Introduction

First you will need to look at the catchment upstream of your proposed scheme. Important information you will need about your catchment is:

• The scheme location (where it is);
• The catchment area (the extent of land where rainfall/precipitation drains downhill into the river draining to the location of the scheme);
• The catchment rainfall (rainfall falling within the catchment area);
• Is there a gauging station nearby? (can you use any nearby measured flow data); and
• Artificial influences (is the flow in your river influenced by the activities of man).

All of this information is important if you would like to estimate your mean flow using the methodologies described within this scoping study.

Scheme location

Your first step is to look at your proposed scheme on an Ordnance Survey (OS) map (the 1:50,000 or finer resolution maps are ideal). You can use websites such as magic or Streetmap, which you can enter the nearest town or postcode. You can obtain the grid reference of your potential outlet from the map.

Catchment area

There are a number of websites that allow you to measure an area by drawing a polygon on the map. Look at the rivers flowing down to your proposed outlet and visualise the highest points in the area surrounding them. A topographic catchment area can be drawn starting at your proposed outlet (scheme location), moving uphill to the highest point (the watershed) and following the highest points around the rivers until you reach your outlet again. Figure 1.2 shows an example of a catchment boundary with the contours showing the shape of the hills surrounding the rivers flowing down to an outlet.

Please note that an accurate definition of the catchment area is an essential step. An inaccurately defined catchment will severely constrain the accuracy of any of the subsequent analyses.

Catchment rainfall

The amount of rainfall falling within your catchment will be a key factor in determining the volume of water which will flow past your proposed scheme. There is a well-established network of rain gauges throughout the UK maintained by the Met Office which has been used to develop national maps of rainfall. As each rain gauge will have data for different periods of time, the national maps have been standardised over set periods of time (SAAR – Standard Average Annual Rainfall); periods commonly used are 1941 to 1970 and 1961 to 1990. Figure 1.3 can be used as a rough estimate of the rainfall in your catchment. Note that the differences in rainfall over these two periods are unlikely to affect the outcome of the rough estimate of river flow calculated within the scoping study.

Is there a gauging station nearby?

There is a network of almost 2000 gauging stations gauging river flows within the UK. Within England and Wales the gauging station network is run principally by the Environment Agency (EA). If there is a gauging station nearby you may be able to use this data to estimate the river flow in your catchment. Daily and monthly river flow data is available for over 1300 of these gauging stations from the UK National River Flow Archive (NRFA), and of these almost 150 of these are in the North West.

Using the NRFA website you can view summary statistics and descriptions of each of the gauging stations. You can view a map of the North West showing the locations of gauging stations (select the UK Gauging Station Network, then regional maps showing gauging station locations and clicking on the North West). From this map you can find the nearest gauging stations to your proposed schemes. This is an interactive map and you can click on the gauging station to view the station's summary statistics and descriptions.

Note that the gauging stations held by the NRFA are those with good quality long term records, but the EA do run further gauging stations. If you wish to gain information on a gauging station not held by the NRFA you can contact the EA (contact information is provided on the EA website).

Artificial influences

The majority of UK rivers are influenced to a greater or lesser degree by the activities of man; termed artificial influences. Common artificial influences include direct abstractions from surface waters, abstractions from ground water, discharge of effluents, impounding reservoirs, canals and inter-basin transfers.
In natural rivers, the magnitude of river flows are determined by climatic and runoff generation processes, amongst which effective rainfall, groundwater recharge and aquifer properties exert a dominant function. However, as a result of the development of rivers and catchments for a variety of purposes, including water supply and transport and dilution of effluent, few rivers now possess natural river flow regimes.

There are a number of ways to find out what is influencing your catchment. The best source of information is the EA Catchment Abstraction Management Strategies (CAMS). CAMS are six-year plans which record how the EA will manage water resources in your area. Your local CAMS document is an invaluable source of information, not only for finding out what already influences your catchment; but how the EA plans to manage water resources over the following six years. You can find out about the CAMS in your area from the EA website.

The NFRA website holds summaries of gauging stations which contain catchment descriptions. If there is a gauging station downstream, or upstream to some extent, of your proposed scheme you could use their descriptions of what is influencing the catchment. Select the gauging station and the Factor Affecting Runoff section will describe any major artificial influences on that catchment.

You could also use local knowledge and OS maps to find, for example, impounding reservoirs, sewage treatment works, factories, etc.

The aim of this section is to see if there is any measured flow data you can use to estimate your mean flow. If measured flow data exists you may be able to transpose the gauged (or analogue) flow data to your proposed schemes catchment, where you have no measured flow.

Generally, to use data from nearby gauging stations you need to ensure that it receives a similar amount of rainfall (climatic conditions) and has similar geology and soils (hydrogeology) so that the catchment responds to the rainfall in a similar way. Considering proximity is usually a good position to start from.

The estimation of river flow for ungauged sites by transposing gauged river flow data from an analogue catchment is a widely used technique requiring the re-scaling of the flow regime to the ungauged target catchment. If there is a short period of measured flow data at the proposed scheme, you can build these data into the transposition methods through the use of regression based relationships between the flows for the target and the analogue catchments (e.g. Shaw, 1988). Please click here (link 1.4.3) for details of transposing flows to short-gauged flow records.

Selection of an analogue catchment

Due to the large spatial variations in rainfall and hydrogeology across the UK, transposition techniques all attempt to adjust the flow record for the analogue for differences in hydrological scale (differences in rainfall and catchment area) between the analogue and ungauged catchments. An analogue catchment is usually a catchment that is:

• Geographically close to the ungauged catchment, hence has the same climatic regime; a general rule of thumb is within 50km. You can use the NRFA website interactive map scale to estimate the distance between your proposed scheme and gauging station, or determine the exact distance using the grid references.
  
  
• Similar rainfall. You can use your estimate of SAAR for your catchment, and SAAR is provided on the NRFA website for gauging stations.
  
  
• Hydrogeologically similar. The geology of the catchments may be found by consulting the relevant Catchment Abstraction Management Strategy (CAMS) documents which should provide descriptions of the hydrogeology within the area of interest (see the EA website). A description of the geology is provided by the NRFA website for gauging stations. Using these sources of information you can use your own judgement as to whether the two catchments are hydrogeologically similar.
  
  
• Similar in size; a general rule of thumb is the catchment size should be within a factor of two. You can use your calculated catchment area and catchment area provided by the NRFA website for gauging stations.
  
  
• Either a natural catchment (no major artificial influences), or a catchment for which there is a naturalised flow record. You can use your assessment of artificial influences in your catchment and a description of influences is provided by the NRFA website for gauging stations.
  
  
• Ideally, the analogue catchment should lie upstream or downstream of the ungauged catchment (termed connected).

In the case of a connected analogue, there is a strong serial correlation between the flow measured at the analogue gauge and the flows at the ungauged site as the water that flows past both points has a common component. It is quite common that there is no connected analogue available, thus a catchment from an adjacent system or tributary for the same system is selected as the analogue for the ungauged catchment. For this case, the analogue would be termed unconnected.

These are relatively stringent requirements and in many cases it will be necessary to relax these criteria with the consequence that the choice of analogue catchment might not be as good. The technical report discusses this further.

You should now have a list of potential analogue gauging stations for your proposed site. If there is more than one potential gauging station, use your judgement to decide which is most similar to your catchment in terms of meteorological conditions, gauging stations directly upstream or downstream would be ideal.

Obtaining the analogue measured flow data

A time series download facility has been developed by the NRFA, to provide access to full daily flow datasets for around 200 representative gauging stations throughout the UK.

The ‘River Flow Data: Time Series Downloads’ is available from the NRFA website.

You can obtain river flow series held by the NRFA but not currently included in this download facility through the NRFA Data Retrieval Service.

If you wish to gain information on a gauging station not held by the NRFA you can contact the EA.

Transposition of flow statistics from an analogue catchment

In the UK it is common practice to transpose flow statistics through rescaling by area and average annual rainfall using:

QX_T = (A_TSAAR_T/A_ASAAR_A) QX_A

Where:

• QX_T = the flow in the target ungauged site, T;
• QX_A = the corresponding flow in the analogue catchment, A;
• A_T = the catchment area for the ungauged site;
• A_A = the catchment area for the analogue catchment;
• SAAR_T = the Standard Average Annual Rainfall for the ungauged site;
• SAAR_A = the Standard Average Annual Rainfall for the analogue catchment.

The uncertainties in this estimate of mean flow are dependent upon both the accuracy of the estimates of catchment area and rainfall and the similarity between your catchment and the analogue gauging station. You could complete some sensitivity testing, through changing your catchment area and rainfall to get an idea of the impact of the uncertainty of these to your mean flow estimate. Unless you have used a connected nearby gauging station and you are confident your estimates of rainfall and catchment area are accurate, then this rough estimate of mean flow should be improved upon using a regional model within Tier 2.

You can calculate a rough estimate of your mean flow using your estimates of catchment area and rainfall:

MF = ((SAAR-PE) x 1000 / 365 x 24 x 3600) A

Where:

MF = the flow in your catchment (m³/s);
SAAR = the Standard Average Annual Rainfall for your catchment (mm);
PE = potential evaporation (mm);
A = your catchment area (km²).

An approximate value of potential evaporation in the North West is 450mm.

The uncertainty in this estimate of mean flow is dependent upon the accuracy of your estimate of catchment area and rainfall, and the approximation of potential evaporation. Please note that this is an estimate of your natural flow and you will need to take account of any artificial influences within your catchment.

Introduction

If you would like an idea of the magnitude of a low flow within your catchment, and hence an idea of the residual flow you can measure the flow in a river using a variety of methods. Please note that these methods will only give you a very approximate low flow. For instantaneous measurements of flow, you should use the velocity-area method where you estimate the cross-sectional area of the river and the velocity through that cross-section. The flow/discharge is the velocity multiplied by the area. To measure a low flow you should wait for a period of at least a week without significant rainfall to obtain an idea of the low flows.

Estimation of cross-sectional area

The simplest method to estimate the cross-sectional area (m²) of your river is to measure the width (m) and multiply by the average depth (m).

Estimation of velocity

The velocity of the river can be estimated using a variety of different methods. Instantaneous measurements can be made using the following methods:

• By float
• By current meter (mechanical, electromagnetic or acoustic doppler)
• By dilution gauging

The simplest method to measure the flow is to use the float methodology. The current meter or dilution gauging methodologies should only be used if you fully understand the methodology; you will normally need to employ a hydrometrist experienced in using these techniques.

The float methodology is a very approximate estimate of velocity. For this rough preliminary survey of flow, the surface velocity can be measured with a float, such as a block of wood or an orange. You will need two people and:

• a measuring tape to measure the distance downstream and width;
• a measuring rod to measure water depth;
• a stop clock; and
• a number of floats of the same size and shape.

Drop the float into the centre of the river and over a known distance, measure the time it takes to float from the start to the finish. Calculate the velocity in meters per second. Repeat this process several times over the same distance and calculate your average velocity.

These measurements only give the surface velocity and a correction factor must be applied to give the average velocity over the depth. British Standard BS 3680 recommends a factor of 0.7 for rivers with 1m depth and 0.8 for rivers with over 6m depth.

Estimation of flow

Once you have your average velocity over the depth (m/s) and cross-sectional area (m²), you can multiply the two together to provide your flow (m³/s). Please note that there is quite a high hydrometric uncertainty associated with this method.

Following the scoping study you will have information about your catchment; either one or two estimates of your mean flow and an estimate of a low flow. This may be fed into the other aspects of the assessment to determine the feasibility of the scheme.

At this point there is very likely to be considerable uncertainty associated with the estimates of both mean and low flows and this should be taken into account within the assessment. As a general rule of thumb, the uncertainty may be halved through proceeding to the next tier.

You can estimate your flow duration statistics using the methodology presented. As stated earlier, the uncertainties associated with this method are dependent upon both the accuracy of the estimates of catchment area and rainfall and the similarity between your catchment and the analogue gauging station.

Unless you have used a connected gauging station with a catchment area within a factor of two and you are confident your estimates of rainfall and catchment area are accurate, then the uncertainty associated with this is much higher than using the LowFlows regional model.

Information on the magnitude and variability of flow regimes, at the river reach scale is a central component of most aspects of water resource and water quality management. The natural variability of flows in the UK is dependent, at the broadest scale, on climatic variables such as rainfall, temperature and evaporation. At a catchment scale, physical characteristics such as hydrogeology also exert control.

For example, flows are less variable in base-flow-dominated chalk catchments than in impermeable clay catchments that respond rapidly to rainfall.

Regionalised models have been developed at CEH to describe the natural variability of flows, in the form of FDCs for any river reach in the UK. This model was implemented within the LowFlows software system. Recent model developments by WHS have been implemented within the latest version of LowFlows (further details available within the Technical Report and on the WHS website.

Following Tier 2, you should now have both the mean flow and flow variability. The uncertainty should be reduced compared with the results developed during the Tier 1 investigation.

These results can be used to assess, with more confidence, the feasibility of the hydropower scheme.

Estimates of the FDC can potentially be further improved through taking spot current meter flow measurements and assigning the flow exceedance percentiles from a suitable gauged natural analogue catchment. The methodology is summarised below. However this method is not error-free, and the uncertainty associated with estimating the flows at specific percentile points is then discussed.

It should be noted that derivation of a flow duration curve from a series of spot current meterings alone would require a very large number of spot current meterings to be made. Furthermore, the metering would need to cover the entire range of flows at the ungauged site. In practice, spot current metering exercises often focus on a number of points on a flow duration curve. Commonly spot gauging will be used in conjunction with the desk based estimates.

The estimation of a Flow Duration Curve

Flow duration statistics can be derived by assigning exceedance percentiles to spot flow measurements. The method consists of the following steps:

• Select an analogue gauging station catchment (link to section 1.1.1 for guidance) with a well established long term flow duration curve which encompasses the period of the spot gauges.
  
  
• For your catchment take spot gauge measurements of flow. Measurement approaches include the use of an impeller type or ultrasonic current meter or dilution gauging (link to section 1.2.3). To enable an approximation of the flow duration curve to be made the flow measurements should be made at a wide range of flows. However, if the specific design problem is in the low flow range then more metering should be carried out at low flows.
  
  
• Exceedance percentiles are derived from the flow percentile corresponding to the flow measured at a suitable gauged natural analogue catchment on the day of measurement. Plot your measured flow against the assigned flow percentile, with your estimated FDC. The flows at the two sites are assumed to be synchronous. Once a suitable analogue catchment has been identified, the uncertainty associated with the estimation of the FDC is a function of the number of spot current meterings taken for a specific percentile point.
  
  
• You will need to take account of the sources of error associated with the estimation of flow duration curves using spot current metering as described in the following section.

Uncertainty and sources of error within the use of spot current meter readings

The technique described for generating an estimate of the flow duration curve using spot gaugings is subject to three sources of error:

• Error 1: The hydrometric error associated with the estimation of river flows using spot current metering techniques.
  
  
• Error 2: The assumption that the spot current meter flow is representative of the mean flow within the day, in the ungauged catchment.
  
  
• Error 3: The error associated with the assumption that the flow within the ungauged catchment corresponds to the same flow percentile observed within the analogue catchment.

Introduction

For larger schemes it is recommended that the developer’s measure flows continuously at the site of interest for as long as possible; no less than 6 months and preferably for a year or more. The longer the period of measurement the more certain the estimate of resource availability.

It is important that the regulator is contacted prior to continuous monitoring to ensure that their requirements are met as monitoring can become both expensive and time-consuming.

Flow duration statistics may be calculated directly from the short record. Whilst these data have undoubted value being direct measurements of flows at the design site, the flow duration statistics compiled from these data are very sensitive to sampling errors when used to represent the long-term resource availability.

The aim of this section is to provide guidance on the methodologies that may be used, but these should only be used by a qualified professional.

Typically the standard error of estimates for MF and Q95 statistics drawn from 1 year of data is between 20% and 50% respectively within the type of catchment used for hydropower development. Use of short records must therefore be analysed in the context of both the seasonal and annual variability of the climatology at the site.

Methodologies

If a suitable analogue catchment exists, which has a long flow record overlapping the short flow record at the site of interest, then it is possible to address the limitations associated with the short record by synthesising a flow record at the site of interest from the long record. The following three methods may be used to extend your short record:

1. Record extension using FDC ratios;
2. Record extension using regression; and
3. Record extension using matching pairs.

Method 2 is very dependent on the synchronicity of the flows within the two catchments and hence is only applicable to the relatively close analogues, whereas Methods 1 and 3 are generally dependent upon the climatology being generally similar over both catchments.

The uncertainty associated with these methodologies is highly dependent upon the length of the record and where possible the length of record should be maximised. As a rule of thumb, the absolute minimum length of record used with these methodologies is one complete year for method 1 and 6 months for methods 2 and 3. These short records should also be representative of an average year and if the period of measurement is particularly dry or wet then measurement should be extended to a longer period of time.

If you are interested in low flows, a fourth method may be used to estimate the Q95 from BFI. For this methodology, the same principles of record length apply with a minimum of one year and preferably two years of continuous measurement.

You should now have an estimate of the mean flow and flow variability for scheme. It is possible that a number of estimates of these are now available and through assessing each of the estimates, with respect to the uncertainties associated with each methodology, a reliable estimate of the resource availability should now be available.

This may be used within the subsequent assessment to improve both an assessment of the feasibility of the scheme, and to fine tune the design of the scheme to ensure that the scheme is developed to be efficient and cost effective.