Coastal Services Center

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Response of Beach Nourishment to Tropical Storms, Hurricanes, and Extra-Tropical Events

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Introduction

Major storms can cause elevated water levels in excess of 20 feet and offshore waves 40 feet high, causing accelerated and modified evolution of beach nourishment projects. In planning for long-term shoreline stabilization by beach nourishment, it is essential that the stakeholders recognize and plan for the accelerated effects of hurricanes and extra-tropical storms. Although the movement of beach material and fluctuations of the shoreline occur continuously, wind-generated waves and currents, and tidal currents created by severe storm events, have sufficient energy to move tremendous quantities of sand in the cross-shore and longshore direction in a short period of time. In the accompanying animation the reaction of the beach profile in response to storm waves is shown where the waves overtop both the Mean Low Water Level (MLW) and the Mean High Water Level (MHW) as shown in the identified stages of the storm interaction with the shoreline.

Overall, beach nourishment projects respond to major storms in the longshore direction, primarily through more rapid spreading of the sediments to the adjacent beaches, and in the cross-shore direction through sediment transported seaward where a longshore bar usually develops. If the landward berm or barrier island is overtopped by the storm tide, overwash processes transport sand in the landward direction and deposits beach material on the roads and onto the landward side of the barrier island. This happened in 1995 in the Florida Panhandle during Hurricane Opal.

In planform evolution, it can be shown that beach nourishment projects respond, in effect, as "integrators of wave energy." The total wave energy expended on a beach nourishment project during a storm will cause the same approximate evolution as if that same total wave energy had occurred over a much longer period of time. Thus, a project constructed on a long, straight beach will simply experience an incremental and irreversible planform evolution, which under more normal wave and water level conditions would require months to years to occur.

The cross-shore transport (seaward and possibly landward) can cause a significant shoreline retreat that may "overshoot" the equilibrium (or expected adjusted beach width) dry beach width that would occur under normal wave conditions. Some of this accentuated loss of beach width may be permanent, primarily due to overwash. However, excess sediment transported seaward by the storm should return landward, albeit over a long period, establishing a profile that is in equilibrium with the normal wave conditions. Accordingly, where a severe storm with high tides and high waves may cause significant erosion in 6 to 48 hours, beach recovery may require weeks or months of normal wave and tidal conditions.

Results from Theory and Numerical Modeling
Theories and numerical models have been developed for representing the planform (that is, longshore) and cross-shore responses to beach nourishment projects. Results from these theories and numerical models are described, followed by examples of three beach nourishment projects that have experienced significant storm events. For theory and background, refer to "Cross-shore and Longshore Transport Models for Large Scale Geological Processes."

Figure 1. Illustration of proportion of placed material transferred to project adjacent beach areas as a function of cumulative wave exerted on project.

Sand Redistribution Following Beach Nourishment on a Long Straight Beach
An illustration of the theory applicable to the planform evolution, or sand redistribution, for beach nourishment projects is shown in Figure 1, which applies for the case of a nourishment project that is placed on a long straight shoreline with no background erosion.

The horizontal coordinate can be interpreted approximately as the cumulative wave energy applied to the beach over time. The vertical coordinate is the proportion of the material (that is, percent) placed that has spread out from the project area to the adjacent shorelines. If a project experiences a long period of low wave activity, the volume lost from the nourishment area will be small, whereas a period of unusually energetic wave conditions will cause substantial quantities of sand to be transported to the project adjacent areas. Stated differently, a particular project can absorb a certain quantity of wave energy prior to losing an associated proportion of the placed volume from the project area. A storm will result in a greater proportion of volume lost from a short project than from a longer project. In addition, as discussed in "Cross-shore and Longshore Transport Models of Large Scale Geologic Processes," it can be shown that for nourishment on a long straight beach with compatible sand, the evolution is relatively insensitive to wave direction. Examples of the effects of wave height and project length are presented. Also evident from Figure 1 is that the effect of a certain quantity of energy expended on a beach nourishment project early in its life will cause much greater evolution, or adjustment, than after the project has "seasoned."

Consider projects exposed to waves of different heights for various times. A storm with an effective wave height of eight feet lasting over a period of two days would cause the same evolution of the beach nourishment project as an average wave height of one foot would over a period of one year. The effects of project lengths can be demonstrated similarly. A project with a length of two miles acted upon by a particular wave height over a one-year period will experience the same evolution that a project of four miles and experiencing the same wave height would in four years. Thus, both the wave heights and project lengths are critical to the effects of storms on beach nourishment planform evolution.

Sand Redistribution of Beach Nourishment Projects Adjacent to Armored Inlets
There are three sub-cases for nourishment adjacent to an inlet: (1) nourishment adjacent to a natural inlet, (2) nourishment adjacent to a deepened but unjettied inlet, and (3) nourishment adjacent to (downdrift of) a jettied inlet. Because these cases are considerably more complex than nourishment on a long straight uninterrupted beach, it should be recognized that modeling of the storm response of these systems is considerably more problematic and subject to greater uncertainty, as shown in Table 1 of "Cross-shore and Longshore Transport Models of Large Scale Geologic Processes."

Sand Distribution of Beach Nourishment Adjacent to Natural Inlets
In this case, it is assumed that the shorelines adjacent to the inlet have established a continuity of sediment transport. Thus, this case is identical to that of nourishment on a long straight beach. As described for structurally stabilized inlets, the project evolution or sand redistribution is relatively insensitive to wave direction.

Beach Nourishment Adjacent to a Deepened But Unjettied Inlet
In contrast to the previous case, the continuity of sediment transport has been severed through inlet dredging and creation of an artificially deepened channel. The best approach in modeling this case is to assume that the shoreline at the inlet is unaffected by the beach nourishment project. In effect, this maintains the shoreline at the location that it would have been without the nourishment project with the placed sand assumed to be drained into the deepened inlet. The loss of sediment from the nourishment area in this case is considerably greater than for nourishment on a long straight beach. An example of this case is presented later in this paper.

Beach Nourishment Adjacent to (Downdrift of) a Jettied Inlet
The presence of the jetty in this case requires that the transport at the jetty be set equal to zero or equal to the value of any artificial sand bypassing, vis-à-vis dredging with downdrift sand placement which occurs. In this case, shoreline evolution is very sensitive to wave direction.

Figure 2. Idealized profile response to increase in water level.

Cross-Shore Beach Profile Evolution
The design of a beach nourishment project includes the depth to which the sediment will be distributed seaward. The planform evolution depends solely on the total energy delivered to a shoreline and the profile response to storms depends on the time variation of wave heights, or simply stated, how long waves of certain heights influence the shoreline, storm tide and duration of these effects and whether or not overwash occurs.

Unlike planform evolution, which is irreversible, profile evolution in response to storms can "overshoot" the equilibrium beach profile and later, under the action of normal wave and tide conditions, the profile will evolve to equilibrium conditions. Sand transported landward as overwash and deposited on the barrier island or on its mainland side represents a permanent loss to the nearshore system unless later returned to the beach by mechanical means. Figure 2 shows an example of the idealized response of a beach profile to an elevated water level and the equilibrium beach profile associated with normal wave conditions.

Examples of Constructed Beach Nourishment Projects

Delray Beach, Florida Project
This project was first nourished in 1973 and has been renourished three times for a total of four nourishments. Results of the monitoring surveys for these projects are shown in Figure 3. To provide a basis for comparison with the monitoring data shown as solid circles, the lines are so-called "blind-folded" predictions for no background erosion (BE = 0) and a background erosion of two feet per year. This project has been subjected to a number of severe storms over the 29-year period since its initial construction, including Hurricane David in 1979, the 1984 Thanksgiving Day Storm, and Hurricane Andrew in 1992. It is seen that although the volume remaining within the project area has varied with respect to the predictions, there are no major lasting effects of the storms. It is noted that these calculations are based on an assumption of a constant wave height. This example illustrates the effects of a relatively large project absorbing the wave energy associated with storms without major and lasting effects.

Perdido Key, Florida Project
This project area was initially nourished in 1989 with the placement of approximately 5.4 million cubic yards of high-quality sand along the shoreline adjacent to Pensacola Bay Entrance, a federally maintained navigation channel. This entrance has been dredged to a depth of 45 feet, considerably deeper than the natural channel depth of 20 feet. This entrance is also stabilized by two very short structures on the west side of the entrance; however, the effects of these structures are believed to be minimal due to their relatively small size and location well landward of the inlet throat (or gorge).

The measured proportion of sand remaining within the eastern and western halves of the project area and the total project volume are shown as the symbols in Figure 4. Also shown by the lines in this figure are the "blind-folded" predictions for the same quantities. This project has been impacted by a number of hurricanes, as shown in Figure 4. The impacts of these storms are evident as reductions in project volume. However, as is the case of Delray Beach, Florida, there are no dramatic long-term effects of the project; thus this project functions as an effective integrator of wave energy.

Manatee County, Florida Project
This beach nourishment project, completed in early 1993, placed 2.3 million cubic yards of beach-compatible sand along a shoreline length of 4.2 miles immediately prior to the so-called "Storm of the Century" (March 1993). Profile evolution was expected due to the severe nature of this storm, as is evident in Figure 5, where it is seen that the average beach width decreased by some 70 feet, thus accomplishing in a single storm the equilibration effects that would normally require two to three years. This is to be compared with the approximate total expected equilibrium beach retreat of some 140 feet.

Summary

Beach nourishment projects have been shown to provide effective storm protection for upland structures. The coastal decision-maker must understand the accelerated shoreline response to tropical storms, hurricanes, and extra-tropical events in planning for long-term shoreline stabilization by beach nourishment.