Engineered Stormwater Controls

Prepared by: AG 473-9
Prepared by Barbara Doll
Coastal Water Quality Specialist
North Carolina Sea Grant College

Published by: North Carolina Cooperative Extension Service

Publication Number: AG 473-9

Last Electronic Revision: March 1996 (JWM)

Complying with the Water Supply Watershed Protection Act

New regulations have been developed to protect water quality in areas of North Carolina where surface waters streams, rivers, and lakes—are used as public drinking water sources. The regulations affect land use and development density. In some cases, they reguire that pollution control measures be used to help keep contaminantsfrom entering waterways. Engineered stormwater controls are a type of control measure that will be required in some situations. Thisfact sheet describes engineered stormwater controls, explains how they work, and discusses their design, construction, operation, maintenance, and costs.

Many North Carolina municipalities use surface waters as their drinking water supply. Public concern about protecting these water sources from pollution has increased in recent years as the state has experienced rapid residential and commercial development. Most of the water in our streams, rivers, and lakes comes from runoff—rainwater that runs off of surrounding land areas.

All of the land that drains into a glven body of water is called a watershed. When a watershed is in its natural state, some of the rain that falls soaks into the soil, and some flows across the surface into nearby bodies of water. The portion that runs off is usually filtered by natural vegetation. When these areas are developed, however, the amount of surface area that water cannot penetrate increases. Rain that falls onto rooftops, parking lots, sidewalks, roads, and other impervious surfaces within a development cannot infiltrate the soil. Instead, it runs off into stormwater drainage systems or directly into nearby bodies of water. As it moves, it picks up pollutants, including sediment, metals, nutrients such as nitrogen and phosphorus, and organic wastes. When this contaminated stormwater enters streams, lakes, and rivers, it degrades their water quality.

Concern about preservmg the quality of water in public drinking water sources led to the passage in 1989 of the Water Supply Watershed Protection Act (WSWPA). The Act empowers the state's Environmental Management Commission (EMC) to develop regulations to protect water quality in areas designated as water supply watersheds - —land areas that drain into lakes and rivers used as public water sources. The Commission has adopted regulations, various parts of which are to become effective during 1993 and 1994.* The regulations restrict development density, limit land uses, and in some cases require the use of pollution controls such as stream buffers and engineered stormwater controls in developing areas.

Counties and municipalities that have water supply watersheds within their land-use jurisdiction are required to develop watershed protec tion plans and ordinances that meet or exceed state guidelines. For planning purposes, water supply watersheds have been divided into five classifications, designated WS-I through WS-V, according to their characteristics. The requirements vary among Figure 1. A these classifications, depending in part on whether the local government chooses to allow high-density or low-density development. For more information on watershed classifications and develop- ment options, see Extension Service publication AG-473-8, Economic Impacts of the Water Supply Watershed Protection Act.

When high-density development is permitted in water supply watersheds classified WS-II, WS-III, and WS-IV, the increased runoff from a newly developed area must be managed by engineered stormwater controls, which consist primarily of wet detention ponds (Figures 1 and 2). These ponds are embankment facilities that collect and hold water after rainfalls. They are designed to contain some water at all times. Although originally developed to prevent flooding, wet detention ponds also protect water quality by removing some of the pollutants from runoff. Because they retain runoff, the ponds allow more time for water to infiltrate the soil, for solid particles to settle out, and for biological processes to remove soluble pollutants such as nutrients and organic wastes. Wet detention ponds are more effective in removing sediment and other pollutant particles than dissolved pollutants. The average percentages of pollutants removed are shown in Table 1.

(*) Legislation was introduced into the North Carolina General Assembly dunng the 1993 session to postpone these deadlines for two years. However, it was not passed.

Figure 1 is not available at this time

Table 1. Efficiency of Wet Detention Ponds in Removing Pollutants


                       Average Removal
Type of Pollutant     Efficiency (%)
--------------------------------------------
Total phosphorus            50

Total nitrogen              30

Lead                        80

Zinc                        50

Copper                      70

Biochemical oxygen demand   30

Sediment                    90

Engineered stormwater controls are usually designed to accommodate the runoff from a single new development, although they can be sized to serve a large area. The land developer normally bears all design and construction costs and is responsible for the operation and maintenance of the system over the long term.

The following sections describe the design, construction, operation, and maintenance of these systems, with attention to costs as well as some of the hazards and constraints involved.

Design Requirements

Responsibilities of Local Governments

Figure 2 is not available
All engineered stormwater controls must be designed in accordance with N.C. Coastal Stormwater Disposal Regulations as described in Administrative Code Section 15A NCAC 2H .1003(g) (2), (i), (k) and (l).

Local governments must include these minimum standards in their water supply watershed protection ordinance if they choose the high-density development option. They may also adopt stricter require-ments. The regulations require that all engineered stormwater controls be designed by a North Carolina registered professional engineer, landscape architect, or land surveyor with appropriate qualifications. Local governments are responsible for approving the final design of all engineered stormwater controls after they have been approved by a registered profes- sional.

Pond Size

Wet detention ponds must be designed to maintain a minimum mean depth of 3 feet, which is called the permanent or normal pool. However, it is permissible for the water level to drop below the permanent pool depth during extended dry periods.

Wet detention ponds must also be designed to store runoff from the first inch of rainfall above the permanent pool depth. The volume of runoff produced by 1 inch of rainfall must be calculated for the entire area that drains into the pond, not the development area only. All existing land use within that drainage area, as well as the proposed development, must be included in this calculation. Therefore, the size of the pond depends upon the size of its contributing drainage area and the land use of that area.

The Division of Environmental Management (DEM) has approved methods for sizing wet detention ponds to remove 85 percent of total suspended solids from the permanent pool and from the runoff collected from the first inch of rain that falls on the pond drainage area during each storm. By this method, the required minimum ratio of permanent pool surface area to the total area that drains into the pond is a function of the mean depth and the percentage of impervious surface within that drainage area. In determining the percentage of impervious surface, the designer must include existing impervious surfaces within the pond's drainag area plus those of the proposed development.

Although it is possible to locate a detention pond within a streambed, the DEM discourages that practice because of the potential for degrading the stream's water quality below the goals of the Clean Water Act. Placing the pond outside of the streambed helps to prevent sediment and pollutants from ever entering the stream. It also helps to prevent collecting runoff from upstream areas outside of the develop- ment, thus minimizing pond size.

Local governments can require all calculations of pond area and runoff volume to be based upon the maximum potential future development (that is, the highest possible percentage of impervious surface) allowed by zoning ordinances within the pond watershed. In many cases, it is easier and less expensive to build the facility large and grow into it rather than upgrade or add new facilities once development takes place upstream. In this case, future upstream developers may have to compensate the pond owner for the cost of building the facility and for annual operation and maintenance expenses.

Inlet Structure and Forebay

The pond inlet structure must be designed to minimize turbulent flow of the water entering the pond. Turbulence can mix the pond water, resuspending sediment and pollutants. Bafffles and other design features are effective in minimizing turbulence. A forebay must be included at the head (input) of the detention pond to trap incoming sediment before it reaches the pond. The forebay can consist of either an excavated settling basin or a headwater segment of the pond that is separated by a weir. The forebay volume should be approximately 20 percent of the total pond volume or able to store the predicted 20- to- 40-year sediment load. It can also help ease the burden of sediment removal. Pond inlet structure and forebay are shown in Figure 2. In addition to the space in the forebay, it may help to provide some sediment storage area in the upper reaches of the pond in order to minimize the frequency of sediment removal.

Outlet Structure

The outlet structure controls the rate of discharge from the pond. It must be designed so that the runoff from 1 inch of rainfall will be retained above the permanent pool for at least two but not more than five days. For safety reasons a trash hood should be included to prevent clogging by litter and debris. A typical pond outlet structure (Figure 2) consists of a riser and barrel system as the primary spillway; a small, negatively sloped pipe for drawdown of the l-inch rainfall; and an emergency spillway for over-flow during very large storms. The riser and barrel should be large enough to prevent clogging.

Pond Dimensions

Although not specified in the regulations, it is suggested that the pond have a minimum length-to-width ratio of 2 to 1 in order to prevent "short-circuiting" —that is, the rapid flow of incoming water through the center of the pond directly to the outlet. Short-circuiting reduces pollutant removal efficiency because the water remains in the pond a shorter time. Experience suggests that ponds are less likely to short-circuit if their length-to-width ratio is 3 to 1 or greater.

Shoreline slopes should be gradual enough to allow safe and easy access and routine mowing. Slopes with a 3-to-1 ratio (horizontal to vertical) or flatter are preferable.

Pretreatment Options

Although wet detention ponds are the primary component of most engineered stormwater controls, they can be used as a secondary treatment device following pretreatment by an infiltration system. Infiltration systems are intended to reduce the pollutant concentration and amount of runoff that reaches the wet detention pond. They are usually effective at minimizing sedimentation and eutrophication of the pond. (Eutrophication occurs when excessive nutrients enter a pond, resulting in overproduction of plant life and subsequent depletion of dissolved oxygen.)

Infiltration systems can consist of such devices as gradually sloped grassed swales or filter strips that drain into the pond.* These devices reduce runoff velocity, allowing time for suspended particles to be deposited and for some of the water to infiltrate the soil. The vegetation acts as a filter to remove some of the nutrients and other pollutants.

It is also beneficial to establish wetland vegetation along the perimeter of the pond to enhance the biological uptake of soluble pollutants, in particular nitrogen and phosphorus. This vegetation also protects against shoreline erosion and provides habitat for wildlife. However, some vegetation, such as cattails, can become so densely established that it significantly reduces the available volume of the treatment pond.

Other Design Elements

Other key design considerations include suitable access for maintenance and inspection, an adequate sediment disposal area, and public safety precautions. If the facility is located close to areas frequented by children, it may be necessary to fence the pond. Most safety hazards can be minimized by keeping the pond fairly shallow, avoiding steep shoreline drop-offs, maintaining gradual side slopes, installing an underwater safety bench, and securing spillway outlets.

(*) Grassed swales are shallow, grass-covered ditches. Filter strips are nearly flat areas of land planted with vegetation selected for its ability to remove pollutants from runoff, which flows over the area as a thin sheet.

Constraints and Hazards

There are several constraints to using engineered stormwater controls. In some regions of the state, such as the coastal plain, small pond drainage areas (less than 25 acres) may not contribute enough runoff to maintain the minimum required mean depth of water. In very large drainage areas, the required detention pond may be too expensive to construct; large facilities may require a dam safety permit. If wet detention ponds are to be located in areas classified as wetlands, wetland disturbance permits will be required.

In developments where land area is limited, there may not be enough room for the necessary stormwater controls. In some cases the pond configuration may be restricted in a way that prevents achieving the desired minimum length-to-width ratio, resulting in short-circuiting of flow.

When runoff is retained in shallow wet detention ponds, water temperatures often rise. Therefore, these ponds may be inappropriate in mountain trout streams and other areas where the discharge of warmer water may be harmful to the aquatic fauna. During summer, there may be periods when the pond's dissolved oxygen is depleted. Release of flow with extremely low oxygen levels may harm aquatic life immediately downstream from the pond.

Construction

Proper construction is vital to the future operation of a wet detention pond and will help ensure low operation and maintenance costs. Although not required, it is advisable for the local government to have a qualified staff member inspect the facility while the pond is under construction. Problems may be visible during construction that cannot be seen during a final inspection.

After construction has been completed, a registered professional with appropriate qualifications and experience must inspect the facility and certify that it was properly built in accordance with the approved plans and specifications. Detailed information on construction components and procedures for wet detention ponds are presented in North Carolina's Erosion and Sediment Control Planning and Design Manual in the section entitled "Riser Sediment Traps"; in Soil Conservation Service (SCS) Technical Guideline 378-1; in the Dam Safety Act; and in chapters 11 and 17 of the SCS Handbook. Key construction elements to check include:

Operation and Maintenance The Operation and Maintenance Plan

The developer must provide a plan or manual that specifies (1) the operation and maintenance activities needed, (2) quantitative criteria to determine when each activity should be performed, and (3) who is responsible for carrying out these activities. The plan must also outline a step-by-step procedure for restoring a stormwater control to its design standard following a failure. The party responsible for each step must be specified in the plan. Local governments are responsible for approving the operation and maintenance plan submitted by the developer.

Routine Inspections

Municipalities are required to inspect engineered stormwater controls at least once a year to determine whether they are performing as they were designed to. The local government can charge a fee for these inspections. It is also advisable to inspect these facilities after major storms. Records of inspections must be maintained on forms approved or supplied by the DEM. A sample inspection sheet is provided in the DEM's model water supply watershed protection ordinance. If stormwater controls are not performing adequately at the time of inspection, the local govern ment must order the facility owner to take corrective action. If the owner does not take the necessary actions, then the local government may impose civil penalties or other legal actions. Local governments are ultimately responsible for operation and maintenance of all engineered stormwater controls.

Maintenance

Wet detention ponds must be properly maintained to prevent future malfunction and to enhance the aesthetics of the pond area. Routine maintenance tasks include mowing, inspection, removal of debris and litter, control of erosion, and management of insect pests, weeds, and odors. Large flow volumes and high velocities can result in eroded channels upstream and downstream of the detention pond. The embankment and the emergency spillway may also slump or erode. It may be necessary to regrade, revegetate, and replace or repair riprap to correct these situations.

How often these maintenance activities must be carried out varies from one site to another, according to pond usage and location. A typical maintenance schedule calls for 2 to 15 mowings per year, one routine inspection per year with additional inspections after major storms, removal of debris and litter 15 to 20 times per year, and erosion and pest control as needed. Routine maintenance is essential and should decrease both the frequency and the extent of nonroutine maintenance. A properly maintained wet detention pond can also enhance the aesthetics of a development and provide wildlife habitat.

Nonroutine maintenance consists of periodic removal of sediment and repairs to the embankment, the inlet, or the outlet structure. The timing of these maintenance activities is highly variable, but they are usually required once every 10 to 25 years. Over time, a detention pond accumulates sediment. If this sediment begins to affect pond dewatering rates, blocks inlets or outlets, or inhibits desired pond functioning, it must be removed. How often the sediment must be removed depends primarily on the amount of land-disturbing activity that occurs within the pond drainage area. The process requires draining the pond and clearing it of accumulated sediment with suitable equipment. In the case of a very large facility, dragline excavation or hydraulic dredging may be required. Currently, on-site sediment disposal is acceptable. However, Environmental Protection Agency investigations of pond sediment contamination hazards may result in more stringent disposal requirements.

Costs and Financing

The cost of designing and constructing individual on-site wet detention ponds is estimated to range from $1,750 to $2,500 per acre of residential development and $2,500 to $3,500 per acre of nonresidential development. The developer is responsible for all design and construction costs unless the local government selects a regional facility approach.

Annual operation and maintenance costs are usually between 3 and 5 percent of the construction cost. The developer is required to post adequate financial assurance in the form of a cash deposit, a bond made payable to the responsible local government, or another acceptable security to assure that the maintenance, repairs, or reconstruction necessary for adequate facility performance will be made. If the developer fails to operate and maintain the facility adequately, the local government is ultimately responsible for carrying out the procedures outlined in the operation and maintenance plan. In that case, the government uses the developer's financial assurance funds to perform operation and maintenance activities. The local government can assume ownership and maintenance responsibility for stormwater controls at any time.

It is likely that financial assurance funds will not last for the life of the stormwater control facility. Therefore, the local government may choose to establish a stormwater utility. With a stormwater utility, property owners pay a monthly user fee to cover the capital, operating, and maintenance costs of the stormwater management program. Stormwater programs can also be funded with general revenues from property taxes, but substantial demands have already been placed on that source of revenues.

Individual Versus Regional Facilities

Engineered stormwater controls may be established on individual development sites or as large regional facilities. Each approach has its advantages and disadvantages.

The Individual Site Approach

Using individual, on-site wet detention ponds for each development is the most common approach to engineered stormwater controls. The owner or developer must finance the design and construction of individual facilities and, initially, is responsible for all operation and maintenance. However, in the case of all facilities, the local government is ultimately responsible for these activities if the owner fails to carry out the necessary operation and maintenance. Small, individual facilities located outside of stream- beds are preferable because they generally cause the least disturbance to the natural system.

There are also disadvantages to individual wet detention ponds. On-site facilities cost more per acre of development than regional facilities. Small ponds are more easily neglected because they are often out of sight and therefore out of mind. If neglected, a facility is less likely to achieve pollutant removal goals and may become an eyesore. Small ponds located within each development also pose a greater safety hazard if not secured properly.

The Regional Approach

In newly developing areas, a local government may choose to install a few strategically located regional facilities within the water supply watershed. If a regional pond is selected, it is advisable to use on-site controls for any heavy commercial or industrial development within the regional pond drainage area. Regional facilities are more appropriate for residential or low-density commercial areas.

Regional facilities are significantly more cost-effective because it is easier and less expensive to build, operate, and maintain one large facility than several small ones. Design and construction of regional facilities are estimated to cost from $1,250 to $2,000 per acre of residential development and $1,750 to $2,500 per acre of nonresidential development. In some states it has been shown that regional ponds are generally well maintained because they are large and highly visible. One large facility poses less of a safety hazard than numerous small ones because it is more visible and is easier to secure.

There are also several disadvantages to regional wet detention ponds. In most cases, the local government would have to provide capital construction funds for a regional facility, including the costs of land acquisition. However, if a downstream developer is the first to build, that person could be required to construct the facility and later be compensated by upstream developers for the capital construction costs and annual maintenance expenditures. Conversely, an upstream developer may have to establish temporary control structures if the regional facility is not in place before construction. Maintenance responsibilities generally shift from the homeowner or developer to the local government when a regional approach is selected. The local government would need to establish a stormwater utility or some other program to fund and implement stormwater control.

A large in-stream pond also poses a greater disruption to the natural flow network of a watershed. Aquatic habitats in the channel network upstream of a large facility may be degraded by bank erosion because they are not protected from large flows and high velocities. The Division of Environmental Management plans to investigate the potential environmental impacts of regional ponds. Some believe that these facilities may fail to maintain stream water quality standards or may inhibit full support of existing uses of the associated upstream waters. Larger facilities are also more likely to affect wetlands within the watershed. Based on these concerns, the U.S. Environmental Protection Agency and the U.S. Army Corps of Engineers have both expressed opposition to regional in-stream stormwater control facilities. Regional in-stream facilities should be avoided if possible and will likely be permitted on a case-by-case basis only.

References

Some of the information in this publication was drawn from the following references. Those marked by an asterisk are particularly useful sources of information on this subject.

Camp Dresser & McKee (CDM). October 15, 19992. "Complying with the State Water Supply Watershed Protection Rules," CDM Workshop, Charlotte, N.C.

* Maryland Department of the Environment, Sediment and Stormwater Administration. 1985. Inspector's Guidelines for Stormwater Management Infiltration Practices. Baltimore, Md. Available from Maryland Department of the Environment, Sediment and Stormwater Administration, 2500 Broening Highway, Bldg. 30,1st Floor, Baltimore, MD 21224. Telephone (410)631-3553

* North Carolina Sedimentation Control Commission, et al 1988. Erosion and Sediment Control Planning and Design Manual. Raleigh, N.C. Available from Division of Land Resources, Land Quality Section, Department of Environment, Health, and Natural Resources, 512 N. Salisbury St., P.O. Box 27687, Raleigh, NC 27611. Telephone (919)733-3833.

N.C. Division of Environmental Management (DEM). Undated. " An Overview of Wet Detention Basin Design." Technical Paper. Raleigh, N.C.

Roenigk, D. J., et al. 1992. Evaluation of Stormwater Maitenance in North Carolina. Water Resources Research Institute of the University of North Carolina, Report No. 267. Raleigh, N.C.

* Schueler, T. R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs. Washington, D.C.: Metropolitan Washington Council of Governments, Department of Environmental Programs Available from Metropolitan Washington Council of Governments Information Center, 777 N. Capitol St. NE, Suite 300, Washington, DC 20062-4226. Telephone (202) 962-3256.

* Schueler, T. R., P. A. Kumble, and M. A. Heraty. 1992. A Current Assessment of Urban Best Management Practices, Techniques for Reducing Non-Point Source Pollution in the Coastal Zone. Washington, D.C.: Metropolitan Washington Council of Governments, Department of Environmental Programs. Available from Metropolitan Washington Council of Governments Information Center, 777 N. Capitol St. NE, Suite 300, Washington, DC 20062-4226. Telephone (202) 962-3256.

Stimpson, L. "Case Study: Guilford County Wet Detention Ponds." Presentation to the Land-of-Sky Regional Council, Stormwater Management Education Advisory Committee, Greensboro, N.C., May 5, 1993. For further information, contact Lee Stimpson, Watershed Protection Engineer, Guilford County Planning Department.

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
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