Coastal Classification Atlas

Coastal Classifications

Figure 2. Coastal geomorphic classification scheme that utilizes morphology and human modifications of the coast as the primary basis for hazard assessment. [larger version]

The coastal geomorphic classification (Fig. 2) emphasizes those physical factors that influence erosion, overwash of sandy beaches and barrier islands, and landward sediment transport during storms along and across those features. The natural coastal attributes that are considered to have the greatest influence on storm impact and landward sediment transport are:

1. presence of overwash areas,
2. dune height (elevation) and continuity,
3. beach width, and
4. presence or absence of emergent sandbars.

1. density and type of development, and
2. the presence of stabilization structures.

Overwash Areas

Figure 3. Overwash terrace. [larger image]

Overwash occurs when storm waters exceed the elevation of the adjacent land and the ocean water flows onshore. The overwash processes commonly transport large volumes of sand onshore where it is deposited as fan-shaped or terrace-shaped features (Fig. 3) (Morton and Sallenger, 2003). Overwash areas, which are indicators of hazards to coastal development, are characterized typically by low elevations adjacent to the backbeach, absence of dunes, and either barren or sparse vegetation. Storm flooding in broad overwash areas is normally by sheetwash, whereas scouring and erosion are common in narrow overwash areas where the flow of waves and currents is restricted. In hurricanes and some winter storms (northeasters), the overwash waves and currents can open new inlets on barrier islands, destroy bridges and roads, or transport sand inland, blocking streets and filling parking lots. The most hazardous areas are located where storm waves have flowed completely across a narrow barrier island or spit. New and closed inlets are also included in the overwash category. Because they are so hazardous, the areas of complete overwash are identified separately from the overwash terrace, which is included in the dune category.

Dune Height (Elevation) and Continuity

Figure 4. Areas where dunes are either absent, or are low, discontinuous, and sparsely vegetated are prime candidates for overwash. [larger image]

The topographies of beaches and barrier islands commonly control the extent of storm-surge impact by preventing overwash in some areas and promoting overwash in others. For example, high, laterally continuous, and densely vegetated dunes can prevent the inland penetration of storm surge. However, the western coast of Louisiana does not have natural, high, continuous dunes because there is an insufficient supply of sand available to construct the dunes. An artificial dune ridge was constructed at Holly Beach to provide some protection against storm waves.

Areas where dunes are either absent, or are low, discontinuous, and sparsely vegetated (Fig. 4) are prime candidates for overwash. Because elevations are low along the western coast of Louisiana, the land adjacent to the beach is overwashed frequently. Dune heights (elevations) on the maps are given in units of decimeters. A decimeter is one-tenth of a meter.

Beach Width

Beaches initially dissipate wave energy during the early stages of storm approach; consequently, beach width can influence the impacts of storms depending on storm intensity and duration. In general, wide beaches (Fig. 5) provide more protection than narrow beaches (Fig. 6).

Figure 5. Wide natural beach. [larger version]	Figure 6. Narrow natural beach. [larger version]

In most coastal areas, natural stable beaches are usually more than 30 m wide. Based on this observation, beach width was classified as being either greater or less than 30 m. The beach-width category is perhaps the most subject to change of all the classifications because beaches are naturally dynamic, and their widths can be altered by human activities. Even since the initial mapping and field verification were completed, some beach segments that were less than 30 m wide may be much wider because of recent beach-nourishment projects. Likewise, some beach segments that were more than 30 m wide at the time of mapping may be significantly narrower because of erosion.

Development

Storm impacts on coastal regions also depend locally on the type and density of coastal construction. This is because artificial structures and topographic modifications tend to complicate wave and current interactions, and they can accentuate the destructive forces of the storm. When high-velocity currents encounter rigid structures, the currents typically are deflected or focused, turbulence increases, and local scouring occurs. Widely spaced and elevated buildings with small footprints cause minimal interactions with storm processes. In contrast, closely spaced concrete pilings or massive foundations of large buildings, swimming pools, and coastal-defense structures locally increase the erosion by focusing the flow between buildings and preventing the wave-dissipating transfer of sand from the dunes to the beach and bars. Because hard structures do not store and release sand like dunes, more sand erodes from the beach to satisfy the capacity of the strong waves and currents.

The map classifications pertaining to infrastructure and residential and commercial development are non-quantitative, and they rely on general differences between end members and intermediate categories. The end members are undeveloped (Fig. 7) and high-density development (Fig. 8). High-density development represents many buildings per unit area with little or no green space (natural or landscaped) separating buildings. In contrast, low-density development represents only a few buildings, usually isolated or scattered throughout an area that consists of natural topography and vegetation or that is maintained. Moderate-density development represents those conditions that fall between high- and low-density development. A similar approach was taken to differentiate among areas dominated by single-family dwellings (houses), areas dominated by multiple-dwelling units (condominiums) or commercial development (hotels, restaurants, shops), and areas where both types of development are interspersed (mixed). Another classification uncertainty arises where extremely large houses are mistaken for small multiple-dwelling units. Normally, the architectural style and geographic location provide other evidence that can be used to make the distinction. Parks constitute a special map classification because they typically consist of large areas that are natural in their morphology and vegetation, but they also commonly contain parking lots, dune-walkovers, bathhouses, concession pavilions, and other man-made features.

Figure 7. An undeveloped area. [larger version]	Figure 8. High-density development. [larger version]

Beach Stabilization and Structures

Figure 9. Revetments and segmented breakwaters constructed to protect the western Louisiana coastal highway from storm waves and beach erosion. [larger image]

Many landowners have attempted to protect their property from storm waves or persistent beach erosion by placing structures along the shore. The structures commonly used to protect individual lots or entire communities are walls, riprap, breakwaters (Fig. 9), and groins.

For mapping purposes, both seawalls and retaining walls (bulkheads) were included in the single generic classification of walls. Distinguishing between seawalls and retaining walls requires knowing their intended purpose and engineering design (holding back the sea or holding back the land). Because these distinctions cannot be determined from field inspection, the term wall is applied to all wall-like features constructed parallel to the shore regardless of their composition and shape. Riprap revetments (Fig. 9) consist of broken rock or sometimes other hard material (concrete) that is placed on the backbeach parallel to the shore. Geotubes are composed of durable textile material formed into long cylinders that are filled with sand. The tubes, which are used instead of hard structures such as riprap, are normally placed in the backbeach parallel to the shore. Walls, riprap, and geotubes can be buried either naturally or artificially beneath sand of the backbeach or dunes. Consequently, there may be short beach segments where shore-parallel structures exist, but they are not visible and therefore are not included on the map. Groins are usually short features composed of concrete, broken rocks, or wood arranged perpendicular to the beach. Groins can be individual structures but they are commonly spaced along the shore to form a field of groins. Breakwaters (Fig. 9) are another shore-parallel structure constructed of concrete or rocks, but they are placed in the water seaward of the beach. They are designed so that the waves lose their energy breaking on the structure and not on the beach. The multiple-structure classification is used to delineate those areas where more than one type of structure is present. Jetties are structures constructed at tidal inlets to prevent sand from entering the navigation channels. They are usually constructed of large blocks of rock and aligned perpendicular to the beach.

A special type of man-made structure composed of sand is an artificial dune ridge. A low dune ridge was constructed at Holly Beach to intercept high storm waves and to block storm surges from overwashing the land. In the coastal-classification statistics, the artificial-dune structure was included in the dune category because it is intended to function as a dune.

continue to: Geologic Setting