Beach Nourishment - Coastal Geology

Beach Nourishment: A Guide for Local Government Officials

Erosion Hot Spots and Causes

Introduction

Since the 1990s, the existence of erosional hot spots (EHSs) has been recognized and studied. Although an appropriate definition of EHSs is still somewhat under discussion, for purposes here they are defined as areas that erode significantly more rapidly than the adjacent beaches, and in the case of nourished beaches, erode more rapidly than anticipated in design.

More than a dozen possible causes of EHSs have been identified (Dean, Liotta and Simon 1999; Kraus and Galgano 2001). On nourished beaches, EHSs can be the result of project construction. This can occur when, for example, nourishment sediment characteristics differ from those of the native, pre-existing shoreline. EHSs can also occur when wave conditions are altered after nourishment as a result of depth changes over the borrow area. Erosional hot spots can also result from natural causes, such as the movement of sand offshore or alongshore.

Tools are available to help the designer anticipate the occurrence of EHSs. These tools include models of equilibrium beach profiles and models of longshore and cross-shore sediment transport. In response to anticipated EHSs, additional sediment can be added to the constructed profile, or hard structures can be included in the constructed beach nourishment project. Unanticipated EHSs can be corrected after the construction of the project by adding more sediment to the profile or by relocating sand within the project area.

The potential for EHSs should be fully evaluated during the planning and design phases of a project. Through better understanding of their causes, it is possible to anticipate and minimize or avoid the occurrence of EHSs in beach nourishment projects.

Causes of Hot Spots

Understanding the underlying coastal processes and the relationship of the beach with the offshore bathymetry and waves is critical to understanding and controlling EHSs (Komar 1998). There have been several attempts to classify their causes. A classification system could lead to consistent efforts to control the effects of these localized erosion areas. The major known causes of hot spots are described below and presented in Table 1.

Table 1. Causes of EHS and possible Mitigation/Remediation Measures
(From Dean, Liotta and Simon 1999)

Cause	Mitigation / Remediation
Dredge Selectivity Losses Over or Through Reefs	Use coarser sand.
Residual Structure Induced Slope	a) Use more sand. b) Use stabilizing structures.
Borrow Site Causes Wave Transformation	Smooth nearshore bathymetry.
Break in Bars	Recognize temporary nature of effect, or if effect is unacceptable, fill break in bar.
Borrow Site Within Active Zone	Possibly use coarser sediment in future renourishments.
Residual Bathymetry	Nourish with enough sand so that effects are acceptable.
Mechanically Placed Fill	Ensure that density of sand is consistent with adjacent beaches.
Profile Lowering in Front of Seawalls	Add more sand.
Headlands	Recognize effect and, if unacceptable, consider use of structures as in 2(b).
Wave Focusing Due to Offshore Translation of Beach Wave Focusing Due to Pre-Existing Bathymetry	Accept effect. No known remediation or mitigation. Possibly consider shore protection structures.

Dredge selectivity. The sand that is dredged from borrow sites is not of uniform quality. Grain size, for example, can vary throughout a borrow site. Before a beach nourishment project commences, samples are taken from various locations in the borrow site to estimate the volume of sand available that is compatible with the sand present on the natural beach. However, sometimes pockets of unsuitable material exist and are dredged from parts of the borrow area that were not sampled. The dredge can also excavate below the design depth, or beyond the borrow site, obtaining undesirable material. When, for these or other reasons, the sand that is placed on the beach contains too much fine material, the resulting beaches will be narrower than anticipated, due to offshore movement of sand or greater localized sand transport.

Residual structure-induced slope. The use of groins to stabilize the shoreline can result in EHSs. Because groins are effective in trapping and holding sediments, a steep slope can develop between the relatively high profile between the groins and a relatively low profile offshore. Often during nourishment, such structures are removed from the active beach system either through extraction or burial. Thus, the nourished profile will be considerably less steep and will require greater sand locally than if the steeper, structure-induced slope were to occur. If this is not recognized in the design, an EHS may occur.

Wave transformations over borrow sites. As waves move shoreward from deeper water and propagate over depth anomalies resulting from removal of material for nourishment, the height, direction, and other characteristics of the waves change. These various transformations, called wave shoaling, refraction, reflection, and diffraction, can significantly increase or decrease the transport of sand along the shoreline, resulting in localized erosion and accretion. Construction of beaches by removal of sand from offshore borrow sites has been documented to cause significant shoreline changes, such as at Grand Isle Beach, Louisiana, where erosion rates decreased at the local shorelines that were immediately landward of the two borrow sites. These two shoreline areas accreted, while adjacent shorelines eroded (EHS) (Gravens and Rosati 1994). Such wave transformations over the borrow sites are complicated and are not currently fully predictable even with the most advanced models.

Gaps in offshore bars. Some shorelines have nearly permanent longshore bars that cause the waves to break before encountering the beach. In the gaps that occur between these bars, larger waves pass through and impinge on the beach. The result is accretion behind the bars with more rapidly eroding beaches between them. Such patterns of erosion and accretion result in EHSs.

Mechanically placed fill. Sand placed on beaches by trucks, earth haulers, or other mechanical means is generally less compact than that placed on beaches hydraulically. This is true because sand compaction is greater when sand is hydraulically (sediment suspended in water that carries it through the pipeline) placed, and thus is more stable. Thus, hydraulically placed material tends to consistently be coarser in nature, less erosive, and perhaps also more suitable for turtle nesting.

Profile Lowering. The beach in front of a seawall or other similar hard structure can erode to an elevation that is significantly lower than the natural adjacent beaches. In severe cases, there may be a substantial water depth adjacent to a seawall. Thus, during nourishment, it is necessary to add sufficient sand to compensate for this deficit, or a localized EHS may occur in front of the seawall.

Headlands. Shorelines that project seaward into the ocean are subjected to higher wave energy than are the adjacent beaches. Such projections can be natural outcroppings called "headlands" or the result of coastal armoring. The placement of sand along headlands or armored projections of land during beach nourishment can result in EHSs. If the projection is very long, the material will last longer, but for the typical length of hardened shoreline, an EHS is likely to develop.

Residual bathymetry. Frequently, offshore contours are not parallel to the shoreline. This can occur naturally or as the result of irregular placement of material during beach nourishment. In either case, this can result in wave refraction that significantly affects the wave energy along the shoreline. This, in turn, will cause EHSs to develop in some areas while others accrete. At Pensacola Beach, Florida, a relict beach ridge extending offshore caused refraction that resulted in EHS and irregular erosion (Browder 2001). A similar relict beach ridge is the likely cause of irregular erosion along the shoreline at Sandbridge, Virginia.

Offshore sinks. Offshore sinks are underwater features that remove sediment from the nearshore littoral system, thus creating EHSs. On the west coast of the United States, littoral sand flows into large underwater canyons that can be thousands of meters deep. On the east coast, sinks can develop as material is drawn from the littoral system into systems of sand bars that develop outside the littoral zone, or when sand is lost from "perched beaches" such as found along the shore parallel reef systems common to Hawaii and at reef locations along the southeast Atlantic coast.

Wave Focusing Due to Seaward Bathymetry. When beaches are translated offshore they may encounter a different, non-uniform wave climate which can cause irregularities in sediment transport creating EHSs. This effect can cause EHSs to occur in areas where erosion was not noticeably different than other parts of the shoreline prior to nourishment.

Borrow Area Within the Active Profile Zone. If the borrow site is located landward of the closure depth, some material placed on the beach will flow seaward into the borrow area's greatest depth. This occurs as the beach profile stabilizes, creating an EHS until the profile reaches equilibrium. The resulting equilibrium profile will be farther landward than planned.

Control of Hot Spots

The best way to control EHSs is to predict their location during the design phase and prevent their occurrence. Numerical and physical models that reflect bathymetric conditions and wave transformation can contribute to this prediction. These models are becoming increasingly sophisticated in capturing the effects of relatively subtle coastal processes. For further information on the recognition of EHSs, see Dean, Liotta, and Simon (1999).

However, even the best of models is no substitute for experience. In order to determine the best mitigation measures for EHSs, the results of modeling must be analyzed by individuals with the experience to recognize the possibilities of EHS occurrence and with an appropriate understanding of the underlying coastal processes. Table 1 of this paper, from Dean, Liotta and Simon (1999), presents a summary of possible mitigation and remedial measures.

It is also important to have the best available knowledge of the borrow site characteristics to identify areas of material (called "fines") that should be avoided by the dredge. Careful monitoring of the dredging operation is also key to ensuring that materials placed on the beach are not finer than the material specified in the design.

Lastly, in some instances, control under proper conditions of hot spots may only be possible through the incorporation of coastal structures in the design. Structures such as groins and detached breakwaters may significantly reduce the formation of EHSs. Groins were added to the design of the beach nourishment project at Folly Beach for an area that historically eroded faster than the adjacent areas. Model tests had shown that the structures would be effective and therefore they were included in the project (Ebersole, Neilans and Dowd 1996).

Conclusions

Through more thorough design and analysis with available methods, some potential erosion hot spots can be identified and prevented. Appropriate design can also help reduce or eliminate the effects of existing EHSs. There is much known about what causes EHSs, although instances remain that are not readily predictable. However, with future anticipated advances in numerical and physical models, and monitoring and analysis of the occurrence of EHSs, it is expected that the types of EHSs described here will become more predictable and can be reduced or eliminated through proper design.

References

Browder, Albert E. 2001. "Bathymetrically-Induced Erosional Hot Spots: Pensacola Beach, L." Report prepared by Olsen Associates, Inc., Jacksonville, FL, for Santa Rosa Island Authority.

Dean, Robert G., Roberto Liotta and Guillermo Simon. 1999. Erosion Hot Spots. Report Number UFL/COEL-99/021, Coastal & Oceanographic Engineering Program, University of Florida, Gainesville, FL.

Ebersole, Bruce A., Peter J. Neilans, and Millard W. Dowd. 1996. "Beach-Fill Performance at Folly Beach, South Carolina (1 Year After Construction) and Evaluation of Design Methods." Journal of Shore and Beach, Volume 64.

Gravens, Mark B., and Julie D. Rosati. 1994. Numerical Model Study of Breakwaters at Grand Isle, Louisiana. Miscellaneous Paper CERC-94-16, ERDC, U.S. Army Corps of Engineers, Vicksburg, MS.

Komar, Paul D. 1998. Beach Processes and Sedimentation. 2nd edition, Prentice Hall, NJ.

Kraus, Nicholas C., and Francis A. Galgano. 2001. Beach Erosional Hot Spots: Types, Causes and Solutions. Coastal & Hydraulics Laboratory Report, ERDC/CHL CHETN-II-44, U.S. Army Corps of Engineers, Vicksburg, MS.

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