ARREST OF A DEBRIS FLOW AT LEMON DAM, LA PLATA COUNTY, COLORADO USING EROSION CONTROL MITIGATION

To minimize erosion from slopes burned by wildfire, erosion control measures such as installation of log erosion barriers (LEBs), construction of check dams and spreading of straw mulch and seed is common practice. All of these measures were implemented at Lemon Dam after the 2002 Missionary Ridge Fire in southwest Colorado to protect the dam's intake structure from being filled with sediment. Implementing these treatments immediately after the fire was under control proved paramount as monsoon rains caused many debris flows in the burned terrain from July through September 2002. In fact two debris flow events occurred in a drainage basin (Basin MR-72) one mile south of Lemon Dam during the rainstorms. A comparison of the physical characteristics between Basin MR-72 and Lemon Dam shows that the erosion control techniques implemented at the dam indeed prevented debris flow at the dam. After the fire, but before the rains, the two drainage basins had similar parameters such as geology, basin area and area burned at moderate to high severity. The only major difference was the steeper channel gradient of the Lemon Dam basin. Since a magnitude of both debris flow events from Basin MR-72 was calculated to be ~3700 cubic yards it is presumed that a similar volume of material was retained in the Lemon Dam drainage basin during the rainfall events. In addition to spreading seed in relatively high concentrations (>60 lbs/acre), straw mulch was crimped into the soil to keep it in place. LEBs were also installed in dense concentrations of 90-250 LEBs/acre depending on slope inclination. Furthermore, since installation of the LEBs, the uphill side has been rehabilitated to increase the life of each LEB during subsequent wet seasons. Though these slope treatments were effective in reducing the input of soil material and water into the main channel, the uppermost check dam caught ~17 cubic yards of material that had been mobilized. Therefore, the inclusion of check dams in the channel may have proven integral to preventing development of debris flows.