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III. DAMS MANAGEMENT MEASURES

The second category of sources for which management measures and practices are presented in this chapter is dams. Dams are defined as constructed impoundments that are either (1) 25 feet or more in height and greater than 15 acre-feet in capacity, or (2) 6 feet or more in height and greater than 50 acre-feet in capacity.

Based on this definition, there are 7,790 dams located in coastal counties of the United States, of which 6,928 dams are located in States with approved coastal zone programs (Quick and Richmond, 1992).

The siting and construction of a dam can be undertaken for many purposes, including flood control, power generation, irrigation, livestock watering, fish farming, navigation, and municipal water supply. Some reservoir impoundments are also used for recreation and water sports, for fish and wildlife propagation, and for augmentation of low flows. Dams can adversely impact the hydraulic regime, the quality of the surface waters, and habitat in the stream or river where they are located. A variety of impacts can result from the siting, construction, and operation of these facilities.

Dams are divided into the following classes: run-of-the-river, mainstem, transitional, and storage. A run-of-the-river dam is usually a low dam, with small hydraulic head, limited storage area, short detention time, and no positive control over lake storage. The amount of water released from these dams depends on the amount of water entering the impoundment from upstream sources. Mainstem dams, which include run-of-the-river dams, are characterized by a retention time of approximately 25 days and a reservoir depth of approximately 50 to 100 feet. In mainstem dams, the outflow temperature is approximately equal to the inflow temperature plus the solar input, thus causing a "warming" effect. Transitional dams are characterized by a retention time of about 25 to 200 days and a maximum reservoir depth of between 100 and 200 feet. In transitional dams, the outflow temperature is approximately equal to the inflow temperature so that during the warmer months coldwater fish cannot survive unless the inflows are cold. The storage dam is typically a high dam with large hydraulic head, long detention time, and positive control over the volume of water released from the impoundment. Dams constructed for either flood control or hydroelectric power generation are usually of the storage class. These dams typically have a retention time of over 200 days and a reservoir depth of over 100 feet. The outflow temperature is sufficient for coldwater fish, even with warm inflows.

The siting of dams can result in the inundation of wetlands, riparian areas, and fastland in upstream areas of the waterway. Dams either reduce or eliminate the downstream flooding needed by some wetlands and riparian areas. Dams can also impede or block migration routes of fish.

Construction activities from dams can cause increased turbidity and sedimentation in the waterway resulting from vegetation removal, soil disturbance, and soil rutting. Fuel and chemical spills and the cleaning of construction equipment (particularly concrete washout) have the potential for creating nonpoint source pollution. The proximity of dams to streambeds and floodplains increases the need for sensitivity to pollution prevention at the project site in planning and design, as well as during construction.

The operation of dams can also generate a variety of types of nonpoint source pollution in surface waters. Controlled releases from dams can change the timing and quantity of freshwater inputs into coastal waters. Dam operations may lead to reduced downstream flushing, which, in turn, may lead to increased loads of BOD, phosphorus, and nitrogen; changes in pH; and the potential for increased algal growth. Lower instream flows, and lower peak flows associated with controlled releases from dams, can result in sediment deposition in the channel several miles downstream of the dam. The tendency of dam releases to be clear water, or water without sediment, can result in erosion of the streambed and scouring of the channel below the dam, especially the smaller-sized sediments. One result is the siltation of gravel bars and riffle pool complexes, which are valuable spawning and nursery habitat for fish. Dams also limit downstream recruitment of suitably-sized substrate required for the anchoring and growth of aquatic plants. Finally, reservoir releases can alter the water temperature and lower the dissolved oxygen levels in downstream portions of the waterway.

The extent of changes in downstream temperature and dissolved oxygen from reservoir releases depends on the retention time of water in the reservoir and the withdrawal depth of releases from the reservoir. Releases from mainstem projects are typically higher in dissolved oxygen than are releases from storage projects. Storage reservoir releases are usually colder than inflows, while releases from mainstem reservoirs depend on retention time and depth of releases. Reservoirs with short hydraulic residence times have reduced impacts on tailwaters (Walburg et al., 1981).

It is important to note that the operation of dams can have positive, as well as negative, effects on water quality, aquatic habitat, and fisheries within the pool and downstream (USEPA, 1989). Potential positive effects include:

  • Creation of above-the-dam summer pool refuge during low flows, an effect that has been documented for small dams built in the upper stream reaches of the Willamette River in the northwest United States (Li et al., 1983);
  • Creation of reservoir sport fisheries (USDOI, 1983); and
  • Less scouring and erosion of streambanks as a result of reduced velocities in downstream areas.
Once a river is dammed and a reservoir is created, processes such as stratification, seasonal overturn, chemical cycling, and sedimentation can intensify to create several NPS pollution problems. These processes occur primarily as a result of the presence of the dam, not the operation of the dam.

Stratification is the layering of a lake into an upper, well-lighted, productive, and warm layer, called the epilimnion; a mid-depth transitional layer, the metalimnion; and a lower, dark, cold, and unproductive layer, the hypolimnion. These layers are separated by a thermocline in the metalimnion, a sharp transition in water temperature between upper warm water and lower cold water (Figure 6-1). This stratification varies seasonally, being most pronounced in the summer and absent in the winter. Between these extremes are periods of less pronounced stratification and spring and fall overturns, when the entire waterbody mixes together. Poor mixing conditions, resulting in stratification, are estimated to occur in 40 percent of power impoundments and 37 percent of non-power impoundments (USEPA, 1989).

Dissolved oxygen levels are tied to the overturn, mixing, and stratification processes. Dissolved oxygen concentration in reservoir waters is the result of a delicate balance between both oxygen-producing and oxygen-consuming processes (Bohac and Ruane, 1990). Dissolved oxygen tends to become depleted in the hypolimnion due to decomposition of organic substances, algal respiration, and nitrification. The epilimnion, however, tends to be enriched with oxygen from the atmosphere and as a product of photosynthesis. The net difference between oxygen consumption and oxygen sources can create anoxic conditions in the lower layer (Figure 6-2).

Anoxic conditions in the hypolimnion may stimulate the formation of reduced species of iron, manganese, sulfur, and nitrogen. Chemical cycling of these elements occurs when they change from one state to another (e.g., from solid to dissolved). Many chemicals enter a reservoir attached to sediment particles or quickly become attached to sediment. As a solid, many chemicals typically are not toxic to many organisms, especially those in the water column. Some chemicals are easily reduced under anoxic conditions and become soluble. The reduced and soluble forms of many chemicals and compounds are toxic to most aquatic organisms at relatively low concentrations. For example, hydrogen sulfide is toxic to aquatic life and corrosive to construction materials at concentrations that are considerably lower than those detectable by commonly used procedures (Johnson et al., 1991). These reduced chemical compounds lead to taste and odor problems in drinking water supplies and toxicity problems for fish.

Hydraulic residence time is defined as the average time required to completely renew a waterbody's water volume. For example, rivers have little or no hydraulic residence time, lakes with small volumes and high flow rates have short hydraulic residence times, and lakes with large volumes and low flow rates have long hydraulic residence times. Reservoirs differ from lakes in that, among other characteristics, their flow is regulated artificially. Hydraulic residence times of reservoirs are generally shorter than those of lakes, giving the water flowing into the reservoir less time to mix with the resident water.

The longer the hydraulic residence time, the greater the potential for incoming nutrients and sediment to settle in the reservoir. Conditions that lead to eutrophication in reservoirs promote increased algal growth, which in turn lead to a greater mass of dead plant cells. In reservoirs with long residence times, a major source of organic sediment settling to the bottom can be dead plant cells. Sediment will settle to the bottom; but, where reservoir releases are taken from the lower layer, they will release colder water downstream that is rich in nutrients, low in dissolved oxygen, and higher in some dissolved species such as iron, manganese, sulfur, and nitrogen.

Management Measures A and B address two problems associated with the construction of dams:

  1. Increases in sediment delivery downstream resulting from construction and operation activities and
  2. Spillage of chemicals and other pollutants to the waterway during construction and operation.
The impacts of reservoir releases on the quality of surface waters and instream and riparian habitat in downstream areas is addressed in Management Measure III.C.

A. Management Measure for Erosion and Sediment Control

  1. Reduce erosion and, to the extent practicable, retain sediment onsite during and after construction, and
  2. Prior to land disturbance, prepare and implement an approved erosion and sediment control plan or similar administrative document that contains erosion and sediment control provisions.

1. Applicability

This management measure is intended to be applied by States to the construction of new dams, as well as to construction activities associated with the maintenance of dams. Dams are defined as constructed impoundments which are either:

  • 25 feet or more in height and greater than 15 acre-feet in capacity, or
  • six feet or more in height and greater than 50 acre-feet in capacity.
This measure does not apply to projects that fall under NPDES jurisdiction. Under the Coastal Zone Act Reauthorization Amendments of 1990, States are subject to a number of requirements as they develop coastal NPS programs in conformity with this measure and will have some flexibility in doing so. The application of management measures by States is described more fully in Coastal Nonpoint Pollution Control Program: Program Development and Approval Guidance, published jointly by the U.S. Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric Administration (NOAA) of the U.S. Department of Commerce.

2. Description

The purpose of this management measure is to prevent sediment from entering surface waters during the construction or maintenance of dams. Coastal States should incorporate this measure into existing State erosion and sediment control (ESC) programs or, if such programs are lacking, should develop them. States should incorporate this measure into ESC programs at the local level also. Erosion and sediment control is intended to be part of a comprehensive land use or watershed management program. (Refer to the Watershed and Site Development Management Measures in Chapter 4.)

Runoff from construction sites is the largest source of sediment in urban areas (Maine Department of Environmental Protection, Bureau of Water Quality, and York County Soil and Water Conservation District, 1990). Eroded sediment from construction sites creates many problems in coastal areas including adverse impacts to water quality, critical instream and riparian habitats, submerged aquatic vegetation (SAV) beds, recreational activities, and navigation.

ESC plans are important for controlling the adverse impacts of dam construction. ESC plans ensure that provisions for control measures are incorporated into the site planning stage of development and provide for prevention of erosion and sediment problems and accountability if a problem occurs (Maine Department of Environmental Protection, 1990). Chapter 4 of this guidance presents a full description of construction-related erosion problems and the value of ESC plans. Readers should refer to Chapter 4 for further information.

3. Management Measure Selection

This management measure was selected because of the importance of minimizing sediment loss to surface waters during dam construction. It is essential that proper erosion and sediment control practices be used to protect surface water quality because of the high potential for sediment loss directly to surface waters.

Two broad performance goals constitute this management measure: minimizing erosion and maximizing the retention of sediment onsite. These performance goals give States and local governments flexibility in specifying practices appropriate for local conditions.

4. Practices

As discussed more fully at the beginning of this chapter and in Chapter 1, the following practices are described for illustrative purposes only. State programs need not require the implementation of these practices. However, as a practical matter, EPA anticipates that the management measure set forth above generally will be implemented by applying one or more management practices appropriate to the source, location, and climate. The practices set forth below have been found by EPA to be representative of the types of practices that can be applied successfully to achieve the management measure described above.

Practices for the control of erosion and sediment loss are discussed in Chapter 4 of this guidance and should be considered applicable to this management measure. Erosion controls are used to reduce the amount of sediment that is lost during dam construction and to prevent sediment from entering surface waters. Erosion control is based on two main concepts: (1) minimizing the area and time of land disturbance and (2) stabilizing disturbed soils to prevent erosion. The following practices have been found to be useful in these purposes and should be incorporated into ESC plans and used during dam construction as appropriate.

Additional discussions of the practices described below can be found in Chapter 4 of this guidance and should be referred to for more information.

  • a. Preserve trees and other vegetation that already exist near the dam construction site.

    This practice retains soil and limits runoff. The destruction of existing onsite vegetation can be minimized by initially surveying the site to plan access routes, locations of equipment storage areas, and the location and alignment of the dam. Construction workers should be encouraged to limit activities to designated areas. Reducing the disturbance of vegetation also reduces the need for revegetation after construction is completed, including the required fertilization, replanting, and grading that are associated with revegetation. Additionally, as much natural vegetation as possible should be left next to the waterbody where construction is occurring. This vegetation provides a buffer to reduce the NPS pollution effects of runoff originating from areas associated with the construction activities.

  • b. Control runoff from the construction site and construction-related areas.

    The largest surface water pollution problem during construction is turbidity resulting from aggregate processing, excavation, and concrete work. Preventing the entry of these materials into surface waters is always the preferable alternative because runoff due to these activities can adversely affect drinking water supplies, irrigation systems, and river ecology (Peters, 1978). If onsite treatment is necessary, methods are available to control the runoff of sediment and wastewater from the construction site. Sedimentation in settling ponds, sometimes with the addition of chemical precipitating agents, is one such method (Peters, 1978). Flocculation, the forced coagulation of fine-grained sediment through agitation to settle particles out of solution, is another method. Chemical precipitating agents can also be used in this flocculation process (Peters, 1978). Filtration with sand, anthracite, diatomaceous earth, or finely woven material, used singly or in combination, may be more useful than other methods for coarser grained materials (Peters, 1978).

  • c. Control soil and surface water runoff during construction.

    To prevent the entry of sediment used during construction into surface waters, the following precautionary steps should be followed: identify areas with steep slopes, unstable soils, inadequate vegetation density, insufficient drainage, or other conditions that give rise to a high erosion potential; and identify measures to reduce runoff from such areas if disturbance of these areas cannot be avoided (Hynson et al., 1985). Refer to Chapter 4 for additional information.

    Runoff control measures, mechanical sediment control measures, grassed filter strips, mulching, and/or sediment basins should be used to control runoff from the construction site. Scheduling construction during drier seasons, exposing areas for only the time needed for completion of specific activities, and avoiding stream fording also help to reduce the amount of runoff created during construction. Refer to Chapter 4 for additional information.

  • d. Other practices

    Many other practices for the control of erosion and sediment loss are discussed in Chapter 4 of this guidance, which should be referred to for a complete discussion where noted. Below are brief descriptions of some of the other practices.

    • Revegetation. Revegetation of construction sites during and after construction is the most effective way to permanently control erosion (Hynson et al., 1985). Many erosion control techniques are also intended to expedite revegetation.

    • Mulching. Various mulching techniques are used in erosion control, such as use of straw, wood chip, or stone mulches; use of mulch nets or blankets; and hydromulching (Hynson et al., 1985). Mulching is used primarily to reduce the impact of rainfall on bare soil, to retain soil moisture, to reduce runoff, and often to protect seeded slopes (Hynson et al., 1985).

    • Soil Bioengineering. Soil bioengineering techniques can be used to address the erosion resulting from dam operation. Grading or terracing a problem stream bank or eroding area and using interwoven vegetation mats, installed alone or in combination with structural measures, will facilitate infiltration stability. Refer to the section on shore protection in this chapter for additional information.

    5. Effectiveness for All Practices

    The effectiveness of erosion control practices can vary based on land slope, the size of the disturbed area, rainfall frequency and intensity, wind conditions, soil type, use of heavy machinery, length of time soils are exposed and unprotected, and other factors. In general, a system of erosion and sediment control practices can more effectively reduce offsite sediment transport than a single system. Numerous nonstructural measures such as protecting natural or newly planted vegetation, minimizing the disturbance of vegetation on steep slopes and other highly erodible areas, maximizing the distance eroded material must travel before reaching the drainage system, and locating roads away from sensitive areas may be used to reduce erosion. Chapter 4 has additional information for effectiveness of the practices listed above.

    6. Costs for All Practices

    Chapter 4 of this guidance contains the available cost data for most of the erosion controls listed above. Costs in Chapter 4 have been broken down into annual capital costs, annual maintenance costs, and total annual costs (including annualization of capital costs).

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    This page last updated October 4, 1999