Delaware Administrative Code
Title 2 - Transportation
2000 - Department of Transportation
2300 - Division of Planning and Policy
2309 - Development Coordination Manual
Chapter 5 - DESIGN ELEMENTS
Section 5.7 - SUBDIVISION DRAINAGE DESIGN
Current through Register Vol. 28, No. 3, September 1, 2024
5.7.1 Subdivision Drainage Design - General
Surface runoff water is a serious threat to both the physical integrity and the serviceability of roadway facilities. Runoff water must be adequately controlled so that it may pass through and be removed from the roadway area without damaging the roadway or adjacent properties, or creating safety issues.
As part of the overall design, the developer's engineer shall provide adequate drainage of the roadway and the site in accordance with all applicable standards. DelDOT maintains the authority to dictate drainage design methods and drainage design systems for all facilities that impact the State right-of-way.
The following sections apply to drainage design within subdivisions. For drainage and stormwater management design of entrances, offsite and frontage road improvements, applicable regulations shall be followed and relevant guidelines, standards and DelDOT policies should be applied to the design process, such as: Chapter 6 Drainage and Stormwater Management of DelDOT's Road Design Manual; and the reference materials found on DelDOT's Website under the Design Resource Center's Hydraulics and Hydrology Tab (http://deldot.gov/information/business/drc/hydrology.shtml).
5.7.2 Subdivision Drainage Design - Drainage Criteria
Drainage criteria for different drainage installations are discussed below and summarized in Figure 5.7.2-a.
Open channels are a commonly used component of a drainage system. They include natural/manmade ditches, streams, median swales and gutters.
Open Channel/ Roadside ditch design within subdivisions shall generally be in accordance with the design methodology described in DelDOT's Road Design Manual. The following specific criteria shall be used in the design of roadside ditches within subdivisions:
A culvert is an open-ended drainage structure which transports water between drainage courses.
Based on the peak flow and watershed area, an appropriate method for determining runoff shall be determined. The following criteria shall be used for culverts:
See Hydraulic Design Series Number 5 (HDS 5), Hydraulic Design of Highway Culverts, September 2001, USDOT, FHWA.
The following criteria shall be used for storm sewers within subdivisions:
See DelDOT's Road Design Manual, Chapter 6 Drainage and Stormwater Management, for additional guidance and information on storm sewer design.
Culverts and storm sewers can be made from Reinforced Concrete Pipe (RCP), Metal Pipe (MP) or High Density Polyethylene (HDPE). Refer to DGM 1-20 Pipe Materials for further information.
Inlet design for subdivision streets shall be in accordance with the design methodology described in DelDOT's Road Design Manual. The following criteria shall be used for inlets:
Appropriate care must be taken in the layout, selection and sizing of drainage components, to ensure that adequate cover is provided over culverts and storm sewers below roadways and at storm sewer connections with drainage structures, to help assist with the cataloguing and review of these elements, DelDOT's Pipe Cover/Pipe Angle Worksheet, availableonline at http://devcoord.deldot.gov > Forms, shall be completed by the engineer and submitted for review. Pipe Materials including, rigid and flexible pipe, shall require a minimum allowable cover, Hmin, as shown on Figure 5.7.2.5-a, which has been adapted from DelDOT's DGM 1-20 Pipe Materials. When rigid pipe is proposed under bituminous concrete (asphalt) pavement, extends into the bituminous concrete (asphalt) pavement section and Hmin is met, then bituminous concrete (asphalt) pavement should be placed directly over the pipe. For this condition, refer to Figure 5.7.2.5-b to see examples for existing and proposed pavements. In addition, use of elliptical reinforced concrete pipe (ERCP) should be considered to increase pipe cover under the pavement when Hmin cannot be provided. On roadways listed on the National Highway System where the minimum allowable cover requirement cannot be met due to field conditions, DelDOT may approve reinforced concrete pipe having AASHTO HL-93 loads as specified in the American Concrete Pipe Association's (ACPA) Concrete Pipe Manual. This will require pipe design calculations to be prepared and submitted for review and approval prior to issuance of construction plan approval. The National Highway System map can be found on DelDOT's website. Under special circumstances, on roadways not listed on the National Highway System, where the minimum cover requirement cannot be met due to field conditions, DelDOT may approve the use of class V RCP.
Figure 5.7.2.5-a Material Based Pipe Cover Requirements (Standard Installation)
Figure 5.7.2.5-b Existing Pavement Shallow Pipe Cover Details
Drainage easements are required for all drainage facilities handling roadway runoff which are not located within a dedicated right-of-way. DelDOT may authorize the use of such drainage easements for accessing and removing downstream blockages that impede roadway runoff from the state maintained portion of the system. Downstream easements do not imply or create a maintenance responsibility on the part of the Department, but communicate the underlying authority to take emergency or corrective action if the Department believes it is appropriate. The Department will not maintain portions of the drainage easement and system that are exempted under Section 3.2.5.1.4 (such as portions that are not directly collecting and conveying the drainage runoff of the proposed State maintained roads and/or rights-of-way, as well as portions that are located upstream or are offsite). Underground drainage facilities shall require a 20-foot drainage easement. The pipe must be located in the center of the easement. Open drainage facilities shall require a width equal to the width of the facility plus a 10-foot easement on one side and a minimum of a 5-foot easement on the other side of the open drainage facility, as measured from the top of slope. Where the top of slope is undefined, measure from the 10-year water surface elevation. Refer to Figure 5.7.2.6-a for additional detail.
Figure 5.7.2.6-a Drainage Easement Requirements
A drainage report shall be submitted with the construction plans to verify pipe sizing, Hydraulic Grade Line (HGL), pipe cover and angles, velocities, channel lining and water spread on the roadway.
A drainage design report containing the following minimum data shall be prepared for each project:
See DelDOT's Design Resource Center, Hydrology & Hydraulics Section, for a sample drainage report.
See Chapter 6 Drainage and Stormwater Management of DelDOT's Road Design Manual for rainfall intensity estimates and depths that are to be used in the preparation of the drainage report.
Figure 5.7.2.7-a Subdivision Streets Drainage Criteria
* The 25-year storm event will also be evaluated for impacts.
Safety grates shall be installed on the inlets of stormwater pipes to improve safety on all stormwater pipes 12 inches and larger with open inlets (i.e., without a grate or drainage inlet) for which full daylight is not visible when looking through the pipe to the other end, regardless of length. Refer to DelDOT Standard Construction Details for applicable safety grate for each installation. Efforts shall be made to minimize use of open end pipe inlets which require safety grates.
5.7.3 Subdivision Drainage Design - Hydrology
Hydrology shall be analyzed using acceptable calculation methods (e.g. Rational Method, TR-55, etc.). Additional guidance on hydrologic calculations can be found in Chapter 6 Drainage and Stormwater Management of DelDOT's Road Design Manual, TR-55, HEC-22, etc.
5.7.4 Subdivision Drainage Design - Hydraulics
Manning's Equation shall be utilized to calculate the flow of water in open channels. Additional guidance can be found in Chapter 6 Drainage and Stormwater Management of DelDOT's Road Design Manual.
