An older sediment pond near Old Middleton Rd
Sediment ponds--also known as settling basins--are important tools for protecting our lakes. The ponds are designed to collect stormwater before it reaches natural water bodies, slowing the current and allowing sediment to settle. After 5 or 10 years, the sediment build-up is removed.
Yet sediment ponds are controversial in Madison. There's no question they protect water quality. The issue is whether other, more natural means, would be more cost-effective.
For example, ponds require many acres of undeveloped land. Often, these are wild areas which local residents have come to love. During the recent construction of two ponds on Old Middleton Rd, over 200 trees were cut to make way for the ponds. Many more trees were cut than were revealed in meetings with residents.
Sediment ponds can fail. "Secret Pond," at the west end of the UW Arboretum was built in 1985, yet by 2011, it had to be moved and rebuilt because it had completely filled with sediment. One of the new ponds on Old Middleton Rd nearly overflowed during a heavy storm, before it was finished. If it had overflowed, the unprotected side could have collapsed, dumping tons of sediment into an adjacent creek leading to Lake Mendota.
Another drawback of sediment ponds is that in many cases, they are located in low-lying areas. Hence, the water they hold can't help replenish the groundwater.
Finally, while they are designed to protect waterways from sediment, their construction itself may cause much erosion, as happened at Old Middleton Rd (below).
There are other ways to remove sediment from stormwater, such as boxes in the storm sewers that filter stormwater or trap sediment.
Perhaps the best alternative is to reduce runoff by trapping it in rain gardens, before the rain even becomes stormwater. Of course, hundreds of small rain gardens might be required to equal one sediment pond. But they could be built on street terraces or other scraps of land, and would have the added benefit of beautifying neighborhoods, and replenishing groundwater.
Of course, rain gardens require maintenance to remain beautiful. But sediment ponds also require periodic maintenance.
Links to previous articles on ponds
Work on Secret Pond reconstruction begins
Erosion control error will cause extra work for Rawson
Another sediment spill by Rawson at Old Middleton Rd
Sediment pond under construction at risk during storm