Hydraulic and earth engineering – Bank – shore – or bed protection – Wave or flow dissipation
Patent
1996-11-12
1999-08-10
Graysay, Tamara L.
Hydraulic and earth engineering
Bank, shore, or bed protection
Wave or flow dissipation
405 15, F02B 304
Patent
active
059348275
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the regeneration of tidal mud flats, that is, unvegetated muddy shorelines.
The value of sandy shorelines has long been appreciated and a number of methods have been employed to counter the erosion of such shorelines, particularly in the case of so-called amenity beaches in holiday resorts. These measures range from the building of groynes to check erosion to the nourishing of beaches with sand dredged from offshore to replace the sand lost by erosion.
Muddy shorelines, on the other hand, have not been afforded protection unless the land behind the shore was considered to be of value for agriculture, habitation or industry. Traditionally, muddy shorelines have been protected against erosion by the planting of salt-tolerant vegetation to encourage the development of salt marshes which trap mud and raise the profile of the shore to build a natural rampart against the sea. More recently, a policy of strategic retreat has been adopted in areas where shore defences are not viable, leading to the abandonment of land.
There is a need for an effective method of managing eroding or degraded muddy shorelines which can be employed, for example, when the development of a salt marsh is not acceptable for environmental reasons and where strategic retreat is not possible because the land behind the shore represents a valuable industrial or residential investment.
The object of the present invention is to fulfil this need. The invention aims to make use of the results of recent hypsographic measurements of muddy shorelines in estuaries and bays. These measurements have revealed that the profile of a shoreline, taken from the sub-tidal or low-tide zone to the high-tide or supra-tidal zone, is low and concave in the case of an eroding or degraded muddy shoreline at equilibrium but is high and convex-tending in the case of an accreting muddy shoreline at equilibrium. If the profile of a tidal mud flat can be changed from a low, concave, degraded shape to a high, convex, accretionary shape, the stability of the shoreline itself and of any artificial coast defences behind the mud flat will be increased.
Accordingly, the present invention provides a method of regeneration, by addition of replenishment material thereto, of a shoreline comprising tidal mud flats which are degrades or are being eroded, characterised in that the method comprises the steps of ascertaining the ambient tidal range and wave climate of a mud flat to be regenerated, deriving from these data a regenerated convex profile for the mud flat and a modified wave climate under which said regenerated profile would be stable, and depositing replenishment material comprising densified or dewatered mud or clay on the mud flat to build the convex profile; further characterised in that breakwater or barrier means are placed to modify the wave climate and sedimentary regime of the mud flat so as to render the mud flat receptive to deposition and retentive of the deposited mud or clay, whereby a new shoreline profile is established which is in equilibrium with the modified wave climate.
The regeneration of the entire profile of the shoreline and the establishment of a new profile which is in equilibrium with the modified wave climate according to the invention is preferable to attempting to regenerate only that part of the profile of a shoreline between the mean tidal level and the mean high tide level, which would result in an unstable profile out of equilibrium with the ambient wave climate and would leave the shoreline vulnerable to erosion at lower tidal levels.
Typically, the modification of the wave climate, and hence of the sedimentary regime, involves the suppression of high and short-period waves which would otherwise cause erosion of the mud flat and its reversion to the pre-regeneration profile which, in the absence of any modification, would be the profile closest to equilibrium with the ambient wave climate.
The new profile for the mud flat to be regenerated may be derived empirically by the identification and copying of a n
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Polytechnisch Tijdschrift; Bouwkunde Wegen-En Waterbouw, vol. 28, No. 22, pp. 723-730, Ing. L. Sterkman `oeververdediging, oeverontsluiting en recreatie` see figures, Oct. 1973.
Friedrichs C.T. & Aubrey D.G. 1996 `Uniform bottom shear stress and equilibrium hypsometry of intertidal flats`, in Pattriaratchi C. (Ed.) `Mixing Processes in Estuaries & Coastal Seas`, Coastal & Estuarine Studies Series, American Geophysical Union, Washington, D.C. 1996.
Graysay Tamara L.
Lagman Frederick L.
Novick Harold L.
Ravensrodd Consultants Limited
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