Tidal Propagation and Water Exchange in Mtwapa Creek, Kenya

Abstract

Mtwapa creek is a tropical tidal creek, surrounded by mangrove dominated mudflats. The entrance channel is long and deep. Time series of sea levels, current velocities and water temperatures were used to study tidal propagation within the creek and at the entrance. Harmonic and spectral analysis showed that the tide is typically semi-diurnal (F::=0.2). A 10% decrease in the semi-diurnal tidal range including appearance of shallow water tides towards the inner end of the creek indicate a weak choking caused by channel friction. Maximum channel currents lag high tide by 3.45 hours and there is a phase lag of 15-20 minutes in maximum sea level between the creek and the entrance. The currents show an assymetry of ebb-dominance, with both higher ebb velocities and shorter ebb periods, which fits with the topography of this creek. Water exchange was calculated from salinity measurements in the uppermost part of the creek. The residence time for water in that end was estimated to 12 days, compared to 3 days as an average for the whole creek. This latter estimate was derived from heat flux and temperature measurements. The water exchange is totally dominated by the tide. Therefore, regarding an average of 3 days there is a considerable variation between spring and neap. A simple linear model [Pugh, 1979] was applied to the creek. The model assumes that except near slack water at the mouth where local acceleration is important, the principal force balance within the channel is between the pressure gradient force and bottom friction. Though it has several shortcomings, it describes the inner tide reasonably well. It also estimates the rate of dissipation of tidal energy.