Bank Erosion By Wind-Generated Waves I: Development of The Wind-Wave Sub-Model In Bstem-Dynamic

Wind-generated waves can have a significant impact on the banks of large, wide, impounded rivers and embayments. Recurrent wave action may contribute to undercutting and eventual failure of the upper part of the bank. Estimation of the relative contribution of wind-generated waves to streambank erosion requires the prediction of the wave properties on the bankline for varying wind conditions and their impact on the streambank. To provide reliable tools for the analysis of bank-retreat processes and effective mitigation strategies in these settings, both a wind-generated wave-prediction module and a wave erosion module were developed and incorporated to the BSTEM-Dynamic code (v. 2.4). A separate sheet is provided for wind-wave data input/output. To calculate the wind-generated wave properties (i.e. significant wave height, Hm0 and peak-wave period, Tp) for a given set of wind speed and fetch length, the Joint North Sea Wave Project (JONSWAP) wave-prediction method was incorporated into BSTEM-Dynamic. The estimated Hm0 and Tp, were compared with field measurements at two sites located along a reach of Tennessee River between Pickwick Dam and Savannah, TN. At each site, waves were measured by 3 m-long capacitance type wave staffs with a 30 Hz measurement frequency. Wind characteristics were measured using a HOBO RXW Davis Wind Speed & Direction Sensor. The fetch was obtained by measuring the straight-line distance in the wind direction from the site to the bankline. The wave-erosion module uses wave properties along with the river stage to calculate the additional shear stress due to wind- generated waves and combines it with the flow shear stress for a given time step. In this new version, both the existing boat-wave module and the new wind-wave module can handle near-shore processes including wave shoaling, wave refraction, wave breaking and wave runup based on the most recent bank profile. The new model was tested along 29 km reach of the Tennessee River between Pickwick Dam and Savannah, TN for the period between 1985 and 2016 using the wind data from Muscle Shoals Airport, AL.This paper presents the theoretical background for the wind wave prediction method and shear-stress estimation procedures.