§ 13.3.1. Design types

There are four common types of detention basin: in-line detention storage, off-line detention, on-line detention, and on-site detention.

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In-line detention: This type of storage occurs within a channel right-of-way and only near the headwaters of a watershed or sub-area, with only the immediate landowner(s) draining to it. The channel is either oversized and/or changed to elevate the water surface inside it by a control structure or increasing roughness in order to slow the storm water and prevent downstream flooding.

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Off-line detention: This type of detention diverts a portion of a hydrograph from a nearby channel only when specific parameters are met. These usually are adjacent to a channel and have a side weir as a control structure, allowing overflow from the open channel.

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On-line detention: This type of detention passes the entire hydrograph through itself. This is often used to delay the time-to-peak discharge and is the best at controlling the rising limb of the hydrograph. These can be on-site detention basins as well, with those that are open to a channel being referred to as "flow-through" detention basins.

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On-site detention: This type of detention is within the development itself, usually only accepting storm water from the development itself (unless the development is right in the path of the areas' storm water, which makes it on-line) and restricting the outfall to the receiving channel. Mandatory detention areas tend to be this type.