Erosion Reduction By Air Entrainment Downstream Of A Stilling Basin By Ted M. Champagne , CEE Dept.

1. Erosion Reduction By Air Entrainment Downstream Of A Stilling Basin By Ted M. Champagne, CEE Dept. Abstract- The South Florida Water Management District (SFWMD) manages and operates various water control structures that are subject to scour. In an effort to reduce scour downstream of these gated structures, laboratory experiments were performed to investigate the effect of a bubble curtain introduced downstream of the terminal structure of a gated spillway on the depth of the scour hole. Experiments were performed in a laboratory channel consisting of a 1:30 scale model of the SFWMD S65E spillway structure. Experiments were conducted without air entrainment and for high, medium, and low air entrainment rates for high and low headwater conditions. For the cases with no air entrainment it was found that there was excessive scour downstream of the structure due to a downward roller formed upon exiting the downstream sill of the stilling basin. When air was introduced vertically just downstream of, and at the same level as, the stilling basin sill it was found that air entrainment does reduce scour depth depending on the air flow rate by up to 58%, but shifts the deepest scour location to the sides of the channel bed instead of the center. Future work will focus on identifying diffuser geometry that minimizes the air required to reduce bed scour. Concept- Air injection near the end sill is being considered as a means of scour reduction. The water flume in Dillman 110 was used to model its effectiveness. A diffuser pipe was positioned downstream of the model structure near the end sill. Holes were drilled into the pipe so that the air would be injected upwards, perpendicular to water leaving the stilling basin. A section of uniformly graded fine sand was placed downstream from the stilling basin. This section will erode during the experiment run and is expected to behave similar to the bed at the actual structure. Experiments were ran with and without air using various headwater and tailwater configurations in an effort to investigate typical operation conditions. Experiments were ran until bed scour stabilized. Data Collection- Scour was measured periodically for the entire time to reach equilibrium with the depth function of a Nortek Vectrino Acoustic Doppler Velocimeter at ten-hertz intervals. These data were then filtered by eliminating erroneous data identified visually and also by eliminating data which failed to satisfy minimum quality criteria. For ease of plotting and data analysis, the filtered data were then averaged in two-minute bins for presentation. Flow was run until sufficient time had elapsed for equilibrium scour conditions to occur, as determined by there being a change in scour depth over a 2-hr period of less than 0.5 percent, after which water surface and bed profile measurements were taken. Water surface plots revealed that air injection did not reduce the conveyance of the structure. Bed elevations were collected on a uniform grid using point gauges. Results- Pictures show the model flume during operation S65E during high flow conditions Background- The South Florida Water Management District is Florida’s largest water management district. The agency operates and maintains 2,600 miles of canals and levees, 63 pumping facilities, and 1,300 water control structures. They are committed to improving flood control and securing a water supply for the 7.5 million residents in its jurisdiction. The agency is also dedicated to improving water quality and restoring ecosystems in areas such as the Southern and Northern Everglades. No Air With Air Injection (25cfm) Subsequent bed profile downstream of model structure. Collected after scour equilibrium is established. Problem- Water control structure S65E is a gated weir stilling basin located approximately 10 miles Northwest of Lake Okeechobee on the Kissimmee River in Glades County. The area surrounding S65E is largely agricultural. There is a residential neighborhood within 200 ft of S65E consisting of about 125 homes. The structure is SFWMD’s largest measuring over 180 feet wide. During maintenance, significant erosion was discovered near the structure. A large scour hole approximately 26 feet below the floor of the stilling basin had formed downstream of the structure. If the scour hole were left unchecked, it could lead to structure failure due to undermining the foundation. No Air With Air Injection (25cfm) https://my.sfwmd.gov/portal/page/portal/pg_grp_sfwmd_wrac/portlet_subtab_wrac_arc_rpt_lok/tab22276105/lo_structures_emerg_action.pdf Conclusion- It was evident that experimental runs with a high headwater to tailwater ratio resulted in the worst bed erosion. A 58% reduction in erosion was achieved in this case by utilizing air entrainment at a rate of 25 cfs. Although scour depth is not directly scalable between model and prototype due to cohesion of small particles, the scour depths found in the model show significant depths comparable qualitatively to those found in the SFWMD S65E spillway structure, thereby providing confidence in the proposed concept and laboratory setup. Sustainability- Because of the size of S65E, it was deemed cost prohibitive to make changes to the structure to prevent continued bed erosion. The environmental impact caused by demolition and construction of a new structure could be avoided through the use of innovative scour reducing techniques. A “bubble curtain” positioned downstream of the structure was proposed to reduce erosion potential. The effectiveness of this concept will be established on a model flume. Future work- Although scour reduction via air injection was proven conceptually, further research is required in order to define its true potential. Future work will investigate the effectiveness of various diffuser material, geometry, and orientation. The goal is minimize bed scour while using the smallest amount of air. This will reduce the operational costs of air injection to be inline with the cost of routine bed maintenance. Acknowledgements- Thanks to the South Florida Water Management District for funding this research project and to my advisor Dr. Brian Barkdoll for his guidance.