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CHAPTER 1 OVERVIEW, FINDlNGS AND RECOMMENDATIONS Various methods are used for the control of storm water runoff. In some areas of the country, systems of separate storm sewers are used. A few large cities with systems of combined sewers have investigated the possible use of underground tunnels to store combined sewage overflows for later treatment, Other cities have considered the use of underground tanks for detaining sewage flows in combined sewers. Many urban governments have studied the problems of peak sewer flows as they are aggravated by inflows from roof drains. As a result, some cities have required that all roof drains be disconnected from combined or sanitary sewers. A number of urban governments also require connections of building foundation drains to be removed. Urban governments and other local public agencies having responsibility for stormwater drainage and local flood control have been giving increasing attention to practical and economical means of reducing the losses and inconveniences caused by flooding from surface runoff. In many areas, local jurisdictions have adopted legislation requiring that land developers take steps to assure that peak runoff flow rates after development are not in excess of those that prevailed prior to development. To meet the requirements of local public agencies for controlling runoff rates, some developers of commercial and industrial properties have installed devices on roof drains of buildings with horizontal roofs to reduce flow rates into the drains, thereby minimizing the effect of roof drainage on peak sewer flows where downspouts are connected to sewers. Man y developers of commercial properties have found it advantageous to construct parking lots with restricted surface drains designed to detain stormwater runoff. Often, this is the only practical means for meeting requirements of the local jurisdiction for limiting the peak runoff flows. This is especially true for developments where little open space is available for constructing detention ponds or basins. Developers of single-family residential subdivisions and multifamily residential complexes in many parts of the country have constructed ponds and basins in the open spaces of these developments for capturing and detaining storm water runoff flows. In some areas, these ponds are also intended to reduce soil erosion and the accompanying siltation. In addition to the above ways of detaining runoff on the site where it originates as rainfall, new concepts for handling and using runoff are being investigated and developed. These include: the use of porous pavements fOf absorbing rain water and snowmelt, the capturing of storm water for beneficial uses, and new ways to detain rainfall on rooftops. Storm Water Management Problems The problems of urban stormwater management that were investigated can be classified in three major groups: (I) local flooding; (2) pollution resulting from combined sewer overflows, storm sewer discharges and soil erosion; and (3) problems associated with the beneficial use of storm water. Local Flooding: Although no data on the evaluation of flood losses.is available, estimates of the American Public Works . Associa tion in their publication, Urban Drainage Practices, Procedures and Needs, 1966, put the cost at over $1 billion per year. This estimate is said to indicate only the general magnitude of the losses and it is expected to mount .as urbanization increases. The major losses from flooding include the damage of real and personal property, loss of human and animal life and disruption of natural streams and fish habitats, Flooding also causes inconvenience, nuisances and delays; and storm sewer discharges and unavoidable overflows of combined sewers into receiving waters can produce health hazards, To document local flood losses sustained in urban areas, an example is given below of extensive flooding that resulted from I