Pool and riffle characterisation: a peculiarity of the bedform differencing technique

A final comment on the comparability of the techniques relates to a peculiarity of the bedform differencing technique in relation to varying profile slope. This technique characterises pool depths and riffle heights by the difference in elevation from the previous bedform (Figure 2.14). Working in a downstream direction (as per O'Neill and Abrahams, 1984), the height of a riffle equals the vertical elevation difference relative to the upstream pool trough, and the depth of a pool equals the vertical elevation difference relative to the upstream riffle crest. Consequently, profile slope can influence the pool depth characteristics. This influence is greatest on steeper reaches, resulting in apparently very deep pools despite the fact that the actual pool depths are relatively shallow. For example, pool x in Figure 3.40a (centreline profile for reach 1 in the Clyne River) has a 'depth' of 1.2m relative to the preceding riffle, even though the low flow water

depth in pool x is only 0.14m. Most of this drop is attributable to the steeper slope, the influence of which reduces downstream.

Figure 3.40b shows the influence of profile slope on reach averaged bedform height and depths in the Clyne River. Due to the influence of the profile, pools are on average 30% deeper than the riffles are high. Riffle heights also increase fi’om reach to reach, although pool depths remain relatively constant. This results fi-om an increase in actual pool depths being compensated by the reducing profile slope. Should pool depths form the focus of the investigation, it would be advisable to identify the depth relative to the downstream riffle crest as this ponds back the water.

a)

Distance dow nstream (m)

1.0 - 0.5 ? 0.0 Ô -0.5 S. -1.0

b) _{• Pool (centreline profile)} A Riffle (centreline profile)

o Pool (thalw eg profile) A Riffle (thalw eg profile)

à 1 2 Upstream 4 5 6 Reach number 7 8 9 Dow nstream

Figure 3.40 a) Influence of profile slope on pool amplitude when evaluated by the bedform differencing technique

b) Influence of profile slope on pool depths determined through bedform differencing for the Clyne River reaches. An increase in riffle height is not matched by an increase in pool depth.

3.11 Summary

Comparing variable form characteristics produced from the different techniques is not straightforward as they require different subjective decisions to be made at different stages of the identification process. These decisions are not directly responsible for all the variability however, as some techniques reveal similar characteristics for some reaches and different characteristics for others (Figure 3.34 and 3.35). This results from a diverse bed morphology which differentially affects the techniques \^ ic h approach the task of form identification in different ways.

Based on a theoretical examination of a sinusoidal morphology, a minimum of 4 data points per bedform is required so as not to unduly smooth the morphology. Unfortunately, even if the bankfull width can be consistently identified between river systems, the data quality may not be consistent as other parameters, such as profile slope, can also influence morphological characteristics. Variations in bedform shape can also vary from reach to reach and further reduce the consistency of data quality. Moreover, variations in data quality can affect pools and riffles to different extents if the bed morphology is 'domed' in nature. Data analysis is also affected by variations in longitudinal bedform shape, affecting both the generation as well as impact of levying a tolerance value.

Systematic morphological variability also reduces the integrity of the bedform identification techniques. Unfortunately, segmenting river profiles raises further concerns relating to the comparability of results obtained from the different reaches where tolerance values are applied. Morphological characteristics also vary significantly depending on vfrether the centreline or thalweg profile is assessed. Differences between form characteristics derived from centreline and thalweg form characteristics also occur, the magnitude of which also varies depending on the technique applied. The variation in cross-sectional morphology with distance also questions the comparability of morphological characteristics derived from the same profile on the same river at different locations. A consistent data sampling and analytical approach will not, therefore, necessarily provide results of a comparable nature. Despite these complications, a number of broad recommendations can be suggested (Table 3.7).

If only one profile is assessed then the thalweg profile is recommend as it has a greater geomorphological significance than the centreline profile. If possible, both profiles should be evaluated, as inferences can be made about the variations in cross-sectional asymmetry without the need for individual cross sections to be assessed.

Where pool-rifiQe morphology is being assessed, the data-sampling interval should equal O.Swy (where the bankfull width is defined in an objective manner).

Local boundaries or breaks in the profile need to be identified to improve the integrity of the bedform identification techniques (Equations 3.1 to 3.6 provide a basis for this). Pool-riffle characteristics derived through spectral analysis and AR(2) modelling should be evaluated with caution as they do not identify individual pools and riffles but simply characterise the undulations in the long profile.

Pool-riffle characteristics derived through AR(2) modelling should be evaluated with caution, as they are similar to those produced through zero-crossing without a tolerance value.

Zero-crossing is recommend over AR(2) modelling as individual bedforms are identified. Zero-crossing therefore provides a much richer data set.

Bedform differencing is recommended over zero-crossing as bedforms may be

preferentially removed with tolerance application in the latter technique inhibiting form characterisations.

If bedform differencing technique is applied, this study recommends a tolerance value multiplier of 1 (none), although it is recognised that this may not be suitable for all river systems.

The selection of an appropriate tolerance value can be aided by techniques outlined in Figure 3.37.

Bedform shape should be identified through assessing the shape of the fi’equency distribution of residuals around a regression, as this provides an insight in to the comparability of the results between and within river systems.

The distance between the first and last form identified, rather than the reach length, should be used to assess characteristics such as average bedform spacing.

The most important recommendation is that the integrity of the techniques varies both within and between river systems as a result of the spatial variability of the bed morphology. Any results should therefore be viewed with caution and techniques should be explicitly stated in any publications. If possible, a range of bedform identification techniques should be applied in order to obtain a more informed evaluation of bedform character.____________________________________________ Table 3.7 Recommendations for the identification of pool-riffle bedforms in natural

alluvial river systems.

CHAPTER 4

Morphological diversity of a natural river system;