Design Considerations

Wave Climate

As wave action is the primary mechanism for coastal erosion, determining the wave climate is essential for development of a suitable approach for coastal protection. In locations protected from wind generated waves, proximity to powerboat marinas and navigational channels can cause the shoreline to be adversely impacted by boat wakes. Many researchers consider fetch, the distance that wind can travel over open water, to be one of the most important parameters in determining the feasibility of a living shoreline approach because longer fetches can result in larger wave heights making areas less suitable for living shorelines applications.

Hardaway and Byrne (1999) described shoreline wave energy as a function of average fetch. Very low and low energy shorelines have fetches of less than 0.5 miles and 0.5 - 1 miles, respectively and are typically found along tidal creeks and small tributaries. Shorelines along main tributaries with average fetches of 1 - 5 miles are medium energy. High energy shorelines are found at the mouth of tributaries and have fetches ranging from 5 - 15 miles, while very high energy shorelines have fetches greater than 15 miles (Hardaway et al., 2010).

Hardaway et al. (2010) and USACE (1980) recommend calculating an average fetch and a longest fetch to provide design wave conditions. Assuming the wind transfer energy to the water in +/- 45^o of the direction of the wind, an effective fetch can be calculated by taking the cosine weighted average of all the rays within a certain sector on either side of the fetch ray (Malhotra and Fonseca, 2007).

Tidal Range

The local tidal range is an important parameter in determining the height of a structure or the width of a marsh or beach necessary to provide protection over the range of water levels at the site (Hardaway et al., 2010). Tidal ranges in the Chesapeake Bay range from 1 – 4 ft. New England tidal ranges are significantly larger; areas in Maine experience tidal ranges of nearly 20 ft. Tidal range is a significant design variation when applying guidelines from other locations.

Ice

Ice can be an extremely destructive force in Connecticut marshes. Ewanchuk and Bertness (2003) suggest that after wrack disturbance, ice is the most important natural disturbance in Connecticut marshes. Wrack primarily affects high marsh, while ice disturbance affects low marsh due to tidal fluctuations. Despite its importance, there is very little in the literature on design guidelines for living shorelines in ice impacted climates; most of what exists is anecdotal.

Storm Surge

The predicted surge for storms of different statistical probability is critical to the design of effective protection against coastal erosion and inland flooding. Surge elevations help identify areas at risk of inundation, volume of floodwaters expected and provide an estimate of the expected level of protection an approach can provide.

Nearshore Bathymetry

The nearshore bathymetry determines the height of the waves approaching the site. A gradual nearshore slope will cause incoming waves to break as they approach the shore, resulting in less erosive wave energy. Steeper nearshore bathymetry will allow larger waves to reach the shoreline. Tidal flats and sand bars can attenuate wave heights reaching the shoreline. Sandy intertidal regions and sand bars indicate sediment available in the systems for natural beach and dune nourishment. Thus, the nearshore bathymetry will determine the size and feasibility of shore protection structures.

Shoreline Geometry

Shorelines can be categorized into three major geomorphologic types: beaches and dunes, rocky and soft bluffs, and mudflats and vegetated communities. In addition to the types of shorelines, Connecticut shorelines are highly variable; they can be long and straight, bounded by rocky headlands, or highly irregular. Pocket beaches tend to be crenulated; the waves diffract as they approach the shoreline and sediment transported tends to remain within the pocket beach system. Linear shorelines and headlands are more exposed to erosive wave action, while irregular shorelines are interrupted by headlands, marshes or coastal structures tend to receive reduced wave effects.

The coastal morphology also determines the ability of the substrate to support protective structures. The composition of the substrate is critical to the types of vegetation and its ability to support a structure, such as a sill or oyster reef.

Shoreline Change

“Understanding how a shore reach has evolved is important to assessing how to manage it,” (Hardaway et al., 2010). The appearance of erosion does not necessarily indicate an erosion problem. An undercut bank may be stable so a landscaping approach may be sufficient, or it could be a result of recent storm effects and with time, naturally restore itself. Thus, the rate of shoreline change must be assessed to determine the appropriate approach.

Site Characteristics

While many parameters for assessing the feasibility of various coastal protections can be evaluated from maps and online sources, it is usually necessary and frequently valuable to visit the site. Although the length of the site may influence the impact of boundary effects, a primary concern is how the site is bounded. Neighboring eroding shorelines may provide sediment while adjacent harden shorelines will limit the amount of sediment available. Headlands and groins may provide wave protection but limit sediment transport into the site.

The upland usage must be considered when evaluating shoreline protection techniques. The importance of coastal development and infrastructure, the cost and feasibility of moving upland structures, and the risk of erosion and flooding all affect the level of protection required.

For a shoreline with no existing shoreline structure or hardening, the condition of the backshore or bank can provide an indication of its stability. Sandy beaches without shoreward dunes may indicate an overwash area that is susceptible to coastal flooding, while a healthy dune system indicates an adequate supply of sediment for repairing storm induced damage. A gradually sloping bank covered with salt tolerant vegetation is a good indicator of bank stability. Steep banks devoid of vegetation frequently exhibit signs of undercutting or slumping. The slumped sediment may act as a buffer to wave action and thus temporarily reducing erosion, but once the slumped sediment is eroded, the toe of the bank is once again exposed to waves and thus susceptible to slumping (Hardaway et al., 2010).  The composition of the bank material will affect its erodibility. Rocky bluffs are obviously less susceptible to wave action than unconsolidated sand banks but may be affected by splashover effects. 

Vegetation

The presence of marsh plants and nearshore submerged aquatic vegetation (SAV) demonstrates that site conditions are suitable for vegetative protection but also may affect the regulatory acceptability of certain erosion control structures.

Shellfish Beds

Nearshore shellfish beds are a clear indication that the conditions are suitable for shellfish; however, their existence may also preclude the creation of some types of Living Shorelines. The construction and presence of Living Shorelines may negatively impact the shellfish beds. For instance, fill material can bury shellfish; sills and breakwaters can damage shellfish beds.

Shore Zone

Existing Coastal Structures

Shoreline Usage

The level of protection and the anticipated shoreline use must also be considered when selecting shoreline protection approaches. The maximum winds, waves and surge conditions from which protection is desired form the design parameters; however, site conditions, permitting and costs may necessitate a revised design which could result in overtopping, failure of the structure and loss of protection during storm conditions.

The table shows the compatibility of shoreline protection approaches with shoreline usage (USACOE, 1981).