Virginia CZM Program: 2006 Coastal Grant Description and Final Summary

Project Task:

94.04

Grantee:

Virginia Institute of Marine Science

Project Title:

Site Survey and Water Quality Assessment for Better Sill Design

Project Description as Proposed:

This project will measure the performance of sills in <st1:place w:st='on'>Chesapeake Bay in support of developing design guidance.  In particular, it will determine how the windows and gaps in some sills affect their value for shore protection and water quality.  The approach will utilize both field data collection (e.g., site assessment and survey, water quality data collection) and hydrodynamic modeling methodology.  Two sites, varying in construction design, will be analyzed at St. Mary’s City, Maryland on the St. Mary’s River.  Site 1 is about 1,000 ft of shoreline with a gapped sill that was built in 2002.  This sill will be compared and contrasted with Site 2, a 1,000 ft non-gapped sill built in 1998 that is adjacent to Site 1.  Previous data exists for these sites, which include the implemented construction plan and the as-built survey.  Modeling methodology will be used to assess residence time and age of water that flushes through sill structures and associated fringing wetlands.  The primary goal of the effort is to increase our understanding of shoreline structures on nearshore habitat water quality, specifically temperature and dissolved oxygen.  Once calibrated and verified using near-continuous field collected data, the model can be used to determine effects of structure design changes.  This project will be divided into two separate subtasks.

SubTask 1.  Site Assessment and Survey

A shoreline and nearshore survey will be performed at the sill site.  A Trimble 4700 Real-Time Kinematic Global Positioning System (RTK-GPS) will be used to set site control and acquire shore data.  The 4700 receiver utilizes dual-frequency, real-time technology to obtain centimeter accuracy in surveying applications.  In addition, a Trimble 5600 Robotic Total Station will be used to acquire data in the nearshore.  We have the existing conditions, the plan and the as-builts.  This will be the performance survey.  Generally, the surveys included the following elements:

1.  Dimensions of the sill;

2.  Mean High Water (MHW) and Mean Low Water (MLW); survey extends to approximately the -3 ft MLW contour; and

3.  Base of bank, where appropriate and possible.  In addition, the condition of the base of bank will be assessed in terms of stability/wave scarping since both sites were impacted by Hurricane Isabel in September 2003.

SubTask 2.  Water Quality Assessment and Modeling 

The Unstructured, Tidal, Residual Intertidal, and Mudflat model (UnTRIM) will be used to assess residence time and age of water (a measure of

spatial variation of retention time) of water that flushes through

sill structures at Site 1 and 2.  UnTRIM is a general three-dimensional model capable of simulating both 2-dimensional (vertically averaged) and 3-dimensional hydrodynamics and transport processes.  Its orthogonal, unstructured grid allows for a better fit regarding study site boundaries and grid refinements to meet the needs of resolving spatial resolution in numerical modeling tasks; Photo C shows an example of orthogonal grid.  A robust wet-dry algorithm is implemented in the model that is capable of inundation simulation.  Field observations (e.g., surface elevation, velocity, and salinity) will be used for the model calibration and verification.  In conjunction, near-continuous (15 minute intervals) measurements of water quality parameters relevant to habitat criteria (e.g., temperature and dissolved oxygen) will be made at Sites 1 and 2 in order to assess spatial and temporal variations associated with varying sill designs.  Once calibrated, a number of scenarios will be conducted to evaluate how changes in sill structure design impacts nearshore water quality behind the structures.

Federal Funding:

$45,000.00

Project Contact:

Scott Hardaway; (804) 684-7277; hardaway@vims.edu

Project Status:

10/1/2006 - 9/30/2007; Project Completed

Final Product Received:

Performance of Sills: St. Mary's City, Maryland (PDF)

Project Summary Provided by Grantee:

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.  A common living shoreline design in Chesapeake Bay includes a low offshore rock sill to absorb wave energy with an emergent wetland landward of the sill to enhance erosion control, provide critical habitat, and improve water quality condition.  This study is part of a larger, ongoing project to (1) evaluate erosion control effectiveness and the sustainability of offshore sill and fringing marsh design and structure, (2) evaluate ecological services (e.g., habitat value, water quality remediation) provided by the various components of the living shoreline design, and (3) develop design criteria that may enhance services provided by living shoreline designs in low and moderate energy environments. 

A shoreline and nearshore survey was performed at the sill site in May 2007.  This data, as well as previous surveys, showed that the 2002 project has evolved over the past five years to a be a viable system for shore protection and habitat creation.  Variations in landscape due to increases and decreases in elevation only serve to diversify site vegetation communities.  The site has been impacted by several high water events that significantly exceeded the design elevations.  This has caused only minor bank scarping, mostly within some of the window areas with no evidence of bank failure.  Window 9, Type 5, with revetment, appears to be the best in terms of maintaining tidal flow across and adjacent to the opening and providing for protection of the bank in the midbay region.  The inclusion of cobble and gravel enhanced shore protection and allowed much less berming around the bay perimeter than those windows with the standard sand fill requirement.

The model for flushing of these types of systems showed that more windows allow for better flushing if there is no interchange through the sills. The seepage model allows for significant exchange between the river and silled marsh.  The reality is that water moves through the rock void and that the porosity of the rock is as important, if not more important, than the window opening.  Oversized stone may even be preferred.

The data collected during three field visits to monitor water quality, quantify groundwater discharge, and measure nekton activity indicated that the ecological services provided by a stone sill system is significant, especially from a fisheries perspective.  Access to the fringe marsh behind the sill occurs through three pathways: 1) the sill windows, 2) macro-pores in the sill, and 3) overtopping by tidal waters.  Results indicate that marsh minnows reside within the filled pore spaces of the St. Mary’s sill during low water and move with rising water into the intertidal marsh region.  Aggregating within the sill structure during low water conditions may serve as a behavioral adaptation to minimize predation risk.  Having some part of the sill below MLW may be an important design component for sills.  

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 toVirginia.Witmer@deq.virginia.gov

 


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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000


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