The dependence of estuarine turbidity on tidal intrusion length, tidal range and residence time

Abstract It is shown that there is a marked tendency for long, strongly tidal estuaries to have greater suspended particulate matter (SPM) concentrations within their high-turbidity regions than shorter estuaries with comparable tidal ranges at their mouths, or weakly tidal estuaries. Using consistently derived data from 44 estuaries in Europe and the Americas, contours of the logarithm of maximum estuarine SPM concentration are shown to be reasonably smooth when plotted against the logarithm of mean spring tidal range (at the estuary mouth) and the logarithm of estuarine tidal length. Predictions from the plot are compared with published observations made in the Delaware, Scheldt, Rio de la Plata, Gironde, Bay of Fundy, Changjiang (Yangtze), Amazon, Patos Lagoon and the Hawkesbury Estuary and it is shown that, qualitatively, there are no serious discrepancies. Short, weakly tidal estuaries are predicted to have very low ‘intrinsic’ SPM concentrations. High SPM concentrations in these estuaries would most likely be the result of either locally generated wave resuspension, high freshwater sediment loads due to freshets, or intruding seawater carrying suspended sediments derived from wave activity in the coastal zone. Application of a generic tidal model demonstrates that longer estuaries possess faster tidal currents for a given tidal range at their mouth and, in the presence of a supply of erodable fine sediment, therefore (by implication) produce greater concentrations of SPM that can be accumulated within a turbidity maximum. The same is true if the tidal range is increased for estuaries of a given length. These features are illustrated by comparing surveys of SPM data from two large estuaries possessing greatly different tidal ranges (the microtidal, medium turbidity Potomac and the macrotidal, highly turbid Humber-Ouse) and a third, much smaller but strongly tidal estuary (the low-turbidity Tweed). It is demonstrated that longer estuaries tend to have longer flushing times for solutes than shorter systems and that larger tides tend to reduce flushing times, although the tidal influence is secondary. Short, rapidly flushed estuaries quickly lose their erodable fine sediment to the coastal zone during freshets and during the ebbing currents of spring tides. Turbidity is therefore small during low runoff, low wave activity conditions. Very long, very slowly flushed estuaries are unlikely to lose a significant fraction of their resuspended sediments during freshets or individual ebb tides and are therefore able to accumulate large and increasing amounts of fine sediment in the long-term. Turbidity within them is therefore high during the fast currents of large spring tides.

[1]  Richard Wood,et al.  Estuarine Nutrient Fluxes to the Humber Coastal Zone, UK, during June 1995 , 1999 .

[2]  M. Newson.  River basin engineering. Fluvial geomorphology , 1986 .

[3]  Carl T. Friedrichs,et al.  Stability shear stress and equilibrium cross-sectional geometry of sheltered tidal channels , 1995 .

[4]  G. Fillmann,et al.  Nutrients and Suspended Matter Behaviour in the Patos Lagoon Estuary (Brazil) , 1999 .

[5]  James N. Kremer,et al.  A coastal marine ecosystem : simulation and analysis , 1978 .

[6]  柳 哲雄,et al.  Interactions between estuaries, coastal seas and shelf seas , 2000 .

[7]  C. Neal,et al.  Major, minor, trace element and suspended sediment variations in the River Tweed: results from the LOIS core monitoring programme , 1997 .

[8]  Chen Zhang,et al.  Sediment resuspension and implications for turbidity maximum in the Changjiang Estuary , 1998 .

[9]  W. Johns,et al.  Physical oceanography of the Amazon shelf , 1996 .

[10]  Philip W. Balls,et al.  Nutrient inputs to estuaries from nine scottish east coast rivers; influence of estuarine processes on inputs to the North Sea , 1994 .

[11]  Graham J.L. Leeks,et al.  Monitoring and preliminary interpretation of in-river turbidity and remote sensed imagery for suspended sediment transport studies in the Humber catchment , 1997 .

[12]  M. Pilson.  On the residence time of water in Narragansett Bay , 1985 .

[13]  James E. Cloern,et al.  Turbidity as a control on phytoplankton biomass and productivity in estuaries , 1987 .

[14]  R. Uncles,et al.  Behaviour of Turbidity Maxima in the Tamar (U.K.) and Weser (F.R.G.) Estuaries , 1997 .

[15]  J. Zwolsman.  Geochemistry of trace metals in the Scheldt estuary , 1999 .

[16]  A. W. Morris,et al.  Seasonality of the Turbidity Maximum in the Humber–Ouse Estuary, UK , 1999 .

[17]  A. J. Mehta,et al.  Estuarine Cohesive Sediment Dynamics , 1986, Lecture Notes on Coastal and Estuarine Studies.

[18]  Hubble,et al.  Dynamics of the turbidity maximum zone in a micro‐tidal estuary: Hawkesbury River, Australia , 1998 .

[19]  J. V. D. Kreeke,et al.  Physics of shallow estuaries and bays , 1986 .

[20]  K. Pye.  Sediment transport and depositional processes , 1994 .

[21]  G. Perillo,et al.  Estuaries of South America : their geomorphology and dynamics , 1999 .

[22]  D. Eisma,et al.  Intertidal Deposits: River Mouths, Tidal Flats, and Coastal Lagoons , 1998 .

[23]  R. Uncles,et al.  Dynamics of turbidity in the tweed estuary , 1997 .

[24]  G. Nagy,et al.  Hydrography and Sediment Transport Characteristics of the Río de la Plata: A Review , 1999 .

[25]  Y. Monbet,et al.  Control of phytoplankton biomass in estuaries: A comparative analysis of microtidal and macrotidal estuaries , 1992 .

[26]  R. Sternberg,et al.  Fluid-mud processes on the Amazon continental shelf , 1996 .

[27]  J. Syvitski,et al.  Geomorphic/Tectonic Control of Sediment Discharge to the Ocean: The Importance of Small Mountainous Rivers , 1992, The Journal of Geology.

[28]  J. Salomon,et al.  Effects of tides on mixing and suspended sediment transport in macrotidal estuaries , 1980 .

[29]  F. Kestner,et al.  Physical oceanography of estuaries (and associated coastal waters) , 1977 .

[30]  W. Geyer,et al.  Observations of currents and water properties in the Amazon Frontal Zone , 1995 .

[31]  W. Boynton,et al.  The fate of nitrogen and phosphorus at the land-sea margin of the North Atlantic Ocean , 1996 .

[32]  J. Milliman,et al.  Transfer of Water and Sediment from the Yangtze River to the East China Sea, June 1980 , 1983 .

[33]  R. Uncles,et al.  Salt Intrusion in the Tweed Estuary , 1996 .

[34]  A. W. Morris.  Removal of trace metals in the very low salinity region of the Tamar Estuary, England☆ , 1986 .

[35]  Carl T. Friedrichs,et al.  Tidal propagation in strongly convergent channels , 1994 .

[36]  O. Brown,et al.  Study of the Río de la Plata turbidity front, Part 1: spatial and temporal distribution , 1996 .

[37]  James P. M. Syvitski,et al.  Estimating fluvial sediment transport: The rating parameters , 2000 .

[38]  Gerardo M. E. Perillo,et al.  Geomorphology and sedimentology of estuaries , 1995 .

[39]  J. Schubel.  Tidal variation of the size distribution of suspended sediment at a station in the chesapeake bay turbidity maximum , 1971 .

[40]  E. A. Delo,et al.  Estuarine Muds manual , 1988 .

[41]  M. Nichols.  Effects of Fine Sediment Resuspension in Estuaries , 1986 .

[42]  K. Dyer,et al.  Coastal and Estuarine Sediment Dynamics , 1986 .

[43]  M. C. Davies,et al.  A morphogenic approach to world shorelines , 1964 .

[44]  R. T. Cheng,et al.  Time scales of circulation and mixing processes of San Francisco Bay waters , 1985, Hydrobiologia.

[45]  P. Castaing,et al.  Hydrographical Characteristics of the Estuarine Area of Patos Lagoon (30°S, Brazil) , 1999 .