Drainage Design Manual - the City of San Luis Obispo

The Minimum Interference AlternativeIf a minimum amount of work is done to “clean and dress” the problem, will the fluvialsystem heal itself at this location in a reasonable amount of time without causing orcontributing to other problems?Flexible Boundary AlternativeCan biotechnical stabilization measures secure this bank? Is there sufficient capacity toconvey the design flow with an increase in vegetative roughness? Is there right-of-wayflexibility to accommodate natural geomorphic processes? Is restoration of this reachvaluable as mitigation for another problem area?Rigid Boundary AlternativeIs some form of hard bank protection required due to hydraulic constraints, public safetyconcerns, or to protect from the loss of private or public property, and can theconsequences of this approach be kept on-site? Can some vegetation be accommodatedwith the hard structures?Integrated Solution AlternativeHow high up the bank is a rigid structure required? How far laterally? Can a combinationof approaches, such as a hard toe and soft vegetated top work? Can the project transitionto stable areas using a soft approach?Short-term vs. Long-term AlternativeIs this a reach that could be better served by inclusion in future, larger scale projects? Isthe risk imminent; are there interim solutions that long-term projects can build upon?• Alternatives Screening and Project EvaluationMost channel modifications to a creek system can have consequences upstream and down streamfrom the project area. These consequences should be anticipated and evaluated prior toconstruction. Impingement into the stream by hard structures can cause a local backwater effect,raising water surface elevations and/or causing sediment accumulation. Channel constrictionsfrom bank repair projects can also accelerate flow immediately downstream, causing scour andincreasing bank erosion risk. Hard structures can force and deflect flow against the oppositebank. The hydraulic model constructed to define existing conditions should also be used toevaluate the consequences of alternative channel modifications. Unfortunately, standard onedimensionalhydraulic models cannot determine the consequences of possible flow deflection;this evaluation must be left to the judgment and experience of the project team.In addition to hydrologic evaluation, geotechnical review and an assessment of potential impactson riparian habitat, fisheries, and water quality and aquatic species should be completed as aprelude to Conceptual Design so that impacts are minimized.To be considered potentially acceptable and worthy of further consideration, the potential bankstabilization approach must successfully meet the following criteria: Environmental. The least environmentally damaging alternative consistent withsubsequent criteria (cost and durability) should be selected. There should be no net lossof aquatic, wetlands, or riparian habitat from the project design.SLO Creek Drainage Design Manual 63 February 2003