Stormwater Ponds
Introduction
Stormwater ponds serve two primary functions: 1) retain stormwater runoff so that it takes longer to reach the Cannon River; and 2) allow sediment and other contaminents to settle out of the water before it continues to the river. As part of the city's stormwater infrastructure the focus of city maintenance is on the functionality of the ponds. Aesthetic concerns are generally secondary unless they contribute to the functionality of the pond.
The pond is designed to have a permant water storage to collect water after rain events. A permanent drain allows water to flow out once the water surface is higher than the permanent storage level. During a flood event the pond will store additional water as it fills with water faster than the drain can let it out. The overflow drain may be an additional pipe of larger capacity, or it may allow water to overflow to a drainage ditch that routes the water to the stormwater system further downstream.
As sediment accumulates in the pond the permanent storage volume may be reduced. If too much sediment accumulates the normal drain may be obstructed and the pond will overflow on a regular basis.
Ongoing Maintenance
Regular maintenance of the ponds focuses on maintaining vegetation along the pond margins through mowing and controlled burns in order to maintain access to the ponds and minimize impacts on water storage volume. Willow and other shrub growth can still persist and may require cutting and removal; however, the shrubs also provide habitat to wildlife that is generally considered desirable.
Ongoing maintenance is funded through a portion of the stormwater utility fund.
Large-Scale Maintenance/Rehabilitation
Since the ponds are meant to collect sediment (dirt, soil, leaves, etc.) from stormwater runoff they require periodic work to restore them to the orignal storage volume and performance. Note that these projects aim at removing sediment build-up in the bottom of the pond, not the removal of algae or aquatic weeds that may grow in the pond.
Large-scale dredging and rehabilitation of a pond generally occurs every 20-30 years, but may be more frequent if there is a problem with sediment build-up (in which case the city will seek out the source of sediment to resolve the erosion problem). These projects are funded through a portion of the stormwater utility fund.
In 2010-2011 the City of Northfield hired Barr Engineering to conduct a survey of the six ponds that appear to have the greatest sediment accumulation using the rating system described at the top of this page. Barr surveyed the ponds, collected sediment cores, and assessed the performance of each pond. The final report (47MB PDF) prioritized the work needed for each pond and estimated the cost of necessary large-scale maintenance.