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r I I I I I I I I I I I I I I I I I I I losses from direct damages, other flooding losses are those incurred by businesses and industries when production or shipments are interrupted. A cost that is usually unnoticed and unrecorded is the public cost of flood cleanup operations and restor- ation of services in urban areas. Structural solutions including storage reservoirs, channelization, levees and dikes, have dominated flood control programs in past years. Many persons feel there are better solutions available which would not present hazards to local water resource ecosystems, as are inherent with many structural solutions. Non-structural measures include land treatment, flood-proofing, land use controls, and various standards and requirements for drainage system design, devised to limit peak stl'rm- water runoff rates. Soil Erosion, Sedimentation and Water Quality Impacts Soil erosion and the accompanying sedimentation are major concerns in many ?rbanized areas, particularly where highly-erosive, silty surface soils are preva- lent. Steep topography and sparse or poor ground cover also are factors which in- fluence erodability and potential sediment loads. In areas where land development is underway, grading and excavation operations on development sites can multiply eroded soil quantities and sediment deposits by hundreds or thousands of times. Runoff flows denude valuable iand, and huge quantities of sediment are often de- posited regularly on downstream floodplains and in receiving streams, l.akes and bays. In the Potomac River at Washington, D.C., 20 percent of the total sediment is said to come from upstream urban areas that comprise only two percent of the total upstream tributary drainage area. Statistics indicate that farm land near Washington, D.C. having an annual soil loss of 0.078 tOns/acre can increase by a factor of one thousand (to 78 tons/acre) when the same land is developed. Accepted good practice calls for limiting soil l.oss to approximately 15 tons/acre. Studies made in recent years have shown that many of the substances causing pollution of receiving waters are transported by eroded soils and deposited as sediment. Control of soil. erosion is the key el.ement in a resource management pl.an to combat soil loss, unwanted soil buildups and associated water degradation. Non- structural measures can be applied prior to, and during, land development to keep soil erosion and subsequent sedimentation within tolerable limits. Structural. measures, although they are usually more costly and sometimes produce other unex- pected prOblems, can also be implemented effectively. Local conditions and cir- cumstances will ordinarily favor one approach over another. Unfortunately, the techniques applied are often selected without evaluation of alternative approaches; and decisions are made in favor of methods that can be adopted expeditiously with little advance planning. Overflows of combined sewers, during wet periods when surface runoff rates are excessive, produce serious pollution of local waterways and receiving waters. The surcharged sewers often spill their contents into streets, highway underpasses and basements of buildings--causing flooding, pollution, health threats, inconvenience and economic losses associated therewith. Different approaches have been used in attempting to reduce combined sewer overflows. Washington, D.C. initiated a pro- gram of sewer separation to eliminate many ~f its combined sewers. The M~tropoli- tan Sanitary District of Greater Chicago has a massive pollution control program underway involving construction of six thousand acre-feet of conveyance tunnels and associated pumping facilities. San Francisco, Cleveland, Cincinnati and Boston are other cities that have either initiated or planned programs for control of com- bined sewer overflows. The different approaches being used have one major similar- ity--"high cost". There appears to be no simple, low-cost solution. 3