DESIGNING FOR POST FIRE EROSION AND SEDIMENTATION: A COLORADO CASE STUDY

A large residential subdivision was planned near the base of North Table Mountain (Golden, CO). The flanks of North Table Mountain are steep and are well known for landslide and rockfall hazards. In addition, several drainages collect runoff from the from the top and steep side slopes of North Table Mountain and discharge onto the subdivision site. It was clear from the incised nature of the drainages and the presence of thick alluvial fans at the base of the drainages that significant volumes of storm water and sediment can move through the drainages.

Wildland fires and severe intensity storms are common events on North Table Mountain. A moderate storm event on North Table Mountain, after a fire, could produce large volumes of sediment that could significantly affect the subdivision. A geologic assessment of the site determined that the primary source of sediment would be from sheet and rill erosion rather than mass wasting.

In order to design a sediment or debris basin to protect homes and public infrastructure, it was necessary to determine the volume of sediment that might be generated from a design storm following a wildland fire. Sediment loss was generated by incorporating parameters found in the Modified Universal Soil Loss Equation (MUSLE) and the SCS Curve Number Method.

Capturing sediment from a bare ground condition requires the use of very large sediment basins. Construction of a large basin above the subdivision was not feasible given existing slope stability and other land use concerns. In addition an estimate of soil loss from a bare ground condition does not consider that effect of root structure in helping to prevent erosion and may result in an overestimation of sediment loss.

One of the important factors used in MUSLE to determine sediment loss is cover-management or C factor. The C factor represents the effect of plants, soil cover, roots and soil-disturbing activities on soil loss. There is little research on C factor conditions after a wildland fire; however, there is some C factor data for mined lands and construction sites that can be used to estimate C factor conditions after a low intensity wildland fire. The paper discusses the use of MUSLE to determine sediment loss/ yield after a wildland fire and in designing sediment containment structures.