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Pennsylvania Handbook of Best Management Practices for Developing Areas§3. Planning Concepts §3.1 Some Important Principles of Effective Site
Planning Avoiding the adverse effects of development requires the preparation of a comprehensive watershed management program. In addition to structural and nonstructural BMPs, elements of a watershed management program include growth management, land-use planning, long-term operation and maintenance, public education, and dedicated funding sources. This section presents techniques for site planning that can enhance land values while reducing the water-resource effects of pending development. §3.1 Some Important Principles of Effective Site Planning A central premise of site planning is that effective site layouts and designs can minimize the need for conventional structural measures, such as storm sewers, thereby reducing the costs of development. Site planning also benefits from an appreciation of the inherent value of natural features in providing essential hydrologic functions and enhancing a site’s aesthetic and recreational value. Efficient site planning can be facilitated by local government ordinances that are flexible in allowing innovative layouts or clustering of development to avoid intruding on sensitive areas or natural drainage features. Similarly, it is helpful if both developers and local governments are open to alternative landscaping approaches, which can both lower long-term maintenance costs and reduce offsite impacts. A truly comprehensive program for watershed management involves extensive planning by state and local government and coordination with potential developers. A comprehensive program might include:
Site planning and design is a complicated process involving many components. Traditional site planning must address zoning, densities, setbacks, access, traffic patterns, and a number of other factors. Additional site planning elements necessary to meet water-quality and sensitive-area objectives, include:
§3.2 Resources for Identifying and Mapping Sensitive Areas The site planner can preliminarily identify some sensitive areas by using existing mapping resources available from federal, state, regional, and local entities. Below is a partial list of resources:
The above resources may be available from Planning Commissions, Municipal Offices, and County Conservation Districts. In general, the materials from these resources are appropriate only for preliminary planning. In most cases, the delineation or quality of a sensitive area can be determined only through on-site evaluation. In particular, proper identification of wetlands requires knowledge of hydrology, soil, and vegetation as mandated by current federal wetland-determination methods. A wetlands scientist can be consulted to provide standard field identification practices to identify wetland and riparian plant and animal species and hydrologic conditions of wetlands and wetlands soil. Overlay mapping techniques and the use of geographic information systems (GIS) are useful approaches for identifying the most critical areas in need of protection during development. The sensitive areas must be identified early in the site planning process. Working from a map that compiles information from several "layers" encourages innovative site layout to prevent conflicts with critical areas. Such avoidance early in the project prevents costly reevaluation and redrawing of site plans after the conflict is identified by a permit reviewer or third party. §3.3 Important Functions of Sensitive Areas and How They are Best Protected Certain sensitive areas have unique hydrologic, habitat, or pollution-mitigation characteristics that warrant special protection. The areas are particularly susceptible to damage during site development. Five categories of sensitive areas are:
Stream corridors include waterways and adjacent riparian lands. Natural waterways provide habitat for fish, aquatic plants, and benthic (bottom dwelling) organisms. Development in waterways may destroy aquatic organisms and introduce large loads of sediment and pollutants into the waterways. Modifying waterways to accommodate development also may destroy the physical features essential to a good habitat, including: stable stream banks and bottom substrates, pools and riffles, meanders, and spawning areas. Vegetated riparian land adjacent to streams stabilizes the stream bank, filters pollutants from storms and floods, and provides habitats for a variety of amphibians, aquatic birds, and mammals that depend on the proximity to water for their life functions. Development in riparian corridors can impair the functions and subject structures to damage from flooding and the meandering of natural streams. A filter strip or riparian forested buffer should be preserved or created along the banks of streams, where possible. Furthermore, consideration should be given to establishing setbacks for intensive development (e.g., buildings, parking lots, roadways). This will minimize the potential for sediment releases to the streams, as well as maintain the corridor to achieve flood control, water quality, and habitat enhancement objectives. If a development site contains a highly channelized stream, the best interest of both the developer and the aquatic resource may be served by restoring the stream corridor. Shorelines of ponds, lakes, and wetlands provide many of the same functions as riparian stream corridors provide for streams. Stable vegetated shorelines are particularly valuable in preventing erosion caused by wave action. Protection of shorelines should be considered when developing water dependent development, such as piers and marinas. Wetlands provide unique habitats for both plants and wildlife, including many sensitive and endangered species. As a consequence, wetlands are valued for aesthetic and recreational reasons. Wetlands also provide valuable flood storage, groundwater recharge, and pollutant-filtering functions. Wetlands are widely scattered throughout Pennsylvania and commonly are encountered on development sites. Protecting the natural functions of wetlands is a critical element of the site planning process. For moderate- to high-quality wetlands, which are very difficult to replace, avoidance is recommended. If the site contains scattered, small, low-quality wetlands, which are more readily replaced, mitigating the wetlands at a central location may be more appropriate, thereby enhancing wetland functions and reducing a potential constraint to development. §3.3.3 Steep Slopes and Highly Erodible Soils From an erodibility standpoint, the definition of steep can vary depending on surface soil type and underlying geology. In general, extra caution is warranted on a slope exceeding 10 percent (1 foot of vertical drop per 10 feet of horizontal distance). However, even flatter slopes that has soil classified as highly erodible should be identified as steep. Disturbing steep slopes with development causes instability of the soil on the slopes. Development destroys vegetation, root systems, and soil structures. High runoff velocities from exposed steep slopes result in destructive and unsightly erosion, denuded slopes that may be difficult to revegetate, and sediment deposition in sensitive areas both on and off the site. A general rule to be followed in site development is to minimize the area and time of disturbance and to fit the development to the natural terrain. Stabilizing vegetation should be protected to the maximum extent practicable and disturbed areas should be immediately revegetated. Karst bedrock areas are underlain by bedrock containing soluble minerals. Karst areas develop voids and solution channels as groundwater gradually dissolves the bedrock. In these terrains, groundwater flow can be extremely rapid and unpredictable. Furthermore, the concentration of runoff may stimulate the formation of sinkholes. Sinkholes can develop as flowing water exposes and then washes into the mouths of the near surface openings of subterrain channels and caverns. Rapid degradation of groundwater resources can result when sediment or pollutant- laden runoff percolates into karst bedrock aquifers. Large areas of Pennsylvania are underlain by limestone, dolomite, or marl carbonate rocks, which are potentially are susceptible to the development of karst conditions. Before introducing site alterations, which could concentrate or pond runoff, the presence or absence of carbonate bedrock should be established. If carbonate rocks do occur, a professional geologist or civil engineer should be consulted to determine whether sink hole activity is likely. The United States Geological Survey is a good source of information on karst bedrock in Pennsylvania. If an area is prone to sink hole development, site drainage should be planned to minimize the concentration of runoff. This can be accomplished by reducing the hydraulic connectivity of impervious surfaces and by the use of filter strips. Where they are required, channels or ponds should be lined. BMPs for the recharge of groundwater in karst areas provide infiltration opportunities over a very large area. Examples are filter strips, large bioretention facilities, and permeable pavement. These practices mimic the natural process by which rainfall enters the subsurface. Point sources of infiltration, such infiltration trenches or dry wells, should be avoided. §3.4 Preserving Natural Hydrologic Conditions Natural hydrologic conditions and pollutant-filtering mechanisms may be altered radically by poor development practices. Deleterious activities include introducing impervious surfaces, destroying existing drainage swales, constructing storm sewers, and changing local topography. A traditional drainage approach of development has been to remove runoff from the site as quickly as possible. To provide this convenience, substantial resources have been invested to convey runoff from developing areas. This approach leads ultimately to the expenditure of additional resources for detaining and managing concentrated runoff at some downstream location. In the meantime, developed areas, starved for rainfall infiltration, are deprived of perennial streams and natural habitat. The recommended alternative approach is to minimize postdevelopment runoff rates, thereby minimizing needs for artificial conveyance and storage. To maintain predevelopment hydrologic conditions, opportunities must be preserved for infiltrating water directly into the ground and to pond runoff on the ground surface from which it is ultimately evaporated or infiltrated. Beneficial results include more stable baseflows in receiving streams, improved groundwater recharge, reduced flood flows, reduced pollutant loads, and reduced costs for conveyance and storage. Preserving natural hydrologic conditions requires both implementing appropriate stormwater BMPs and practicing alternative site design. Alternative site design measures, which are described below, are essential for limiting increases in the volume of runoff and better controlling runoff quality. Site design practices include minimizing impervious surface area, reducing the hydraulic connectivity of impervious surfaces, preserving natural drainage features, and protecting natural depression storage. A well-designed site will contain a mix of structural BMPs and site design BMPs. §3.4.1 Reducing or Disconnecting Impervious Surface Areas Minimizing impervious surface areas is probably the most effective way to preserve predevelopment hydrology. Techniques include:
§3.4.2 Preserving Natural Drainage Features Protecting natural drainage features, particularly vegetated drainage swales and channels, is desirable because of their ability to infiltrate and attenuate flows and to filter pollutants. However, this objective is often not accomplished in modern developments. In fact, commonly held drainage philosophy encourages just the opposite pattern. Streets and adjacent storm sewers typically are located in the natural headwater valleys and swales, thereby replacing natural drainage functions with a completely impervious system. Runoff and pollutants generated from impervious surfaces flow directly into storm sewers with no opportunity for attenuation, infiltration, or filtration. One method of preserving natural drainage features is to use cluster development to avoid disturbing major swales. Another recommended approach is to develop site plans that keep roads and parking areas higher in the landscape and locate existing swales along back lot lines within drainage easements. §3.4.3 Protecting Natural Depressional Storage Areas Depressional storage areas have no surface outlet or drain very slowly following a storm event. They can be commonly seen as ponded areas in farm fields during the wet season or after large runoff events. Traditional development practices eliminate these depressions by filling or draining, thereby obliterating their ability to reduce surface runoff volumes and trap pollutants. The volume and release-rate characteristics of depressions should be protected in the design of the development site. The depressions can be protected by simply avoiding the depression or by incorporating its storage as additional capacity in required detention facilities. §3.5 Site Planning and Land-Use Techniques to Minimize Efforts of Development The surest way to minimize disturbances to sensitive areas and natural features is to avoid them. However, absolute avoidance is not always practical. Further, avoidance alone may not be sufficient for protecting beneficial functions. In understanding the critical functions of sensitive areas, site planners and designers, in cooperation with local zoning officials and plan reviewers, can implement planning concepts that both protect the resource and add to the value of the development and the community. Some of the concepts most useful for protecting sensitive areas include:
A setback is the area between intensive development (i.e., buildings, parking lots, roads) and a protected area, such as a wetland. Setbacks are necessary for:
For example, a highway or parking lot built directly on the edge of a high-quality wetland may adversely affect water quality and wildlife habitat from pollutant runoff or spray and traffic noise. Setback requirements for structures, particularly adjacent to streams, reflect the fact that streams naturally meander or expand over time. Placing structures in the natural path of a meandering stream virtually guarantees that expensive stabilization measures will be needed in the future as the stream approaches building foundations, threatening their collapse. Only limited activities are recommended for approval in a setback. The types of activities include minor improvements, such as walkways, foot bridges, and observation decks; roadways necessary for crossing a waterbody; maintenance and repair of existing roads and utilities; and the establishment of landscaped lawns or parks. In general, major modifications to the land surface should be avoided in setbacks. Limiting activities in a floodway to appropriate uses is similar to a setback requirement. A floodway is the part of the floodplain, centered on the stream, that will convey most of the flow during a high water event. Appropriate uses exclude most buildings and structures. However, other uses that are allowed may adversely affect water quality and habitat. These include:
Within a setback, a buffer strip is the transitional vegetated area closest to the waterbody or wetland. The purposes of a buffer are to:
Buffers reflect that natural aquatic systems may not function well in isolation and that a gradual continuum exists from natural riparian or wetland systems to upland. Ideally, a buffer should be maintained or planted in native riparian vegetation to maximize pollutant filtering, soil stabilization, and habitat functions. One of the best site planning techniques for minimizing the disturbance of sensitive areas and natural drainage features while allowing for reasonable economic use of the land is to use cluster developments. Cluster development maintains the gross density of the site but clusters the development (i.e., roads, buildings, parking lots, manicured landscape) onto only a part of the site, thereby protecting sensitive areas with no loss in the number of lots. In the traditional development, the entire subdivision is composed of either lots or streets. In the cluster development, natural areas are maintained between clusters of lots. Although the individual lots are smaller in the cluster development, often the impression is one of lower density because of the intermixing of natural areas and green space in the developed areas. Cluster development may be readily accomplished under the provisions of a planned unit development (PUD). In a PUD, a municipality may allow higher net densities as a tradeoff for protecting sensitive areas, as long as the gross density meets zoning requirements. This approach requires flexibility from both the developer and the local government and should be accompanied by a resource management plan to ensure long-term management and maintenance of sensitive features and common areas. Ideally, cluster development will allow environmental objectives to be achieved without contributing to suburban sprawl, and without unduly reducing the property owner’s return on land value. The best application of the cluster concept is to avoid sensitive areas. By using clustering, only the areas most suited for development are subject to grading and modifications to accommodate the development and the sensitive areas are set aside and not modified. To meet the development goals in terms of the number of units required, the density is increased in the areas that are most suited for development. This reduces development costs for "engineering" the land to accommodate the development and for mitigation that generally is required for disturbance of streams and wetlands. Clustering has additional benefits in terms of improved aesthetics, increased open space, and reduced infrastructure costs. When cluster developments are designed, all offsite impacts, including environmental, must be considered. For example, although PUDs often are typified in reference materials by dead-end "cul-de-sac" streets, it is important to traffic flow that all auto trips not be routed from local streets to major arterials. By using loop streets and collector streets to connect adjacent clusters, the traffic pressure on the arterials can be reduced. Some local governments place explicit zoning restrictions on wetlands, stream corridors, and woodlands. This handbook endorses the concept of a lowland conservancy overlay district. Using this approach, a municipality identifies sensitive areas on its zoning map. Tree ordinances are becoming more popular with municipalities. The ordinances protect both woodlands and individual trees. Another useful tool for protecting sensitive areas is a conservation easement. A conservation easement incorporates legal provisions into a property deed that limits the use of the property. Conservation easements allow for the continued private ownership of the land but restrict land uses to current uses or to nondamaging activities. The legal concession may be donated by or purchased from the owner. The land owner also may be compensated by reduced property taxes on the land in the easement. §3.5.5 Development Designed to Fit Site Topography Too often sites are extensively graded to create site topography to fit a plan that was designed in the office rather than creating a design for the site to avoid the need for major changes in the elevation contours. Not only is mass grading expensive, it requires stripping, stockpiling, and replacing the top soil and results in compaction of the soil, destruction of natural drainageways, and loss of site diversity. By varying lot sizes and building styles and by using at least limited clustering, the need for mass grading can be reduced substantially.
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