Friction Factors in Storm Surges over Inland Areas

In many recent models of hurricane-generated surges, a quadratic law is used to describe the bed friction. When models include overland flooding, friction factor values throughout the solution domain often reflect ocean bed conditions. A theory is presented to develop model friction factors based on a Darcy-Weisbach type relationship that gives the coefficients spatial variability, dependent on bed roughness, obstructions and their spacings, and local depths. An equivalent friction factor is introduced which combines overland values, derived by considering an assumed prevailing logarithmic velocity distribution, with values derived by considering the theoretical form of the head loss due to drag forces acting on uniformly distributed obstacles. The effects of the equivalent friction factor were studied for hurricane-generated surges on the west coast of Florida. It was shown that its introduction, based on natural vegetation and manmade obstructions, attenuated and lagged the peak surge at the project site, compared with surges simulated using a uniform value based on the ocean bed.