Wave Overtopping Resiliency of Grass and Turf Reinforcement Mats on Sandy Soils

Summary wave overtopping tests conducted in the CSU Wave Overtopping Test Facility examined the resiliency of grass and reinforced grass slopes founded on sandy soils. Soil loss was quantified by comparing surveys of the slope surface to the initial survey. Grass failure is shown to correlate with the cumulative excess work done by the individual overtopping wave volumes, even though the hydraulic loading sequences between test samples were significantly different in time. Grass strengthened with turf reinforcement mats can sustain 2-4 times the hydraulic loading of grass-only slopes. These results are strictly limited to the soils and grass species tested in the wave overtopping simulator. Introduction state of Florida in the United States builds dikes, levees, and embankments using native soil high in sand content that often contains shell fragments. Erosion resistance of these soils is improved by vegetation, but generally the improvement is less than erosion resiliency of grass-covered cohesive slopes. One of the most significant flood protection structures in Florida is the 9-m-high Herbert Hoover Dike that almost completely surrounds the 1,900 km 2 area of Lake Ockeechobee. Seiching of the lake, and waves caused by hurricanes passing over the lake, can be extreme. At least 2,800 people perished during the late 1920s due to lake surges overtopping the previous inadequate dikes. The Jacksonville District, Corps of Engineers, sponsored full-scale wave overtopping tests at Colorado State University's Wave Overtopping Test Facility (Thornton et al. 2011; Van der Meer et al. 2011). The purpose of the testing was to determine acceptable wave overtopping limits that could be tolerated by South American Bahiagrass (Paspalum notatum Flugge) planted on the sandy soils typically used for construction of dikes and embankments in Florida. These resiliency limits will be used to set levee and embankment crest elevations. In addition, two wave overtopping tests examined the increased slope stability afforded by turf reinforcement mats (TRMs) populated with Bahiagrass. This paper presents erosion results and initial analyses that relate the cumulative eroded sediment volume to the cumulative hydraulic loading represented by the excess work in the individual overtopping waves above a critical threshold. After a brief overview of the test tray preparation and testing procedures, the concept of cumulative excess work is introduced. Results are presented for two grass-only tests and two grass+TRM tests, and the observed erosion is related to the applied hydraulic loading for each test. Preliminary limits for tolerable grass damage for this specific soil and grass species are presented in terms of cumulative excess work. The steep landward-side slope caused the unsteady overtopping flow to be supercritical most of the time.