Managing Stormwater to Protect Water Resources in Mountainous Regions of Colorado PREPARED FOR: Clear Creek County Berten Weaver, Community Development Director P.O. Box 2000 Georgetown, CO 80444 PREPARED BY: Ralf Topper, CPG July 2009 Table of Contents Acknowledgements ............................................................................................................................... iii 1. Introduction .................................................................................................................................. 1 Applicability .................................................................................................................................. 3 Cited References ........................................................................................................................... 4 2. Water Resource Impacts of Development .................................................................................... 5 Recharge concepts/definition ...................................................................................................... 5 Surface and subsurface infiltration .............................................................................................. 6 Why stormwater matters ............................................................................................................. 8 Managing stormwater to protect water resources .................................................................... 10 Socioeconomic Benefits .............................................................................................................. 11 3. Hydrogeology of Clear Creek County .......................................................................................... 13 Climate Considerations ............................................................................................................... 13 Average Annual and Mean Monthly Precipitation ..................................................................... 15 Rainfall Frequency Spectrum ...................................................................................................... 17 Geology ....................................................................................................................................... 20 Groundwater .............................................................................................................................. 21 Groundwater Recharge .............................................................................................................. 25 Cited References ......................................................................................................................... 26 4. The Regulatory Environment ...................................................................................................... 29 Federal (EPA) .............................................................................................................................. 29 State (CDPHE) ............................................................................................................................. 30 Local (Clear Creek Co.) ................................................................................................................ 30 Ramifications of Colorado Water Law ........................................................................................ 33 5. Maintaining Pre‐Development Hydrology – Nonstructural Controls .............................................. 35 Land Use Planning ...................................................................................................................... 35 Low Impact Development Strategies .......................................................................................... 37 Evaluation of Costs and Benefits ................................................................................................ 38 Cited References ......................................................................................................................... 41 6. Structural Control Measures/Best Management Practices ........................................................ 43 Filtration ..................................................................................................................................... 45 Infiltration ................................................................................................................................... 46 i Detention .................................................................................................................................... 47 Flow Control ............................................................................................................................... 48 7. Considerations for BMP selection in mountainous terrain ........................................................ 51 Appendix A: Groundwater Recharge Potential Analysis Appendix B: Filtration Based Best Management Practices Appendix C: Infiltration Based Best Management Practices Appendix D: Detention Based Best Management Practices Appendix E: Flow Control Based Best Management Practices ii Acknowledgements This project was made possible by the vision and efforts of Clear Creek County’s Community Development Director, Mr. Berten R. Weaver. In a cooperative effort with the Colorado Geological Survey, Mr. Weaver applied for funding through the Colorado Water Conservation Board’s 2007‐2008 Severance Tax Trust Fund Operational Account. In December 2008, the State of Colorado issued a purchase order in the amount of $50,000.00 to fund the work proposed in the original grant application. In February 2009, Clear Creek County entered into an agreement with the Colorado Geological Survey to implement the tasks specified in the scope of work. This manual represents the culmination of the work effort on this project. We would like to acknowledge the contributions of Clear Creek County staff in providing information, support, and coordination functions for this project. In particular, Mr. Bert Weaver, Community Development Director; Mr. Frederick Rollenhagen, Planning Director; and Mr. Matt Taylor, GIS Director. A special thanks is extended to Clear Creek County’s Site Development Inspector, Mr. Tim Vogel whose proactive approach to stormwater management is leading the county to meet its sustainable goals. The Colorado Geological Survey was fortunate to contract with Mr. Gregory Zimmerman for identifying and compiling existing stormwater management literature, establishing a database of best management practices, and creating fact sheets for structural BMPs. Completion of this project would not have been possible without the assistance of Mrs. Michelle DeLaria‐Kutschera whose expertise in stormwater management along Colorado’s Front Range is well recognized. Locally, Michelle has been a key proponent of managing stormwater to mimic a site’s predevelopment hydrology. Contributing staff of the Colorado Geological Survey include: Karen Berry, Eric Oerter, and Nick Watterson. iii iv 1. Introduction Like much of the Front Range of Colorado, Clear Creek County is defined by its landforms, climate, vegetation, and focused population centers (Fig. 1‐1). The physiographic characteristics of the county include high peaks, great relief, rugged terrain, steep slopes, and shallow soils. The climate varies from semi‐arid to alpine with elevations above 10,000 feet receiving an average of 25 inches or more of moisture per year. Vegetation varies from alpine tundra above timberline to thick stands of evergreen forest with interspersed mountain meadows and communities of aspen trees. The population in this mountainous county is sparse with 39 or fewer residents per square mile. However, because of the rugged terrain and dominance of federally owned land, most of its residents are concentrated along mountain flanks and stream valleys. The residents of the unincorporated portions of the county and certain municipal and commercial entities rely largely on groundwater for their water supply. With the exception of narrow alluvial aquifers along stream systems, this water supply is limited by the fractured, crystalline rock aquifers that store this resource. Increasing population growth and development have raised concerns about the sustainability of the groundwater resource. The purpose and focus of this study is to identify and promote stormwater management technologies and practices that may be implemented locally to protect and conserve water resources through mitigation of detrimental impacts caused by land disturbances and modifications associated with land development. Natural runoff results when rainfall or snowmelt exceeds the capacity of the ground to absorb the additional moisture. The tendency of an area to produce runoff as overland or channelized flow depends upon vegetation, soils, geology, slope and climate. Naturally vegetated or undisturbed ecosystems produce little to no runoff from the vast majority of precipitation events. For example, on undeveloped property on the plains of the Denver Front Range area there is less than one runoff event a year (Urbonas, 2006). The native environment has and continues to evolve towards optimum management of excess water through evapotranspiration, infiltration, and natural runoff. While runoff is part of the natural hyrologic cycle, man‐made hydrologic modifications that accompany development have increased the volume of stormwater runoff significantly. Land use modifications by human activities, particularly urbanization