Flooding in Deltaic Plains

Low-gradient deltaic plains are flat, low-lying depositional surfaces formed where rivers divide into distributaries before entering the sea. Their weak slope, restricted drainage and interaction with coastal waters allow floodwater to spread widely and cause prolonged inundation rather than drain away rapidly.

Weak natural drainage

The small difference in elevation between inland areas and the sea produces little gravitational force for rapid runoff.

• Slow-moving water readily spills from distributaries and occupies extensive flood basins and depressions.
• Fine silt and clay deposited in backswamps can reduce infiltration, while a shallow water table limits underground storage.
• Natural levees may obstruct the return of floodwater to river channels, leaving water trapped behind them.

Interaction of rivers and the sea

Deltaic flooding is often a compound process in which river discharge, rainfall and coastal water levels act together.

• Heavy rainfall and upstream discharge can exceed the capacity of distributary channels, particularly where sedimentation has reduced their depth.
• High tides create a backwater effect that slows the discharge of rivers and drains into the sea.
• Cyclonic storm surges can push seawater inland and upstream, while associated rainfall adds freshwater flooding.

Factors that prolong ponding

The duration of inundation increases when natural drainage paths and temporary water-storage areas lose capacity.

• Embankments, roads and canals may obstruct cross-drainage unless adequate sluices and openings are provided.
• Settlement and construction on wetlands and flood basins reduce natural storage and increase surface runoff.
• Channel encroachment, sediment accumulation and blocked drains create drainage congestion, delaying the recession of floodwater.