An Innovative Control of Shoreline Erosion

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In addition to the traditional problems of agriculture-related erosion, erosion control for natural ecosystems

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Figure 1 Severe soil erosion in a wheat field near Washington State University, USA. Photographer: Jack Dykinga, http:// www.ars.usda.gov/is/graphics/photos/k5951-1.htm.
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Figure 2 Wind erosion on Ulen fine sandy loam, Grand Forks, North Dakota, USA. Photographer: Adrian Fox, http:// www.nrcs.usda.gov/TECHNICAL/ECS/agronomy/ Photo%20File/FrontAgr3.jpg.
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Figure 3 Yangtze River showing the sediment-rich water, the Three Gorges, Hubei Province, China. From http:// www.soilerosion.net/.

(e.g., national parks) has been recently a critical issue. In particular, the border area between land and water (e.g., shoreline, riparian zone) is extremely sensitive to erosion. Proper management of erosion in this type of ecotone is essential for achieving sustainability of ecosystems (e.g., biodiversity, efficiency in energy production) and for obtaining maximum socioeconomic benefit from environmental amenity.

In this article, an innovative control of shoreline erosion is introduced to demonstrate how efficient ecosystem management could be optimized between nature conservation and environmental engineering. Protection of recreational beaches along ocean coasts and inland lakes, bays, and inlets, as well as finding a beneficial use for dredge material, has become a sensitive issue to a diverse public. It is often difficult to find a solution that makes good engineering sense while maintaining environmental responsibility.

Current conventional methods used to retard shoreline erosion include the installation of breakwaters, groins, and jetties. Sand replenishment is often used in conjunction with these methods when shorelines are being extended or restored. These techniques, though often functional, are costly and can detract from the natural environment. Dredge material management was viewed as a 'necessary evil' associated with overdevelopment of coastal areas in the past. Through the years, hundreds of metric tonnes of sediment have been dredged annually for commercial and recreational purposes and subsequently discharged into land-based disposal facilities or into oceans, estuaries, rivers, and lakes. As the space for disposal facilities reaches capacity, and discharge into water bodies becomes more of an ecological concern, the problem arises as to what to do with this material.

The purpose of this article is to describe in detail how Presque Isle State Park, located along the shoreline of Lake Erie in Pennsylvania, implemented a unique erosion protection project, which also included the beneficial use of dredge material. This low-cost, innovative demonstration project minimized erosion in the lesser-energy zone of Misery Bay in Presque Isle State Park by utilizing native plants, bioengineering, dredge material placement, and nonconventional erosion practices.

Outline of the Park and Erosion

Presque Isle State Park is a 1295 ha migrating sand spit that juts 11.3 km into Lake Erie and is a major recreational landmark for approximately 4 million visitors each year. The park, a National Natural Designated Landmark, is particularly environmentally sensitive with its constantly evolving shoreline (Figure 4) and the presence of

Figure 4 Presque Isle State Park. (a) Aerial view of Gull Point and the park from the east. (b) The shore on the bay side, looking north. (a) Photographer: Robert K. Grubbs, http:// en.wikipedia.org/wiki/Image:PresqueIsleStatePark.JPG; (b) http://en.wikipedia.org/wiki/Image:PresqueIsleBay-lookingN.JPG.

Figure 4 Presque Isle State Park. (a) Aerial view of Gull Point and the park from the east. (b) The shore on the bay side, looking north. (a) Photographer: Robert K. Grubbs, http:// en.wikipedia.org/wiki/Image:PresqueIsleStatePark.JPG; (b) http://en.wikipedia.org/wiki/Image:PresqueIsleBay-lookingN.JPG.

numerous plants recognized as being of exceptional value. Presque Isle is rated as one of the top birding areas in the Northeast, as birds use the distal end of the spit for a resting and feeding area.

Protection of the spit has been an ongoing process since 1828. Along the Lake Erie shoreline, a series of conventional erosion control techniques such as groins, bulkheads, seawalls, and beach nourishment have been used with varying degrees of success. Between 1989 and 1992, many of the previous structures were removed and 55 offshore rubble mound breakwaters were constructed. Since completion of the breakwaters, shoreline maintenance has been limited to an annual beach nourishment program. Construction of the breakwaters has decreased sand purchased for annual nourishment by c. 85%, from approximately 231 000m before breakwater installation to c. 30 764 m3 after breakwater construction.

Since 1975, the beaches along the lakeside of the park have been nourished annually; nourishment amounts varied based on fluctuating lake levels and storm severity. The prevailing winds along the lake are from the west, and, as a result, the beach sand is in continual motion as it moves in response to longshore transport. While most of this transient sand is redeposited in offshore bars in the lake, some of the sand is carried around the distal end of the spit into the back bay area. Accumulation of this finegrained material in the back bay has been a continual problem as these shallow areas become choked with sediment. As a result, the park struggles with the problem of dredging these areas and finding a suitable disposal option for the dredged material.

Historically, protection of the shoreline from erosion had been accomplished along the Presque Isle Bay side of the park by utilizing large stones to riprap the shoreline. Although this process was very effective in preventing shoreline erosion and was quite suitable in the more developed recreational areas of the park, because of its 'non-natural' appearance, riprap did not concur with the desired results and appearance specified by the management plan for the low density and natural areas.

As a result of the fragile ecosystem of the spit, specific erosion problems along the bay, and the development of a sand bar within the park's back bay area, the decision was made to seek funds to advance an innovative solution to these problems. With this goal in mind, the Department of Conservation and Natural Resources, Bureau of State Parks - Presque Isle State Park, in conjunction with the Presque Isle Partnership, secured funding via a matching grant from the Great Lakes Commission. The project coordinated efforts between state and federal government units, as well as private, nonprofit volunteer organizations to design, implement, and provide construction services for the project. This project brought forward a concept that provided the park with the protection needed for the infrastructure as well as creating a shoreline appearance that resembled natural shorelines along environmentally sensitive areas of the park. Additionally, the project provided a beneficial use of dredge material from the back bay sand bar.

In order to realize the goals of the project, the decision was made that rather than solely utilizing conventional riprap, the project would incorporate a combination of riprap as well as indigenous vegetation, bioengineering, dredge material, and innovative landscape architecture to retard shoreline erosion along a heavily used multipurpose trail. Completion of this project has provided valuable information to other parks and recreational facilities in the Great Lakes area (especially along bay inlet areas), which are also faced with the challenges of minimizing erosion and sedimentation as well as finding a beneficial use for dredge material.

Problem Areas

There are numerous recreational features within the park. One of these is a 15.4 km multipurpose trail. This trail, designated as a National Recreation Trail, begins at the park entrance and completes a 21.7 km loop throughout the park. This is the most popular trail within the park, and is heavily used by bicyclists, joggers, roller bladers, and is wheelchair accessible. Because of its popularity, protection of the trail from erosion is paramount.

A portion of this trail lies along the southern shoreline of the peninsula within Presque Isle Bay, Misery Bay, Marina Lake, and Thompson Bay; this area had been exhibiting significant erosion. Because this area was adjacent to Presque Isle's ecological reservation area, the standard riprap remedy was not appropriate because it did not match the park's designated management prescriptions for maintaining a natural shoreline appearance.

Another popular tourist attraction is the Perry Monument, dedicated to Commodore Perry. The area surrounding the monument, located along Misery Bay, receives widespread use for shoreline fishing as well as the launching of recreational boats. Normal wave energy along the shoreline of Presque Isle Bay, fluctuating water levels and currents, caused a significant sand bar to develop off the northeast tip of Perry Monument. The sand bar measured approximately 91m long by 8 m wide by 1.5 m deep (1100 m ) and severely restricted recreational boat usage. Removal ofthe sand bar was essential to preserve the recreational activities at the monument.

A small portion ofthis sand undoubtedly was the beach sand along the Lake Erie side of the spit. The probable source for the remainder of this sand was the erosion of the shoreline around Misery Bay. Historical photos of this area show that the east shoreline of Misery Bay has eroded several hundred feet since the late 1800s. Suitable disposal of this sand would be difficult because ofits susceptibility to erosion no matter where it would be placed. Rather than following the standard disposal options of this dredged material, the park wanted to find a constructive use for this sand.

Project Description

The first phase of the project was to remove some of the sand from Perry Monument. After passing the mandated state tests for disposal of dredge material, and in accordance with all applicable permits and regulations, approximately 917 m3 of material from the sand bar was dredged via a clam shell and placed in an unused gravel parking lot within the park so it could naturally dewater; the water could naturally seep through the gravel into the ground. The remaining sand bar was then graded to provide a suitable launch/mooring area for canoes and shallow boats.

The next phase involved the creation of a stabilized area on the backside of Misery Bay where the multipurpose trail is located adjacent to an ecologically sensitive area of the park (an area where the natural habitat is to be maintained and little to no development is to occur). In this area, significant erosion had occurred, to the point that water was only 3-4.5 m from the trail.

Initially, the project proposed to install 10-25-cm-sized riprap c. 7-9 m from the existing shoreline. However, based on design criteria for the worst-case scenario for wave height, which in this area would be 1m, the decision was made to place 30-61-cm-sized riprap offshore; it was felt that this sized riprap would provide better protection for the shoreline. Placement of this riprap below the water line created an artificial stone shoreline that protected the multipurpose trail by functioning as an erosion/wave energy dissipater.

Once the rock was in place, the third phase began. In this phase, the park placed the dewatered dredge material (sand) over the newly placed riprap, creating a higher-elevation dune line. This subsequently provided a buffer of c. 8-9 m between the water and the trail. Next, to enhance the 'natural' appearance of the shoreline, randomly spaced downed trees and stumps from the park (25-91 cm in diameter), minus the limbs, were used as timber groins. To function as groins, the tree root bases were anchored behind the riprap in the fill, and the trunks extended out past the riprap and into the water, also serving as sediment catch basins.

After the fill had been placed and prior to planting, the decision was made to use geotextile (comprised of coconut fiber cured to aid in longevity) in conjunction with wattles (poles interwoven with slender branches) in order to augment the vegetative plant rooting and further stabilize the dredged sand. The woven geotextile material is biodegradable, but it was anticipated that the plants should be well established before it decomposed.

Within the fill material, several trenches, parallel to the shoreline, were dug. First, the geotextile was laid in the trench, and then the wattles were placed on top - end to end and parallel to the shoreline, approximately at the average high-water mark. The geotextile was then rolled back over the wattles and staked with live saplings. The wattles and geotextile were then further secured by placing sand on top; the sand helped to anchor the entire apparatus against the wind. The geotextile and wattles provided extra erosion protection to the shore zone area, as well as ensuring a stabilized area in the fine-grained sand for plant rooting.

Prior to planting of the indigenous vegetation, plant community goals were established to ensure that the plants would thrive in the newly created environment. The plant community goals were developed by reviewing historical records of plant community structure in that area, consulting local and regional plant experts, and considering wildlife uses of the site. After the goals were established, a vegetation planting plan was prepared. Final preparation of the site prior to planting included the addition of topsoil to the upper layer of sand, and shaping of the dune line.

The final phase was the vegetative planting. For this phase, local sources of plant material within the park were identified for transplanting onto the dune. These included beach grass (Ammophilia breviligulata), Indian sea oats (Chasmanthium latifolium), switch grass (Panicum virgatum), choke cherry (Prunus virginiana), bayberry (Myrica pensylvanica), and black oak (Quercus velutina). Additionally, driftwood from local sources was collected and was dispersed in the restored area to provide shelter for the young seedlings. Local sources of emergent wetland plants were located, and these were then transplanted into shallow water below the wattle trenches. Transplanted aquatic plants included species that enhanced the establishment of desirable native emergent communities. These species, such as branching bur reed (Sparganiuma androcladum), duckweed (Spirodela oli-gorrhiza), and soft-stem bulrush (Scirpus validus) were also beneficial to waterfowl by providing a native food source.

After the native species were established, invasive species, such as purple loosestrife (Lythrum salicaria) and common reed (Phragmites australis), targeted in the Presque Isle Partnership report, were mechanically removed as they were encountered throughout the restored areas. Roundup, a glyphosate, was applied as necessary to eliminate invasive species that could not be controlled by mechanical means. The final objective was to achieve at least 50% vegetative cover in both the shoreline and dune habitats - this goal had been achieved within 6 months of project completion. Annual plane surveys have shown that since project completion, erosion has been reduced by approximately

90%, thus providing protection for the heavily used multipurpose trail.

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Homeowners Guide To Landscaping

Homeowners Guide To Landscaping

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