Revitalization of a Runoﬀ Canal with ACBs
With neighborhoods being threatened with flooding and erosion, Florida Department of Transportation used articulating concrete blocks to redirect stormwater runoff.
Even in sunny Orlando, FL, the rains come, and when they do, flooding is a primary concern. Runoff from roads and highways flows rapidly through ditches such as State Road 434 Out Road Canal, installed years ago by the Florida Department of Transportation (FDOT). Florida has been regulating stormwater discharge and management since the 1980s with the intention of protecting its surface waters.
The Florida Department of Environmental Protection (FDEP), regional water management districts and local governments all have oversight responsibilities for stormwater runoff. FDEP requires 80 to 95 percent reduction in the annual loads of pollutants from rainwater discharge. Meanwhile, the City of Orlando maintains its streets to keep them clean, safe and in good repair. The City also ensures that the drainage facilities perform as intended and that receiving water bodies meet state and federal water quality standards.
The aging drainage ditch that served a portion of State Road 434 had been eroding over time, according to Dale Mudrak, project engineer with Gregori Construction and Engineering.
Local properties in the residential neighborhoods where the water runs through were threatened with erosion and potential flooding. In order to prevent further damage, FDOT applied to the regional water district for a permit to make improvements on the 434 Canal drainage ditch, which serves as a conveyance system to channel runoff brought in from the State Road, two adjacent subdivisions and a lake at its head. That runoff ultimately flows into Little Wekiva River and through Wekiva River Buffer Conservation Area, with its fragile wetlands that are often flooded in the rainy season. The local water district officials recognized the need for improvements to the outdated infrastructure. However, the contractor was required to include vegetation along the canal bank and maintain the plants for one year to mitigate the loss of natural vegetation during construction.
Like much of Florida, the canal banks were mostly sand, so any rapid flow in the drainage canal would lead to erosion. Flowing stormwater added to the canal from the adjacent residential properties causes the erosion to increase, especially in areas with no vegetation or where the area lacks stabilization.
ACBs meet site challenges
Ready mixed concrete was first considered for the project, but there were site restrictions that could not be met with that solution. Access to the drainage ditch near the two subdivisions was restricted; too small to get a ready mixed truck on-site. In addition, the FDOT was seeking a sustainable solution allowing plants to grow in the revetment area. “So they went with articulating concrete block (ACBs) instead of ready mix,” Mudrak said.
The proprietary ACBs selected for this 25,000-square-foot (2322-m2) project are 6-inch units (152-mm). They were installed as mats, which required a crane for placement. Mudrak says the contractor got permission from two homeowners to bring in a crane between their properties, and the clearance was so tight that the crane’s retractable wheels had to be withdrawn.
Steel sheet piles were used across the canal to slow down the speed of the runoff. When the velocity of the stormwater is decreased, particles in it have time to settle out on the bottom, helping to clear the water. “The ACBs are extremely important in periods of high flow after a storm event as water will flow around or over the sheeting weirs, actually increasing water speed,” says Mudrak. When that happens, the ACBs will keep the bottoms and sides of the canal from eroding away.
The Florida Administrative Code’s Water Resource Implemen- tation Rule sets design criteria for stormwater systems. In situa- tions like the State Road 434, where the runoff discharges into the conservation area of Wekiva River, the rules require a 95 percent reduction in pollutant loads. Water quality is a big issue for the drainage project and one of the critical factors why FDOT chose ACBs, according to Mudrak.
Technically considered a conveyance system, not a treatment system, the ACBs on the canal actually accomplish some of both. The combination of the steel weirs and extensive ACB mats offer some water treatment benefits. By slowing down the water flow with the steel sheeting, there is less erosion and some of the sediment carried in the runoff has time to settle to the bottom.
As for treatment, the ACBs allow vegetation to grow in the open cells along the canal’s banks. The vegetation then absorbs nutrients from the water which cleans it. The open cells on the bottom of the canal allow sand to settle and keep it from continuing down stream. Once established, the natural vegetation can hide the ACBs that are stabilizing the slopes. The life expectancy of this drainage conveyance system, with its treatment qualities, is 50 years.
“We see ACBs as one of the solutions to erosion and slope stabilization issues. They are an engineered product and in many cases provide better performance than natural products. We expect to see continued lower overall cost utilizing ACBs compared to other products which is due to limited amounts of natural resources and the increased cost of trucking them. The quality of the ACBs combined with design flexibility and production sizes—both thickness and mat size—make it preferred by owners, designers and contractors,” says project engineer Mudrak.