Drainage Design Manual - the City of San Luis Obispo

6.1 GeneralSECTION 6.0ANALYSIS & DESIGN OF BANK STABILIZATION STRUCTURESThis section provides the required approach for the design and implementation of all public andprivate bank repair and bank protection projects within the San Luis Obispo watershed. Due tothe great variability in conditions within the watershed, streambank erosion control and bankstability design is to be based on site-specific conditions, including:• Site biological resource values;• Physical conditions such as bed slope, channel roughness, location (inside or outside ofchannel bend), soil type and geotechnical stability requirements;• Hydraulics, including flow velocity and shear forces, and;• Characteristics of the channel and adjacent site including the available right-of-way, andwhat is located at top of bank that needs to be protected.The general approach to be used in the SLO Creek watershed is sometimes referred to as the“Tractive Force” method, as outlined in Federal Highway Administration Manual HEC 15SI(Federal Highway Administration, 2000). This is available athttp://www.fhwa.dot.gov/bridge/hydpub.htm. This method compares the shear stress or tractiveforces acting on a site with the shear strength of bank soils and shear resistance of bankprotection materials in selecting appropriate channel bank lining. Alternative methods ofanalysis, such as “Permissive Velocity” are subject to the approval of the City Engineer orCounty Public Works Director.Emphasis shall also be placed by the design team on an assessment and understanding of theoverall geomorphic stability of the stream reach where the project is located, especially withrespect to channel bed conditions (i.e., reach is either stable, is aggrading sediment, or isdegrading and eroding) and overall erosion and sediment transport dynamics.6.2 Design Goals and Design RequirementsThe goals of the design process are to insure selection, design, and construction of a project:1. That is stable over the long term.2. That is the least environmentally damaging and the “softest” approach possible.3. That does not in turn create upstream or downstream flooding or induce other localstream instabilities.4. That minimizes impacts to aquatic and riparian habitat and insures that all identifiedimpacts are adequately mitigated, preferably onsite.Detailed biological, geotechnical and geomorphic and hydraulic studies should be completedprior to design of large bank repair structures to insure that these goals are met.As specified in Section 3.6, vegetative or Biotechnical slope repair and bank stabilizationtechniques will be the preferred approach in the San Luis Obispo Creek watershed. Wheregeomorphic, hydraulic, and geotechnical studies indicate that “hard” elements such as rock rip-SLO Creek Drainage Design Manual 50 February 2003