Determining heterogeneous bottom friction distributions using a numerical wave model

This paper describes a method for estimating spatially variable bottom roughness lengths (kb) in friction‐dominated coastal regions where dense measurements of the significant wave height are available. The method utilizes a numerical wave model to calculate wavefields. The model‐predicted significant wave height is compared to a control simulation with a known kb field, which is a proxy for measured wave heights. The error is used in combination with an influence matrix to successively correct the bottom roughness field. This predictor‐corrector calculation is completed in a series of analysis cycles. The method is demonstrated in an idealized basin with different kb distributions. The test cases simulate swell propagating over a sloping beach. The original kb fields are recovered in a reasonable number of analysis cycles but the method is limited by the influence of bottom friction on the wave height. The inversion is shown to be robust in the presence of errors in the measured wavefield as well as random bathymetry errors. However, the inversion fails if bathymetry errors are large and/or systematic because the friction error is not substantially greater than the error from bathymetry, which is also a key parameter for calculating the wave height. Thus it is important to select parameters and variables that have well‐defined dependencies in the numerical wave model for this procedure to be effective.