The Response of Turbidity Maximum to Peak River Discharge in a Macrotidal Estuary

Yuhan Yan,Dehai Song,Nan Wang,Xianwen Bao

doi:10.3390/w13010106

Abstract

The Ou River, a medium-sized river in the southeastern China, is examined to study the estuarine turbidity maximum (ETM) response to rapidly varied river discharge, i.e., peak river discharge (PRD). This study analyzes the difference in ETM and sediment transport mechanisms between low-discharge and PRD during neap and spring tides by using the Finite-Volume Community Ocean Model. The three-dimensional model is validated by in-situ measurements from 23 April to 22 May 2007. In the Ou River Estuary (ORE), ETM is generally induced by the convergence between river runoff and density-driven flow. The position of ETM for neap and spring tides is similar, but the suspended sediment concentration during spring tide is stronger than that during neap tide. The sediment source of ETM is mainly derived from the resuspension of the seabed. PRD, compared with low-discharge, can dilute the ETM, but cause more sediment to be resuspended from the seabed. The ETM is more seaward during PRD. After PRD, the larger the peak discharge, the longer the recovery time will be. Moreover, the river sediment supply helps shorten ETM recovery time. Mechanisms for this ETM during a PRD can contribute to studies of morphological evolution and pollutant flushing.

Highlights

Estuaries have been recognized as areas where buoyancy forcing from river discharge alters the water density from that of the adjoining ocean [1]
DDiissttrriibbuuttiioonn ddiiaaggrraamm ooff bboottttoomm ssaalliinniittyy ((fifilllleedd ccoonnttoouurr, uunniitt:: PPSSUU)) uunnddeerr ((aa––dd)) tthhee llooww--ddiisscchhaarrggee ccoonnddiittiioonn aanndd (n(eee––ahhp)) tPPiRdReDD, acconondnddriitigitoihontnpaatatnhehiglishgahwrewatdaeutrersrilnasgclakscpaknraidnngldotwildowwe.aWwteahrtisetleracasklrae.caTksh.ienTdshaiecliasntaietliytniiidstyailnifs1la-itPn.S1U-PiSnUterivnatlesr.vLaelsft. pLaenfteplsaanreelsduarreindgunreinapg tide, and right panels are during spring tide
Under the peak river discharge (PRD), the tidally averaged stratification decreases from the landward head to 21.7 km during neap tide and to 20.9 km during spring tide, but the PRD cannot alter the mixing type

Summary

Introduction

Estuaries have been recognized as areas where buoyancy forcing from river discharge alters the water density from that of the adjoining ocean [1]. Tidal straining, which mixes suspended sediment higher up during floods than during ebbs and drives an exchange flow downstream near the surface and upstream near the bottom, causes another process of ETM formation [16,17]. Different from the first part [32], the model was run starting on 9 December 2006, as we do not have the measured SSC data in 2005 for validation In this part, the model was initiated with zero water level and velocity and was first run for 120 days with a fixed river discharge, including both OR and NR (the daily average on 8 April 2007) as well as the open boundary salinity (the monthly average in April 2007), to reach a quasi-equilibrium state. To validatme2the numerical model, the RS6R.2, which is the raMtioehoftathaendroMotcmAenaanllysq[u46a]re error (RMSE) nno2rmalized by the standard dev1i.a6tion of the obserMvaetihotna [a4n8d] iMs ucsAenda, lwlyh[ic4h6]is calculated as: C0

Validation

Experimental Design

Spring-Neap Modulation

Conclusions