news from the waterfront by Tim Matson
WEATHER EXTREMES AND POND STREAMS
As June ends, Vermont's rainfall for the month has already gone way over normal: 8.33 inches recorded at Montpelier, which is 233% above average. It's been a cold and rainy spring and early summer so far, which seems like a continuation of the extreme cold and snow we had during winter. Across the country the weather continues to hit extremes from drought and heat to flood. Since it seems our lot in the northeast is weirdly wet weather, I thought it might be useful to review a couple of pond designs that help offset high water events. These thoughts are condensed from my book Earth Ponds A-Z, An Illustrated Encyclopedia.
Also known as sediment ponds, storm water ponds, and retention ponds, they're used for flood and erosion control as well as pollutant and nutrient catchments. Detention ponds are generally required by environmental agencies in connection with construction projects and developments, where builders must comply with requirements to manage storm water runoff, soil erosion, non point pollution, and hazardous material spills. They are also popular with architects required to include them in developments, who soon discover that they become an aesthetic focus as well as a water management technique. Developers and architects have learned that water sells property.
A well-designed detention pond serves the practical purpose of water-quality control during construction, and upon completion adds a landscaping asset. How does it work? Runoff above the pond is usually absorbed and filtered by soil. When soil is replaced by pavement, buildings, drains, and so on, runoff and pollution increase and move faster. If directed into a detention pond, however, silt and contaminants settle to the bottom, and the cleaner discharge overflows into the downstream watershed. Result: a natural filtration system, especially when augmented by pollutant-absorbing plants.
One of the most common problems with ponds is siltation. The buildup of silt and sediment is usally due to erosion in the watershed leading to an inflow of suspended solids that settle out and accumulate. Siltation often causes problems with turbidity, and loss of depth and storage capacity. Siltation can also involve contamination by pesticides or too much phosphorous (and nutrients in general), depending on use of the watershed. Siltation is usually a natural process that may no cause a significant problem until changes in watershed use trigger increased erosion. Ponds that depend on a direct inflow of stream water will often experience more problems with siltation than ponds fed by springs, wells, and bypass systems.
One of the best ways to minimize siltation is to construct a silt pool in the feeder stream just above the pond. Sometimes called detention basins, these pools help trap sediment before it reaches the pond. Silt pool design depends on the size and velocity of the inflow stream, which in general means, the bigger and faster the stream, the bigger the pool. Pools should be at least one yard deep, and engineering formulas are available to help match pool size to watershed. Ponds should be designed so they can be periodically cleaned out. Pools are sometimes simply pockets excavated in the streambed, while others may be more elaborate constructions or prefabricated devices such as well tiles or wood boxes. Builders sometimes use filter fabric or plastic to reinforce silt pools, but they are vulnerable to being undercut by the stream. Stone is sometimes used, but may complicate cleanouts. A series of two or more silt pools is sometimes the most effective sediment preventive.
Flooding streams threaten ponds. I'd like to add a couple of points to these Earth Ponds A-Z book entries. As far as sediment pools go,in situations where you don't want an open stream running into the pond, the silt pool may include a piping system so discharge runs below ground to the main pond. This might be a culvert or a standpipe. I've seen these designs used where a road ran between the pond and the silt pool, or where a seamless shore was preferred. Also, any work on the stream feeding the silt pool and the pond should be undertaken to minimize damage to the stream. I've seen overgrown/silted streams cleaned out and left vulnerable to erosion: the first big rain that came along loosened enough soil to fill the entire silt pool and overflow sediment into the pond. The stream should have been lined with stabilizing stone, or fabric, or both; and perhaps an additional silt pool dug in the stream. As noted in the June pondology, it may be possible to create pools that both filter water and support aquatic plants, between a water source and the main pond. Bottom line: your water supply stream deserves as much attention as the pond itself.
July 1, 2015