10/30/2002 Env Bd Packet - Laserfiche WebLink

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reduce pollutant loading and reduce impacts to benthic biota may raise the allowable imperviousness. However, even when effective practices are widely applied, the threshold of imperviousness is eventually crossed, which results in a degraded condition. It is, therefore, critical that local government units (LGUs) address the impacts of imperviousness very early on by aggressive land use policies. There are many policies that can be adopted on a local level to reduce the impacts of imperviousness. Narrower streets, smaller parking requirements, swales instead of curb and gutter, and a host of other practices are outlined by documents from numerous centers, associations and agencies. One of the many growing practices is "Cluster development." This is defined as the grouping of all residential structures of a development on a portion of the available land, reserving a significant amount of the site as protected open space. Many communities in Minnesota and across the United States are updating their comprehensive plans and establishing ordinances to guide the development and construction of cluster developments. New ordinances are requiring design standards, and identifying open space and density standards. These key changes have prompted some communities to opt for more descriptive terminology, such as "open - space development" or "conservation subdivision design," instead of the more traditional "cluster development." While this use of different terminology has created some confusion, each still maintains the three basic goals of cluster development: (1) preserving open space, (2) protecting critical ecological habitat and (3) preserving agricultural land. The useable open space created by a cluster development can serve to meet a number of community goals, such as the protection of critical ecological resources, protection of wooded areas or the preservation of farmland. Obviously, these goals overlap and have the potential to conflict with one another. For example, the protection of wildlife habitat may be incompatible with the preservation of agricultural land. However, the key benefit is the quality of life preserved by the availability of open space made possible through the clustering of units. POLICIES TO PROTECT GROUND WATER When development occurs, the problems of runoff need to be addressed; often this is by "management policies" or "infiltration devices." Management policies, in this context, means reducing impervious surfaces, discharging impervious surfaces over pervious areas, disconnecting roof drains from the storm water system or other measures. Management policies are encouraged and are essential; however, general policies may require special considerations in industrial areas or other unusual cases. The other category of activity is called infiltration devices. This is everything from filter strips and swales to large infiltration ponds or infiltration trenches, tubes or other devices that conduct the runoff into the ground. In most cases the types of devices that are of most concern are devices that bypass the zone of aeration above the ground water table (vados zone) and conduct surface runoff directly into the ground. For example, swales and ditches are generally of less concern, while devices that conduct into deep aquifers are generally of greater concern. Note that these are generalizations that need to be evaluated on a site - specific basis. A site analysis should be conducted before implementing infiltration devices on project or in a community. Infiltration devices, such as basins and trenches, are controversial as BMPs for storm water management. Literature indicates ( e.g. see Pitt et al. January 1994) that operation of infiltration devices is a concern for two reasons: (1) failure to Urban Runoff 11 -13