LARIMER COUNTY RURAL AREA ROAD STANDARDS

CHAPTER 6 – STORMWATER MANAGEMENT6.1 GENERALA. Streets are intended for vehicular traffic and not the conveyance of storm runoff. Theobjective of roadway drainage is to convey storm runoff away from the road surfaceto a safe point of discharge, rather than to act as an element of the overall drainagesystem. This is accomplished by using proper cross slopes, longitudinal grades, andcross drainage structures to convey storm water into the drainage system.B. Planning and design of the overall drainage system for a rural development shouldbe done simultaneously with the road or street layout and planning and design ofslope gradients. This will help the designer route storm water away from theroadways and into the overall drainage system. Also, coordination of the road orstreet grades with the finished lot elevations is very important to assure that positivelot drainage can be achieved.C. The main reference source for the planning, design, and construction of drainagefacilities for roads or streets, either on the existing County Road System or in adevelopment, will be the Larimer County Stormwater Design Standards.D. In addition to the submittal requirements found in the Larimer County StormwaterDesign Standards, a culvert design analysis (including an analysis for driveways)shall be submitted with each rural street design. The analysis shall show culvert size,configuration and material specifications (see Drawing 7 and 11 in Appendix C).This information is required for assuring continuity in the roadside drainage system.Driveway culvert information will be used at the time of building permit issuance toinform the homeowner or builder of the particular driveway culvert requirements.6.2 CROSS FLOW DRAINAGE STRUCTURESA. Cross flow drainage structures shall be designed and built to pass and withstand thefollowing design frequencies. Chapter 7 contains additional information pertinent tobridges and major drainage structures.1. The design discharge used in an area that has a FEMA floodplain shall be the100-year discharge.2. The design discharge used in an area that has an adopted storm drainage basinmaster plan shall be as defined in the plan.3. Cross flow drainage structures for new roadway construction in areas notcovered by a FEMA floodplain or storm drainage basin master plan shall bedesigned and built to pass the standard design frequencies in Table 6-1.Larimer County Rural Area Road Standards Page 6-1October 22, 2007