7. Effects of changing estuarine geometry (15 pts): Most estuaries have been mad

Question

7. Effects of changing estuarine geometry (15 pts): Most estuaries have been made narrower, deeper and straighter, with a more regular bed, so that larger ships can enter, and to provide shoreline space for industry, ports, agriculture, etc. We will discuss the dynamics effects of these changes during the course, but it is worthwhile thinking about this qualitatively. How do you think changing the entrance depth of the Columbia River estuary from about 6m deep and 10km to 18m deep and 5 km, with a arrow and quite regular channel has affected the following: Bed friction: higher or lower a) b) Salinity intrusion: more or less salt in the system Tidal range c) Explain your reasoning

Explanation / Answer

(a) The bed friction is going to be lower:

Several studies that have explored the transverse or lateral structure of along-estuary flows driven by density gradients under bathymetric influences, studies have shown that net inflows tend to be concentrated

in channels while outflows appear over Shoals. One scientist characterized the estuary–ocean exchange in terms of

The competition between friction and the earth’s rotation As captured by the vertical Ekman number E. under large frictional influences inflow from surface to bottom in the channel and outflow entirely occupying the

Flanks. As frictional effects decreased, net inflow was still found in the channel but now restricted to a bottom

ekman layer, whereas outflow occupied the entire near-surface layer. Therefore, estuary–ocean

exchange shifted from large transverse variability under strong friction to large vertical variability under weak friction.

(b) The salinity intrusion is going to be high in the system:

For the given estuary , if the depth is increased from 6m to 18m, and length from 10 km to 5Km, for a narrower deeper estuary, the salinity intrusion is going to be higher provided that the discharge of the estuary should remain constant, if the discharge also increases, then it may change.

(c) The tidal range is going to be higher:

Deepening and narrowing by embankments (land reclamations, and loss of intertidal area), sand mining and large scale sediment displacements by dredging and dumping. One of the consequences of these human interventions is an increase in tidal range, which also affects natural sediment movements in the estuary. In some of these estuaries, changes have been so large, that they led to a regime shift: developing into hyper-turbid systems with large amounts of fluid mud and serious water quality problems (depletion of oxygen).

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