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fev,rer), the less likely that on-street parking \vill need to be provided as part of the road \vidth. The Bucks County (Pennsylvania) Planning Commission street \vidths for cluster subdivisions were set at 22 feet for streets with an average daily traffic volume of 2,000 vehicles or less, and 24 feet for streets \vith traffic volumes greater than 2,000 vehicles per day. Both measurements include curbs. The issue of access by emergency vehicles is often raised \vhen a community seeks to reduce residential street \vidths in all or part of its jurisdiction. Clustering houses is an effective way of keeping floodplains largely free from development \vhile allmving the same level of density that vvould be permitted under normal conditions. The variable street configurations in such developments are beneficial because they tend to require less land area to be devoted to roads and impervious surfaces than in conventional subdivisions and they can be aligned in a manner that best accommodates natural drainage patterns. (The cluster development concept is described in detail in Chapter 3.) Detention and retelltio1t basins. In upland areas, detention basins of sufficient size and depth are used to hofd and gradually release runoff. [t is up to the regulating jurisdiction to dictate the size, location, storage capacity, and release rate of detention basins. A typical requirement is enough capacity to capture the two-year, 24-hour event and hold it for at least 24 hours before it is released into a \vetland or other watercourse. Retention basins, sometimes called wet ponds, are subject to similar requirements although they are not usually designed to release storm water at a particular rate other than \vhat naturally occurs through evaporation and seepage. Retention basins can double as \vildlife habitat and as recreational facilities. The Urban Land Institute nm\' recommends the inclusion of retention basins in large developments for both their storm water management ability and their attractiveness to buyers and tenants (Urban Land 1992). Infiltration devices. Infiltration devices are designed to promote percolation of stormwater runoff from roofs, do\vnspouts, driveways, and large lawns into the ground before that runoff can reach a \\'ater body (e.g., a stream). These devices can reduce the flow of \vater do\vnstream, help preserve natural vegetation, and potentially lower development costs by limiting the size of the se\'ver system needed. The most common infiltration devices arc trenches and basins. lnfiltration trenches are three. to eight-foot- deep gravel-filled trenches into which runoff is diverted from home sites. Runoff collected in the trench is then released into the subsoil. The trenches can be used in conjunction v....ith s\vales. Natural depressional areas. \Vatersheds naturally contain topographic depressions that collect storm1,.vater runoff and delay its release to stream and other outlets. These areas are ~also created by road\\'ay embankments. Some depressional storage areas release rainfall excess only via evaporation or by slow infiltration into groundwater, thus preventing runoff from contributing to the flood. Site grading often results in the loss of these depressional storage areas and can have serious effects on the rate and volume of site runoff. The initial site analysis and subsequent drainage plan should identify depressional storage areas so that they are not eradicated through grading and site preparation. Swales. A swale is a grass-lined natural depression or wide shallow ditch used to temporarily store, route, or filter runoff. (See Figure 4-2.) In low-density residential areas, swales are a very common alternative to curbs and gutters. "lan-made s\vales located along natural drainage path and on the lower portions and edges of a development site can collect or redirect flow from yards or streets. The sides of swales should be shallow to allow for the slmv, even flow of runoff, which enables the swale to also absorb \vater. Grasses should be planted in swales to help slov.' runoff, stabilize soil, and slow weed growth. The use of swale drainage is advocated as part of a total storm water management program in a number of states, including I\~ew Jersey, Maryfand, Ohio, and Florida. Riparian buffers. Buffers are forested or vegetated areas adjacent to \vater bodies designed to stabilize banks, limit erosion, reduce runoff volume, preserve wildlife habitats, and create open space and recreation areas. (A greem.vay, as described in Chapter 3, is a linear open space established along a natural corridor, such as a riverfront, stream valley, or ridgeline, or over land along a railroad right-of-way or canal route, which has been converted to recreational use. \Vithin a greenway, a riparian buffer \vmIld be the area immediately adjacent to the \vatercourse.) The appropriate \-vidth of a buffer depends on the site conditions (e.g., slope and vegetation), the nature of storm water flo\-v from adjacent properties (e.g., the quantity and velocity of \vater, and the existence of channels), and the cost and benefits of the buffer to the developer and the community. To assemble a riparian buffer, a local government may purchase the land or it may require a developer to set aside lands immediately adjacent to \-vaterfronts for buffers. Such requirements may be provided \-vithin a storm\-vater management ordinance or in a separate riparian buffer ordinance. Buildillg setbacks. A building setback line further protects the \vatenvay from erosion and water-quality degradation due to runoff caused by development. In many cases, a setback of IOO to 200 feet from the watC'rway will serve to channel the drainage in the opposite direction of the stream or river and thus reduce pollution. A number of states and communities have applied setback standards to designated streams, rivers, and lakes through their shoreland management programs. For example, the Michigan Department of Natural Resources plan for the Betsie River in Michigan establishes a 20D-foot setback, with a reduction of five feet for everyone foot of upward slope, with a minimum setback of 150 feet. NFIP standards for the tloodway can f1ct as a de facto setback in that they prOhibit-any buildings or other structures from J encroaching into the flood way if they result in any increase in flood levels during a IOO-year flood. 31