Fluid Mud Layer and Its Effects on the Hydrodynamics of Benthic Boundary Layers

Xiao Hua WangSchool of Geography and Oceanography

University of New South Wales@Australian Defence Force Academy

Canberra, Australia

1. Introduction

2. Model description2.1. Hydrodynamic model2.2. Sediment transport model

3. Experiment setup

4. Model results 4.1. Benchmark experiment4.2. Resuspension hysteresis—the nepheloid layer4.2.1. Sensitivity to Mellor-Yamada turbulent closure assumptions4.2.2. Effects of flocculation4.2.3. Effects of freshwater runoffs4.3. Hyperpicnal plume4.4. Bottom drag coefficient 4.5. Model sensitivity to vertical resolution

5. Summary and conclusions

Hydrodynamic model—Princeton Ocean Model with following new features: (1) C

s

ww )1(

(2) 2

0 ]/)ln()1/(

1[

zzH

ARC b

fd

where A=5.5 (Adams & Weatherly, 1981) and Rf is flux Richardson number Model forcing

22

22

2sin

2sin

SS

MM T

ta

T

ta

at x=60 km

where am2=0.4 m, as2=0.1 m, Tm2=12.42 hours and Ts2=12 hours

S e d i m e n t t r a n s p o r t m o d e l

C

t

x

uC

y( vC )

z

[ w w s C ] z

( K h

C

z) F C

Ez

CK h

a t z = - H

A c c o r d i n g t o A r i a t h u r a i & K r o n e ( 1 9 7 6 )

(12)

if )1(

if )1(0

cbc

bsb

cbc

b

wC

EE

E0=10-4 kg m-2 s-1, ws=-10-4 m s-1, c=0.02 N m-2

Setup of numerical experiments

Experiment )(C ws x10-4

(m s-1)

spring

(m)

R

(m3 s-1)

(zb+H)/z0

1a-1b C=0, C0 -1 0.5 0 100

2a-2b C0 -0.5, -2 0.5 0 100

3a-3b C=0, C0 -1 0.5 800 100

4a-4b C0 -1 0.4, 0.6 0 100

5a-5c C0 -0.5

0.5, 0.4,

0.6

0 100

6a-6b C=0, C0 -1 0.5 0 1000

7a-7b C=0, C0 -1 0.5 0 4000

Summary & conclusions This work describes exploratory numerical modelling

results of sediment transport processes in both neutrally and sediment-induced stratified tidal estuary. The focus was placed on description and prediction of the dynamics of a fluid mud layer developed in the estuary due to the coupling effect of the seawater and resuspended sediment concentration.

It can be concluded that the establishment of the fluid

mud layer in the BBL causes: 1) a significant reduction in the total sediment transport

along the estuary; 2) occurrence of resuspension hysteresis event that

forms a highly concentrated fluid-mud bottom wall layer;

3) variability in the bottom drag coefficient Cd.