A MUD BALANCE FOR BELGIAN-DUTCH COASTAL WATERS BETWEEN 1969 AND 1986

Mud transport and mud-balances are usually calculated from multiplication of suspended matter concentrations and depth-averaged residual water transport. The results are assumed to underestimate the actual mud transport because the suspended matter concentrations are measured at the sea surface, mostly during calm weather. In addition, these kinds of budgets are sensitive to small variations in the estimated residual current velocity. These problems can be overcome by comparing the calculated transports with the fluxes that result from balancing amounts of erosion and deposition. The last figures integrate spatial differences in transport over time to a large extent. Following this approach it is shown that between 1969 and 1986 variations in the mud budget occurred as a result of large-scale human interference (deepening the approach channel to Zeebrugge, closure of Dutch Delta estuaries). The variations were small compared with the annual longshore mud transport, which is dominated by the amount of mud that passes from Dover Strait to Belgian-Dutch waters (c. 8.5 106 tons per year), but were relatively large when compared with the mud transport in the 20 km wide coastal zone. It is demonstrated that fluctuations in the annual frequency and duration of gales can cause the annual flux of mud to double in some years. © 1990.