LARIMER COUNTY RURAL AREA ROAD STANDARDS

Chapter 6 – STORMWATER MANAGEMENTSection 6.2 Cross Flow Drainage StructuresTable 6-1Minimum Standard Design FrequenciesPeak Flow at Structure(cfs)Recurrence Interval for Design(years)Q 50 < 500 10500 CFS < Q 50 < 4000 25Q 50 ≥ 4000 50B. After sizing a drainage facility using the peak flow for the design frequency, it isnecessary to evaluate this proposed facility with a review flood. This is done toinsure that no unexpected flood hazards are inherent in the proposed facility. Thereview flood shall be the 100-year event.C. Single access areas shall have a minimum 50-year design frequency with no morethan 1 foot of flow across the road. Frontage roads will have the same designfrequency as the road they serve.D. All structures designed for a 25-year or higher recurrence interval shall be designedand constructed such that they will withstand the 100-year flood without sustainingmajor structural damage. This analysis shall take into account the tractive force andscour potential of flood waters during the 100-year flood.E. Downstream and upstream water surface elevations before and after replacement oraddition of a cross flow structure shall be analyzed determine whether a rise in floodwater surfaces would result and for potentially detrimental effects to public andprivate property. These conditions will be analyzed for the design flood as well asthe 100-year flood. Structure replacement or addition should include any stepsnecessary to mitigate major damages in these areas (see Chapter 7 for additionalcriteria on structure design)Larimer County Rural Area Road Standards Page 6-2October 22, 2007