A Model Study of the Estuarine Turbidity Maximum a

Authors: Kyeong Park Harry V Wang SungChan Kim JeongHwan Oh

Publish Date: 2007/12/18

Volume: 31, Issue: 1, Pages: 115-133

PDF Link

Abstract

A threedimensional intratidal sediment transport model is developed for the estuarine turbidity maximum ETM in the upper Chesapeake Bay The model considers three particle size classes including the fine class mostly in suspension in the water column the medium class alternately suspended and deposited by tidal currents and the coarse size suspended only during the times of relatively high energy events Based on the results of a box model depthlimited erosion with continuous deposition is employed for the medium and coarse classes by varying the critical shear stress for erosion as a function of eroded mass For the fine class mutually exclusive erosion and deposition is employed with a small constant value for the critical shear stresses for erosion and deposition to assure quick erosion of recently deposited fine particles but without allowing further erosion of consolidated bed sediments The model is run to simulate the annual condition in 1996 and the model generally gives a reasonable reproduction of the observed characteristics of the ETM relative to the salt limit and tidal phase The model results for 1996 are analyzed to study the characteristics of the ETM along the main channel of the upper bay in intertidal and intratidal time scales Under a low flow condition local erosion/deposition and bottom horizontal flux convergence are the main processes responsible for the formation of the ETM with the settling flux confining the ETM to the bottom water Under a high flow condition a distinctive ETM is formed by strong convergence of the downstream flux of sediments eroded from the upstream of the null zone and the upstream flux of sediments settled at the downstream of the null zone Intratidal variation of the ETM is mainly controlled by erosion and the tidal transport of eroded sediments for a low flow condition Under the direct influence of a high flow event the ETM is mainly formed by erosion during ebbing tidal current strengthened by large freshwater discharge and by convergence of ebbing freshwater discharge and flooding tidal current During the rebounding stage of a high flow event intratidal variations are mainly controlled by tidal asymmetry caused by the interaction between tidal currents gravitational circulation and stratificationWe are thankful to LP Sanford at the Horn Point Laboratory Center for Environmental Science University of Maryland for providing his data that have been used for model validation and to W Panageotou at the Maryland Geological Survey for sharing the information of dredging in the main channel of the upper Chesapeake Bay We also are thankful to AY Kuo and three reviewers for their helpful comments on the manuscript This study was funded in part by the Maryland Department of Environment MDE and we would like to thank N Panday at the MDE for his administrative and technical support throughout this studyBox model results for continuous deposition solid line and exclusive erosion and deposition dashed line for type I depthlimited and type II unlimited erosion for the fine 3 μm medium 18 μm and coarse 65 μm size classes The top panel shows the applied bed shear stressFor the fine size class 3 μm with type I erosion EED results in a virtually constant sediment concentration indicating that the limited amount of erodible bed sediments is quickly resuspended and kept in suspension in the water column all the time because of relatively slow settling and short time duration of deposition CD gives the same pattern although with a very small tidal fluctuation For type I erosion EED with constant τ ce = τ cd = 002 Pa not shown gives virtually the same results of constant sediment concentration We hence conclude for the fine particles that the modeling method of erosion/deposition is not important as long as it assures quick resuspension of deposited fine particles For type II erosion the concentrations keep increasing in both EED and CD because of the unlimited erodible bed sediments and very slow settling In reality however the condition of type II erosion is not likely for the fine particles with relatively short residence time in the bedFor the medium size class 18 μm with type I erosion EED results in little tidal fluctuation in sediment concentration The maximum concentration remains constant over the time period of relatively large τ b during which too large a τ b suppresses deposition and the lack of erodible bed sediments inhibits erosion The behavior of the constant maximum concentration maintained into the decelerating tide does not agree with the observation in Sanford and Halka 1993 that the concentration decreases as τ b starts to decrease EED for type II erosion shows that the concentrations keep increasing because of the unlimited erodible bed sediments and the relatively short time duration of deposition CD on the other hand gives the concentrations with distinct tidal fluctuation for both types of erosion For type I erosion the concentration increases soon after τ b starts to increase and all of the erodible bed sediments are eventually resuspended before reaching the maximum τ b Then as CD continuously removes suspended sediments from the water column the maximum concentration occurs earlier than the time of the maximum τ b The model behavior that the maximum erosion occurs at the midpoint in the accelerating tide and that erosion has ceased by the time τ b reaches its maximum agrees with the previous observations Sanford and Halka 1993 Sanford and Maa 2001 In type II erosion in which erosion is not limited by the availability of erodible bed sediments the maximum concentration occurs near the time of the maximum τ bFor the coarse size class 65 μm EED shows the prolonged time duration of constant maximum concentration for both types of erosion CD exhibits the concentration variations with tidal fluctuation The maximum concentration occurs earlier than the maximum τ b for type I erosion but near the time of the maximum τ b for type II erosion as in the medium size class CD also represents the characteristics of the coarse particles well in that they stay at the bed for a relatively long time period and spend relatively little time in suspension in the water column

Keywords: