Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 301

the ontogenic time scales of the young of many estuarine animals. This is an area of
active research, particularly in the context of understanding how the physical envir-
onment supports commercially important coastal fisheries.
As the zone of first impact of whatever contaminants are carried by estuarine
waters, the water quality in many ROFIs is monitored closely. The idea of the coastal
ocean being a limitless receiver of unwanted domestic, industrial and agricultural
effluents has now been superseded by the knowledge that the physics of these regions
tends to limit dispersion into the wider coastal ocean. Contaminants that adsorb onto
sediments will be transported by the onshore near-bed flow back into the source
estuary, or into estuaries downstream in the coastal buoyancy current. The often high
nutrient loads of estuarine water can, in stratified ROFIs, drive high surface primary
productivity and are often seen to correlate with growth of toxic species of phyto-
plankton. As that organic material eventually sinks into the bottom layer and begins to
be consumed by bacteria, high biological oxygen demand can severely deplete the
bottom waters of dissolved oxygen, with major impacts on coastal organisms.
ROFIs are host to a complex set of processes, making them perhaps the most
difficult of all the regimes of the shelf seas to predict. While we have a first order
understanding of many of the processes involved, combining them in models to fully
simulate their physical and biological behaviour remains a major challenge.

FURTHER READING

de Boer, G. J., et al. SST observations of upwelling induced by tidal straining in the Rhine
ROFI. Continental Shelf Research, , 263-277, 2009.

Hill, A. E. Buoyancy effects in coastal and shelf seas, In: The Sea, vol.

29(1)

10 , ed. K. H. Brink and

A. R. Robinson, John Wiley & Sons, Chichester, 21-62, 1998.
Hypoxia in the Gulf of Mexico: http://www.gulfhypoxia.net/ .

Problems

..................................................................................
9.1. Along the Dutch coast at latitude 52.25 N, the density increases outwards from
the coast with a depth uniform gradient of 1.5 10 4 kg m 4 . Estimate the

cross-shore surface slope and hence the alongshore surface current at a point
where the water depth is 18 m assuming that the flow is in steady geostrophic
balance and that there is zero pressure gradient at the bed. Indicate the form of
the velocity profile and estimate the net transport per unit width along the coast.
Compare your answer with (i) the result of integrating the thermal wind
( Equation 3.28 ), and (ii) the steady flow obtained numerically with the
ROFIZ Simulation Module using a realistic value of the eddy viscosity

0.02 m 2 s 1 ).

(say N z ¼

9.2. In a ROFI where the tide is predominantly a standing wave, the water column is
well mixed at high water. If the subsequent ebb flow has a peak velocity of