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Title: COMPUTER MODELING OF A LEAST TERN ISLAND

Author
item GEZA, MENGISTU - OKLAHOMA STATE UNIV
item Hunt, Sherry
item DEMISSIE, TESFAYE - OKLAHOMA STATE UNIV
item SCHNEIDER, SCOTT - USDA-NRCS
item BARFIELD, BILLY - OKLAHOMA STATE UNIV
item KAHN, A - UNIV OF MISSISSIPPI

Submitted to: International Hydro-Science & Engineering International Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/1/2004
Publication Date: 5/30/2004
Citation: Geza, M., Britton, S.L., Demissie, T., Schneider, S., Barfield, B., Kahn, A. 2004. Computer modeling of a least tern island. In: Proceedings of the 6th International Conference on Hydro-Science and -Engineering. Advances in Hydro-Science and -Engineering, May 30 - June 4, 2004, Brisbane, Australia. 2004 CDROM.

Interpretive Summary: The Interior Least Tern was listed on the endangered bird species list in 1985. Least Terns prefer to nest on island with very little or no vegetation. The U.S. Army Corp of Engineers has conducted various studies on the habitation and breeding success of the Least Tern. Observations documented by the Tulsa Audubon Society in Tulsa, Oklahoma from two island habitats located in the Arkansas River showed dramatic decreases in the number of young per nest over the past decade. The negative impacts of vegetation overgrowth, flooding, and human activities are listed reasons for the declining numbers of the Interior Least Terns. One option to create a suitable habitat for the birds is to develop small islands using river training structures. Evaluating the possibility for an island development using physical modeling approach alone is resource consuming. Computer modeling can help determine geometry, size and location of a river training structure suitable for island development. The objective of this study was to validate the possibilities for island formation using the CCHE2D computer program and to compare the results to physical model data collected at the USDA-ARS Hydraulic Engineering Research Unit. The test compares the computed and experimental results of flow patterns and soil movement along the channel as affected by two gravel structures placed one after another. The computer simulation yielded a drop in flow velocity and bed movement behind the structure, causing deposition of soil behind the structure. The geometry, size, and location of the structures are important when developing an island. Among the structures tested, a chevron or V-shaped structure placed downstream of a straight, broad rectangular structure performed better than other geometries tested. A combination of structures helped protect the downstream structure from scour damage in the front and showed better performance as compared to a single structure. The distance between the structures is also important in that the downstream structure should be placed as far away from the upstream structure as possible to increase the total area of deposition. However, it should also be close enough to the upstream structure, so it lies within the low velocity region created by the upstream structure to minimize scour in front of the structure and to create a continuous deposition pattern.

Technical Abstract: The Interior Least Tern was listed on the endangered bird species list in 1985. Least Terns prefer to nest on island with very little or no vegetation. The U.S. Army Corp of Engineers has conducted various studies on the habitation and breeding success of the Least Tern. Observations documented by the Tulsa Audubon Society in Tulsa, Oklahoma from two island habitats located in the Arkansas River showed dramatic decreases in the number of fledged young per nest from 1.44 in 1992 to 0.29 in 2002. The negative impacts of vegetation overgrowth, flooding, and human activities are listed reasons for the dwindling numbers of the Interior Least Terns. One option to create a suitable habitat for the birds is to develop small islands using river training structures. Evaluating the possibility for an island development using physical modeling approach alone is resource consuming. Computer modeling can help determine geometry, size and location of a river training structure suitable for island development. The objective of this study was to validate the possibilities for island formation using the CCHE2D computer program and to compare the results to physical model data. The test compares the computed and experimental results of velocity and shear distribution, location and magnitude of erosion, and sediment deposition as affected by two gravel structures placed one after another which were set in a rectangular laboratory flume at the USDA-ARS Hydraulic Engineering Research Unit. A discharge of 0.06 m**3/s and downstream water surface level of 0.45 m were used as boundary conditions for the simulation. The channel width was 1.8 m, and the height of the gravel structure varied from 0.42 m to 0.47 m. The computer simulation yielded a drop in velocity and bed shear behind the structure, causing deposition of sediment behind the structure. The velocity dropped from 0.32 m/s down to 0 m/s as the flow entered behind the structure. The maximum velocity and shear obtained were 0.58 m/s and 13.7 N/m**2, respectively. Consequently, this was the location of observed maximum. The geometry, size, and location of the structures are important when developing an island. Among the structures tested, a chevron or V-shaped structure placed downstream of a straight weir performed better than other geometries tested. Using a cascade of structures can help protect the downstream structure from scouring in the front, and a combination of structures can help protect the downstream structure from scouring in the front, and a combination of structures showed better performance as compared to a single structure. The distance between the successive structures is also important in that the downstream structure should be placed as far away from the upstream structure as possible to increase the total area of deposition. However, it should also be close enough to the upstream structure, so it lies within the low velocity region created by the upstream structure to minimize scour in front of the structure and to create a continuous deposition pattern.