Credit: Photos: Orange County
Prior to construction of a retention pond, the banks of Orange County, Fla.'s Anderson Road Outfall Ditch eroded rapidly, thanks to high stormwater runoff, peak timing of runoff, and volume of discharge.
A newly constructed inlet and dissipater in Anderson Pond handles stormwater runoff from Anderson Road.
Many areas within Florida's Orange County contain secondary outfalls that discharge into landlocked lakes. The county has no easement agreement or right of way (ROW) for these outfalls, which historically conveyed floodwater from upstream areas where most of the stormwater system predates stormwater regulations, and has no stormwater facilities.
These secondary drainage systems are vital to the proper functioning of the internal stormwater system. Most of the upstream areas have undergone extensive development with newer stormwater facilities. However, considerably higher runoff coefficient, peak timing of runoff, and volume of discharge degrade the integrity of these earthen outfalls. Many systems cannot be properly maintained due to the lack of easement agreements or ROWs. In addition, the unwillingness of property owners to donate easements, county budget constraints, newer regulations, treatment philosophies, and lakefront owners' expectations all impact flood mitigation solutions and timeliness of retrofits.
A typical example of a secondary system with such general constraints is the Anderson Road Outfall Ditch, located near the intersection of Anderson and Conway Gardens Roads. The ditch runs south from Anderson Road to Lake Anderson. The landlocked lake outfalls to Lake Conway through a pump station. The outfall ditch was eroding near the lake, and the degradation was moving upstream. What was once a small ditch is a now ditch with a drop exceeding 6 feet in some areas, with vertical sides because of the flow and erosion.
The negative aspect of this erosion was that the sediment eroding from the ditch's banks accumulated into Lake Anderson, forming a sandbar at the mouth of the ditch. And this erosion moved northward with every heavy storm and would eventually have reached Anderson Road, resulting in catastrophic roadway washout.
In order to rectify the sedimentation build-up caused by the erosion of the Anderson Road outfall ditch and to prevent the collapse of Anderson Road, Orange County's Road & Drainage (R&D) Division hired a consultant to study available options. Public safety was top priority, but the costs/benefits of the ditch, its constructability, the probable effect on sedimentation, and the project's environmental impact were all taken into consideration.Changing Options
The consultant recommended an alternative: re-route the untreated water to a retention pond across from the intersection of Anderson Road and the north end of the Anderson Road Outfall Ditch. The floodwater would then outfall to Lake Anderson through a 72-inch pipe, which would replace the existing 500-foot earthen ditch. This approach would:Minimize sediment transport from the canal's banks and bottom, thus increasing the potential for better water quality in the receiving lakesEliminate the unstable side slopes of the canal's banks, minimizing the potential for bank failure and side yard subsidenceProvide better accessibility for maintenance due to easement acquisition.
The suggested proposal would hinge on the owner donating an easement, which encompassed the ditch located on his property. The owner of the private property—where more than 90% of the ditch flowed—decided not to donate the easement to the county after many months of negotiating for the easement acquisition, so the piping alternative was abandoned.
Subsequently, the Orange County Environmental Protection Division (EPD) became involved in the project because EPD had been working with the homeowners' association to improve the water quality by reducing the pollution loading from the upstream areas into Lake Anderson. Based on the concerns encountered, the R&D Division and EPD met and decided that it would be in the interest of the community to construct a retention pond, in lieu of enclosing the existing ditch. The retention pond would reduce the nutrient loading and treat the water prior to it entering Lake Anderson.
Because of the proposed lake water quality improvements and the potential safety risks posed by the ditch undercutting and reaching the road, the EPD determined that the best approach would be to purchase the 3.9-acre property where the ditch currently flowed. The consultant then designed a pond to fit in the site that could closely meet the current St. Johns River Water Management District regulations.
The pond was designed as a 25-year, 24-hour wet detention pond that provided a permanent pool with a residence time of 14½ days. The proposed pond provided ½ inch of water quality volume and reduced the existing nutrient loading rates to the lake by 50%. The existing ditch would be filled and the upstream flow would be diverted to the retention pond, where a control structure would regulate the runoff flow to Lake Anderson.
The project was bid and awarded to the lowest bidder. Construction began in May 2005 and was successfully competed the end of August 2005. The total cost of the project, including the design, plan preparation, land purchase, and construction was $922,320.
On the whole, this project was beneficial for everyone involved because it enabled reduction of pollution loading to the lake, mitigated undercutting and stability of the bank, curtailed upstream flooding, and provided the necessary safety and health improvements and ecosystem preservation.
— Budhu is manager and Bermudez is engineer II with the Orange County, Fla., Roads & Drainage Division.