Sills (Figure 3-9) are generally semicontinuous structures built to reduce wave action and thereby preserve, enhance, or create a marsh grass fringe for shore erosion control. The sill is often built along an existing marsh fringe to maintain its integrity and enhance the protection afforded by the marsh in controlling erosion on the adjacent upland. The addition of sand with marsh grass plantings provides a stable marsh fringe system in low to moderate wave energy environments. Breaks or windows in the sills are recommended to allow the ingress and egress of marine fauna.

Building a sill system requires encroachment bayward or riverward, usually beyond Normal High Water or Mean High Water (MHW), constituting the property limit in most states and complicating the process for obtaining permits for installation. There is often a trade-off of habitats in constructing a sill system. The eroding bank, narrow beach and nearshore are converted to a stable bank, marsh and stone sill. In addition, the sill system may reduce the sediment supply to adjacent shores.

The sill structure is often composed of stone, but treated wood and other materials are also used. The sill is usually low, designed to trip or break storm waves before they cascade across the marsh fringe, thus dissipating wave energy before it reaches the upland bank and minimizing marsh toe erosion. A typical sill is illustrated in Figure 3-9, where sand fill has been added to create a gentle seaward slope from the bank to the back of the sill structure at mid-tide elevation. Once the sill has been properly constructed, appropriate wetlands grasses are planted to establish or supplement the marsh.

For landowners in particular, creating and maintaining a beach and dune for shore protection is often the most desirable option. Trapping and adding sand or gravel creates an effective shore planform and cross-section for shore protection. Structures installed perpendicular to the shore (e.g., groins) and parallel to the shore (e.g., breakwaters) are used to trap sand, frequently in conjunction with projects that add sand to the shoreline. Groins will reduce the volume of sand transported downstream, potentially depriving these areas of sand needed to maintain a beach.

Beach nourishment is the addition of sand to a shoreline to enhance or create a beach area, offering both shore protection and recreational opportunities. Beach nourishment studies, design, and projects have been focused mostly on ocean coasts (NRC, 1995a). On sheltered coasts, beach nourishment is most often used on public lands to promote recreational use. Sometimes, beach nourishment

Front Matter (R1-R14)

Summary (1-10)

1 Introduction (11-24)

2 Understanding Erosion on Sheltered Shores (25-43)

3 Methods for Addressing Erosion (44-77)

4 Mitigating Eroding Sheltered Shorelines: A Trade Off in Ecosystem Services (78-97)

5 The Existing Decision-Making Process for Shoreline Protection on Sheltered Coasts (98-123)

6 A New Management Approach for Sheltered Shorelines (124-137)

References and Bibliography (138-150)

Appendix A Committee and Staff Biographies (151-158)

Appendix B Acronyms (159-160)

Appendix C Workshop Agenda and Participants List (161-166)

Appendix D Potential Federal Regulatory Requirements (167-169)

Appendix E Glossary (170-174)