Melissa Parsons, Martin Thoms and Richard Norris
Cooperative Research Centre for Freshwater Ecology
University of Canberra
Monitoring River Health Initiative Technical Report Number 22
Environment Australia, 2002
ISBN 0 642 54888 9
In a geomorphological survey, thorough description of the physical characteristics of a stream reach generally includes measurement of several cross-section profiles (Gordon et al., 1992). A channel cross-section is essentially a "slice" through the channel, made at right angles to the flow (Gordon et al., 1992). Data collected at a cross-section provides information on linear and areal channel dimensions. Aspects of channel dimension are related to discharge character and sediment transport, and can also be used to examine changes that occur in the channel profile as a result of anthropogenic or natural events. Aspects of channel dimension can also be used to calculate complex geomorphological or hydrological parameters such as Mannings n or stream power, although these are not included in the current protocol.
Channel cross-sections can be measured using survey equipment, although in the current protocol, equipment is kept to a minimum (Figure 5.23) and channel cross-sections will be taken using measuring tapes. Regardless of the equipment used, the procedure for measuring cross-sections involves taking vertical measurements at several points across a horizontal transect-line. At each point, both the horizontal distance across the channel and the vertical distance to the streambed are recorded (Figure 5.24). Specific components of the cross-section will be discussed further in the next section.
In wadeable streams, cross-sections are relatively easy to measure because the entire width of the stream can be accessed, even in pools. Accessibility makes cross-sections slightly more difficult to measure in deep pools and large lowland rivers, however, there are many simple ways to sample cross-sections in these types of rivers. For example, in large rivers a boat can be used to access the width of the river or in medium sized rivers, a canoe, small boat or sometimes even a Li-Lo (air mattress) can be used to access the centre of deep pools. When weather, flow and water quality conditions are safe, cross-sections can be performed by swimming across the stream. At some sampling sites, cross-sectional measurements may also be made from a bridge. To take depth measurements along a horizontal transect, a marked pole (e.g. a metre ruler, survey staff or custom made device) can be used, or in slow flowing areas a weighted tape measure or weighted and marked rope is also suitable. Another method that has been used successfully in the River Habitat Audit Procedure (Anderson, 1993a) is to rig a depth sounder onto a rubber flotation board, that can be pushed, pulled or placed across the river to take depth measurements at the required intervals.
It is important that the required numbers of cross-sections are measured at every sampling site. Thus, preparation for a field trip should include planning of the logistics and equipment required to make cross-sectional measurements at all sampling sites, even those located on large or deep rivers. Health and safety issues must be taken into consideration when planning cross-section sampling in any type of river.
The components that must be measured at each cross-section are detailed in Figure 5.24 and are described as follows:
Stream width at the water surface is the width of the water surface at the time of sampling.
Baseflow stream width is the width of the stream at a point corresponding to baseflow conditions. The baseflow water mark is evidenced by the limit of terrestrial vegetation, eroded area or a break in bank sediment. Under baseflow conditions, baseflow stream width will be equivalent to stream width at the water surface.
Bankfull channel width is the width of the channel between the top of the banks. Bankfull level is the point at the top of the channel where under high flow conditions, the water level would be even with the top of the banks, or in a floodplain river, at the point just before water would spill over onto the floodplain. Further information on the identification of bankfull level is provided in Figure 5.31.
Bank height is the height of the bank measured from the baseflow water mark to the top of the banks. Bank height is measured at both the left and right banks.
Bank width is the width of the bank, extending from the edge of the stream (at the watermark) to the bankfull point. Bank width is measured at both the left and right banks.
Vertical distance between water surface and baseflow water mark is the height difference between the water surface and the baseflow water mark. Vertical distance between water surface and baseflow water mark is measured to compensate for conditions where flow is below normal levels at the time of sampling. This component is measured at both the left and right banks.
Vertical water depths and horizontal distances are measured together at several points across the width of the stream. At each horizontal distance from the edge of the stream, water depth is recorded.
Each of these components are used in various combinations to calculate cross-sectional variables (see section titled 'What variables are derived from a cross-section?') and thus, it is vital to make all of these measurements at each cross-section.
The field procedure for measuring a cross-section is as follows:
In summary, a cross-section requires measurement of the following components:
The number and placement of cross-sections at each sampling site is dependent on the relative heterogeneity of the channel.
Two cross-sections should be measured at sampling sites that have a relatively uniform channel shape and sediment composition (Figure 5.29). These types of sampling sites generally correspond to large, low gradient rivers without riffles. The two cross-sections should be placed close to the upstream and downstream boundaries of the sampling site. These cross-sections should not be located on the apex of a bend.
Three cross-sections should be measured at sampling sites that have a relatively complex channel shape and sediment composition (Figure 5.30). These types of sampling sites generally correspond to small to medium wadeable rivers with a cascade or riffle-pool flow character. The three cross-sections should be placed to represent the different types of bedform units present at the sampling site (ie. riffles, pools, runs) and must include at least one pool. In most streams, appropriate placement of cross-sections would include one riffle, one run and one pool, spread throughout the entire length of the sampling site. Again, cross-sections should not be placed on the apex of a bend.
Accurate identification of the bankfull channel and baseflow water mark levels is fundamental to the measurement of cross-sections. The placement of bankfull and watermark levels at cross-sections located in different channel types is explained in Figure 5.31.
Bankfull channel level is the point within the stream channel where the water level would fill the channel to the tops of the banks. The 'tops of the banks' varies according to channel type (Figure 5.31).
The baseflow water mark level is generally evidenced by the limits of terrestrial vegetation, scour lines, growth of macrophytes or abrupt changes in bank slope (Figure 5.24). However, the baseflow water mark level can be difficult to identify in some situations, using the above criteria. An additional method that can be used to aid the identification of the baseflow water mark level is residual pool depth (Lisle, 1987). Residual pool depth is the difference in depth or bed elevation between a pool and the downstream riffle crest. Residual pool depth is measured by surveying a pool with a tape measure and ruler and subtracting the depth of the riffle crest from those in the pool. A detailed description of the residual pool depth method is available to down load from the United States Forest Service website at http://www.rsl.psw.fs.fed.us/projects/water/Lisle87.pdf
Confined channels have no floodplain development and are generally found in upland areas with steep valleys. Under undisturbed conditions, bankfull width is usually not much larger than baseflow width. The bankfull level in a confined channel is evidenced by the limit of terrestrial vegetation, the growth of macrophytes, the presence of moss or lichen, the presence of scour marks or an abrupt change in bank slope. (Figure 5pt31 - 1 of 6)
This type of stream is found where islands (i.e. bars) have formed within the channel. The bars may be vegetated or unvegetated. The placement of a cross-section should run across the bars. Bankfull width should include the bar portion, but baseflow width should break around the bar portion. (Figure 5pt31 - 1 of 6)
|Channel with instream bars|
This type of stream occurs where bars have formed and are attached to the banks. The placement of a cross-section should run across the bars. Bankfull width should include the bar portion, but baseflow width should not include the bar portion if it is not within the baseflow area. (Figure 5pt31 - 1 of 6)
Terraced channels are channels in which the banks are characterised by bench formations. Terraced channels generally occur in lowland floodplain rivers. Regardless of the number of benches present, bankfull width is always measured to the top of the first bench only. (Figure 5pt31 - 1 of 6)
|One bank higher than the other|
When one bank is higher than the other, bankfull width is measured to the top of the lowest bank. This is because the top of the lowest bank represents the point where water would overtop the bank and spill onto the floodplain. (Figure 5pt31 - 1 of 6)
Braided channels contain multiple channels that diverge and converge around many islands. Banks may be poorly defined in these types of channels, although the lateral limit of the channel can often be identified. Cross-sectional bankfull width and baseflow width of should be measured across all the threads of a braided channel. (Figure 5pt31 - 1 of 6)
Figure 5.31. Identification of bankfull level in different channel types. Baseflow water mark level is also drawn on for context, however, the actual position of the water mark level relative to the bankfull level can only be determined after examination in the field.
The variables derived from data collected at the cross-sections are given in Table 5.4. Several of these variables are derived directly from field cross-section measurements, several are derived following office-based adjustments and several are derived using the AQUAPAK1 computer package. Instructions for the calculation of each variable are provided in the following pages.
|Bankfull channel width||Calculated directly from the cross-section data collected in the field|
|Bankfull channel depth||Cross-section data collected in the field is adjusted in the office|
|Baseflow stream width||Taken directly from the cross-section data collected in the field|
|Baseflow stream depth||Cross-section data collected in the field is adjusted in the office|
|Bank width||Calculated directly from the cross-section data collected in the field|
|Bank height||Calculated directly from the cross-section data collected in the field|
|Bankfull width:depth ratio||Calculated using the bankfull channel width and bankfull channel depth variables|
|Bankfull cross-sectional area2||Calculated in AQUAPAK using adjusted cross-section data|
|Bankfull wetted perimeter2||Calculated in AQUAPAK using adjusted cross-section data|
|Baseflow cross-sectional area2||Calculated in AQUAPAK using adjusted cross-section data|
|Baseflow wetted perimeter2||Calculated in AQUAPAK using adjusted cross-section data|
In addition to these variables, substrate composition, bank material, riparian zone width, filamentous algae cover, periphyton cover, moss cover and detritus cover will also be measured in the immediate vicinity of the cross-section. Further information on these variables are provided on each instruction sheet.
1 AQUAPAK is a package of stand-alone, IBM compatible computer programs which supplement the text "Stream Hydrology: An Introduction for Ecologists" (Gordon et al., 1992). Among other functions, AQUAPAK calculates the channel cross-sectional area and wetted perimeter variables. AQUAPAK can be purchased for $20 from the Centre for Environmental and Applied Hydrology, Department of Civil and Environmental Engineering, The University of Melbourne, Victoria, 3010. An order form is provided in Appendix 3.
2 Bankfull cross-sectional area and baseflow cross-sectional area are the same variable but calculated for the bankfull and baseflow areas of the channel respectively (Figure 5.24). Likewise, bankfull wetted perimeter and baseflow wetted perimeter are also calculated the same way.