Dam Seepage

Most dams have some water seeping through the face of the dam. The amount of seepage depends on the elevation of water in the dam, the elevation of water in the tailwater, the integrity of the dam itself, and other factors. In some situations, seepage from the pool through the dam and into the tailwater can be a significant source of discharge that must be modeled. Less commonly, water in the main channel downstream may seep through the levee or dam face and enter the pool. Both of these situations can be represented using the dam seepage structure. Only one seepage structure can be added to any reservoir, so all sources and sinks of seepage must be represented collectively.

It is assumed that all reservoir seepage ends up in the river downstream of the reservoir. Seepage into the reservoir is taken from a global source and only will occur when tailwater is lower than the reservoir elevation. When water seeps out of the reservoir, the seepage is automatically taken from the reservoir storage and added to the main tailwater discharge location. This is the mode of seepage when the pool elevation is greater than the tailwater elevation. Seepage into the reservoir happens when the tailwater elevation is higher than the pool elevation. In this mode the appropriate amount of seepage is added to reservoir storage, but it is not subtracted from the tailwater. Dam seepage is also commonly used with pump stations outside levees. One side must be declared to be tailwater.

Currently the only dam seepage method available is Tabular Seepage. This is similar to modeling an "Unknown Spillway". The user gives two elevation-discharge curves.

Tabular Seepage

The tabular seepage method uses an elevation-discharge curve to represent seepage. Usually the elevation-discharge data will be developed through a geotechnical investigation separate from the hydrologic study. A curve may be specified for inflow seepage from the tailwater toward the pool, and a separate curve can be specified for outflow seepage from the pool to the tailwater. The same curve may be selected for both directions if appropriate. If a curve is not selected for one of the seepage directions, then no seepage will be calculated in that direction.

In order to determine which table to use, the pool elevation elev_pool is compared to the tailwater elevation elev_tw. Positive values of the seepage head h_s between the pool and tailwater indicate seepage out of the dam, as long as the pool elevation is above a defined minimum. Negative values of h_s indicate seepage from the tailwater into the pool. Once the direction of seepage is determined, the seepage value is simply a matter of looking up the seepage from the seepage table.

h_s = elev_pool – elev_tw