engineering of dunes - sea...

Protection Benefits of Coastal Sand Dunes

In addition to providing habitat for a variety of species,coastal sand dunes are an integral part of a well-planned coastal defense system. Coastal sand dunes actas reservoirs of sand that help the beach maintain itsequilibrium and preserve the ability of the beach torespond naturally to storm events. Beaches evolveduring a storm by taking on a more dissipative statethat causes waves to break farther offshore, reducingthe wave energy near the shoreline. During thistransition, the beach slope is reduced and one or moresand bars may form. e bars are formed as sand istransported offshore during the peak of the storm andis deposited near the region of most intense wavebreaking. During smaller storms, the waves don’t reach

the base of the dune, and the erosion is limited to thebeach face (berm) itself. e dunes only become activeduring moderate to large storms when the dissipationcreated by the bars is insufficient to prevent the wavesfrom attacking the base of the dune. As a dune erodes,it releases a portion of its built-up reservoir of sandinto the littoral system, where it contributes to barformation and the development of a more dissipativeprofile, ultimately reducing damage to inlandinfrastructure. Larger dunes can withstand more waveactivity and therefore provide more protection to areasbehind them. In the simplest terms, the sand storedin a dune buys time and provides protection fromsevere storms.

FEMA Dune Standards

Although dunes of nearly any size are beneficial, inorder to be considered as a barrier to coastal floodingfor flood insurance purposes, coastal sand dunes needto meet the size criteria established by the FederalEmergency Management Agency (FEMA). Based onan analysis of hurricane-related dune erosion, FEMAidentified the amount of sand stored within the cross-sectional area of the frontal half of the primary duneabove the 100-year stillwater elevation (Figure 10A) asthe critical parameter for protection against a 100-yearstorm event. It has been established that a dune that ispredominantly below the 100-year stillwater elevationwill be rapidly overwashed and eroded and will notprovide significant protection. As the dune erodes,sand is transported both seaward into the littoral driftand landward into the backdune, resulting in alandward migration of the dune. Along developed

shorelines, this process will ultimately cause the loss ofthe dune. A dune that is only slightly above the100-year stillwater elevation will typically undergosignificant erosion and will deflate, reducing in heightand width. Studies by Hallermeier and Rhodes (1986)and by Dewberry & Davis, LLC (1989) found that540 cubic feet of sand per linear foot of dune(equivalent to 20 cubic yards per foot) was required toresist the 100-year storm. FEMA’s current V-zonemapping procedures (FEMA 1995) are based on theseobservations and require this quantity of material bepresent in the cross-sectional area of the frontal half ofthe primary dune, above the 100-year stillwaterelevation (Figures 10A and 10B), to be consideredsubstantial enough to withstand erosion during a baseflood event. is criterion is commonly referred to asthe FEMA 540 rule.

12 Dune Manual

Engineering of Dunes

Dune Removal

100-Year Stillwater Elevation

Dune Retreat

100-Year Stillwater Elevation

Erosion

Dune Toe

Initial beach profile

Changed Segment

(Eroded Profile)

50’

1’

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Ero

sio

n

Figure 10A. FEMA's current determination of dune retreat and removal.

Figure 10B. Factors to be considered in determining dune failure and V-Zone mapping.

More recently, post-storm surveys have indicated that aneven larger volume of sediment is necessary to withstandsignificant erosion events. e revised FEMA CoastalConstruction Manual (FEMA 2000) recommends aminimum frontal dune volume (measured the same wayas the 540 rule) of 1,100 cubic feet per linear foot(equivalent to 40.7 cubic yards per foot) above the 100-

year stillwater level. Since the National Flood InsuranceProgram (NFIP) regularly encourages communities toestablish standards above the minimum NFIP standards,New Jersey has taken the proactive step of referencingthis higher standard in its Coastal Zone ManagementRegulations, although it is not yet required in the rules.Currently, very few communities in New Jersey even

Dune Manual 13

Human-Induced Modifications of Dune Systems

e ability of a dune to withstand the extreme forces of asevere storm event can be either enhanced or diminishedby human modifications to the dune system. Dunes aredynamic features that naturally erode during extremestorm events and then recover when storm conditionssubside. Although robust dunes have been shown to beextremely effective in minimizing damage during evenvery large storm events (Barone et al. 2014), several NewJersey communities have sought to supplement theprotection provided by these natural systems byincorporating a non-erodible core to their existing orplanned dunes. Some of the materials that have beenutilized and/or proposed include geotextile cores, rock,and steel/vinyl sheet pile. e objective is to combine theaesthetic and habitat benefits of a dynamic beach anddune system with the robust storm protection providedby a structural core.

Although constructing a dune core can have significantbenefits from a storm-protection standpoint, there areseveral important factors that must be considered. Hardstructures that are routinely exposed to wave activityhave been shown to accelerate erosion in front of andalong the edges of the structure; therefore, dunes with astructural core must be properly maintained. After stormevents, the beach fronting a dune with a structural coreshould be restored as soon as possible to ensure that theexposed core does not create additional erosion. Whenconstructed on eroding beaches, dune cores should onlybe considered within the context of a larger beachmanagement strategy that addresses the long-termsustainability of the beach in front of the dune, includingthe potential impacts to habitat for rare and listed species(evaluated in coordination with the ENSP and the U.S.Fish and Wildlife Service).

Examples of Engineered Dune Stabilization Methods

GeocoreGeocore refers to one of several approaches in whichthe natural beach sand is encased in a geotextile fabric.Geotextile is a synthetic woven product that is used inmany soil-stabilization projects to increase the strengthof the soil and maintain proper drainage. e twomost popular forms of geocores used in beachstabilization projects are geotubes (Figure 11) andgeocubes (Figure 12). In both cases, the geotextilecontainers are manufactured offsite and are shipped to

14 Dune Manual

meet the lower standard. However, federal beachnourishment programs are authorized for the majority ofNew Jersey's Atlantic coast communities. In Ocean,Atlantic, and Cape May counties, the approved federalprojects include construction and maintenance of dunesof specific dimensions, with the dune crest typically 12to 22 feet above sea level (U.S. Fish and WildlifeService 2005). ese approved dunes, along with theassociated beach berm, have been determined by the

Army Corps of Engineers to provide sufficient stormprotection. Projects to enlarge or modify a Corps-builtdune should be reviewed by the ENSP and the U.S.Fish and Wildlife Service before starting any work.Likewise, any proposals for dune building inMonmouth County should be reviewed by the ENSPand the U.S. Fish and Wildlife Service, since theCorps-approved beach nourishment program inMonmouth County does not include any dunes.

the installation location where they are filledmechanically with sand from the beach on which theyare being deployed. Geocores are frequently viewed asa quasi-soft approach to shoreline stabilization and insome locations may be preferred over rock and/orsheetpile.

Geotubes come in a variety of shapes and sizes;however, the most common for dune-core applicationsis an oval-shaped tube with a diameter of between 4

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Figure 13. Exposed dune rock core after SuperstormSandy in Bay Head, New Jersey.

and 8 feet. e geotube casing is typically filled bypumping a water and sand mixture, known as a slurry,into the casing through a fill port. As the geotextilematerial is porous, the water percolates through thefabric, leaving behind a large-diameter, sand-filledtube. Geotubes have been used in several locations inNew Jersey, including Strathmere, Atlantic City, andOcean City on the ocean coast, and at Mordecai Islandin Barnegat Bay. e installation in Ocean City wascompleted just prior to Sandy, and although the mostsevere impacts of the storm were experienced wellnorth of the project site, the geotube core performedits intended function and successfully absorbed thewave impacts.

Geocubes are a relatively new and less commonly usedapproach for stabilizing coastal dunes. A singlegeocube consists of a series of interconnectedrectangular compartments that can be filled with an

Dune Manual 15

Generally, rock dune cores are constructed as rockrevetments backing an already existing beach. estructures are usually covered in sand, and in somecases, planted with dune grass to improve the overallaesthetic appeal of the project. Current practice is todesign rock cores independently as traditional revetmentsin the event they become exposed during a storm.Design guidance for revetments can be found in both theCoastal Engineering Manual (U.S. Army Corps ofEngineers 2002) and the Rock Manual (CIRIA; CUR;CETMF 2012). After Superstorm Sandy, manycommunities in New Jersey pointed to the effectivenessof Bay Head’s relict rock seawall in protecting thecommunity (Walling et. al. 2014; Irish 2014) as rationalefor their own proposed dune enhancements. During the2015-2016 winter, several sections of the Bay Headseawall were undermined, illustrating a potentialshortcoming in the traditional design philosophy andhighlighting the importance of maintaining a wide beachin front of proposed dune core projects.

excavator (Figure 12). e manufacturer of theproduct suggests that one of the advantages of theproduct compared to geotubes is that if a single unit orcompartment fails, the integrity of the structure as awhole is generally preserved. Currently, the onlyknown geocube installation in New Jersey is anexperimental system deployed in Ocean City afterSuperstorm Sandy.

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Figure 11. Exposed geotube after Superstorm Sandy inOcean City, New Jersey.

Most modern sheet pile dune core projects areconstructed using steel or vinyl sheet pile; however, inNew Jersey there are many older timber bulkheads thatessentially perform the same function. Steel/vinyl sheetpile is typically used due to its durability and ease ofconstruction. Similar to other dune core stabilizationapproaches, the intent of the sheet pile wall orbulkhead is to function as a last line of defense duringsevere storms. e sheet pile technique has beenincorporated at several shoreline locations in NewJersey, including Sandy Hook and Mantoloking. eMantoloking bulkhead was constructed in response tothe breach that occurred during Sandy thatcompromised a major coastal evacuation route. eproject successively resisted the erosive forcesassociated with the 2015 Joaquin nor’easter and winterstorm Jonas, however, the beach in front of the walleroded significantly. In several places, 15 to 20 feet ofwall were exposed after the storms, highlighting theimportance of maintaining a wide beach in front ofthe structure.

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Beach Nourishment and DunesIn New Jersey, beach nourishment projects have beendesigned and constructed both with and withoutdunes. Projects that include a dune generally specify atrapezoidal cross-section with regularly spaced plantings(typically American beachgrass). Such a dune provides

Sheet Pile/Bulkhead

16 Dune Manual

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Figure 15. A photo set showingthe development of a dunesystem after a nourishmentproject at a severely erodedbeach.

Considerations to Dune Engineering

Walkways/CrossoversAlthough stabilized dunes with no gaps degrade habitatsfor rare, threatened, or endangered (RTE) species, theprotective value of a coastal dune for humaninfrastructure is maximized when the dunes arecontinuous. Any break or gap in a dune becomes an areawhere the erosive power of storm surge and waves areconcentrated. Typically, water seeks the path of leastresistance and is funneled toward any gap or low spot ina dune. Unfortunately, since natural dunes are made oferodible material, the concentration of the flowaccelerates erosion along the flow path. In order to avoidcreating this hazardous condition, dune walkovers areconsidered the preferred means of beach access, sincethey allow the dune line to remain continuous.

Dune Manual 17

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Figure 16A. Wooden dune walkover (preferred method); B. angled walkway through dune; C. shore-perpendicularwalkway through dune (least preferred method).

uniform storm protection at the expense of thecreation of more natural dune forms. While there hasbeen some discussion recently about modifying projectdesign specifications to allow for the creation of morenatural features, there has been no formal adoption ofthese suggestions. Beach nourishment projectsdesigned without a dune typically have higher andwider berms to compensate for the absence of thedune. ese berms, which can be 150 to 300 feetwide, often provide a unique opportunity for dunedevelopment in areas where the natural sedimentsupply is scarce and the beaches are naturally narrow(Figure 15). e rate at which the dunes will grow onthese constructed beaches depends on the rate at whichthe placed sand is transported onto the dune from the

beach berm and on the effectiveness of the vegetation,fencing, and/or the dune itself in retaining the sand.

Both primary and secondary dune features (multipledune lines) have been found on nourished beachesbecause of the enhanced sediment supply (Nordstromand Mauriello 2001). As an example, a 6.6 millioncubic yard beach-fill project that created a 100-foot-wide berm in Ocean City, New Jersey, successfully re-established the natural cycle of dune growth(Nordstrom and Mauriello 2001). By following acarefully crafted protocol that allowed the dune todevelop naturally, both a primary and a secondarydune were developed within 5 years in regions whereonly small foredunes previously existed.

Dune walkovers are typically timber structuresconsisting of stairs and/or ramps on both the front andbackside of the dunes, with a flat deck-type structureacross the dune crest. Ideally, the walkover structure isconstructed in such a manner that it does not interferewith natural dune processes, including the movementof sand and the growth of dune vegetation. Dunewalkovers are often used in areas where there is heavyfoot traffic, and typical dimensions are dependent onthe intended use of the structure. Guidance forconstructing walkover structures in New Jersey can befound in the New Jersey Coastal Zone ManagementRules, section N.J.A.C.7:7E. In general, for multiplefamily or public beach accesses, the walkover structuremust not exceed 6 feet wide in overall dimension, and

also be used to construct footpaths over existing duneswhen walkovers are not possible. On-grade footpathsare not considered appropriate in locations where anescarpment exists between the dune structure andbeach berm (Beach and Dune Walkover Guidelines).

In addition to their effects on the storm protectionfunctions of the dune system, dune crossings areimportant in managing human disturbance andtrampling of rare, threatened, and endangered species,because human density is strongly and inverselycorrelated with distance from access points onrecreational beaches (Tratalos et al. 2013). e number,spacing, and locations of dune crossings should becarefully evaluated for their individual and cumulativeeffects to rare, threatened, and endangered species.

Environmental ConcernsAny hard structure introduced into the beachenvironment is likely to degrade habitat for wildlife ingeneral, but specific concerns about rare, threatened, andendangered species are necessary to address. Proponentsof such projects should contact Endangered and Non-Game Species Program (ENSP) and U.S. Fish andWildlife Service representatives early in planning. Evenin areas that do not currently support rare, threatened, orendangered species, hard structures are likely to precludethe formation of the most optimal habitats during futurestorm events. In some locations, hard structures may bethe only cost-effective alternative to safeguard humanlives and property from future storms. However, theadverse effects of such structures in precluding theformation of optimal habitats for federally listed speciesmust be accounted for under the Endangered Species Actthrough coordination with the U.S. Fish and WildlifeService. For example, for the recent installation ofsheetpile in Mantoloking, project proponents (inconsultation with Service representatives) providedoffsite compensatory mitigation.

All developed portions of the New Jersey coastdesignated as piping plover nesting areas, including all“protected zones” and “precautionary zones,” are

18 Dune Manual

must meet a minimum clearance of 3 feet 10 inchesabove the dune crest. Walkovers for single family useare limited to 4 feet wide and require a minimum of 3feet of clearance above the dune crest. Structures areintended to pose the least possible amount ofdisruption to the natural dune, and thus are requiredto terminate at either 10 feet seaward of the line ofpermanent beach dune vegetation or at the toe of thefrontal dune. Support posts are not to be encased inconcrete, should have a minimum soil penetration of 5feet, and should allow for the erosion of sand during astorm event (Beach and Dune Walkover Guidelines).

Although dune walkovers are preferable from a coastalprotection standpoint, there are some locations andsituations where they are not possible or necessary,such as areas of minimal dune development, sparsevegetation, low foot traffic, or critical habitat value. Inthese cases, there are several alternatives that may beused to provide access at grade. e least preferredmethod is a shore-perpendicular street-end cut. Shore-perpendicular entrances provide minimal resistance towaves and surge, and they provide a conduit forfunneling high-velocity flow directly inland. If beachaccess must be provided at grade, two preferablealternatives, which provide at least some additionalstorm protection, include angling the entrances to atleast 45 degrees with respect to the shoreline, orincorporating a small blockading feature to deflect thesurge from flowing straight through the access way. Inboth cases, the intent is to deflect or divert thepotential storm surge. State regulations require thaton-grade footpaths be limited to 6 feet in width, andbe constructed of materials other than solid concreteor stone, which may become dangerous projectiles in astorm event. e use of geotextile fabrics or cabledwood planks is reviewed on a case-by-case basis. Analternative that has been utilized with some success inAvalon, as well as in several other communities, is theconstruction of a mixed-sediment footpath. e mixedsediments are selected to be more resistant to erosionthan the native sand, provide a stable base, preservenatural aesthetics, and have a relatively minorecological impact. e mixed-sediment approach can

Adverse effects can be further reduced if conservationcommitments include periodic thinning of plantedbeach grass (see targets in Table 1). Both mechanical andchemical (herbicidal) thinning treatments can beconsidered during those times of year when rare,threatened, and endangered species are not present.Managers should also avoid creating dunes that wouldprovide suitable den or burrow sites for predators such asfoxes (Vulpes vulpes) and ghost crabs (Ocypode quadrata),and avoid installation of solid posts or other structuresthat provide perches for avian predators such as crows(Corvus spp.) and gulls (Laridae spp.). Woody speciesshould not be planted in or near rare, threatened, andendangered species areas (not even in backdune areas), asthey provide perches for avian predators.

Section 7(a)(2) of the Endangered Species Act (ESA)requires Federal Agencies that authorize, fund, or carryout projects within coastal ecosystems to consult with theU.S. Fish and Wildlife Service if a proposed action mayaffect any federally listed species. Section 9 of the ESAapplies to both Federal and non-Federal activities andprohibits unauthorized taking of listed species, includingsignificant habitat modification or degradation thatresults in the killing or injury of listed wildlife bysignificantly impairing essential behavioral patterns, suchas breeding, feeding or sheltering. e U.S. Fish andWildlife Service and the ENSP assist local beachmanagers in complying with the Endangered Species Actand related laws and regulations. Most municipalitieshave worked with the ENSP and the U.S. Fish andWildlife Service to develop a BMP for rare and listedspecies.

delineated in the Beach Management Plans (BMPs)(Figure 17). ENSP and the U.S. Fish and WildlifeService also recommend the following generalconservation measures as means for achieving the habitattargets listed in Table 1. Where they are deemed essentialto protection of beachfront developments or otherinfrastructure (and where overwashes or blowouts havenot formed), sand fences should be placed as farlandward as possible to minimize the amount of sparselyvegetated and gently sloping beach that will be replacedwith steep dunes and dense vegetation. e number ofrows of sand fence should be minimized to decrease theextent of habitat loss. Beach managers should work withENSP staff on site-specific sand fenceconfigurations.Only nativeherbaceousvegetation shouldbe planted, and theareal extent anddensity of plantingsshould beminimized.

Dune Manual 19

Figure 17. Portions of the New Jersey coastdesignated as piping plover nesting areas. Basedon FWS NJ map.

Dune Characteristic Target

Dune slope < 13% (management intervention at > 17%)

Dune height < 1.1 m (management intervention at > 1.6 m)

Vegetative cover: primary dune 13% (management intervention at > 22%)

Vegetative cover: back beach < 10% (management intervention at > 17%)

Shell/pebble cover: back beach 17-18%

Table 1. Management targets for Dune Characteristics in all developed portions of the New Jerseycoast designated as piping plover nesting areas (Figure17), including all “protected zones” and“precautionary zones” (based on Maslo et al. 2011). Typically, these targets are best achieved byleaving the beach/dune system to evolve on its own without any vegetation planting or sandfencing.

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