Wetlands and Flood Moderation

Wetlands are areas where water is the main factor controlling the environment and associated plant and animal life. In a catchment, they function as natural hydrological buffers by receiving, storing and slowly releasing excess water. This moderates floods by reducing the speed and, where sufficient storage exists, the peak of downstream flows.

Temporary storage and delayed release

Wetlands create space in which floodwater can spread and remain temporarily instead of moving immediately downstream.

• Floodplain wetlands receive overbank flows, while depressional wetlands collect rainfall and surface runoff.
• Temporary storage delays the movement of water through the catchment, producing flood attenuation and potentially lowering peak discharge downstream.
• Water retained after the flood peak may be released gradually to streams or transferred to groundwater.

Slowing and dispersing flood flows

Wetland soils, vegetation and uneven surfaces increase resistance to flowing water and lengthen its travel time.

• Dense vegetation creates hydraulic roughness, which reduces flow velocity and dissipates energy.
• Where soils and geology permit, infiltration and groundwater recharge divert part of the surface flow into slower subsurface pathways.
• Vegetation stabilises banks and soils, while wetlands trap sediments that might otherwise contribute to channel obstruction and degradation.
• Coastal wetlands such as mangroves and salt marshes dissipate wave energy and moderate the effects of storm surges.

Conditions governing effectiveness

Flood moderation depends on the wetland's storage capacity, location within the catchment and hydrological connection with rivers and floodplains.

• A wetland that is already saturated before heavy rainfall has less capacity to store additional water.
• Drainage, filling, encroachment and disconnection from floodplains remove natural water-storage and flow-dispersal functions.
• Wetlands reduce flood risk but cannot by themselves prevent all floods, especially extreme events; they complement sound watershed and floodplain management.

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