Town of Ocean View, DE Drainage and Storm Sewers

When open channels are proposed, an engineering analysis shall be submitted supporting the proposal with respect to feasibility, capacity design, and soil stabilization within the channel.

The engineering analysis shall adhere to the following requirements:

(1)

Open channels shall be designed to adequately convey a twenty-five-year storm frequency. The depth of the water in the ditch shall not be higher than six inches below the edge of the proposed top of bank for contributory drainage areas less than five acres and not higher than one foot below the proposed top of bank for contributory drainage areas greater than five acres;

(2)

Documentation of the seasonal high water table elevation with respect to the proposed channel depth shall be provided to evaluate the potential of the ditch intersecting the groundwater table causing the channel to receive greater flows than design capacity and the possibility for continually wet surfaces within the channel;

(3)

Channels shall be trapezoidal or parabolic in shape. In either case, accessibility for maintenance shall be considered in the design;

(4)

Maximum side slopes: three feet horizontal to one foot vertical (3:1) for unpaved (vegetated side slopes) and two feet horizontal to one foot vertical (2:1) for paved side slopes. Paved side slopes shall only be implemented in locations where the required conveyance capacity requires paved side slopes, or existing conditions physically prohibit the width requirements of three feet horizontal to one foot vertical (3:1) side slopes;

(5)

Minimum bottom width shall be three feet where the depth exceeds one foot;

(6)

Minimum design velocity shall be two feet per second, unless otherwise approved by the Town Engineer;

(7)

The maximum design velocity in a grassed channel shall not exceed the permissible values stated in the Standards and Specifications for Grassed Waterways section of the Delaware Erosion and Sediment Control Handbook for Development, except as provided with structural measures as detailed therein. Also as noted therein, sustained wet conditions in a channel are not amenable to the establishment of adequate vegetative cover; therefore, where conditions will exist which indicate a base flow in the channel, the channel shall be provided with a stone center according to the specification noted above;

(8)

If designed with a paved bottom, the bottom shall be reinforced Portland cement concrete with a minimum six-inch thickness;

(9)

All other standards and specifications contained in the Grassed Waterway and Lined Waterway or Outlet sections of the Delaware Erosion and Sediment Control Handbook for Development shall apply;

(10)

The maximum flow depth for the design storm event in an open channel on a residential lot and/or the easement adjacent to a residential lot shall be one foot. The maximum channel depth shall be two feet. The depth at any given cross section shall be measured from the bottom or the channel to the height at which the side slope becomes less than five feet horizontal to one foot vertical; and

(11)

A backwater analysis will be required by the Town to assess the impact of post-development flows if apparent constrictions exist downstream.

Cover over pipe shall meet the manufacturer's recommended cover requirements. In no case shall the minimum soil cover over pipe be less than one foot without approval of the Town.

Inlet and grate specifications design shall conform to DelDOT standards.

Rock outlet protection, designed per the Rock Outlet Protection section of the Delaware Erosion and Sediment Control Handbook, shall be provided at each outfall.

Drainage easements of a minimum twenty-foot width shall be provided where closed drainage systems are proposed. In subdivisions, storm sewers located outside of the Town right-of way shall be located on properties lines unless otherwise approved by the Town.

A backwater analysis may be required by the Town to assess the impact of post-development flows if apparent constrictions or high tail water conditions exist downstream.

Conveyance systems shall outfall to a stormwater management facility or watercourse capable of accepting the design runoff.

The following criteria shall be used for storm sewers:

(1)

A twenty-five-year storm frequency shall be used;

(2)

The hydraulic gradient shall be no higher than one foot below the top of the grate for twenty-five-year storms, unless otherwise approved by the Town;

(3)

The following criteria shall be used in calculating HGL:

(a)

Tailwater elevation of the outfall, if it is higher than the normal crown of the outfall pipe; or

(b)

Normal crown of the outfall pipe. For wet ponds, the pipe invert elevation shall be equal to or higher than the normal pool elevation of the pond, unless otherwise approved by the Town Engineer.

(c)

Minimum pipe size shall be 15 inches in diameter, unless otherwise approved by the Town Engineer.

For a storm drain system discharging into a stream, the pipe invert elevation of the discharging pipe shall be no lower than the level of the base flow. If the stream is dry most of the time, the invert shall be at least a foot above the stream bottom, unless otherwise approved by the Town Engineer. The HGL shall start from the crown of the pipe.

A "culvert" is a drainage structure that transports water from a natural drainagecourse. The following criteria shall be used for culverts:

(1)

A twenty-five-year storm frequency shall be used;

(2)

The maximum headwater elevation shall be one foot below the edge of the proposed roadway shoulder or other embankment. The resulting ponding shall not negatively impact the street or the adjacent property;

(3)

Outlet protection, according to the Rock Outlet Protection Section of the Delaware Erosion and Sediment Control Handbook for Development, must be provided at all culverts.

(4)

The minimum pipe size shall be 18 inches in diameter.

Culverts and storm sewers can be made from reinforced concrete pipe (RCP) or high-density polyethylene (HDPE). Metal pipe will only be allowed if coated (such as aluminized pipe) in order to provide a minimum fifty-year life span. Minimum pipe cover shall be in accordance with the pipe manufacturers' minimum cover requirements. The actual cover and minimum cover requirements shall be provided in the drainage calculation report.

Inlet design shall be in accordance with the following criteria:

(1)

A twenty-five-year storm frequency.

(2)

The spread of water shall be no greater than eight feet from the flow line of the curb in streets. For areas other than streets or paved areas, inlets shall be located to ensure that a minimum of 95% of the flow is captured by the inlet.

(3)

Maximum spacing of inlets is 300 feet.

(4)

In order to maintain structural integrity, inlet box corners shall not be cut. In order to ensure proper size, DelDOT tables for angle of deflection for pipe entering and exiting inlet boxes shall be used.

Drainage design report containing the following minimum data shall be prepared for each project:

(1)

Drainage area plan.

(2)

Time of concentration.

(3)

Weighted runoff coefficient.

(4)

Design discharge.

(5)

Type and slope of drainage facility.

(6)

Spacing of drainage inlets.

(7)

Erosion protection methods: riprap sizing calculations.

(8)

Inlet spread calculations.

(9)

Culverts: headwater elevations.

(10)

Hydraulic grade line (HGL) calculations.

(11)

Full flow pipe velocity.

(12)

Actual flow pipe velocity.

(13)

Difference between inlet grate elevation and HGL elevation.

Personnel grates shall be installed on pipe inlets to improve safety by preventing people, animals and debris from entering 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.

(1)

Since safety grates become blocked by debris, thus potentially decreasing drainage flow and increasing maintenance needs, engineers shall evaluate designs to avoid open inlets to stormwater pipes. One alternative to consider is creating a separation of pipe runs by realigning pipes in a long pipe run into relatively short, straight runs, with daylight visible from the openings.

(2)

A personnel grate for a pipe inlet shall be considered as the last resort in designing inlets for storm drain systems. Grates shall be secured to prevent entry by the public but allow access to the storm drainpipe for maintenance and repair. Construction details and specifications shall be included in the construction plans.

The Rational Method of estimating the storm runoff, shown in Figure 1, shall be utilized for all storm sewers and waterway openings equal to or less than 19 square feet. The proposed method for computing the storm runoff requiring a waterway opening greater than 19 square feet shall be approved by the Town Engineer.

Manning's Equation, shown in Figure 2, shall be utilized to determine the flow of water in open channels.

Any structure, including supports, erected over a depression or an obstruction, such as water, a highway or a railway, for carrying vehicular or pedestrian traffic will be reviewed by the Town Engineer. All structural designs shall be in accordance with DelDOT's Bridge Design Manual and AASHTO's (American Association of State Highway Transportation Officials) Load and Resistance Factor Design (LRFD) Bridge Design Specifications. If there are structural designs required on a plan and not included in the DelDOT Standard Construction Details, shop drawings signed and sealed by a professional engineer registered in the State of Delaware shall be submitted for review and approval.

The outfall shall be carried to a point of positive outfall in order to prevent downstream flooding. A detailed hydraulic and stormwater analysis downstream to a point where the first downstream tributary and/or closed pipe system whose drainage area meets or exceeds the contributing area from the property to be developed shall be required to determine the impacts to the drainage system and to ensure that stormwater impacts for surrounding property owners is minimized.

New or improved drainage conveyance systems shall be designed and constructed to require economical maintenance. Improvements to watercourses in existing developments must be designed and constructed to retain the character of the surrounding area as much as practicable. Adequate rights-of-way must be provided for access for construction and afterwards for maintenance.

An as-built topography plan of open channels that drains an area greater than one acre shall be provided for review and approval by the Town. The as-built shall include profiles and cross sections at fifty-foot stations and computations that demonstrate that the channel meets design objectives. All stormwater piping systems shall be as-built upon completion of construction. As-built drawings shall be signed and sealed by a registered professional surveyor and submitted to the Town for review and approval prior to the issuance on building permits.

Stormwater management basin/pond as-builts shall meet the requirements of the Sussex Conservation District.

Figure 1. Rational Method of Design
Q = C/A
Where:
	Q	=	Rate of runoff in cubic feet per second.
	C	=	Weighted runoff coefficient (average of the coefficients assigned to the different types of contributing areas).
	I	=	Average rainfall intensity, inches per hour, for the selected frequency and for duration equal to the time of concentration.
	A	=	Drainage area, in acres, tributary to the point under design.
Values of runoff coefficient (C) for various types of contributing areas, indicated in Table 1 and Table 2, shall be utilized in the solution of this method of design.

Table 1. Runoff Coefficient (C) for Use in Rational Method
Type of Surface	Runoff Coefficient (C)	Notes
Rural areas	0.15 to 0.3	1. For flat slopes or permeable soil, use the lower values. For steep slopes or impermeable soil, use the higher values. 2. Contributing drainage areas shall be evaluated based on the fully developed land in accordance with the existing or proposed zoning.
Concrete or sheet asphalt pavement	0.8 to 0.9
Asphalt macadam pavement	0.6 to 0.8
Gravel roadways or shoulders	0.4 to 0.6
Bare earth	0.2 to 0.9
Steep grassed areas (2:1)	0.5 to 0.7
Turf meadows	0.1 to 0.4
Forested areas	0.1 to 0.3
Cultivated fields	0.2 to 0.4

	Table 2. Runoff Coefficient (C) for Different Types of Surfaces
	Type of Surface	Runoff Coefficient (C)
	Flat residential, with about 30% of area impervious	0.40
	Flat residential, with about 60% of area impervious	0.55
	Moderately steep residential, with about 50% of area impervious	0.65
	Moderately steep built-up area, with about 70% of area impervious	0.80
	Flat commercial, with about 90% of area impervious	0.80

Figure 2. Manning's Equation
V = 1.49 R^2/3 S^1/2/n
Where:
	V	=	Velocity in feet per second.
	n	=	Manning's coefficient of channel roughness.
	R	=	Hydraulic radius in feet.
	S	=	Slope, in feet per foot.
Values of Manning's coefficients (n) for various types of channel linings shall be used as indicated in Table 3.

Table 3. Manning's Roughness Coefficients (n)
Type of Conduit		Roughness Coefficient (n)
Closed Conduits
	Reinforced concrete pipe (RCP)	0.012
	Corrugated metal pipe (CMP) or pipe arch plain or fully coated (unpaved)	0.024
	High-density polyethylene (HDPE)	0.011
	25% of circumference paved	0.021
	Fully paved	0.012
Lined Open Channels
	Concrete (float finish)	0.014
	Plain riprap	0.040
	Grouted riprap	0.035
Swales And Channels With Maintained Vegetation
	Grass (mowed to 2 inches)	0.045
	Grass (good stand: 12-inch height)	0.09
Street Gutters
	Concrete	0.012
	Asphalt	0.013