Resource Directed Measures for Protection of Water Resources: River Ecosystems

Location of cross-section/s

• 
  Firstly, each specialist present at the site selection should identify and justify the location of ‘non-hydraulic’ cross-sections;
  

• 
  Secondly, the positions and importance of all cross-sections should be assessed with a view to combining cross-sections without loss of essential information; and
  

• 
  Finally, additional cross-sections for hydraulic purposes (positioned at changes in the water slope and channel geometry) should be located by the hydraulician, and the purpose of their inclusion explained to the specialists on the team.
  

When selecting the additional hydraulic sections, the hydraulician must bear in mind that hydraulic controls (i.e. determinants of the relationship between discharge and flow depth) are a function of discharge. These must therefore be selected at a discharge appropriate to IFR-type application; that is, with greater emphasis on low flows.

2. 
   Intermediate Ecological Reserve
   

Site selection for an IER is undertaken by the hydraulician and river ecologist who jointly represent the site specific needs of the various IFR specialists, and therefore must be adept with the morphological and biological characteristics required of IFR sites. Resource constraints usually dictate that one cross-section per site is surveyed, and it is consequently essential that the profile is correctly positioned from both a hydraulic and biotic perspective. Due the minimal time available between site selection and the on-site specialist meeting, it is necessary and cost-effective to undertake the survey immediately following site selection. This implies that the hydraulician should have sufficient expertise, and the necessary equipment, to carry out the survey.

R17.4.2 Site survey

The purpose of the survey from the hydraulics viewpoint is to define the cross-sectional profile of the river channel in sufficient detail to enable hydraulic measurements and analysis to be undertaken at sufficient levels of resolution, and to identify features of interest to river scientists.

R17.4.3 Measurement of discharge

It is essential that, to be of any use in the IFR determination, parameters such as flow depth and velocity, specified by the other specialists in the process, are related to a known discharge (flow rate).

Various methods exist for the measurement of discharge, including the use of existing rated sites (natural river sections and structural gauges) and manual techniques including the velocity-area and dilution methods.
A gauging weir or rated cross-section located in close vicinity to the IFR site provides a useful means of obtaining discharge data. The integrity of data derived from hydrological stations must not however be taken for granted, and should be checked with the authority responsible for its operation. The gauge should be sufficiently close to the site so that intervening inflows and losses may be neglected. Furthermore, care must be exercised during unsteady flow conditions, to account for the travel time and attenuation of flow between the local IFR site and the remote gauging station. A method for synthesising rating relationships based on the measurement of an unsteady flow event is provided by Birkhead and James (1998).
The velocity-area method is undoubtedly the most commonly applied manual technique for determining the flow rate in natural, medium to large watercourses, whereas dilution techniques are better suited to turbulent rivers where other methods are difficult to apply (e.g. a rock-strewn river of high bed slope). Details and standards for the application of these manual gauging techniques are given in the British Standard for the Measurement of Liquid Flow in Open Channels (BS 3680), and must be consulted to ensure correct application of the methods. See also Gordon et al. (1992).
Point velocities recorded during manual discharge measurement may be used to determine the distribution of velocity across the channel and, if depth requires more than one depth-averaged velocity measurement, vertically in the water column. The hydraulician should ascertain if this velocity data will be of use to any of the other specialists, notably the fish biologist and the fluvial geomorphologist, in their deliberations. Reference is made in 12 to further developments and application of velocity data in IFR determination.
In order to observe as wide a range of discharges as possible, it is highly desirable to undertake discharge and related measurements, by whatever method, over at least one hydrological season. This, however, does not necessarily guarantee that a suitable range of flows will be encountered due to the possibility of unfavourable climatic conditions (e.g. a failed wet season or unseasonally high flows in the dry period). Under such exceptional circumstances the IFR specialist meeting may be postponed or, if this is not possible, additional data collection and refinement of the hydraulics and IFR recommendations will be required following the specialist meeting. For an IER, time constraints will often dictate that two observed rating points do not cover a wide range of discharges, and this should be taken into account when assessing the confidence in the extrapolated hydraulic relationships.
Measuring discharge by direct methods during high flows in most rivers, where depth and velocity of flow militate against safely entering the water, requires the use of boats or other techniques, and demands high standards of safety to avoid accidents. Dangers from the natural inhabitants of the river such as hippopotami and crocodiles, and the risk of contracting river-related diseases such as giardia and bilharzia, must also be taken into account.
When high flows make entry into the river, by whatever means, impossible or inadvisable, stage levels at the banks should be recorded at the cross-sections, as well as upstream and downstream of the sections as discussed in 4.2. Where possible, floats should be used to measure surface velocities. The use of surface velocities to obtain an estimate of discharge is described in BS 3680.

R17.4.4 Data analysis and modelling

This component of the hydraulics task is concerned with converting observed cross-sectional and flow data, the latter often for a limited range of flow rates, into relationships between flow rate and the more biologically useful parameters for the entire range of flows of interest in the IFR exercise.

When sufficient observed rating data exists, it is possible to establish rating functions based entirely on field measurements by fitting relationships of the form given by Birkhead and James (1998) to the observed data. (Refer to Fig. 2). Care must be exercised with the extrapolation of the modelled rating relationship above and below the highest and lowest recorded flow, respectively. The validity of the extrapolation can be assessed by computing inferred resistance coefficients and average flow velocities beyond the range of observed data, and comparing these with reasonable values based on experience, and resistance coefficients given in the literature (e.g. Barnes, 1967; Hicks and Mason, 1991 and Ven Te Chow, 1959).
F
igure 2: Example plots of a rating relationship on log-normal scale showing the observed data, and a cross-section profile.
Synthesis of a rating relationship essentially involves interpolation between sparse data points and extrapolation beyond the limit of recorded data. As discussed previously with reference to Manning’s resistance coefficient, flow resistance in natural watercourses is generally a function of stage, particularly at low stages. The rating relationship may therefore be synthesised between observed data by interpolating the flow resistance coefficient. The selection of a suitable resistance relationship and corresponding coefficient (e.g. Darcy-Weisbach, Chézy or Manning) is considered arbitrary in the existing application and should be based on pragmatic considerations such as for example the relevant resistance equation applied in available software (if this is to be used in the analysis), experience and familiarity. This is because, although certain relationships are theoretically more rigorous than others, it is illogical to apply the most rigorous modelling approach in a situation where the resistance coefficient is essentially a ‘composite’ calibration factor based on field data. This factor and the energy losses cannot be derived solely from consideration of the measurable physical dimensions of the resistance components (e.g. size of the roughness elements, vegetation type and density, channel plan form, etc). Extrapolated rating relationships may be developed by extrapolating the resistance coefficient based on calibrated data (i.e. fitting a relationship to the resistance coefficient versus discharge (or stage) data and extrapolating). Calibrated resistance coefficients are usually determined based on non-uniform flow profile computations (i.e. backwaters), or by assuming linear variation of the energy slope for a site represented by a single cross-section. Once again, care must be taken by judicially assessing the values of the extrapolated coefficient and comparing these with values based on experience, and published in the literature.
A major difficulty with low-flow hydraulic analysis at many IFR sites (pools, for instance), is the estimation of the stage of zero discharge; that is, the water level at which the flow ceases. The most appropriate method for estimating the stage of zero discharge (in the absence of observed flow data) is to survey the longitudinal profile downstream of the cross-section within the deepest portion of the active channel to ascertain the level of the downstream bed which causes the upstream backup. Alternatively, extrapolation of the observed rating data to zero discharge may also provide a useful, albeit approximate, estimate of the stage of zero discharge.
Once the rating relationship for a river section has been developed, the relationships between discharge and other hydraulic determinants (e.g. average flow velocity, wetted perimeter and average flow depth - refer to Fig. 3) may readily be computed using the cross-sectional geometry.





Figure 3: Example plots of discharge against flow depth (maximum and average), average velocity and wetted perimeter on normal scale axes.
Hydraulic analysis and modelling must only be carried out by skilled practitioners who are familiar with low flow techniques and problems, as the errors inherent in the application of the more traditional approaches of analysis, more suited to high flows, resonate throughout the entire process. For instance, as discussed earlier, the values of the resistance parameter Manning’s n which must be applied to low flows in a riffle are considerably higher than the range of values used in high flow analyses. Application of inappropriately low n values results in significant underestimation of flow depths, and concomitant overestimation of velocities, for specific flow rates. This in turn prompts overestimation of flow rates to achieve particular flow depths and velocities for, for instance, fish passage, and thereby inflates the instream flow requirement.
Although the hydraulic information supplied for use within IFR assessments should be ‘best estimates’, it is considered advisable to err on the side of conservatism when uncertainties are present due to limited observed data. This is particularly applicable for an Intermediate determination where only two rating observations are allowed for, and procured within a short time period. Conservatism in this respect implies overestimating the discharge required to provide specific flow depths - a consequence of underestimating the resistance coefficient as discussed above. If a CER is undertaken at a later stage and recommends significantly higher flows, then it may be very difficult to provide for such recommendations if the balance of the resource has been allocated to other users.

R17.5 Minimum, acceptable and ideal data sets

R17.5.1 Site selection

It is absolutely essential for the hydraulics specialist to visit, inspect, photograph and carry out (or oversee) the site survey for both an Intermediate and Comprehensive Ecological Reserve. As emphasized previously, it is completely impractical, when dealing with low flows, to work from contour maps or contoured site plans alone.

R17.5.2 Site Survey

The accuracy of the survey (cross-sectional profile and water levels) must be comparable to the level of accuracy with which results are presented in the starter document and at the specialist meeting. For example, it is fallacious to predict changes in discharge due to 1cm variations in river stage when water levels are accurate to, say, only +5cm.

R17.5.3 Observed flow data - stage and discharge

In general, the more observed stage versus discharge data available, the higher the confidence is in the hydraulic relationships derived. Every possible opportunity should be taken to visit the site and gather such data.

Sites characterised by prismatic river cross-sections and uniform flow conditions require relatively fewer observed data points than more complex sites containing rapids and riffles, since the hydraulic relationships for the former are relatively easier to synthesize analytically - and consequently more appropriate for an Intermediate level of determination (refer to 4.1).
Ideally, observations should be made at significantly different flow rates. The flow range 0-5 m³/sec is better characterised by flow rates of 0.2, 0.5, 1,2 and 5 m³/sec, than by 0.1, 0.12, 0.15 and 5 m³/sec. This requires careful planning of site visits to maximise the likelihood of procuring a range of discharges. The range of flows is river dependent, and a function of how much each flow increment influences the depth of flow. Some flood-related information (which can sometimes be derived from recording the level of flood-borne debris on the river banks or in trees, or from information obtained from residents living close to the river, related to known flows from the hydrological record) is useful in fixing a high point of the curve in extrapolating the observed low flow data.

Intermediate Ecological Reserve

• 
  Absolute minimum: one data point at an appropriate low flow, plus stage of zero discharge.
  

• 
  Acceptable: absolute minimum condition, plus one data point within the intermediate to high flow range.
  

Comprehensive Ecological Reserve

• 
  Absolute minimum: one data point at an appropriate low flow, plus stage of zero discharge
  

• 
  Acceptable: three points distributed over the low flow range of interest, plus stage of zero discharge, plus some flood-related data if possible.
  

• 
  Ideal: six data points, good distribution, plus stage of zero discharge, plus some flood-related data.
  

R17.6 Summary of what to include in starter documents, and how this is used by specialists

The following is a list of what is required in the starter document:

• 
  Brief summary of the method used to determine the rating relationships.
  
• 
  Record of techniques used to collect discharge data.
  
• 
  Tabulated observed discharge versus flow depth and resistance coefficient data.
  
• 
  Plots of the rating relationships on log-normal scale showing the observed data, and the cross-sections with annotated salient features (refer to Fig. 2) for different flow ranges (i.e. low, intermediate and high).
  
• 
  Plots of discharge against flow depth (maximum and average), average velocity and wetted perimeter (refer to Fig. 3) on normal scale axes.
  
• 
  Explicit estimate of the accuracy in the rating relationships over the range of observed data. This may be assessed by calculating the average absolute difference in flow depth (or discharge) between the observed and modelled data.
  
• 
  An assessment of the confidence in the extrapolated rating relationships.
  

R17.7 Roles and responsibilities at the specialist meeting

• 
  Explanation of data and information presented. Interpretation of data and assistance to other specialists in deriving useful and appropriate information from the hydraulic data.
  
• 
  Assessment of levels of confidence in data.
  
• 
  Assessment of the limitations of data.
  
• 
  Transfer of technology.
  

R17.8 Roles and responsibilities after the specialist meeting

At this stage in the development of the IFR process its credibility with water resource managers depends to a large extent on the quality and reliability of the hydraulic information. The fundamental importance of reliable hydraulic information to the work of the other specialists involved in the process demands that results of the highest possible quality, in terms both of completeness and confidence, should be presented at the specialist meeting. Shortcomings in the hydraulic information will almost always militate against a successful outcome, and it should never deliberately be assumed that hydraulic inadequacies can be made good at some later stage.

Post-specialist meeting hydraulic work in connection with monitoring the effectiveness of the specified IFR to achieve the objectives of the modified flow regime is discussed in 13 following.

R17.9 Example of terms of reference for hydraulics specialists

The terms of reference for the hydraulics specialist have been discussed previously, in the descriptions of the tasks necessary to carry out the study. The terms of reference should, however, in addition, provide an explicit breakdown of the allocation of resources (time and rate of remuneration) for the essential tasks, including:

• 
  Site selection (CER - 5 days/4 sites; IER - 3 days/2 sites, including survey)
  
• 
  River channel cross-sectional and longitudinal profile surveys (CER - 1 day/site)
  
• 
  Field trips for collection of hydraulic data (CER - 3 days/4 sites, IER - 1.5 days/2 sites )
  
• 
  Reduction of survey and hydraulic data (CER - 0.5 day/site visit/4 sites, IER - 0.5 day/2 sites)
  
• 
  Hydraulic analysis and modelling (CER - 2 days/site, IER - 2 days/2 sites)
  
• 
  Reporting (CER - 2 days/4 sites, IER - 0.5 day/2 sites)
  
• 
  Specialist meeting (refer to supporting documentation for specialist meetings)
  
• 
  Post-specialist meeting activities (e.g. additional data collection and refinement, monitoring and scenario meetings).
  

Notes:

• 
  The time allocations for tasks involving field work (site selection and data collection) are dependant on the location of the river system and site accessibility, and the allocations provided above are therefore approximate.
  
• 
  Allowance must be made for an assistant in undertaking the survey and hydraulic data collection. The minimum time required for collection of hydraulics data is half a day per site, with a reasonable allocation of one day per site to complete the all of the tasks described in this document.
  

R17.10 Minimum and optimal specialist training

If the results of the hydraulics task are to successfully support the work of the other specialists, all of its aspects - site selection, site survey, discharge measurement, and the analysis of hydraulic data - require a high degree of specialist knowledge, expertise and experience, particularly in the requirements of low flow work. Furthermore, the limited hydraulics data used for the generation of hydraulic information in the IER (2 rating observations) is likely to place greater reliance on specialist expertise and previous experience than for a CER where allowance is made for 4 rating observations over a longer time frame thus improving the likelihood of recording flow related data over a wider range of discharges.

It is however possible that local non-specialist assistance can be enlisted in gathering water level and flow rate data, and a photographic record. This can provide additional observed data points whilst reducing the number of site visits by the hydraulician. Extreme care must however be taken with the selection and training of such assistance, since experience has shown that the integrity of data cannot be assured, with potential dire consequences for the IFR study.

To illustrate, one such attempt was made in which the staff of a nature reserve area, through which the river in question ran, were asked to observe and record relevant data. The intention was to supplement a limited number of on-site observations made by specialists, to obtain a record for the whole annual flow cycle. Although considerable efforts were made to define the necessary sequence of tasks (verbally on site, and in writing using illustrations, as simply as possible, the resulting data proved to be of virtually no use in augmenting data gathered by hydraulics specialists. This was by no means due to lack of intelligence on the part of the people involved, who were all well-educated environmentalists, nor from a lack of understanding of what the objectives of the exercise were, but arose from an understandable unfamiliarity with the hydraulic concepts involved. Such attempts should not be made lightly, nor should great reliance be placed on the results.

R17.11 Potential pitfalls

The potential pitfalls have been discussed in the relevant sections of this document, with the following concerns deserving re-emphasis:

• 
  Inappropriate location of cross-sections (biotic and hydraulic).
  
• 
  Surveys undertaken in insufficient detail and not linked to a common datum.
  
• 
  Inability to relocate fixed bench marks.
  
• 
  Inaccurate stage and discharge measurements.
  
• 
  Insufficient data over a limited flow range.
  
• 
  Inappropriate hydraulic analyses, particularly for low flows.
  
• 
  Sites too hydraulically complex to achieve a reasonable level of confidence for an IER (refer to Fig. 1).
  

R17.12 Further developments

As discussed previously, spatially linked two-dimensional cross-sections are used to describe the river geometry and relationships between discharge and various hydraulic determinants. Consequently, the determination of the IFR is strongly focussed on the geometry of two-dimensional river sections. In order to provide a less spatially fixed description of the site, recently (Birkhead, 1998) flow depth data have been analysed as frequency distributions of available depth across river sections as a function of discharge.

Similar analyses for depth-averaged velocity would provide useful information for invertebrate and fish ecologists, but is only advisable using measured velocity data, due the complexity of velocity distribution across non-prismatic channel sections. In order to further develop the role of hydraulics in IFR determination using the BBM, is it therefore necessary to invest the time required to collect these data.
The application of full three-dimensional modelling techniques in order to obtain an improved characterisation of the hydraulic conditions across the sites is prohibitive, primarily in terms of the data collection requirements for an IFR type analysis. There is potential, however, to extend the existing hydraulic modelling to provide data on habitat availability in quasi three-dimensions by integrating the hydraulic characterisation with habitat mapping between cross-sections. Broadly, habitat mapping provides a plan description of the habitat conditions at the sites, and the hydraulic analysis a continuous longitudinal water surface profile between cross-sections. Consequently, it is possible to model (based on a common or water surface profile datum) the changes in flow depth and velocity at the mapped sites.

R17.13 Monitoring

The overall objective of monitoring is to assess the effectiveness of the specified Instream Flow Requirements in maintaining the existing condition, or to achieve the desired condition of the river. Two phases are envisaged: establishment of baseline conditions, followed by monitoring to detect changes in the baseline conditions.

Ideally the baseline conditions, defining the hydraulic characteristics of all the sites for a full range of flows, should have been determined prior to the specialist meeting. If this was not possible for any reason, baseline monitoring should include collection of additional data to complete the hydraulic characterisation. This could relate to supplementary observations to augment an incomplete flow range, or repeat observations to raise confidence in a data set in which doubt arose at the specialist meeting. Additional data collection at IER IFR sites following the specialist meeting may not only be used to refine the flow recommendations, but also serves to provide information for a CER that may be undertaken at a later stage. If very high flows occur subsequent to the specialist meeting and are believed to have altered the morphology of the channel, and consequently the site’s hydraulic characteristics, resurvey followed by full hydraulic analysis will be required to establish a new baseline condition.
Long-term monitoring involves repeat visits (regular, and also after high flow events) to the sites to resurvey the cross-sections to detect morphological changes and collect flow-related data, followed by re-evaluation of the hydraulic relationships.
Hydraulic monitoring relates closely to hydrological monitoring, the aim of which is to determine the degree of concurrence between the flow regime specified by the IFR and that which actually eventuates.

R17.14 Conclusions

Experience with the determination of the Instream Flow Requirements of South African rivers has repeatedly shown that the availability of reliable hydraulic information is of quintessential importance to the success of the process as a whole. If biologically relevant data in terms of depths and velocities of flow cannot be related to rates of flow, the instream flow requirements cannot be quantified. Although the work required to gather and analyse hydraulic data can be costly and time-consuming, especially in rivers in remote areas, or where access is difficult, the investment pays rich dividends in confidence. It is infinitely preferable to undertake this work prior to the IFR specialist meeting, where it can be confidently used by the other disciplines, rather than to enter the meeting with inadequate or unreasonably low confidence information, thereby having to subsequently revisit the entire procedure with improved hydraulic information.

References & bibliography

Barnes, H.H. (1967). Roughness Characteristics of Natural Channels. U.S. Geological Survey Water Supply Paper 1849. U.S. Geological Survey, Washington D.C., 1-9.

Birkhead, A.L. (1998). Mkomazi River Instream Flow Hydraulics. In: Site Visit Document for the 1998 Workshop to Identify the Instream Flow Requirements of the Mkomazi River, Scottburgh, Kwa-Zulu Natal, 24-27 March 1998.
Birkhead, A.L. and James, C.S. (1998). Synthesis of rating curves from local stage and remote discharge monitoring using non-linear Muskingum routing. Jour. Hydrology, 205, 52-65.
BS 3680, British Standard for the Measurement of Liquid Flow in Open Channels. Part 2: Dilution Methods. Part 3A: Velocity-area Methods.
Gordon, N.D., McMahon, T.A. and Finlayson, B.L. (1992). Stream Hydrology: An Introduction for Ecologists. John Wiley and Sons. 420 pp.
Henderson, F.M. (1966). Open Channel Flow. Macmillan. N.Y. 522 pp.
Hicks, D.M. and Mason, P.D. (1991). Roughness Characteristics of New Zealand Rivers. Water Resources Survey Paper, DSIR, 1-13.
Rowlston, B., Jordanova, A. and Birkhead, A.L., in prep. Hydraulics chapter for the Building Block Methodology used in Instream Flow Reqirement assessment. Water Research Commission Report.
Tharme, R. (1996). Review of international methodologies for the quantification of the Instream Flow Requirements of rivers. Draft report.
Ven Te Chow (1959). Open Channel Hydraulics. McGraw-Hill, N.Y. 680 pp.

Glossary of terms

Flow/flow rate/discharge - Volumetric flux per unit time past a fixed point along the river (m³/s).

Flow velocity - Speed at which water moves per unit time past a fixed point in a given direction (m/s).
Flow depth - Vertical distance between the water surface and bed at a given point in the river channel, or the maximum depth along a cross-section (m).
Resistance - Overall resistance to flow imposed by the river channel, including all resistance components, e.g. bed roughness, vegetation, channel plan form, etc. (Manning’s n - s/m^1/3; Chézy’s C - m^1/2/s; Darcy-Weisbach’s f - dimensionless).
Roughness - Resistance imposed by the fluvial sediments.
Stage/water level - Elevation of the water surface relative to a datum (m).
Surface width - Horizontal extent of the water surface, measured along the channel cross-section (m).
Wetted perimeter - Amount of channel in contact with flow, measured along the cross-section (m).

1 Editor’s note: Use of terminology IFR/BBM/Reserve is inconsistent and must be reviewed for version 1.1

2 Editor’s note: Here IFR is used to indicate “the water quantity component of the ecological Reserve”

3 Editor’s note: See note 1 above.

4 Editor’s note: Author to check and correct cross-references.

5 Editor’s note: Author to check all cross-references

R17/
Department of Water Affairs and Forestry, South Africa

Version 1.0: 24 September 1999

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