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Steep Slopes FACT SHEET : 12 FACT SHEET: 12 Steep Slopes iTRaC is the Nashua Regional Steep slopes are legally defined as hillsides Figure 1: Example of a 15% slope Planning having a 15 foot, or greater, vertical rise Commission’s over 100 feet of horizontal run, or 15% slope 15% slope new approach 75’ to community (Figure 1). They are often undesirable ar- planning that eas for development due to the difficulty focuses on 500’ integrating of building on steep grades. On the other transportation, hand, these slopes can provide wildlife land use and environmental habitat, recreational opportunities, and Steep slopes have a ≥15 ft vertical rise over a planning. The scenic views, preserving the unique and 100 ft horizontal run, or a 15% slope. program was developed to culturally valuable environmental qualities assist that people treasure in New Hampshire. communities in dealing with the challenges of growth in a Difficulties Developing Steep Slopes coordinated Strategies for Developing Steep Slopes way that sustains Erosion ~ The loss of vegetation and Vegetation can stabilize soil and prevent erosion community disruption of natural drainage patterns on steep slopes by binding loose soil with roots character and brought about by development on a sense of and slowing the passage of water down the place. steep slopes can cause erosion problems slope. Replanting disturbed locations after con- leading to potential flooding, stream struction with a combination of trees, shrubs, sedimentation, and slope instability. and groundcover is key. Infrastructure ~ Providing infrastructure to hillside development can be expensive Berms are long earthen mounds running per- to engineer and construct. The typically pendicular to the fall line of the slope. They slow shallow, poor-draining soils on slopes are runoff so it can be absorbed into the soil instead not suitable for septic systems. High of washing away and are less expensive than failure rates of septic systems on steep retaining walls (Figure 2). slopes are a serious threat to ground and surface water quality. Retaining walls also help to hold unstable slopes. A series of natural retaining walls can Road construction ~ NH DOT & form terraces down the hillside, creating more municipalities regulate the gradient of useable space on the slope. roads, common driveways, and sometimes private driveways. When Nashua Regional building on steep slopes, switchbacks & Planning Commission extensive regrading are required to assure motorist safety. 115 Main Street PO Box 847 Nashua, NH 03061 Phone: (603) 883-0366 Figure 2: Berms help to slow runoff from steep slopes. Fax: (603) 883-6572 Source: Innovative Uses of Compost: Erosion Control, www.nashuarpc.org Turf Remediation and Landscaping. Environmental Protection Agency, Oct 1997 Updated 11/6/07 Continued on back FACT SHEET: 12 Regulating Steep Slopes Since the 1950’s, when steep slope ordinances were first adopted, 3 methods have emerged as the most effective in regulating development: Slope/Density Ratio Soil Overlay Overlay Districts The Slope/Density Ratio bases the This method uses soil maps A hillside overlay district, similar to the allowable density of develop- provided by the Natural Re- more familiar wetlands overlay ment on the steepness of the source Conservation Service to district, can provide a set of guiding grade to effectively reduce the determine which slopes can principles for all future hillside intensity of development as the safely support development development within a jurisdiction. slope increases. The cap is gen- based on the stability, depth, These regulations are often flexible erally 25%, when slopes become and type of soils found in a given and can be tailored to specific pro- too steep for safe and cost- location. For more info visit: jects and slopes. effective development. soils.usda.gov. Enabling legislation authorizing communities to regulate slope development is Top 10 Steep Slopes Criteria found under RSA 674:16—Grant of Power; RSA 674:21—Innovative Land Use 1. Topography Controls; and 674:21.I(j)—Environmental Characteristics Zoning. As of 2006, 27 2. Slope stability NH municipalities had implemented some form of steep slope regulations. 3. Drainage & erosion In order to develop the most effective steep slope regulations, 10 criteria 4. Infrastructure should be considered including: topography, slope stability, drainage and 5. Access erosion, infrastructure, access, aesthetics, natural qualities, fire risks, recreation 6. Aesthetics opportunities, and open space. These ten criteria can be used as a frame- 7. Natural qualities work to build a solid justification for regulating steep slopes, hillsides, and ridge- 8. Fire risks lines. For more information on these topics and how to regulate development 9. Recreation opportunities on steep slopes consult Planning for Hillside Development by Robert Olshansky 10. Open space (available at: www.planning.org/APAStore). Steep Slopes Enabling Legislation RSA 674:16 Grant of Power RSA 674:21 Innovative Land Use Controls RSA 674:21.I(j) Environmental Characteristics Zoning Don’t let this happen to you... Protecting Steep Slopes One way to keep development off of steep slopes is to protect them. Steep slopes can be valuable community resources for recreation and wildlife habitat. Possible mechanisms for steep slope pro- tection include creating greenways, wildlife habitat preservation areas, and conservation areas. Sample language and a model steep slopes ordi- nance prepared by the NH Department of Environ- mental Services can be found online at: www.des.nh.gov/REPP/ilupth/Steep_Slopes.doc Source: Center for Coastal Studies, University of California, Santa Cruz For more details visit www.nashuarpc.org/itrac. There you will find fact sheets, resource cards, and helpful links for a variety of planning topics as well as our lending library, training schedules and materials, best practices guidelines, frequently asked questions, and much more. You may also contact Camille Pattison, iTRaC Program Manager, at [email protected] or 603-883-0366 x14. Nashua Regional Planning Commission in partnership with the NH Department of Transportation & the NH Office of Energy and Planning .
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