Water: Coastal Zone Act Reauthorization Amendments
Management Measure for Physical and Chemical Characteristics of Surface Waters - II. Channelization and Channel Modification Management Measures
One form of hydromodification is channelization or channel modification. These terms (used interchangeably) describe river and stream channel engineering undertaken for the purpose of flood control, navigation, drainage improvement, and reduction of channel migration potential (Brookes, 1990). Activities such as straightening, widening, deepening, or relocating existing stream channels and clearing or snagging operations fall into this category. These forms of hydromodification typically result in more uniform channel cross sections, steeper stream gradients, and reduced average pool depths.
The terms channelization and channel modification are also used in this chapter to refer to the excavation of borrow pits, canals, underwater mining, or other practices that change the depth, width, or location of waterways or embayments in coastal areas. Excavation of marina basins is addressed separately in Chapter 5 of this guidance.
The term flow alteration describes a category of hydromodification activities that result in either an increase or a decrease in the usual supply of fresh water to a stream, river, or estuary. Flow alterations include diversions, withdrawals, and impoundments. In rivers and streams, flow alteration can also result from undersized culverts, transportation embankments, tide gates, sluice gates, and weirs.
Levees along a stream or river channel are also addressed by this section. A levee is defined by the U.S. Army Corps of Engineers (USACE) as an embankment or shaped mound for flood control or hurricane protection (USACE, 1981). Pond banks, and other small impoundment structures, often referred to as levees in the literature, are not considered to be levees as defined in this section. Additionally, a dike is not used in this guidance to refer to the same structure as a levee, but rather is defined as a channel stabilization structure sited in a river or stream perpendicular to the bank.
For the purpose of this guidance, no distinction will be made between the terms river and stream because no definition of either could be found to quantitatively distinguish between the two. Likewise, no distinction will be made for word combinations of these two terms; for example, streambank and riverbank will be considered to be synonymous.
The following definitions for common terms associated with channelization activities apply to this chapter (USACE, 1983). Other definitions are provided in the Glossary at the end of the chapter.
Channel: A natural or constructed waterway that continuously or periodically passes water.
Channel stabilization: Structures placed below the elevation of the average surface water level (lower bank) to control bank erosion or to prevent bank or channel failure.
Streambank: The side slopes of a channel between which the streamflow is normally confined.
Lower bank: The portion of the streambank below the elevation of the average water level of the stream.
Upper bank: The portion of the streambank above the elevation of the average water level of the stream.
Streambank stabilization: Structures placed on or near a distressed streambank to control bank erosion or to prevent bank failure.
Based on the above definitions, the difference between channel stabilization and streambank stabilization is that in streambank stabilization, the upper bank is also protected from erosion or failure. This additional protection guards against erosive forces caused by high-water events and by land-based causes such as runoff or improper siting of buildings. Levees are placed along streambanks to prevent flooding in adjacent areas during extreme high-water events.
Effects of Channelization and Channel Modification Activities
General Problematic Effects Channel modification activities have deprived wetlands and estuarine shorelines of enriching sediments, changed the ability of natural systems to both absorb hydraulic energy and filter pollutants from surface waters, and caused interruptions in the different life stages of aquatic organisms (Sherwood et al., 1990). Channel modification activities can also alter instream water temperature and sediment characteristics, as well as the rates and paths of sediment erosion, transport, and deposition. A frequent result of channelization and channel modification activities is a diminished suitability of instream and riparian habitat for fish and wildlife. Hardening of banks along waterways has eliminated instream and riparian habitat, decreased the quantity of organic matter entering aquatic systems, and increased the movement of NPS pollutants from the upper reaches of watersheds into coastal waters.
Channel modification projects undertaken in streams or rivers to straighten, enlarge, or relocate the channel usually require regularly scheduled maintenance activities to preserve and maintain completed projects. These maintenance activities may also result in a continual disturbance of instream and riparian habitat. In some cases, there can be substantial displacement of instream habitat due to the magnitude of the changes in surface water quality, morphology and composition of the channel, stream hydraulics, and hydrology.
Excavation projects can result in reduced flushing, lowered dissolved oxygen levels, saltwater intrusion, loss of streamside vegetation, accelerated discharge of pollutants, and changed physical and chemical characteristics of bottom sediments in surface waters surrounding channelization or channel modification projects. Reduced flushing, in particular, can increase the deposition of finer-grained sediments and associated organic materials or other pollutants.
Levees may reduce overbank flooding and the subsequent deposition of sediment needed to nourish riverine and estuarine wetlands and riparian areas. Levees can cause increased transport of suspended sediment to coastal and near-coastal waters during high-flow events. Levees located close to streambanks can also prevent the lateral movement of sediment-laden waters into adjacent wetlands and riparian areas that would otherwise serve as depositories for sediment, nutrients, and other NPS pollutants. This has been a major factor, for example, in the rapid loss of coastal wetlands in Louisiana (Hynson et al., 1985). Levees also interrupt natural drainage from upland slopes and can cause concentrated, erosive flows of surface waters.
The resulting changes to the distribution, amount, and timing of flows caused by flow alterations can affect a wide variety of living resources. Where tidal flow restrictors cause impoundments, there may be a loss of streamside vegetation, disruption of riparian habitat, changes in the historic plant and animal communities, and decline in sediment quality. Restricted flows can impede the movement of fish or crustaceans. Flow alteration can reduce the level of tidal flushing and the exchange rate for surface waters within coastal embayments, with resulting impacts on the quality of surface waters and on the rates and paths of sediment transport and deposition.
Specific Effects Depending on preproject site conditions and the extent of hydromodification activity, new and existing channelization and channel modification projects may result in no additional NPS problems, additional NPS problems, or benefits.
The following are major categories of channelization and channel modification effects and examples of associated problems and benefits.
Changed Sediment Supply. One of the more significant changes in instream habitat associated with channelization and channel modification projects is in sediment supply and delivery. Streamside levees have been linked to accelerated rates of erosion and decreased sediment supplies to coastal areas (Hynson et al., 1985). Sherwood and others (1990) evaluated the long-term impacts of channelization projects on the Columbia River estuary and found that changes to the river system resulted in a net increase of 68 million cubic meters of sediment in the estuary. These changes in sediment supply can include problems such as increased sedimentation to some areas (an estuary, for example) or decreased sediment to other areas (such as streamside wetlands or estuarine marshes). Other changes may be beneficial; for example, a diversion that delivers sediment to eroding marshes (Hynson et al., 1985). Another example of a beneficial channel stabilization project might be one that results in increased flushing and the elimination of unwanted sediment in the spawning area of a stream.
Reduced Freshwater Availability. Salinity above threshold levels is considered to be a form of NPS pollution in freshwater supplies. Reduced freshwater availability for municipal, industrial, or agricultural purposes can result from some channelization and channel modification practices. Similarly, alteration of the salinity regime in portions of a channel can result in ecological changes in vegetation in the streamside area. Diversion of fresh water by flood- and hurricane-protection levees has reduced freshwater inputs to adjacent marshes. This has resulted in increased marsh salinities and degradation of the marsh ecosystem (Hynson et al., 1985). A benefit of other diversion projects was a reduction of freshwater inputs to estuarine areas that were becoming too fresh because of overall increases in fresh water from changes in land use within a watershed. Increases in oyster harvests have been attributed to a freshwater diversion in Plaquemines Parish, Louisiana. Over the 6-year period from 1970 to 1976, oyster harvests increased by over 3.5 million pounds (Hynson et al., 1985). Potential problems with diversions include erosion, settlement, seepage, and liquefaction failure (Hynson et al., 1985).
Accelerated Delivery of Pollutants. Channelization and channel modification projects can lead to an increased quantity of pollutants and accelerated rate of delivery of pollutants to downstream sites. Alterations that increase the velocity of surface water or that increase flushing of the streambed can lead to more pollutants being transported to downstream areas at possibly faster rates. Urbanization has been linked to downstream channelization problems in Hawaii (Anderson, 1992). It is believed that the deterioration of Kaneohe Bay may be caused by development within the watershed, which has increased runoff flows to streams entering the Bay. Streams that once meandered and contained natural vegetation to filter out nutrient and sediment are now channelized and contain surface water that is rich in nutrients and other pollutants associated with urban areas (Anderson, 1992). Some excavation projects have resulted in poor surface water circulation along with increased sedimentation and other surface water quality problems within the excavated basin. In some of these cases, additional, carefully designed channel modifications can increase flushing rates, which deliver accumulated pollutants from the basin to points downstream that are able to assimilate or otherwise beneficially use the accumulated materials.
Loss of Contact with Overbank Areas. Instream hydraulic changes can decrease or interfere with surface water contact to overbank areas during floods or other high-water events. Channelization and channel modification activities that lead to a loss of surface water contact in overbank areas also may result in reduced filtering of NPS pollutants by streamside area vegetation and soils. Areas of the overbank that are dependent on surface water contact (i.e., riparian areas and wetlands) may change in character and function as the frequency and duration of flooding change. Erickson and others (1979) reported a major influence on wetland drainage in the Wild Rice Creek Watershed in North and South Dakota. Drainage rates from streamside areas were 2.6 times higher in the channelized area than in undisturbed areas during preliminary project activities and 5.3 times higher following construction. Schoof (1980) reported several other impacts of channelization, including drainage of wetlands, reduction of oxbows and stream meander, clearing of floodplain hardwood, lowering of ground-water levels, and increased erosion. Channel modification projects such as setback levees or compound channel design can provide the overbank flooding to areas needing it while also providing a desired level of flood protection to adjoining lands.
Changes to Ecosystems. Channelization and channel modification activities can lead to loss of instream and riparian habitat and ecosystem benefits such as pathways for wildlife migration and conditions suitable for reproduction and growth. Problematic flow modifications, for example, have resulted in reversal of flow regimes of some California rivers or streams, which has led to the disorientation of anadromous fish that rely on flow to direct them to spawning areas (James and Stokes Associates, Inc., 1976). Eroded sediment may deposit in new areas, covering benthic communities or altering instream habitat (Sherwood et al., 1990). Orlova and Popova (1976) researched the effects on fish population resulting from altering the hydrologic regime with hydraulic structures such as channels. The effects assessed by Orlova and Popova (1976) include:
- Deterioration of spawning habitat and conditions, resulting in lower recruitment of river species;
- Increases in stocks of summer spawning river species; and
- Changes in types and amounts of food organisms.
Many channel or streambank stabilization structures provide increased instream habitat for certain aquatic species. For example, Sandheinrich and Atchison (1986) reported increases in densities of epibenthic insects within revetments and stone dike areas and more suitable substrate for bottom-dwelling insects in revetment areas.
Instream and Riparian Habitat Altered by Secondary Effects. Secondary instream and riparian habitat alteration effects from channelization and channel modification projects include movement of estuarine turbidity maximum zones (zone of higher sediment concentrations caused by salinity and tide-induced circulation) with salinity changes, cultural eutrophication caused by inadequate flushing, and trapping of large quantities of sediment. Wolff and others (1989) analyzed the impacts of flow augmentation on the stream channel and instream habitat following a transbasin water diversion project in Wyoming. The South Fork of Middle Crow Creek, previously ephemeral, was beneficially used as a conveyance to create instream habitat as a part of impact management measures of the transbasin diversion project. Discontinuous channels, high summer water temperature, and flow interruptions and fluctuations were identified as potential limiting factors for the development of such practices for this particular project. Modeling results, however, indicated that as the channel develops, the effects of the first two limiting factors will be negligible. Following 2 years of increased flow in the 5.5-mile section of stream channel (reach) used in this study, the volume of stream channel had increased 32 percent and more channel areas were expected to develop on approximately 67 percent of the stream reach. The total area of beaver ponds had more than doubled. The brook trout with which the beaver ponds were stocked were reported to be surviving and growing.
The examples described above illustrate the range of possible effects that can result from channelization and channel modification projects. These effects can be either beneficial or problematic to the ecology and surrounding riparian habitat. The effects caused by changed sediment supplies provide an excellent example of these varying impacts. In one case, sediment supplies to coastal marshes are insufficient and the marshes are subsiding (problem). In another case, sediment supplies to an estuary are increasing to the point of causing changes to the natural tidal flow (problem). A final example showed decreased sediment in a streambed, which has resulted in better conditions for native spawning fish (benefit). Thus, depending on site-specific conditions and the particular channelization or channel modification practices used, the project will have positive or negative NPS pollution impacts.
Another confounding factor is the potential for one project to have multiple NPS problems and/or benefits. Assuming that a channelization or channel modification project was originally designed to overcome a specific problem (e.g., channel deepening for navigation, streambank stabilization for erosion control, or levee construction for flood control), the project was intended to be beneficial. Unfortunately, planners of many channelization and channel modification projects have, in the past, been myopic when considering the range of impacts associated with the project. The purpose of the management measures in this section is to recommend proper evaluation of potential projects and reevaluation of existing projects to reduce NPS impacts and maximize potential benefits.
Proper evaluation of channelization and channel modification projects should consider three major points.
- Existing conditions. New and existing channelization and channel modification projects should be evaluated for potential effects (both problematic and beneficial) based on existing stream and watershed conditions. Site-specific stream conditions, such as flow rate, channel dimensions, typical surface water quality, or slope, should be evaluated in conjunction with streamside conditions, such as soil and vegetation type, slopes, or land use. Characteristics of the watershed also need to be evaluated. This phase of the evaluation will identify baseline conditions for potential projects and can be compared to historical conditions for projects already in place.
- Potential conditions. Anticipated changes to the base (or existing) conditions in a stream, along the streambank, and within the watershed should be evaluated. By examining potential changes caused by new conditions, long-term impacts can be factored into the design or management of a channelization or channel modification project. Studies like that of Sandheinrich and Atchison (1986) clearly show that short-term benefits from hydromodification activities can change to long-term problems.
- Watershed management. Evaluation of changes in watershed conditions is paramount in the proper design of a channelization or channel modification project. Since the design of these projects is based on hydrology, changes in watershed hydrology will certainly impact the proper functioning of a channelization or channel modification structure. Additionally, many surface water quality changes associated with a channelization or channel modification project can be attributed to watershed changes, such as different land use, agricultural practices, or forestry practices.
The two management measures presented in this section of the chapter promote the evaluation of channelization and channel modification projects. Channels should be evaluated as a part of the watershed planning and design processes, including watershed changes from new development in urban areas, agricultural drainage, or forest clearing. The purpose of the evaluation is to determine whether resulting NPS changes to surface water quality or instream and riparian habitat can be expected and whether these changes will be good or bad.
Existing channelization and channel modification projects can be evaluated to determine the NPS impacts and benefits associated with the projects. Modifications to existing projects, including operation and maintenance or management, can also be evaluated to determine the possibility of improving some or all of the impacts without changing the existing benefits or creating additional problems.
In both new and existing channelization and channel modification projects, evaluation of benefits and/or problems will be site-specific. Mathematical models are one type of tool used to determine these impacts. Some models provide a simple analysis of a particular situation and are good for screening purposes. Other models evaluate complex interactions of many variables and can be powerful, site-specific evaluation tools. There are also structural and nonstructural practices that can be used to prevent either NPS pollution effects from or NPS impacts to channelization and channel modification projects. Interpretation of design changes, model results predicting changes or impacts, or the effects of structural or nonstructural practices requires sound biological and engineering judgment and experience.
The first three problems listed above are usually associated with the alteration of physical characteristics of surface waters. Accordingly, they are addressed by Management Measure II.A in the section below. The last three problems listed above can be grouped to represent problems resulting from modification of instream and riparian habitat. They are addressed by Management Measure II.B in the subsequent section below.
A. Management Measure for Physical and Chemical Characteristics of Surface Waters
- Evaluate the potential effects of proposed channelization and channel modification on the physical and chemical characteristics of surface waters in coastal areas;
- Plan and design channelization and channel modification to reduce undesirable impacts; and
- Develop an operation and maintenance program for existing modified channels that includes identification and implementation of opportunities to improve physical and chemical characteristics of surface waters in those channels.
This management measure is intended to be applied by States to public and private channelization and channel modification activities in order to prevent the degradation of physical and chemical characteristics of surface waters from such activities. This management measure applies to any proposed channelization or channel modification projects, including levees, to evaluate potential changes in surface water characteristics, as well as to existing modified channels that can be targeted for opportunities to improve the surface water characteristics necessary to support desired fish and wildlife. 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 management measures and will have some flexibility in doing so. The application of this management measure 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.
The purpose of this management measure is to ensure that the planning process for new hydromodification projects addresses changes to physical and chemical characteristics of surface waters that may occur as a result of the proposed work. Implementation of this management measure is intended to occur concurrently with the implementation of Management Measure B (Instream and Riparian Habitat Restoration) of this section. For existing projects, the purpose of this management measure is to ensure that the operation and maintenance program uses any opportunities available to improve the physical and chemical characteristics of the surface waters. Changes created by channelization and channel modification activities are problematic if they unexpectedly alter environmental parameters to levels outside normal or desired ranges. The physical and chemical characteristics of surface waters that may be influenced by channelization and channel modification include sediment, turbidity, salinity, temperature, nutrients, dissolved oxygen, oxygen demand, and contaminants.
Implementation of this management measure in the planning process for new projects will require a two-pronged approach:
- Evaluate, with numerical models for some situations, the types of NPS pollution related to instream changes and watershed development.
- Address some types of NPS problems stemming from instream changes or watershed development with a combination of nonstructural and structural practices.
The best available technology that can be applied to examine the physical and chemical effects of hydraulic and hydrologic changes to streams, rivers, or other surface water systems are models and past experience in situations similar to those described in the case studies discussed in this chapter. These models, discussed in detail under the practices of this section, can simulate many of the complex physical, chemical, and biological interactions that occur when hydraulic changes are imposed on surface water systems. Additionally, models can be used to determine a combination of practices to mitigate the unavoidable effects that occur even when a project is properly planned. Models, however, cannot be used independently of expert judgment gained through past experience. When properly applied models are used in conjunction with expert judgment, the effects of channelization and channel modification projects (both potential and existing projects) can be evaluated and many undesirable effects prevented or eliminated.
In cases where existing channelization or channel modification projects can be changed to enhance instream or streamside characteristics, several practices can be included as a part of regular operation and maintenance programs. New channelization and channel modification projects that cause unavoidable physical or chemical changes in surface waters can also use one or more practices to mitigate the undesirable changes. The practices include streambank protection, levee protection, channel stabilization, flow restrictors, check dam systems, grade control structures, vegetative cover, instream sediment control, noneroding roadways, and setback levees or flood walls. By using one or more of these practices in combination with predictive modeling, the adverse impacts of channelization and channel modification projects can be evaluated and possibly corrected.
This management measure addresses three of the effects of channelization and channel modification that affect the physical and chemical characteristics of surface waters:
- Changed sediment supply;
- Reduced freshwater availability; and
- Accelerated delivery of pollutants.
Selection of this management measure was based on the following factors:
- Published case studies of existing channelization and channel modification projects describe alterations to the physical and chemical characteristics of surface waters (Burch et al., 1984; Erickson et al., 1979; Parrish et al., 1978; Pennington and Dodge, 1982; Petersen, 1990; Reiser et al., 1985; Roy and Messier, 1989; Sandheinrich and Atchison, 1986; Sherwood et al., 1990). Frequently, the postproject conditions are intolerable to desirable fish and wildlife.
- The literature also describes instream benefits for fish and wildlife that can result from careful planning of channelization and channel modification projects (Bowie, 1981; Los Angeles River Watershed, 1973; Sandheinrich and Atchison, 1986; Shields et al., 1990; Swanson et al., 1987; USACE, 1981; USACE, 1989).
- Increased volumes of runoff resulting from some types of watershed development produce hydraulic changes in downstream areas including bank scouring, channel modifications, and flow alterations (Anderson, 1992; Schueler, 1987).
As explained 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 implementation of 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.
- a. Use models/methodologies as one means to evaluate the effects of proposed channelization and channel modification projects on the physical and chemical characteristics of surface waters. Evaluate these effects as part of watershed plans, land use plans, and new development plans.
Mathematical Models for Physical and Chemical Characteristics of Surface Waters, Including Instream Flows
Over the past 20 to 30 years, theoretical and engineering advances have been made in the quantitative descriptions and interactions of physical transport processes; sediment transport, erosion, and deposition; and surface water quality processes. Based on these theoretical approaches and the need for evaluations of proposed surface water resource engineering projects, a variety of simulation models have been developed and applied to provide technical input for complex decision-making. In planning-level evaluations of proposed hydromodification projects, it is critical to understand that the surface water quality and ecological impact of the proposed project will be driven primarily by the alteration of physical transport processes. In addition, it is critical to realize that the most important environmental consequences of many hydromodification projects will occur over a long-term time scale of years to decades.
The key element in the selection and application of models for the evaluation of the environmental consequences of hydromodification projects is the use of appropriate models to adequately characterize circulation and physical transport processes. Appropriate surface water quality and ecosystem models (e.g., salinity, sediment, cultural eutrophication, oxygen, bacteria, fisheries, etc.) are then selected for linkage with the transport model to evaluate the environmental impact of the proposed hydromodification project. Because of the increasing availability of relatively inexpensive computer hardware and software over the past decade, rapid advances have been made in the development of sophisticated two-dimensional (2D) and three-dimensional (3D) time-variable hydrodynamic models that can be used for environmental assessments of hydromodification projects (see Spaulding, 1990; McAnally, 1987). Two-dimensional depth or laterally averaged hydrodynamic models are economical and can be routinely developed and applied for environmental assessments of beneficial and adverse effects on surface water quality by knowledgeable teams of physical scientists and engineers (Hamilton, 1990). Three-dimensional hydrodynamic models, usually considered more of an academic research tool, are also beginning to be more widely applied for large-scale environmental assessments of aquatic ecosystems (e.g., EPA/USACE-WES Chesapeake Bay 3D hydrodynamic and surface water quality model).
The necessity for the application of detailed 2D and 3D hydrodynamic models for large-scale hydromodification projects can be demonstrated using detailed simulation models to hindcast the long-term surface water quality and ecological impact of projects that have actually been constructed over the past 20 to 40 years. Sufficient data are available from a number of large-scale hydromodification projects in the United States and overseas that can provide data sets for the development of hindcasting models to illustrate the capability of the models to simulate the known adverse long-term ecological consequences of projects that have actually been operational for decades. The results of such hindcasting evaluations could provide important guidance for resource managers, who use good professional judgment to understand the level of technical complexity and the costs required for an adequate assessment of the long-term ecological impacts of proposed hydromodification projects. In the Columbia River estuary, for example, Sherwood and others (1990) used historical bathymetric data with a numerical 2D hydrodynamic model (Hamilton, 1990) to document the long-term impact of hydromodification changes on channel morphology, riverflow transport processes, salinity intrusion, residence time, and net accumulation of sediment.
When models are not suited to evaluate a particular situation, examining existing conditions and using best professional judgment are another way to evaluate the effects of hydromodification activities. For example, in cases where water supplies need to be restored to wetlands that have historically experienced a loss of water contact, models can be used to ensure that the length of time of renewed water exposure is within the tolerance of the wetland plants for inundation, since excessive inundation of wetland plants can be as destructive as loss of water contact. Surface water quality monitoring and procedures such as Rapid Bioassessment Protocols (see Management Measure B in this section for more information) are examples of methods to examine existing conditions.
Table 6-1 (14k) lists some of the available models for studying the effects of channelization and channel modification activities. Listed below are examples of channelization and channel modification activities and associated models that can be used in the planning process.
- Impoundments. A hydrodynamic model coupled with a surface water quality model (e.g., WASP4) can be applied to determine changes in surface water quality due to an increased detention of storm water runoff caused by the upstream dams. Changes in sediment distribution in the estuary caused by a reduction in the sediment source (due to the trap efficiency of an upstream impoundment) are difficult to determine with modeling.
- Tidal Flow Restrictions. Restrictions of tidal flow may include undersized culverts and bridges, tide gates, and weirs. One potential modeling technique to determine the flow through the restriction is the USGS FESWMS-2DH model. Once the flows through the restriction are defined, then WASP4 can be applied to compute surface water quality impacts.
- Breakwaters, Jetties, and Wave Barriers. Construction of these coastal structures may alter the surface water circulation patterns and cause sediment accumulation. Physical hydraulic models can be used to qualitatively determine where sediment will accumulate, but they cannot reliably determine the quantities of accumulated sediment. Finite element (CAFE) or finite difference (EFDC) models can be used to determine changes in circulation/flushing caused by the addition or modification of coastal structures. The WASP4 model can be applied to determine surface water quality impacts.
- Flow Regime Alterations. Removing or increasing freshwater flows to an estuary can alter the hydraulic characteristics and water chemistry. The WASP4 model can be used to determine surface water quality impacts.
- Excavation of Uplands for Marina Basins or Lagoon Systems. Depending on the magnitude and frequency of water-level fluctuations, this activity may result in poorly flushed areas within a marina or lagoon system. Finite element or finite difference models (e.g., CAFE/DISPER and EFDC) can be used to determine a design that will result in adequate flushing. The WASP4 model can be applied to determine surface water quality (e.g., dissolved oxygen or salinity) impacts.
Although a wide range of adequate hydrodynamic and surface water quality models are available, the central issue in the selection of appropriate models for an evaluation of a specific hydromodification project is the appropriate match of the financial and geographical scale of the proposed project with the cost required to perform a credible technical evaluation of the projected environmental impact. It is highly unlikely, for example, that a proposal for a relatively small marina project with planned excavation of an upland area would be expected or required to contain a state-of-the-art hydrodynamic and surface water quality analysis that requires one or more person-years of effort. In such projects, a simplified, desktop approach requiring less time and money would most likely be sufficient (McPherson, 1991). In contrast, substantial technical assessment of the long-term environmental impacts would be expected for channelization proposed as part of construction of a major harbor facility or as part of a system of navigation and flood control locks and dams. The assessment should incorporate the use of detailed 2D or 3D hydrodynamic models coupled with sediment transport and surface water quality models.
In general, six criteria can be used to review available models for potential application in a given hydromodification project:
- Time and resources available for model application;
- Ease of application;
- Availability of documentation;
- Applicability of modeled processes and constituents to project objectives and concerns;
- Hydrodynamic modeling capabilities; and
- Demonstrated applicability to size and type of project.
The Center for Exposure Assessment Modeling (CEAM), EPA Environmental Research Laboratory, Athens, Georgia, provides continual support for several hydrodynamic and surface water quality models. Another source of information and technical support is the Waterways Experiment Station, U.S. Army Corps of Engineers, Vicksburg, Mississippi. Although a number of available models are in the public domain, costs associated with setting up and operating these models may exceed the project's available resources. For a simple to moderately difficult application, the approximate level of effort varies from 1 to 12 person-months (Table 6-2).
Factors that need to be considered in the application of mathematical models to predict impacts from hydromodification projects include:
- Variations in the accuracy of these models when they are applied to the short- and long-term response of natural systems;
- The availability of relevant information to derive the simulations and validate the modeling results;
- The substantial computer time required for long-term simulations of 3D hydrodynamic and surface water quality process models; and
- The need for access to sophisticated equipment such as the CRAY-XMP.
- b. Identify and evaluate appropriate BMPs for use in the design of proposed channelization or channel modification projects or in the operation and maintenance program of existing projects. Identify and evaluate positive and negative impacts of selected BMPs and include costs.
Several available surface water management practices can be implemented to avoid or mitigate the physical and chemical impacts generated by hydromodification projects. Many of these practices have been engineered and used for several decades not only to mitigate human-induced impacts but also to rehabilitate hydrologic systems degraded by natural processes.
In general, the design of streambank protection may involve the use of several techniques and materials. Nonstructural or programmatic management practices for the prevention of streambank failures include:
- Protection of existing vegetation along streambanks;
- Regulation of irrigation near streambanks and rerouting of overbank drainage; and
- Minimization of loads on top of streambanks (such as prevention of building within a defined distance from the streambed).
Several structural practices are used in the protection or the rehabilitation of eroded banks. These practices are usually implemented in combination to provide stability of the stream system, and they can be grouped into direct and indirect methods. Direct methods place protecting material in contact with the bank to shield it from erosion. Indirect methods function by deflecting channel flows away from the bank or by reducing the flow velocities to nonerosive levels (Henderson and Shields, 1984; Henderson, 1986). Indirect bank protection requires less bank grading and tree and snag removal.
Direct methods for streambank protection include stone riprap revetment, erosion control fabrics and mats, revegetation, burlap sacks, cellular concrete blocks, and bulkheads. Indirect methods include dikes, wire or board fences, gabions, and stone longitudinal dikes. The feasibility of these practices depends on the engineering design of the structure, the availability of the protecting material, the extent of the bank erosion, and specific site conditions such as the flow velocity, channel depth, inundation characteristics, and geotechnical characteristics of the bank. The use of vegetation alone or in combination with other structural practices, when appropriate, would further reduce the engineering and maintenance efforts.
Innovative designs of streambank protection tailored to specific environmental goals and site conditions may result in beneficial effects. Several innovative channel profiling and revetment design considerations were reviewed by Henderson and Shields (1984), including composite revetments for deep channels with flow concentrated along the bank line, windrow revetments for actively eroding and irregular banks, and reinforced revetments (stone toe protection) to control underwater activities adjacent to high banks. Composite revetments placed along the Missouri River were built with a combination of stone, gravel, clay, and flood-tolerant vegetation to protect the streambank (USACE, 1981). The different materials were selected to match the erosive potential of the streambank zones. Beneficial environmental impacts that can be achieved by this type of design include higher densities and abundance of riparian vegetation on the top bank, allowing flood-tolerant species to colonize the clay and gravel of the splash zone. The design was reported to provide better access to the channel by wildlife, and it had a greater aesthetic value.
An excavated bench (compound channel) streambank protection design, based on streambed stabilization, was used to control erosion activities on the Yazoo River tributaries in Mississippi. These tributaries were experiencing extensive bed degradation and channel migration. The design consisted of structural protection to the water elevation reached during 90 to 95 percent of the annual storm events, a flattened bench excavated just above the structural protection to provide a suitable growing environment for wood vegetation and shrubs, and a grass-seeded upper bank, which could be succeeded by native species. This practice has been reported to be successful in controlling streambank erosion (Bowie, 1981).
Streambank protection structures may impact the riparian wildlife community if the stabilization effort alters the quality of the riparian habitat. Comparison of protected riprapped and adjacent unprotected streambanks and cultivated nearby areas along the Sacramento River showed that bird species diversity and density were significantly lower on the riprapped banks than on the unaltered sites (Hehnke and Stone, 1978). However, benthic microorganisms appear to benefit from stone revetment. Burress and others (1982) found that the density and diversity of macroinvertebrates were higher in the protected bank areas.
Many valuable techniques can be used, when applied correctly, to protect, operate, and maintain levees (Hynson et al., 1985). Evaluation of site-specific conditions and the use of best professional judgment are the best methods for selecting the proper levee protection and operation and maintenance plan. According to Hynson and others (1985), maintenance activities generally consist of vegetation management, burrowing animal control, upkeep of recreational areas, and levee repairs.
Methods to control vegetation include mowing, grazing, burning, and using chemicals. Selection of a vegetation control method should consider the existing and surrounding vegetation, desired instream and riparian habitat types and values, timing of controls to avoid critical periods, selection of livestock grazing periods, and timing of prescribed burns to be consistent with historical fire patterns (Hynson et al., 1985). Additionally, a balance between the vegetation management practices for instream and riparian habitat and engineering considerations should be maintained to avoid structural compromise (Hynson et al., 1985). Animal control methods are most effective when used as a part of an integrated pest management program and might include instream and riparian habitat manipulation or biological controls (Hynson et al., 1985). Recreational area management includes upkeep of planted areas, disposal of solid waste, and repairing of facilities (Hynson et al., 1985).
Channel Stabilization and Flow Restrictors
Channel stabilization using hydraulic structures to stabilize stream channels, as well as to control stream sediment load and transport, is a common practice. In general, these structures function to:
- Retard further downward cutting of the channel bed;
- Retard or reduce the sediment delivery rate;
- Raise and widen the channel beds;
- Reduce the stream grade and flow velocities;
- Reduce movement of large boulders; and
- Control the direction of flow and the position of the stream.
Check Dam Systems
The Los Angeles River Watershed (1973) evaluated the cost-effectiveness of check dam systems as sediment control structures in the Angeles National Forest. In general, the check dam systems were found to be marginally cost-effective and were able to provide some beneficial sediment-reduction functions.
Swanson and others (1987) described the use of 71 check dams in the headwaters area of a perennial stream in northwestern Nevada. Watershed management problems, such as a history of overgrazing, led to riparian habitat degradation in streamside areas and severe gullying. The problem was ameliorated with changes in watershed management practices (livestock exclusion in streamside areas or limited grazing programs) and structural practices (check dams). Loose rock check dams, designed for 25-year floods, were selected for their ability to retard water velocities and trap sediment.
Benefits of this planned channel modification project include both instream and streamside changes. Sediment was trapped behind the dams (average of 0.9 foot in 2 years), and small wetland areas were established behind most dams. Additionally, over one-half of the channel length was vegetated in the deepest areas and the entire channel was at least partially vegetated. Streamside benefits included increased bird and plant diversity and abundance.
Grade Control Structures - Streambank and Channel Stabilization
Grade control structures (GCS) are hydraulic barriers (weirs) installed across streams to stabilize the channel, control headcuts and scour holes, and prevent upstream degradation. These structures can be built with a variety of materials, including sheet piling, stone, gabions, or concrete. Grade control structures are usually installed in combination with other practices to protect streambanks and direct the stream flow. Grade control structure design needs to account for stream morphologic, hydrologic, and hydraulic characteristics to determine the range of stream discharges for which the structure will function. Additionally, the upstream distance influenced by the structure, changes to surface water profiles, and the sediment transport capacity of the targeted stream reach need to be considered.
Shields and others (1990) evaluated the efficiency of GCS installed on Twentymile Creek (northeast Mississippi) to address channel instability. Effects on bank line vegetation were assessed using a before-and-after approach. Benefits of the GCS included local channel aggradation for about 1 mile upstream of each structure, increased streambank vegetation, locally increased fish species diversity downstream from the GCS, and the creation of low-flow velocities and greater pool depths downstream from the GCS. The primary problem associated with the project was the continued general streambed degradation after the structures were installed.
Streambank protection using vegetation is probably the most commonly used practice, particularly in small tributaries. Vegetative cover, also used in combination with other structural practices, is relatively easy to establish and maintain, is visually attractive, and is the only streambank stabilization method that can repair itself when damaged (USACE, 1983). Appropriate native plant species should be used. Vegetation growing under the waterline provides two levels of protection. First, the root system helps to hold the soil together and increases overall bank stability by forming a binding network. Second, the exposed stalks, stems, branches, and foliage provide resistance to the streamflow, causing the flow to lose part of its energy by deforming the plants rather than by removing the soil particles. Above the waterline, vegetation protects against rainfall impact on the banks and reduces the velocity of the overland flow during storm events.
In addition to its bank stabilization potential, vegetation can provide pollutant-filtering capacity. Pollutant and sediment transported by overland flow may be partly removed as a result of a combination of processes including reduction in flow pattern and transport capacity, settling and deposition of particulates, and eventually nutrient uptake by plants.
Instream Sediment Load Control
Instream sediment can be controlled by using several structural practices depending on the management objective and the source of sediment. Streambank protection and channel stabilization practices, including various types of revetments, grade control structures, and flow restrictors, have been effective in controlling sediment production caused by streambank erosion. Significant amounts of instream sediment deposition can be prevented by controlling bank erosion processes and streambed degradation. Channel stabilization structures can also be designed to trap sediment and decrease the sediment delivery to desired areas by altering the transport capacity of the stream and creating sediment storage areas. In regulated streams, alteration of the natural streamflow, particularly the damping of peak flows caused by surface water regulation and diversion projects, can increase streambed sediment deposits by impairing the stream's transport capacity and its natural flushing power. Sediment deposits and reduced flow alter the channel morphology and stability, the flow area, the channel alignment and sinuosity, and the riffle and pool sequence. Such alterations have direct impacts on the aquatic habitat and the fish populations in the altered streams (Reiser et al., 1985).
Farm, forestry, and other rural road construction; streamside vehicle operation; and stream crossings usually result in significant soil disturbance and create a high potential for increased erosion processes and sediment transport to adjacent streams and surface waters. Road construction involves activities such as clearing of existing native vegetation along the road right-of-way; excavating and filling the roadbed to the desired grade; installation of culverts and other drainage systems; and installation, compaction, and surfacing of the roadbed.
Although most erosion from roadways occurs during the first few years after construction, significant impacts may result from maintenance operations using heavy equipment, especially when the road is located adjacent to a waterbody. In addition, improper construction and lack of maintenance may increase erosion processes and the risk for road failure. To minimize erosion and prevent sedimentation impacts on nearby waterbodies during construction and operation periods, streamside roadway management needs to combine proper design for site- specific conditions with appropriate maintenance practices. Chapter 3 of this document reviews available practices for rural road construction and management to minimize impacts on waterbodies in coastal zones. Chapter 4 outlines practices and design concepts for construction and management of roads designed for heavier traffic loads and can be applied to planning and installation of roads and highways in coastal areas.
Setback Levees and Flood Walls
Levees and flood walls are longitudinal structures used to reduce flooding and minimize sedimentation problems associated with fluvial systems. They can be constructed without disturbing the natural channel vegetation, cross section, or bottom slope. Usually no immediate instream effects from sedimentation are caused by implementing this type of modification. However, there may be a long-term problem in channel adjustment (USACE, 1989).
Siting of levees and flood walls should be addressed prior to design and implementation of these types of projects. Proper siting of such structures can avoid several types of problems. First, construction activities should not disturb the physical integrity of adjacent riparian areas and/or wetlands. Second, by setting back the structures (offsetting them from the streambank), the relationship between the channel and adjacent riparian areas can be preserved. Proper siting and alignment of proposed structures can be established based on hydraulic calculations, historical flood data, and geotechnical analysis of riverbank stability.
Costs for modeling of channelization and channel modification activities range from $1,500 to over $5,000,000 (see Table 6-3). Generally, more expensive modeling requires custom programming, extensive data collection, detailed calibration and verification, and larger computers. The benefits of more expensive modeling include a more detailed analysis of the problem and the ability to include more variables in the model. Less expensive models, in general, have minimal data requirements and require little or no programming, and they can usually be run on smaller computers. The difference in cost roughly corresponds to the detail that can be expected in the final analysis.