from my understanding, pilings would be driven in the waterside bottom out about 40 feet from the dam in a U shape, then wooden beams slid down to create a barrier to prevent the water from coming in to this area. Sometimes it can be done with a soil "dam, then ultimately pumped out to remove the seep in water. I have sen them drain canals in Venice for repair this way, and I know many times in seaport areas this technique is used to isolate some feature to repair, docks, etc
