Backwater Effect in River Systems

The backwater effect is the rise of a river's water surface upstream because a downstream condition obstructs or slows the normal passage of flow. During floods, this causes a higher upstream water level than would occur if the river could drain freely, even without additional local rainfall.

How the effect develops

In subcritical flow, water levels are influenced by conditions downstream. A high downstream stage reduces the hydraulic gradient, slows drainage and creates an upstream-rising water-surface profile called a backwater curve.

• At a confluence, a swollen main river can retard the discharge of a tributary and cause water to accumulate in its lower reaches.
• Backwater usually means reduced flow velocity and ponding; it does not necessarily imply a complete reversal of river flow.

Conditions that produce backwater

The controlling condition may be temporary, such as an exceptionally high main-stem flood stage, or persistent, such as a structure or channel constraint.

• Simultaneous flood peaks near river confluences can intensify the effect.
• Dams, barrages, bridges, embankments, channel constrictions and sediment accumulation can reduce channel conveyance and raise upstream levels.
• High tidal or storm-driven water levels at a river mouth can impede river discharge and extend the effect inland.

Flood and disaster-management significance

Backwater can expand floodplain inundation, overtop embankments and prolong waterlogging in tributary valleys and low-lying areas. Flood assessment must therefore consider downstream water levels, channel geometry, structures and the timing of flood waves, rather than local rainfall or discharge alone.

• Forecasting requires linked observations of river stage and discharge along both the main channel and its tributaries.
• Floodplain zoning, adequate bridge and barrage openings, sediment management and coordinated reservoir operation can reduce exposure or aggravating constraints.