Mitigating Shore Erosion Along Sheltered Coasts (2007) / Chapter Skim
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3 Methods for Addressing Erosion
3 Methods for Addressing Erosion
Pages 44-77
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From page 44... ...
This chapter provides an overview of techniques and technologies commonly used to address erosion, followed by a discussion of important design elements and criteria that should be considered in selecting an approach to address erosion on sheltered coastlines. APPROACHES TO EROSION Techniques used to address erosion along sheltered coasts may be placed into broad categories, such as those proposed by Nordstrom (1992)
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From page 45... ...
It is common for some combination of techniques to be applied at any particular location of a sheltered coast. For instance, if a decision is made to vegetate a site with a fringe marsh on a low to moderate wave energy coast, a combination of marsh plantings (vegetate)
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From page 46... ...
, creation of a marsh fringe will require the addition of elements such as sand fill (to provide a better substrate or planting terrace, see Figure 3-1) with or without some type of sill to attenuate wave action (see Figure 3-9)
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From page 47... ...
. Marsh grass was planted on sand fill and short, stone groins were placed (middle, 3-1 3 months after installation)
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From page 48... ...
. Water levels tend to be higher than normal during storm events and the capacity of seagrasses to attenuate waves (and provide shore protection)
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From page 49... ...
Traditional shoreline hardening design involves methods applied at a local or regional scale, often utilizing local materials such as stone, wood, and concrete, and built using techniques familiar to local marine contractors and property FIGURE 3-2 A dune beach along Virginia's Chesapeake Bay. Note the fencing and dune grass plantings.
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From page 50... ...
Wooden and concrete walls are common around the sheltered coasts of Florida and Alabama. In Mobile Bay, wood bulkheads have been used so extensively that a "bath tub" effect has been created: Even at low tide there is no beach; the "shore" is a bulkhead (Douglass, 2005a)
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From page 51... ...
which begins to interfere with the nearshore processes, causing vertical erosion of sediment in 3-4 front of bulkhead (C) , which leads to loss of the intertidal habitat (area between mean high and low water)
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From page 52... ...
As more and more of the shore becomes hardened, the impacts become greater. The cumulative impacts to sheltered coasts include permanent removal of sand from the littoral system creating oversteepened shorefaces, loss of intertidal zones, and intertidal and beach habitat loss.
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From page 53... ...
METHODS FOR ADDRESSING EROSION FIGURE 3-6 Landward placement of bulkheads leaving existing marsh and beach intact, at least for the near term. SOURCE: Tanski, 2005.
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From page 54... ...
Seawalls Seawalls (Figure 3-7) differ from bulkheads in that they are designed to withstand greater wave energy and are more likely to be constructed on open coasts to protect against ocean wave climates.
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From page 55... ...
A breakwater or breakwater system may or may not include the addition of sand to the system depending on the design, site conditions (whether there is abundant sand in the reach) , and the level of shore protection required (a more detailed discussion is provided later in this chapter)
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From page 56... ...
MITIGATING SHORE EROSION ALONG SHELTERED COASTS feet FIGURE 3-8 Stone revetment shortly after construction on the Potomac River, Virginia, and a cross-section of elements necessary for proper stone revetment design. SOURCE: Hardaway and Byrne, 1999.
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From page 57... ...
The sill is often built along an existing marsh fringe to maintain its integrity and enhance the protection afforded by the marsh in controlling erosion on the adjacent upland. The addition of sand with marsh grass plantings provides a stable marsh fringe system in low to moderate wave energy environments.
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From page 58... ...
Mary's County, Maryland, and North Carolina Patches of existing marsh were incorporated into the plan. SOURCE: VIMS shoreline photo archive, and North Carolina Coastal Federation.
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From page 59... ...
encourage the addition of clean sand to the littoral system because existing natural beaches along many sheltered coasts are rare, typically low and narrow, and often too small for effective shore protection. Groins A groin is a barrier-type structure, used on a variety of coasts including sheltered shores and open coasts, that traps sand by interrupting longshore sand transport.
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From page 60... ...
0 FIGURE 3-10 Cape Charles, Northampton County, Virginia, beach fill and dune field development. SOURCE: VIMS shoreline photo archive.
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From page 61... ...
METHODS FOR ADDRESSING EROSION A B FIGURE 3-11 Groin field on the Rappahannock River, Virginia, with an adequate sand 3-11 supply to provide a protective beach zone to upland property (A) and an inadequate supply along the shore reach (B)
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From page 62... ...
An attached or headland breakwater, as the name suggests, is connected to the mostly sandy beach shoreline, often with beach fill. The headland breakwater system is composed of a series of pocket beach and breakwater units designed to maintain the adjacent beach in a predictable shore planform for shore protection or to create a recreational beach.
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From page 63... ...
METHODS FOR ADDRESSING EROSION FIGURE 3-12 A detached breakwater system at Bay Ridge on Chesapeake Bay in November 2002. Definition sketch of beach morphology of tombolo and salient as they 3-12 relate to offshore breakwaters (after Dally and Pope, 1986)
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From page 64... ...
FIGURE 3-13 Kingsmill on the James, James City County, Virginia. Shore protection system utilizing primarily headland breakwaters and beach fill with wetland vegetation, bank grading with upland vegetation, and an interfacing low-crested breakwater and revetment.
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From page 65... ...
found on straight stretches of sheltered shorelines and the corresponding cross-sections (B) of the engineered breakwaters.
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From page 66... ...
Beach nourishment projects have frequently included structures to help retain the sand. For example, breakwaters can maintain a beach in an equilibrium planform that is effective in dissipating surf-zone energy during storm events.
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From page 67... ...
This section provides a general outline that discusses design elements including the level of protection, and damage and risk. The fundamental causes of shore erosion and how these causes affect how erosion is addressed are discussed in the Chapter 2 section on "Implications of Geomorphic Setting for Erosion Mitigation Strategies." Design methods can be found in numerous publications such as "Shore Protection Manual: Low Cost Shore Protection, a Guide for Engineers and Contractors," (USACE, 1981)
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From page 68... ...
Quantifying storm waves and storm surge impacts and their return intervals or frequency is an issue that needs to be addressed when designing erosion mitigation procedures. Most coastal localities have Federal Emergency Management Agency (FEMA)
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From page 69... ...
In other words, doing just about anything will provide some erosion control over the current condition. Many unproven devices will provide some shore erosion control but may not provide shore protection.
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From page 70... ...
, with a 64 percent chance of failure during that time interval. EROSION CONTROL STRATEGIES IN APPLICATION The following hypothetical example is offered to illustrate how site conditions affect the range of effective erosion control options available to the homeowner.
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From page 71... ...
MLW. Note significant bank scarping due to Hurricane Isabel when the combination of storm surge and wave runup reached 4 meters (approx.
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From page 72... ...
Larger, less frequent storms will cause storm surges of over 5 feet. Bank erosion is associated with storms and associated high water and wave action.
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From page 73... ...
There are many places along the upper reaches of the creek where just trimming trees and planting the existing substrate could significantly enhance a protective marsh fringe. However, the 3 mile fetch to the northeast at the homeowner's site will make the marsh vulnerable to storm-driven waves, offering minimal erosion protection during major storms.
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From page 74... ...
P is the probability of a particular extreme wave condition occurring during design life N years. SOURCE: Derived from British Standards Institution, 1991.
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From page 75... ...
The sill system will continue in front of neighbor L's lot but the bank grading would not be needed at the side that contains a marsh fringe. Some of the original marsh will be affected, but the new marsh fringe should compensate for the loss.
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From page 76... ...
• Managing land use has long-term individual and cumulative benefits that extend beyond those produced by other types of erosion control. • There are different strategies for shore protection, but the final design choice depends on landowner's goals, level of protection, risk, site assessment,
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From page 77... ...
Matching any of the many approaches to the appropriate setting then becomes the fundamental challenge. • Many engineers, contractors, and property owners are unaware of the range of options available for controlling erosion.
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