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Channel Processes: Degradation

The lowering of the streambed abandons floodplains, lowers the water table, and increases bank height, which adds to bank erosion and often leads to long-term instability. Causes of degradation are complex and can be related to many sources. Clear water discharge below reservoirs, urban storm drains, excess shear stress due to changes in flow regime, straightening of the channel alignment that alters slope, headward advancement (headcut) of base level shifts due to downstream alteration and excess shear through contraction and bed scour from bridges and culverts can all contribute to channel degradation. Examples of degradation in stream channels are shown in Figure 36 and Figure 37.

Figure 36

Figure 36. Example of a gully created due to degradation caused by high shear stress and stream power below the "double-barrel shotgun" effect of the culverts - Maryland.

Figure 37

Figure 37. Headward advancement of a degraded gully in a meadow - Colorado.

Field evidence of degradation is a combination of a lowered width/depth ratio and an increased bank height ratio. Bank height ratio is defined as the height of the lowest bank divided by the maximum depth at the bankfull stage (Rosgen 2001b). If the ratio is greater than 1, then flows greater than bankfull discharge are required to over-top the bank. The longitudinal profile survey, where elevations of the bed, water surface, bankfull stage and bank height are plotted in the downstream direction, provides valuable information as to the direction and extent of the incision. If the "wedge" or the bank height ratio is increasing in the downstream direction, then a headcut is advancing. If the wedge is decreasing in the downstream direction, then the source of the incision is upstream. This field technique helps to determine the source and extent of channel incision.

An increase in bank height ratio generally results in increased shear stress and stream power and a potential for continued lowering of the streambed. Additional evidence of degradation is associated with undermining the root mass of riparian species and the collapse of the upper bank with trees and other vegetation. Generally streams do not incise unless the width/depth ratio is less than 10 (Rosgen 2001b). This corresponds with higher shear stress and unit stream power, thus, incision is generally associated with a shift in stream type from E or C to a G (gully). Often E channels are straightened in meadow environments to drain the meadow for agricultural purposes. The results of the increased slope and low width/depth ratio are to increase shear stress and stream power, causing incision. Any disruption in the natural energy balance and sediment transport that is reflected in the morphological variables of dimension, pattern and profile often leads to serious long-term adjustments such as incision/degradation.

Prediction of potential degradation uses competence calculations and critical depth computations (Equations II-8 and II-9). If the shear stress and depth/slope relations are sufficient to move particle sizes larger than what is contained in the bed, there is a high probability of excess shear and bed scour leading to channel incision and eventual degradation. The potential for degradation becomes an essential and critical stability evaluation due to the severity and long-term adverse adjustments of stream channels once they become incised or entrenched (vertically contained) as a result of degradational processes. The sediment supply associated with degradation can be extremely high, as presented by Simon (1989).

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