Effects of Bulkheads on Estuarine Shores: an Example from Fire Island National Seashore, USA

Transcription

1 Journal of Coastal Research SI ICS2009 (Proceedings) Portugal ISSN Effects of Bulkheads on Estuarine Shores: an Example from Fire Island National Seashore, USA K.F. Nordstrom, N.L. Jackson, P. Rafferty, N. A. Raineault, R. Grafals-Soto Institute of Marine and Coastal Sciences Dept. of Chemistry and Environmental Science National Park Service Rutgers University New Jersey Institute of Technology Northeast Region Newark, NJ 07102, USA 120 Laurel Street New Brunswick, NJ , USA Patchogue, NY 11772, USA ABSTRACT NORDSTROM, K.F., JACKSON, N.L., RAFFERTY, P., RAINEAULT, N.A., and GRAFALS-SOTO, R., Effects of Bulkheads on Estuarine Shores: an Example from Fire Island National Seashore, USA. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), Lisbon, Portugal, ISSN Bayside bulkheads on Great South Bay, New York at Fire Island National Seashore are evaluated to determine their impact on unprotected areas adjacent to them and to identify alternatives for future protection from coastal erosion. Bulkheads extend along about 18% of the 67.3 km-long shoreline. Annual topographic surveys conducted at four bulkheads and two control sites reveal that annual retreat can be as great as 3.3 m yr -1 in the upland and 6 m yr -1 on the foreshore. Local bulkhead-influenced sand starvation appears to extend up to 68 m alongshore. Sand starvation due to bulkheads is only one aspect of the erosion problem. Lack of fresh inputs of sediment to the bayshore by inlets, overwash and dune migration all contribute to shoreline transgression. Beach nourishment can restore the sediment budget in places but should be introduced in a way that minimizes burial of benthic habitat or creation of large scale exotic environments. The inherent dynamism of natural systems and the adaptation of species to this dynamism prevent restoration of landforms or habitats to stable target states. The closest approximation of a permanent solution would be to restore the sediment budget by creating feeder uplands by placing fill sediment at about the height of local natural formations and close to bulkhead ends. Dredging of navigation channels appears to provide a ready source of compatible fill. ADITIONAL INDEX WORDS: Beaches, beach nourishment, coastal habitat, shore protection structures. INTRODUCTION Shore-parallel walls (bulkheads, seawalls and revetments) are a commonly used method of protecting estuarine shores because they are affordable, provide protection in limited space, and need not alter the bay bottom (NORDSTROM, 1992; JACKSON and NORDSTROM, 1994). They are a response to sediment starvation, but they also contribute to local sand starvation by preventing erosion of the upland that would otherwise provide sediment to the longshore transport system. They also increase wave reflection, which has been hypothesized as creating greater turbulence and scour. If placed across the active beach, their shore-perpendicular tie-back extensions function as sediment traps and create localized erosion and accretion and change beach profile response (NORDSTROM and JACKSON, 1992). The structures eliminate beach habitat (for dwelling, spawning, and foraging) by replacing the beach during construction or preventing new beach from forming as the shore is displaced landward through erosion. They also create exotic habitat as a hard structure in a sand or gravel environment. Shore-parallel structures stabilize the land behind them, which makes the continued erosion of adjacent shorelines even more apparent than prior to their construction. Erosional scarps in adjacent headlands provide evidence that erosion occurs near these structures, but the extent to which bulkheads are responsible for accelerating erosion, and the spatial limits of these local effects, are not clear because there are few measurements of topographic changes near bulkheads on estuarine beaches. This study was conducted to assess the effects of bayside shoreparallel protection structures on resources on the estuarine shore of Fire Island National Seashore. These structures are nearly all bulkheads, which are walls of lighter construction than seawalls and revetments and are normally built to hold land in place rather than resist direct wave attack. They are usually the primary protection on estuarine beaches because wave energies are relatively low (NORDSTROM, 1992). SITE CHARACTERISTICS Great South Bay (Figure 1) is a narrow basin where fetch distances for generation of waves are short, usually less than 15 km in the direction of the dominant northeasterly and northwesterly winds. Water depths in the bay are often less than 1.0 m within 1 km of the shoreline. Wave heights are lower and wave periods are shorter than in many estuaries because of the short fetch distances and shallow depths (NORDSTROM and JACKSON, 2005). Mean spring tidal range near the middle of the bay shore is 0.24 m (NOAA, 1995). The strongest and most 188

2 Estuarine Bulkheads Figure 1. Study area. frequent onshore winds blow from the northwest, but strong northeast winds occur during passage of low-pressure centers. Beach sediments are in the range of medium sand (SHERMAN et al., 1994) and are predominantly quartz and feldspar. The irregular orientation of the bay shore is inherited from past episodic additions of sediment delivered from the oceanside by overwash, dune migration, and inlet flood tides. Several residential communities remain as developed enclaves within the park. Building and stabilizing dunes on the ocean side to protect the homes reduces the likelihood of overwash and inlet formation and migration of dunes across the island that would provide sediment to the bayside. Upland erosion, indicated by scarps and fallen trees (Figure 2), is common along the bayshore. LEATHERMAN and ALLEN (1985) indicate that much of the bay shoreline at Fire Island is eroding, with an average rate of about 0.3 m yr -1 and a maximum of over 1.0 m yr -1. The fallen trees can function as small groins (shore-perpendicular protection structures), trapping sand moving alongshore. determined using a straight-line measurement of the bayward face of the structures, excluding shore-perpendicular tie back distances. Data on vegetation resources were obtained from the National Wetland Inventory GIS data files supplied by the Conservation Management Institute. Annual topographic surveys were conducted near bulkheads at Fire Island Pines, Cherry Grove, Sailors Haven and Kismet (Figure 1) and at control sites east and west of Fire Island Pines that are considered far enough away from the structure (325 and 167 m) to be unaffected by it. One profile line was surveyed on the east and west sides of each bulkhead segment, except at Cherry Grove East, where three lines were surveyed to better define spatial impacts. The survey sites are either at wooded uplands or grassy barriers fronting wetlands. Surveys were conducted over several days in October and November 2004, 27 October 2005, 26 October 2006, 1 November 2007, and 12 September Erosion and accretion along topographic profiles was used to determine the amount of vegetation resources directly influenced by bulkheads. Bulk sediment samples, taken to a depth of 5 mm, were obtained at profile sites on the east and west sides of the bulkhead at Fire Island Pines (one in each subenvironment) and the west side of the Sailors Haven bulkhead (two in the cliff and four on the foreshore) in 2007 and Two additional samples were obtained from sediment dredged from the navigation channel leading to the Sailors Haven marina to evaluate the suitability of dredged material as beach fill. Sediments were washed, dried and sieved at 0.5 intervals in a sonic sifter and analyzed using FOLK S (1974) inclusive measures. RESULTS Figure 2. Fire Island Pines East during calm spring high water conditions, showing eroding remnant dune upland and fallen trees that interfere with longshore transport. METHODS The length, number, type and location of bulkheads on the bayshore were obtained from aerial photographs from 2003 (produced by the US Army Corps of Engineers, New York District) and satellite imagery from GoogleMap. Data from these sources was mapped on USGS 1:24,000 scale topographic quadrangles to determine the scale of features. Lengths were Inventory of bulkheads and resources affected About 18% of the 67.3 km-long bay shore of Fire Island is protected by 43 bulkhead segments. Most bulkheads are sheet-pile structures. The longest extent of bulkhead is 1.85 km. The most common natural environments along the developed and undeveloped shoreline (last column, Table 1) are sparsely vegetated sand and low salt marsh. Two environments landward of bulkheads or influenced by bulkheads (dune shrubland and beach heather/dune) are poorly represented along the shore. Over one third of the maritime deciduous scrub forest is landward of bulkheads and an additional 17.4% is influenced by bulkheads. Maritime holly forest, such as the Sunken Forest (a valued park resource west of Sailors Haven marina that is several thousand years old), is considered globally rare (SIRKIN, 1972 in KLOPFER et al., 2002). 189

3 Nordstrom et al. Table 1. Length of distinctive shoreline environments along the bayshore of Fire Island. The amount of shore influenced by bulkheads is based on a distance of 68 m from the end of each structure, determined by topographic profiles. Environment Armored Influenced Total (m) % (m) % (m) Human-modified Sand path/road Paved road ,052 Residence/building Boardwalk/dock 2, ,011 Lawn Mosquito ditch ,183 Bare Sand Natural Dune shrubland Northern salt shrub Beach heather dune N. beach grass dune 4, ,625 Maritime holly forest Jap. black pine forest N. mar. decid. forest ,620 Sparsely veg. sand 2, , ,291 Open beach ,413 Low salt marsh ,331 Reedgrass marsh ,727 High salt marsh ,183 Beach topography at representative bulkheads Annual changes in beach profiles (Figure 3) reveal that the top of the active foreshore at most sites is only about 1 m above the elevation of the low tide terrace (0 m elevation at each site). This small vertical distance is due to the relatively low wave energies and low tidal range. The greatest annual changes occurred 2004 to 2005, with up to 3.3 m of cliff erosion at Fire Island Pines East and 6.0 m of foreshore accretion at Cherry Grove East, Line W. The pronounced accretion there, just east of the bulkhead, indicates that the net transport direction was to the west. The relatively great amount of erosion at Fire Island Pines East 2004 to 2005 could be either because the bulkhead is too far to the west (65 m) for accretion to ameliorate losses at the profile site, or temporary sand traps updrift of it (such as fallen trees) interfered with longshore tranport. The minimal amount of change at Fire Island Pines West, especially relative to the control site nearly 100 m farther from the bulkhead imply that the zone of structuralinduced erosion falls within a distance of 68 m from the bulkhead. Beach profile changes at these two sites in the three subsequent years differed little. Profile changes at Sailors Haven East and Kismet East 2004 to 2005 occurred without a net loss of sediment. A high cliff cut into a remnant dune just east of Sailors Haven East is a major potential source of sediment and may be the reason for no net loss here. The beach at Kismet is compartmentalized by nearby bulkheads on both the east and west and it is possible that this configuration retains sediment. The sheltering by the islands offshore of the site (Figure 1) could diminish losses as well. Net transport direction along the bayshore 2005 to 2006 is apparently to the east as revealed by the erosion close to the bulkhead at Cherry Grove East, Line M. Erosion on the foreshore at Kismet East and Sailors Haven East imply that the bulkheads interfere with transport to the east. Lack of accretion at Kismet West 43 m updrift of the bulkhead in 2006 and lack of accretion at Cherry Grove E, Line E 43 m updrift of that bulkhead 2004 to 2005 (when transport was to the west) indicate that the trapping effect updrift of bulkheads of those sizes occurs over a relatively short distance. Less accretion occurred at Sailors Haven West 2005 to 2006 than might be expected, given the proximity of the site to the bulkhead and the net transport from west to east at profiles for other sites. From less change occurred near the bulkhead at Cherry Grove East (Line M), impyling no clear dominant transport direction. In general, erosion continued 2006 to 2007 but at a lower rate. Little erosion of upland occurred 2007 to Profiles west of some structures show accretion 2007 to 2008, while the diagnostic profiles at Cherry Grove East show erosion of the foreshore. The upland and foreshore at Fire Island Pines East Control Site retreated more than Fire Island Pines East in both 2006 to 2007 and 2007 to 2008, indicating that vulnerability to upland erosion occurs well beyond bulkhead ends. Annual changes reflect the influence of 1) major events (storms) occurring within each year; 2) the most recent event occurring prior to the field surveys; and 3) local, highly specific, conditions in the eroding upland. As a result, direct cause-effect relationships are hard to conclude. Nevertheless, the surveys reveal that bulkheads have considerable local effect on the sediment budgets. Sand starvation over one-year periods may extend as far as 48 to 68 m, but any accretion on the opposite sides resulting from this net drift appears to occur over shorter distances. Local sources (in eroding formations) and local sinks (living and fallen trees functioning as groins) introduce site specific differences that do not allow for more precise determination of distances. Sediment characteristics Sediments in the two samples from the eroding cliffs on the east and west sides of the bulkhead at Fire Island Pines (mean 0.31 mm, sorting 0.53 ) are similar to the two foreshore samples at those locations (mean 0.28 mm, sorting 0.45 ), implying that much of the volume of sediment supplied by these cliffs remains in the foreshore subenvironment. Sediments in the two samples from the eroding cliffs at Sailors Haven (mean 0.22 mm, sorting 0.47 ) are finer than the four foreshore samples taken there (mean 0.41 mm, sorting 0.52 ). One of these foreshore samples was unusually coarse (mean 0.75 mm). The means of the other three foreshore samples (0.29 mm) were similar to the foreshore samples at Fire Island Pines. The two samples dredged from the navigation channel (mean 0.30 mm, sorting 0.58 ) are slightly coarser and similar to foreshore sediments, implying that the dredged sediment is compatible with placement on the beach. DISCUSSION AND MANAGEMENT IMPLICATIONS The following conclusions can be used to inform management decisions: 1. Upland vegetation landward of bulkheads is not threatened by erosion, although faunal interaction with the bay is restricted. 2. Upland vegetation next to bulkheads is threatened by bulkhead-influenced erosion. 3. Annual retreat can be as great as 3.3 m in the upland and 6 m on the foreshore. 4. Greater rates of upland erosion can occur at sites removed from bulkheads than sites closer to the structures. 5. Foreshores reveal alternating periods of erosion and accretion, even if uplands landward of them erode through time. 6. Upland erosion rates provide a better diagnostic of long-term vulnerability of coastal resources than beach erosion rates. 7. Fallen trees and roots create sediment traps that result in highly variable rates of erosion over distances of a few meters. 190

4 Estuarine Bulkheads Figure 3. Annual beach profile changes east and west of representative bulkheads. Numbers in parentheses are distances from the ends of structures. Control sites were not established the first year. The datum monument at Fire Island Pines West was lost between 2004 and 2005 and Cherry Grove East, Line W was buried under wrack in 2006, so no profiles are presented for those times. 8. Erosion occurs along most of the bay shore; sand starvation at bulkheads is only one aspect of the erosion problem. 9. Eroding cliffs can supply sediment compatible with a foreshore location. 10. Maintenance dredging of navigation channels can supply sediment compatible with placement on the foreshore. The problem of shoreline erosion and sediment budgets Guidelines for shore-protection strategies must be placed within the framework of the larger problem of the overall negative sediment budget of the bayshore. The former inlet, overwash and dune deposits now forming the eroding shore on the bayside are remnants that are not being replaced. Many bulkheads have been built on the high ground that would normally erode and provide sediment to nearby beaches, so even the former deposits from the ocean side are unavailable as sources of beach sand in many places. The lack of new sediment accelerates the rate of erosion of the unarmored upland. Many bulkheads have been in place so long that the foreshores fronting them have been eliminated, leaving the structures on the low tide terrace. The shoreline setbacks adjacent to these bulkheads act as sediment traps, causing the beach to accrete on the updrift side and erode on the downdrift side, thus functioning as groins. Shore protection options The principal means of addressing shoreline erosion are: 1) removal of threatened human infrastructure (retreat); 2) no action; 3) building static engineering structures; and 4) augmenting natural protective features using vegetation or artificial beach nourishment. Removal of buildings and infrastructure is theoretically possible but difficult to employ due to reluctance of landholders to move and the great cost of compensating them for their property. The no-action alternative may be a preferred strategy for addressing bulkhead-induced erosion where there are no structures or rare species or habitats. The relatively great length of beach grass/dune and sparsely vegetated sand environments adjacent to many bulkheads (Table 1) may fall into this category, but less common and more valuable environments, such as the maritime holly forest, may require protection. Structural solutions, such as groins, sills and breakwaters are not compatible with the National Park Service mission to favor natural processes, and they do not resolve the biggest problem, which is the lack of beach sediment. Extending bulkheads farther alongshore would protect the eroding formations next to them but eliminate exchanges of sediment and biota between upland and bay and displace the zone of accelerated erosion in unprotected areas alongshore. Using vegetation plantings to provide protection in low energy environments can be successful (WOODHOUSE et al., 1976; KNUTSON, 1978), but not where underlying substrate is sandy and subject to direct wave attack, such as most of the bay shore of Fire Island. Beach nourishment is the current preferred shore protection alternative on ocean beaches, and it can maintain beach widths on estuarine shores, but placing beach fill on the low tide terrace covers benthic habitat. This is a special concern on estuarine beaches, where fauna are not acclimated to rapid surface change and burial, as in the nearshore of ocean beaches. The perceived losses are often cited as the primary reason for lack of 191

5 Nordstrom et al. acceptability of nourishment projects in estuaries (US ARMY CORPS of ENGINEERS BALTIMORE, 1980; US ARMY CORPS of ENGINEERS SEATTLE, 1986), and the New York Department of Environmental Conservation has a policy against use of beach fill on the low tide terrace. One potential alternative is to place the fill above the low tide terrace, creating an eroding, feeder upland, so less benthic habitat will be buried in the initial operation. Alternatively, beach fill could be placed landward of bulkheads that are reduced in elevation to make them controlled-erosion headlands that would supply sediments to eroding downdrift areas (ZELO and SHIPMAN, 2000). The lower portions of bulkheads could be left as sills to keep sediment from moving onto the low tide terrace. Bulkheaded segments modified as controlled-erosion areas would only remain effective if the modified segments are periodically nourished. Spoil materials dredged from channels in the bays are mostly sand and would make good beach material. Fill materials brought in from outside the nourished area may differ in color or size from local sediments, create different growing conditions for vegetation, and have different packing, porosity and permeability that affect burrowing of organisms and groundwater flow. The inherent dynamism of natural systems and the adaptation of species to this dynamism prevent the possibility of finding a permanent solution by restoring landforms or habitats to specific target states and maintaining a stable shoreline condition, but at least the sediment budget interrupted by structures can be restored. When the concept of restoration refers to a sediment budget, assurances that specific target species will be planted or have a predicted survival rate are not required. Compatible species may have to be planted to temporarily stabilize the surface of the fill, but the new environment should be considered expendable. A feeder upland should mimic the natural configuration of the shore as much as possible. By placing the fill at about the height of the natural formations and placing it close to bulkhead ends, the sediment deficit caused by the bulkhead can be compensated at the location most adversely affected, and the fill sediment can feed into adjacent portions of the shore by natural processes. The Sunken Forest west of the marina at Sailors Haven is a good location to test a feeder upland because 1) the eroding upland habitat has great resource value and justifies direct protection; 2) the upland species there do not require direct contact with the bay waters; 3) the need to maintain the navigation channel and dispose of the dredged sediment provides a ready source of fill; 4) the dredged sediment is compatible with foreshore sediment; and 5) mobilization costs for dredge equipment for beach nourishment would be minimized because the dredge would already be on site to perform channel maintenance. CONCLUSIONS Sediment losses caused by bulkheads take on added significance when the contributions of inlets, overwash and dune migration are prevented. Removal of existing bulkheads to increase the availability of sediment to the beaches is unrealistic under present conditions of human development. The sediment starvation problem can be partially mitigated by nourishing beaches. Introducing this sediment at bulkheads would help overcome the site-specific impacts of these structures while allowing adjacent natural areas to be nourished at a rate corresponding to natural sediment transport rates. Large scale nourishment projects, although desirable on economic grounds, would bury too much benthic habitat and create large exotic upland environments. Use of sediment dredged from nearby navigation channels provides suitable source materials that can be delivered relatively cheaply and at a rate that more closely approximates natural losses. LITERATURE CITED FOLK, R.L The Petrology of Sedimentary Rocks. Austin TX: Hemphill Publishing Co. JACKSON, N.L. and K.F. NORDSTROM, The mobility of beach fill in front of a seawall on an estuarine shoreline, Cliffwood Beach, New Jersey, USA, Ocean and Shoreline Management 17: KLOPFER, S.D., A. OLIVERO, L. SNEDDON, and J. LUNDGREN Final Report of the NPS Vegetation Mapping Project at Fire Island National Seashore. Conservation Management Institute. KNUTSON, P.L Designing for bank erosion control with vegetation, Coastal Sediments 77. New York: American Society of Civil Engineers, pp LEATHERMAN, S.P., and J.R. ALLEN (eds.), Geomorphic Analysis: Fire Island Inlet to Montauk Point, Long Island, New York - Reformulation Study. New York: U.S. Army Corps of Engineers. NATIONAL OCEANIC and ATMOSPHERIC ADMINISTRATION (NOAA) Tide Tables 1995, East Coast of North and South America. Washington, DC: U.S. Department of Commerce. NORDSTROM, K.F Estuarine Beaches. London: Elsevier Science Publishers. NORDSTROM, K.F. and N.L. JACKSON, Two dimensional change on sandy beaches in estuaries, Zeitschrift für Geomorphologie 36: NORDSTROM, K.F. and N.L. JACKSON, Bay Shoreline Physical Processes (Fire Island Synthesis Paper). Technical Report NPS/NER/NRTR-2005/020. Boston, MA: National Park Service. SHERMAN, D.J., K.F NORDSTROM, N.L. JACKSON, and J.R. ALLEN, Sediment mixing-depths on a low-energy reflective beach, Journal of Coastal Research, 10: U.S. ARMY CORPS of ENGINEERS, BALTIMORE DISTRICT Beach Erosion Control Colonial Beach, Virginia: Detailed Project Report. Baltimore: U.S. Army Corps of Engineers. U.S. ARMY CORPS of ENGINEERS, SEATTLE DISTRICT Lincoln Park Shoreline Erosion Control Seattle Washington: Final Detailed Project Report and Final Environmental Assessment, Seattle: U.S. Army Corps of Engineers. WOODHOUSE, W.W. Jr., E.D. SENECA and S.W. BROOME Propagation and use of Spartina alterniflora for shoreline erosion abatement, US Army Corps of Engineers, CERC Technical Report ZELO, I. and H. SHIPMAN, Alternative Bank Protection Methods for Puget Sound Shorelines. Ecology Publication Olympia WA: Washington Department of Ecology. ACKNOWLEDGEMENTS We are grateful to the National Park Service for providing funds for the project, to Wesley Brooks for help in the field, and to Fabiola Nelson for work in the lab. 192