Change in channel capacity

Natural channels will often incise in response to human impacts, such as watershed development, channel straightening, removal of vegetation, or overgrazing. The incision is a lowering of the channel bed, that in effect increases the channel size and capacity. Often, the overbank dries out due to a falling water table. This lowered water table can cause wetlands to shrink and adjacent productive lands to depend on irrigation. For projects in which overbank soil moisture is a concern, the duration of flow is often more important than the peak discharge. Inchannel flow can have a significant effect on overbank soil moisture if it is near bankfull for a sufficient duration.

Example problem: Change in overbank duration

Problem: A channel has, in the span of 10 years, incised by several feet and increased the bankfull flow area from 84 square feet to 107 square feet. The channel slope has increased from 0.0020 to 0.0025. The wetted perimeter increased from 29.4 feet to 42 feet. The vegetation has suffered to the extent that composite n value has decreased from 0.045 to 0.038. Approximate the change in duration of overbank flooding, given the season-long hydrograph in figure 16.

Solution: Using a uniform flow assumption and Manning’s equation, the original channel capacity was:

With the changed hydraulic parameters:

Looking at the hydrograph, then, the new channel condition fully contains the hydrograph, since the peak is less than 390 cubic feet per second: no days of overbank flooding occur. The previous channel capacity was 250 cubic feet per second, and overbank flow would have occurred four separate times for a total of about 16 days.