Case study in cost-based risk assessment for selecting a stream restoration design method for a channel relocation project

A low-risk, stream restoration design includes methods that validate design assumptions, incorporate uncertainty in the decision-making process during the project design phase, and reduce uncertainty by checking the final design. A two-step method of incorporating uncertainty and risk in stream restoration design has been developed as a combination of design failure modes and effects analysis (DFMEA) and risk quantification. As a first step, DFMEA is applied to identify risk in terms of ratings with respect to consequence of failure, the likelihood of occurrence of a failure, and the ability to detect a failure. Due to its evolutionary nature, the DFMEA can be revised to account for design modifications and relative ratings are reevaluated to examine reductions in uncertainty, and thereby, risk. The second step of the method is quantifying risk using initial and expected failure costs. The two-step, risk-based method is illustrated through application to a stream relocation project in Pennsylvania. Both sediment transport capacity and supply analysis and alluvial channel modeling design methods were shown to reduce uncertainty and risk by detecting design deficiencies that the initial design using incipient motion tests overlooked. However, the alluvial channel model method was favored due to its ability to simulate the combined effects of flow hydraulics, sediment transport, and river channel adjustment.