 21, HEC-22 • Open Channel Flow Schematic • Stability Factor Selection Criteria o Figures 6 and 7, HDS-4 Chart 3, HEC-11 • Manning's Roughness Coefficients n for • Correction Factor for Riprap Size Open Channels o Chart 2, HEC-11 o Figure 6-16 • Riprap Size Relationship o Table 1, HDS-3 o Chart 1, HEC-11 o Tables 12 and 14, HDS-4 Computer Software o Tables 2.1 and 2.2, HEC-15 • HYDRAIN • Channel Geometry Equations o Appendix B, HEC-15 • HY7 WSPRO o Table 2-1, Chow • HEC-RAS • Nomograph for Solution of Manning's Equation 6.7.2 OVERVIEW o Chart 83, HDS-3 Most roadside drainage design is assumed o Appendix C, Chow to be steady, uniform flow. The designer • Solution of Manning's Equation for assumes that the channel cross section, slope, Channels of Various Side Slopes alignment and surface roughness are o Chart 16, HEC-12 reasonably constant over a sufficient length. • Capacity of Trapezoidal Channels For steady, uniform flow in open channels, o HDS-3 and HDS-4 Manning's equation (Equation 6.7) is used to determine the velocity. • Geometric Design Chart for Trapezoidal Channels The capacity of the channel is determined o HDS-3 by: Appendix B, Chow o Q = VA (6.16) • Uniform Flow in Trapezoidal Channels by Manning’s Equation Combining the two equations results in: 49.1 o Table B.1, HEC-14 = AQ sR 2/13/2 (6.17) • Open Channel Design Procedure n o Figure 6-12 In the above equations: • Maximum Non-Scouring Ditch Grades V = Mean velocity (ft/s) with Grass Lining n = Manning's roughness coefficient • Permissible Velocities for Channels Lined with Grass

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R = Hydraulic Radius (ft), the ratio of • To collect runoff from the highway and flow area of cross section (A) to the adjacent areas which may sometimes wetted perimeter (P), A/Pw extend outside the right-of-way, and s = Slope of the energy grade line (ft/ft). dispose of the accumulated runoff at Usually it is parallel to the water suitable outlet locations, surface or the profile of the channel • To drain the pavement structure and the bottom. top layer of supporting subgrade to Q = Rate of flow (ft3/s) prevent saturation and loss of support for A = cross sectional area of the flow (ft2) the pavement, and to protect it from damage from freeze-thaw cycles, and Manning's roughness coefficient n may • To control the quantity and quality of vary with the depth of flow. A range of n roadway runoff while minimizing any values for different types of open channels is impact to adjacent properties. Draining provided in the referenced resources. runoff onto adjacent properties should To simplify solving the equations a only be considered when absolutely conveyance factor (K) can be developed in a necessary; in this case, temporary and/or tabular or curve form for the more commonly permanent easements may be needed. used channel cross sections based on the following equation: 6.7.3.1 DESIGN CRITERIA 49.1 In additional to any controls set in other K = AR 3/2 (6.18) n chapters of this manual, the following criteria is to be used in the design of roadside ditches: As a result, substituting K into Manning’s equation the rate of flow, Q, would be: 1. The rational method is used to compute = KsQ 2/1 (6.19) the design runoff, and Manning's equation for capacity. Figure 6-1 has design frequencies. 6.7.3 ROADSIDE DITCHES 2. Roadside ditches shall conform to clear Roadside ditches are trapezoidal or V- zone criteria and legal requirements. shaped open channels that usually parallel the Safety of motorists, minimum roadway and are lined with grass or non- maintenance requirements, aesthetic erodible materials required for erosion values, and avoiding potential damage to protection. Even in curb sections the designer abutting properties as well as adverse may need to include a roadside ditch in the environmental impacts are essential drainage design. Median ditches and other considerations in design. Refer to small-excavated channels are designed using AASHTO’s Roadside Design Guide. the same principles as for roadside ditches. 3. Maintain a minimum freeboard of 0.5 ft Roadside ditches provide an opportunity to minimum, 1 ft preferred, below the edge reduce runoff pollutants with infiltration of shoulder. through permeable (vegetative) surfaces that 4. For adequate subbase and pavement allow the pollutants to be absorbed into the drainage, the bottom of the roadside ditch underlying soils and reduce the peak flow should be at least 2.5 feet below the velocity. The swales and ditches include stone shoulder edge of the traveled way. check dams, vegetated beds, and banks with 5. Side slopes for grass-lined ditches shall be shallow longitudinal slopes and relatively 3:1 or flatter to facilitate mowing shallow side slopes. One of the controlling operations. The designer should review factors is the design flow. the current roadside vegetation to ensure Roadside ditches serve several purposes: the proper selection of a roughness coefficient.

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6. At the top of cut slopes, an intercepting capacity under the assumed final ditch ditch should be investigated. Every effort lining conditions. shall be made to prevent sediment and 3. After checking these two conditions and debris from filling in the ditch. deciding on the final dimensions, the 7. All roadside ditches are to be lined with design criteria freeboard is added. grass or special materials for scour Refer to Sections 6.10, 11 and 12 for a protection. The ditch linings are to be discussion on designing roadside ditches as a designed to withstand potential erosion. part of stormwater management. These 8. The minimum ditch grade is 0.3%, but sections also more fully describe the phased 0.5% is preferred. approach to ditch design. 9. Gradual transitions should be used for cross section changes. Normally, The following design procedure is used for transition rates are 25 ft for every foot roadside ditches. change in the ditch bottom width and 100 1. A preliminary layout of the roadside ditch ft for increasing or decreasing the value of system with flow patterns shall be drawn z (horizontal component of the side slope) on the construction plans along with other by 1. drainage facilities (existing and proposed), 10. After completing the design, the designer utilities, right-of-way and other specific should ensure that the proposed right-of- details that may influence or restrict the way and easements are adequate to ditch design. construct the selected ditch sections. 2. In general, the grade of the roadside ditch should closely follow the grade line of the 6.7.3.2 DESIGN PROCEDURE highway. Deep ditches and frequent Since the channel’s n value changes with breaks in grade should be avoided. The time, typically (but not always) becoming distance between break points should higher as vegetation is established, the design preferably be at least 100 ft. Use Figure 6- section shall be checked for velocity 13 as a guide for limiting the maximum constraints based on the lowest n value for the grade of roadside ditches. Working plans newly constructed channel that will yield the showing the ditch layout should also highest velocity. The channel shall also be include the ditch profile and all features checked with a higher n factor for the mature above and below ground that may be growth stage to establish the minimum potential obstructions to the proposed capacity of the channel. The vegetative ditch. retardance factor is directly related to the 3. Highway cross sections, usually at 50-ft selected n value. Curves for these relationships intervals, showing the natural ground and can be found in Chow’s Open Channel the proposed highway section including Hydraulics, HEC-15, and the NRCS all roadside ditches used in establishing Engineering Field Handbook. the construction limits within the proposed The design procedure consists of three right-of-way. phases: 4. Schematic diagrams of the ditch plan, including geometric elements of the ditch, 1. Design the ditch for stability. Determine topographic features, and basin divides the dimensions using a low retardance shall be drawn to facilitate the factor, since the permanent stand of grass computation of peak discharges at the has not been established. design frequency. 2. Design for the maximum capacity by 5. An approved spreadsheet (such as the one determining the increase in depth of flow in Example 4, Attachment B, should be necessary to provide the maximum used for hydraulic computations. Manning’s roughness coefficient can be

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assumed to be 0.05 for the grass surface of In preparing an H&H report, the post- the channel. construction ditch section is analyzed. Usually 6. If the computed velocity is lower than the most surfaces will have grass or some other permissible velocity for soil with grass type of vegetation. As a result, the permissible linings as stipulated in Figure 6-14, then velocity increases on all types of soil used in permanent grass seeding and mulching per construction. In spite of the growth of DelDOT's Standard Specifications should vegetation, the permissible velocity may be suffice. exceeded in certain cases that would 7. If the computed velocity exceeds the necessitate the use of a permanent lining in permissible velocity on grass surface, then lieu of seeding and mulching. This permanent the designer must increase the ditch cross ditch lining, which is capable of withstanding section and flatten the profile to lower the erosion, may be either rigid or flexible. The velocity of flow below the permissible rigid lining usually consists of cast-in-place limit or use a special channel lining for concrete paving, fabric formed concrete permanent erosion control as described in revetment mattress, bituminous (asphaltic) Section 6.7.4. concrete paving or grouted riprap. For flexible lining, alternatives include riprap with or 6.7.4 DITCH EROSION CONTROL without geotextile fabric and wire enclosed riprap (gabions). 6.7.4.1 OVERVIEW Rigid linings can sustain high velocities up When the flow velocity of a roadside ditch to 20 ft/s, and should be considered when the exceeds the allowable velocity on bare earth of channel bed is not expected to undergo the channel (see Figure 6-15) during appreciable settlement. The cost of construction, then a temporary ditch lining, construction is a very important factor in the such as an erosion control blanket, is needed selection of this type of lining. Soil reinforcing to prevent erosion of the ditch. For surface mats are a cost-effective alternative to rigid erosion due to overland flow, the permissible linings. velocities of Figure 6-14 should also apply. Section 6.7.4.2 briefly discusses the

See DelDOT’s ES2M Design Guide, procedure for design a flexible lining. HEC-11 Standard Construction Details, and the and HEC-15 contain detailed information on Delaware Erosion & Sediment Control the design, example problems and Handbook for the requirements and specifications for flexible lined channels. The construction details for erosion and sediment following design procedures are based on control items to be used during construction. those publications.

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Figure 6-12 Open Channel Design Procedure 49.1 Open Channel Design using = AQ sR 2/13/2 n Step 1 Select channel cross section (typically Step 5 Determine required depth1 trapezoidal) Step 2 Determine slope (normally the Step 6 Determine subcritical or supercritical centerline profile grade line ≤ 3%) flow2 Step 3 Select lining (initial trial assumed to be Step 7 Calculate design velocities for grass; slopes greater than 3% may need erodible lining sections-check different lining) criteria3 (reevaluate section by section as necessary) Step 4 Determine roughness coefficient n (use Step 8 Add minimum free board an average value from Figure 6-16) 1 Use charts from HDS-3, nomograph solution for Manning’s equation, and/or charts for hydraulic elements of channel sections, partial area of flow in a circular channel and depth of flow in a trapezoidal channel (also available on DelDOT’s web site). 2 Avoid supercritical flow by adjusting design parameters. 3 Use a conservative retardance, i.e., C for channel capacity and D for flow velocity.

Figure 6-13 Maximum Non-Scouring Ditch Grades with Grass Lining1 Soil Type Flow Depth (in) Grade (%) 6 3.0 Erosive 12 1.5 18 0.8 6 6.0 Average 12 3.0 18 1.6 6 10.0 Non-Erosive 12 5.0 18 3.0 1 Slopes greater than 5% are not recommended.

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Figure 6-14 Permissible Velocities for Open Channels Lined with Grass Permissible Velocity Cover Type Slope Range (ft/s) (%) Erosion- Easily eroded resistant soils soils 0-5 8 6 Bermuda grass 5-10 7 5 >10 6 4 0-5 7 5 Buffalo grass, Kentucky bluegrass, 5-10 6 4 Smooth brome and Blue grama >10 5 3 0-5 5 4 Grass mixture 5-10 (max) 4 3 Lespedeza sericea, Weeping lovegrass, Ischaemum (Yellow bluestem), Kudzu, 0-5 (max) 3.5 2.5 Alfalfa and Crabgrass Annuals-used for temporary erosion control such as Common Lespedeza or 0-5 (max) 3.5 2.5 Sudangrass

Figure 6-15 Maximum Allowable Velocities for Open Channels Maximum velocity (ft/s) Soil type AASHTO Classification Bare Earth Grass Lining

Fine sand A-3, Beach Sand 2.0  Sandy loam A-2-4 Non-plastic 2.0 4.0 Silt loam A-4 Non-plastic 2.5 5.0 Ordinary firm loam A-7-5 Plastic, silt clay sandy 3.0 5.0 Fine gravel Granular 4.0 

Alluvial silt (colloidal) Plastic topsoil 4.0 6.0 Stiff clay A-7-6 Clay 4.5 7.0 Graded loam to cobbles Non-plastic soil and rock 4.5 7.0 Graded silt to cobbles Plastic soil and rock 5.0 8.0 Coarse gravel Creek Gravel 6.0 

Cobbles and shingles Soft rock 7.0  Shale and hardpan Medium rock 8.0  Bedrock Rock 20.0  Paved lining

Concrete 18.0

Asphalt 15.0 Grouted riprap 18.0

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Figure 6-16 Manning’s Roughness Coefficients (n) for Open Channels Type of Channel and Surface Description Range of n values Normal Maximum Natural Streams 1. Fairly regular section a. Some grass and weeds, little or no brush 0.030 0.035 b. Dense growth of weeds, depth of flow materially greater than weed height 0.035 0.050 c. Some weeds, light brush on banks 0.035 0.050 d. Some weeds, heavy brush on banks 0.050 0.070 e. Some weeds, dense willows on banks 0.060 0.080 f. For trees within channel, with branches submerged at high stage, increase all the above values by 0.010 0.020 2. Irregular sections, with pools, slight channel meander Increase respective values above by 0.010 0.020 3. Mountain streams, with pools, no vegetation in channel, bank usually steep, trees and brush along banks submerged at high flood stage a. Bottom gravel, cobbles, and few boulders 0.040 0.050 b. Bottom cobbles with large boulders 0.050 0.070 Excavated of Dredged Channels Normal Maximum 1. Earth, straight and uniform 2. Earth, winding and sluggish a. Clean, recently completed 0.018 0.020 a. No vegetation 0.025 0.030 b. Clean, after weathering 0.022 0.025 b. Grass, some weeds 0.030 0.033 c. Gravel, uniform section, 0.025 0.030 c. Dense weeds or aquatic plants in 0.035 0.040 clean deep channels d. With short grass, few 0.027 0.033 d. Earth bottom and rubble sides 0.030 0.035 weeds e. Stony bottom and weedy sides 0.035 0.050 f. Cobble bottom and clean sides 0.040 0.050 3. Dragline  excavated or 4. Rock cuts dredged 0.028 0.033 a. Smooth and uniform 0.035 0.040 a. No vegetation 0.050 0.060 b. Jagged and uniform 0.040 0.050 b. Light brush on banks 5. Channels not maintained, weeds and brush uncut a. Dense weeds, high as flow depth 0.080 0.120 b. Clean bottom, brush on sides 0.050 0.080 c. Dense brush, high flood stage 0.100 0.140

Lined Open Channels 1. Temporary Erosion 2. Permanent Erosion Protection Protection a. Concrete 0.013 0.015 a. Soil reinforcing mat 0.025 0.036 b. Asphalt 0.016 0.018 b. Excelsior blanket 0.035 0.066 c. Compacted gravel 0.033 0.044 c. Jute net 0.022 0.028 d. Plain riprap 0.035 0.045 e. Grouted riprap 0.030 0.040

Highway channels & Swales with maintained Vegetation (Values are for velocities of 6 and 2ft/s) Bermuda, Kentucky Good stand, any grass Fair stand, any grass bluegrass and Buffalograss Normal Maximum

Normal Maximum 1. Depth of flow up to 0.7 foot i. Mowed to 2 in 0.045 0.070 ii. Length 4 to 6 in 0.050 0.090 iii. Length about 12 in 0.090 0.180 0.080 0.140 iv. Length about 24 in 0.150 0.300 0.130 0.250 2. Depth of flow 0.7 1.5 feet i. Mowed to 2 in 0.035 0.050 ii. Length 4 to 6 in 0.040 0.060 iii. Length about 12 in 0.070 0.120 0.060 0.100 iv. Length about 24 in 0.100 0.200 0.090 0.170 3. Grass linings (general) 0.030 0.060

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6.7.4.2 DESIGN PROCEDURE FOR The value of C may alternatively be FLEXIBLE LININGS obtained from Chart 2, HEC-11. The remaining expression of equation 6.21 can be The normal treatment for reducing the obtained from Chart 1, HEC-11. chance of future erosion is to use stone lining (riprap) in the erodible ditch section. The The corresponding weight W50 (lbs.) for general design procedure involves the D50 (ft) stone is given by: following steps: 3 W50 = 67.32 50 SD s (6.23) 1. The depth of flow (d) and velocity (V) in the channel corresponding to the known However, for rock riprap, Manning’s peak discharge are computed with the roughness coefficient varies with the mean Manning's roughness coefficients for riprap size D50 as given in the equation:

from Figure 6-16 as illustrated in Example 6/1 4, Attachment B. n = 0395.0 D50 (6.24)

2. The required D50 riprap size is obtained For a more accurate solution, the value of n from the equation: corresponding to D50 should be obtained from

3 equation 6.25 and used in Steps 1 and 2 for a 0.001Va refined value of D50. D50 = C 5.0 5.1 (6.20) ()d avg K 1 3. With the required mean stone size (D50), the type of riprap can be specified from the where gradation data in DelDOT’s Standard

D50 = the size of stone (ft) such that 50% Specifications. Also refer to the Standard of all stones in the riprap are smaller Construction Details. than D50 4. In addition, the following criteria shall be C = a correction factor for the specific taken into considered when specifying the gravity of stone and the stability factor layer thickness: to be applied a. The layer thickness should not be less Va = Average velocity (ft/s) than the spherical diameter of D100 stone davg = Average depth of flow (ft) or less than 1.5 times the spherical

K1 = Bank angle correction factor diameter of D50 stone whichever is the greater thickness. D denotes that C can be calculated by using the equation: 100 100% of the riprap stones are smaller 1.5 ⎡ SF ⎤ than this size. = C = 61.1 ⎢ ⎥ (6.21) b. The layer thickness should not be less ⎣S s −1 ⎦ than 12 inches for practical placement. where c. The thickness determined by the above SF = Stability factor to be applied, Table criteria shall be increased by 50% when 1, HEC-11 (Default = 1.2) the riprap is placed under water due to uncertainties associated with this type of Ss = Specific gravity of stones in the riprap (Default = 2.65) placement. d. An increase in thickness of 6 to 12 K1 can be calculated form the equation: inches accompanied by an appropriate 2 1/2 K1 = (1 - 2.27 sin θ) (6.22) increase in stone sizes should be where provided where riprap revetment will be subjected to floating debris, ice or θ = Angle of the bank with the horizontal waves. axis

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To prevent the riprap from settling, a Geotextile Filter Fabrics. The design of geotextile filter fabric is placed between the filter fabric is based on specifications riprap and the underlying soil. This prevents conforming to its functional requirements. migration of fine particles through voids in the DelDOT’s Standard Specifications specify riprap, distributes the weight of the riprap filter fabrics for use under riprap for erosion units to provide a more uniform settlement, protection. For good performance, a properly and permits relief of hydrostatic pressures selected filter fabric should be installed as within the soil. recommended by the manufacturer, the Standard Specifications and the Standard Please refer to the Standard Construction Construction Details. Details for more information on the layers of the ditch section.

Figure 6-17 Channel Lining Design Procedure Step 1 Hydrologic computations: includes Step 7 Determine hydraulic resistance using determining the drainage area, rainfall HEC-11 charts with known R, A and design year, rainfall intensity and dmax or calculate runoff coefficient Step 2 Design flow - temporary and/or Step 8 Determine velocity using HEC-11 permanent charts with known R and So Step 3 Soil erodibilty; retardance Step 9 Determine the allowable flow rate, Q Step 4 Channel description including Step 10 If Qallowable >> Qdesign, the channel is geometry; includes bottom width, side over designed slopes, flow line slope, top width and If Qallowable << Qdesign, the channel is area under designed See Figure 6-18 for a useful computation chart. Step 5 Trial lining type Step 11 Based on above results, if necessary, perform Steps 3 thru 10 with new parameters Step 6 Determine permissible maximum Step 12 If a temporary lining is to be used depth of flow, dmax, (using HEC-11 until establishment of permanent Charts) cover, it may be prudent to repeat steps 6 thru 9 using temporary lining characteristics

Figure 6-18 Channel Lining Design Computation Chart Lining d B d /B A/Bd A R/d V Q = AV T Remarks Type max max

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6.8 PAVEMENT DRAINAGE AND TABLES, CHARTS AND FIGURES STORM DRAINS Access to the following references will be In certain situations, especially for urban needed to perform this analysis, most of which highways, it may not be practical to construct are in HEC-12 or HEC-22. HEC-12 has been roadside ditches (open channels) to collect superseded by HEC-22 but still may be useful pavement runoff and convey it to an outfall in design. point. The options are to provide curbs or curb • Manning's Roughness Coefficients n for and gutter combinations along the shoulder Pavements and Gutters edges for containing the runoff and o Table 4-3, HEC-22 channelizing the flow into inlets through • Allowable Water Spread which the pavement runoff is removed. o Figure 6-2 Gutter flow along the pavement is confined • Inlet and Gutter Sections to a width and depth that will not obstruct nor o Figure 6-19 cause a hazard to traffic. The ponding width will have a corresponding depth. The gutter • Nomograph for Velocity in Triangular flow capacity depends upon the contributing Gutter Sections drainage area, gutter width, cross slope, o Chart 4B, HEC-22 longitudinal slope, and roughness. • Flow in Triangular Gutter Sections o Chart 29, HDS-3 Usually the stormwater flowing along o Chart 1B, HEC-22 median swales or in open areas of parking lots is also removed through inlet structures. A • Frontal Flow to Total Gutter Flow Ratio storm drain receives the runoff from inlets, o Chart 2B, HEC-22 conveys it through a closed conduit, and • Flow in Composite Gutter Sections discharges at a suitable outfall. This is referred o Chart 5, HEC-12 to as a closed drainage system as opposed to • Curb-Opening and Slotted Drain Inlet open channels/roadside ditches. The design of Length for Total Interception storm drains involving curbed drainage and o Figure 35b, HDS-4 inlets will be generally described in this o Chart 9, HEC-12 section. • Curb-Opening and Slotted Drain Inlet 6.8.1 REFERENCES Interception Efficiency PUBLICATIONS o Figure 36, HDS-4 • Design Charts For Open-Channel Flow, o Chart 10, HEC-12 Hydraulic Design Series No. 3 (HDS-3), o Chart 8B, HEC-22 FHWA 1973 (Reprint 1980) • Grate Inlet Capacity in Sump Conditions • Introduction to Highway Hydraulics, o Chart 11, HEC-12 Hydraulic Design Series No. 4 (HDS-4), o Chart 9B, HEC-22 FHWA, 1983 and 2001 • Depressed Curb Opening Inlet Capacity in • Drainage of Highway Pavements, Sump Conditions Hydraulic Engineering Circular 12, (HEC- o Chart 12, HEC-12 12), FHWA, 1984 o Chart 10B, HEC-22 • Urban Drainage Design Manual, • Orifice Flow in Depressed Curb-Opening Hydraulic Engineering Circular 22, (HEC- Inlet 22), FHWA, 2001 o Chart 14, HEC-12 o Chart 12B, HEC-22 • DelDOT Design Guidance Memorandum Number1-20R. • Slotted Drain Inlet Capacity in Sump Locations o Chart 15, HEC-12

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o Chart 13B, HEC-22 6.8.2.1 MANNING'S ROUGHNESS • Ratio of Frontal Flow to Total Flow in COEFFICIENT Trapezoidal Channel Manning's roughness coefficient varies o Chart 17, HEC-12 slightly for different types of pavement and o Chart 15B, HEC-22 gutter surfaces. For simplicity, use n = 0.016 • Inlet Clogging Factor of Safety for all surfaces of pavements and gutters. o Figure 6-3 6.8.2.2 LONGITUDINAL SLOPE • Gutter and Inlet Design using HEC-22 o Figure 6-20 The longitudinal roadway slope SL (ft/ft) is initially established by following the ground • Frontal Flow Interception factor, Rf o Figure 6-21 profile. The roadway profile is adjusted to balance the cut and fill sections to maximize • Side Flow Interception Factor, Rs use of existing soil. Excavation required for o Figure 6-22 drainage also enters into this decision. Flat • Min. Pipe Slope for Full Flow at 3 ft/s slopes create flooding problems in a curbed o Figure 6-24 drainage system. Therefore, a minimum o Table 7-7, HEC-22 gradient of 0.30% within 50 ft of the PI station • Circular Pipe Conveyance Factor (K) in the sag or crest of vertical curves should be o Figure 6-25 used in design. Warping the shoulder by gradually changing its cross slope shall be • Wall Thickness and Approximate Weight considered when the profile grade falls below of Circular Concrete Pipe 0.3% on vertical curves. Also for a curbed Figure 6-26 o drainage system, the roadway profile shall • Manning’s Roughness Coefficients (n) for have a minimum gradient of 0.3% on tangent Pipe sections. o Figure 6-27 In the design of roadways, equal tangent • Inlet Spacing Computation Form parabolic vertical curves are commonly used o Example 5, Attachment B for setting the profile. Several basic equations COMPUTER SOFTWARE apply when designing drainage for this type of • HYDRAIN using the HYDRA module profile. To determine an elevation at a desired point on the vertical curve use the equation: • InRoads Storm & Sanitary − gg • HY22 Urban Drainage Design Programs EE += 12 2 + xgx (6.26) ax 2L 1 6.8.2 PAVEMENT DRAINAGE where The primary source for this section is HEC- E = Elevation of a point at a distance x 22. The flow capacity of the curb and gutter x from the PVC (ft) section of a roadway pavement depends upon: E = Elevation PVC (ft) 1. Manning's roughness coefficient of the a surface, g1 = Entering grade (%) 2. Longitudinal slope of the roadway, g2 = Exiting grade (%) 3. Cross slope, x = Distance measured from PVC (ft) 4. Allowable spread, and divided by 100 5. Inlet spacing. L = Length of vertical curve (ft) divided by 100

July 2008 Highway Drainage and Stormwater Management 6-35 DelDOT Road Design Manual

Note: g1 and g2 are positive for ascending 6.8.2.5 CURB AND GUTTER FLOW grade and negative for descending There are two types of gutter sections used grade to control drainage along curbed roadways. To determine the turning point on a vertical One section has a uniform cross slope and the curve (highest point for a crest curve or lowest other has a composite cross slope. Their point for a sag curve) use the equation: design is considered a shallow open channel. Refer to Figure 6-19, DelDOT’s Standard 1Lg Construction Details, and Figure 4-1, HEC- xt = (6.27) − gg 21 22, for a description of the shape, details, dimensions and terminology used in designing where gutter and inlet drainage.

xt = Location of the turning point from PVC (ft) 6.8.2.5.1 GUTTER WITH UNIFORM CROSS SLOPE Other notations are as previously defined. The elevation of the turning point can be obtained As illustrated in Figure 6-19, triangular open channels with uniform cross slope are from equation 6.30 by setting x equal to xt. characterized by: The slope of a point (S) on a vertical curve is determined using the equation: • T = Width of water spread (ft) • d = Depth of flow at curb (ft) − gg 12 S = + gx 1 (6.28) • S = Cross slope (ft/ft) L x • SL = Longitudinal roadway slope (ft/ft) S is the slope in (%) at a point on the curve • n = Manning's roughness coefficient that is a distance x from the PVC. The other 2 notations are as previously defined. • A = Cross-sectional area of flow (ft ) Manning's equation for flow rate in an open 6.8.2.3 CROSS SLOPE channel has to be modified for use with gutter Adequate cross slope on roadways is sections because the depth of flow is very essential for rapid movement of surface water shallow and wide. The governing equations to reduce sheet flow and potential for average velocity (V, ft/s) and flow (Q, ft3/s) hydroplaning. On a normal section, the cross in a triangular gutter section are: slope on a traffic lane is 2% and the shoulder 12.1 0.5 0.67 0.67 cross slope is 4%. Where these minimum V = S L S x T (6.29) cross slopes are not provided (as in transitions n between superelevated and normal sections, or 0.56 0.5 1.67 2.67 on cross streets and near curb ramps) all Q = SL S x T (6.30) efforts shall be made to minimize the sheet n flow by providing additional drainage Also for a uniform triangular gutter section, appurtenances and/or with suitable geometric adjustments. = TSd x (6.31)

2 d 2 6.8.2.4 ALLOWABLE WATER SPREAD STA x == 5.05.0 (6.32) S x It is imperative that the computed width of water spread does not exceed the allowable Therefore, the flow rate is given by: water spread as stipulated in Figure 6-2. The 67.2 drainage report shall include a comparison of 56.0 5.0 d Q = S L (6.33) the two values to ensure compliance with the n S x criteria.

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See Charts 1B and 4B, HEC-22, for the 6.8.2.5.2 GUTTER WITH COMPOSITE nomographs to solve these equations. From CROSS SLOPE Chart 4B, HEC-22, the flow velocity (V) can Composite cross slope results when a break be determined when the width of water spread exists in the cross slope of the gutter section. (T) is given (and vice versa) with other known See Figure 6-19 for typical gutter sections and parameters. nomenclature. A composite cross slope is The width of water spread (T) varies normally associated with the use of Integral between inlets in a curbed drainage system P.C.C. Curb & Gutter, Type 1. The hydraulic and so does the velocity of flow (V). If T1 is efficiency of a gutter section with composite the spread at the trailing end of an inlet, T2 is cross slope is significantly higher than one the spread at the approach end of the with uniform cross slope. Therefore, the use of downstream inlet, and Ta represents the Integral P.C.C. Curb & Gutter, Type 1, is average width of spread between the two inlets preferred in the design of a curbed drainage on the triangular gutter section, then Ta can be system. obtained from Table 4-4, HEC-22. Composite curb and gutter sections are Ta is useful in estimating the travel time of characterized by the following terms that are surface water flow on a triangular gutter also used in the gutter flow equations: section with drainage inlets. The velocity V a • T = Width of water spread (ft) corresponding to Ta is the average velocity that can be obtained from Chart 4B, HEC-22. • Ts = Width of section with normal cross Once Va is known, the average travel time is slope, equal to the total spread minus the determined by dividing the length of travel gutter pan width

(distance between the two inlets) by Va. • Sx = Cross slope (ft/ft) of adjacent traveled Chart 1B, HEC-22, provides the solution way or shoulder for the rate of flow Q in a triangular gutter • d’ = Depth of flow at cross section break section. When Q is known, T can be found and • W = Gutter pan width of greater slope vice versa, provided n, S, and S are also x • S = Slope of gutter section known. This chart can also be used for solving w the flow rate of V-shaped gutter sections by • d = Depth of flow at curb (ft) combining the two cross slopes of the V- • SL = Longitudinal roadway slope (ft/ft) sections into a single cross slope as illustrated • n = Manning's roughness coefficient on the chart. • A = Cross-sectional area of flow (ft2)

July 2008 Highway Drainage and Stormwater Management 6-37 DelDOT Road Design Manual

Figure 6-19 Inlet and Gutter Sections

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Figure 6-20 Gutter and Inlet Design using HEC-22 Capacity (Gutter and Inlet)

Step 1 Determine design parameters

L = Length of design strip Sx = Slope of adjacent shoulder or pavement A = Area of contribution Sw = Slope of gutter section C = Runoff coefficient of contributing area n = Roughness coefficient (0.016 for concrete) tc = Time of concentration T = Allowable spread width of flow I = Design year rainfall intensity W = Proposed gutter width 1 Q = Peak flow using Q = CIA LE1 = Length of grate

Calculate the following ratios needed Determine E (the total efficiency Step 2 Step 9 for using charts: Z = 1/Sx, W/T and of the inlet) using: Sw/Sx E = (RfEo) + Rs(1-Eo)

Find Qn, the flow in the shoulder or Find Q (the inlet interception Step 3 Step 10 i pavement (area outside of W) from flow) using: Chart 1B Qi = EQ

Find gutter velocity V from Chart 4B Determine Q (the inlet flow Step 4 Step 11 b using Sw/Sx ratio bypass) from Qb = Q - Qi

Find E from Chart 2B using factors Using the design flow values, Step 5 o Step 12 Sw/Sx and W/T determine the depth of water at the curb line for any curb overflow2 and actual spread.

Find Q (the gutter flow rate over the If Step 12 results do not meet the Step 6 w Step 13 grate) using: criteria, than repeat the procedure until the correct E = Q /Q o w parameters and results are met.

Find R (frontal flow interception rate) Add the inlet flow bypass to the Step 7 f Step 14 from Figure 6-21 or Chart 5B. next Q for the next inlet downstream.

Find R (side flow interception factor) Determine the location of the Step 8 s Step 15 from Figure 6-22 or Chart 6B. first inlet by iteratively finding the area generating a flow equal to Qi.

1. The flow to the all inlets is the sum of flow from all contributing areas that will reach the inlet being analyzed. 2. Don’t overflow the curb. 3. See Example 5 for complete form and instructions, also HEC-22 Figure 4-19.3.

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In the hydraulic analysis of a composite They are used on roadways, median swales, section, the area is broken into three triangular roadside ditches and parking areas. sections. The modified Manning's equation is applied to each section. The rate of flow in a 6.8.2.6.1 INLET TYPES gutter section with a composite cross slope is There are a variety of inlets available for given by the equation: use in drainage. Many have a unique shape 67.2 67.2 and application. The Department’s basic inlets ⎛ 67.2 ⎡ ' ' ⎤⎞ 56.0 5.0 ⎜ d d d ⎟ Q = S ⎢ −+ ⎥ and grate types are shown in the Standard L ⎜ ⎟ Construction Details. These details have a n ⎜ S w ⎢ S x S w ⎥⎟ ⎝ ⎣ ⎦⎠ series of inlet top unit designs (drainage inlet (6.33) assembly) to fit the available types of curbs and for swale locations. where Inlet capacity is improved by combining a d = T S + WS (6.35) s x w grate inlet with a curb opening, particularly at ’ d = TsSx (6.36) sag points, and is relatively free from clogging A = 0.5[T 2 S + 2 T W S + W2 S ] (6.37) by debris. This type of inlet is commonly used s x s x w on projects. Charts 1B and 2B, HEC-22, can be used in conjunction with each other to obtain Q for a Curb-inlets have no grate opening and are composite gutter section. The notations are used particularly on narrow paved medians defined in the respective charts. and curbed sections were the driver might shy away from a grate. They work best where Chart 5, HEC-12, provides the hydraulic grades are flatter than 3 percent. The inlet is solution of composite gutter sections. When depressed 2 inches below the gutter flow line Q, n, SL, W, Sx and Sw are known, T can be with transitions at both ends. It is best to use found from this chart. Likewise, Q can be Integral P.C.C. Curb and Gutter, Type 3, with determined when T is given. curb-inlets since it only has a one-foot gutter pan. The minimum length of these inlets is 5 6.8.2.5.3 GUTTER FLOW DESIGN ft; the total length shall be specified in 5 ft TABLES multiples. The hydraulic analysis of gutter sections for both uniform and composite cross slopes 6.8.2.6.2 INLET GRATES can be performed in accordance with the The types of inlet grates used on projects equations and charts referred to in this section are shown in DelDOT’s Standard as well as available software. For performing Construction Details. All the grates are 20 in gutter flow design by hand, drainage tables for by 36 in. A description of the grates follows. pavements of varying geometric characteristics that are usually incorporated in Type 1 grate has an opening area of 320 2 road design are provided on DelDOT’s web in , approximately 44% of the total area. The site; the variables in these tables are the width rounded bars intercept flow more efficiently. It of water spread (T), the flow rate (Q), and the is used adjacent to curb with or without average velocity (V). integral gutter where bicycle traffic can be anticipated. 6.8.2.6 DRAINAGE INLETS Type 2 grate has an opening area of 370 2 Drainage inlets for intercepting surface in , approximately 51% of the total area. This runoff should be properly designed with grate is used adjacent to a curb in controlled respect to their location and capacity so that access highways or in median swales where the drainage system functions efficiently. bicycle traffic is restricted.

6-40 Highway Drainage and Stormwater Management July 2008 DelDOT Road Design Manual

Type 3 grate has an opening area of 295 • The rate of flow of stormwater in2, approximately 41% of the total area. This approaching the inlet (the total gutter type of grate is used in open parking areas, flow, Q). median swales, and along roadsides where bicycle traffic can be expected. These grates • The rate of flow intercepted by the inlet are intended to intercept the surface runoff in (Qi). sump conditions and shall not be used beside • The carryover or bypass rate of flow (Qb). curbs. Therefore, Type 4 vane grate has an opening area of 2 215 in , approximately 30% of the total area. b = − QQQ i (6.38) This type of grate has a higher hydraulic The efficiency (E) of the inlet is: capacity and lower weight than the other types. It may be used where bicycle traffic Qi can be expected. It is not recommended for E = (6.39) Q use in sump locations. An inlet on continuous grade can be a grate 6.8.2.6.3 HYDRAULIC inlet, a curb inlet or a slotted drain inlet. CHARACTERISITICS OF INLETS Grate Inlet The total gutter flow Q In addition to size and shape, the consists of two components: interception capacity and efficiency of the inlet depend upon its location. The intended Q = Q + Qsw (6.40) hydraulic function is also a selection and where design consideration. Q = Frontal flow over width W of the The hydraulic analysis of inlets is described w grate, and for their three most common uses. These are to intercept flow: Qs = Side flow. • Along a curb on a continuous grade, The ratio of frontal flow to total gutter flow (E ) is defined by the equation: • Along a curb in a sag or surface runoff in o Qw 2.67 a sump location, and Eo = = 1- (1-W/T) Q • In a swale (ditch). (6.41) 6.8.2.6.4 INLET INTERCEPTION ON Where: CONTINUOUS GRADE Q = Total gutter flow (ft3/s) HEC-22 is the primary resource to be used Q = Flow over width W of grate for designing inlet interception on a w (ft3/s) continuous grade. The following is a brief discussion of this procedure. The chart W = Width of depressed gutter or references throughout Sections 6.8.2.6.2 and grate (ft) 6.8.2.6.3 are found in HEC-22. For design T = Total spread of water in the gutter purposes, the capacity of a combination inlet (ft) is analyzed based on the grate efficiency and the curb opening is considered a relief for Solutions for Eo are presented in Chart 2B clogging conditions. W S w for various and ratios. The three elements of runoff (Figure 6-19) T S x associated with the inlet capacity on a continuous grade are:

July 2008 Highway Drainage and Stormwater Management 6-41 DelDOT Road Design Manual

The efficiency E is given by, Qwi Let: R f = (6.42) Qw = + (1− ERERE osof ) (6.44) where Therefore,

Rf = Frontal flow Interception factor, [ Q = EQ Qi = EQ = Q [ R E + R (1 - Eosof )] (6.45) Qwi = Frontal flow Intercepted by the inlet. The value of Eo from Chart 2B is used in the above equations. Rf and Rs depend upon the Also, approach velocity V that corresponds to Q for the total gutter flow. Whether the flow takes Qsi Rs = (6.43) place on either a uniform cross slope or a Qs composite cross slope, V can be obtained by solving the equations or from the nomographs. where Having known V, the values of Rf and Rs can Rs = Side flow interception factor, be obtained from Figures 6-21 (for the Type 1 grate only) and 6-22, or Charts 5B and 6B, Q = Side flow intercepted by the inlet. s respectively.

Figure 6-21 Type 1 Grate Frontal Flow Interception Factor, Rf* V < 6 7 8 9 10 11 12 13 14 15

Rf 1.0 0.92 0.83 0.73 0.63 0.56 0.47 0.38 0.28 0.19

*Adapted from Chart 5B, HEC-22

Figure 6-22 Side Flow Interception Factor, Rs* V 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Sx Rs

0.02 0.63 0.32 0.19 0.12 0.08 0.06 0.05 0.04 0.03 0.03 0.02 0.20 0.02 0.01 0.01

0.03 0.71 0.42 0.26 0.17 0.12 0.09 0.07 0.06 0.05 0.04 0.03 0.03 0.02 0.02 0.02

0.04 0.77 0.49 0.32 0.22 0.16 0.12 0.09 0.07 0.06 0.05 0.04 0.04 0.03 0.03 0.02

0.05 0.81 0.55 0.37 0.26 0.19 0.14 0.11 0.09 0.07 0.06 0.05 0.05 0.04 0.03 0.03

0.06 0.83 0.59 0.41 0.29 0.22 0.17 0.13 0.11 0.09 0.07 0.06 0.05 0.05 0.04 0.04

1 *Using equation Rs = 8.1 3.2 []+ () )/(15.01 x LSV

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Curb-Opening Inlet. For uniform cross Thus for depressed curb-opening inlets: slope, the length of curb opening for total 0.6 interception of gutter flow is given by the 0.42 0.3 ⎛ 1 ⎞ 0.6 = = 0.6 Q S ⎜ ⎟ (6.50) equation: LT L ⎜ ⎟ ⎝ n S e ⎠ 0.6 0.42 0.3 ⎛ 1 ⎞ Slotted Drain Inlet The equations for LT = 0.6 Q SL ⎜ ⎟ (6.46) ⎜ ⎟ solving for LT and E and Chart 7B for curb- ⎝ n S x ⎠ opening inlet along with the nomograph in where Chart 5B for interception efficiency are applicable to slotted drain inlets on a L = Length of curb opening in feet for total T continuous grade. interception of gutter flow (100% efficiency of interception). 6.8.2.6.5 INLET INTERCEPTION IN SAG Other notations are as defined previously. LOCATIONS

The efficiency E for inlets shorter than LT is Inlets on sag vertical curves of roadway expressed by the equation: pavement, in open parking areas, and on

1.8 median swales, operate in a sump condition. - 1 = E = 1 - ()1 - L/ LT (6.47) As the surface runoff approaches from some or all sides of the inlet, the interception takes This equation is also valid for depressed curb- place as if the flow occurs over a weir or an opening inlets. orifice depending on the depth of flow. The Charts 7B and 8B provide graphical inlet operates as a weir with low head and as solutions of these equations. an orifice with relatively high head. However, curb-opening inlets must be Note that when using Chart 9B the depressed to significantly increase their following applies: interception of gutter flow. This results in a composite cross slope as illustrated on Chart • Inlet Type 1─Opening ratio = 0.44 7B. The equation is: • Inlet Type 2─Opening ratio = 0.51 • Inlet Type 3─Opening ratio = 0.41 S = S xe S+ 'W E o (6.48) • Inlet Type 4─Opening ratio = 0.30 Where, Inlet grates in sag locations are Se = Equivalent cross slope for depressed analyzed for two conditions: opening (ft/ft) Condition 1 - Inlet Grate Operating as a S = Pavement cross slope (ft/ft) x Weir S'w = Cross slope of the gutter measured from the edge of pavement (ft/ft) The flow rate intercepted by a grate inlet is

Eo = Ratio of flow over the depressed 1.5 Q = C w P d (6.51) width to the flow over the total gutter i section (Qw/Q) where 3 As shown in Chart 7B, W is the depressed Qi = Flow intercepted by the inlet (ft /s) width or gutter width (ft) and a equals the Cw = Weir coefficient = 3 gutter depression (in). P = Perimeter of the sides of inlet Therefore, approached by runoff (ft) a d = Average depth of water adjacent to SW′ = (6.49) the inlet (ft) (see Figure 4-17, HEC- ) W (12 W ) 22)

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Chart 9B has the graphical solution to this where equation. As indicated on the chart, if the flow L = Length of depressed curb opening is hindered on one side of the inlet due to the (ft) curb, then that side has to be omitted in calculating P. For practical purposes W and L W = Lateral width of depression (ft) shall be taken as the outside dimensions of the d = Depth of water (ft) at curb grate inlet. measured from normal cross slope Generally, the Type 1 grate operates as a In this case, weir in a curbed drainage design. The d = T S (6.54) permissible depth of flow at the curb is limited x to the height of curb and the allowable spread. where However, when used in parking areas and T = Water spread swales, the Type 1 grate may operate as an orifice because a greater depth of flow is Sx = Slope of adjacent pavement likely to be accommodated. For the depressed inlet to operate as a weir: a Condition 2 - Inlet Grate Operating as an d ≤ [ h + ] (6.55) Orifice 12 This condition exists when the depth of where water exceeds 8 in. The flow rate intercepted h = Height of curb-opening inlet (ft) by the grate inlet is given by: a = Depth of depression (in) Q = A 2gd 0.5 (6.52) i Co () Case 2 - Depressed Curb-Opening Inlet Where, Operating as an Orifice 3 The inlet interception Qi (ft /s) is given by: Co = Orifice coefficient = 0.67 A = Clear area of grate opening (ft2) 0.5 Qi = 0.67 h L (2 gd o) g = Acceleration due to gravity (32.2 0.5 ft/s2), ⎡ ⎛ h ⎞⎤ 2g A 0.67 = = 0.67 A ⎢2g ⎜d i - ⎟⎥ (6.56) d = Average depth of water adjacent to ⎣ ⎝ 2 ⎠⎦ the inlet (ft) (see Figure 4-17, HEC- 22) where Chart 9B presents the solution for equation do = Effective head on the center of 6.52 using the given inlet opening ratios. orifice throat (ft) A = Clear area of the opening (ft2) When the depth of water is in transition between weir and orifice flow, the perimeter h = Height of depressed curb-opening line on the lower half of the figure should be inlet (ft) connected by a smooth curve with the di = Height of depressed curb-opening corresponding clear area line of the upper half orifice (ft), expressed as: for Qi in transition. a TSd += (6.57) Depressed Curb-Opening Inlet in Sag xi 12 Location a = Depth of depression (in) Case 1 - Depressed Curb-Opening Inlet Operating as a Weir For the depressed curb-opening inlet to operate as an orifice, the depth of flow should 3 The inlet interception Qi (ft /s) is given by: be greater than 1.4 h. 1.5 Qi = 2.3 ()+ 8.1 WL d (6.53)

6-44 Highway Drainage and Stormwater Management July 2008 DelDOT Road Design Manual

Chart 10B gives the graphical solution of However, in sump conditions where equations for depressed curb-opening inlets. clogging is anticipated, the factors of safety to Chart 12B gives the solution for inclined and be applied are: vertical orifice throats. • for curb on one side use 1.5 (Sf1) Slotted Drain Inlet • for no curb, i.e., the interception takes The slotted drain inlet in sump condition place from all four sides, use 2.0 (Sf2). operates as a weir when the depth of flow is For weir flow, the clogging shall be up to 0.2 ft and as an orifice when the depth of assumed to occur lengthwise, so that a reduced water exceeds 0.4 ft. Chart 13B provides the width of grating is taken into consideration to solution both for weir and orifice conditions account for the factor of safety. For example, with dashed lines for transition. Note the consider a standard grate that operates as a multiplying factor involving the width of slot weir. If it is placed alongside a curb, then the at the top of the figure. perimeter to be considered for computing the interception with factor of safety is given by: 6.8.2.6.6 INLET IN OPEN CHANNEL ⎛ W ⎞ Grate inlets are installed in median swales P ⎜ += L⎟ 12/2 (6.58) or roadside ditches to intercept the flow of ⎜ ⎟ ⎝ S f 1 ⎠ these open channels. They can be either on a continuous grade or in a sump depending on ⎛ 20 ⎞ P 2⎜ += ⎟ = 22.512/36 ft the profile of the channel. For the sump 5.1 condition, the methodology outlined in ⎝ ⎠ Section 6.8.2.6.5 applies. For installations on a If the curb does not exist, then the interception continuous grade, the analysis for interception would occur from all four sides. Therefore, by grate inlets in Section 6.8.2.6.4 is valid, except for E that was defined by equation ⎛ W ⎞ o P = 2⎜ + L⎟ 12/ (6.59) 6.41. ⎜ ⎟ ⎝ S f 2 ⎠ Eo for trapezoidal channels can be obtained from Chart 15B. The intercepted flow Q is ⎛ 20 ⎞ i P = 2⎜ + ⎟ 12/36 = 7.67 ft determined by using equation 6.45. ⎝ 2 ⎠ 6.8.2.6.7 FACTOR OF SAFETY FOR When a grate inlet in a sump condition CLOGGING OF GRATE INLETS operates as an orifice, then its clear open area (A) should be divided by the factor of safety Because grate inlets may become clogged specified in the foregoing to compute the with debris, the theoretical interception actual interception capacity. capacity should be properly reduced in drainage design. 6.8.2.6.8 INLET LOCATIONS Generally a grate inlet on continuous grade is placed beside a curb that has an opening. 6.8.2.6.8.1 DESIGN CRITERIA This opening, disregarded in the interception 1. Drainage inlets shall be designed for the analysis, should alleviate the effect of runoff frequency and spread as defined in clogging that seldom occurs on a continuous DelDOT’s design criteria. The design involves grade. Therefore, a factor of safety to reduce determining the spacing of inlets based on the theoretical interception capacity of grates their interception capacity. on continuous grade may be deemed unnecessary. 2. Aside from inlet locations as required by their hydraulic design, additional inlets shall be provided in strategic areas to avoid the

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concentration of sheet flow by intercepting the specified in Section 6.8.2.6.7. In approved runoff in advance. Some of those strategic locations, parallel bar grates are preferred in locations are: sump conditions. • Upstream of median breaks, entrance and 6. The criteria for using curb-opening inlets exit ramp gores, curb ramps, crosswalks are in Section 6.8.2.6. When the roadway and street intersections. profile is steeper than 3%, curb-opening inlets • Upstream and downstream of bridge should not be used. approaches. 7. Gutter inlets should not be used to • Upstream of superelevation tangent runout intercept runoff from areas adjacent to the and cross slope reversal. roadway, neither within nor outside the right- of-way. The surface runoff from those areas • End of channels in cut sections. may be more efficiently intercepted in • Behind curb and sidewalk in low areas. advance, and sometimes disposed of by other • At the low point of a sag vertical curve. means such as roadside ditches.

The inlet(s) at the low point should be 6.8.2.6.8.2 SPACING OF DRAINAGE designed to keep the water spread within the INLETS allowed limit. The preferred method is to design flanking inlets to act in relief of the low Inlet spacing begins at a required distance point inlet if it gets clogged or the water from the crest of roadway subject to spread exceeds the limit due to a less previously defined location criteria. The flow infrequent storm. The flanking inlets shall be rate of stormwater intercepted by the first inlet located where the gutter elevation is and the bypass flow rate are computed and its approximately 0.2 feet higher than the low efficiency is checked. If the inlet efficiency is point or where allowed spread would be found to be lower than the prescribed exceeded if a sump drain clogged. The method minimum, then the inlet location is moved described in HEC–22 is used to design upstream to achieve the required 70% flanking inlets. efficiency. Then the computation proceeds to the next inlet where the discharge from its Equations for elevation and low point on a watershed (downstream of the first inlet) is sag vertical curve are included in Section added with the bypass from the first inlet. This 6.8.2.2. The distance of each flanking inlet total gutter flow rate is used to compute the from the low point x (feet) is given by: water spread, which cannot exceed the design = x = ()4000L/A 0.5 (6.60) criteria, and the efficiency of this inlet. The procedure is repeated as the roadway profile Where: moves into a sag curve where the storm water is ultimately removed from the inlet at the low L = Length of vertical curve point, which operates in a sump condition at A = Algebraic difference in approach 100% efficiency. The following equations are grades used for spacing inlets on grade. It should be noted that these equations are based on flow 3. The maximum spacing of inlets shall be data obtained using the procedure outlined in 300 ft. Figure 6-20. 4. The efficiency of inlets on continuous Spacing of First Inlet from Crest. grade shall be at least 70%. However, inlets provided at strategic locations should intercept Q1T x 43,560 100% of the gutter flow. L1 = (6.61) C1 I 1 W 1 5. The factor of safety for potential clogging of inlet grates in sump conditions are where

6-46 Highway Drainage and Stormwater Management July 2008 DelDOT Road Design Manual

L1 = Distance of the first inlet from All inlets of the drainage system shall be crest (ft) numbered and the computed results shall be presented in a tabular format. Use a procedure Q1T = Total gutter flow capacity upstream of the first inlet within such as described in Example 5, Attachment the width of permissible water B. spread that conforms to the required efficiency 6.8.3 STORM DRAINS

C1 = Weighted average runoff The hydraulic design of a system is coefficient of the drainage area of performed after the preliminary layout the first inlet locating inlets, storm drains, and outfalls. The Rational Formula is used to determine the I1 = Rainfall intensity corresponding to the time of concentration for the peak discharge for sizing storm drains. The first inlet (in/hr) hydraulic design is a two step process: (1) select a preliminary pipe size based on W1 = Mean width of the first inlet's hydrology and simplified hydraulic drainage area (ft) computations and (2) compute the hydraulic gradeline (HGL) for the system. The second Spacing of Subsequent Inlets step refines the preliminary pipe size based on the hydraulic losses in the system. [Q - Q 1-nB,nT ] x 43,560 Ln = (6.62) HEC-22 is the primary resource for the Cn I n W n design of storm drains. Since this is a trial and where error process, the use of approved computer software is recommended. The designer Ln = Distance of n'th inlet from the (n- 1)'th inlet (ft) should check that the output is in an approved format for inclusion in the H&H Report. It is Q = The total gutter flow capacity nT particularly important that the results are upstream of n'th inlet within the compared to DelDOT’s drainage criteria in the width of permissible water spread output data where applicable. that conforms to the required inlet efficiency (ft3/s) Each pipe segment of the storm drain system is designed individually for size and QB,n-1 = Bypass flow rate from the (n- 1)'th inlet (ft3/s) slope from inlet to inlet or junction to junction. The Rational Formula is used to compute the C = Weighted average runoff n peak discharge at the inlet in the beginning of coefficient of the drainage area of each segment. From the uppermost end of the the n'th inlet storm drain system the design begins and In= Rainfall intensity corresponding to continues downstream for each segment to the the time of concentration for the outfall point. When all pipe segments are n'th inlet (in/hr) designed, it may be necessary to determine the W = Mean width of the n'th inlet's hydraulic gradeline elevation to check the n operation of the system under the design storm drainage area (ft) frequency by computing head losses in the For non-uniform drainage areas, the system starting from the design high water solutions to these equations are obtained by elevation at the outfall and proceeding trial and error. The spacing distances are upstream (backwards) considering every pipe assumed and the mean widths of drainage segment up to the beginning inlet. This may be areas are computed along with other a trial and error procedure that is simplified by parameters until the equations are satisfied using computer software. with the assumed spacing of the drainage inlets.

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The storm drain design process begins with 3. In order to prevent damage to the pipe the collection of the following data: surface, the maximum velocity at full flow should not exceed 15 ft/s. Slopes flatter 1. Hydrologic data of the watershed than 10% are normally used. including the outfall data. 4. To prevent water bubbling out of inlets 2. Locations of underground utilities, and manholes, particularly at low points, existing or proposed. the hydraulic gradeline shall remain at 3. Topographic data (including runoff basin least 1 ft below the gutter elevation at any divides, streets, developments, and lot inlet or top of manhole. lines) of the entire watershed draining into 5. For concrete pipe, Class IV is most the storm drain system with arrows on the commonly used along with Class V for maps denoting all directions of flow from greater load bearing locations, such as each lot and street. This information where there is minimal cover. Other pipe should be superimposed on the working materials (such as corrugated metal and drawings of preliminary plans. High Density Polyethylene (HDPE)) must 4. Preliminary roadway plans for the project also be considered as per federal including inlet locations, roadway typical regulations. The designer has to ensure sections, profiles, and cross sections. that the pipe material can bear site-specific Choosing inlet locations is nearly complete loads, perform well under the site before the actual storm drain design. The next conditions and comply with the project’s step is to develop a layout plan for the expected service life criteria. conveyance of the intercepted runoff through 6. The preferred minimum cover over a main storm drains and laterals that connect all storm drain is 3.0 ft. Refer to DelDOT’s outlets to a suitable outfall point. This system Standard Construction Details, Standard layout plan shall indicate locations of inlets, Specifications, AASHTO LRFD Bridge manholes and junction boxes; all pipe Design Specifications, and manufacturer’s segments with directions of flow; and existing recommendations for proper bedding and or proposed underground utilities such as cover requirements under roadway water, gas, cable and sanitary sewer. Then a pavements. rough profile gradeline from inlet to inlet of 7. A simple initial approach for preliminary the storm drain system leading to the outfall is design and evaluation to ensure the system established, the pipe segments are designed, will operate as open channel flow is to lay potential utility conflicts (if any) are resolved, out each pipe segment at its friction slope the hydraulic gradeline is checked, and final under full flow condition and use adjustments are made for approval of the equations 6.64 and 6.65. design. 8. The layout of pipe junctions at inlets, 6.8.3.1 DESIGN CRITERIA manholes and junction boxes shall be carefully planned for smooth flow with Storm drains ⎯- The following general minimum head loss. Pipes entering or criteria and those in Figure 6-3 shall apply to exiting a junction cannot exceed an angle storm drain design: of 45o. The designer shall ensure that the 1. The storm drain trunk line should not be pipes fit into the proposed structure, under a traveled way but in the median, a horizontally and vertically. Refer to the Standard Construction Details for shoulder area or behind the curb. minimum clearances to the drainage 2. The minimum velocity should be 3 ft/s structure walls. flowing full to prevent the deposition of 9. At changes in pipe size, the soffits (top sediments and debris inside the storm inside surfaces of the two pipes) should be drain system. See Figure 6-24.

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maintained at the same level by dropping Outfalls ⎯- The following general criteria the invert of the larger pipe. should be considered when determining storm 10. The size of any pipe segment shall not drain outfalls. decrease in size in the downstream 1. Compliance with the NPDES. direction even if a change in design such 2. Compliance with the Delaware Erosion as increasing the slope would permit the and Sediment Control regulations. use of a smaller sized pipe. 3. Coordinated with the locations of any 11. For cleanout and maintenance inspection, required stormwater quantity and quality access to storm drains shall be provided management systems. through drainage inlets or manholes within the 300 ft maximum distance. 4. Preferably discharges into a natural drainage course. 12. If a storm drain or ditch carrying highway runoff extends beyond the right-of-way, 5. Ensuring that overland flow occurs before then a permanent easement or additional entering any wetlands. right-of-way to the outfall shall be 6. The storm drain outfall flow should not acquired for future maintenance. scour the downstream reach. 13. Curved alignment of storm drains 7. The outfall should desirably operate at the following the general alignment of the design storm flow level under inlet control roadway may be permitted. The and as a free (non- submerged) outfall. manufacturer’s recommendations for pipe 8. The design of the system should be layout should be followed. refined to avoid adverse hydraulic 14. The design criteria for strength and conditions such as excessive velocity, hydraulic analysis of pipes shall conform erosion, or excessive scouring requiring to the provisions of Section 6.9. the need for expensive energy dissipaters, 15. The designer’s initial site evaluation or bank overflow protection. should include the prevailing ground 9. Embankment slopes and headwall designs water elevation level for estimating for the outfall shall be in conformance dewatering costs. with the Roadside Design Guide. 16. Pressure flow design should not be considered.

Figure 6-23 General Guidelines for Culvert Outfall Treatment* Pipe outfall design velocity Channel treatment required Max. allowable velocity above those in Figures Sod or stabilization 6-14 and 6-15 up to 10 ft/s 10 ft/s to 14 ft/s Dumped riprap for a distance of 15 to 25 feet Greater than 14 ft/s Special treatment, such as a plunge pool, stilling basin or energy dissipater *Note: The outfall design is determined by the stormwater control regulations and design procedure.

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Figure 6-24 Minimum Pipe Slope to Ensure a 3.0 ft/s Velocity in a Storm Drain Flowing Full Inner Pipe Full Pipe Minimum Slope (%) Diameter Flow n = 0.011 n = 0.012 n = 0.013 n = 0.020 n = 0.024 (in) (ft3/s) 12 2.4 0.32 0.38 0.44 1.04 1.49 15 3.7 0.24 0.28 0.32 0.78 1.11 18 5.3 0.18 0.22 0.26 0.60 0.87 21 7.2 0.15 0.18 0.21 0.49 0.71 24 9.4 0.12 0.15 0.17 0.41 0.59 27 11.9 0.11 0.13 0.15 0.35 0.51 30 14.7 0.09 0.11 0.13 0.30 0.44 36 21.2 0.07 0.09 0.10 0.24 0.34 42 28.9 0.06 0.07 0.08 0.20 0.28 48 37.7 0.05 0.06 0.07 0.16 0.23 54 47.7 0.04 0.05 0.06 0.14 0.20 60 58.9 0.04 0.04 0.05 0.12 0.17 66 71.3 0.03 0.04 0.05 0.11 0.15 72 84.8 0.03 0.03 0.04 0.09 0.14

6.8.3.2 DESIGNING STORM DRAINS The design method consists of computing Open channel design shall be used for the the design discharge to be carried by each pipe design of a storm drain system. For the rare segment and determining the size and slope of occasions when pressure flow design is used pipe necessary for carrying this discharge. due to the topographic constraints of the site, This is accomplished by proceeding step by the engineering justification and calculations step from the beginning of a storm drain line for pressure flow design shall be included in downstream to its terminus, which may be a the project’s drainage report and must be junction point where one trunk line discharges approved by DelDOT’s Assistant Director - into another line or the outfall point. It is Design. essential that the soffit elevation of the outlet pipe at the point of discharge is at or above the 6.8.3.2.1 OPEN CHANNEL DESIGN design tailwater elevation at the outfall. Under most conditions a roadway’s storm The outfall point is a discharge point into a drain system flows under atmospheric pressure natural watercourse, excavated channel, and gravity. The various pipe segments are retention pond or another storm drain system sized assuming that they will flow practically for disposing of the stormwater collected from full under design discharge such that the the highway. Estimating the design tailwater surface of the design flow in the pipe is open elevation is based on hydrologic to atmospheric pressure. One of the considerations relating to the downstream controlling design criteria for pipe design is channel characteristics as well as the ratio of the minimum slope necessary to maintain 3 watershed area of the storm drain system ft/s flowing full to reduce silting. (tributary) to the total drainage area of the outfall (main stream). The tailwater elevation The design depth of flow should be lower can be affected by differences in the design than the full depth (diameter) of the pipe. A frequencies and peak discharge rates of the circular pipe flowing at a depth of 93% of its watersheds, including downstream structures. diameter is able to carry the flow equal to its full depth capacity. One simple initial approach for preliminary design and evaluation to ensure the system will operate as open channel flow is to lay out

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each pipe segment at its friction slope under downstream watersheds is large enough that full flow condition and use equations 6.64 and the tailwater depth would be the normal flow 6.65; also see Section 6.8.3.2.4. Placing the depth of the receiving channel. pipe at its friction slope results in determining Then working upstream along the entire the normal depth of uniform flow under pipe system, compute friction loss for each constant discharge. Essentially this is the slope individual pipe segment, losses due to at which the friction and gravity forces in the curvature or bends, and junction losses at the direction of flow are equal but acting in various structures to determine the HGL. different directions. Hydraulically this means that at normal depth the slope of the pipe, the For pressure flow design, refer to Example slopes of the HGL and EGL are numerically 6, Attachment B, for a form and procedure for equal and parallel to each other. Normal depth tabulating computed results. Computer is a function of the discharge flow rate, the programs such as HYDRA make the channel geometry, the channel/pipe slope, and computation of a system’s EGL and HGL the resistance due to friction. The equations in simple. However, it is important that the Section 6.8.3.2.4 use these factors to find the designer understand the process to judge if the correct pipe size and slope. Figure 6-28 shows input and output are valid. pipe flow capacity for various pipe sizes at its friction slope using Manning’s equations for n 6.8.3.2.3 SAG POINTS equals 0.012. Available software maximizes DelDOT's design criteria require that at sag the hydraulic efficiency of the system based points where the only drainage available is the upon the input parameters selected for the best storm drain, the pipe draining the sag point and most economical final design. and inlet(s) should be sized for a 50-yr The open channel design of a storm drain frequency rainfall. Although software is system can be performed following the available for most of these calculations, they procedure and filling out a form similar to that can be done by computing the bypass flow in Example 6, Attachment B. occurring at each inlet during a 50-yr frequency rainfall and accumulating it at the 6.8.3.2.2 HYDRAULIC GRADE LINE sag point. For sizing the downstream storm PROCEDURE drain, one method of determining the additional bypass flow is to convert the flow Since the design criteria specifies that the to an equivalent using the product of the HGL elevation at any structure be one foot weighted runoff coefficient and contributing below the structure top elevation, it is area, CA, that can be added to the design CA; necessary to compute the elevations of the see discussion on the rational formula and the HGL in a storm drain system whether under inlet design procedure in Example 5, open channel or pressure flow conditions. The Attachment B. This equivalent CA is found by computations are based on the design dividing the 50-yr bypass by the I10 in the pipe frequency discharges. at the sag point. Normally the HGL is assumed to be outlet Sag point drainage is an important design control. The HGL is determined by first consideration due to flooding of the roadway establishing the design tailwater elevation at and adjacent properties as well as the potential the outfall and comparing it to the average of creating a hydroplaning area. In order to critical depth (dc) and the height of the pipe prevent this condition from occurring, the use (dc+D)/2. The larger of these values is selected of flanking inlets is recommended. as the beginning elevation. Depending upon the outfall channel, establishing the tailwater 6.8.3.2.4 HYDRAULIC PROCEDURES elevation could involve analysis of joint or The hydraulic capacity of a storm drain coincidental occurrence of storm events. system is determined with equations 6.16 Normally, the ratio of the upstream and

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through 6.19.For circular storm drains flowing 463.0 K = D 67.2 (6.69) full and R = D/4, and the equations become: n

590.0 2/13/2 For concrete pipes (n = 0.012) flowing flow: V = SD (6.63) 2 n Q S f = 3/8 2 463.0 1490()D (6.70) Q = SD 2/13/8 (6.64) n where The head loss due to friction Hf is determined by the equation: V = Velocity (ft/s) = LSH (6.71) D = Pipe diameter (ft) ff S = Storm drain slope (ft/ft) where n = Roughness coefficient L = Length of pipe Q = Pipe flow (ft3/s) However, most design will be done using software. In the references there are tables that The references listed in Sections 6.8 and 6.9 show of the results of solving these equations contain nomographs and charts that can be for different pipe materials, slopes, and used to solve these equations during design. roughness coefficients. These tables allow for Another approach to finding pipe flow is to a preliminary evaluation of pipe size and slope develop a conveyance factor, K, which as well as checking software output. The full represents the physical characteristics of the flow capacity is calculated by multiplying the channel/pipe; see equations 6.18 and 6.19. square root of the slope times the K value Using a combination of these equations, the obtained from Figure 6-25. friction slope, Sf, of the conveyance surface 6.8.4 FLARED END SECTIONS can be used to determine the values for the Flared end sections (FES’s) are to be used velocity and flow capacity of the pipe. The outside of the clear zone at intakes requiring equations developed are: personnel safety grates and outfalls. 2 ⎛ Vn ⎞ n 2 ⎛V 2 ⎞ (Personnel safety grates are only placed on the S f = ⎜ ⎟ = 29 ⎜ ⎟ intake end of a pipe run that is not linear; refer 3/2 3/4 ⎜ 2g ⎟ ⎝ 49.1 R ⎠ R ⎝ ⎠ to DelDOT Design Guidance Memorandum (6.65) No. 1-15.) They are not recommended for For full flow conditions: roadside drainage installations or on the 184n 2 ⎛V 2 ⎞ ends of driveway pipes where crashworthy S f = ⎜ ⎟ (6.66) treatments are needed; please refer to 3/4 ⎜ 2g ⎟ D ⎝ ⎠ AASHTO’s Roadside Design Guide. During Or, installation, the FES shall be placed at the 2 ⎛ Q ⎞ flow line grade of the pipe as with any pipe S f = ⎜ ⎟ (6.67) section. FES’s are not manufactured to fit ⎝ K ⎠ skewed slopes. For a skewed pipe culvert, the where K is defined as FES is placed in line with the pipe, and the fill 49.1 slope is warped to fit the FES. When K = AR 3/2 (6.68) n calculating the quantity of pipe, the FES is not Or, included in the length of pipe, and is a separate pay item.

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Figure 6-25 Circular Pipe Conveyance Factor (K) D A R 49.1 K = AR 3/2 Pipe Area Hydraulic n Diameter (ft2) Radius (in) (ft) n =0.011 n =0.012 n = 0.013 12 0.785 0.250 42.2 38.7 35.7 15 1.227 0.313 76.6 70.2 64.8 18 1.767 0.375 125 114 105 21 2.405 0.438 188 172 159 24 3.142 0.500 268 246 227 27 3.976 0.563 367 337 311 30 4.909 0.625 486 446 411 33 5.940 0.688 627 575 531 36 7.069 0.750 790 725 669 42 9.621 0.875 1192 1093 1009 48 12.566 1.000 1702 1560 1440 54 15.904 1.125 2330 2136 1972 60 19.635 1.250 3086 2829 2611

Figure 6-26 Wall Thickness and Approximate Weight of Circular Concrete Pipe Class IV with Type B Wall Thickness Size Wall Approximate Size Wall Approximate (in) Thickness Weight (in) Thickness Weight (in) (lbs. per ft) (in) (lbs. per ft)* 12 2.0 93 36 4 524 15 2.25 127 42 4.5 686 18 2.5 168 48 5 867 21 2.75 214 54 5.5 1068 24 3 264 60 6 1295 30 3.5 384 66 6.5 1542 33 3.75 451 72 7 1811 * Pipe sections are 8 ft in length.

Figure 6-27 Manning’s Roughness Coefficients (n) for Pipe Type of Pipe Recommended n Concrete - 0.012 Round and elliptical Corrugated Metal - Annular (2 2/3 x ½ in) 0.024 Helical (2 2/3 x ½ in) 0.020 Spiral Rib 0.012 Plastic - Polyvinyl 0.011 High Density Polyethylene 0.012 Polyethylene-Double Walled 0.012

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Figure 6-28 Friction Slope (ft/ft) for n = 0.012, Full Flow

Q Pipe Diameter (in) (cfs) 12 15 18 21 24 27 30 36 42 48 54 60 2 0.003 4 0.011 0.003 6 0.024 0.007 0.003 8 0.043 0.013 0.005 10 0.067 0.020 0.008 0.003 12 0.097 0.029 0.011 0.005 14 0.132 0.040 0.015 0.007 0.003 16 0.052 0.020 0.009 0.004 18 0.066 0.025 0.011 0.005 0.003 20 0.082 0.031 0.014 0.007 0.004 24 0.118 0.044 0.020 0.010 0.005 26 0.138 0.052 0.023 0.011 0.006 25 0.048 0.021 0.010 0.006 0.003 28 0.061 0.027 0.013 0.007 0.004 30 0.069 0.031 0.015 0.008 0.005 32 0.079 0.035 0.017 0.009 0.005 34 0.089 0.039 0.019 0.010 0.006 35 0.095 0.042 0.020 0.011 0.006 36 0.100 0.044 0.022 0.012 0.007 38 0.111 0.049 0.024 0.013 0.007 40 0.054 0.027 0.014 0.008 0.003 42 0.060 0.029 0.016 0.009 0.003 44 0.066 0.032 0.017 0.010 0.004 45 0.069 0.034 0.018 0.010 0.004 46 0.072 0.035 0.019 0.011 0.004 48 0.078 0.038 0.020 0.012 0.004 50 0.085 0.042 0.022 0.013 0.005 55 0.103 0.050 0.027 0.015 0.006 0.003 60 0.122 0.060 0.032 0.018 0.007 0.003 65 0.070 0.038 0.021 0.008 0.004 70 0.082 0.044 0.025 0.009 0.004 75 0.028 0.011 0.005 80 0.032 0.012 0.005 0.003 85 0.037 0.014 0.006 0.003 90 0.041 0.016 0.007 0.003 95 0.046 0.017 0.008 0.004 100 0.051 0.019 0.008 0.004 105 0.056 0.021 0.009 0.005 110 0.061 0.023 0.010 0.005 0.003 115 0.067 0.025 0.011 0.005 0.003 120 0.073 0.028 0.012 0.006 0.003 125 0.079 0.030 0.013 0.006 0.003 130 0.086 0.032 0.014 0.007 0.004 135 0.092 0.035 0.015 0.008 0.004 140 0.099 0.038 0.016 0.008 0.004 145 0.106 0.040 0.018 0.009 0.005 0.003 150 0.043 0.019 0.009 0.005 0.003 160 0.049 0.022 0.011 0.006 0.003 170 0.055 0.024 0.012 0.006 0.004

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6.9 PIPES AND PIPE CULVERTS the American Iron and Steel Institute, and the design manual of the Plastics Pipe Institute. 6.9.1 REFERENCES All of these give pipe size data, general hydrology and hydraulic design, structural Publications design, durability and recommended installation methodology. Much of this The primary publications for understanding information is not available in other the principles for the analysis of pipes and publications. pipe culverts are: • Design Charts For Open-Channel Flow, Tables and Charts Hydraulic Design Series No. 3 (HDS-3), Access to the following tables and charts FHWA 1973 (Reprint 1980) are needed. Most of these can be found in the • Introduction to Highway Hydraulics, referenced publications. Hydraulic Design Series No. 4 (HDS-4), FHWA 1983 and 2001 • Defining Energy Concept in Pipe Flow o Figure III-8 and III-9, HDS-5 • Hydraulic Design of Highway Culverts, o Figure 7-8, HEC-22 Hydraulic Design Series No. 5 (HDS-5), FHWA, 2005 • Manning's Roughness Coefficient n for Pipes • Hydraulic Charts for the Design of o Figure 6-27 Highway Culverts, Hydraulic Engineering o Table 14, HDS-4 Circular No. 5 (HEC-5), FHWA, 1980 o Table 4, HDS-5 • Capacity Charts for the Hydraulic Design o Tables 3-4 and 7-1, HEC-22 of Highway Culverts, Hydraulic • Friction Slope for Circular Pipe, Full Flow Engineering Circular No. 10 (HEC-10), o Figure 6-28 FHWA, 1972 o Charts 53 and 54, HDS-3 • Drainage of Highway Pavements, • Hydraulic Elements of Circular Sections Hydraulic Engineering Circular No. 12, for Various Flow Depths (HEC-12), FHWA, 1984 o Figure 40, HDS-4 • Hydraulic Design of Improved Inlets for o Charts 26 and 27, HEC-22 Culverts, Hydraulic Engineering Circular • Uniform Flow in Circular Sections No. 13 (HEC-13), FHWA, 1972 Flowing Partly Full • Hydraulic Design of Energy Dissipaters o Table B.2, HEC-14 for Culverts and Channels, Hydraulic • Velocity in Pipe Conduits o Charts 35 thru 51, HDS-3 • Engineering Circular No. 14 (HEC-14), • Velocity in Elliptical Pipes FHWA, 1975 o Chart 74, HDS-3 • Urban Drainage Design Manual, • Critical Depth of Flow - Circular Pipes Hydraulic Engineering Circular No. 22, o Chart 4B, HDS-5 (HEC-22), FHWA, 2001 o Appendix B, HEC-14 In addition to these references there are • Critical Depth for Elliptical Pipes manuals available from the different pipe o Chart 31B, HDS-5 manufacturers, including the Concrete Pipe o Figure B.2, HEC-14 Design Manual (viewable online at • Entrance Loss Coefficients http://www.concrete-pipe.org/techdata.htm) o Table 12, HDS-5 and the Handbook of Concrete Culvert Pipe o Table 7-5b, HEC-22 Hydraulics by the American Concrete Pipe Association, Modern Storm Sewer Design by

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• Headwater Depth for Concrete Pipe o Figure 6-24 Culverts with Inlet Control o Table 7-7, HEC-22 o Chart 1B, HDS-5 • Determining Culvert Size with HEC-5 o Chart 28B, HEC-22 o Figure 6-29 • Head for Concrete Pipe Culverts Flowing • Culvert Design Form Full o Figure 7-7, HEC-22 o Chart 5B, HDS-5 Computer Software • Headwater Depth for C.M. Pipe Culverts with Inlet Control • WinHY-8 Hydraulic Design of o Chart 2B, HDS-5 Highway Culverts o Chart 29B, HEC-22 • HEC-RAS • Head for Standard C.M. Pipe Culverts • NFF Flowing Full o Chart 6B, HDS-5 6.9.2 HYDRAULIC DESIGN OF PIPE CULVERTS • Headwater Depth for C.M. Pipe-Arch Culverts with Inlet Control Culverts addressed in this chapter are pipe o Chart 34B, HDS-5 culverts and are defined as a separate category • Head for Standard C.M. Pipe-Arch in drainage design. Their primary function is Culverts Flowing Full to carry a watershed’s peak discharge flow from one side of a roadway embankment to an o Chart 39B, HDS-5 acceptable downstream outfall. The important • Headwater Depth for Oval Concrete Pipe considerations in locating and designing a Culverts with Inlet Control culvert are: o Chart 29B, HDS-5 • Topography • Head for Elliptical Concrete Pipe Culverts Flowing Full • Debris potential o Chart 33B, HDS-5 • Design storm frequency • Allowable Fill Heights for RCP Pipe • Allowable upstream water depth o DelDOT’s web site • Allowable downstream water depth • Allowable Fill Heights for Round • Allowable outfall velocity Corrugated Steel Pipe o DelDOT’s web site • Minimum culvert flow velocity • Allowable Fill Heights for Pipe Arch • Risk of overtopping the roadway Corrugated Steel Pipe • Geometric and safety criteria o DelDOT’s web site • Erosion and sediment criteria • Allowable Fill Heights for Round For a project, pipe culverts may be one or a Corrugated Aluminum Pipe combination of several designated materials o DelDOT’s web site depending upon the service life criteria for the • Allowable Fill Heights for Pipe Arch project’s highway functional classification, the Corrugated Aluminum Pipe project scope, location within the roadway o DelDOT’s web site section, environmental factors, manufacturer’s • Typical Cross-Sections Elliptical Concrete installation requirements, and economics. Pipe Culverts may be single or multiple barrels o Concrete Pipe Design Manual, with the following shapes: Illustrations 5.1 and 5.3 1. Box • Minimum Slopes for Full Flow in Pipes at 3 ft/s 2. Circular

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3. Elliptical (for concrete pipes only, also comparison of the calculated values with the called oval concrete pipes), and design criteria. 4. Pipe arch Factors that determine the performance, capacity and required culvert size include the 6.9.3 DESIGN CRITERIA following: The design frequency is selected from 1. D = Height of inside barrel opening (ft) Figure 6-1. The allowable headwater shall remain at least 6 inches below the edge of 2. HW = Headwater depth at the culvert roadway embankment, preferably one foot. entrance However, in flood plain areas, the headwater 3. AHW = Allowable headwater depth at the shall conform to the applicable regulations. inlet permitted by design criteria or other Since the outlet velocity is invariably greater safety considerations. than the velocity in the natural channel, 4. dc = Critical depth (ft) is the depth of flow erosion control provisions for the outlet with minimum specific energy. The velocity should be used. Altering the hydraulic specify energy is the sum of the depth plus characteristics of the culvert can reduce the velocity head for a given discharge. outlet velocity. 5. ke = Entrance loss coefficient for full flow The allowable headwater and tailwater 6. L = Length of culvert depths are significant factors in culvert design. Some the concerns inaccurately determining 7. Sf = Head loss per length of culvert due to friction (ft) these heights are: 8. n = Coefficient for surface roughness of • Damage to upstream and downstream the pipe wall (see Figure 6-27) properties.

9. So = Slope of pipe • Flooding of travel lanes. 10. TW= Tailwater depth at the culvert outfall • Relationship to low point(s) in the 11. V = Mean velocity at any depth (ft/s) roadway profile. The design method for sizing pipe culverts • DHW ≤ 5.1/ is based on procedures in HEC-5, HEC-10 and • Proposed/probable future land use affects other publications. The following discussion on the drainage watershed. describes the basic concept for culvert design. • Possible damage to the roadway support Computer programs such as HEC-RAS, NFF structure. and HY-8 automate the design methods. However, the designer should have an • FEMA 100-yr flood plain regulations. understanding of the basis used to develop the • Potential for debris clogging and possible software. bypassing of culvert to the sag point. When using any approved procedure, the • Effect on wetlands and other results must be summarized to include the environmentally sensitive areas. projected flow capacity, the pipe size, the • Stability of the downstream discharge area required slope, discharge velocity and a beyond the point of outfall.

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Figure 6-29 Culvert Size Determination Using HEC-5

If TW≥D, ho = TW. If D>TW, then

Step 1 Enter known data, i.e., Q, L, S , and c + Dd o Step 6 h = or TW, whichever is AHW, in an acceptable culvert design o 2 form. For the initial trial, use the larger. Obtain dc from the charts. friction slope for So. Using the principles of open channel Compare the two values of HW. The flow; determine the design storm Step 2 Step 7. higher value indicates what the flow depth of flow; assume this is the TW condition is, i.e., outlet or inlet unless downstream conditions control control this depth. If inlet control prevails, the type of Select a trial size assuming inlet culvert, the size and entrance type control, HW/D = 1.5; use appropriate Step 8. Step 3 being evaluated are correct. Other HW nomograph. If HW>AHW, try combinations of pipe materials and larger size pipe until HW

6.9.4 DESIGN PROCEDURE SUMMARY greater rate than the water can flow through it. Capacity is affected by energy Flow Controls losses beginning at the outlet with the Flow through the culvert is determined by backwater affect of the tailwater depth, the available energy differential between the proceeding upstream with the losses due inlet and outlet. Depending upon this to culvert barrel characteristics including relationship, the culvert may flow under two the size, shape, type of material, types of flow. The design must consider each roughness, slope and length. At the inlet, type of flow to select a culvert type and size. losses due to the inlet geometry and headwater depth affect the capacity. The • Inlet Control: The culvert size, inlet culvert can flow either with partial or full geometry and headwater depth determine flow. the flow capacity. Inlet control exists as long as water can flow through the pipe at The following is a brief overview of the a greater rate than it enters. This type of two most commonly used methods for flow is not affected by downstream determining culvert sizes. One method is conditions nor the roughness, length and based on a series of culvert capacity charts and slope of the culvert. Primarily the barrel the other is based on a series of nomographs shape, cross-sectional area, inlet edge and using inlet and outlet control as the primary headwater depth affect inlet control. parameters. These methods involve manually Culverts flowing under inlet control will determining the required culvert size by trial always flow partially full. and error. The designer may choose to use the HY8 Culvert Analysis Microcomputer • Outlet Control: Outlet control exists as long as water can enter the culvert at a

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Program, HEC-RAS or other approved The control with the higher headwater governs software. and is used to continue the design. The steps for design are easy and provide a Determine outlet velocity reliable design and/or a check on computer A. Outlet Control⎯Outlet velocity equals generated designs. The methods are limited the flow divided by the flow cross sectional and do not address hydrograph routing or area at the outlet. If the outlet is not velocity concerns involving energy dissipators submerged, the flow area is usually based on a or outlet scour potential. depth of flow equal to the average of the Critical depth is an important consideration critical depth and the pipe’s vertical height. in the design of culverts. For the available energy head, the maximum discharge occurs at B. Inlet Control⎯The outlet velocity is critical flow and the depth of flow at this point calculated using Manning’s formula. Capacity is defined as the critical depth. A culvert under charts for solving this equation for full flow inlet control does not have the hydraulic losses and partial flow values are found in the associated with outlet control and has its references. maximum capacity at critical flow. Selection Report For inlet control the critical depth occurs in An important step is to summarize the the barrel near the culvert entrance and normal results showing the selected pipe size, pipe flow and depth is reached before the outfall. material, required headwater and outlet Conversely under outlet control the critical velocity. The design criteria shall be compared depth occurs near the outfall and the tailwater with the design results to ensure compliance. depth becomes an important factor. Nomograph Method HEC-5 Capacity Chart Method - HEC-10 Define Design Data Define Design Data 1. Establish the design Q. 1. Establish the design Q. 2. Approximate the length, L, of the culvert. 2. Approximate the length L of the culvert. 3. Select a roughness factor n and preliminary 3. Select a roughness factor n and preliminary culvert slope. slope. 4. Determine the allowable headwater depth. 4. Determine the allowable headwater depth. 5. Determine the allowable outfall velocity 5. Determine the allowable outfall velocity based on the proposed outfall conditions, i.e., based on the proposed erosion characteristics erodible or non-erodible discharge point. of the discharge point. 6. Select a trial culvert, including a barrel 6. Select a trial culvert, including a barrel cross sectional shape and entrance type. cross sectional shape and entrance type. Divide the allowable headwater depth by a Approximate the initial trial pipe size by factor of two for the initial trial size using the dividing the allowable headwater depth by a friction slope for So. factor of two. Find headwater for trial culvert Determine culvert size A. Inlet Control. Select the appropriate capacity chart for the trial culvert size. Following the chart (1) Given the design Q, size and type of directions, find the headwater depth using inlet culvert, follow the use directions and the control. If this exceeds the allowable appropriate inlet control nomograph to find headwater depth, then try a larger pipe. the HW. Conversely, if it is less than the allowable, (2) If the HW is greater than the allowable, then try a smaller pipe. Next compare the then try another trial size until HW is headwater depths for outlet and inlet control. acceptable for inlet control.

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B. Outlet Control The goal of stormwater management is to restrict the peak rate or volumetric rate of (1) Given Q, size and type of culvert, and stormwater runoff after the project area is estimated tailwater depth (TW, ft), above developed to the same as or less than it was the invert at the outlet for the design originally, and to preserve or improve the frequency flood condition in the outlet quality of the runoff. channel: (a) Locate the appropriate outlet control Restricting the post-development peak flow nomograph for the type of trial culvert. rate reduces the potential for increased downstream erosion and flooding. Typically Find the entrance loss coefficient, ke. this quantity control is accomplished using (b) Find the head (H) following the wet ponds or infiltration. These ponds are instructions for using the nomograph. designed to provide sufficient storage volume (2) For tailwater (TW) elevation equal to or so that a restrictive outlet will release no more greater than the top of the culvert at the than the pre-development rate. Stormwater outlet set ho equal to TW and find HW by ponds are also used for stormwater quality the equation: management with sediment forebays and HW = H + ho - SoL (6.72) permanent non-draining pools that encourage (3) For tailwater elevations less than the the deposition of runoff-borne pollutants, most of which are either sand sized or are bonded to top of the culvert at the outlet, use ho = dc+ D/ 2 or TW, whichever is the greater, where sand sized particles. This allows the removal of an estimated 80% of generated pollutants dc (the critical depth in feet) is determined from the appropriate critical depth chart. by controlling the flow rate and letting them naturally settle to the bottom. C. Compare the headwaters previously found for inlet and outlet control. The higher If a higher degree of treatment is required, headwater governs in determining the flow specialized designs are available but they are control for the given conditions and the expensive and generally require extensive, selected trial culvert. regular maintenance. D. If HW is higher than acceptable, select a DelDOT’s Stormwater Engineer is the larger trial culvert size and find HW as responsible authority for ensuring a project’s previously described. stormwater management control and erosion control features are adequate and in Determine outlet velocity compliance with the latest policies, rules and regulations. This section is a general A. Inlet control ⎯ Determine the outlet discussion of these subjects to inform and velocity using Manning’s equation. supplement, but not replace any approved B. Outlet control –⎯ The outlet velocity is design guides, manuals, memorandums of equal to the discharge flow divided by the agreement, or other requirements. flow cross sectional area at the outlet. Sections 6.10, 6.11 and 6.12 are an Selection Report overview of DelDOT’s ES2M Design Guide, DNREC’s Green Technology: The Delaware Record the results including the selected Urban Runoff Management Approach, culvert size, type, headwater depth, and outlet DURMM: The Delaware Urban Runoff velocity. Where necessary, compare with the Management Manual, and NRCS’s Pond design criteria and indicate whether or not the Code 378. In addition, there are Internet web criteria have been met. sites with extensive information on stormwater management concepts and design 6.10 STORMWATER methodologies. MANAGEMENT

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In stormwater management, a commonly • Design of Roadside Channels with used term is Best Management Practices Flexible Lining, Hydraulic Engineering (BMP’s). BMP’s are policies, practices, Circular 15 (HEC-15), FHWA procedures or structures used to mitigate the • Hydrology, Highway Engineering Circular adverse impacts on surface water quality 19 (HEC-19), FHWA resulting from changes in the watershed. BMP’s are subject to the available area and • HEC-22, Urban Drainage Design Manual site conditions for implementation. Second Edition One stormwater management strategy is • Open-Channel Hydraulics, Ven Te Chow, 1959 Low Impact Development (LID). LID emphasizes conservation and the use of on-site • National Engineering Handbook, NRCS natural features integrated with engineered, • Various State DOT sites i.e. VDOT, small-scale hydrologic controls in order to MDOT, WSDOT and IDOT more closely mimic the pre-development hydrologic function of the site. LID strategies and techniques can be used to reduce the COMPTER SOFTWARE impact of runoff on the receiving waters. • Delaware Urban Runoff Management Some LID principles include: Model (DURMM) • Maximize retention of native forest cover • HEC-2: Floodplain Determination and restore disturbed vegetation to • TR-20: Large Drainage basin hydrology intercept, evaporate and transpire • TR-55: Hydrology, including hydrograph precipitation. generation • Retain and incorporate topographic • FLOWMASTER: Hydraulic design for features and patterns. various cross-sections such as trapezoidal, • Create a hydrologically rough landscape v-shape, rectangular, and closed pipes. that slows storm flows and increases the • PondPack Hydraulics including pond time of concentration. routings • Provide multiple or redundant LID • HydroCad measures in order to increase reliability. • Other approved software on DNREC’s 6.10.1 REFERENCES Drainage and Stormwater web site PUBLICATIONS 6.10.2 EROSION CONTROL AND • ES2M Design Guide, DelDOT STORMWATER MANAGEMENT • Green Technology: The Delaware Urban The regulations involving Stormwater Runoff Management Approach, DNREC Management, Erosion, and Sedimentation • Delaware Erosion & Sediment Control Control are intended to minimize the adverse Handbook, DNREC impact of projects that change the existing • Hydraulic Design of Highway Culverts, natural environment as well as manmade Highway Design Series 2 (HDS-2), features due to increased flow and degraded FHWA water quality. • Design of Riprap Revetment, Hydraulic Erosion control measures are used during Engineering Circular 11 (HEC-11), construction to manage surface scour and FHWA washouts created by uncontrolled runoff through the construction site and the • Hydraulic Design of Energy Dissipaters subsequent deposit of sediment and other for Culverts and Channels, Hydraulic pollutants on natural areas and adjacent Engineering Circular 14 (HEC-14), properties. Permanent stormwater measures FHWA

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are frequently incorporated into the temporary 6.10.4 GENERAL CRITERIA erosion control plans. The following is based on the Delaware Typically, DelDOT projects affect the time Sediment & Stormwater regulations. of concentration, rate and volume of stormwater peak runoff from the site due to an • Stone check dams are used in swales, increase in the amount of impervious surfaces ditches and channels to control velocities such as a larger roadway pavement section, and erosion until sufficient cover has been sidewalks, and concrete gutters. established. . • Outlet protection is required at all Erosion control practices include diversion discharge points of pipes, channels and of water entering the site by the construction spillways. of diversion berms and swales on the uphill side to route water around or away from the • Erosion control matting is required on work area; and/or piping clean water all the slopes of 3:1 or greater and all concentrated way through the project and discharging flow locations. below the work area. Limitations on total • Sediment traps and basins shall be utilized work area allowed to be stripped at any time, and sized to accommodate 3600 ft3 of and stabilized construction entrances are also a storage per acre of contributing drainage form of erosion control, as is frequent interim area until project stabilization is complete. mulching, and permanent post construction These structures shall be located at the base vegetative stabilization. of the drainage area. The required 6.10.3 SEDIMENT AND STORMWATER minimum length to width ratio is 2:1. MANAGEMENT • Drainage calculations provide pre- and post-development velocities, peak rates of Sediment control involves the controlled discharge, and inflow and outflow deposition and removal of materials eroded hydrographs of stormwater runoff at all during various construction activities. existing and proposed points of discharge Sediment control practices include placing silt from the site for the 2-yr and 10-yr and fencing at designated locations on the 100-yr frequency storm. construction site; sediment traps; sediment basins; stone check dams in the flow lines of • Hydrological data is to be developed using swales and ditches; and filters around storm TR-20 or TR-55 with a storm duration drain inlets (catch basins). Early construction based on the 24-hour rainfall event. For phasing of permanent features such as wet and projects south of the C&D Canal, the dry stormwater ponds with sedimentation Delmarva Unit Hydrograph shall be used. forebays, riprap-stilling basins at culvert • All ponds are to be designed and outlets, biofiltration swales, and vegetative constructed in accordance with approved filtration strips are also used. All permanent version of NRCS Small Pond Code 378. facilities are cleared, cleaned and • For water quality, ponds are sized to store reconstructed as necessary at the end of the runoff from a 2-yr storm with a construction activities. maximum of 1 inch of runoff released over Stormwater management, erosion, and a 24-hour period. sediment control, are highly interrelated. • Post-development peak rates of discharge DNREC and DelDOT combine these elements for the 2-yr and 10-yr (and the 100-yr into one set of plans termed a Sediment and frequency storm events for projects north Stormwater Management Plan that of the C&D Canal) and in some cases post- accompanies the Stormwater Management development volumetric rate shall not Report. exceed the pre-development peak rates of discharge based on the same storm frequency criteria.

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• Set aside areas for disposal of sediment groundwater recharging, creation of wildlife removed from the stormwater management habitat, and other environmental benefits. facilities must be provided. These areas are Since much of stormwater design is to be large enough to accommodate at least generated through computer analysis, there are 2% of the stormwater management facility several practical considerations, including the volume to the elevation of the 2-yr storage following: volume elevation, have a maximum depth of one foot, and have a slope not to exceed 1. Field review of the site to ensure the 5%. stormwater can physically and economically be directed to the selected • Wet ponds are to have a forebay to act as a location. Just designating an area on the sediment trap and two ten foot (10 ft) wide plans is not adequate without a field safety benches, one at one foot above and check. the other at one foot below the normal pool elevation. 2. Locate the water table to ensure the pond will have stormwater capacity. If the 6.10.5 PROJECT DESIGN AND REVIEW pond will be normally filled to capacity CHECKLIST with groundwater then it will provide

DelDOT’s ES2M Design Guide and the little detention value. Delaware Erosion & Sediment Control 3. Take soil borings to ensure the site can be Handbook, are the primary references for economically excavated or at least have preparing the Erosion, Sediment and the proper excavation bid items, e.g., Stormwater plans. An approved checklist is rock excavation. submitted to the Stormwater Engineer at each 4. Ensure there are provisions for periodic plan review stage for any project disturbing maintenance, e.g., access. over 5,000 square feet of land. A completed checklist is required for each plan submission. Another important consideration is that The items that do not apply to the project shall many stormwater management facilities are in be marked "N/A". or adjacent to developed areas and may not be considered a community asset. Therefore, the 6.11 STORMWATER QUANTITY design should consider aesthetics and safety. MANAGEMENT Ponds become an attraction particularly when 6.11.1 REFERENCES they are flowing full. It is important that the outlet structure be designed for the safety of For publications and computer software, those who may use it for recreation. The most see Section 6.10. economical or hydraulically efficient design 6.11.2 GENERAL may not be the safest alternative for the site conditions. Stormwater quantity control is based on the requirement that the peak rate of discharge A landscape plan to accompany each pond from an area after development cannot exceed design is desirable. Woody vegetation, shrubs, the peak rate of discharge from that site before or trees planted on the embankment are not development. Both infiltration techniques and allowed. Fencing is discouraged because of stormwater detention pond with a restrictive maintenance and perceived safety issues. outlet structure achieves this goal. The Methods of vegetating ponds vary. measures to control stormwater quantity also 6.11.3 POND DESIGN can be valuable in maintaining the quality of Stormwater ponds fill with water when the project runoff and are usually designed rate of inflow exceeds the rate of outflow. The concurrently. One such measure is a wet pond volume of water that is temporarily stored that serves both purposes. The permanent pool during a runoff event can be expressed of water allows for filtration of contaminants, graphically as the area between the inflow and

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outflow hydrographs. Figure 6-30 outlines the Step 5. Fit pond to available location. steps to design a wet pond. Sketch the trial pond on the plans to determine 6.11.3.1 POND SIZING its limits and if it is viable. Establishing the bottom elevation of the pond is the starting The hydraulic analysis of detention ponds point. Selecting this elevation is based on generally involves the following steps: hydraulics and geotechnical considerations. Step 1. Define design criteria. Determine Hydraulic limitations involve locating and the required storm frequency events and other setting the invert of a positive discharge criteria as mandated by DNREC. In addition, outfall such as an existing stream or ditch, follow the design criteria based on the figuring back to the pond location at a nominal functional highway classification as defined in 2% slope. The pond has to be able to drain, so Figure 6-1. the outlet elevation has to be above the discharge point. Geotechnical considerations Step 2. Select sites. Based on the include an analysis of the types of soil or rock topography of the project area select feasible that may be encountered. Excavating preliminary pond locations. Select these unsuitable material or rock is expensive. The locations based on: proximity to the project; determination of the normal groundwater level an acceptable outfall location to drain the at the proposed pond location is extremely water (such as a natural stream); adequate important. First, the storage capacity will be space to construct a structure; moderate to flat decreased by the continual inflow of slopes so grading steep slopes doesn't groundwater. Second there could be a need to consume all the space; suitable geotechnical protect the existing aquifer from conditions; outside floodplains and wetlands; contamination. This also is not inexpensive. etc. Other considerations include setting tentative elevations for the bottom, permanent Depending upon the design, additional pool level, principal spillway including the space may be required for sedimentation various storm frequency structure(s), and the forebays or safety benches. For the design of emergency spillway level. small to moderate ponds (most serve only a few acres in area), allowing an extra 30 to Step 3. Determine peak flows. Using TR- 50% of surface area would be appropriate for 55, TR-20 or another approved version of the initial sizing. Current regulations may also NRCS Soil Cover Complex Method, require buffer areas around the pond for access determine the peak rates of runoff for existing or wetlands protection. conditions (pre-development) from the drainage areas contributing to each selected The grading for the pond involves pond location. These rates cannot be exceeded embankment slopes typically 4:1, but no by post-construction flows. steeper than 3:1. The pond shape is an important part of its function. Irregular shapes Step 4. Determine trial pond size. As a with ratios of length to width of at least 2 to 1 design tool, TR-55 has a manual method of are preferred. For safety purposes, there are determining an initial trial pond size for the two 10 ft wide safety benches located one foot designer to assess possible pond location. The above the normal pool and one foot below the method estimates the storage volume required normal pool level. The upper bench can be to allow for the control of the peak outflow eliminated if the side slopes are 4:1 or flatter. discharge. The pre- and post-development The recommended depth for a wet pond is 3 to flows as developed by TR-55 and using TR-55 6 ft. The top of the embankment should be 10 Figure 6-1 allows the designer to quickly ft wide. A 10 ft buffer for access around the estimate the storage volume. This value is then toe is preferred. The design top elevation geometrically related to the available area and should allow for future embankment depth at a selected site. settlement.

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Obviously ponds consume considerable Step 7. Check emergency spillway. The land area and early site selection is very maximum stage (elevation) reached in the important. pond during the storm routing (particularly the Step 6. Verify the trial pond size. The 100-yr) is also an important part of the size approximated in Steps 4 and 5 must be analysis. Regulations require one foot (or checked through the use of inflow and outflow more) of freeboard (height remaining) at peak hydrographs, analyzing the discharge capacity stage. If the freeboard is less than one foot, of alternative outflow structures and routing regrading the pond is needed either by raising the various storm flows through the pond. the embankment or regrading the pool to Because several design storms and types of increase the storage, lowering the detention outfall structures must be routed through the elevation. Sometimes the pond is so small that pond, approved stormwater management the water overtops the dam; this is software is used to generate the data. The unacceptable and must be remedied by input data for the software must comply with regrading to make the pond considerably the current regulations. larger, by about a third as a first approximation. If the maximum routed The process is to develop stage-storage elevation is significantly under the freeboard relationships using the estimated available requirement, a reduction in pond size might be storage at various pond elevations, then appropriate. determine what type of spillway is the best outlet structure. Two typical types are a Step 8. Structural review and hazard rectangular concrete box or concrete wall with classification. After the final embankment assorted weirs and orifices to control any details are determined, a structural review and water quality flow as well as several storm hazard classification as to the potential frequency flows. Computer software quickly damage that might occur due to a major determines the discharge characteristics of breach or failure of the structure is performed. most of the combinations. The discharge The analysis is done using national dam safety characteristics of the outlet structure constitute standards based on the ultimate land use that the stage discharge curve. Size the outlet may occur during the life of the structure. structure for water quality in accordance with 6.11.3.2 TYPES OF OUTLET the current DNREC regulations. STRUCTURES Computer software combines the stage- The effectiveness of a storage basin is very storage curve and the stage-discharge curve dependent upon the outlet structure. An outlet based on their common factor stage. The structure consists of the principal spillway inflow hydrograph is then routed through the with provisions for emergency overflow. The pond for each of the required frequencies. principal spillway should have the capacity to The routed outflow peak rate is compared convey the design discharge to with the pre-development peak rate or other predevelopment levels as mandated by mandated criteria. Currently, it must be equal DNREC from the storage basin over a 24-hr to or less than existing for all frequencies. If duration without allowing the flow to pass the routed outflow is greater or considerably through the emergency spillway. less then allowed, then a change in the pond Principal spillways include a combination size and/or outlet structure is evaluated. Since of weirs, orifices, drop inlets, and pipes. The it is recognized that there is a rather large error typical principal spillway consists of either a (25% for TR-55) in developing stormwater weir that controls the flow over the management volumes, downsizing ponds embankment or a multistage riser structure should not be done without careful study of functioning as a drop inlet with one or more both the input and generated data. weirs and outlet conduits that are designed to control one or more selected storm flows. The multi-stage riser system uses separate

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openings of various sizes placed at different opening is typically provided to extend elevations to control the rate of discharge from detention flow. the watershed for the specified design storms. When the water rises above the top of the The type of spillway depends upon opening, the discharge through an orifice is topography, the size of the storage basin, and determined using: the drainage characteristics of the downstream 5.0 conveyance system. The principal spillway is = (2gHACQ oooo ) (6.73) sized to carry the design storm without where allowing flow to enter the emergency spillway 3 using extended detention devices. Qo = Discharge through the orifice (ft /s)

If site conditions do not allow for an Co = Orifice entrance coefficient 2 emergency spillway, then it would be Ao = Cross-sectional area of orifice (ft ) necessary to design the principal spillway to g = Acceleration due to gravity (32.2 ft/s2) carry the 100-yr storm flow without H = Total head of water (ft), measured from overtopping the facility. Sizing an outlet o water surface to the center of the orifice. If structure is based on hydrologic routing the orifice outfall is submerged, then H is calculations as well as the potential threat to o the elevation difference between the downstream life and property. The crest headwater and tailwater surfaces. elevation of the principal spillway must be at least 1.0 ft below the crest elevation of the Co is a variable coefficient that depends emergency spillway. upon the orifice diameter, edge shape and its wall thickness as follows: The placement of emergency spillways should be on original ground and avoid • Sharp-edged orifice with walls thinner than downstream features such as residences and the diameter: Co = 0.60 other facilities. • Sharp-edged orifice with walls thicker than 6.11.3.3 OUTLET HYDRAULICS the diameter: Co = 0.80

A stormwater management facility has an • Rounded edge orifice: Co = 0.92 outlet system that controls water flow and 6.11.3.3.2 CONCRETE WALL quality. The hydraulic design depends on SPILLWAY WITH WEIR whether the storage basin is intended to be dry or permanently wet. Frequently it is intended Weirs are used in lieu of a riser/barrel that the facility function as an extended system for shallow basin spillways. The weir detention system that provides control over outlet is a channel, not a pipe, which operates both water quantity and water quality. This more efficiently with less maintenance and means it allows the pre-development flow to allows for greater opportunity for improving slowly be released over a 24-hr period into the water quality. The top of the spillway weir outfall channel while a permanent pool of wall is used to control the storage volume and water is retained in the pond. Preferred design the outlet flow. It is important that weir alternatives and criteria per current regulations structures constructed on an embankment be should be verified before beginning the design protected from undermining and erosion. to determine the method(s) of stormwater Weirs are also commonly used in a management. The three most commonly used concrete box riser system. (See Section outlets are discussed briefly in the following 6.11.3.3.3.) The shape, size and number of the sections. weirs may vary to control the runoff for 6.11.3.3.1 ORIFICES various storm conditions. In addition, concrete box risers allow for the easy installation of Orifices are small openings in an outlet stormwater quality orifices and, if necessary, structure. For stormwater quality, a circular an opening at the bottom to drain the pond.

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Two common types of weirs are the broad is just slightly above the top elevation, the crested weir and the V-notch weir. inlet operates as a weir. When the water The discharge over a broad crested weir is: increases and begins flowing into the inlet from all directions, the flow actually becomes 5.1 unstable and the inlet operates as an orifice. = HLCQ wwww (6.74) For a circular pipe riser, this flow condition where usually occurs when the height of flow (H) is Q = Discharge over the weir (ft3/s) 1.2 to 1.5 times the diameter (D). Eventually, w the head can be high enough that the riser Cw = Broad-crested weir coefficient flows full and operates as culvert flow. L w = Length of the weir (ft) Normally the spillway design prevents this

Hw = Head of water over the weir crest (ft) level of flow from occurring.

Cw varies based upon a relationship There is a point in orifice flow where between the width of the weir and the head vortex flow occurs, resulting in severe measured at a designated point upstream from turbulence and reduced inlet capacity. This the weir. An average value is 3.1 or 3.3. condition is to be avoided by ensuring the riser is large enough to operate under the weir flow The discharge over a V-notch weir is given condition, the barrel is at full flow with as low by the equation: a head over the crest as practical, and before ⎛θ ⎞ 5.2 orifice flow develops. vw = CQ vw tan⎜ ⎟H vw (6.75) ⎝ 2 ⎠ Hydraulically the ideal flow condition occurs when the outlet conduit and riser are where operating at full flow. The riser should operate 3 Qvw = discharge (ft /s) at weir flow with the lowest head over the top C = weir coefficient, usually 2.5 as practical. In the design of the riser, vw determine the elevation over the riser when the θ = angle of the notch at the apex (degrees) flow transitions from weir flow to orifice flow Hvw = total energy head (ft) and then make sure that the barrel controls the In the design of multi-stage risers, notched flow at that elevation. Since this transition or rectangular weirs are frequently used to point is difficult to accurately determine, it is control the flow from one or more design usually sufficient to assume that the transition years. When the depth of flow rises above the occurs where the weir flow equation value is top elevation of the weir, the weir may be equal to the orifice flow equation value. subject to orifice flow conditions. See Section If the H/D ratio is less than 1.5, conduits 6.11.3.3.3. smaller than 12 inches in diameter are usually 6.11.3.3.3 DROP INLETS analyzed as a submerged orifice. The barrel is usually designed as a pipe culvert flowing full Drop inlets are used in spillway design and with inlet control using the procedure outlined may be square, rectangular or circular. The in HDS-5. vertical portion of a drop inlet is referred to as a riser. The inlet is a horizontally positioned The drop inlet spillway is treated as a opening through which water enters, drops combination of weir, orifice and pipe flow. vertically through a shaft, and then flows Various levels of flow are analyzed using the through a horizontal pipe culvert (referred to weir and orifice equations. If the head is high as a barrel) and then discharged at the enough (normally avoided), the drop inlet acts downstream outlet. as a pipe and the flow is determined by the equation: A drop inlet spillway may operate under three conditions depending upon the height of flow over the inlet. When the water elevation

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5.0 strips are vegetative patches that intercept ⎡ 2gH ⎤ = AQ ⎢ ⎥ (6.76) sheet flow. The design should emphasize ⎣⎢1 +++ feb Lkkk ⎦⎥ biofiltration, rather than transporting flow with the greatest possible hydraulic efficiency. The where design is therefore based on criteria that H = difference between headwater and promote sedimentation, filtration, and other tailwater elevations (ft) pollutant removal mechanisms.

kb = bend loss coefficient, use 0.6 Biofiltration Swale — Grass

ke = entrance loss coefficient, use 0.5 This BMP is designed to convey

kf = barrel friction loss coefficient stormwater at a non-erosive velocity while L = length of pipe (ft) improving water quality through infiltration, sedimentation and filtration. Check dams can The other variables are as previously defined. be used to slow the flow and create small Since multi-stage risers have to be analyzed temporary ponding areas to promote based on several design storms, various infiltration. This option is one of the least spillway combinations and flow conditions, expensive as it involves no engineered filter computer software is used. A stage-discharge fabric, imported pervious materials or outlet curve is developed plotting the discharge structures. The design considerations are versus head for each of the flow conditions. channel capacity; erosion; a runoff velocity of The minimum flow for any given head is the 1 fps for the design storm; and a total length discharge used in continuing the evaluation of providing the desirable residence time. These the selected spillway, pond size etc. swales usually involve a typical section that has a wider bottom width, flatter side slopes Example 7, Attachment B, contains a and denser vegetation than a normal open simplified pond design example. channel. 6.12 STORMWATER QUALITY Filter Strips MANAGEMENT Filter strips are relatively flat graded areas that are heavily vegetated. They are designed 6.12.1 GENERAL to treat runoff and remove pollutants primarily The preferred methods of protecting and through vegetative filtration. They are not improving water quality are referred to as intended as a stand-alone solution but as part Green Technology BMP’s. DNREC’s Green of a treatment system and/or pre-treatment for Technology: The Delaware Urban Runoff another BMP. Filter strips are effective in Management Approach provides detailed providing a buffer between incompatible land- discussion, design guidance and construction uses, can be landscaped and made details for designing nonstructural BMP’s. aesthetically pleasing, and provide These include the biofiltration, bioretention groundwater recharge in highly pervious soil and infiltration design concepts. BMP's are areas. Filter strips may be constructed to allow most effective and require the least treatment of sheet flow directly from a site or maintenance if designed as a system involving natural areas that are left around drainage several types of BMP’s working together. channels. The flow characteristics for natural areas are similar to shallow concentrated flow. 6.12.2 BIOFILTRATION Constructed filter strips are more effective Biofiltration techniques are intended for if the entering flow is spread out maintaining improving post-construction water quality. sheet flow onto the filter and a shallow There are two basic types of biofiltration ponding area is created through the use of devices: swales and filter strips. Swales are berms. shallow ditches that carry flow, whereas filter

6-68 Highway Drainage and Stormwater Management July 2008 DelDOT Road Design Manual

6.12.3 BIORETENTION clogging infiltration, BMP’s are usually constructed after a site has been stabilized and Bioretention BMP’s are structural has an established vegetative cover with an stormwater controls that accept and upstream BMP for removal of sediment. The temporarily store the water quality volume two types of infiltration techniques are using special soils and vegetation in shallow infiltration trenches and basins. basins or landscaped areas. Their function is to remove pollutions, not control runoff. Some Infiltration Trenches advantages are they can be easily integrated Since they occupy the smallest area into a site’s landscape plan, are aesthetically infiltration trenches are the BMP most pleasing and need low maintenance. commonly used to treat the water quality Bioretention Areas volume and recharge the groundwater. These trenches are long, narrow and filled with a Bioretention areas are engineered, freely draining stone aggregate. The trapped landscaped, shallow stormwater basins. water is filtered into the underlying soil. Stormwater collected in the upper layer of the Normally there is no engineered outlet. In basin is filtered through surface vegetation, a order to prolong their effectiveness and reduce mulch layer, and a pervious soil layer, and maintenance there must be a positive sediment then temporarily stored in a stone aggregate trap system above the site. Although the base layer. The water quality volume is trenches can process fine sediment, coarse drained from the aggregate base by infiltration sediment can quickly clog the trench. into the underlying soil or/or removed to an outlet through a perforated pipe subdrain. Infiltration Basin These systems work best if they are combined If more area is available, an alternative is to with a perimeter grass filter strip or grass provide an infiltration basin. Essentially this is swale to reduce the sediment and velocity a dry pond to trap water and discharge it into prior to entering the biorentention area. groundwater flow through infiltration. This 6.12.4 INFILTRATION concept uses heavy vegetation to control velocities and increase the percolation. Most BMP’s partially rely on infiltration for their effectiveness. Stand-alone infiltration Infiltration techniques are more BMP’s are designed specifically to control a maintenance intensive and site specific. There selected volume of the runoff, retain it, and must be highly pervious underlying soils, no infiltrate all or part of it into the ground. rock and no potential for contaminating the existing groundwater. In addition, construction Infiltration depends upon the underlying scheduling must be adjusted to allow for soils to provide significant capacity to absorb project completion prior to their installation. the proposed flow. In addition, there must not be a possibility of adversely affecting the existing groundwater or aquifer. This technique is the most site-sensitive, most maintenance-intensive and most likely to fail of the BMP’s. Because of the potential of

July 2008 Highway Drainage and Stormwater Management 6-69 DelDOT Road Design Manual

Figure 6-30 Design Steps for a Wet-Extended Detention Stormwater Pond Step 1 Determine hydrologic data for the site and confirm pre-developed flow rates.

Step 2 Compute runoff water quality volumes and compute release rates.

Step 3 Estimate the permanent pool volume and extended detention volume.

Step 4 Determine forebay requirements and preliminary locations and size(s).

Step 5 Prepare a preliminary grading plan for the stage-storage curve.

Step 6 Field review the project. Locate a site area and confirm a stormwater pond is practical using: the estimated pond size; configuration; forebay locations; maintenance access; set aside areas; and safety features.

Step 7 Design the water quality orifice or weir.

Step 8 Set the permanent pool volume and elevation.

Step 9 Size the 2-yr control orifice or weir.

Step 10 Check for performance of the 2-yr opening.

Step 11 Size the 10-yr control opening.

Step 12 Check the performance of the 10-yr opening.

Step 13 Perform a hydraulic analysis of the facility including the forebay, the pond, the riser flow control, the barrel inlet flow and the barrel outlet.

Step 14 Size the 100-yr release opening or emergency spillway.

Step 15 Check total discharge and performance of the 100-yr control opening.

Step 16 Design any required outlet protection as per HEC-14.

Step 17 Perform any required buoyancy calculations.

Step 18 Determine the required seepage control.

Step 19 Check final design of inlets, sediment forebays, outlet structures, maintenance access, and safety features making sure all criteria has been met.

Step 20 Based on the final grading and design, determine the pond hazard classification.

Step 21 Develop a landscape plan. Steps 7 to 12 are used to determine the appropriate outlet structure and stage-discharge curve for the storm events as mandated by DNREC.

6-70 Highway Drainage and Stormwater Management July 2008 DelDOT Road Design Manual

Chapter Six ATTACHMENT A DRAINAGE DESIGN AIDES

GENERAL DELDOT DESIGN CRITERIA Design Frequency...... DelDOT Figure 6-1 Allowable Spread on Pavement Cross Section...... DelDOT Figure 6-2 Miscellaneous Design Criteria ...... DelDOT Figure 6-3 RECOMMENDED PUBLICATIONS • AASHTO - Model Drainage Manual

• DelDOT - ES2M Design Guide • DNREC o Delaware Erosion & Sediment Control Handbook o Green Technology: The Delaware Urban Runoff Management Approach • FHWA o HDS-2 Highway Hydrology (FHWA-NHI-02-001) o HDS-3 Design Charts for Open-Channel Flow (FHWA-EPD-86-102) o HDS-4 Introduction to Highway Hydraulics (FHWA-NHI-01-019) o HDS-5 Hydraulic Design of Highway Culverts (FHWA-NHI-01-020) o HEC-2 Water Surface Profiles Software and User’s Manual o HEC-11 Design of Riprap Revetment (FHWA-IP-89-016) o HEC-10 Capacity Charts for the Hydraulic Design of Highway Culverts, 1972 o HEC-12 Drainage of Highway Pavements, 1984 (replaced by HEC-22) o HEC-14 Hydraulic Design of Energy Dissipaters for Culverts and Channels, 1975 o HEC-15 Design of Roadside Channels with Flexible Lining (FHWA-IF-05-114) o HEC-19 Hydrology, 1985 (replaced by HDS-2) o HEC-22 Urban Drainage Manual, 2001 (FHWA-NHI-01-021) • NRCS o Urban Hydrology for Small Watersheds, Technical Release No. 55 (TR-55), 2003 o National Engineering Handbook, 1997 RECOMMENDED INTERNET SITES • DelDOT’s Web Site (http://www.deldot.gov/) - Design Resource Center • FHWA Hydraulics Engineering (http://www.fhwa.dot.gov/engineering/hydraulics) • New Jersey Department of Transportation (http://www.state.nj.us/transportation/eng/) • University of Wisconsin Cooperative Extension (http://learningstore.uwex.edu/) - Wisconsin Storm Water Manual: Grassed Swales (G3691-7) • US Army Corps of Engineers Hydrologic Engineering Center (HEC) (http://www.hec.usace.army.mil)

July 2008 Highway Drainage and Stormwater Management, Drainage Design Aides A6-1 DelDOT Road Design Manual

• Virginia Department of Transportation (http://www.virginiadot.org) - Drainage Manual RECOMMENDED SOFTWARE • HY7 WSPRO (2001) - Gradually varied flow in open channels • HY8 Culvert Analysis - Methodology and data found in HDS-5, HEC-14 and HEC-15 • HY22 Urban Drainage Design Program - Highway pavement drainage, open channel flow, critical depth solutions, storm stage-storage computations and reservoir routing • TR-55 Urban Hydrology for Small Watersheds • TR-20 • WSP-2 HYDROLOGY REFERENCES • FHWA o HDS-2 Highway Hydrology (FHWA-NHI-02-001) o HDS-4 Introduction to Highway Hydraulics (FHWA-NHI-01-019) o HEC-19 Hydrology, 1985 (replaced by HDS-2) o HEC-22 Urban Drainage Manual , 2001(FHWA-NHI-01-021) • NRCS - TR-55 Urban Hydrology for Small Watersheds, Technical Release No. 55, 2003 DESIGN AIDES: FIGURES, NOMOGRAPHS AND TABLES DelDOT’s Criteria for Design Frequency...... DelDOT Figure 6-1

Frequency Factor, Cf ...... Section 3.2.2.1, HEC-22 Delaware’s Rainfall Intensity Estimates...... DelDOT Figures 6-5 thru 6-7 Runoff Coefficients...... DelDOT Figure 6-8; Table 5.7, HDS-2 Appendix B, Tables 11 and 12, HDS-4; Table 3-1, HEC-22

Velocities for Upland Method of Estimating Tt1 and Tt2 ...... Figures 37 and 52, HEC-19 Figure 3-1, TR-55; USDA Hydrology Technical Note N4, Time of Concentration (1986) Soil Group Descriptions for TR-55 Method ...... DelDOT Figure 6-9 Hydrologic Soils Descriptions for TR-55 Method Common to Delaware...... DelDOT Figure 6-10 Delaware’s 24-Hour Rainfall Depths...... DelDOT Figure 6-12 Runoff Curve Numbers for Urban Areas...... Table 5.4, HDS-2 Manning’s Roughness Coefficients n (TR-55) for Sheet Flow ...... Table 2.1, HDS-2 Table 3-4, HEC-22; Table 3-1, TR-55

Unit Peak Discharge (qu) for NRCS Type II Rainfall Distribution ...... Exhibit 4-II, TR-55

Adjustment Factor (Fp) - Pond & Swamp Areas Spread In Watershed...... Table 5.6, HDS-2 Table 3-9, HEC-22; Table 4-2, TR-55 OPEN CHANNEL FLOW AND ROADSIDE DITCHES REFERENCES • FHWA o HDS-3 Design Charts for Open-Channel Flow (FHWA-EPD-86-102) o HDS-4 Introduction to Highway Hydraulics (FHWA-NHI-01-019)

A6-2 Highway Drainage and Stormwater Management, Drainage Design Aides July 2008 DelDOT Road Design Manual

o HEC-11 Design of Riprap Revetment (FHWA-IP-89-016) o HEC-14 Hydraulic Design of Energy Dissipaters for Culverts and Channels, 1975 o HEC-15 Design of Roadside Channels with Flexible Lining (FHWA-IF-05-114) o HEC-22 Urban Drainage Manual, 2001 (FHWA-NHI-01-021) • Open-Channel Hydraulics, Ven Te Chow, 1959 DESIGN AIDES: FIGURES, NOMOGRAPHS AND TABLES Open Channel Flow Schematic ...... Figures 6 and 7, HDS-4 Manning's Roughness Coefficients n for Open Channels ...... Table 1, HDS-3 Tables 12 and 14, HDS-4; Tables 2.1 and 2.2, HEC-15 Formulas for Various Channel Geometrics...... Appendix B, HEC-15; Table 2-1, Chow Nomograph for Solution of Manning's Equation ...... Chart 83, HDS-3; Appendix C, Chow Solution of Manning's Equation for Channels of Various Side Slopes...... Chart 16, HEC-12 Capacity of Trapezoidal Channels...... HDS-3 and HDS-4 Uniform Flow in Trapezoidal Channels by Manning’s Equation ...... Table B.1, HEC-14 Geometric Design Chart for Trapezoidal Channels ...... HDS-3; Appendix B, Chow Trapezoidal Channel Geometry...... HDS-3 Maximum Nonscouring Ditch Grades with Grass Lining...... DelDOT Figure 6-13; Tables 2-3, HDS-3 DelDOT’s Roadside Ditch Design Form ...... DelDOT Attachment B Permissible Velocities for Open Channels ...... DelDOT Figures 6-14 and 6-15

Kb Factor for Maximum Stress on Channel Bends...... Chart 21, HEC-22 Correction Factor for Riprap Size ...... Chart 2, HEC-11 Riprap Size Relationship ...... Chart 1, HEC-11 PAVEMENT DRAINAGE AND STORM DRAINS REFERENCES • FHWA o HDS-3 Design Charts for Open-Channel Flow (FHWA-EPD-86-102) o HDS-4 Introduction to Highway Hydraulics (FHWA-NHI-01-019) o HEC-12 Drainage of Highway Pavements, 1984 (replaced by HEC-22) o HEC-22 Urban Drainage Manual , 2001(FHWA-NHI-01-021) DESIGN AIDES: FIGURES, NOMOGRAPHS AND TABLES Manning's Roughness Coefficients (n) for Pavements and Gutters ...... HEC-22 Allowable Water Spread...... DelDOT Figure 6-2 Curb, Gutter and Inlet Selection...... DelDOT Standard Construction Details Typical Gutter Sections ...... Figure 4-1, HEC-22 Nomograph for Velocity in Triangular Gutter Sections...... Chart 4B, HEC-22 Average Water Spread Width in Triangular Gutter Section.... Chart 29, HDS-3; Table 4-4, HEC-22 Flow in Triangular Gutter Sections ...... Chart 29, HDS-3; Chart 1B, HEC-22 Ratio of Frontal Flow to Total Gutter Flow ...... Chart 2B, HEC-22 Flow in Composite Gutter Sections...... Chart 5, HEC-12 Gutter Flow Rate Q on Uniform Cross Slope (ft3/s)...... DelDOT Web Site

July 2008 Highway Drainage and Stormwater Management, Drainage Design Aides A6-3 DelDOT Road Design Manual

Gutter Flow Q on Composite Section (ft3/s)...... DelDOT Web Site Average Gutter Flow Velocity for Uniform and Composite Sections...... DelDOT Web Site Spread at Average Velocity in a Reach of Triangular Gutter ...... Table 4-4, HEC-22

Frontal Flow Interception Factor, Rf ...... Chart 5B, HEC-22

Side Flow Interception Factor, Rs...... Chart 6B, HEC-22 Curb-Opening and Slotted Drain Inlet Length for Total Interception ...... Figure 35b, HDS-4 Chart 9, HEC-12 Curb-Opening and Slotted Drain Inlet Interception Efficiency ...... Figure 36, HDS-4 Chart 10, HEC-12; Chart 8B, HEC-22 Grate Inlet Capacity in Sump Conditions ...... Chart 11, HEC-12; Chart 9B, HEC-22 Distance to Flanking Inlets in Sag Vertical Curve...... Table 4-7, HEC-22 Depressed Curb Opening Inlet Capacity in Sump Conditions...... Chart 12, HEC-12 Chart 10B, HEC-22 Orifice Flow in Depressed Curb-Opening Inlet...... Chart 14, HEC-12; Chart 12B, HEC-22 Slotted Drain Inlet Capacity in Sump Locations...... Chart 15, HEC-12; Chart 13B, HEC-22 Ratio of Frontal Flow to Total Flow in Trapezoidal Channel...... Chart 17, HEC-12 Chart 15B, HEC-22 Inlet Clogging Factor of Safety...... DelDOT Figure 6-3 Minimum Slopes for Full Flow in Pipes at 3 ft/s...... DelDOT Figure 6-24; Table 7-7, HEC-22 Inlet Spacing Computation Form...... DelDOT Attachment B; Figure 4-19, HEC-22 Storm Drain Computation Table (Open Channel) .... DelDOT Attachment B; Appendix D, HEC-22 Hydraulic Gradeline Computation Table (Pressure Flow Design) ...... DelDOT Attachment B Appendix D, HEC-22 PIPES AND PIPE CULVERTS REFERENCES • FHWA o HDS-3 Design Charts for Open-Channel Flow (FHWA-EPD-86-102) o HDS-4 Introduction to Highway Hydraulics (FHWA-NHI-01-019) o HDS-5 Hydraulic Design of Highway Culverts (FHWA-NHI-01-020) o HEC-5 Hydraulic Charts for the Design of Highway Culverts, 1980 o HEC-10 Capacity Charts for the Hydraulic Design of Highway Culverts, 1972 o HEC-12 Drainage of Highway Pavements, 1984 (replaced by HEC-22) o HEC-13 Hydraulic Design of Improved Inlets for Culverts, 1972 o HEC-14 Hydraulic Design of Energy Dissipaters for Culverts and Channels, 1975 o HEC-22 Urban Drainage Manual , 2001(FHWA-NHI-01-021) In addition to the above references there are manuals available from various pipe manufacturers, including the Concrete Pipe Design Manual (CPDM) (viewable online at http://www.concrete- pipe.org/techdata.htm) and the Handbook of Concrete Culvert Pipe Hydraulics by the American Concrete Pipe Association; and Modern Storm Sewer Design by the American Iron and Steel Institute. These give pipe size data, general hydrology and hydraulic design, structural design, durability, and recommended installation methodology. Much of this information is not available in other publications.

A6-4 Highway Drainage and Stormwater Management, Drainage Design Aides July 2008 DelDOT Road Design Manual

DESIGN AIDES: FIGURES, NOMOGRAPHS AND TABLES Energy Concept in Pipe Flow...... Figure III-8 and III-9, HDS-5; Figure 7-8, HEC-22 Manning's Roughness Coefficients n for Pipes ...... DelDOT Figure 6-27; Table 14, HDS-4 Table 4, HDS-5; Tables 3-4 and 7-1, HEC-22 Friction Slope for Circular Pipe, Full Flow...... Charts 53 and 54, HDS-3 Circular Section Hydraulic Elements at Various Flow Depths ...... Figure 40, HDS-4; Charts 26 and 27, HEC-22 Circular Pipe Conveyance ...... Table 3, CPDM Flow for Circular Pipe Flowing Full ...... Figures 2 thru 5, CPDM Flow for Horizontal Elliptical Pipe Flowing Full...... Figures 6 thru 9, CPDM Uniform Flow in Circular Sections Flowing Partly Full ...... Table B.2, HEC-14 Velocity in Pipe Conduits...... Charts 35 thru 51, HDS-3 Velocity in Elliptical Pipes...... Chart 74, HDS-3 Solution of Manning’s Equation for Flow in Storm Drains ...... Chart 25B, HEC-22 Flow for Circular Pipe Flowing Full ...... CPDM Flow for Horizontal Elliptical Pipe Flowing Full...... CPDM Flow Coefficients for Circular Pipe...... CPDM Uniform Flow for Pipe Culverts...... Del DOT Web Site Uniform Flow for Concrete Elliptical Pipes...... Del DOT Web Site Critical Depth of Flow for Circular Conduits...... Chart 4B, HDS-5; Chart 27B, HEC-22 Critical Depth for Elliptical Pipes ...... Chart 31B, HDS-5 Energy Loss Coefficients ...... Table 12, HDS-5; Table 7-5b, HEC-22 Culvert Design Form ...... Figure 7-7, HEC-22 Headwater Depth for Concrete Pipe Culverts with Inlet Control...... Chart 1B, HDS-5 Chart 28B, HEC-22 Head for Concrete Pipe Culverts Flowing Full ...... Chart 5B, HDS-5 Headwater Depth for C.M. Pipe Culverts with Inlet Control...... Chart 2B, HDS-5 Chart 29B, HEC-22 Head for Standard C.M. Pipe Culverts Flowing Full ...... Chart 6B, HDS-5 Headwater Depth for C.M. Pipe-Arch Culverts with Inlet Control...... Chart 34B, HDS-5 Head for Standard C.M. Pipe-Arch Culverts Flowing Full...... Chart 39B, HDS-5 Headwater Depth for Oval Concrete Pipe Culverts with Inlet Control ...... Chart 29B, HDS-5 Head for Elliptical Concrete Pipe Culverts Flowing Full...... Chart 33B, HDS-5 Allowable Fill Heights for RCP Pipe ...... DelDOT Web Site Allowable Fill Heights for Round Corrugated Steel Pipe ...... DelDOT Web Site Allowable Fill Heights for Pipe Arch Corrugated Steel Pipe...... DelDOT Web Site Allowable Fill Heights for Round Corrugated Aluminum Pipe...... DelDOT Web Site Allowable Fill Heights for Pipe Arch Corrugated Aluminum Pipe ...... DelDOT Web Site Typical Cross-Sections Elliptical Concrete Pipe...... CPDM, Illustrations 5.1 and 5.3 Elliptical Concrete Pipe Characteristics ...... Figure 7-2, HEC-22

July 2008 Highway Drainage and Stormwater Management, Drainage Design Aides A6-5 DelDOT Road Design Manual

Minimum Slopes for Full Flow in Pipes at 3 ft/s...... DelDOT Figure 6-24; Table 7-7, HEC-22 Friction Slope for Concrete — Full Flow ...... DelDOT Figure 6-29 Circular Pipe Conveyance...... DelDOT Web Site Flow Coefficients for Circular Pipe...... DelDOT Web Site STORMWATER QUANTITY MANAGEMENT - DETENTION PONDS REFERENCES • DNREC o Delaware Erosion & Sediment Control Handbook o DURMM: The Delaware Urban Runoff Management Model Technical Manual - Green Technology: The Delaware Urban Runoff Management Approach User’s Manual Computer Program

• DelDOT - ES2M Design Guide • FHWA o HDS-2 Highway Hydrology (FHWA-NHI-02-001) o HEC-11 Design of Riprap Revetment (FHWA-IP-89-016) o HEC-14 Hydraulic Design of Energy Dissipaters for Culverts and Channels, 1975 o HEC-15 Design of Roadside Channels with Flexible Lining (FHWA-IF-05-114) o HEC-19 Hydrology, 1985 (replaced by HDS-2) o HEC-22 Urban Drainage Manual , 2001(FHWA-NHI-01-021) • NRCS o Engineering Field Handbook, 1984 o National Engineering Handbook, 1997 o Pond Code 378 o Urban Hydrology for Small Watersheds, Technical Release No. 55 (TR-55), 2003 • Open-Channel Hydraulics, Ven Te Chow (1959) • City of Tacoma Surface Water Management Manual • Low Impact Development⎯–Technical Guidance Manual for Puget Sound • Various Internet sites • TP-61 Handbook of Channel Design for Soil & Water Conservation • City of Tacoma Surface Water Management Manual Volume V Runoff Treatment BMP’s • "Sand Filter Design for Water Quality Treatment" Shaver, E., and Baldwin, R., 1991, Stormwater Conference Proceedings, Mt. Crested Butte, CO, ASCE, Washington, D.C. (Also see "A Catalog of Ultra Urban Best Management Practices Using Sand Filters" Bell, W., 1992, City of Alexandria, Alexandria, VA.) • Grass Swales: "Biofiltration Systems for Storm Runoff Water Quality Control" Horner, R., 1988, Washington State Department of Ecology, Seattle, WA. • Website link - http://lowimpactdevelopment.org DESIGN AIDES: FIGURES, NOMOGRAPHS AND TABLES Design Steps for a Wet Extended Detention Stormwater Pond...... DelDOT Figure 6-30 Degree of Retardance...... Table 5, HDS-3

A6-6 Highway Drainage and Stormwater Management, Drainage Design Aides July 2008 DelDOT Road Design Manual

Chapter Six ATTACHMENT B EXAMPLE PROBLEMS

Introduction: These example problems are being presented to show the basic steps for drainage design. It is expected that the designer will use current available software and spreadsheets as much as possible to facilitate drainage calculations. To ensure compliance with the criteria and expedite review, the designer should ensure that all final calculations, size selections, water spread, etc., are compared with the Department’s approved drainage criteria and are highlighted in the summary and/or within the drainage report itself. EXAMPLE 1─RATIONAL METHOD PROBLEM STATEMENT The watershed area in Figure 6B-1 has a shown in figures 6B-2 and 6B-3. The design composite watershed of 36.5 acres and drains problem is to determine the peak discharge at to a culvert at E on a local road in Kent point E for a 25-yr storm frequency using the County. The surface characteristics of the Rational Method. watershed and the stream flow path are as Figure 6B-1 Watershed for Rational Method Example

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-1 DelDOT Road Design Manual

Solution

Step 1. Determine tc Figure 6B-2 Flow Path of Watershed Segment Description Slope % Length (ft) A-B Residential lawn 1.0 50 B-C Forest and meadow 2.0 50 C-D Shallow gully 0.8 250 D-E Open channel 0.8 475

Compute the sheet flow time, Tt1*, for segments A-B and B-C. Use equation 6.4: nL )(42.0 8.0 T = 1 t1 5.0 4.0 24 sP where:

Segment A-B Segment B-C n = 0.15 from Table 3-2, HEC-22 n = 0.30 from Table 3-2, HEC-22 L1 = 50 ft L1 = 50 ft P24 = 3.3 from Figure 6-11 P24 = 3.3 from Figure 6-11 s = 0.01 s = 0.02

[])50)(15.0(42.0 8.0 [ )50)(30.0(42.0 ] 8.0 T = + =+= min96.1665.931.7 t1 5.0 )01.0()3.3( 4.0 5.0 )02.0()3.3( 4.0 *Note: A great deal of the literature indicates that sheet flow should be limited to 100 ft. Compute the shallow concentrated flow 2 x 2 + 3 = P = 3 + 2 x 2 2 +14 = 49.19 ft time, Tt2 for segment C-D. Use the Shallow (Gutter) Gully Flow Line of Figure 6B-2 and Therefore using equation 6.7a: 0.8% watercourse slope. From Figure 37, A 22 HEC-19, V2 = 1.35 f/s. Using equation 6.5 and R === 13.1 ft V2 = 1.35 ft/s Pw 49.19

L2 250 With s = 0.008, n = 0.045 and R = 1.13, T t2 = = = 08.3 minutes 60 V 2 60 x 1. 35 then V3 = 3.21 ft/s from Chart 83, HDS 3, or equation 6.7: Compute the open channel flow time, 49.1 5.067.0 Tt3, for segment D-E. From survey data, the V = sR small stream has a base width (B) of 3 ft with n 4:1 side slopes (z = 4), banks approximately 2 49.1 67.0 5.0 V = = 21.3)008.0()13.1( ft high (d), and a streambed slope of 0.008 3 045.0 ft/ft. This meandering channel has some weeds L3 475 with light brush. Therefore, n = 0.045 (Table Tt3 === min47.2 1, HDS-3, or Table 3-4, HEC-22). 60 3 xV 21.360 Using equation 6.8: The streambed cross sectional area (A) and wetted perimeter (P) from Figure 5-6, HEC- c = + = TTTt 321 = ++ 47.231.296.16 = 22: 21.74 rounded to 22 min x ( + 2 x 3 = A = 3 x 2 + ( x 22 ) = 224 ft 2

B6-2 Highway Drainage and Stormwater Management, Example Problem July 2008 DelDOT Road Design Manual

Step 2. Determine I Equation 6.2 and runoff coefficients for the respective ground characteristics (Figure 6- Using Figure 6-6, for t =22 minutes c 8). (duration) and 25-yr frequency, I =4.06 in/hr.

Step 3. Determine C AC xx f C = ∑ A From HEC-22, Cf=1.10 for a 25-yr total frequency. 13.6 x 0.25 + 16.9 x 0.40 + 6.0 x 0.35 x 6.0 + 0.40 x 16.9 + 0.25 x 13.6 C = Step 4. Determine the weighted average 6.0 + 16.9 + 13.6 runoff coefficient using Figure 6B-2, C = 0.34

Figure 6B-3 Surface Characteristics of Watershed* Area (ac) Description Runoff Coefficient

A1 = 6.0 Residential, single family, large lots 0.35 A2 = 16.9 Residential, single family, small lots 0.40 A3 = 13.6 Woods, light vegetation, loam soil 0.25 *Assumed for this example.

Step 5 Calculate Q25, peak discharge, by Trapezoidal using Equation 6.1. Symmetrical 2 Q25 = C f (CIA) = 1.10x0.34 x 46.4 x36.5 += zdBdA 3 = 60.9, rounded to 61 ft /s zdBP 2 ++= 12 Useful formulas for various channel shapes Unsymmetrical are as follows. (Also see Appendix B, HEC-15 2 ( + 21 )dzz or Figure 5-6, HEC-22). z is dimensionless BdA += and represents the horizontal ditch slope rate, 2 2 2 for a 2:1 slope z = 2; 3:1 slope z=3. ( 1 zzdBP 2 ++++= 11 ) V-Shape Parabolic 2 Symmetrical = TdA 2 3 = zdA 8d 2 2 TP += zdP += 12 3T Unsymmetrical Approximation 0

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-3 DelDOT Road Design Manual

EXAMPLE 2─NRCS TR-55 METHOD PROBLEM STATEMENT The watershed area shown in Figure 6B-4 The designer should be aware that when has a composite watershed area of 36.5 acres using the TR-55 Method the methodology, draining into a culvert at E on a local road in variables and approach to solving this problem New Castle County. The design frequency is is exclusive to this method. Mixing of 25-yr. Estimate the peak discharge at point E variables, runoff coefficients etc. from the using the NRCS TR-55 Graphical Peak Rational Method will lead to inaccurate results. Discharge method.

Figure 6B-4 Watershed for NCRS Method Example

SOLUTION

For this problem since the watershed has = FqAqQ pdmu homogenous areas and has no stormwater management facilities requiring analysis of First, the runoff depth (qd) is calculated, several storm frequencies and routing second, the time of concentration is calculated hydrographs, the Graphical Peak Discharge and used to obtain the unit peak discharge,(qu), Method is applicable. a pond and then a swamp factor (Fp) is The peak discharge equation 6.14 is: selected and then Q determined.

B6-4 Highway Drainage and Stormwater Management, Example Problem July 2008 DelDOT Road Design Manual

Step 1. Obtain the 24-hr rainfall for the 25- ∑( × ACN mm ) yr frequency from Figure 6-11. P = 6.0. CN = 24 A ∑ m Step 2. Establish hydrologic soil groups for the composite watershed and obtain CNT. The results are: The respective hydrologic soil groups were 75.2696 obtained from the Soils Maps for New Castle CN == 88.73 , Rounded to 74 50.36 County, the curve numbers were obtained from Figure 6B-5 based on the land use, and Since impervious areas exist only on the weighted average curve number is residential areas, and the CN values for computed using the data from Figure 6B-5 and residential areas, depending on their sizes, the equation: have accounted for these impervious areas. Therefore, for this example CNT = 74

Figure 6B-5 Watershed Data Soil Type Hydrologic Land Use CN Area Soil Group (ac)

Am C Residential - 1 Ac 79 2.50 Am C Residential - 1/2 Ac 80 12.60 AdA C Residential - 1/4 Ac 83 0.75 AdA C Woods 70 1.10 (Good hydrological Conditions) TaB2 B Residential - 1/4 Ac 75 1.00 TaB2 B Woods 55 9.00 (Good hydrological Conditions) WcA D Residential - 1 Ac 84 1.75 WcA D Residential - 1/2 Ac 85 4.30 WcA D Woods 77 3.50 (Good hydrological Conditions)

Other than the given information all the other data is the same as in Example 1.

Step 3. Calculate Runoff Depth q from 2 d ⎡ ⎛1000 ⎞⎤ Equations 6.9 and 6.10. ⎢ − 2.00.6 ⎜ −10⎟⎥ ⎣ ⎝ 74 ⎠⎦ 2 q = = 2.3 in ()− SP )2.0 d q = 24 ⎛1000 ⎞ d + 8.00.6 ⎜ −10⎟ P24 + 0.8S ⎝ 74 ⎠ 1000 S −= 10 Step 4. Calculate tc Note that TR-55 uses CN hours when calculating and using tc.

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-5 DelDOT Road Design Manual

Overland (Sheet) Flow (Two segments: exclusively for use when using the TR-55 A-B & B-C) Refer to the topographic data on method for sheet flow time of concentration. Figure 6B-6 for T , using equation 6.11: t1 For segment A-B, n = 0.24 for lawn from 8.0 Table 3-1, TR-55 nL1 )(007.0 T t1 = 5.0 4.0 For segment B-C, n = 0.40 for woods (light 2 sP underbrush) from Table 3-1, TR-55. Manning’s runoff coefficient n in this equation is selected from Table 3-1, TR-55. These coefficients have been developed

Figure 6B-6 Flow Path of Watershed Segment Description Slope % Length (ft) A-B Residential lawn 1.0 50 B-C Forest and meadow 2.0 50 C-D Shallow gully 0.8 250 D-E Open channel 0.8 475

Therefore: L2 250 0.8 T t = = = 05.0 min 0.007().24 x 500 2 V 60 x 44.1033600 T t = 2 1 5.0 )01.0()2.3( 4.0 Open Channel Flow 0.8 0.007 ()0.40 x 50 The travel time for the open channel + = 34.0 hr )2.3( 0 5. )02.0( 4.0 segment calculation involves the same procedure as in the Rational Method. From Shallow Concentrated Flow (C-D) Example 1, Tt3 = 2.47 minute or 0.04 hr. TR-55 has three equations that are Therefore, from equation 6.8: acceptable for determining T . The designer, t2 after field reviewing the watershed, may = + + TTTt tttc 321 decide that using the equation, tc = + + = 43.004.005.034.0 hr 5.0 2 = 8.32 ksV and assigning a k factor from Figure 6-4 will give the most realistic velocity Step 5. Determine Unit Peak Discharge qu value. In this case: from equation 6.15 First, determine Ia/P using 0.5 equation 6.10 and that Ia is assumed to be 20% V2 = 32.8(0.457) (0.008) = 1.34 ft/s of S: The other two equations develop an I ⎛ 200 ⎞ ⎛ 200 ⎞ average k for unpaved areas resulting in a ⎜ −= 2⎟ ⎜ −= 2⎟ equation 6.12: P ⎜ ⎟ P24 0.6 ⎝ CNT ⎠ ⎝ 74 ⎠ 5.0 Unpaved V2 = s)(10.16 = 12.0

5.0 V2 = = /44.1)008.0(10.16 sft Ia/P can also be obtained from Table 4-I, Select the higher of the velocities, since it TR-55. will result in a more conservative peak This value and tc are used to obtain the discharge, to find Tt2 and use equation 6.5 value of qu from Exhibit 4-II, TR-55, by modified to calculate hours:

B6-6 Highway Drainage and Stormwater Management, Example Problem July 2008 DelDOT Road Design Manual

interpolating between the curves for Co, C1, C2 are coefficients derived from Ia interpolation of Table F-1, TR-55. P = 12.0 Figure 6B-7 23 Coefficients for Unit Peak Discharge u = ( //575 inmisftq )

Step 6. Obtain Pond Adjustment Factor, Fp. Ia/P Co C1 C2 Since the percent of pond and swamp area 0.10 2.55323 -0.61512 -0.16403 is zero for the watershed, Fp = 1.0 from Table 5.6, HDS-2, Table 3-9 HEC-22, or Table 4-2, TR-55. 0.30 2.46532 -0.62257 -0.11657 Step 7. Calculate Q from equation 6.14: 0.35 2.41896 -0.61594 -0.08820 = FqAqQ pdmui 0.40 2.36409 -0.59857 -0.05621

Am = Drainage Area = 36.5/640 = 0.057 square miles. Therefore, the peak discharge is: 0.45 2.29238 -0.57005 -0.02281 Q = 575 x 0.057 x 3.20 x 1.00 0.50 3.20282 -0.51599 -0.01259 = 105 ft3/s The designer may find it simpler to use the Exhibit 4-II, which is not easily or accurately equation and coefficients. read, is based on the equation:

2 )log( ou += 1 c + 2 [ tCtCCq c )log()log( ]

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-7 DelDOT Road Design Manual

EXAMPLE 3—OPEN CHANNEL HYDRAULICS PROBLEM STATEMENT A grass-lined, unsymmetrical trapezoidal depth of flow, velocity, and if the flow is channel with a base width of 4 feet, a 4:1 supercritical. There are charts and nomographs foreslope, a 2:1 backslope, a stream bed slope available in HDS-3, HEC-15 (1986 or 1988 of 0.012 ft/ft (1.2%), and n = 0.05, carries a version) and HEC-22 that can be used to design discharge of 115 ft3/s. Determine the quickly solve for these two values. SOLUTION Using the channel equations given at the 49.1 V = 3/2 2/1 = /17.4)012.0()445.1( sft end of Example 1, the depth of flow (d) will 05.0 be obtained by trial and error. Trial 1. = xQ = 3 /60.11916.475.28 sft Try d = 2 ft (greater than 115 ft3/s) ()+ dzz 2 BdA += 21 Trial 3. 2 Try d = 2.45 feet 2 2 + 2)42( 2 ( + ) 45.224 ()xA 24 += = 20 ft ()xA 45.24 += = 81.27 ft 2 2 2

2 2 P = + x = 58.19)45.2359.6(4 ft ( 1 zzdBP 2 ++++= 11 ) 2 2 = = 42.1 ftPAR P ( ++++= 141224 ) 49.1 V = 3/2 2/1 = /125.4)012.0()42.1( sft P =+= 718.16)359.6(24 ft 05.0 20 PAR === 196.1 ft 718.16 = xQ = 3 /71.114125.481.27 sft 3 (equation 6.7a) ≈115 ft /s) 49.1 Therefore: V = sR 2/13/2 n Depth of flow d = 2.45 ft and 49.1 V = 4.12 ft/s = 3/2 2/1 = /58.3)012.0()196.1( sft 05.0 Instead of performing the calculations, (equation 6.7) Chart 1, HEC-15, or Chart 22, HEC-22, could xAVQ === 3 /6.7358.320 sft have been used. From these charts and using: 3 (too low for 115 ft /s) d 45.2 == 6125.0 and, zz =+ .6 0 Trial 2. B 4 21 Try d = 2.5 feet R/d is estimated at 0.58. ()+ x 5.224 2 Therefore: ()xA 5.24 += = 75.28 ft 2 2 R = 0.58 x 2.45 = 1.421ft. P += x = 898.19)5.2359.6(4 ft A/Bd = 27.81/ (4) (2.45) = 2.84, and == 445.1 ftPAR A = 2.84 x 2.45 x 4 = 27.83 ft2.

B6-8 Highway Drainage and Stormwater Management, Example Problem July 2008 DelDOT Road Design Manual

The designer can also use Chart 83, HDS- proceed horizontally until z = 3. That point of 3, or Chart 14B, HEC-22, to solve Manning’s intersection corresponds to d/B = 0.615 from equation, connect the slope-scale at 0.012 with which d = 4 x 0.615 = 2.46ft. the n-scale of 0.05 then draw a line passing Froude Number: through R = 1.42 and the point of intersection of s and n at the turning line, and extend it to V Fr = the velocity scale which is read at 4.15 ft/s. gD Therefore, Q = 27.44 x 4.15 = 113.88 ft3/s 3 which is close to 115 ft /s, and the assumed d A 81.27 = 2.45 ft is O.K. D == = 49.1 ft xT + 445.26 For example, assume z1 + z2 = 3. On Chart 3, HEC-15 (May 1986), join s = 0.05 with Qn 12.4 Fr = <= 160.0 = 115 x 0.05 = 5.75 and extend to the turning x 49.12.32 line. Connect that point of intersection with B = 4 and let the line intersect z = 0 from there Therefore, the flow is subcritical.

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-9 DelDOT Road Design Manual

EXAMPLE 4—ROADSIDE DITCH PROBLEM STATEMENT Figure 6B-8 shows a roadside ditch along Compute the velocities of flow along the a Sussex County rural collector with ditch, and determine if additional erosion trapezoidal cross section, 2-foot bottom protection is necessary. From Figure 6-1, the width, and 4:1 side slopes. The ditch lining design frequency shall be for a 10-yr period. is average grass in sandy loam. Figure 6B-8 Schematic Diagram for Roadside Ditch

SOLUTION

Enter the following critical stations on a Station 20+00 (Back) Roadside Ditch Design computation form: Step 1. From Sta. 10+00 to Sta. 20+00, Station 20 + 00 (Back) - Pervious Area = (400)(1000)/43560 = Station 20 + 00 (Ahead) 9.18 ac. Station 32 + 00 (Outfall Point) - Impervious Area = (24)(1000)/43560 = Leave blank spaces for intermediate 0.55 ac. stations at this time. The calculations - Total Area = 9.18 + 0.55 = 9.73 ac proceed as follows. The sequence numbers Step 2. Select C values from Figure 6-8. for each station pertain to the respective columns on the computation sheet. - Pervious: For residential areas with 2- acre lots, use C = 0.40.

B6-10 Highway Drainage and Stormwater Management, Example Problem July 2008 DelDOT Road Design Manual

- Impervious: For asphalt pavement (hot- Therefore: mix roadway & shoulder), C = 0.95. 1000 T t3 = = 4.17 min Step 3. Calculate total CA 60 x 4 Use CA = CpAp + CIAI and, using equation 6.8:

CA = (0.40)(9.18) + (0.95)(0.55) = 4.19 tc = T t1 + t 2 + TT t3 = 17.71+1.79+4.17 = 23.67 min Step 4. Determine tc Step 5. Find I from Figure 6-7 for the 10- (a) Since the watershed is homogenous year frequency and t = 23.67 minutes and not complex, the Rational Method can c be used to find to the find the discharge I = 3.83 in/hr (interpolating between 15 flow. Therefore, use equation 6.4: and 30 min) 8.0 Step 6. Calculate Q using equation 6.1: nL1 )(42.0 Tt1 = 5.0 4.0 Q =Cf CIA = Cf (CA) I 24 sP where: Q = 1.00 x 4.19 x 3.83 = 16.05, rounded to 16 ft3/s n = 0.40 from Figure 6-16 Step 7. Enter ditch cross sectional data L1 = 100 ft P24 = 3.4 in/hr from figure 6-11 z1 = 4, B = 2 feet, z2 = 4, and S = 0.025 s = 0.03 Step 8. Select Manning's n

Therefore: n = 0.05 for grass from Figure 6-16 [])100)(40.0(42.0 8.0 Step 9. Calculate d T = t1 5.0 )03.0()4.3( 4.0 Use Chart 3, HEC-15 (1986), or Chart = 17.71 min 14B, HEC-22. For s = 0.025, Qn = 16 x 0.05 The remaining 300 ft is to treated as shallow = 0.8, and B = 2, d/B reads 0.45 when z = 4. concentrated flow using equations 6.5 and Thus d = 0.45 x 2 = 0.9 feet. 6.6: L Step 10. Calculate Average Velocity T = 2 t 2 Use V = Q/A 60v2 5.0 2 2 2 2 = 8.32 ksV A = zddB x +=+ 4 x = 04.59.00.29.0 ft The latter equation has been modified into 16 sft = = V = = /18.3 sft the condition for unpaved areas in equation 04.5 6.12: NOTE: Check the computed velocity 0.5 Unpaved V2 = 16.10(s) against the assumed velocity used to determine Step 4. If the velocities differ by Therefore: more than 20%, repeat Steps 4 through 9 5.0 using the velocity calculated in Step 10. At V2 = = /79.2)03.0(10.16 sft 300 this location, the velocities differ by ≈14%. T t 2 = = 79.1 min x 60 x 79.2 Step 11. Check the computed velocity in Step 10 against the permissible velocity (b) For 1000 feet open channel flow, Tt3, from Figures 6-13 through 6-15. If the assume maximum permissible velocity of computed velocity is greater than the 4.0 ft/s for grass on sandy loam from Figure maximum permissible velocity, a stronger 6-15. protective lining is required.

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-11 DelDOT Road Design Manual

Step 12. In this case, V = 3.18 ft/s and Vmax )2200)(24( A = = 21.1 ac = 4 ft/s. Since V

Pervious Area The remaining 300 ft is to treated as shallow + )400)(600(2/1)400)(600( concentrated flow using the following Ap = equations 6.5 and 6.6: 43560 L T = 2 and, = 8.26 ac t 2 60v2 5.0 APtotal =+= 44.1726.818.9 ac 2 = 8.32 ksV

Impervious Area

B6-12 Highway Drainage and Stormwater Management, Example Problem July 2008 DelDOT Road Design Manual

This equation has been modified for For s = 0.025, Qn = 21 x 0.05 = 1.05, and unpaved areas in equation 6.12: B = 2, d/B reads 0.51 when z = 4

0.5 Unpaved V2 = 16.10(s) Thus d = 0.51 x 2 = 1.02 feet

Step 10. Calculate Average Velocity Therefore: Use V = Q/A 5.0 V = = /79.2)03.0(10.16 sft 2 2 2 2 A = =+ xzddB 0.202.1 + 4 x 02.1 = 20.6 ft 300 21 T t 2 = = 79.1 min V = = /39.3 sft x 60 x 79.2 20.6 Assuming a grass lining from Sta. 20+00 NOTE: Check the computed velocity to Sta. 32+00 (1200 feet open channel flow) against assumed velocity used to determine for which the maximum permissible velocity Step 4. If the velocities differ by more than is 4 ft/s (Figure 6-15), the travel time is: 20%, repeat Steps 4 through 9 using the velocity calculated in Step 10. At this 1200/(60 x 4) = 5 min location, the velocities differ by about 22%. Add this to the travel time at Sta. 20+00, The second iteration yields the results in which is 23.67 min. Figure 6B-9. Thus using equation 6.8, If the velocity had been greater than 4.0 ft/s, it would be necessary to begin at an t = 23.67+5+8.93+1.79 = 39.39 min. c intermediate location after Station 20+00 Step 5. From Figure 6-7, I = 2.94 in/hr. and determine the station at which a Step 6. Calculate Q as before from equation protective lining, ditch cross section change 6.1 or energy dissipater should be introduced. This is a trial and error process and could be Use Q = (CA) I worked from the outfall back or from an Q = (7.25) (2.94) = 21.32, rounded to 21 upstream station. Since the drainage ft3/s procedure has several assumptions, it is a matter of engineering judgment in defining a Step 7. Enter ditch cross sectional data point at which an alternative ditch lining and/or riprap would be considered. In this z1 = 4, B = 2 feet, z2 = 4, and s = 0.025 problem, based on the data given, it appears Step 8. Select Manning's n that the grass lining would be sufficient. n = 0.05 for grass from Figure 6-16 However, it would be prudent from an environmental and erosion control Step 9. Calculate d perspective to place riprap for at least the last 25 ft. Use Chart 3, HEC-15 (1986), or Chart 14B, HEC-22.

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-13 DelDOT Road Design Manual

B6-14 Highway Drainage and Stormwater Management, Example Problem July 2008 DelDOT Road Design Manual

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-15 DelDOT Road Design Manual

EXAMPLE 5 ─ INLET SPACING PROBLEM STATEMENT Figure 6B-11 shows data for a 4-lane divided New Castle County urban arterial with a C of 0.88. Figure 6B-11 – Typical Section, Plan and Profile

B6-16 Highway Drainage and Stormwater Management, Example Problem July 2008 DelDOT Road Design Manual

The following describes the procedure for 1 E = determining inlet spacing based on the o ⎧ ⎫ equations in Section 6.8.1, the hydraulic data ⎪ ⎪ given, use a C = 0.88, and DelDOT’s standard ⎪ / SS ⎪ 1+ xw 20”x36” Type 1 grate. This example has been ⎨ 67.2 ⎬ / SS simplified and does not include site-specific ⎪ ⎡ xw ⎤ ⎪ ⎪ ⎢1+ ⎥ −1⎪ factors, such as maximum spacing based upon ⎩ ⎣ WT −1/ ⎦ ⎭ access for maintenance, location of commercial and residential entrances, curb Step 4. Find the grate interception capacity ramps, superelevation transitions, curb height, using the following equations 6.45 and from etc. HEC-22 provides a comprehensive Figure 6-22: description of the procedure for determining = [ ()1−+ ERERQQ ] inlet capacity and placement. HEC-12 (1984) i sof o presents a more basic understanding of the 1 concept of gutter flow and for determining Rs = 8.1 3.2 inlet spacing using charts and nomographs. []+ ()/15.01 x LSV

Solution: Rf = 1.0 if V ≤ 6.0 Step 1. Select the design frequency. From DelDOT’s web site: 3 From Figure 6-1, the design frequency is Qi = 4.99 ft /s ( 10-yr return period for an urban arterial. Step 5. Therefore E = Qi / QT = 4.99/8.2 Step 2. Select the allowable spread. = 0.61 ≤ 0.70 From Figure 6-2, T = 10 ft. Try T = 8 ft for which: W/T = 0.25 Note: Tables for values of Qt and Qi with Eo = 0.6 variables of SL, SX, and Sw can be found on DelDOT’s web site. These values can be Qs = 1.94 used to check the calculated values but are Q = Q / (1-E ) based on untested performance data t s o Q = 4.85 ft3/s The given composite cross section has an t 3 Sx = 0.04, Sw =- 0.08, W = 2 ft, SL = 0.01, Qi = 3.32 ft / from DelDOT’s web site W/T = 0.2 and Sw/Sx = 2 E = 3.32 / 4.85 =0.68 < 0.70 Step 3. From Charts 1B and 2B, HEC-22: Try T = 6 for which W/T = 0.333

Eo = 0.485 Eo = 0.73

Qs = 4.2 Qs = 0.66

Qt = Qs / (1-Eo) Qt = Qs / (1-Eo) 3 Qt = 4.2 / (1- 0.485) = 8.2 ft /s 3 Qt = 2.44 ft /s Chart 1B represents solutions to the 3 1.67 0.5 2.67 Qi = 1.96 ft / from DelDOT’s web site equation: Q = (Ku/n) Sx SL T where Ku= 0.56. This is used in conjunction with E = 1.962 / 2.44 =0.80 > 0.70 Chart 2B together they solve the equation 3 Based on the various assumptions used in for composite gutter flow. Qt = 8.25 ft /s drainage calculation, using T = 6 is probably using the equation. Using the design aid 3 too conservative. chart Qt = 8.26 ft /s. Therefore, use T = 8 ft and Qt =4.85 Eo for composite gutter sections can be derived from the equation: QB = QT – Qi

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-17 DelDOT Road Design Manual

3 QB = 4.85 – 3.32 = 1.53 ft /s Sx, Sw, and W are the cross sectional elements of the gutter section from the Spacing of Inlet 1 constructions plans and Standard Construction Assume tc = 5 minutes Details. For concrete gutter sections the From Figure 6-5, I = 6.42 in/hr recommended n = 0.016. Refer to Section 6.8 for details on the procedure, applicable

iT xQ 43560 x4356085.4 equations and additional references. L1 = = WIC 111 xx 6042.688.0 The drainage design begins by using a set of preliminary construction plans and marking L1 = 623.25 ft (round to a more convenient preliminary drainage areas and known inlet 625 ft locations such as curb ramps, entrances, sags The location of Inlet 1 equals station: etc. as specified in Section 6.8.1.8. The initial area is 300 to 500 feet long below the high (10 + 00) + (6 + 25) = 16 + 25 point. Check tc: V = 2 ft/s for shallow gutter flow on The primary control is the spread. The a 1% slope, so using equation 6.5: upstream inlet will define the initial spread L 625 and the design continues by locating inlets as Tt === min21.5 necessary to control the spread as defined by 60V )2)(60( the design criteria. To locate these inlets

Therefore, tc = 5.21 min as this is the incremental areas of flow are selected and minimum recommended calculations are performed. The flow for the area is determined and including any flow that I = 6.36 in/hr has bypassed previous inlets and the next inlet Spacing of Inlet 2 placed to intercept this flow before the design spread is exceeded. Using equation 6.61, The form and the following description − QQ L = 2T B1 x43560 allow for an orderly determination and 2 recording of this process. For a continuous 22 WIC longitudinal slope the process is relatively − 53.185.4 = x43560 = 66.430 simple if the contributing subsequent areas xx 6036.688.0 have uniform runoff characteristics. After determining the spacing between the first two The location of Inlet 2 equals station: inlets all downstream inlets are similar spaced (16+25) + (4 +30) = 20+55 until there is a change in the defining drainage Subsequent inlets shall be spaced at 430 data. feet as long as the geometric controls remain Procedure: unchanged and there is clean-out access at a 1. Enter in Col. 1 the inlet number. All inlets minimum distance of 300 ft (Figure 6-3). in a storm drain shall be numbered in Stationing of inlets can be rounded to the sequence starting with the first inlet from nearest foot for construction convenience and the crest down to the inlet at the low point can be adjusted slightly to accommodate other of the sag curve where the interception is project requirements. An exaggerated degree 100%. of precision in calculations is rarely justified. 2. Enter in Col. 2 the inlet’s location by its All inlets of the drainage system shall be baseline station and whether it is left or numbered and the computed results shall be right of the baseline. presented in a tabular format using the Inlet Spacing Computation sheet. 3. Enter in Col. 3 the partial drainage area, ∆A (ac), which discharges into the inlet. It Inlet Spacing Design Form

B6-18 Highway Drainage and Stormwater Management, Example Problems July 2008

DelDOT Road Design Manual

is the uppermost part of the entire 13. Enter in Col. 13 the width of the gutter or drainage area for the first inlet, or the grate. partial area above the inlet up to its 14. Enter in Col. 13 the width of water spread, upstream inlet for subsequent inlets. Note T, on the gutter section, due to Q (ft). T that in some cases areas outside the T can be calculated for the gutter section pavement may have to be included in ∆A. with either a uniform cross slope or 4. Enter in Col. 4 the runoff coefficient of composite cross slope as explained in the drainage area ∆A of the inlet. If the Sections 6.8.1.5.1 and 6.8.1.5.2. T must drainage area is composite, then a not exceed the permissible water spread weighted value of C using Equation 6.2 specified in the design criteria. If T is should be used. C values are to be selected excessive, reduce QT to the amount that from Figure 6-8. can be accommodated within the permissible T. Use a trial and error 5. Enter in Col. 5 the travel time T (min) for t procedure by decreasing the distance the surface runoff to travel distance, L, as between inlets resulting in a smaller ΔA to in Section 6.6. find the matching QT 6. Enter in Col. 6 the intensity of rainfall I 15. Enter in Col. 15 the ratio of gutter/grate (in/hr) for the inlet corresponding to t for t width divided by the spread. This ratio is the 10-yr frequency, the values of I are needed when using the charts in HEC-22 obtained from Figures 6-6, 6-7 or 6-8. to determine the interception efficiency If drainage inlets are located on a depressed and solving for other design values. roadway, then a 50-yr frequency must be 16. Enter in Col. 16 the inlet type, be sure to used for most types of highways as indicate if using other than the standard indicated in the design criteria. grate types 7. Enter in Col.7, the gutter flow, Q, the 17. Enter in Col. 17 the intercepted flow, Q . product of Columns 4, 5and 6. i Compute Qi as explained in Section 8. Enter in Col.8 the longitudinal slope of the 6.8.1.6.2.1. For standard grates on grade, gutter at the inlet, SL (ft/ft). For vertical Qi can be obtained from DelDOT’S web curves determine SL from the slope site. equation for a parabolic curve. This value 18. Enter in Col. 18 the by-pass flow, Q , by of S is used to determine the gutter flow B L subtracting column 17 from column 11. rate and the water spread on the pavement. 19. Enter in Col. 18 remarks such as sump 9. Enter in Col. 9 the cross slope of the condition, 50-yr frequency, etc gutter/pavement section. One of the final checks is to determine the 10. Enter in Col. 10 the bypass flow rate from interception efficiency, E, of the inlet. E is the up stream inlet as shown in column 18 equal to Q in column 17 divided by Q in of the previous row. i T column 11. If E is less than 0.7, use a trial and 11. Enter in Col. 11 the total gutter flow, QT, error procedure by decreasing T (see 14) and approaching the inlet. QT is equal to ΔQ in repeating the computations until E ≈ 0.7. Column 7 plus the bypass flow rate of the upstream inlet QB of Column 18 of the previous row of the table. 12. Enter in Col. 12 the depth of flow at curb, d (ft). It must be 0.5 ft or less. Follow the procedure in 14 if d controls T so that T may be decreased.

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-19 DelDOT Road Design Manual

B6-20 Highway Drainage and Stormwater Management, Example Problems July 2008

DelDOT Road Design Manual

EXAMPLE 6 ─ STORM DRAINS Detailed examples, acceptable forms, and HEC-22 and other references in Sections 6.8 available software for the design of storm and 6.9. Below is an acceptable process for drains are found in HDS-3, HDS-4, HDS-5, using the forms. PROCEDURE FOR COMPLETING STORM DRAIN COMPUTATION FORM Col. 1 ─ Pipe Unit Number. Enter the Col. 7 ─ Cumulative Product. Enter the sum designated number of the pipe unit. The of entries of Column 6 applicable to the pipe computations begin with the first pipe unit in unit. Determine CA for the n'th pipe unit as the uppermost location of the drainage area, follows: gradually proceeding downstream in sequence. (CA)n = (CA) 1-n + (C x ΔA )n Col. 2 ─ Upper Inlet Number. Enter the inlet (junction) number at the upper end of the pipe Col. 8 ─ Time of Concentration. Enter the unit under consideration. Below this inlet time of concentration for the pipe, tc (min), number and within the space, indicate its which is the travel time of stormwater along baseline station and whether to the left or right the hydraulic length to reach the inlet in of the baseline. Column 2. Analyze all possible routes. Use the route with the longest travel time. Col. 3 ─ Lower Inlet Number. Enter the inlet (junction) number at the lower end of the pipe For the first pipe, the hydraulic length being designed. As in Column 2, indicate its includes only surface flow from the uppermost location. areas of the watershed that drains into the inlet in Column 2. The travel time for surface flow Col. 4 ─ Drainage Area. Enter the is computed by using Equation 6.3, 6.4 and/or incremental area, ΔA, of the (portion of) 6.5. watershed draining into the inlet in Column 2. For the first pipe, ΔA is the uppermost area of For subsequent pipe segments, the the watershed where the storm water flows on hydraulic length (surface flow) is underground the surface into the inlet. For subsequent along the storm drain. Thus the time of pipes, it is the drainage area bounded between concentration for a pipe is the time of the inlet in Column 2 and the previous concentration for stormwater to travel from upstream inlet. the upstream pipe plus the time required by storm water to travel the length of the pipe. Col. 5 ─ Runoff Coefficient. Enter the runoff For the n’th pipe, it is represented by the coefficient C for the drainage area in Column equation: 4. It is the composite runoff coefficient for the drainage area ΔA as in Equation 6.2: = ncnc −1 + TTt )()()( nt −1

∑ AC xx On the form, tc is entered by adding C = Column 8 and Column 16 of the previous row. A Total The minimum period for tc is 5 minutes. Col. 9 ─ Rainfall Intensity. Enter the The incremental drainage area may be intensity of rainfall corresponding to the time assumed to be composed of only pervious and of concentration in Column 8, I (in/hr). Use impervious runoff coefficients. Select the Figures 6-6, 6-7 or 6-8 for the design appropriate runoff coefficients from Figure 6- frequency. 8 and compute the weighted average for the composite runoff coefficient, C. Col. 10 ─ Design Discharge. Enter the design discharge for the pipe unit, Q. It is the product Col. 6 ─ Product. Enter the product of of the entries in Columns 7 and 9. Column, 4 and 5, which is C x ΔA.

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-21 DelDOT Road Design Manual

Col. 11 ─ Pipe Size. Enter the pipe diameter Col. 15 ─ Flow Velocity. Enter the velocity of in inches. Use Figure 6-29 to size concrete flow in the pipe unit, V. See Section 6.9. pipes for the design discharge at friction slope Col. 16 ─ Travel Time. Enter the travel time or slightly steeper. Otherwise the equations for the flow to travel over distance L in the and referenced resources can be used to find a pipe, T (equation 6.5): suitable pipe size and an acceptable slope. See t equations, charts and tables referenced in L (Col.12) T t = = Section 6.9. 60V ()(Col.60 15 ) Col. 12 ─ Length of Pipe. Enter the length of Col. 17 ─ Upper Invert Elevation. Enter the the pipe, L (ft). invert elevation of the pipe at the inlet in Col. 13 ─ Friction Slope. Enter the friction Column 2. The elevation for the first pipe is slope of the pipe unit, Sf (ft/ft). Use Equations normally based on the minimum cover 6.64 and 6.65. requirement for the selected type of pipe. Col. 14 ─ Actual Slope. Enter the actual slope Col. 18 ─ Lower Invert Elevation. Enter the of the pipe, S (ft/ft). Note that S ≥ Sf. invert elevation at the lower end of the pipe. Col. 18 = Col. 17 – (Col 12 x Col. 14)

B6-22 Highway Drainage and Stormwater Management, Example Problems July 2008

DelDOT Road Design Manual

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-23 DelDOT Road Design Manual

PROCEDURE FOR COMPLETING HYDRAULIC GRADELINE COMPUTATION FORM Col. 1─Junction Number. Enter the junction A = Area of outflow pipe (ft2) number that represents the catch basin, g = 32.2 ft/s2. manhole or junction box immediately upstream of the outflow pipe. HGL Add Hc to Hf and enter the sum as total Hf computations begin at the outfall and are in Column 8. calculated for each junction in the storm drain. Col. 9─Outflow Velocity. Enter the velocity Col. 2─Baseline Station. Enter the baseline of flow in the outflow pipe, Vo (ft/s), as station for the junction indicating whether it is determined in Step 8. to the left or right of the baseline. Col. 10─Contraction Head Loss. Enter the Col. 3─Outlet Water Surface Elevation. contraction head loss, Ho, by using the Enter the outlet water surface elevation. For formula: the first junction it is the outfall design 2 tailwater elevation; for subsequent junctions it 0.25 V o H o = is the inlet water surface elevation of the 2g previous downstream junction. Col. 11─Inflow Pipe Velocity. Enter the Col. 4─Outfall Pipe Diameter. Enter the velocity of flow in the inflow pipe that flows diameter of the outflow pipe unit, Do (in). into the junction, Vi (ft/s). Vi equals the design 3 Col. 5─Design Discharge. Enter the design discharge for the inflow pipe (ft /s) divided by its area (ft2). discharge for the outflow pipe, Qo. Col. 6─Outflow Pipe Length. Enter the When more than one pipe is flowing into length of the outflow pipe, L . the junction, Vi is determined as follows. For o each pipe flowing into the junction, compute Col. 7─Friction Slope. Enter the friction the product of its velocity of flow and the slope of the outflow pipe flowing full, Sfo discharge. The pipe that has the greatest (ft/ft). Refer to Figure 6-29 for concrete pipes, product will be considered; its Vi will be or use equations 6.67 and 6.69 to derive: entered in Column 11. In this situation, the time of concentration that influences the H 2 f ⎛ Qn ⎞ design discharge may have to be adjusted S fo == ⎜ 67.2 ⎟ Lo ⎝ 46.0 D ⎠ depending on the controlling pipe whose Vi is entered in Column 11. Col. 8─Friction Head Loss. Enter the head Column 12─Expansion Loss. Enter the loss due to friction for the outflow pipe, Hf by using equation 6.71: controlling expansion loss, Hi using the equation: = SLH 2 foof 0.35 V i H i = On curved alignment, the additional head 2g loss due to friction, Hc is given by: Col. 13─Skew Angle. Enter the angle of skew 2 between the controlling inflow pipe Δ 0.002 ∅ V o H c = (degrees). 2g Where, Col. 14─Bend Loss. Enter the bend loss, HΔ, which is obtained from the equation: ∅ = Angle of curvature in degrees, 2 K V i V = Velocity in the outflow pipe (ft/s) =H o Δ 2g Q V = o Where: o A

B6-24 Highway Drainage and Stormwater Management, Example Problems July 2008

DelDOT Road Design Manual

K is a loss coefficient, Column 17─Gross Total Head Loss. Enter the gross total head loss, H, which is the sum The values of K are listed at the bottom of of friction loss, H , in Column 8 and adjusted form for various skew angles. f junction loss H t , in Column 16. Col. 15─Total Head Loss. Enter the total head loss at junction, Ht, which is equal to the Col. 18─Potential Water Surface Elevation. sum of Ho in Column 10, H1 in Column 12, Enter the sum of the elevation in Column 3 and HΔ in Column 14. That is: and H in Column 17. This elevation is the potential water surface elevation for the = Ht = H + H io + H Δ junction under consideration. Col. 16─Adjusted Head Loss. Enter the Col. 19─Top Elevation of Structure. Enter adjusted total head loss at junction, H t , the top of manhole elevation or the inlet determined by multiplying Ht with a elevation on the gutter flow line (elevation at coefficient as follows: the top of catch basin). If this elevation is not • If the junction incorporates a drainage at least one foot higher than the elevation in inlet that intercepts at least 10% of Column 18, then the hydraulic gradeline must be lowered to satisfy the design criteria. mainline flow, then the coefficient is 1.3. The HGL can be lowered by increasing • When the interception is under 10%, the slopes and sizes of pipe, or by reducing the coefficient is 1.0. design discharge that can be achieved through • If the junction incorporates a manhole or a proper stormwater management. junction box, then the coefficient is 0.5.

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-25 DelDOT Road Design Manual

B6-26 Highway Drainage and Stormwater Management, Example Problems July 2008

DelDOT Road Design Manual

EXAMPLE 7 ─ POND DESIGN PROBLEM STATEMENT: Design a stormwater detention pond for the project is located above the C&D Canal, Example 2 using the TR-55 method. The area the design will be based on the TR-55 Tabular to be commercially developed is classified as Hydrograph Method for developing inflow Am and a detention pond with a release and out flow routing and stage-discharge structure will control stormwater runoff relationships that allow for determining the increases back into the existing channel due to pond size and most efficient control structure. the construction of the commercial site. Since Figure 6B-15 Location of Pond

This example is simplified to illustrate the at the location in this example would have to basic concept of controlling stormwater runoff be designed combining each subarea’s inflow through the use of a retention basin. In reality, and outflow hydrographs and routings to the the regulations would require the new outfall. The analysis would be made for the 2-, commercial site to provide onsite stormwater 10- and 100-yr Type II 24-hr storms. Bridge quantity and quality features to limit the site Design would be requested to analyze the discharge to the pre-developed peak flow and downstream culvert and channel for possible reduce the total suspended solids to 80%. The flooding, as channel stability and wetland current State regulations should be consulted encroachment. for design criteria. A detention pond permitted

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-27 DelDOT Road Design Manual

Figure 6B-16 Watershed for Example 6

Figure 6B-17 Flow Path of Watershed Segment Description Slope % Length (ft)

A-B Commercial and 1.0 50 Business

B-C Forest and 2.0 50 meadow

C-D Shallow gully 0.8 250

D-E Open channel 0.8 475

B6-28 Highway Drainage and Stormwater Management, Example Problems July 2008

DelDOT Road Design Manual

The first step is to establish all of the 0.8 ( 0.007 ( nL1 ) hydrologic parameters and regulatory design T t1 = 5.0 0.4 controls. The design data for this problem is 2 sP based on Example 2. The area to be developed is the upper watershed, segment A-B, From Table 3-1, TR-55, nA-B = 0.011 (asphalt) designated as Am that is to be developed from and nB-C = 0.40 (woods, light underbrush). So, "1 ac Residential" to "Commercial and 0. (0. x 50011007 )0.8 Business". A 25-yr frequency is to be used. T t 1 = 5.0 4.0 + The data developed in Example 2 applies. (0.01))2.3( The pre-development design data from 0.007 (0.40 x 50 )0.8 = 22.0 hr Example 2 is: 5.0 0.4 2 )2.3( (0.02 ) Atotal = 36.5 ac or 0.057 mi Shallow Concentrated Flow (segment C-D) = CN = 74 qd = 3.2 in assumed to remain the same. Flow depths for this type of flow are assumed to be in the 4 to tc = 0.43 hr Ia/P = 0.12 6 inch range. 3 3 qu = 575 ft s/mi/in Q = 105 ft /s 250 Determine the post-development data. T t = = 0480 hr. 2 3600 x 1.44 The weighted average curve number due to the proposed construction is computed as: Open Channel Flow (segment D-E) Σ (CN x A) 2734.25 The travel time is calculated the same as = CN = = = 74.9 ≅ 75 for the Rational Method, so Tt3 = 0.041 hr. Σ A 36.50 Therefore, from equation 6.8: Calculate Runoff Depth qd from equations 6.9 and 6.10: tc = 0.22 + 0.048 + 0.041 = 0.31 hr ()− SP )2.0 2 DETERMINE UNIT PEAK DISCHARGE = = 24 qd qU P24 + 0.8S The unit discharge, qU is determined from 1000 S −= 10 Exhibit-II, TR-55. CN Find the ratio Ia/P using Table 4-1, TR-55, or 2 using equation 6.10 and that I is assumed to ⎡ ⎛1000 ⎞⎤ a ⎢ − 2.00.6 ⎜ −10⎟⎥ be 20% of S: ⎣ ⎝ 75 ⎠⎦ q = = 28.3 in 200 200 d ⎛1000 ⎞ - 2 - 2 + 8.00.6 ⎜ −10⎟ I a CN 75 ⎝ 75 ⎠ = = = 0.11 P P24 0.6 CALCULATE t C Therefore, from Exhibit 4-II: Use the topographic data from Figures 6B- q = 660 ft3/s /sq mi/in 16 and 17. u Pond Adjustment Factor F remains F = 1.0 Sheet (Overland) Flow (segments A-B & B- p p C) this type of flow is assumed to be no CALCULATE Q greater than 3 inches in depth. Drainage Area Am = 36.5/640 = 0.057 sq mi Find Tt1 using equation 6.11: From equation 6.14:

Q25-developed = qu Am qd Fp

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-29 DelDOT Road Design Manual

= 660 x 0.057 x 3.28 x 1.00 = 123 ft3/s Therefore, the developed condition peak 3 discharge Q25 equals 123 ft /s.

Figure 6B-18 Curve Number Computations Soil Type Hydrologic Land Use Area, A CN CN x A Soil Group (Acres) (Good Hydrological Conditions)

Am C Business 94 2.50 235.00 Am C Residential, ½ Acre 80 12.60 1008.00

AdA C Residential, 1/4 Acre 83 0.75 62.25 AdA C Woods 70 1.10 77.00

TaB2 B Residential, 1/4 Acre 75 1.00 75.00

TaB2 B Woods 55 9.00 495.00

WcA D Residential, 1 Acre 84 1.75 147.00 WcA D Residential, 1/2 Acre 85 4.30 365.50 WcA D Woods 77 3.50 269.50

Total 36.50 2734.25

Figure 6B-19 Impervious Area Computations Land Use Acres Percent Impervious Impervious Acres Woods 13.60 12* 1.63 Residential 1 acres 1.75 20 0.35 Residential ½ acres 16.90 25 4.23 Residential ¼ acres 1.75 38 0.67 Business 2.50 85 2.13 Total 9.01 Percent 25 *Assumes at least a minimum future change in land use

B6-30 Highway Drainage and Stormwater Management, Example Problems July 2008

DelDOT Road Design Manual

RETENTION/DETENTION DESIGN POND VOLUME DETERMINATION The problem statement is to design a The TR-55 approximation method consists storage facility and an outfall structure that of four steps: discharges at the pre-development rate. Step 1. Based on the design frequency, Storage basins are frequently designed to determine the peak discharges Q and Q for function as a wet pond (retention facility for 1 2 the pre-and post development conditions. quantity and quality control) and a detention Based on the existing criteria, Q is used as the facility releasing runoff volume at an 1 allowable downstream peak discharge. allowable discharge rate. The design is based on hydrologic routing of inflow and outflow Step 2. Calculate the ratio Q1/ Q2 and based on several design storms and various obtain the ratio Vs/Vr from Figure 6-1, TR-55, types of outfall structures. For the example the using the Type II curve. For areas below the pond is being sized for the 25-yr storm that canal, this value would be somewhat less for may or may not be in conformance with the ratios above 0.3 and would give a greater current requirement. storage volume than is probably required. Vs is the required storage volume in acre-feet and Computer models are readily available to V is the total runoff volume in acre-feet. perform all the design steps. Even though the r example has limited the problem to a single Step 3. The runoff volume is calculated storm event, this is still a very time consuming using the equation: and substantial undertaking if done manually. V = 53.33 q A ac-ft The example will describe the process in r d m designing a stormwater management pond. Where:

The following subsection will allow the qd = Runoff depth in inches for the post- designer to manually size a trial pond to use in development condition assessing whether a proposed location is even Am = Drainage area in mi2 physically feasible and should be further evaluated through the software. The method is Therefore for this example: from TR-55. As pointed out in the document Q1 = 105 ft3/s Q2 = 123 ft3/s this method is only about 25% accurate and is to be confirmed by using complete hydrograph Q1/Q2 = 0.85 routing and volume analysis through a Previously calculated: software program. 2 qd = 3.28 in Am = 0.057 mi For generating runoff hydrographs, the State has been divided into two parts with the From TR-55 Figure 6-1, Vs/Vr = 0.2 Chesapeake and Delaware Canal as the The equation for determining this value dividing line. The reasoning is that the general is: hydrological soil conditions below the canal V provide better initial absorption rate for runoff s −= QQ + QQ )/(64.1)/(43.1683.0 2 than above the canal and that the average 21 21 Vr watershed slope is <5% thus increasing the 3 time to reach peak discharge. Therefore, the − QQ 21 )/(804.0 NRCS Type II Unit Dimensionless Using this equation the value is 0.16 (use Hydrograph is used above the Canal and the 0.2) Delmarva Unit Hydrograph is used below the Using the equation for finding V : Canal. r Vr = (53.33)(3.28)(0.057) = 9.97 ac-ft

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-31 DelDOT Road Design Manual

Step 4. Determine the storage volume, Vs liner up to the desired pond depth depending in acre-feet by multiplying the known Vs/Vr upon the potential adverse affect to the ratio of Step 2 with Vr obtained from Step 3. underlying aquifer. Therefore, The estimating procedure for sizing a pond is (1) assume a trapezoidal shape with a length V = (9.97)(0.2) = 2.0 ac-ft or s (in the direction of flow) to width ratio of 2:1 3 Vs = 86,859 ft or greater, (2) side slopes not to exceed 3:1, Using this value an approximation of the preferably greater, and (3) provide at least 1-ft average release rate can be determined as of freeboard above the highest design water follows: elevation. It is generally preferred that ponds be irregular in shape with the longest Vs is to be released over a 24-hr period, dimension being parallel to the flow. therefore: The formula for determining the volume of 86,859 ft3/ (24 hrs x 3600 sec/hr)=1.0 ft3s a trapezoidal basin is: Step 5. Determine an initial pond size using 4 DZ 32 Equation 8-8 and 8-9 in TR-55. Assume that )( ZDWLLWDV 2 +++= the side slopes are 3 to 1, therefore Z = 3. 3 Because, we have selected this slope ratio a where: safety will be incorporated in the final pond 3 layout. Slopes greater than 4:1 do no currently V= Volume (ft ) require a dry safety bench. However, ponds L = Length of bottom (ft) with side slopes 4:1 or less require a 10-foot wide aquatic bench one foot below the W = Width of bottom (ft) permanent pond level and a dry 10-foot safety D = Depth of basin (ft) bench 1 foot above the permanent pond level Z = Side slope factor (ratio Based on the site survey data there is 7 ft of horizontal to vertical) topographic relief available at the proposed pond location and the water table level allows An approximate method to estimate a for the following: trial basin knowing the storage volume is to use the equation: Bottom elevation 93.0 Aquatic bench elevation 95.0 L = 5.0 Permanent pool elevation 96.0 ⎡ 2 2 2 4rV ⎤ rZD ++− ⎢ rZD −+ )(33.5)1()()1( rZD + ⎥ Outlet structure elevation 96.0 ⎣ D ⎦ 2r Safety bench elevation 97.0 r = Ratio of width to length of basin Design water surface level 98.0 Using this equation and the available data Emergency Outfall 100.0 find the approximate size of the basin. The It should be pointed out that because the data is: 3 designer would like this pond to be a wet pond Z = 3 D = 2 ft r = 0.5 Vs = 86,569 ft and to ensure there is no rock or other Results: L = 290 ft W = 145 ft problems in constructing the pond at this location, soil borings locating the normal The depth of 2 ft is based on the depth water table and type of soils were conducted. available between the permanent pool level If there had been a possibility that the natural and the proposed outfall structure. Because of aquifer would be contaminated, it would have the inherent assumptions and approximations been necessary to provide an impervious clay used in determining stormwater values, the

B6-32 Highway Drainage and Stormwater Management, Example Problems July 2008

DelDOT Road Design Manual

additional storage created by the use of a The design has set elevation 96.0 as the bench(s) is frequently ignored and the simple elevation for any outfall structures with trapezoidal shape is used in the calculations. storage between elevation 96.0 and 98.0. The allowable storage must be at least equal to the In this example the benches have been used calculated volume of 86,859 ft3. for determining available storage volumes etc. At elevation 95.0 L and W increase by 32 ft or Elev. 99 L= 322 ft and W = 177 ft. at elevation 97.0 L Data: L = 354 ft W = 209 ft D = 2 ft Z = 3 and W increase by another 32 ft or L= 357 ft 3 and W = 209 ft. V99 = 58,503 + 154,824 = 213,329 ft or 4.9 ac-ft. Using these dimensions, calculate the available volumes at various elevations. Elev. 100 Elev. 95.0 Data: L = 354 ft W = 209 ft D = 3 ft Z = 3 Data: 3 V100 = 58,503 + 237,483 = 295,986 ft L = 290 ft W = 145 ft D = 2 ft Z = 3 or 6.8 ac-ft. V95 = 89,416 or 2.05 ac-ft At this point a review of the initial basin Elev. 96.0 Permanent Level configuration would indicate that the storage Data: volume between elevations 96.0 and 98.0 is L = 290 ft W = 145 ft D = 2 ft Z = 3 3.15 ac-ft and is considerably larger than the 2.0 ac-ft required. Therefore iterations of V = 89,416 or 2.05 ac-ft plus the 96 several dimensions would probably be volume between elev.95.0 and 96.0 using L necessary. One of the easiest solutions would = 322 ft W = 177 ft D= 1 ft and Z= 3 the be to use the initial V divide by the allowable additional volume is 58,503 for a total s depth to find the required surface area at the volume at the permanent pool elevation of design storage elevation. Than using the 147,919 ft3 or 3.4 ac-ft. desired between length to width of at least 2:1 For the storage calculations using the as selected in the example, L = 2W in the wet pond concept the available storage equation L x W = Surface area and making the volume to be released at elevation 96.0 is substitution, 2W2 = Area. Therefore: 0.0. The storage volume available from V /2 =A or 86,569/2 = 43285 ft2 Elevation 96.0 to 97.0 is: s 2W2 = 43285 or W = 147 and L = 294.

Elev. 97.0 These two dimensions apply at elevation 98.0 on a trapezoidal shape having a bottom Data: L = 322 ft W = 177 ft D= 1 ft and elevation of 93.0. At this elevation, the two Z= 3 dimensions would be reduced by 30 feet 3 V97 = 58,503 ft or 1.27 ac-ft. would result in a trial size of L = 260 and W = 130. The volume equation for a trapezoid Next, find the volume at Elevation 98.0 allows for a quick check of available storage Elev. 98.0 between 93.0 and 96.0 V = 2.36 and between Data: 93.0 and 98.0 V = 4.49. The available storage The volume at 98.0 includes the volume would be the difference between these two available at 97.0 (58,503 ft3) plus the volume values or 2.13 ac-ft. This value is closer to the between these two elevations. required volume. The proposed two benches Data: L = 354 ft W = 209 ft D = 1 ft Z = 3 will provide the additional storage to meet the inherent margin of error recognized in 3 V98 = 58,503 + 75,687 = 134,190 ft drainage design. Figure 6B-21 shows the or 3.1 ac-ft. tabulated values using the second trial size.

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-33 DelDOT Road Design Manual

Figure 6B-20 Trial 1 (L=290 ft & W=145ft) Stage Storage Elevation Surface Area Cumulative Storage Usable Storage (ac) (ac-ft)* (ac-ft)** 93 0.97 0.00 0.00 94 1.03 1.0 0.00 95 1.09 2.05 0.00 95.1*** 1.31 2.19 0.00 96 1.38 3.39 0.00 97 1.45 4.8 1.41 97.1 1.70 5.05 1.66 98 1.78 6.54 3.15 99 1.86 8.35 4.96 100 1.94 10.25 6.86 *Assumes pond is completely empty and is used for calculating quality control storage later. **Assumes pond is full to Elevation 96.0 (permanent pool). ***Often on the first trial the bench elevations are not known. In this problem, assume there is a requirement for a permanent pool at Elevation 96.0, so the benches go at 95.0 and 97.0.

Figure 6B-21 Stage-Storage Trial 2 (L=260 ft & W=130 ft) Elevation Surface Area Cumulative Storage Usable Storage (ac) Volume (ac-ft)* Volume (ac-ft)** 93 0. 78 0.00 0.00 94 0.83 0.80 0.00 95 0.88 1.66 0.00 95.1*** 1.08 1.77 0.00 96 1.15 2.78 0.00 97 1.21 3.96 1.18 97.1 1.44 4.11 1.33 98 1.52 5.44 2.66 99 1.59 7.0 4.22 100 1.66 8.62 5.84 *Assumes pond is completely empty and is used for calculating quality control storage later. **Assumes pond is full to Elevation 96.0 (permanent pool). ***Often on the first trial the bench elevations are not known. In this problem, assume there is a requirement for a permanent pool at Elevation 96.0, so the benches go at 95.0 and 97.0.

If the designer is confidant that the initial pond, generate an outflow hydrograph based trial size is suitable for the designated site, on the selected principal and emergency then the design can be refined and verified spillway configuration. The designer selects through the appropriate computer software. and inputs spillway types and sizes considered The necessary input data for the software has appropriate for the site and downstream been generated and the program will develop conditions. This structure could be; a concrete and route an inflow hydrographs through the wall built into the embankment with one or

B6-34 Highway Drainage and Stormwater Management, Example Problems July 2008

DelDOT Road Design Manual

series of broad crested or v-notch weirs; or a met. If not adjustment in input data are made riser pipe with an outfall pipe (barrel); or a and the program rerun. rectangular riser structure with one or more Now that the first goal of sizing the pond weir and orifice openings at different and determining the spillway design has been elevations to control a combination of storm met, the other required post-development frequencies and an outfall pipe. Risers with a storm flows have to be routed through the pipe outlet are frequently referred to as drop pond to verify that the design also meets the inlets. criteria set for them. Normally, the analysis There are several types of flow calculations will include the 2-, 10-, and 100- year storms that have to be made. The pipe discharge flow are evaluated. Current regulations set which capacity of any outfall pipe, the riser flow one of these will control the pond size. The based on weir or orifice flow conditions, and stage-discharge data for all the storms are used weir flow when using either rectangular or to confirm any predetermined pond elevations trapezoidal (V-notch) shaped openings in the such as benches. Any requirement for riser structure. In undeveloped areas with a providing a permanent pool volume also natural outfall stream or channel, the broad enters into this evaluation. Usually the 100-yr crested weir or the V-notch weir without a storm flow sets the emergency spillway riser structure is more economical and can be capacity and required total depth of the pond. constructed as part of the pond embankment. As initially stated using computer software Some of these flow analysis have to include is almost a necessary in order to select the structure losses, i.e. friction and various most feasible and economical design. tailwater conditions. The software will include However, the designer’s input data and an analysis of the emergency spillway based choices in alternative design selections are on the designer’s input data. The final product crucial in making this happen. HEC-22 and is the stage-discharge relationship shown as a the other resources listed in Section 6-10, 11 curve or tabulation that sums all flows through and 12 have detailed examples for pond both spillways based on the water depth in the design. pond. Using this data, the designer can determine if all design parameters have been

July 2008 Highway Drainage and Stormwater Management, Example Problems B6-35