Culvert Size Culverts should preferably be at least 400mm in diameter to minimise the risk of blocking with woody debris and sized appropriately for the catchment area. Culvert or Bridge Capacity Culverts or bridge crossings that are too small can block in storm events and subsequently ‘blow out’, causing severe down-stream damage. Branches and logs from harvest operations can make forestry culverts and bridges more susceptible to blocking. Ensure the culvert or bridge for a stream crossing is big enough for storm flows of at least a 1 in 50 year storm event. For catchments greater than 120ha you should get an engineer to calculate the size of structure for the stream crossing. It is also good to direct excess water to protect culverts in the event of a storm. Construct a broad swale at one end of the stream crossing. The flood flow can then go to a stable part of the stream, with the water returning back to the stream bed. 11 FlooFd lowPath