The effect of wave-induced turbulence on intertidal mudflats: Impact of boat traffic and wind

Abstract

Semi-diurnal and fortnightly surveys were carried out to quantify the effects of wind- and navigation-induced high-energy events on bed sediments above intertidal mudflats. The mudflats are located in the upper fluvial part (Oissel mudflat) and at the mouth (Vasière Nord mudflat) of the macrotidal Seine estuary. Instantaneous flow velocities and mudflat bed elevation were measured at a high frequency and high resolution with an acoustic doppler velocimeter (ADV) and an ALTUS altimeter, respectively. Suspended particulate matter concentrations were estimated by calibrating the ADV acoustic backscattered intensity with bed sediments collected at the study sites. Turbulent bed shear stress values were estimated by the turbulent kinetic energy method, using velocity variances filtered from the wave contribution. Wave shear stress and maximum wave–current shear stress values were calculated with the wave–current interaction (WCI) model, which is based on the bed roughness length, wave orbital velocities and the wave period ( T S). In the fluvial part of the estuary, boat passages occurred unevenly during the surveys and were characterized by long waves ( T S>50 s) induced by the drawdown effect and by short boat-waves ( T S<10 s). Boat waves generated large bottom shear stress values of 0.5 N m −2 for 2–5 min periods and, in burst of several seconds, larger bottom shear stress values up to 1 N m −2. At the mouth of the estuary, west south-west wind events generated short waves ( T S<10 s) of H S values ranging from 0.1 to 0.3 m. In shallow-water environment (water depth <1.5 m), these waves produced bottom shear stress values between 1 and 2 N m −2. Wave–current shear stress values are one order of magnitude larger than the current-induced shear stress and indicate that navigation and wind are the dominant hydrodynamic forcing parameters above the two mudflats. Bed elevation and SPM concentration time series showed that these high energy events induced erosion processes of up to several centimetres. Critical erosion shear stress ( τ ce) values were determined from the SPM concentration and bed elevation measurements. Rough τ ce values were found above 0.2 N m −2 for the Oissel mudflat and about 1 N m −2 for the Vasière Nord mudflat. These results demonstrate the advantages of combining the measurement of instantaneous velocity and bed elevation to determine in situ the erosion and deposition processes as a function of bottom shear stress variations.