All dams have some amount of seepage as the water impounded by the dam seeks paths of least resistance through the dam and its foundation. While this seepage is natural, it becomes a concern if it carries material from the dam itself with it. It’s important therefore to not only capture seepage flows, but also to measure their rates and composition.
Usually located at foundation drain outlets when seepage flows can be piped, primary devices (flumes and weirs) can be used to measure the flow rates and trends. While flumes can provide flow rate data, they are designed to pass solids – exactly what you don't want when monitoring for breakthrough.
Weir boxes, however, solve both needs as they can not only quantify the flow rate and trends, but also serve as a capture point for breakthrough sediments.
The key to this capture is the upstream weir pool.
Weir pools are used to condition the flow prior to it passing over the weir crest. In conditioning (slowing) the flow, sediments drop out of the flow stream and collect behind the weir plate. For most applications, this sediment capture is a source of maintenance that users would otherwise would want to avoid – here, however, it is exactly what is needed.
Sediment collection efficiency can be increased in weir boxes through the use of overflow / underflow baffles which serve to further condition seepage flow prior to them entering the weir pool. Combinations of underflow / overflow baffles can be used to settle out finer materials suspended in the flow stream. To maximize collection efficiency tube or plate settlers can be mounted in the weir box upstream of the weir plate.