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Channel Processes: Channel Enlargement

Enlargement of channels can be caused by combined processes of incision, bank erosion and direct modification by construction activities. Lateral erosion may occur in stable streams, but the point bar follows at the same rate, thus the stream does not get wider over time. This contrasts with enlargement, where the width of the stream gets wider over time due to lateral erosion, often concurrently on both banks. The results of enlargement are increased sediment supply from the bed and banks, increased deposition due to decreased shear stress and stream power, loss of habitat, increased water temperatures, and a shift in evolutionary state of morphological stream types. Often a C4 stream type (single-thread meandering channel), due to enlargement is shifted to a D4 (multiple-thread, braided channel). Increased flows due to watershed changes, trans-basin diversions (imported water), storm drains from urban runoff, power generation due to "ramping flows" from reservoir releases and contraction scour below culverts and bridges can all contribute to channel enlargement. Combined processes of incision, degradation, aggradation, and lateral accretion can be associated with enlargement.

Often channels are mechanically enlarged to increase flood flow capacity for bridge construction and/or flood control projects. These over-widening projects generally result in excess deposition in the channel, actually decreasing channel capacity due to reduction in sediment transport. A consequence of these projects is increased bank erosion, thus, many of these projects have high maintenance for dredging sediment and stabilizing streambanks. An example of channel enlargement is shown in Figure 38.

Figure 38

Figure 38. An over-width gravel bed stream evolving from a C4 to D4 stream type. Enlargement due to combined bank erosion on both banks and excess coarse sediment deposition from upstream source.

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