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Is it possible to make some generalizations about the hydrographic effects of dredging a channel into an estuary? Or is it too complex a problem and each case must be considered independently?

Let me pose a what may be a fairly common scenario: an estuary is connected to the ocean via a channel which allows a reasonable amount of tidal flow - the tide causes currents of 3 to 4 kts in both directions (it is not blocked at any time). The estuary is fed by small creeks but no major river, so the tidal flow is the major influence. There is a proposal to dredge the channel to increase the depth and width to more easily accommodate small vessels.

Does dredging change the volume of water that flows through the channel during a tide cycle?

If so, does this imply that there is a change in the max and min tide heights in the estuary or is the tidal height unaffected?

How does this effect the rate of water flow? (in a simple venturi, it would be slower).

What other hydrographic effect might occur? (ignoring biological etc.)

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    $\begingroup$ Is the estuary in question a natural sediment trap? $\endgroup$
    – Spencer
    Apr 19 at 19:52
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    $\begingroup$ Spencer - being a data systems engineer, not a hydrologist I don't fully understand your question and over what timeframe. Some sediment washes in from rainfall and small creeks. Some sand gets blown in from dunes and some gets washed in from incoming tide. But generally i think it is pretty well flushed from the tide and sediment collects fairly slowly and minimally. Once in a great while a storm might blow in a considerable amount of sand and re-arrange things. $\endgroup$
    – tkreyche
    Apr 19 at 20:14
  • $\begingroup$ The question being asked is a valid question & an interesting one but because it is asking about a potential human alteration to an environment system it could also be construed as an engineering question, in the realm of coastal engineering. If you don't get an answer here I would suggest you close the question here & re-post it on SE Engineering. $\endgroup$
    – Fred
    Apr 20 at 5:42
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    $\begingroup$ I haven't had reason to explore SE Engineering, but really doubt that there'd be such specialized knowledge of such topics. I'd think people here in oceanography fields would be more likely to be versed in the subject. I'll post it on their chat just in case, but I would think it's better here. $\endgroup$ Apr 20 at 23:36

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Although no information is given about the particular estuary that is the subject of this question, considering any estuary in a general manner can provide some conceptual views for clarification of processes. Generalizations can be made, and with regard to a more detailed understanding of the interactions of inflow and outflow, a simple hydraulic model can be developed. A recommendation would be to use a simple backwater curve approach for the general dimensions of the inflow/outflow channel of the estuary.

Is it possible to make some generalizations about the hydrographic effects of dredging a channel into an estuary?

Answer: yes. In general, dredging a channel into an estuary will improve the efficiency of the tidal inflow and receding-tidal outflow from the estuary. The consequence of this improvement may be undesirable because water residence times within the estuarine environment will be shortened, particularly in the upstream portion above the inflow channel.

Does dredging change the volume of water that flows through the channel during a tide cycle?

Answer: yes, most likely. Because dredging will improve the inflow and outflow efficiency of the channel, more water will flow into the upper portion of the estuary. Under pre-dredging conditions, the channel was less efficient. Consequently, as high tide was staged, less water would flow into the estuary before high tide started to recede. Further, as high tide receded, less water would flow out of the estuary before high tide again began to stage. Now, presuming the channel was dredged, under these same tidal conditions more water would flow into the estuary. This is because the inflow channel (being now dredged) would be more efficient in moving water upstream as the channel had been deepened, and perhaps widened, thereby offering less hydraulic resistance to the inflow. And further, the outfall would be more efficient. Consequently, the interchange of tidal water within the estuary would be increased, which may be undesirable.

If so, does this imply that there is a change in the max and min tide heights in the estuary or is the tidal height unaffected?

Answer: this depends... If conditions in the inflow channel offer relatively high resistance to tidal inflow, the upper portion of the estuary may not have come to equilibrium with the elevation of the high tide before the tide begins to recede. Consequently, the upper area does not reach the full height of the tidal staging. Neither does the upper area fully drain to the stage of the low tide before high tide begins. Under these natural conditions, specific environments develop within the upper area of the estuary, and in the fore-beach oceanward section of the estuary outfall where the outflow reaches and mixes with the sea. But if the channel is deepened, less hydraulic resistance to this inflow and outflow is present and the interchange of this water becomes more efficient. The range across the max and min tide heights may be affected. This can have adverse consequences to the estuarine environment.

How does this effect the rate of water flow?

Answer: Dredging will improve the water inflow and thereby increase the rate of inflow and outflow in response to tide. This is because deepening of the channel reduces the channel's hydraulic resistance and thereby improves (increases) the rate of inflow, and outflow. Depending on the size of the estuary, and the response to tidal inflow and outflow, the expected consequence would be that somewhat more volume would accrue on the inflow in response to high tide.

What other hydrographic effect might occur?

Answer: This is unknown. In some instances the consequences can be dramatic. For example, estuarine marshland can be adversely affected by the post-dredging influx of more saline water.


Disclaimer: This answer is in regard to general observations, only. No consultation or practice liability is assumed.

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  • $\begingroup$ Thanks, Thomas for your comprehensive and thoughtful answer, especially for such a generalized question....Tom Kreyche $\endgroup$
    – tkreyche
    Apr 25 at 22:44

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