Virginia Institute of Marine Science
Shoreline Management: Better Sill Design 2
Project Description as Proposed:
Rock sill systems consist of a line of rock placed just offshore of an eroding shoreline/coast with sand fill placed between the sill and the eroding bank upon which marsh grasses are planted to create a protective marsh fringe. The wider and higher the sill system, the greater is its ability to provide shore erosion control. One issue questioned by the Virginia Marine Resources Commission (VMRC) has been how far these systems have to encroach onto state-owned bottom to provide the desired shore protection. The purpose of this project is to develop encroachment guidelines for sill type systems based on 1) level of protection desired 2) storm return interval of that shore protection and 3) sill system width needed to attain that level of protection.
On September 18, 2003 Hurricane Isabel stormed up the Chesapeake Bay pushing a storm surge that has not been experienced since the 1933 Hurricane. All sill and marsh fringes were overtopped, and storm waves impacted the upland banks. Bank scarping by wave attack occurred, but bank failure did not, and the damage "healed" over the next year or two as vegetation returned to the bank face. In some cases, additional marsh planting was required. Post storm analyses have been few and generally without an eye toward the actual performance for erosion control relative to storm intensity and surge. Hurricane Isabel created an important storm benchmark of 25 years to 50 years and even higher depending on project location. Therefore, it is the purpose of this project to re-visit several representative sill sites to measure pertinent project parameters in order to develop minimum encroachment guidelines for sill construction.
Two sites will be assessed: Jefferson Patterson Park & Museum in Calvert County MD has several sill segments that have been installed since 1987 and Webster Field, installed in 2002 in St. Mary's County MD, has numerous sill openings along its project shore. These assessments will include a survey of selected sills and cross-sections at each site as well as a hydrodynamic and storm surge analysis. In order to develop the preliminary design elements, an extensive literature search will be performed.
C. Scott Hardaway, (804) 684-7277; email@example.com
10/01/07 - 9/30/08 : Project Completed
Final Product Received:
Encroachment of Sills onto State-Owned Bottom: Design Guidelines for Chesapeake Bay (pdf)
Project Summary Provided by Grantee:
Recent efforts have sought to expand the use of "Living Shorelines" by waterfront property owners in Virginia and Maryland to combat tidal shoreline erosion. Living shorelines represent a shoreline management option that combines various erosion control methodologies and/or structures while at the same time restoring or preserving natural shoreline vegetation communities. Some regulatory agencies and non-governmental organizations prefer living shorelines over "traditional" shore hardening using bulkheads or stone revetments because these structures create a "barrier" or disconnect between the upland and marine environments.
Typically, creation of a living shoreline involves the placement of sand, planting marsh flora, and, if necessary, construction of a rock structure on the shoreline or in the nearshore. When any type of material, sand and/or rock, is placed below/beyond mean high water (MHW) two situations occur: 1) encroachment onto regulated lands necessitates a permit and 2) one habitat is traded for another -- non-vegetated wetlands and/or nearshore bottom for marsh fringe and rocky substrate. Encroachment beyond mean low water (MLW) in Virginia (MHW in Maryland) is onto state-owned bottom. This latter point is of concern to the Virginia Marine Resources Commission (VMRC) which manages this region. Its concern is the determination of how much encroachment onto public state bottom is necessary for a shore protection project. The Virginia Chesapeake Bay Local Assistance Program (CBLA) regulates the area above MHW and the landward limit of tidal wetlands. They are concerned about how much encroachment landward is required for bank stabilization. The goal of this report is to offer some guidance toward these concerns, most particularly as it pertains to state bottom. Specifically, it is the intent of this report to look at encroachment primarily bayward of MHW/MLW for sill-type systems installed for shore protection.
In order to determine the encroachment of the sill site on subaqueous areas and the overall performance, three sites were examined. They were selected for review because of the availability of pre-construction data, design data, and the length of structures. They represent sill installations in low, (fetch <1 miles), medium (1-5 miles) and high energy (>5 miles) environments. The sites are St. Mary's, Jefferson Patterson Park & Museum (JJPM), and Webster Field Annex, respectively. St. Mary's was surveyed in 2007 and JPPM and Webster Field in 2008. These projects were constructed in 2002, 1999, and 2003, respectively.
The long shoreline systems designed for St. Mary's, JJPM, and Webster used shore morphology to create, enhance, and maintain diverse coastal habitats comprised of marshes and beaches and secured by stone and vegetation. The design of each of the three study sites was based on a "standard" fill slope from the base of the eroding bank to about mid-tide level at the land side of the stone sills. In the case of St. Mary's (smaller fetch) the slope was 8:1 while at both JPPM and Webster the slope averaged 10:1 with certain site modifications for each. After experiencing different but direct effects of Hurricane Isabel and Tropical Storm Ernesto, the sills at each site performed as expected with minimal bank scarping and no bank failure. This suggests that this method of sill design should be a reasonable standard to begin the design process. To minimize encroachment, systems should be designed to the needed level of protection elevation and then be graded on an average slope to the back of the sill.
Disclaimer: This project summary provides the federal dollars initially awarded to the grantee. Due to underexpenditure or reprogramming of grant funds, this figure may change. For more information on the allocation of coastal grant funds, please contact Laura McKay, Virginia Coastal Program Manager, at 804.698.4323 or email: Laura.McKay@deq.virginia.gov
A more detailed Scope of Work for this project is available. Please direct your request for a copy to Virginia.Witmer@deq.virginia.gov