# 4 | L-shaped cofferdam | Support on a bridge pile
This is the typical installation for structures with one or more piers.
Start from the bank and go around the pier upstream, ensuring that you have:
- a long enough barrier to ensure a watertight seal against the bridge pier
- an opening between the supports wide enough for the dam to pass through
Depending on the slope and the force of the current, it may be necessary to set up a second installation downstream of the bridge.
In this case, do not go past the chosen pier. Attach the dam to the pier facing the starting bank.
