Drainage Design Manual - the City of San Luis Obispo

SECTION 7.0DESIGN OF CONDUITS, CULVERTS, AND OPEN CHANNELS FOR MINORWATERWAYSThis section outlines the design criteria for closed conduits, and for minor waterways includingroadside ditches that have a design discharge of less than 1.0 cubic meters per second (36 cubicfeet per second). Minor waterways have a catchment area of less than 2.6-km 2 (1-mile 2 ) andshall be designed for a minimum storm recurrence interval of years, with 0.3-m (1-ft.) offreeboard (Section 5.2).This section also addresses the hydraulic analysis and design of runoff that drains onto andacross improved surfaces such as streets, residential and commercial areas and parking lots andthen into storm drains via drop or curb inlets. The storm drainage is then conveyed into a naturalor improved open channel.The design discharge and street flow hydraulics are to be determined following the procedurescontained in the Federal Highway Administration’s Urban Drainage Design Manual, HEC-22(FHWA 2001, 2 nd edition). This document includes procedures for determining runoff frompaved areas, gutter flow, and the hydraulics of drop inlets and curb inlets, storm drains andculverts, pump stations, and minor open channel flow using normal depth methods of analysis.In some situations, a backwater hydraulic model should be used in the hydraulic analysis. TheFHWA Design Manual, HEC-22, is available online at http://fhwa.dot.gov/bridge/hec22.pdf.Street and their associated underground drainage system must be designed so that they conveythe 100-year runoff such that the water surface elevation is at least 1-foot below the finish floorof adjacent structures, in conformance with the provisions of Section 3.5.As part of this analysis, the engineer shall consider the overall safety of adjacent developments.This process includes understanding what happens for events larger than the 100-year, and alsowhat happens if critical drainage devices become blocked with debris. Drainage overflow pointsor secondary outlets that anticipate such occurrences shall be designed into the overall drainagesystem.All stormwater system elements (including, but not limited to conveyance systems, stormwaterdetention facilities, and vegetated corridors) shall be designed and constructed in accordancewith all applicable rules and regulations of the City and County stated in this Manual. Anyinterpretations, must be consistent with applicable referenced technical guidance manuals, andwith all applicable federal, state and local statues and rules.The designer of the storm drain system will have to use good professional judgment whendealing with the constraints and conflicts with existing infrastructure. When the designengineers have to deviate from the standards and criteria contained in the Manual, they shouldcontact the City or County as soon as possible to explain the situation and come to an agreementon the solution.SLO Creek Drainage Design Manual 85 February 2003