Slope
Overview
Slope
Slope refers to changes in the physical features of the land: its elevation, orientation, and topography.
Construction on hillsides often requires alteration of the slope, especially where changes in the visual character of the site may occur and where slope instability, erosion, and/or drainage problems may result. In some localities, hillsides are likely to house native plant communities which could be lost as a result of topographic alteration.
Improper grading often alters the surface water flow and may cause flooding for the site and the surrounding property owners. Excessive grading may also alter the groundwater level, which may cause the slow death of trees and ground cover, and in turn destroy wildlife habitats.
Since erosion, slope stability, and drainage characteristics depend not only on the steepness of the slope but also on the materials of which it is composed, any analysis of slope conditions must consider soil suitability.
As discussed under the Stormwater section, certain projects require that the developer produce a Stormwater Pollution Prevention Plan (SWPPP), which contains project-specific strategies to control onsite stormwater (including strategies that prevent onsite stormwater from running offsite). In addition, an SWPPP, which is required as part of the National Pollutant Discharge Elimination System (NPDES) process, considers erosion, slippage, settlement, subsidence, or other related problems and incorporates measures to reduce issues related to soil suitability.
Important Considerations
- Is the site on a slope? If so is the slope slight, moderate, severe, or very severe (see chart below)?
- Does the area have a history of slope failure?
- Do visual indications exist of previous slides or slumps in the project area, such as cracked walls or tilted trees or fences?
- Does the city or county have a soil survey that mentions that slopes are unstable for any of the soils on the site?
- Has a geotechnical report that includes soil boring information been previously developed for the site?
- Does the proposal call for development on a steep slope and, if so, does the design plan include measures to overcome potential erosion, slope stability, and runoff problems?
- Will slope modification activities remove micro-climatic conditions that facilitate the growth of unique natural habitats (e.g., northwest-facing slopes occupied by plant communities from cooler regions)?
- Will the slope modification activities affect social and cultural resources?
Analysis Techniques
First, determine the slope. This information can be found in soil surveys or similar reports and studies, as well as contour or topographic maps. Next, complete a field investigation using an Abney Level, a clinometer, or a phone application. If in the field, measure the slope using the overland flow path profile that represents the topography of one-third to one-quarter of the most erodible part of the field (USDA-NRCS). Compare the slope at the project site to the slope suitability chart. If the slope at the site is in the marginal or unsatisfactory rating, consult project engineers and city development to determine if there are any restrictions, design changes, or mitigation measures needed.
For example, a restrictive soils zoning district proposed by the Metropolitan Council in the Twin Cities area in Minnesota would prohibit commercial and industrial development on slopes steeper than 12% and would require that developers of residential property on such slopes prove that construction techniques employed would overcome the site’s limitations. The following table presents slope suitability standards for urban areas.
Slope Suitability for Urban Development: Slopes Suitable for Development by Land Use Type
| Limitations | Suitability Rating | Residential | Commercial | Industrial Park |
|---|---|---|---|---|
| Slight | Optimum | 0–6% | 0–6% | 0–2% |
| Moderate | Satisfactory | 6–12% | 6–12% | 2–6% |
| Severe | Marginal | 12–18% | 12–18% | 6–12% |
| Very Severe | Unsatisfactory | >18% | >18 % | >12 % |
Adapted from Keifer, Ralph W. "Terrain Analysis for Metropolitan Fringe Area Planning" Journal of Urban Planning Division, Proceedings of the American Society of Civil Engineers, December 1967. Moechnig, Howard, Inventory and Evaluation of Soils for Urban Development (St. Paul HRA C.P. District 6 - North End), Ramsey Soil and Water Conservation District.
Field Observation: Visual Indications of Unstable Slopes
- Indications of previous slides or slumps in the project area
- Cracking of top of slope shows movement
- Movement or tilting of fences, retaining walls, utility poles, or trees
- Slowly developing and widening cracks in the ground or paved areas
- Uneven, mounded, or irregular soil on middle to lower slopes
- Breakage of underground utility lines
- New cracks in plaster, tile, brickwork, or foundations
- Outside walls, walks, or stairs pulling away from the building
- Leakage from swimming pools
- Doors or windows that stick or jam may be caused by slope movement
Additional Considerations
HUD Minimum Property Standards (Minimum Property Standards for Housing, 1994 Edition Handbook (4910.1) Section 203 Site Conditions and Section 301-1 Topography, superseded in part by HUD Handbook 4000.1) establish requirements for the stability of slopes and embankments; however, this only applies to HUD’s Office of Housing programs.
HUD Housing Quality Standards for the Housing Choice Voucher (HCV) program defines “standard housing” and established minimum criteria for the health and safety of program participants. This includes requirements for all housing types, including single and multi-family dwelling units, as well as specific requirements for special housing types such as manufactured homes, congregate housing, single-room occupancy, shared housing, and group residences.
Some states and localities have established slope construction regulations. These usually address a combination of factors: hillside management in relation to land use, lot size, drainage, foundation design, and sewage disposal.
24 CFR 92.251 Property Standards specify that housing that is newly constructed with HOME Investment Partnerships Program (HOME) funds must meet all applicable state and local codes, ordinances, and zoning requirements.
Mitigation Measures
- Avoid development on steep slopes
- If developed, keep the densities very low and avoid grading
- Work with architectural and geotechnical engineers to addresses site problems adequately
- Consult with the appropriate local agency (e.g., building inspector, city engineer, or city building department) and comply with local hillside regulations, including retention of open space and critical habitats