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Beach Nourishment Profile Equilibration: What to Expect After Sand Is Placed on a Beach By

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Beach Nourishment Profile Equilibration: What to Expect After Sand Is Placed on a Beach By ASBPA WHITE PAPER: Beach nourishment profile equilibration: What to expect after sand is placed on a beach By Kenneth Willson, Gordon Thomson, Tiffany Roberts Briggs, Nicole Elko, and Jon Miller ASBPA Science & Technology Committee March 2017 each nourishment is a commonly implemented solution to mitigate long-term erosion, provide habi- tat,B and reduce storm-damage to coastal communities. During a beach nourish- ment project, large volumes of sand (with similar properties as the native sand) are added to the beach from upland, offshore, or nearby inlet sources to establish a de- signed level of protection and/or restore sand that has eroded. During the construction of a beach nourishment project, sand is brought to the beach by dredges or truck hauling to Figure 1. Diagram showing the basic features of a beach profile typical of widen the beach. Bulldozers are then used many U.S. beaches. to grade the sand into a pre-determined refers to the typically dry area between the natural processes by including a vol- construction template. Nourishment the (high) water level and dune/upland ume of sand intended to be transported projects are designed and constructed region. Seaward of the beach is the surf offshore (Figure 2). to take advantage of the natural forces, zone, extending beyond the region where The constructed beach nourishment such as waves and currents, to move sand waves break. The dune, beach, surf zone, project includes a total volume of sand, offshore. This process results in a natural and dune comprise the beach profile “constructed fill,” which consists of sloping beach within the littoral zone, and (Figure 1). The seaward boundary of several components each with different is referred to as profile equilibration(or the beach profile, just beyond where the functions. When engineers design a profile adjustment). largest waves break, is the closure depth. beach nourishment project, they deter- The closure depth depends on the local The process of profile equilibration, mine how much sand is needed to meet wave climate and the size of the sand, which typically occurs within 12 months the stated design goal. A typical goal may and represents the seaward-most point following sand placement (depending on be to prevent waves overtopping the dune at which noticeable changes in the beach storms), dramatically decreases the width during a storm event. Since this is the profile occur. Sand placed on the dry of dry beach from the very wide beach basis of the design, it’s called the “design beach during nourishment is expected to observed immediately after nourishment. fill” (Figure 2). be redistributed across the entire beach This decrease in beach width (profile profile by the waves and currents, filling The anticipated storm event, and basis equilibration) is often misunderstood by in the underwater portion of the beach for determining the design fill, may oc- some of the public as the failure of the profile as far offshore as the closure depth. cur several years after construction. In beach nourishment project because they the time between construction and the perceive “all the sand washed away.” The Beach nourishment involves place- major storm event, the beach will lose objective of this paper is to explain the ment of sand along the upper (dry) por- sand due to average waves and smaller process of profile equilibration in in a tion of the beach profile either by dump storm events at a fairly predictable rate non-technical way to inform coastal com- truck or dredge. Once the sand is placed, (long-term erosion rate). The design munities and increase public understand- bulldozers redistribute the sand across the must account for this expected sand loss. ing of how beach nourishment works. dry portion of the beach. Conventional To ensure that the design fill is always construction equipment cannot physi- BEACH BASICS present and available to protect the up- cally place sand to a designed configura- One of the first steps in understand- land infrastructure, additional sand is tion 15-25 feet under water, which is the ing how beach nourishment works is to included in the construction fill called the common range of the closure depth. It is define “the beach profile.” For purposes “advanced fill.” The advanced fill erodes for this reason that beach nourishment of this discussion, the beach (Figure 1) at the long-term rate (e.g. x ft/yr). Ideally, projects are designed to take advantage of Page 8 Shore & Beach Vol. 85, No. 2 Spring 2017 Figure 2. Beach nourishment projects are designed to allow sand movement across the profile, while maintaining a certain dry-beach width once the advanced fill has dispersed and effective in breaking larger waves further shoreline retreats rapidly, the shoreline the design fill is beginning to erode, the offshore. The movement of the sand from retreat rate slows down as the profile next project (called a renourishment) will the steep construction slope to this flatter, approaches equilibrium. begin. The goal is to ensure that when a more stable shape typically occurs within REAL-WORLD EXAMPLE major storm occurs, at a minimum the 12 months after construction, depending The 2015 North Topsail Beach (NC) design fill is present to reduce damage on how many storms impact the area. post-nourishment profile (constructed to the community. Although profile equilibration is fac- fill) and equilibration profile (equili- A less intuitive volume of sediment in- tored into the design of beach nourish- brated fill) provides a real-world example cluded in beach nourishment design and ment projects, the public may negatively illustrating the equilibration process construction is called the “equilibration perceive this process. The immediate, (Figure 3). The constructed fill (Figure fill.” The equilibration fill is actually part rapid loss of newly placed (and often 3A) equilibrated over a period of seven of the advanced fill and design fill. Dur- quite expensive) sand may discourage months transporting sediment to the ing construction, the dredging contractor project stakeholders if the process of surf zone (Figure 3B) and adjusting the places sand at a slope steeper than the profile equilibration is not explained. It overall beach profile (slope). As shown in natural beach slope because it’s cost pro- is critical that stakeholders understand the example beach profile, the equilibra- hibitive to shape the beach profile beyond that this loss of sand does not represent tion fill is often deposited as a sandbar, the area where the waves are breaking. a failure. In fact, the offshore transport which helps to trip the waves, and further After construction, the waves and cur- of equilibration fill (Figure 2) is part of reduce the amount of wave energy reach- rents naturally soften the slope and move the engineering design. Most often the ing the beach. the sand offshore to fill the deeper parts of beach profile hasn’t lost any sand, the ENVIRONMENTAL the beach profile. During this process, no sand has just moved offshore as intended CONSIDERATIONS sand has been lost from the beach profile; where it is performing an important role Understanding sediment transport it has rather been redistributed to create dissipating wave energy. After this initial pathways of the constructed fill is com- a more stable beach shape, which is more equilibration process during which the Shore & Beach Vol. 85, No. 2 Spring 2017 Page 9 plex and is dependent on a number of fac- tors. Therefore, when designing a beach nourishment project, careful consider- ation must be given to potential impacts of the equilibrating beach profile on ad- jacent resources. The movement of sand (equilibrated fill) from the constructed template seaward into the surf zone typically occurs over a period of months. Mobile benthic organisms are resilient in this highly dynamic setting and can adapt to frequent changes in the elevation of the seafloor. On the other hand, sessile organisms (that cannot move) may be more susceptible to impacts associated with sand transported offshore. SUMMARY The objective of this paper is to explain the process of profile equilibration to inform coastal communities and increase the public’s understanding of beach nourishment. During beach nourish- ment projects, conventional construc- tion equipment cannot physically place sand within the surf zone, and therefore projects are designed to include a volume of sand that the waves and currents will transport offshore to fill in the lower parts of the beach profile. During this process, the dry beach appears as though it has eroded, however this sand has moved offshore by design, where it is more effec- tive at dampening the effects of the waves. ASBPA encourages coastal communities to share this white paper with coastal stakeholders before beach nourishment project construction begins. Figure 3. The North Topsail Beach (NC) beach profile immediately following nourishment construction (A), and seven months later once the profile equilibrated (B). Page 10 Shore & Beach Vol. 85, No. 2 Spring 2017.
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