2. MARCO REFERENCIAL
2.2.2. La Documentación de los Sistemas de Gestión de la Inocuidad de los Alimentos: La ISO 9001/2000 presenta los requisitos para los sistemas de gestión
The hydraulic data is also used in addition to the longitudinal morphological analysis, to assess flow diversity and behaviour associated with different types and styles of pool-riffle sequences at different stages. This provides a basis by which the relative ecological benefits of an enhanced over a naturally developed bed morphology can be evaluated. It also provides a basis for comparing the relative flow behaviour and ecological benefits provided by the two distinct styles of rehabilitation approach used in the Pool River. To compare cross sectional pool and riffle flow characteristics at different discharges, measures which are independent of scale are required. This study applies two such measures, one directed toward assessing cross sectional asymmetry and one toward cross sectional diversity. The asymmetry ratio, 'a', relates the mean flow velocity on one side of the cross section to that on the other.
a = v ,r v , (2.7)
where:
V, = mean velocity (an average of the two velocity recordings made near the left bank)
= mean velocity (an average of the two velocity recordings made near the right bank)
Milne (1982b), applies a similar measure to assess variations within and between pool-riffle bedforms in a natural gravel bed river. For the present study, the recording made in the centre of the channel is omitted for the assessment of asymmetry ratios. The diversity measure, 'b', relates the sum of the recorded velocities made in each cross section to the average velocity in the cross section.
b = v / + \ 2 + V3^ + V4^ + Vs^ (2.8)
V^ *n where:
V = depth averaged velocities recorded across the section (5 verticals, v, to V 5 ) V = average velocity for all cross sections
n = number of recordings (in this case 5)
Values nearing unity indicate uniform form or flow. Progressively higher values are associated with increasing lateral diversity. This is a more involved measure than those for asymmetry as it identifies variability between all values. Clifford and Richards used this equation to study the relative behaviour of pools and riffles in a natural alluvial river (1992, p54. Figure 2.6). They found the flow to be less diverse in riffles relative to pools. Furthermore, for both pool and riffle bedforms, cross-sectional flow diversity was observed to be relatively high at low discharges, and relatively low at high discharges.
B oth asymmetry ratios and diversity measures are independent of scale. These measures allow for flows at different stages to be compared, as well as flow characteristics derived from different reaches. Unfortunately, the fact that they are not scale-dependent means that they may not identify aspects of variability between pools and riffles which may be ecologically important. For example, although the flow velocities in the riffle cross section in Table 2.2 have a larger range of values than the pool, the diversity coefficients and asymmetry ratios are the same. Asymmetry ratios and diversity measures are also potentially more erratic when applied to small- scale variations in form and flow. For example, form characteristics from riffle sites are more sensitive to ratio measures, as these cross sections are relatively symmetrical. A small degree of sampling error will, therefore, have a greater impact relative to the same amount of error on asymmetrical pools. Flow characteristics are also affected. Ratios for intermediate and high flows are relatively stable, in that if they were recorded again, ratios would remain similar. In contrast, measures derived from low flow recordings are less stable, as aspects such as equipment limitation have more of an impact (accuracy of EMCM = ±0.05m®"', Valeport Model 802 operation manual, 1998, pp 6). Given the small numbers involved at low flow, the values of the ratios produced could also be extremely high.
Average flow velocity (m®"')
Data collection position Pool Riffle
V3 from left bank 0 0
% from left bank 0.1 1
Mid channel 0.2 2
% from right bank 0.3 3
V3 from right bank 0.4 4
Diversity coefficient 2.5 2.5
Asymmetry ratio 0.14 0.14
Table 2.2 Similarity in diversity and asymmetry ratio coefficients as they are independent of scale.
Another difficulty with asymmetry ratios relates to the division of alternating lower and higher denominators and numerators as the flow meanders throughout the channel. The analysis could always divide the higher value by the smaller value, although the directional components to flow and form would be lost. Logging the values or graphically representing the values logarithmically overcomes this problem. However, where reverse flow occurs in the cross section (i.e. recirculating flow moving upstream), negative values arise which cannot be plotted. This preferentially sensitises the low flow data collected in pools. These aspects of data collection and analysis make it difficult to characterise flow behaviour and compare behaviour in different reaches.
Scale-dependent measures do not suffer from the limitations outlined for the ratio measures. However, these measures are not directly comparable between flows of different magnitudes as they will tend to increase with discharge. Bearing this in mind, scale-dependent measures can provide insights into the behaviour of pool and riffle flow at different stages. This behaviour can also be related to that identified in other reaches if it is assumed that the three recorded flow stages (low, intermediate and high) are broadly comparable in terms of occurrence for those sites. For this analysis, another asymmetry measure 'c' is applied. However, instead of dividing flow velocities recorded on either side of the channel, they are subtracted.
C = V ; - Vr (2.9)
where:
V] = mean velocity (an average of the two velocity recordings made near the left bank) Vr - mean velocity (an average of the two velocity recordings made near the right bank)
In addition to these flow characteristics, cross sectional form characteristics of the pool-riffle units are also assessed in terms of asymmetry (ratios and magnitudes) as well as diversity. This morphological assessment is based on the 5 cross sectional recordings of flow depth at low flow (and only at low flow). These measurements were sampled at the same time the low flow velocity recordings were made and assume the cross sectional water surface is horizontal (i.e. no super elevation at baseflow or excessive drawdown). To reduce the effect of the variable water depths between pool and riffle bedforms, the shallowest depth recording is set to zero in each cross section. For the cross sectional flow recordings, ratio measures were potentially more erratic when characterising the pool velocities at low flow. For these form recordings, ratio measures are potentially more erratic wiien characterising riffle cross sections. This is due to the relatively small differences in cross sectional elevation in comparison to the pools. A summary of the morphological and hydraulic indices used in this investigation is presented in Table 2.2 and 2.3.
Morphological analysis conducted on river long profiles (Section 2.4)
• Local boundary hunting (Section 2.4.5). • Zero-crossing technique (Section 2.4.1). • Bedform differencing technique (Section 2.4.2). • AR(2) modelling (Section 2.4.3).
• Spectral analysis (Section 2.4.4).
Natural alluvial Rehabilitated non-alluvial Naturally developed non-alluvial
Clyne River Pool River Ravensbourne River
Task: Task: Task:
• Identify local boundaries in • Identify local boundaries in the • Identify local boundaries in the profile
the profile profile • Assess pool-riffle morphology at a variety
• Assess pool-rifiQe morphology at a variety of scales.
• Assess pool-riffle morphology at a variety of scales.
of scales.
Objective: Objective: Objective:
• Insight into diversity of • Insight into ability to recreate • Insight into naturally developed non-
natural alluvial river form. natural alluvial river form. alluvial river form.
• Provide a basis for • Highlight pros and cons of • Provide a basis for assessing form
‘natural’ design adaptation. current approaches (including the
ability to create a diverse habitat). • Provide a foundation for design improvements.
diversity (habitat potential) and hence necessity of enhanced intervention. • Provide a basis for harmonious design principles (integration).
Table 2.2 Summary of morphological investigations carried out in the present study to river long profiles.
Morphological and hydraulic analysis conducted on pool-riflle cross-sections (Section 2.5.1 and 2.5.2)
• Cross-sectional area variation with stage (Section 2.5.1). • Cross-sectional flow velocity with stage (Section 2.5.1).
• Cross-sectional form and flow asymmetry (ratio) with stage (Section 2.5.2). • Cross-sectional form and flow asymmetry (magnitude) with stage (Section 2.5.2). • Cross-sectional form and flow diversity with stage (Section 2.5.2).
NB: Cross-sectional form and flow characteristics are not required from the natural alluvial
Clyne River site as: a) diversity measurements are used to assess the relative benefits of enhanced rehabilitation as opposed to natural recovery and, b) there is already much data on the velocity reversal mechanism in natural river environments.
Rehabilitated non-alluvial Pool River Task:
• Evaluate the potential for velocity reversal (identify area reversals).
• Identify cross-sectional pool-riffle form and low characteristics with stage.
Objective:
• Insight into ability to recreate form.
• Insight into ability of implemented form to support form maintenance processes (velocity reversal). • Insight into ability to recreate processes responsible for form maintenance (specifically velocity reversal). • Highlight pros and cons of current approaches (including the ability to create a diverse habitat). • Provide a focus for design improvements.
Naturally developed non-alluvial Ravensbourne River Task:
• Evaluate the potential for velocity reversal (identify area reversals).
• Identify cross-sectional pool-riffle form and low characteristics with stage.
Objective:
• Insight into naturally developed non-alluvial river form and form maintenance processes (velocity reversal).
• Provide a basis for assessing form diversity (habitat potential) and necessity for enhanced intervention. • Provide a basis for harmonious design principles (integration).
Table 2.3 Summary of morphological and hydraulic investigations carried out in the present study to pool-riffle cross-sections.