Types of river channel and engineering significance

River channels are classified by their plan shape (pattern) as:

 Straight:       ----------------------
                 ======================

 Meandering:      ___      ___
                 /   \    /   \
              __/     \__/     \__

 Braided:         ___   ___   ___
                 /   \_/   \_/   \
                 \___/ \___/ \___/

1. Straight channel: nearly straight, sinuosity less than 1.05. It is rare and short, found in steep, narrow, bedrock valleys. Flow is fast; the thalweg still swings from bank to bank.
2. Meandering channel: single, sinuous channel with sinuosity above 1.5. It occurs on gentle slopes in fine, cohesive sediment. Erosion occurs on the outer (concave) bank and deposition on the inner (convex) bank as point bars. Cut-offs form ox-bow lakes.
3. Braided channel: many small channels dividing and rejoining around bars. It occurs on steep slope, heavy bed load, easily erodible banks and flashy flow, e.g. the Koshi, Narayani and Karnali rivers below the hills.
4. Anastomosing channel: several stable, interconnected channels separated by vegetated islands, on very low slopes.

Engineering significance (hydropower, bridges and river training)

• Straight reach: stable, so good for a headworks, intake, weir or bridge site. Bedrock gorges give a strong abutment and a high head.
• Meandering reach: banks migrate, so bridges, intakes and embankments need bank protection and spurs. Bend flow causes scour on the outer bank, so the intake should be placed on the outer bank side (to avoid sediment) and structures kept away from migrating bends. Cut-offs can leave an intake dry.
• Braided reach: unstable, shifting channels and high bed load. Bridges need long spans and guide bunds; intakes silt up; embankments are at risk. Needs river training works.
• General: study of channel type guides selection of dam, weir, intake and bridge sites, estimates of sediment load, flood level and scour depth, and the design of bank protection.