Erosion, transport, and sedimentation are the processes by which wind and water wear away the land surface or move it to another location.

While commonly considered an agricultural problem, erosion in the urban context, resulting from land clearance and construction, can be equally serious. In urbanized areas, erosion can cause structural damage in buildings by undermining foundational support. It can pollute surface waters with sediment and increase the possibility of downgradient flooding by sediment-laden water entering the drainage system or watercourses. It can also increase the possibility of slope collapse.

Erosion results from:

  • The interaction of physical characteristics (topography, soil type, groundcover)
  • Wind and water action
  • Human use at any one site

Some soils are less stable than others and are consequently more susceptible to erosion. Loosely consolidated soils (e.g., sands) and those of small particle size (e.g., fine silts) are more susceptible to erosion. By contrast, soils with high moisture and clay content are more resistant to erosion. Wind erosion is most likely to occur in arid or semi-arid regions where the low moisture content reduces the cohesiveness of indigenous soils.

A key factor in erosion is the land cover. Undisturbed vegetated areas are less susceptible to erosion than surfaces with less vegetation. The greater the slope, the more likely the occurrence of erosion because steep slopes (often defined as >12%) increase the velocity of runoff.

Important Considerations

  1. Is there evidence of erosion or sedimentation?
  2. Does the project involve the development of an erosion-sensitive area (near water, on a steep slope, or on sandy or silty soil)?
  3. If the site requires clearance, what are the effects of the removal of vegetation? How will erosion be managed and controlled? How many acres will be cleared and for how long?
  4. Is an erosion control plan included as part of construction and the construction contract?
  5. Will the project site significantly affect or be affected by erosion or sedimentation conditions? If so, does the design plan include measures to overcome potential erosion problems?
  6. Could erosion from the project adversely impact a downslope development or natural environment?
  7. Does the proposed project involve the steepening of slopes?

Analysis Techniques

A variety of secondary sources, as listed below, provide guidance on assessment techniques. In addition, field observation can help indicate a site’s erosion potential. Silty or sandy soils and high slopes are indications of erosion potential. Active rills or gullies, the collapse of stream banks, the wash out of previously installed erosion control materials like riprap, sediment fans, debris left on hard surfaces, muddy water due to siltation, and/or storage for temporary repair material for roads usually are evidence of active or past erosion. A natural resources map identifying soils, vegetative cover, and resources protected may be used to assess impacts of erosion and sedimentation from disturbed slopes.

Slope and soil suitability play a role in erosion potential. Compare the slope and soils in the project location with field observation. If the slope is severe or very severe and there is the presence of silty or sandy soil, a potential for erosion exists. The potential for erosion can be calculated with the Revised Universal Soil Loss Equations (RUSLE). For the equation, the average soil loss is calculated taking into account rainfall erosivity, soil erodibility, slope length, slope steepness, land cover, and proposed activity. 

Additional Considerations

As described in more detail under the Stormwater section, the National Pollutant Discharge Elimination System (NPDES) process and Stormwater Pollution Prevention Plan (SWPPP) preparation address erosion concerns. In addition, many communities have local building codes, subdivision regulations, and hillside zoning ordinances that address required erosion control techniques during site preparation and actual construction.

In order to determine locations with serious erosion potential, consult both soil classification and topographic maps. If the community does not have data available on these topics, review the information under Resources to Reference/Experts to Contact.

HUD’s Community Resilience Toolkit lists a number of options that communities can take to improve resilience against erosion.

Mitigation Measures

If a location has a potential erosion problem, review project plans to determine if they properly address the need for erosion control measures. In addition, an SWPPP, prepared by a civil engineer or other professional who is qualified to prepare such a plan, will address construction-related erosion and sediment-laden runoff from the site. Therefore, mitigation measures might suggest elements that could be incorporated into the SWPPP as temporary and permanent erosion and sediment control measures.

Good site design and construction practice should meet local codes as well as:

  1. Include a plan that fits the contours of the site and keeps grading to a minimum
  2. Retain vegetative cover until construction start-up
  3. Clear only the area needed for construction at any one time
  4. Stockpile removed topsoil for revegetation
  5. Provide temporary cover such as grass, sod, mulch, burlap, or plastic when extended exposure is unavoidable
  6. Implement a revegetation plan (including long-term monitoring, maintenance, and replanting if necessary) after construction is complete
  7. Use soil sediment control measures (silt fences, sediment basins) to prevent sediment runoff to offsite slopes, drains, or water bodies

Despite these precautions, some erosion may be inevitable. Consider adding to the impact mitigation effort sediment control measures (such as the construction of sediment barriers, traps, and basins) to help reduce potential damage.

Four basic approaches exist to reducing potential wind erosion:

  1. Maintain soil cohesiveness (by wetting disturbed areas and by avoiding unnecessary traffic on construction sites)
  2. Create or maintain vegetation or ground cover
  3. Reduce wind action (by scheduling construction to avoid high wind seasons, planting or preserving tree lines or hedgerows perpendicular and upwind of the construction site, and erecting artificial wind barriers such as snow fences as needed)
  4. Cover on-site stockpiles of spoils and fill

Resources to Reference/Experts to Contact