Channel Assessment Procedure Field Guidebook

Table of Contents

Determining the type of channel morphology

The seven channel types used in the CAP are summarized in Table 2. The type of morphology is determined by using the field measures, nomogram (Figure 5), and by referring to Figure 6. First, the relative width is calculated by entering the measured values of D and Wb on Graph 1 of Figure 5. Second, the relative roughness is determined by entering the measured values of D and d on Graph 2. Third, the respective D/Wb and D/d values are transferred onto Graph 3, which calculates their product. Finally, the product of D/Wb and D/d is transferred onto Graph 4, with the intersection of this value and s giving the type of channel morphology. If the point of intersection between s and (D/Wb)(D/d) does not lie on the diagonal line, follow the shortest line distance back to the shaded band.

Table 2. Channel types and associated characteristics

Code Morphology Sub-code Bed material LWD

RP riffle-pool RPg-w gravel functioning
RP riffle-pool RPc-w cobble functioning
CP cascade-pool CPc-w cobble present, minor function
CP cascade-pool CPb boulder absent
SP step-pool SPb-w boulder present, minimal function
SP step-pool SPb boulder absent
SP step-pool SPr boulder-block absent

Figure 5. Nomogram used to determine channel morphology.

Figure 6. Channel morphologies of small- and intermediate-sized channels.

As an example of the use of Figure 5, use:

D = 10 cm
d = 120 cm
Wb = 20 m
s = 1.5%

Following the lines in the nomogram produces a riffle-pool morphology with predominately gravel-textured materials (RPg). Large woody debris (LWD) is important in these channel types (see Figure 7).

Figure 7. Determining the influence of LWD on channel morphology.

If either the CP or SP morphologies are determined from Figure 5, it is necessary to determine if LWD is expected in the particular channel. The importance of LWD to channel functions depends on the width of the channel (also stream power, but this is considered implicitly in Figure 5). The functional role of LWD is given in Figure 7. For instance, if a channel is determined to be cascade-pool morphology, from Figure 7 it is apparent that when the channel is less than 30 m wide, LWD should be present. When the channel is wider than 30 m, LWD is present but not functioning. (Note that since LWD characteristics are to be used as field indicators of disturbance, it is necessary to know when LWD should, or should not, be present in channel.)

The nomogram is a tool to assist in determining the type of channel morphology. If, in the field, the nomogram indicates a type of morphology that appears incorrect, the field measures should be re-taken. For instance, if a step-pool morphology is determined from Figure 5, but the channel is clearly a riffle-pool morphology, the field values used (with emphasis on channel slope) should be checked.

Evaluating channel disturbance

Each of the three main channel morphologies assessed in the field (step-pool, cascade-pool, riffle-pool) respond differently to disturbances caused by changes in streamflow discharge and sediment/debris loads. In general, the nature of the morphological disturbance expected is associated with channel degradation and aggradation (Figure 8 and Appendix 2).

Figure 8a. Small-sized channel morphology matrix showing levels of disturbance (degradation and aggradation).

Figure 8b. Intermediate-sized channel morphology matrix showing levels of disturbance (degradation and aggradation).

Field indicators

Field evidence of channel degradation and aggradation is summarized in Figure 9. The changes in sedimentological characteristics are related to both sediment supply and transport limitations. Bank impacts are related to recent erosion as evidenced by collapsing or freshly removed materials. Morphological features considered are primarily the relative abundance of pools and steps or riffles.

Disturbances associated with LWD are assessed by considering the functional role of debris in controlling the morphology (see Hogan et al., in press, for additional details concerning LWD functions). In certain channels, LWD controls the patterns of sediment erosion and deposition within the channel zone. The channel will adjust to any change if LWD is altered in its dimensions, orientation, or storage patterns. LWD plays an insignificant stream-forming role in other channel types (Figure 7).

Figure 9a. Field indicators of channel disturbance-sedimentological features.
Figure 9b. Field indicators of channel disturbance-bank features.
Figure 9c. Field indicators of channel disturbance-morphological features.
Figure 9d. Field indicators of channel disturbance-LWD features.

Channel keys

The field indicators of disturbance described in Figure 9 are used together with the diagnostic keys (Appendix 2) to determine the level of disturbance present along a reach of a particular channel type. The keys integrate all the individual properties of disturbance and incorporate the field indicator evidence to show the overall disturbance pattern. Without the keys it would be necessary to physically measure the attributes of disturbance (many of which are problematic due to streamflow stage dependency), and then compile these in some statistically rigorous way to determine the overall level of channel disturbance.

Several points should be noted concerning the channel keys. First, the keys are organized so that the undisturbed channel is located in the middle of each set of channel types. Three levels of aggraded channels are found in front of the undisturbed channel and three levels of degraded channels are located after the undisturbed channel. This series shows the progression of channel changes that occur if sediment and/or water quantity are increased (aggrading, for example, downstream of a landslide entry point to the channel) or decreased (degrading, for example, downstream of a barrier to sediment transport, such as a landslide-dammed channel). This series is given in Appendix 2. The three levels represent degrees of disturbance severity.

Second, the text associated with each channel disturbance level often indicates that a feature is becoming "more," "less," "coarser," "finer," etc. These terms refer to the progression of changes away from the undisturbed state (e.g., see title page of each channel type in Appendix 2). They indicate the sequence of changes that make up the disturbance.

Third, the keys provide generalized, typical examples of channel conditions along a reach. They are not intended to be exact duplicates of the conditions encountered (do not expect your field situation to be exactly the same as conditions shown in the keys). Finally, the "typical field indicators" listed in the keys do not have to be found in every case for that particular level of disturbance to be assigned. The indicators are included to show which indicators should, but not must, be present for the level of disturbance.

In the field, the morphology of each reach is determined according to the channel attributes measured in the field and with Figure 5. The level of disturbance is then assessed by inventorying the field indicators of channel disturbance and reviewing the deviation from the stable state, as shown in Appendix 2. All data are recorded on Field Form 1, and the proportion of each reach with disturbed morphology is calculated on Field Form 2. Finally, the information summarized on Field Form 2 is transferred to Form 8 of the Channel Assessment Procedure Guidebook.

Field equipment

Recommended field equipment for conducting the CAP include:


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Hogan, D.L., S.A. Bird, and S. Rice. In press. Channel morphology and recovery processes. In Proceedings of the Carnation Creek and Queen Charlotte Islands Fish/Forestry Interaction Program: Applying 20 years of coastal research to management solutions. Chatwin, S., Hogan, D.L., and Tschaplinski, P. (eds.). B.C. Min. For. Victoria, B.C.

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