Sorry, wandering off topic again, but no need to drain the pound above. The upper gate with paddles shut will keep enough water back to keep the water level up. While the upper gate leaks, the leaks will not exceed the inflow rate to the pound above (well, not normally), and more importantly will be less than the outflow rate from the lock below if the latter has all paddles (and gates, if necessary) open. Some water may come round the bypass weirs, but again can be dealt with by keeping the gates of the lower lock open.