Drainage Design Manual - the City of San Luis Obispo

• Geotechnical Analysis:In addition to a geomorphic and hydraulic evaluation, a geotechnical review should be completedby a California Registered Geologist, Engineering Geologist or Geotechnical Engineer wheneverdeep-seated or mass failure is suspect at a site, for proposed vertical retaining structures, orwhere slopes steeper than 2.5H:1V are proposed. The City/County may require such aninvestigation, based on their review of the site and surrounding area. The geotechnical analysisrequirement should include mapping surficial features and failure areas, and a program ofdrilling and laboratory testing to determine index soil properties and soil shear strengthsnecessary for geotechnical analysis and design. The drilling and laboratory analysis programmay be necessary to describe foundation bearing conditions and drainage requirements,especially where retaining walls and crib walls are under consideration. Use of a slope stabilitymodel such as Slope-W is an efficient way to determine geotechnical stability for banks over 3.5meters (12 feet) high. Soil shear strength and engineering index properties, weak soil conditionsand potential failure surfaces should be identified and described in a Geotechnical Report, or as ageotechnical section of a overall bank stabilization Conceptual Design Report.• Biological Analysis:An assessment of existing biological conditions and potential project impacts on riparian habitat,fisheries, and water quality and aquatic species should be completed prior to preparing aconceptual design so that impacts are minimized. Special attention should be paid tocharacterizing the occurrence of shaded pools and overhanging banks, and the presence ofwoody debris in the vicinity of the project site. Some of the stream biology information isavailable in the creek inventories that were completed as part of the studies for the WaterwayManagement Plan. Depending on the creek reach, a protocol search for endangered species mayalso be required, and appropriate mitigations developed.A Biological Assessment should be prepared that identifies and maps jurisdictional wetlands,endangered species, aquatic habitat and fisheries protection needs. For small projects this can beincluded as a section of the Conceptual Design Report. The Assessment should also includerecommendations for aquatic habitat enhancement, for instance by recommending the locationand placement of such in-stream structures as root-wads or channel boulders to create stable poolfeatures. The California Salmonid Stream Habitat Restoration Manual (Flossi, 1998) should bereferred to for design considerations for these in-stream structures. The project biologist can alsohelp to determine plant suitability based on local microclimates and soil conditions for riparianhabitat mitigation and biotechnical planting selection and specification (see Section 11.0).• Construction FactorsThe design team should also identify site and channel access and work conditions, restrictedseasons, mobilization and materials storage areas and needs, and the need for and allowablemethods of flow diversion and dewatering. Do not leave these factors up to the constructioncontractor, but include construction details, limits of work, diversion methods, access andmobilization sites, etc. in the Conceptual Design Report and detail them in the project Plans andSpecifications.SLO Creek Drainage Design Manual 61 February 2003