ASBPA WHITE PAPER: Best management practices for coastal inlets By Nicole Elko,1 Kimberley McKenna,2 Tiffany Roberts Briggs,3 Nicholas Brown,3 Michael Walther,4 and Dawn York5 1) ASBPA, P.O. Box 1451, Folly Beach, SC 29439; [email protected] 2) Stockton University Coastal Research Center, 30 Wilson Avenue, Port Republic, NJ 08241 3) Florida Atlantic University, Department of Geosciences, 777 Glades Road, SE43, Boca Raton, FL 33431 4) Coastal Tech–G.E.C. Inc., 3625 20th Street, Vero Beach, FL, 32960 5) Moffatt & Nichol, 272 North Front Street, Suite 204, Wilmington, NC 28401 ABSTRACT Coastal inlets separate individual barrier islands or barrier spits merical modeling, and other science-based methods, demon- and adjacent headlands (Hayes and Fitzgerald 2013). Inlets strate that BMPs improve management of sand resources and modify longshore transport and store sediment in flood and reduce impacts associated with tidal inlet dredging. For some ebb shoals leading to dynamic adjacent shorelines. For example, inlet conditions, BMPs include use of inlet sediment sinks as 80% to 85% of the beach erosion in Florida can be attributed to cost-effective and eco-friendly sand sources for beach nourish- inlets (Dean 1991). In some cases, structured inlets are designed ment projects located close to the inlet. to trap sand in a preferred location to minimize interference For optimal coastal inlet management, the ASBPA Science with navigation and facilitate its removal through dredging. and Technology Committee recommends the following BMPs Sound coastal engineering practice requires that this sand be and conservation measures: placed on adjacent eroding beaches (NRC 1995) to protect • Limit frequency and duration of impacts, coastal resources. • Follow environmental windows, This paper provides a brief overview of coastal inlet man- • Implement regional sediment management, agement and identifies Best Management Practices (BMPs) • Use beach-compatible sand, intended to balance human needs for inlet navigation with the • Conduct pre-, during-, and post-dredging monitoring, natural systems adjacent to tidal inlets. Today’s conservation • Modify dredging equipment/practices, and measures, which are a result of considerable monitoring, nu- • Design rechargeable, low-impact inlet borrow sites. ur nation’s inlets and water- resulted in serious beach erosion impacts ways play important ecologic, KEYWORDS: Tidal inlet, conserva- at some coastal inlets (Charleston Harbor, economic and national security tion measures, beach nourishment, SC; Cape Canaveral, FL; Ocean City Inlet, dredging, Coastal Barrier Resource Oroles by providing entrances to estuarine MD). A better management practice is to Act. nursery grounds and cycling nutrients place dredged sands on adjacent beaches as well as sustaining international and to mitigate beach erosion that may have domestic trade and providing harborage. governments, as well as local beach com- been exacerbated by the navigation chan- Coastal inlets interact with the natural munities, commonly support periodic nel; however, implementing this BMP has alongshore drift of sand but also provide placement of sand on eroding beaches been a coastal management policy chal- critical hydrodynamic linkages of ocean — a process called beach nourishment. lenge. A difficult and time-consuming and estuarine waters that sustain coastal USACE beach nourishment projects are challenge for some USACE Districts has ecosystems and communities. A key typically designed to (a) provide upland been proving that beach placement is agency mission of the U.S. Army Corps protection in the potential future event of a cost-effective and sound engineering of Engineers (USACE) is the maintenance a 10-year to 25-year return interval storm, practice to avoid downdrift impacts. of navigable waterways. Optimal inlet and and (b) be maintained over a lifespan of Local governments may fund the cost waterway management is vital to sustain 50 years as authorized and funded by difference between offshore placement the coastal ecosystem and to support Congress. State and local beach nourish- and a type of placement closer to shore commercial and recreational uses of inlet ment projects are commonly designed that is beneficial to eroding beaches such resources. for differing criteria without USACE as beach nourishment or placement in funding. a nearshore feeder berm (South Padre Adjacent to coastal inlets, the beach Island, TX). and dune system provide important Over the last century, many coastal national infrastructure that reduces risk inlets have been jettied and routinely For many inlets, beach and inlet main- from coastal flooding while enhancing dredged to facilitate navigation. The tenance expenditures can be reduced by opportunities for tourism and critical dredged sediment was often placed in a combining a navigation and beach nour- habitat restoration. To maintain this cheap, offshore disposal area. This remov- ishment project. For example, the USACE critical infrastructure, federal and state al of sediment from the littoral system has Jacksonville District has identified the Shore & Beach Vol. 88, No. 3 Summer 2020 Page 75 potential value to the nation of over $100 million across the southeastern United States by managing sediments across projects and business lines, and opti- mizing regional dredging activities and schedules (USACE RSM RCX 2019). This concept has such potential to reduce Figure 1 (right). Basic wasteful federal expenditures that a US- geomorphic features of ACE “systems approach” called Regional coastal inlets. Sediment Management (RSM) was cre- Figure 2 (below). Channel ated. The USACE RSM program receives avulsion model modified several million dollars of funding annu- from Hubbard 1977, ally to study and encourage projects that courtesy of Coastal Science save money, treat sediment as a resource, and Engineering Inc. and enhance ecological benefits (USACE RSM 2019a). Similar cost savings can be realized at state and local beach nour- ishment projects that utilize inlet and waterway sand sources. Sand used for beach nourishment is ideally as similar as is practically possible to the native sand in terms of composi- tion (carbonate vs quartz), grain size distribution, and color. The sand source can be offshore, nearshore (inlet shoals or navigation channels) or from upland sand mines. Sand sources, which are com- monly referred to as “borrow areas,” are carefully selected and undergo extensive study for not only sediment quality, but also for avoidance and minimization of ecological impacts. Each year, the USACE oversees the dredging of over 140 million cubic yards (Mcy) of sediment along the Atlantic, Gulf of Mexico, Great Lakes, and Puerto Rico coasts from about 360 inlets and segments of the Intracoastal Waterway (USACE RSM 2019b). Approximately 100 of these inlets are located at least partially within units established by the Coastal Barrier Resources Act (CBRA) enacted in 1982. Of the 140 Mcy of federally dredged sediment in this region, about 7 Mcy of sand is dredged from federal naviga- tion channels and placed on proximate beaches every year. Other beneficial use of finer sediments include placement elsewhere in the littoral system such as nearshore berms or to restore marsh habitat (USACE RSM 2019a). COASTAL INLETS 101 Coastal (tidal) inlets occur at breaks in the shoreline and provide for the ex- change of water between the inner shelf and back-barrier environments (Figure 1). Inlets are found within all tidal ranges worldwide. General geomorphological characteristics (Figure 1) include a main Page 76 Shore & Beach Vol. 88, No. 3 Summer 2020 inlet channel with marginal side (flood) shoal bypassing involves channel avul- the maintenance dredging of harbors, channels, and sand bodies on both the sion (Figure 2), which occurs when the inlets, and waterways along, or seaward flood (landward) and ebb (seaward) main channel migrates in the direction of of, the adjacent oceanfront shorelines ends. These flood- and ebb- tidal deltas littoral drift and begins to infill (Sexton of barrier islands. This practice helps to (also called shoals) act as ephemeral and Hayes 1983). Meanwhile, a second- maintain healthy beaches and littoral sediment sinks by temporarily retaining ary channel forms in the outer ebb delta sediment resources in the sand-sharing sands delivered via longshore and cross- which is oriented in-line with the inlet barrier-inlet system (Beck 2019). This is shore transport processes. Inlets with throat (Traynum and Kaczkowski 2015). a common operating procedure when the jetties commonly include an additional At some point, the main channel closes lowest cost disposal area is the adjacent sediment sink in the form of an updrift and its sand shoals migrate onshore. beach (e.g. Folly River, SC; St. Mary’s King fillet — sand impounded against the The secondary channel takes over as the Bay, FL; Tampa Harbor, FL) or nearshore updrift jetty. Significant weather events dominant channel. Coastal inlets have placement areas (e.g. Barnegat Inlet, NJ; commonly alter the forcing mechanisms displayed similar, dynamic sediment Burns Harbor, IN). to reshape the tidal deltas, channels, and transport processes throughout geologic Inlets or waterways are also dredged to fillets. Sediment infilling of the channel history (Hein et al. 2016). provide a sufficient volume of sediment can impede navigation and is the primary In some cases, this channel avulsion (on the order of 1 Mcy) for a large-scale reason jetty structures