Obtaining accurate and precise values of the shear stress imparted onto the bed from free surface flows is crucial for calculating sediment transport, seabed scour, sediment composition and classifying habitats. Numerical models such as TELEMAC provide this information using the law of the wall method in both two and three-dimensional models. Objects such as bridge piers and monopiles are of particular interest as this infrastructure often induces complex flow fields, flow separation and turbulence which can lead to scour. Yet the law of the wall method used to estimate the bed shear stress (τ_b), which drives scour, is only applicable in fully developed open channel flow, and not flows with adverse pressure gradients and shear in the flow. Consequently, the values for τ_b these models produce can have orders of magnitude errors compared to the “real” τ_b. This paper addresses this issue by investigating a method to improve model derived values for τ_b, using the turbulent kinetic energy (TKE) method usually applied to oceanographic field data, instead of the law of the wall approximation.
To have an accurate baseline to compare the model results against, we utilize a laboratory experiment in which a geometrically down-scaled offshore wind turbine monopile (including rock armoring, and a sandy bed) had created a field of bedforms in its turbulent wake and where τ_b was above the threshold of motion of the sand. A 3D grid of Acoustic Doppler Velocimeter (ADV) data around the monopile was collected during this flume experiment which, using the TKE method, correctly indicated that τ_b exceeded the threshold of motion where the bedforms existed, and was under the threshold of motion where they did not.
Here we compare various methods of using the TKE from a TELEMAC 3D simulation of the laboratory experiment against this laboratory data. The value of TKE taken from 10% of the flow depth (above the bed) was found to compare best with the laboratory data. Whilst the modelled τ_b based on a law of the wall failed to capture the enhanced τ_b in the monopile wake, and thus would not predict the existence of bedforms.
We present a new method of integrating the more accurate TKE method for calculating τ_b within the TELEMAC 3D modelling system.