In-line or off-line storage devices act as flow control devices by providing for storage of excessive upstream flows thereby attenuating and lagging the wet weather flow hydrograph from the upstream area. Examples of in-line storage are dry and wet detention ponds, and lakes.

Conceptually, storage junctions are "tanks" of constant surface area over their depth. A storage "tank" may be placed at any node in the system. The elevation of the top of the tank is specified in the storage junction data and must be at least as high as the highest pipe crown at the junction. If this condition is violated, the system will go into simulated surcharge before the highest pipe is flowing full.

Note that the only difference between a storage junction and a regular junction is that added surface area in the amount defined for the storage node (ASTORE) is added to that of the connecting pipes. Note also that the conduit obvert (ZCROWN) is set at the top of storage. When the hydraulic head at junction exceeds ZCROWN, the junction becomes surcharged, and a small slot (Priessman slot) as described elsewhere in this section is used by the model.

The surface area of the storage junction is either constant or variable. If variable the relationship of area versus depth may be defined by a power function:

Area = Carea × Depth ^ Earea

where:

Area = area [acres, hectares],

Carea = coefficient [acres, hectares], and

Earea = exponent.

An arbitrary stage-area-volume relationship may be input using the dialogs to represent detention ponds and lakes. The depth-area data is integrated to determine the depth-volume relationship for the junction. Routing is performed by ordinary level-surface reservoir methods. This type of storage facility should not be allowed to surcharge. The user should guarantee that the pond will not surcharge by setting the ground elevation of the node to the topmost stage of the pond.

Note the large variation in surface area units between constant and variable storage areas. Constant storage has units of ft^2 or m^2 and variable storage has units of acres or hectares.