B08_Attachment_10_Shoreline_Mgmt_Plan.pdf

6 REACHI

6.1 Physical Setting

6.1.1 Reach Boundaries, Geology, and Shore Change

Reach I extends from F elgates Creek eastward to Indian Field Creek, a distance of about 1. 7 miles.

Shoreline change is depicted by two baselines on Plate 1 and Plate 2 (Appendix A). On Plate 1 for the first approximately 3,000 feet of shoreline, shore change has been slightly erosional with the exception of the area near the mouth ofFelgates Creek which has accreted since 1937. Farther east on Plate 2, shore change varies at the position of low water (the digitized shoreline), but the bank has been stablized since 1960 by shore hardening. The underlying geology along this reach is the Shirley Formation.

6.1.2 Upland Characteristics, Shore Zone Conditions, and Storm Impacts

The upland bank along Reach I has been hardened with stone revetments since before 1960 to protect the Colonial Parkway from erosion. The bank face is fairly stable but Hurricane Isabel over topped the revetment which has crest elevations of +6.0 ft mean lower low water (MLL W) and created a wave cut scarp along the entire reach (Appendix C, page C2). Some minor slumping also occurred. Overall, the bank face remains stable along most of the reach but the base of bank was interpreted as transitional because of Isabel impacts (Appendix B, page Bl). Vegetative growth along the base of the bank has masked most of the Isabel wave scarping.

The area near Felgates Creek has a low, marshy strand with intermittently eroding marsh fringe headlands and small pocket beaches (Figure 6-lA). The marsh strand protects the adjacent upland from erosion but also is eroding. Revetment overtopping and erosion of the grassed upland at the opposite end of the reach at Indian Field Creek is also evident (Figure 6-lB).

The 6 ft depth contour is about 3,900 feet offshore near Felgates Creek and gets closer toward Indian Field Creek where the 6 ft depth contour is only about 230 feet offshore.

6.1. 3 Energy

Reach I is exposed to waves driven by winds from the northwest, north, and northeast. The western end of the reach is partially protected from northwest and north wind/waves by Penniman Spit which, as previously mentioned, is in state of erosive decay.

6.2 Management Recommendations

Reach I has been adequately protected with a stone revetment along most of its length but was overtopped during Isabel. This has not significantly impacted the stability of the bank face or threatened the parkway. However, another storm of that magnitude or greater might create more severe scarping with potential bank slumping. Also, the sand/marsh strand on the west end will continue to erode thereby reducing its ablility to attenuate wave action during severe storms (Appendix B, pages Bl, B6, and B7).

No Action: A reasonable option at this time. Bank conditions should be assessed after severe storms. Also upland runoff may play a greater roll in bank erosion until vegetation is fully reestablished.

Defensive: The strategy of choice over the past 50 years yielding good service. The existing revetment is in good shape but could be more effective for the next 50 years if it was raised 2 feet or

so. This would prevent wave scarping along the base of bank for Isabel type storms. In addition, the effective elevation of the revetment has decreased as sea level has risen approximately half a foot since the revetment was created.

Offensive: In the case of Reach I installing a sill along the low sand/marsh strand would not only enhance stability but would enhance beach and marsh fringe habitat.

Headland Control: The Reach I coast is not really conducive to this approach, mainly because of the extensive stone work. However, a long the beach area between an existing revetment and marsh headland may qualify.

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7 REACH II

7.1 Physical Setting

7.1.1 Reach Boundaries, Geology, and Shore Change

Reach II extends from the mouth of Indian Field Creek eastward for about 1.4 miles to the west pier of the Yorktown Naval Weapons Station (YNWS). It starts on Plate 2 and carries over to Plate 3 (Appendix A). The underlying geology is the Shirley Formation, but the shoreline lies along a low terrace or plain of sandy and shelly fill that averages about 5 feet MLW along the river. Historically, from 1937 to 2002, the net shore change has been almost O (Appendix A, page AIO). However, variability exists along much of the reach particularly at Sandy Point where a large spit feature grew over time. From 193 7 to 1978, the spit called grew into the York River. After that, it became erosive.

7 .1.2 Upland Characteristics, Shore Zone Conditions, and Storm Impacts

The low upland coast is hardened with a stone revetment near Indian Field Creek that extends downriver for about a 1,000 ft (Appendix B, page B2). This shoreline has intermittent marsh toe sills along much of its length. The areas without a sill are eroding marsh fringe headlands and beaches (Figure 7-1 ). The upland bank is stable where the marsh fringe has a sill or is wide enough to naturally attenuate moderate storm wave action. Otherwise the low bank is eroding and more recently from Hurricane Isabel impacts. Hurricane Isabel overtopped the entire shoreline uprooting trees and creating an obvious wrackline along the grassy terrace (Appendix C, page C4). The small broken concrete breakwaters (called the Parkway Breakwaters) are marginally protective and were overtopped by Isabel and the upland bank was significantly eroded (Appendix C, page CS).

At the location of the Parkway Breakwaters, the upland bank begins to increase in elevation. Downriver of the breakwaters, a sandy strand and intermittent marsh fringe occurs. It is wide enough to offer protection to the adjacent upland bank. The base of the bank has been denoted transitional with minor impacts from Isabel (Appendix B, page B2). This area is also partially protected by the long west pier at the YNWS which denotes the downriver boundary of Reach II.

The nearshore region is wider off Sandy Point as the 6 ft depth contour goes out about 400 ft. It draws back closer to the shoreline and is only about 260 ft offshore of the Parkway Breakwaters.

�proximately 5 kilometers along the parkway from Yorktown (also known as KS), th€ parkway ]comes very close to the York River (Figure 7-2). During Hurricane Isabel, the combination of storm surge and wave action severely eroded the upland banks, exposed the banks, and moved the top of the bank to within 6 ft of the road base. Emergens.y measures are being implemented using a stone revetment to secure the area (Figure 7-3).

7.1. 3 Energy

Reach II is exposed to a wind/wave climate to the northwest, north, and northeast. The northwest wind waves tend to drive shoreline sediments downriver and constantly erode the marsh fringe. Frequent northeasters create wind/waves that approach the coast in a sub-parallel manner causing significant erosion and moving the sand in an onshore/offshore direction. The northwest winds frequently follow the passage of northeasters which impacts a recently stressed shoreline.

7 .2 Long-Term Monitoring Data: Parkway Breakwaters

This site includes five, broken, concrete breakwaters that were constructed in 1985 just upriver from Yorktown Naval Weapons Station pier (Figure 2-7). The site, called the National Park Service (NPS) breakwaters, was surveyed before and after installation then two times per year until 1990. This site was reoccupied and surveyed in 1997, 1998 and post-Isabel in 2003.

The distance to the mean high water (MHW) line from the baseline is plotted for five surveys made at the NPS breakwater site (Figure 7-4). The downriver portion of the site had retreated between postinstallation and 1998. However, the MHW line did not change much between 1998 and 2003 except in the middle of the site. Breakwaters 4 and 5 have become detached, and while breakwater 5 still slightly influences the wave climate at the site, breakwater 4 has become transparent to waves and does little to influence the shape of the shoreline. The bank also has eroded, particularly in response to storms when elevated water levels impact it directly. Hurricane Isabel caused severe erosion particularly above MHW. The NPS breakwaters are still adjusting, by upland erosion, into equilibrium embayments. The breakwaters are only 50 ft long and illustrate that the shorter breakwater units (shorter, relative to the impinging wave length) are less effective than longer structures in maintaining equilibrium embayments.

Figures 7-5 and 7-6 show the cross-section profiles of the NPS breakwaters through time. A great deal of erosion occurred along the entire beach profile between 1990 and 1997 even directly behind the breakwaters. However, between 1997 and 1998, much of the erosion occurred above +5 ft mean low water (ML W) indicating that elevated water levels allowed the waves to act directly on the bank. The upriver end of the site was relatively stable (P2) as the breakwater became fully attached; severe bank erosion occurred along much of the site likely due to Hurricane Isabel. After Isabel, breakwater 3 is detached (i.e. attached subaqueously). The downriver end of the site (P26) actually accreted on the bank and eroded at MHW between 1998 and 2003.

One interesting feature revealed by analysis of data obtained by Hardaway et al. (1991) at the NPS breakwater site is the difference between the beach and upland planforms. The tangential section of the embayed beach between breakwaters generally faces north-northwest and is controlled, in large part, by the northwest wind-generated wave climate. During a typical northeaster, the storm originally has winds blowing from the northeast and elevated water levels, but as the storm moves away from the area, the winds shift to the northwest leaving exposed shorelines orientated into this wind-wave condition. However, when water levels are elevated during northeast storms, the wave action is up against the bank, and the tangential section of the bank planform faces northeast in response to the major component of the storm's windgenerated wave climate.

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