The City of Kissimmee is often subjected to severe rainfall events such as hurricanes and flash floods. Storm events of this magnitude often cause extensive damage to existing property and land in the City, and are the source for creating major safety and environmental hazards.
The City experienced extensive flooding on July 11, 2009, from a flash flood where unofficial recordings of 7” of rain in two hours were reported. The nearest City rain gauge reported 4.35” in two hours. The flash flood was caused by two colliding fronts that converged over a 1,200-acre area of the City. The subsequent runoff severely eroded a major conveyance ditch, named the West City Ditch which drains half of the City into Lake Tohopekaliga. The ditch is designed to convey 1500 cfs for a 100-year storm event; however, the flash flood was equivalent to a 500-year storm event and the resulting flow overtopped the ditch banks by more than two feet along certain reaches of the canal, flowed over three collector roads crossing the ditch and severely eroded the ditch banks.
Due to the severe damage caused by the flash flood and the desire to prevent this type of damage in the future, the City applied for an Emergency Watershed Restoration Grant with the United States Department of Agriculture – Natural Resource Conservation Service (NRCS). They were awarded the grant to make the necessary repairs to restore the ditch banks and install erosion protection measures. Initially, the parties desired a cost-effective and “green” solution instead of using traditional methods such as rip-rap. Having several positive experiences with engineered solutions and services supplied by Contech Construction Products Inc. and Propex for streambank stabilization, the NRCS recommended the City use a combination of two products. The City utilized 20,000 square yards of ArmorMaxTM Anchored Reinforced Vegetation System and 20,000 square feet of ArmorFlex® hard armor to restore the eroded streambanks and to provide future erosion protection measures.
ArmorMax anchored reinforced vegetation system is manufactured by Propex and composed of a High Performance Turf Reinforcement Mat and earth percussion anchors. ArmorMax is an engineered solution and provides additional factors of safety against erosion and scour. It can also be engineered to provide surficial slope stabilization to resist shallow plane failures when needed. The ArmorMax System utilized on the West City Ditch was not only used to prevent surface erosion, but upon further investigation, it was determined that the ditch embankments were deemed unstable, and therefore a structural ArmorMax System was needed to prevent erosion due to surficial slope failures. Propex provided the necessary engineering/ slope stability analysis needed to complete the design, and 6 ft, 9 ft and 12 ft anchors were used to stabilize the slopes, while the Pyramat component of the System provided for permanent surface erosion protection.
ArmorFlex Concrete Articulated Block Mats, manufactured by Contech, make a flexible matrix of concrete blocks with uniform size, shape and weight. ArmorFlex blocks are designed to have specific hydraulic capacities and are laced longitudinally with cables to ease handling and installation. The blocks come in an open-cell or closed-cell configuration, depending on the aesthetic and vegetation needs. The blocks were designed to withstand the hydraulic forces that would be anticipated in the next severe storm event and multiple block sizes and thicknesses were used on the project, ranging from 4”-6” and using both open and closed-cell configurations.
The project was constructed within a stringent construction schedule and the flexibility of the products allowed the contractor to easily address any issues that arose during construction. The City and NRCS were very pleased with the final project and felt that by utilizing both ArmorFlex and ArmorMax, they were able to stabilize a larger area for the same amount of funds as the previously used streambank stabilization methods.
“By utilizing both ArmorFlex and ArmorMax, we were able to stabilize a larger area for the same amount of funds as the previously used sand bag riprap stabilization methods,” stated Lawrence M. Clough, PE, CFM, Engineer IV with the City of Kissimmee Public Works and Engineering Department. “We ended up with a ‘greener,’ more aesthetically pleasing and more sustainable project.”