HR: 14:00h
AN: OS52F-01 INVITED     [PDF]
TI: How Tides, River Flow, Saline Intrusion and Alluvium Control Sediment Trapping, Sorting and Turbidity Maxima Whilst Maintaining Stable Estuarine Bathymetries
AU: * Prandle, D
EM: dp@pol.ac.uk
AF: Proudman Oceanographic Laboratory, Joseph Proudman Building 6 Brownlow Street, Liverpool, L3 5DA United Kingdom
AB: New theories have been developed and translated into characteristic responses for: tidal propagation, saline intrusion and sedimentation trapping, sorting and turbidity maxima - all consistent with stable estuarine bathymetries. By integrating these responses into an 'analytical emulator' new expressions are derived linking dynamics, sediment motions and bathymetry. The principal objective is to explain existing bathymetric conditions in terms of the balance with prevailing tides, river flow and alluvium in strongly tidal, funnel-shaped estuaries The familiar assumption of a single predominant (M$_{2}$) tidal constituent is utilised. The adoption of a `synchronous' estuary with a triangular cross-section is an expedient that provides a direct relationship between localised tidal dynamics and the slope of the sea bed, S. Integration of the latter provides an estimate of estuarine length, L. Moreover, these approximations enable salient features of estuarine tidal dynamics and related levels of stratification to be illustrated directly as functions of D and $\hat{\zeta}$. The related dynamics of saline intrusion were examined for the case of a vertically and temporally constant axial salinity gradient. An expression for the length of saline intrusion was derived. Intrusions respond to changes in tidal range and river flows by adjustment both to length and by axial migration. These twin adjustments explain some of the difficulties in reconciling observations and theory. Utilising these derivations for estuarine length and saline intrusion length, an expression was derived for estuarine depth, D$_{O}$ at the estuarine mouth in terms of the river flow Q and side slope of the triangular cross section. By incorporating typical observed ranges for this side-slope, an estimate for Do is obtained directly in terms of Q. Comparison of the associated envelope of D as a function of Q (over a range covering almost all estuaries) again accords sensibly with observed values. Using existing theories on the nature of locally resuspended sediment regimes in tidally dominated regimes, it was shown that significant estuarine siltation is most likely to occur via the entrainment of fine marine sediments. Moreover, the transport of such sediments in estuaries (of the kind considered) approximates that of a conservative tracer such as salt. Thence by combining theoretical estimates of time and depth averaged sediment concentrations with estimate of salinity flushing times (based on L$_{I}$ and U$_{O}$), maximum rates of estuarine in-filling were estimated. Factoring these results from observed rates of sediment capture, it was concluded that bathymetric changes over decadal time scales are likely to be minor for all but the shortest and shallowest of estuaries. Further recent developments integrate the above explicit formulations into an analytical emulator, which provides insight, scaling analyses and sensitivities into the processes of sediment trapping, sorting and turbidity maxima. By requiring conditions to be consistent with estuarine stability, estimates of suspended concentrations and net fluxes again accord with observations. Moreover, the variability of these parameters over cycles of spring-neap tides, high and low river flows and smooth and rough beds are indicated. This approach also reveals a stable feed-back mechanism linking these details of localised sediment movement to whole-estuarine dynamical response.
DE: 4203 Analytical modeling
DE: 4235 Estuarine processes
DE: 4263 Ocean prediction
DE: 4560 Surface waves and tides (1255)
SC: Ocean Sciences [OS]
MN: 2004 Ocean Sciences Meeting