Pressure difference: Keswick flood defences

This picture was taken from the path into Keswick from Stormwater Bridge. We were standing outside the flood back on fields that are designed to food. Look closely at the dam. There are flaps in it. Flood water pushing from the outside in would push the flaps flat shut so the water could not get in. Water that had somehow got in over the banks would be able to get out if the pressure on the inside pushing out was greater than the pressure on the outside pushing in. Water did indeed get inside the banks. It has to be let out somehow. The pressure difference can only surely be high enough when the waters on the outside have receded. But there must have been quite a flow of water outwards through the flaps judging by the trail of stones left on the field outside the flaps.