Fundamentals of Fluvial Geomorphology and Channel Processes
Each quadrant of the stability diagram is characterized by geotechnical and hydraulic stability
number pairs, and stream reaches that plot in each quadrant have common characteristics with respect to
stability, flood control, and measures that may be implemented to achieve a project goal.
Quadrant 2 (Ng > 1, Nh > 1) streams are severely unstable; the channel bed is degrading and
channel banks are geotechnically unstable. Grade control must be used to reduce bed slope, transport
capacity, and Nh. Both flood control and bank stability must be considered when determining the height
to which grade control should be constructed. A series of grade control structures can reduce bank height
enough to stabilize the banks, but a combination of grade control and bank sloping may better resolve flood
control while meeting stability objectives. Quadrant 1 (Ng < 1, Nh > 1) is not as severe as Quadrant 2; the
channel bed may be degrading or may be incipiently degradational, but the channel bank is not yet
geotechnically unstable. Bank erosion is occurring only locally and bank stabilization measures such as
riprap, dikes, or vegetation could be applied. However, local stabilization would not be successful if bed
degradation continued, moving to Quadrant 2, and destabilized the channel stabilization measures. If flood
control is a project goal, almost any channelization measure or construction of levees would increase the
Nh instability, shifting the value to the right and increasing the opportunity to make Ng > 1. Flow control
using a reservoir can address flood control and improve stability if the new flow duration curve reduces
cumulative sediment transport; however, changing the flow duration curve and reducing the available
sediment supply are potentially destabilizing. Each of these factors should be considered in projects
involving Quadrant 1 channels. Bed stabilization through the use of a grade control structure or bed
stabilization element may be desirable.
Quadrant 3 (Ng > 1, Nh < 1) has a severe and dynamic problem with gravity driven bank failure,
but without continued bed degradation. Bank sloping could be effective without grade control
emplacement, but usually both measures should be considered. Local bank stabilization measures in either
Quadrant 2 or 3 are unlikely to be successful. Flow control in these two quadrants could be beneficial, but
must be considered in the context of extreme reach instability and grade control is likely to be required.
Quadrant 4 (Ng < 1, Nh < 1) is characterized by general aggradation. Local bank stabilization
measures will be effective. As Nh decreases in this quadrant, the potential for channel aggradation-related
flood control problems increases.
The desirable range for long-term channel stability is for Ng to be less than one, and for Nh to be
approximately one (Ng < 1, Nh 1). If flood capacity is not sufficient as Ng approaches 1.0, levees or
a compound channel should be considered.
The USACE (1990) used the channel stability diagram in discussions of Nelson, Beards, Catheys,
and James Wolf Creeks stability, as shown in Figure 3.17. Figure 3.18 depicts the change in plotting
positions of the result of channel stabilization measures that move two streams from degradation to
aggradational (Stream A), and from degradational and unstable banks to aggradational and stable banks
(Stream B). The proper characteristics for long-term stability are neither aggrading nor degrading, with
stable banks.
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