Virginia Coastal Program: 2006 Coastal Grant Project Description and Final Summary
Project Task:
93.02
Grantee:
Virginia Institute of Marine Science - Center for Coastal Resource Management
Project Title:
Tidal Flushing Characteristics in Virginia’s tidal embayments
Project Description as Proposed:
Tidal flushing has important water quality implications that are known to affect numerous estuarine management issues. One current issue is siting for shellfish growing and aquaculture. Successful aquaculture requires regular tidal exchange to cleanse water and replenish food resources.
Tidal flushing is also an important consideration for local government planners who wish to expand waterfront development in a community where economic growth depends on water dependent activities. The ability to maintain a balance between ecological function and economic development is essential. The physical environment can lend important insight if certain characteristics such as flushing can be identified in advance.
Presently, there is no resource for local government planners to consult for information on flushing characteristics across the Virginia coastal zone. The objective of this proposal is to perform a combination of water quality modeling analyses that will evaluate individual systems for general flushing characteristics. The initial focus will be on shellfish growing areas, which constitute a large proportion of the coastal zone. The desired outcome will classify tidal creeks and tributaries using simple designations based on modeling output (e.g. well flushed, moderately flushed, and poorly flushed).
Coastal embayments in the Virginia portion of the Chesapeake Bay will be grouped into three categories based on size and available data. The fist group will consist of small coastal embayments. These embayments are typically well mixed. Therefore, a simple equation based on tide range can be used to compute the flushing time. When salinity data are available, a more sophisticated equation can be used to cover those situations in which the “well mixed” assumption is not appropriate.
The second group will consist of intermediate size embayments. For these, we propose to use a tidal prism model to compute the flushing time and residence time.
The third group consists of large coastal embayments. Previous model applications (1D to 3D) have been conducted in many of these waterbodies, including Lynnhaven River, Onancock Creek, Poquoson River, etc. We will collect these models and use them to compute residence times for these large waterbodies.
For a small waterbody the first-order description of transport is often expressed as “flushing time”. The flushing time (Tf) is a bulk or integrative parameter that describes the general exchange characteristics of a waterbody that is assumed to be well-mixed. The flushing time can be calculated as follows:
(1)
Where V is the volume of the waterbody, Q is the volumetric flow rate through the system. Eq. (1) is often used in lakes and freshwater systems. For coastal embayments, the influence of tide needs to be considered. Therefore, the tidal prism method is often used which can be written as:
(2)
Where T is tidal period, P is tidal prism (the volume between high and low tide), and b is the return ratio that is the fraction (0.0 – 1.0) of ebb water returning to embayment during the flood.
The assumption a waterbody is well mixed is not always valid. To account for spatial variability, salinity can be used to estimate the fraction of freshwater in an embayment. The flushing time can be calculated as:
(3)
where Q is river inflow and f is the mean fractional freshwater concentration in the estuary, given by:
(4)
where S0 and S are the sea water salinity and the salinity in the estuary, respectively.
Residence time is the time it takes for a water parcel to leave the system through its inlet. Consider a parcel of material in a reservoir at time t = 0. Let the amount of the material at t = 0 be R0, and the amount of the material which still remains in the reservoir at the time t be R(t). The residence time distribution function can be defined as:
(5)
It can be further assumed that:
(6)
The average residence time (?r) of the material is defined as:
(7)
Integrating equation (7) by parts gives:
(8)
where r(t) = R(t)/R0 is called the remnant function.
These equations are used in a tidal prism model to calculate flushing time and residence time.
For a large coastal embayment with multiple branches, using Eqs. (2) and (4) to calculate flushing time will not provide accurate results. Therefore, 1D and 2D numerical models, and the tidal prism model can be used to calculate the flushing time and residence time. An embayment can be segmented into multiple tidal prism segments, then using the numerical models, residence time can be calculated.
Federal Funding:
$60,000
Project Contact:
Julie Herman, 804-684-7535; herman@vims.edu
Project Status:
10/1/2006 - 9/30/2007; Project Completed
Final Product Received:
"Tidal Flushing Characteristics in Virginia’s Tidal Embayments" (PDF)
Project Summary Provided by Grantee:
The objective of this project was to perform a combination of water quality modeling analyses that evaluated individual systems for general flushing characteristics. Residence time is the average length of time that a parcel of water remains in an estuary. Residence times for water bodies in the Virginia coastal zone were calculated using three water quality models, depending upon the complexity of the water body. Results showed that residence times range from 0.1 to 29 days. One outlier has a residence time of 72 days. Approximately 80% of water bodies in the Virginia coastal zone are dominated by quickly flushed tidal creeks and tributaries with residence times less than about 5 days.
Descriptive statistics were used to analyze the data, and the residence times were grouped into tidal flushing categories using mean depth. Shallower water bodies flush more quickly than deeper water bodies. The flushing categories (quickly, intermediately, and slowly flushed) reflect a relative time frame in which a water body is flushed. Residence times and flushing categories were joined to a geographic information system layer (GIS) in order to spatially display and analyze results. The geographic distribution of flushing categories shows that regardless of location, in general small, simple water bodies flush quickly and complex water bodies flush slowly.
On the CCRM website
http://ccrm.vims.edu/tidal_flushing/tidal_flushing.html
there are 4 items that are available to download (the GIS datalayer and Coastal GEMS Fact Sheet will be available on the Virginia Coastal GEMS website):
- Final Report
- Maps (jpgs) from final report
- GIS data layer
- Coastal Gems fact sheet
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
