In many cases a municipality has a complete storm sewer system. When it rains, the storm water enters the large sewer pipes through catch basins which are located along the sides of the street and is eventually discharged into a holding pond.
Many designs of stormwater holding ponds incorporate stormwater tanks which are called a forebay. This forebay is a large tank which is before the stormwater enters into the pond. The forebay may be natural or it may be a constructed tank. Although adding one increases the cost of stormwater management, they serve a very useful purpose and in the long run greatly reduce maintenance and the associated expenses.
Stormwater tanks that are installed before the holding pond serve a number of purposes, the major benefit being that there is a buffer when there is a storm surge from excessive rain. The tank collects the water first and releases it into the pond in a controlled manner. When the forebay has been designed for this purpose it is usually quite shallow but a large surface area.
The forebay also is the first place the water goes so a great deal of the sediment that is carried with it falls here. This is where the long term savings come in; it is much easier and less expensive to de-silt the small pond than the large one furthermore the water that does enter the main pond is significantly cleaner which allows the pond to be used for recreation and as a source for fresh water which will be processed and end up in the municipal water supply.
The forebay makes a wonderful natural habitat for fish, aquatic plants and animals. These stormwater tanks often are installed just to create a better and more environmentally friendly way of stormwater management.
These stormwater tanks can be of any size, shape and capacity. It is the overall design that dictates whether the tank will be below grade or above. They do not necessarily have to abut the main holding pond; they can be located some distance away and feed the pond via pipes, culverts or channels. When the forebay is connected to the main pond in this way, the flow of water can be managed only allowing water into the reservoir when needed.
