Economic Optimization of Flood Bypass Capacity

Flood bypasses are used in regional flood management to reduce flood risk and damage by routing substantial flood flows to a supplementary river channel to lower flood stages in the usual river mainstem. Design, operations, and modifications to flood bypasses commonly employ hydraulic analysis and evaluation. Economics of flood bypasses is usually considered only in terms of feasibility, and bypass design is rarely optimized economically. The literature lacks formal integration of economics, engineering, and hydrology for flood bypasses. Economically analyzed flood bypass systems better incorporate bypasses into flood management, reflecting and balancing the range of costs, impacts, and opportunities of bypasses. This paper presents a method to integrate economics, hydrology, and engineering for flood bypass planning. An economic optimization model is developed based on a cost-benefit analysis. The optimization suggests and explores a preferred economic flood bypass capacity. A preliminary analysis is developed, with a base case that includes benefits from flood risk reduction only. Costs include for a levee setback, weir widening, and land use. The optimization model is applied preliminarily to California's Yolo Bypass. With the simplification of unlimited land availability, the model results suggest an optimal Yolo Bypass capacity expansion of approximately 5,800 m3/s. The expansion suggested is close to the Department of Water Resources (DWR) stated objective in their 2017 Central Valley Flood Protection Plan and part of the Basin-Wide Feasibility Studies Sacramento River Basin, part of the Central Valley Flood Management Program of 2017. The model is also applied to the Mississippi River Basin's, New Madrid Birds Point Bypass and Morganza Floodway. In all cases, actual and theoretically optimal bypass capacities agree in their general behavior. This paper provides a framework for decision analysis. This method can better inform policymakers and stakeholders on bypass design and structural modifications and long-term flood management strategy.