Channel Assessment Procedure Field Guidebook

Table of Contents

Introduction

The channel assessment procedure (CAP) consists of both office and field work. The following is restricted solely to field assessments. The objective of the field guidebook is to collect the relevant data required to complete the CAP. Only the text and field forms directly relevant to field assessments are included; all supplemental information on methods, definitions and background context is included in the Channel Assessment Procedure Guidebook.

Field assessments are conducted in small and intermediate-sized channels; they are not usually required for large channels except to confirm aerial photograph assessments. Field work is done in all reaches along the mainstem where reaches could not be assessed on the aerial photographs (due to riparian vegetation canopies or shadows).

The CAP field assessment is based on determining the type of channel expected (given its location within the watershed and certain channel attributes), a set of field indicators of disturbance, and a series of diagnostic disturbance keys. In general, for those reaches identified on the aerial photographs as requiring field visits, complete the following steps.

  1. Walk the stream reach and determine the average channel gradient (s), depth (d), bankfull width (Wb) and largest stone moved by flowing water (D).

  2. Determine the appropriate channel type (based on s, d, Wb and D).

  3. Refer to the correct series of diagnostic channel keys.

  4. Using the field indicators, identify the types and level of stream disturbance present from the series of diagnostic channel keys.

  5. Evaluate the overall disturbance level for the stream, using the keys to determine the specific level of disturbance and the survey lengths to calculate the spatial extent of disturbance.

Field procedure overview

The steps to complete a field assessment (Table 1) are as follows (all forms are located in Appendix 1). Details associated with each step can be found in the sections to follow.
  1. Proceed to the downstream (or upstream) end of the reach identified on the aerial photograph as requiring field evaluations.

  2. Measure bankfull channel width (Wb) at five locations spaced evenly along the reach and record on Field Form 1.

  3. Measure channel depth (d), using a stadia rod, at five locations spaced evenly along the reach and record on Field Form 1.

  4. Identify the largest sediment particle (D) on the channel bed at five locations spaced evenly along the reach, and measure the b-axis. Record on Field Form 1.

  5. Measure the channel gradient (s), over a distance of several channel widths, at five locations spaced evenly along the reach. Record on Field Form 1.

  6. Calculate D/Wb.

  7. Calculate D/d.

  8. Based on s and the ratios D/Wb and D/d, determine channel morphology using the nomogram. Record on Field Form 1.

  9. Determine if large woody debris (LWD) is important to channel structure.

  10. Refer to appropriate set of seven channel keys corresponding to the determined channel morphology (Appendix 2). For example, if the morphology is identified as a CPc-w (cascade-pool morphology with predominately cobble bed and functional LWD), then the appropriate set of keys is found on pages 50 to 56.

  11. Attach the hip chain thread at the beginning of the reach and begin the channel inventory, using the list of field indicators of disturbance and appropriate diagnostic keys. Walk upstream (or downstream) along the channel thalweg (deepest point along the cross-section if wading is possible) or along the bar tops if the water is too deep or the stream flow velocities are too fast for safe wading. Record the distance (from the hip chain) corresponding to the channel condition as determined from the list of field indicators of disturbance and appropriate keys. Record data on Field Form 1. For instance, if the reach is a RPc, the distance corresponding to each level of disturbance is recorded. An example of the field form is as follows:




    Distance
    (m)


    Bank
    type
    Channel type
    and
    disturbance
    level

    Field
    indicators
    S1  S2  S3 ...  D3

    Photo
    roll and
    frame

    0-53 A3/4 RPc:A2     R3:F7, 8
    53-109 A3/4 RPc:D1     R3:F9, 10

  12. Refer to the "Field operational rules" section for directions to follow during the survey.

  13. At each recording of channel type and disturbance level, determine predominant bank material type and record on Field Form 1 (see Field Form 1 for bank-type codes).

  14. Continue the survey, recording all levels of disturbance to the end of the reach. Continue to refer to the "Field operational rules" section.

  15. Repeat the procedure in the next reach.

  16. Continue to the end of all reaches requiring field inspection.

    Table 1. Summary of the field procedure


    Task Field form Refer to figure(s) Field form

    1. Measure:
      Wb 2 1
      d 3 1
      D 4 1
      s 1
    2. Calculate:
      D/Wb 5 & 6
      D/d 5 & 6
    3. Determine:
      Type of morphology 5 & 6 1
      Importance of LWD 7
    4. Inventory:
      Bank material 1
      Level and extent of disturbance 8 & Appendix 2 1
    5. Summarize:
      Level of disturbance associated with each reach 2

Disturbance level analysis

The steps to assess the level of disturbance are as follows.
  1. Summarize the inventory information collected by completing Field Form 2.
  2. Calculate the percentage of the reach in each disturbance class.
  3. Sum the percentage of the reach in moderate and severe disturbance classes.
  4. Weight the percentage of the reach in both moderate and severe disturbance classes by the total length of the reach.
  5. Sum the weighted length of disturbed stream channel for each reach assessed in the field and enter the result on Form 8 of the Channel Assessment Procedure Guidebook.

Field operational rules

During any field survey there are always numerous decisions to be made; it is important that these decisions are made in a consistent manner. The following operational rules will make field surveys easier by removing procedural ambiguities.
  1. Minimum stream survey length is 1 Wb (no change in level of disturbance will be identified and recorded unless the section of channel with a different level of disturbance exceeds 1 Wb).

  2. Maximum distance along a channel without an assessment is 10 Wb (even if there is no change in the level of disturbance, an assessment, including the listing of disturbance code, field indicators and photographs looking upstream and downstream, must be made every 10 Wb). This rule is ± 1 Wb; if a distance of 10 Wb has been reached but it is evident that a change in disturbance level occurs at, or before, the 11 Wb distance, then it is acceptable to miss the 10 Wb and proceed to the 11 Wb.

  3. It is acceptable to break reaches, as determined from aerial photographs, into shorter reaches, based on field examinations. Because of the resolution of aerial photographs, it is possible that a reach may need to be divided into two or more shorter segments if field conditions warrant (based on changing s, D, d, Wb). The new reaches should be identified as a subset of the reach that is being subdivided (e.g., Reach ;B is broken into Reach B.1 and B.2).

  4. As in Rule 1, if a different type of channel is encountered (e.g., changing from a RPg to a RPc, or from a RPc to a CPc), it must extend for more than 1 Wb to be included as a distinct type (i.e., if the different channel type is <1 Wb it will not be inventoried).

  5. If a different type of channel is encountered (e.g., changing from a RPg to a RPc, or from a RPc to a CPc), and it extends for more than 3 Wb, then a new reach must be designated.

  6. If a channel type not considered in the assessment is encountered (e.g., a bedrock waterfall–drainage network classification code CB3aii), and it is ­3 Wb in length, it is listed as such on Field Form 1. The total stream length includes these entries (i.e., the total length of the channel used to calculate the percentage of disturbed channel morphology includes lengths of channel not assessed by this procedure). The level of disturbance assigned to these channels is "none" so they will not influence the overall reach rating.

  7. If, as in Rule 5, a channel type not assessed by the CAP (e.g., CB3aii) extends beyond 3 Wb in length, a new reach must be designated and assigned a channel class according to Figure 1.

  8. If a survey proceeds from a single channel into zones with multiple channels, the survey is to follow the thalweg (follow the branch with the deepest channel, and usually highest discharge).

Figure 1. Drainage network classification (details are presented in Appendix 3).

Field measurements and diagrams

The main field measurements required for the CAP are channel gradient, bankfull channel width, and depth and size of the largest stone on the bed that is moved by flowing water. The relative channel size and morphological type are based on these measures. Morphological type is determined by the relative roughness (D/d), relative width (D/Wb), and slope of the channel. Measurements are made at five locations spaced evenly along the reach (i.e., at the begining and end of the reach with three locations between). Fewer measurements may be made if the assessor is an experienced geomorphologist with extensive knowledge of fluvial forms. If competent professional judgement is used to determine the morphological type, the number of field measurements is left up to the assessor.

Channel slope

To measure channel gradient:

Channel width

To determine the channel bankfull width (Figure 2), measure Wb using a fibre tape and measuring to ±0.1 m.

Figure 2. Identifying bankfull width (after Church 1992). In this schematic, the lower limit of perennial terrestrial vegetation is used to define the left banktop while the right banktop is defined by a terrace (old elevated floodplain surface).

A number of standard criteria can be used to determine Wb in the field (after Leopold 1994). Only those relevant to the field site need be used. Look for:

Channel depth

To determine the channel depth:

Figure 3. Locations used to determine channel depth. (a) Illustration of step-pool morphology and location to determine channel depth (d). Depth is estimated at the step-pool break from the thalweg to the height of bankfull conditions. Note that D/Wb in this schematic is approximately equal to 0.3.
(b) Illustration of riffle-pool morphology and location to determine channel depth (d). Depth is estimated at the riffle-pool break from the thalweg to the height of bankfull conditions. Note that D/Wb in this schematic is 0.01.

Largest stone moved by flowing water

To determine the size of the largest sediment particle on the channel bed:

Figure 4. Illustration of the largest stone moved by flowing water, relative width, and relative roughness used to determine channel size.


Return to top Continue