Channel Stability Analysis

This step involves a stream channel stability analysis, using departure analysis summaries of morphological variables and collection and analysis of specific channel stability variables. The stability variables are entered into Worksheet 14 (PDF, 59 kb). Previous step results, the relations derived from Step 6, and the outputs of the subsequent steps in this procedure are summarized in this worksheet.

Detailed interpretations and examples for the following categories of stability variables are presented in Chapter 6 of Applied River Morphology (Rosgen, 1996). Some of these are associated with natural, stable rivers and have little anthropogenic influence, yet are very active. It is this reason why reference reach evaluations are conducted to determine differences between genuine departure and natural stable channel adjustment. The general descriptions for these stability variables are

1. Riparian vegetation. Riparian vegetation alteration assessment determines existing composition/density including overstory/understory cover. Departure of the current vegetative community from the potential vegetative community is an important interpretation. Documentation of the riparian vegetation is recorded in Worksheet 15 (PDF, 32 kb, 1 p.). If the composition changes from a woody riparian community to a grass/forb, for alluvial channel types it generally increases the risk of enlargement by stream bank erosion due to the change in rooting depth, and is associated with a high sediment supply due to the channel adjustment.

2. Flow regime. The categories of flow regime evaluated are shown in Worksheet 16 (PDF, 20 kb). Interpretations for sediment supply and stability relations are based on flow regime and vary with stream type. For example a spring-fed E4 stream type has very little bedload and very low suspended sediment. The corresponding riparian vegetation for spring-fed streams, being saturated yearlong, is very dense with little evidence of bank erosion (unless direct alterations occur, which is rated elsewhere). Stormflow-dominated streams, in contrast to snowmelt, ephemeral, and spring-fed systems, have a certain influence for their respective flow category. Flow increases from reservoirs and/or diversions, as well as flow decreases from diversion or reservoir regulation, influence channel and hydrologic processes. Rain-on-snow events, which contribute to mid-winter floods, are evaluated in this worksheet.
   
   
3. Stream order and stream size. These two categories are included in Worksheet 17 (PDF, 22 kb, 1 p.). Strahler stream order gives an approximate reference for source area of contribution. In other words, interpretations for an A3 stream type on an ephemeral first order stream would indicate a much lower risk for sediment supply based on watershed size alone than for a third order, A3 stream type. Stream size expressed as bankfull width ranges also helps summarize data by knowing the approximate width of the channel.
   
   
4. Meander pattern. Meander patterns express adjustment processes from past events or impacts. They also indicate the direction and mode of lateral adjustment. The patterns indicate central tendency for the "wandering river," lateral migration and other related processes, when compared with time-trend aerial photography. The various patterns are summarized in Figure 106.

Figure 106. Meander Pattern variables that influence channel stability. (modified from Galay et al. 1973)

Figure 107. Depositional patterns used for stability assessment interpretations (Rosgen, 1996)

 

5. Depositional patterns. These ratings describe the nature and extent of bar features in rivers. Many of these features are very stable yet active point bars and are not indicators of instability. However, when point bars start to occur on E4 stream types, it generally is associated with accelerated sediment supply and/or increase width/depth ratio. Point bars on C4 stream types, on the other hand, are a stable feature. Excess deposition leading to channel enlargement and/or aggradation is often associated with multiple mid-channel bars and chute-cutoffs, identified by categories in Figure 107.

6. Debris/channel blockages. As with many of these type of variables, a certain amount of large woody debris is desirable for both physical and biological function. When the magnitude and abundance of debris gets to a state that the stream aggrades, loses sediment transport capacity, and provides fish migration barriers, then the consequences of debris is likely adding to sediment supply. Debris-driven sediment supply increases can involve avulsions, lateral migration, or streambank or side-slope rejuvenation, which often accelerates mass failures. Categories of channel blockage also include in-channel structures, check dams, diversion structures or similar installations. The rating also considers different categories of active and abandoned beaver dams. The various categories are shown in Worksheet 18 (PDF, 8 kb, 1 p.).
   
   
7. Width/depth ratio state. Width/depth ratio is a key variable for assessing departure from a stable reference condition. Increases in W/D ratio generally are associated with accelerated streambank erosion rates, excess deposition/aggradation processes, over-widening due to direct mechanical impacts, and other causes. It is rated as a comparison or departure from a reference condition. Examples of width/depth ratio increases from a reference W/D ratio are shown in Figure 108 (PDF, 11 kb, 1 p.).

   It is important to distinguish the differences between W/D ratio increases versus decreases. The decrease category is only rated as a high risk when accompanied by a low bank height ratio (lowest bank height/maximum bankfull depth that determined degree of channel incision). This indicates that as the W/D ratio is decreasing, there is an associated increase in shear stress and unit stream power, and the bank height ratio values larger than 1.0 are indicating incision. Examples of W/D ratio departures are shown in Introduction to Sediment & River Stability, in Chapter 6 of Applied River Morphology, and in Figure 109 (PDF, 16 kb, 1 p.).

8. Modified Pfankuch stability rating. The summary of the ratings (Pfankuch, 1976) are utilized in Worksheet 19 (PDF, 56 kb). Included in these ratings are adjustments in the ratings based on stream type (Rosgen 1996, 2001b). A stability rating of 88 for a C4 would be "good" or due to stable morphological characteristics associated with the rating. The same rating, however, for a B4 stream type would be "poor" and untypical of the stable form of the B4 stream type. Preliminary interpretations are identified for potential vertical stability, width/depth ratio condition and sediment supply.
   
   
9. Bank height ratio. This variable is a field measurement that determines the degree of channel incision. It is calculated by dividing the maximum bankfull depth into the height of the lowest bank. Some rivers may be entrenched (vertical containment of floods), while the bank height ratio variable is indicative of a stream that is lowering it's local base level, but is not yet entrenched. The diagrams in Figure 110 (PDF, 10 kb, 1 p.) indicate various ranges of bank height ratios (degrees of incision). Streams with high bank height ratios generally contribute disproportionate amount of sediment from streambanks and the bed of the channel due to high shear stress. The relation of bank height ratio/stability risk is shown in Figure 111 (PDF, 10 kb, 1 p.).
   

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