LAYING THE GROUNDWORK
A basic tenet of water resources management is that hydrologic systems are interconnected, requiring that effective water project planning take an
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers 2 River Basins and Coastal Systems: The Primary Domains of Integrated Water Resources Project Planning Chapter Highlights Planning, construction, and operation of water projects within our nation’s river, coastal, and estuarine systems are influenced by a wide range of hydrologic, geologic, geochemical, social, ecologic, economic, and political factors. Simple solutions emerge only when project objectives are defined in terms of a very limited set of these factors. However, water projects usually have more than one purpose, with impacts that cascade among factors and interactions that are often difficult to predict. The primary objectives of this chapter are to delineate the two major types of water resources systems (river basins and coastal systems) and to illustrate the interactions among physical, chemical, biological, and social components of water resources systems over a wide range of temporal and spatial scales. The complex, multi-scale interactions among the aforementioned factors provide the essential rationale for integrated water resources project planning. LAYING THE GROUNDWORK A basic tenet of water resources management is that hydrologic systems are interconnected, requiring that effective water project planning take an
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers integrated approach; this approach reduces the possibility that attempts to solve problems in one realm, or subsystem, will cause problems in another (NRC, 1999a). Incorporating a broader view of natural systems in water resources project analysis will increase the nation’s economic productivity and environmental well-being in a sustainable manner by minimizing the potential that project benefits in one location are offset by adverse impacts (costs) on other components of the system. A water resource system is defined by this National Research Council (NRC) panel for the purposes of this discussion as “a set of interrelated physical, chemical, and ecologic components of the hydrospheric environment that act upon, or are acted upon, by one another, and by such interaction thereby determine the unity or whole.” Although the division is neither exact nor complete, water resources systems can be usefully divided into two major categories: river basins and coastal systems. “Watershed” and “catchment” are terms similar to river basin and are often used to describe smaller drainages nested within a larger river basin. Coastal systems are geographic units of the coastal zone that can be delineated based on their hydrology, geology, biology, or a combination of the three. The complex nature of coastal systems makes their boundaries harder to define, requiring flexibility and local knowledge in defining a workable unit for environmental management. Most coastal systems—especially estuaries—are strongly influenced by upland watersheds; hence coastal system analysis should generally include both the watershed and the coastal environment. In addition to linkages between river basins and coastal systems, factors relevant to water project evaluation, particularly its economic, social, and ecologic benefits and impacts, may require consideration across river basin and coastal system boundaries. Water resources project planning that integrates the linkages among the physical, environmental, economic, and societal services of hydrologic systems requires a “systems” approach that is both multi-disciplinary and multi-jurisdictional. RIVER BASINS River basins define a well-established and widely accepted framework for designing and evaluating water resources projects (Loucks, 2003; NRC, 1999a,b). River basins are drainage or catchment areas that collect precipitation and transport water, sediment, and dissolved constituents downstream within a system of connected river channels (Figure 2-1). Each basin is separated from surrounding basins by a drainage divide that is a
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers FIGURE 2-1 Typical components of a drainage basin (reprinted with permission from Blackwell Publishing, and John S. Bridge, Binghamton University; Figure 1.1, Bridge, 2003). physical feature, but not necessarily a political boundary (i.e., political jurisdictions rarely follow such features; consequently, larger river basins typically encompass many federal, state, or local jurisdictions). Nested within this larger physical system are interconnected biological, hydrological, and geochemical subsystems that operate and interact on a variety of temporal and spatial scales, such that changes in one subsystem may trigger changes in others. The river basin concept is a useful framework in which the mass balance of water, sediment, and associated geochemical constituents and their downstream fluxes provide a consistent basis for evaluating system components, their connections, and change. River basins can be divided into areas of erosion and transport in upstream reaches and transport and deposition in downstream reaches. In
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers the headwaters, the rivers are fast flowing and erosive and channels are typically steep with narrow floodplains. Water projects in this region tend to focus on hydroelectric development and water storage for flood control and for subsequent use in lower parts of the basin. As rivers emerge from the steeper uplands, the fluvial mass balance undergoes a transition from dominantly erosive to dominantly depositional, and river channels grow larger with more extensive floodplains. In this region, water projects focus more on channel works (e.g., locks, dikes, dams, channels) to provide flood protection and reliable navigation. At the distal end of the system (lake or ocean), a delta forms where the river divides into smaller channels (distributaries) and deposits its sediment load. Water projects in this region tend to focus primarily on the removal, through dredging, of sediment deposited in harbors and navigable waterways. At the coast, the freshwater runoff system driven by gravity merges with the saltwater system of the coastal zone driven by waves and tides. The two systems are intricately interconnected; changes in discharge and sediment load upstream in the watershed are ultimately felt downstream in the coastal system. River basins serve many purposes such as water supply for domestic, industrial, and agricultural purposes; wildlife habitat; transportation and navigation; energy generation; and recreation. Water projects can rarely, if ever, be optimized for all potential objectives that a project might address. Regulation of water velocity and depth in storage reservoirs and channel works for flood control or navigation can alter aquatic and riparian habitat, reduce hydroelectric generating rates, and reduce some recreation opportunities while enhancing others. The spawning and migration of fish, the hydrology of wetlands important to birds and reptiles, and the thriving of plants and animals on different levels of the food chain are all impacted by regulation of normal fluctuations in river water. Evaluation of water projects must account for a large and diverse range of hydrologic, social, economic, and ecologic factors, many of which are difficult to compare directly and all of which depend on a sound understanding of their interaction over a range of spatial and temporal scales. Watershed Delineation The term watershed, as used in this report, borrows its definition from New Strategies for America’s Watersheds (NRC, 1999a, p. 39): “a drainage area along with its associated water, soils, vegetation, animals, land use, and human activities.” This definition “connotes a relatively small drainage area, while the term river basin is reserved for very large areas. These terms
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers are not scale-specific and should not be limited to particular size classes, however, because each term properly applies to regions ranging in size from less than a small field to almost a third of the North American continent” (NRC, 1999a, p. 34). Each watershed thus has a defining topography with inputs, outputs, and interactions from ecosystem components within that watershed. These open systems may receive water from other watersheds and subsequently transport this water (along with its energy, sediment, nutrients, and contaminants) to downstream watersheds, either naturally or as a result of diversions. The nation’s watersheds have been delineated by hydrologic units that are useful in planning water resources projects. During the 1970s, the U.S. Geological Survey (USGS) and the U.S. Water Resources Council developed a hierarchical hydrologic unit code (HUC) for the United States. The HUC mapping system provides nationwide coverage of surface water drainage with extensive documentation. Attribute tables show hydrologic unit names, numerical codes, and flow direction among cataloging units. The hydrologic units are numerically arranged in a nested fashion, from the smallest watershed (cataloging unit) to the largest (regions; Figure 2-2). During the late 1970’s the Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service) initiated a national program to further subdivide HUCs into smaller watersheds for water resources planning. The 4- to 11-digit HUCs are commonly used by federal and state water resources agencies for water resources inventory and monitoring programs. By the early 1980s, 11-digit HUC mapping was completed for most of the United States. This method of classifying watersheds into progressively finer units provides a basis for addressing the spatial scale of water project evaluations. Corps Activities in the Nation’s Watersheds Through its Civil Works Program, the U.S Army Corps of Engineers (the Corps) plans, designs, constructs, and operates projects in most of the nation’s major watersheds. Project purposes include navigation, flood control, environmental restoration, hydroelectric power, water supply (domestic, industrial, and agricultural), and recreation. The Corps has played an important role in shaping water resources systems in the United States since 1850, when Congress directed the Corps to engage in its first
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers FIGURE 2-2 The hydrologic unit hierarchy, the concept of nested watersheds, and nomenclature for watersheds of various sizes according to the federal hydrologic unit code. Figure courtesy of Bruce McCammon, U.S. Forest Service, Portland, Oregon. large-scale planning exercise—to determine the most practical plan to control flooding along the Lower Mississippi River (Clarke and McCool, 1996). The Corps presently operates 384 dams and reservoirs for flood control, water supply, and navigation. The Corps also operates 75 hydroelectric
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers power plants that produce one-fourth of the nation’s hydroelectric power or 3 percent of the nation’s total electric power supply. The Corps presently maintains more than 12,000 miles (19,200 km) of inland waterways, and it operates 235 locks for navigation. The heart of the nation’s inland waterway system—the Mississippi, Ohio, and Illinois Rivers and the Gulf Intracoastal Waterway—accounts for 91.8 percent of the nation’s commercial navigation tonnage and is dependent on the Corps for maintenance. The Corps also maintains 300 commercial harbors through which each year pass 2 billion tons of cargo, along with more than 600 smaller harbors (U.S. Army Corps of Engineers, 2003c). Corps construction projects in inland waterways consist primarily of floodways, cutoffs, revetments, dikes, dams, and channels. Along the courses of major rivers, Corps projects are generally designed to increase the flood carrying capacity of the channel, protect levees, and improve navigation. In smaller watersheds, projects are generally designed to improve drainage, provide water for industrial and municipal use, and increasingly to promote ecosystem rehabilitation. Understanding the Scope of Water Resource Projects: Examples from the Missouri and Mississippi River Basins Corps projects in the Missouri and Mississippi River basins illustrate the scale, scope, and type of water resources projects implemented by the Corps under numerous authorizations for flood damage reduction and navigation enhancement in the nation’s large river basins. Although comprehensive in some aspects (e.g., hydrologic controls), planning and construction of these projects was, with few exceptions, focused primarily on two functions of these large river systems: conveyance of floodwaters and navigation. The cumulative influence of Corps projects on other functions of riverine systems, such as water quality and wetland maintenance is now receiving increased attention in its analysis of project costs and benefits. The Corps has developed agency-wide environmental objectives that indicate greater emphasis on systems-level analysis of project impacts. After the 1993 Midwest floods, Congress passed legislation to develop a comprehensive plan for the Upper Mississippi River through the Upper Mississippi River Environmental Management Program and to establish an interstate management council. This legislation required coordination among the Corps, the U.S. Fish and Wildlife Service, and other federal agencies on ecosystem management and restoration as well as flood control
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers and navigation studies. The governors of Illinois, Iowa, Minnesota, Missouri, and Wisconsin created the Upper Mississippi River Basin Association (UMRBA) to fill the void left by the dissolution of a federalstate river basin commission in 1981. In a report defining the strengths and weaknesses of Corps planning (UMRBA not dated), UMRBA recommended that the Corps seek authorizations for large-scale regional programs instead of individual projects.2 Case Study: Missouri River. The modern Missouri River basin ecosystem reflects cumulative changes that began with navigation enhancement in the early 1900s and continued with the damming and flow regulation of the mainstem Missouri River in the 1930s. Dam operations have been designed to meet two primary objectives: maintaining navigation and providing flood control. Recreation and irrigation objectives have been subsidiary and generally treated as constraints when designing operating rules to meet the primary objectives. In recent decades, environmental objectives have taken on prominence, particularly through concerns about threatened and endangered bird and fish species. Some Corps activities and their environmental impacts on the Missouri River basin ecosystem are listed below: Damming and channelization on most tributary streams, where at least 75 dams have been constructed, have fragmented the river system and partitioned the watershed into smaller units that function somewhat independently of each other, rather than as an integrated whole. Dams have reset water temperatures, trapped sediment (starving downstream areas of a supply of natural sediment), disrupted fish migration, and altered the natural hydrologic variability of many segments of the Missouri River and tributaries (NRC, 2002b). By design, the amplitude and frequency of natural peak flows have been sharply reduced by dam construction and managed flows. The replacement of natural, high spring discharges and low summer discharges with steady flows to support barge traffic has altered natural processes. For example, regulated flows (together with sediment trapping in reservoirs) restricted the formation and dynamics of river sandbars, which are crucial to spawning of 2 Conversely, the UMRBA report cited the success of the Delaware River Basin Commission for its ability to develop strong, legally binding consensus among agencies involved in water resources management.
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers sturgeon and other fish species and an important habitat for a wide variety of macroinvertebrates and birds. Regulated flows have increased minimum river depths and velocities, increased bed degradation, reduced suspended sediment loads, decreased overbank flooding and associated nutrient supply, and reduced sources of food for wildlife. As a result of the nation’s effort to manage the Missouri, nearly 3 million acres of natural riverine and floodplain habitat have been altered through land-use changes, inundation, channelization, and levee building. The impacts have reduced biodiversity, affecting species ranging from benthic invertebrates to native fish species to cottonwood trees. There is an extensive body of scientific research on the Missouri River ecosystem. More than 2,000 studies have been conducted on this ecosystem during the past 30 years, although very few have taken a systems approach that considers the interrelationships between physical, biological, geochemical, and anthropogenic components of the watershed (NRC, 2002b). There is a need for a more integrated planning and management approach that acknowledges linkages between upstream and downstream parts of the basin as well as between various systems or components that make up the Missouri River Basin. A broader hydrologic issue in Missouri River management is the system-wide effect of levee construction on flood levels. As the proportion of river banks protected by levees increases, thereby increasing the speed with which flood waves propagate downstream, downstream areas could experience increased flooding at the expense of upstream flood protection. Clearly, a system-wide management approach is needed to adequately consider the trade-offs among flood protection, navigation, recreation, and ecosystem services. The Corps has been the acknowledged controlling authority for Missouri River operations since the completion of construction of the major main stem dams in 1963. In 1979, the Corps codified its operating rules for projects within the basin in the Missouri River Main Stem Reservoir System Reservoir Regulation Manual (Master Manual) (U.S. Army Corps of Engineers, 1979). In the 1980s, it became clear that a growing recreation industry, a small shipping industry, and the increased importance attached to environmental impacts of the Missouri water system, required a revision of the operating rules. Since that time, the Corps has been engaged in a long-running effort to revise the Master Manual, which has included extensive modeling of main stem flows, evaluation of benefits and costs
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers among the competing interests dependent on river flow, proposed revisions to the Master Manual, and stakeholder consultation at a range of levels. To date, political differences between upstream and downstream states (particularly regarding the relative importance of navigation, flood control, recreation, and ecosystem restoration) have prevented agreement on revised operating rules and stalled environmental restoration efforts that depend on such revisions. The unresolved conflicts and absence of a revised master plan came to a head in the summer of 2003 when the Corps found itself subject to two conflicting court orders, one requiring dam releases to maintain depth in the shipping channel and another requiring reduced flows to increase habitat for federally protected bird and fish species. The immediate impasse was resolved in favor of lower flows by a third U.S. court convened to adjudicate the conflict, although a substantial delay in flow reduction was introduced and the long-term prioritization of conflicting objectives remains unresolved. Case Study: The Mississippi River and Tributaries Project. The Mississippi River and Tributaries (MR&T) Project is one of the most comprehensive Corps endeavors in inland waters. It involves flood control and navigation improvements in all four major river basins of the Lower Mississippi River Valley: St. Francis in east Arkansas; Yazoo in northwest Mississippi; Tensas in northeast Louisiana; and Atchafalaya in south Louisiana (Figure 2-3). MR&T project work is authorized by the Flood Control Act of 1928 (FCA) and its amendments. The original FCA authorized work that would protect the Lower Mississippi River Valley against Mississippi River floods only, although the tributary streams within the basins caused frequent flood damage and could not be prevented by the main stem Mississippi River protective works. Later amendments to this act authorized work to also alleviate flood problems of the tributaries in all four aforementioned river basins. In March of 2002, Brigadier General E.J. Arnold reported that the MR&T project was 87 percent complete physically. He also estimated that the nation had invested about $10.8 billion for planning, design, construction, operation, and maintenance of the project and that the accumulated savings in flood damages totaled more than $258 billion (Arnold, 2002).
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers FIGURE 2-3 The Mississippi River Drainage Basin (outlined in light gray). The Mississippi River and Tributaries (MR&T) Project area is the lowest portion of the Mississippi River Drainage Basin, beginning in lower Kentucky and ending at coastal Louisiana (see smaller box, lower left). Figure courtesy of U.S. Army Corps of Engineers, New Orleans District. The MR&T project is made up of four construction elements: (1) levees for containing flood flows; (2) floodways for the passage of excess flows past critical reaches of the Mississippi; (3) channel improvement and stabilization of the channel in order to provide an efficient navigation alignment, increase the flood carrying capacity of the river, and protect the levee system; and (4) tributary basin improvements, such as dams and reservoirs, pumping plants, and auxiliary channels, for major drainage and flood control . The Mississippi River levee projects are designed to protect the alluvial valley against the projected flood by confining flow to the leveed channel, except where it enters the natural backwater areas or is diverted purposely into restricted floodways. The main stem levee system, comprised of levees, floodwalls, and various control structures, is 2,203 miles long (Figure 2-4). Some 1,607 miles lie along the Mississippi River itself and 596 miles lie along the south banks of the Arkansas and Red Rivers and in the Atchafalaya basin. The levees were constructed by the federal government
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers FIGURE 2-9 Image of a bald cypress swamp impacted by saltwater intrusion near New Orleans, Louisiana, resulting from the building of the Mississippi River Gulf Outlet, a 76-mile-long, man-made navigational channel connecting the Gulf of Mexico to the city of New Orleans. The U.S. Army Corps of Engineers (1999c) report speculates that 1,500 acres of cypress swamps and levee forests have been destroyed or severely altered as a result of the MRGO. In addition, the Corps (1999c) report estimates the loss of 3,400 acres of fresh or intermediate marsh, 10,300 acres of brackish marsh, and 4,200 acres of saline marsh. Photo courtesy of Rex H. Caffey, Louisiana State University. the importance of integrated water systems planning and the significant multi-jurisdictional challenges faced when managing the coastal zone. Terrestrial impacts on the coastal zone can be seen over a wide range of scales, from water quality and shoreline loss in the Gulf of Mexico, associated with agricultural practice and river regulation. in the Mississippi-Missouri-Ohio River system, to beach erosion and harbor siltation in many smaller coastal watersheds. Hypoxia and eutrophication in many of the nation’s coastal waters illustrate the influence of the delivery and quality of inland surface waters
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers FIGURE 2-10 Image of the 76-mile-long Mississippi River Gulf Outlet, Lake Borgne, and adjacent coastal wetlands. New Orleans urban area located in upper left, adjacent to Lake Pontchartrain. Figure courtesy of John Barras, U.S. Geological Survey, Lafayette, Louisiana. on environmental quality in the coastal zone. Several investigations during the past 10 years have linked chronic bottom-water hypoxia (<2 mg/L dissolved oxygen) in the north-central Gulf of Mexico to increased nutrient loading in the Mississippi River (Bratkovich et al., 1994; Dortch et al., 1994; Justic et al., 1993; Rabalais and Turner et al., 2001; Rabalais et al., 2002). In midsummer, the aerial extent of bottom-water hypoxia may cover up to 20,000 km2, which concerns fishery managers and others interested in estuarine and coastal water quality. Hypoxia is also common in coastal bays and estuaries and is often linked with activities in adjacent watersheds. Approximately half of the 58 estuaries within the U.S. Gulf of Mexico coastal region have evidence of nitrogen and phosphorus loading from upland sources at rates that indicate eutrophic or near-eutrophic conditions. Nutrients from adjacent watersheds have been identified as a major cause of water quality degradation in coastal Florida, Louisiana, and Mississippi (NRC, 2000; USEPA, 1999).
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers Suspended sediment loads carried by the Mississippi River have decreased by one-half since European immigrants settled the Mississippi Valley. Most of the decrease in sediment load has occurred since 1950, because the largest sources of natural sediment supply were cut off from the main stem of the Mississippi River by the construction of large reservoirs on the Missouri and Arkansas Rivers (Meade, 1995). This decrease in riverborne sediments has adversely affected coastal habitat integrity in the Mississippi River delta region, where roughly 1 million acres of coastal wetlands have been lost since 1940. The decrease in suspended sediments and the channeling of sediments into deep water to facilitate navigation have contributed to significant land loss in coastal Louisiana. Proposed coastal protection and restoration projects to address this problem in coastal Louisiana may cost as much as $14 billion (Committee on the Future of Coastal Louisiana, 2002). The trapping of sediment in reservoirs constructed for flood control and water supply can reduce the supply of sediment needed to replenish coastal beaches. Efforts to mitigate the loss of sediment from coastal systems are under way, although few projects have been successfully completed. Difficulties include the cost of sediment removal, the long distances from the reservoirs to the coastline, and the potential for contamination in the deposited sediment and its interstitial waters. Consideration is being given to the removal of some deteriorated dams, which should allow some of the trapped sediments to be transported to the coastline. Land disturbance and erosion in terrestrial watersheds can also increase the supply of fine sediments to coastal waters requiring dredging of navigable coastal waters, and delivering nutrients that degrade the water quality of estuaries. It has been recognized that the recovery of the Chesapeake Bay requires a reduction in sediment and nutrient supply from terrestrial sources (Chesapeake Bay Program, 2000). Projects that alter the structure or function of one component of a natural water resources system will tend to impact not only the immediate project area but other parts of the system as well. Appropriate evaluation of project impacts requires an understanding of the broader system within which the project is placed. If the cumulative effects of reservoir and levee construction on Mississippi River sediment transport had been analyzed as part of the Mississippi River and Tributaries Project, wetland losses in the Mississippi deltaic plain could have been foreseen, allowing for the possibility of earlier and more effective mitigation. A system perspective is needed not only for impact assessment, but also for successful project design. Elements of a river basin or coastal system and its history (as well
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers as other existing or proposed projects) can have important influences on the success of a water resources project. SPATIAL AND TEMPORAL SCALES FOR PLANNING AND MANAGEMENT OF WATER RESOURCES PROJECTS Integrated water systems planning and management require a balanced evaluation of all relevant factors over appropriate scales of space and time, such that multiple objectives can be investigated, unintended consequences can be identified, and the complete costs and benefits of a project may be evaluated. Such an approach may often require that project planning and evaluation be conducted at spatial and temporal scales larger than specifically required for isolated individual projects. Because of the range of factors involved in evaluating modern water projects, it may often be necessary to evaluate different factors at different spatial and temporal scales. Spatial Scales for Water Resources Project Planning As some of the preceding case studies illustrate, water project activities can affect, for better or worse, hydrologic, ecological, and economic conditions beyond the immediate project area. Flood protection levees in one location, or the progressive construction of many levees, can exacerbate flooding in downstream portions of a waterway. Project alteration of hydrological regimes and sediment transport directly affect the magnitude and timing of nutrient and contaminant loadings to downstream systems. Construction of dams and water diversion projects and the dredging of ports affect water chemistry and quality, including water temperature and nutrient loading. Projects thus potentially alter the growth and reproduction of plants and animals, the location of fisheries, and the intensity and type of local commerce and shipping. Corps permitting and projects in rivers and coastal systems also potentially disrupt movements of animals, which in turn can affect the sustainability of fisheries and the trophic structure of aquatic systems. Development or maintenance of navigation channels will influence the relative costs of different shipping alternatives, thereby potentially impacting a wide range of economic activities. Reoperation of storage reservoirs for ecosystem or recreation objectives can influence the timing
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers and amount of hydroelectric generation, which can have repercussions throughout a regional power grid. The complexities of defining the appropriate spatial scale are particularly difficult in the context of environmental stewardship, which requires consideration of the ecosystem affected by the project. Among the basic ecosystem principles noted in Ecosystem-Based Fishery Management, a National Marine Fisheries Service (NMFS, 1998) report to Congress, are the principles that “multiple scales interact within and among ecosystems,” that “components of ecosystems are linked,” and that “ecosystem boundaries are open.” Ecological systems are connected by, among other things, water flow, the movement of people, and the activities of plants (e.g., photosynthesis; the influence on transport and transformation of nutrients) and animals inhabiting the ecosystems. All of these vectors influence the transport of materials, which may indirectly affect an ecosystem’s natural balance within its watershed or coastal system. Water flow transports nutrients, sediments, and contaminants from the landscape to waterways and ultimately downstream to estuaries and the ocean. Many organisms, including economically and ecologically important species and their prey, are dependent on large portions of watersheds to complete their life cycle. Almost without exception, the watershed approach is endorsed by scientists, water planning experts, and the National Research Council (NRC, 1992, 1996, 2000, 2001a, 2002a) as the framework in which the physical and ecological aspects of water resources planning should occur. In the context of coastal systems, a comparable framework would be defined in terms of the region encompassing significant flows of water, sediment, nutrients, and contaminants. Reasons supporting watershed and coastal systems planning include the inherent connectedness of hydrologic and ecological systems; the importance of location within the landscape to both habitat function and the impact of anthropogenic activities; and the localization of potential cumulative effects and unintended consequences within a watershed or coastal system. In the terrestrial realm, a framework for water project evaluation is given by the hydrological unit hierarchy shown in Figure 2-2. Used as a basis for project evaluation, this system makes clear the nested nature of potential project benefits and costs and would provide a consistent basis for project analysis. Use of hydrologic unit codes and “nested watersheds” is supported by numerous studies (Seaber et al., 1987; U.S. Department of Agriculture, 2003). The analogous spatial planning unit for coastal waters is the “estuarial region” and the “coastal unit” (NRC, 1999b).
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers Regardless of the basic heuristic for determining the spatial boundaries on possible project effects, a set of clearly defined temporal and spatial scales for water planning investigations cannot be specifically prescribed for all cases. The hydrologic, ecologic, and economic setting of any particular water resource project will vary from that of other projects. While one might ideally envision a national analysis for every Corps water project to ensure consistency and fit with a national water policy or set of objectives, such an approach would be financially impractical. Nonetheless, the essential starting point for evaluating any water project is the identification of the essential factors and objectives of that project and their relevant spatial scales. There may be instances in which a local focus is sufficient, but there are also cases (e.g., commercial navigation on navigable waterways, waterways that provide critical habitat for migratory endangered birds, projects that contribute significantly to regional electric power grids) where project planning and evaluations must extend beyond watershed boundaries, at least for some essential aspects of the project. Because the spatial scale for project evaluation varies from case to case, flexibility in defining the appropriate spatial scale is essential for effective, integrated water resources management. Current cost-sharing and project time lines can pressure the Corps to limit the general scope and spatial scale of a project evaluation to an area in the immediate vicinity of the proposed project. Revisions to project planning arrangements that would help address these pressures are discussed in Chapter 5. Temporal Scales for Water Resources Project Planning The processes that operate within watershed and coastal systems operate at varying rates. Thus, the magnitude of any measurable parameter that may be important to the design or operation of a water resource project is not fixed and varies through time. This temporal variability has been recognized in engineering practice for decades and has historical been addressed by assuming that important design variables will vary within a predictable range over a given time, allowing designers to build in a margin of safety. However, as the objectives of water resource projects have increased and as efforts to more fully account for the behavior of complex natural systems are made, sensitivity to temporal variation and the scale at which these variations takes place has become increasingly important. For water projects, the most significant factors whose time variability must be accounted for are those associated with climate.
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers Consideration of Climate Variability and Change in Water Resources Planning A large body of research relating to climate variability has been published in water resources planning and management archival literature during the past 15 years (Cayan et al., 1998, 1999; Kahya and Dracup, 1993). This research indicates that there have been significant variations in interannual and interdecadal precipitation and streamflow in selected areas of the United States, most notably in the Pacific Northwest, the Pacific Southwest, and the southeastern United States (Cayan et al., 1999). The causes of these variations have been determined, allowing for prediction with lead times of up to seven to nine months (Goddard et al., 2001). It also has been demonstrated that the incorporation of predictable interannual and interdecadal streamflow variations in the planning and management of water resource systems can lead to considerable increases in hydroelectric and water supply revenues, flood reductions, and better approaches to drought management (Hamlet and Lettenmaier, 1999; Piechota and Dracup, 1996). Longer-term trends in temperature and precipitation are also well documented. Instrumental records indicate that average annual precipitation and temperature increased during the twentieth century in the United States and over most of the northern hemisphere (IPCC, 2001; Melillo et al., 2000; NRC, 2001b). These changes were most pronounced at high latitudes. Alaska has warmed an average of 2° C since 1950, while the lower 48 states warmed roughly 0.6° C during the past 100 years (which is roughly the global average). Although there is a great deal of regional and local variation, average annual precipitation increased during the twentieth century by approximately 5 to 10 percent in the conterminous United States, with much of it due to an increase in the frequency and intensity of heavy rainfall (Karl and Knight, 1998). Climatic changes of this nature have practical significance to water resource managers and numerous implications for the design of water resources projects. Accelerated sea-level rise is regarded as one of the most costly and most certain consequences of global warming. If sea-level rise increases at rates projected by the United Nation’s IPCC (2001) during the next century, many of the world’s low-lying coastal zones and river deltas could be inundated. Along the low-lying southeastern coastal margin, where the majority of U.S. flood losses occur in an average year, effective design and maintenance of flood control works will require the incorporation of long-term sea-level and precipitation trends in project design.
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers Case Study: The California Reservoir System. California water management is charged with meeting many socioeconomic challenges including providing a reliable water supply; protecting developed areas from flooding; and balancing competing water uses related to urban, agricultural, and environmental benefits. California’s hydrology is extremely variable, involving multi-year droughts and short-duration extreme floods, and this variability increases the difficulty of water resources management. Regional water systems have been constructed to dampen the effects of droughts and floods on beneficial use. Two of the major regional systems serving California are the Central Valley Project (CVP) and State Water Project (SWP) systems, each collecting surface water runoff from the northern half of the state for redistribution throughout the state. Operation of the CVP-SWP system is driven by risk management associated with floods and droughts. Management guidelines are not translated into detailed risk assessments but are expressed through general operating principles (Brekke, 2002). These principles include the following: (1) seasonal to annual CVP-SWP operation requires hydrologic anticipation of supply availability and flood potential; (2) information supporting this anticipation comes from past experience (i.e., climatology) and snow survey data collected during winter; (3) the consequence of using supportive information errantly (i.e., over anticipation of supply availability or underestimation of flood potential) is a system operational state where drought and flood vulnerability is increased (without identifying specific consequences of that increased vulnerability); and therefore (4), the management action is to avoid using information errantly by using it very conservatively (e.g., 90th percentile exceedance anticipation of supply availability). Past results of operations have been mostly satisfactory; however, the periodic occurrence of floods (e.g., New Years 1997 in the Sacramento Valley) and drought-induced water shortages (e.g., 1987-1993), coupled with the increasing competition for California water, leads to the question of how to conduct hydrologic anticipation more aggressively while maintaining adequate risk protection and providing a more flexible supply management capability. Improvements in longer-term water management can look to climateweather connections between Pacific basin phenomena (e.g., El Niño Southern Oscillation [ENSO]) and subsequent western U.S. hydroclimatic variations (i.e., weather and hydrologic variations). For example, it is widely understood that when ENSO is in its El Niño phase during summer months, there is likely to be increased winter precipitation in the Pacific Southwest (PSW) and decreased winter precipitation in the Pacific Northwest (PNW). Although ENSO has a relatively weak connection to
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers Northern California hydroclimatic variations compared to the PNW and PSW, this illustration makes the point that distant climatic information in the Pacific can be monitored and would seem to improve our foresight of hydrologic conditions and affect how we manage our water resources. Despite the available data, distant climatic information is seldom used to influence large regional system operations. Planning for Continued Climate Change The changes in climate that are projected to occur during the next century could have a significant impact on the planning, design, and operation of water systems in the United States. Particularly, climate change could have a significant impact on river basin deltas and coastal regions. In spite of current levels of uncertainty associated with General Circulation Models (GCMs) that are used to simulate and predict climatic change, there is general agreement among climate scientists that (1) the amount of greenhouse gases such as carbon dioxide is increasing in the atmosphere; (2) the world wide atmospheric temperature increased approximately 0.5° C (0.6 ± 0.2° C) during the twentieth century; and (3) the average global sea-level rise ranged from 20 to 65 mm from 1910 to 1990. Furthermore, as stated in an assessment of the Intergovernmental Panel on Climate Change, “globally it is very likely that the 1990s was the warmest decade, and 1998 was the warmest year” in the instrumental record (1861-2000) (IPCC, 2001, p. 2). The three parameters of global climate change that will likely have the greatest impact on water resources in the United States are air temperature changes, sea-level rise, and precipitation. Temperature. Based on predictions from nine GCMs, the IPCC reports a potential air temperature increase during the next 100 years ranging from a low of 1.0 percent to a high of 5.2 percent by the year 2100 over a base of 1961 to 1990 values (IPCC, 2001, p. 541). Sea level. Based on the predicted sea-level rise from 14 GCMs, the IPCC assessment reports a potential global 0.10- to 0.87-meter increase in sea level from 1990 to 2100 (IPCC, 2001, pp. 670-671). Precipitation. Based on predictions from nine GCMs, the IPCC reports a potential precipitation increase ranging from a low of 1.0 to a high of 8.9 percent by the year 2100 over a base of 1961 to 1990 values (IPCC, 2001, p. 541).
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers The impact of global climate change (based on increases in air temperature, sea-level rise, and precipitation) potentially could be a significant problem and have negative impacts on Corps planning and management of water resource projects. For example, hurricane protection levees in the New Orleans region are presently designed and maintained by the Corps to protect the city from storm surge flooding. Due to subsidence associated with groundwater withdrawals, levees along the Mississippi River that prevent sediment deposition, global sea-level rise, drainage and oxidation of organic soils, and natural compaction and dewatering of the deltaic sediments upon which the city has been constructed, most of the city of New Orleans lies below mean sea level (MSL). Hurricane protection levee design heights range from about 4.5 to 6 meters above MSL. The levees along the Lake Pontchartrain shoreline are designed at a height that exceeds the surge and waves of a Category 3 hurricane. The design for these levees assumes no increase in MSL (Burkett et al., 2003). The original congressional authorizations for these projects did not provide the Corps with the authority to include sea-level rise in its plan for maintaining these levees. The Corps is planning to review these projects, however, to determine if changes should be made to account for sea-level rise in future authorizations for these and other flood protection projects in the New Orleans region (Alfred C. Naomi, U.S. Army Corps of Engineers, New Orleans District, personal communication, December 4, 2003). SUMMARY The spatial and temporal complexities of river basin and coastal systems discussed in this chapter present significant challenges to effective water resource planning and implementation. Historically, the Corps has been at the forefront of engineering practice and has undertaken much of the pioneering work in relevant civil engineering fields. The Corps has long embraced the underlying philosophies of systems planning and watershed approaches to project implementation and management. As growing awareness of the importance of coastal systems emerged, the Corps has attempted to incorporate greater understanding of these systems in water resource planning in the nation’s coastal regions. However, as the nation’s expectation for such projects has changed and greater emphasis has been placed on maintaining or restoring ecological function in the nation’s river basin and coastal systems, the portfolio of water resource projects planned and implemented by the Corps has changed. As a consequence, uniform application of key systems planning concepts in its water resources planning
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River Basins and Coastal Systems Planning within the U.S. Army Corps of Engineers and implementation efforts is a goal the Corps continues to pursue. The following chapters explore some of these concepts in greater detail and attempt to recommend how these goals may be achieved.
Representative terms from entire chapter: