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Construction BMPs By Michelle DeLaria, Jefferson County, CO and John A. Jehn, Arvada, CO Introduction The standard land development prac- tice of collecting and conveying stormwa- ter runoff removes water from the water- shed. In the fractured and decomposed granite areas of the Rocky Mountains, the effects of conventional stormwater man- agement are more pronounced. Mountain subdivisions in Jefferson County range from homes on 1/3 to 1 acre lots, served by private wells or a local mountain water district, to large upscale subdivisions with 5 to 35 acre lots served by private wells and accessed by long driveways through steep terrain. Conveying concentrated flows through culverts and road side ditches in mountainous terrain accelerates erosion, degrades natural water ways and removes water from mountain watersheds. At the same time, an increasing popula- tion demands more water from a fracture driven water supply. In Jefferson County, Colorado approximately 20,000 homes rely on rain and snowmelt to recharge their household wells. Water quality, water quantity and population growth are com- mon concerns among residents. Historic subdivision Jefferson County is home to a mountain subdivision that began as summer cabins in the early 1900's. Article will be posted in full at www.casfm.org Subdivisions prior to 1970 typically lack detention capacity. A two and a half mile road network serves about 225 homes and 500 people. Stormwa- ter is conveyed through culverts and roadside ditches that have become eroded. Although the hillside (draining north) is heavily impacted, the amount of impervious area per person is less than in typical newer mountain subdi- visions. Many decades of road and rooftop runoff have been conveyed through a series of cascading culverts and a road -side ditch network to a road at the base of the subdivision. There is inadequate right of way to accommodate traditional ditch widen- ing and stabilization. Decreased density It may appear that decreased densi- ties and larger lots would reduce the erosion and sedimentation problems in mountain communities. However, larger lot sizes translate to longer mountain driveways and more dis- turbed area. Access roads can also have a dramat- ic affect on a mountain landscape. Many access roads lack permanent BMPs and have changed the local sur- face hydrology from sheet flow to con- centrated flow conveyed in roadside ditches. At the switch backs, concen- trated runoff cascades onto grass -cov- ered mountain soils and fractured rock, creating extreme gully erosion. Then, material erodes from the site and can be deposited off the property. This level of erosion can be reduced if access road regulations required a series of infiltration structures at switchbacks and a small detention area at the base. The new stormwater Phase II require- ment to ensure long -term function and maintenance of permanent structures would also encourage better site stew- ardship. Best management practices to reduce mountain erosion Erosion during and after construc- tion can be reduced if a combination of detention, infiltration and sheet flow methods are used to mimic predevelop- ment hydrology. Improved stormwater regulations and education are key factors for improving water quality protection. Mountain construction sites are adapt- ing to steeper terrain by using reinforced silt fence or even snow fencing in rocky conditions. In many instances, seeded dirt berms are preferred. Summer 2006 9