Subtidal water level variation controlled by river flow and tides

TLDR

Subtidal water level dynamics in the Berau river, East Kalimantan, Indonesia, exhibit a pronounced fortnightly variation. The study aims to explain the mechanisms driving subtidal water level variation and to dissect the contributions of river flow, tidal interactions, and tidal motions. A local subtidal momentum balance was constructed from field data and continuous discharge estimates derived from an acoustic Doppler current profiler, and a generic decomposition method was introduced to separate friction contributions from river flow, river‑tide interaction, and tidal motions. The analysis shows that spring tides raise daily mean water levels by 0.2–0.6 m, that friction and surface‑gradient terms dominate the momentum balance, and that river‑tide interaction is the primary driver of fortnightly subtidal variation, with tidal‑tide interactions playing a minor role and a regression model enabling prediction of level changes with increased discharge.

Abstract

Subtidal water level dynamics in the Berau river, East Kalimantan, Indonesia, feature a pronounced fortnightly variation. The daily mean water levels at a station about 60 km from the sea are 0.2–0.6 m higher during spring tide than during neap tide. To explain the underlying mechanisms, a local subtidal momentum balance is set up from field data, using continuous discharge estimates inferred from measurements taken with a horizontal acoustic Doppler current profiler. It is demonstrated that terms accounting for friction and variation in the water surface gradient are dominant in the subtidal momentum balance. To further investigate the sources of subtidal water level variation, a generic method of analysis is proposed to decompose the subtidal friction term into contributions caused by river flow, by interaction between tidal motions and river flow, and by the tidal motions alone. At the station under study, mainly the river‐tide interaction term is responsible for generating fortnightly variation of the subtidal water level. The contribution from interaction between diurnal, semidiurnal, and quarterdiurnal tides to subtidal friction is significantly smaller. Provided that the reduction of tidal velocity amplitudes with increasing discharges can be predicted from a regression model, the results presented herein can be used to predict changes in subtidal water levels as a result of increased river discharges.