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Coastal Protection Strategies: Dynamic and Static Armoring

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In dynamic armouring smaller particles are used and they are allowed to adjust by erosion and deposition to the prevailing wave climate, while still protecting the finer particles behind them from being washed away. In static armouring very large stones are placed to lock the shore in place - no movement is anticipated even in the worst storm. What are the advantages and disadvantages in both human terms (use, appearance, cost) and natural terms of the two strategies for coastal protection?

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What are the advantages and disadvantages in both human terms (use, appearance, cost) and natural terms of the two strategies for coastal protection?

I divided this response into two sections, dynamic and static strategies for coastal protection. There is an overlap in some of the material, so I presented the material in both sections.

I. Dynamic armoring processes of coastal protection

1. Natural terms. There is a linkage between the stream-geomorphic processes and salmon and steelhead. Stream shape and the processes that give rise to the shapes define the quality and quantity of the fish habitat. When the shape of the stream is reformed, fish habitat is transformed, and this may be in either a positive or negative direction. Complex macro-habitats such as side-channels, back-channels and slack-water areas can also be the result of these changes, creating diverse and productive habitats for a variety of fish and other aquatic organisms (Rempel 1999). (see full article attached for other ideas).

Similarly Kern (1998) notes that "...[r]iver systems evolve over geologic time periods, governed by the prevailing geologic, tectonic and climatic conditions." The geometry of an evolving stream is the result of interplay amongst flow, the quantity and character of sediment being transport by the stream, the character of the bed (e.g., the slope) and bank material, and the in-stream and riparian vegetation. Because the streams are not static, but dynamic and subject to a continuing suite of inputs, present-day sediment processes define the current salmon and steelhead habitat abundance and health of a stream, and govern the distribution, size and abundance of these particles which determine these parameters. (See --"Sediment Processes" attached for fuller details). Also see online source http://www.fish.bc.ca/html/fish2E16.htm

2. Human use: Aggregate Industry—advantages and disadvantages. Mining of sand and gravel brings in revenue (i.e., wages, miners, making roads using sand and gravel, etc.) while removal of sediment for aggregate purposes has been a problem for fisheries habitat regulators. While sand and gravel extraction and associated aquatic environmental impacts are significant issues in British Columbia, they have consequences that are also found around the world. With the global expansion in human populations over the past 50 years, the needs of the aggregate industry have grown worldwide and have caused environmental problems related to extraction of river-derived sediments. Sear and Archer (1998) indicate that commercial-gravel extraction from riverbeds is a global phenomenon because rivers have historically been an attractive source of gravel supply where they have existed close to the point of use.

About 10-20% of the sand and gravel mined in the United States in 1974 came from streams (Meador and Layher 1998). Construction utilizes about 96% of the material, while around 43% of this amount is used for buildings in that country. To place aggregate use in context, about 91,000 kg of sand, gravel or crushed stone are required to construct a six-room house and 14 million kg of aggregate are required to construct a school or hospital. Like British Columbia, road building in the United States accounts for a high level of aggregate use, with 24% of the sand and gravel volumes directed to this purpose. Around 60 million kg of aggregate are normally used in building 1.6 kilometres (1 mile) of a typical four-lane US highway (Meador and Layher 1998). (See http://www.fish.bc.ca/html/fish2E18.htm for more details).

3. Human Use: Effects of Removal of Floodplain Sediments for Flood Protection (Advantages and disadvantages)

While removal of sediment for aggregate purposes has been a problem for fisheries habitat regulators, a second and potentially more difficult issue has arisen in recent years with regards to the removal of in-stream or floodplain sediments in order to increase stream capacity for flood protection.

This is particularly an issue for flood-prone streams near populated areas with high recruitment of bedload. The buildup of material in the stream bed can cause dike over-topping during flood events, with the attendant destruction of life and property. A simple-minded way to deal with the problem has been to remove sediment from the channel in order to increase the water conveyance. Often, in the cause of expediency and cost, this method is undertaken by floodplain managers. In centers of high population, where gravel supplies are already short and the demand is high, the aggregate industry is more than happy to remove the material at minimal cost, or even pay a significant royalty for the material. Regulatory agencies are pressured to allow the removal of gravel from within the floodplains of salmon and steelhead streams in excess of what realistically may be needed. Further complicating matters, the aggregate industry readily provides its removal capability to government agencies for the ostensible purpose of reducing an actual or perceived threat of flood. The resulting alliance between industry and government officials often undermines the support that fisheries regulators require to protect salmon and steelhead habitat.

In British Columbia, the removal of sand and gravel in order to increase the floodway capacity for the purpose of public safety and flood control occurs under the aegis of the provincial Water Management Branch, and usually with some level of input from the senior fisheries agencies and the local communities. (http://www.fish.bc.ca/html/fish2E18.htm).
In the United States Colins and Dunne (1989) recorded the gravel-extraction impacts to three gravel-bed streams in southwestern Washington. They found that the amounts of material removed from these rivers were exceeded for up to three decades, and often by a volume of more than an order of magnitude greater than the natural recruitment. For the Lower Mississippi River, various studies have shown that in recent years there were significant changes in both the size and gradation of the bed material due to mining (Lagasse et al. 1980). Much of this change seems to be related to the removal of the coarse fraction of the bed material that results from the dredging of the limited gravel resources
( http://www.fish.bc.ca/html/fish2E18.htm).

TABLE 3:
Summary of the effects of gravel extraction in streams. From Collins and Dunne (1990) and Sear and Archer (1998).
Impacts to Habitat and Natural-Stream Attributes
• In-stream mining causes disruption of bed sedimentology.
• Removal of sand and gravel from streams causes disruption of sediment movement continuity.
• Extraction of bed material in excess of replenishment by transport from upstream causes the bed to lower (degrade) upstream and downstream of the site of removal.
• Degradation may change the morphology of the riverbed, which constitutes one aspect of the aquatic habitat.
• Degradation can deplete the entire depth of the gravelly bed material, exposing other substrates that may underlie the gravel, which in turn, affects the quality of the aquatic habitat.
• If a floodplain aquifer drains into the stream, groundwater levels can be lowered as a result of bed degradation.
• Lowering of the water table can destroy riparian vegetation.
• The supply of overbank sediments to floodplains is reduced as flood heights decrease.
• Rapid bed degradation may induce bank collapse and erosion by increasing the heights of banks.
• The reduction in size or height of bars can cause adjacent banks to erode more rapidly or to stabilize, depending on how much gravel is removed, the distribution ...

Solution Summary

Referring to dynamic and static armoring along the shores, this solution discusses the advantages and disadvantages in both human terms (use, appearance, cost) and natural terms of the two strategies for coastal protection. Supplemented with related articles.

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