Hydraulic Resistance due to Emergent Wetland Vegetation

Models to estimate hydraulic resistance due to vegetation in emergent wetlands are crucial to wetland design and management. Hydraulic models that consider vegetation rely on an accurate determination of a resistance parameter such as a friction factor or a bulk drag coefficient. At low Reynolds numbers typical of flows in wetlands, hydraulic resistance is orders of magnitude higher than fully turbulent flows and resistance parameters are functions of the flow regime as well as the vegetation density and structure. The exact relationship between hydraulic resistance, flow regime and vegetation properties at low-Reynolds number flows is unclear. The project goal was to improve modeling of emergent wetlands by linking vegetation and flow properties to hydraulic resistance. A 12.2-m x 1.2 m vegetated flume was constructed to evaluate seven models of vegetated hydraulic resistance through woolgrass (Scirpus cyperinus (L.) Kunth), a common native emergent wetland plant. Measurements of vegetation geometry and structure were collected after each set of flume runs. Study results showed at low stem-Reynolds numbers (<100), the drag coefficient is inversely proportional to the Reynolds number and can vary greatly with flow conditions. Empirical models that were developed from data collected in natural wetlands predicted flow velocity most accurately. Using data from this flume study, regression models were developed to predict hydraulic resistance. Results indicated stem Reynolds number, stem diameter, and vegetation area per unit volume were the best predictors of friction factor. Vegetation flexibility and water depth were also important parameters but to a lesser extent. The spatial distribution of hydraulic resistance was estimated in a small floodplain wetland near Stephens City, VA using the regression models developed from the flume data. MODFLOW was used to simulate a 4-hour flood event through the wetland. The vegetated open water surface was modeled as a highly conductive aquifer layer. On average, MODFLOW slightly underpredicted the water surface elevation. However, the model error was within the range of survey error. MODFLOW was not highly sensitive to small changes in the estimated surface hydraulic conductivity caused by small changes in vegetation properties, but large decreases in surface hydraulic conductivity dramatically raised the elevation of the water surface.

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Biological Systems Engineering

Department dc:contributor.department

Biological Systems Engineering

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2010

Author dc:creator

• Piercy, Candice Dawn

Chair dc:contributor.committeechair

• Wynn-Thompson, Theresa

Committee members dc:contributor.committeemember

• Hession, W. Cully
• Benham, Brian L.
• Daniels, W. Lee
• Diplas, Panayiotis

• wetlands
• vegetated flow
• hydraulic resistance
• vegetation
• Modeling

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Dc Identifier Other

etd-04072010-173437

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/37590