The effect of cross-sectional velocity and concentration variations on suspended sediment transport rates in tidal creeks

Abstract

Instantaneous suspended sediment transport rates over numerous tidal cycles have been determined for two tidal creeks in South Carolina: Town Creek, North Inlet, and Bass Creek, Kiawah Island. Three different calculation methods were employed: (1) area-integrated product of sediment concentration, velocity, and sample subarea (Method I, the true transport rate); (2) product of instantaneous channel cross-section area, cross-sectionally averaged velocity, and cross-sectionally averaged concentration (Method II); and (3) product of instantaneous discharge and sediment concentration at a single, near-surface, mid-channel sample location (Method III). Logistical and financial constraints at times dictate use of Method III rather than the more rigorous Method I. Deviations between Method II (and/or III) and Method I as a percentage of the Method I transport rates can be large (up to 120%). Fortunately the largest percent deviations occur at times of low Method I transport. As a result, linear regressions of Method I transport rates against those of Methods II or III yield correlation coefficients r2 > 0·9. Thus Method II or III transport rates can be corrected by regression analysis to give reliable estimates of the Method I transport rate. Accordingly, it may be possible to calculate tidal residual suspended sediment transport rates for the Long-Term Ecological Research (LTER) site at North Inlet using daily water sample data, providing an accurate hypsometric model can be developed for estimating instantaneous discharge from continuous tide data.