Flood risk

Flood risk therefore depends on both the amount of precipitation and how quickly water moves through the drainage basin—the area drained by a river and its tributaries. When large quantities arrive together, discharge rises rapidly and may exceed channel capacity.

Dredging can also increase flood risk for downstream communities. It may make banks unstable, damage fish spawning grounds and disturb contaminated sediment, affecting water quality downstream. A response intended to help one location can therefore have costs elsewhere in the catchment.

Flood risk involves both the likelihood of flooding and the harm it could cause. A flood across undeveloped land has different consequences from one reaching homes, roads and businesses. Risk can therefore rise because more water reaches a river, because more people and property occupy vulnerable land, or both.