CEN/TR 17798:2022

SLOVENSKI STANDARD
01-junij-2022
Optimalno načrtovanje hidrometričnih omrežij
Optimal design of hydrometric networks
Hydrometrisches Datenetz und Optimierung
Conception optimale des réseaux hydrométriques
Ta slovenski standard je istoveten z: CEN/TR 17798:2022
ICS:
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN/TR 17798
TECHNICAL REPORT
RAPPORT TECHNIQUE
April 2022
TECHNISCHER BERICHT
ICS 07.060
English Version
Optimal design of hydrometric networks
Conception optimale des réseaux hydrométriques Hydrometrisches Datenetz und Optimierung

This Technical Report was approved by CEN on 27 March 2022. It has been drawn up by the Technical Committee CEN/TC 318.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 17798:2022 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Nomenclature . 6
4.1 Categories . 6
4.2 Geophysical and other restraints originating from the river basin . 6
4.3 The need for data and information . 6
4.4 Technical and economic considerations due to data capture, data processing and data
archival . 7
5 Strategic considerations . 7
5.1 The need for hydrometric data . 7
5.2 Network requirements . 8
5.3 Existing and potential users of the catchment data . 8
5.4 Water resources utilization and demand for water . 9
5.5 National and international needs . 10
5.6 Consideration of other hydrological monitoring networks . 11
6 Factors affecting hydrometric design . 11
6.1 Patterns of runoff . 11
6.2 Catchment morphology . 11
6.3 Winter conditions and snow melt . 11
6.4 Availability of surrogate gauged catchments . 12
6.5 Environmental and legislative constraints . 13
7 Technical considerations . 13
7.1 Use of temporary networks . 13
7.2 Choice of measuring technique . 14
7.2.1 General. 14
7.2.2 Open channel flow measuring techniques . 14
7.3 Permanent flow measuring structures . 15
7.3.1 General. 15
7.3.2 Accessibility . 16
7.3.3 Length and quality of established data records . 16
7.3.4 Distribution of gauging stations . 16
7.3.5 Representative basins . 16
7.3.6 Coastal floodplains and other low gradient environments . 17
8 Methods of network design . 17
8.1 User survey. 17
8.2 Prioritization . 17
8.3 Physiographic impacts . 18
8.4 The use of deterministic models to inform hydrometric network design . 18
8.5 Statistical techniques . 19
8.6 Optimization and review . 20
9 Addressing uncertainty in network design . 21
9.1 The concept of uncertainty . 21
9.2 Uncertainty inherent in hydrometric networks . 21
10 The socio-economic importance of the network . 22
10.1 Techniques to justify a hydrometric network – use of cost benefit analyses . 22
10.2 Techniques to justify a hydrometric network – evaluating the strategic value of a
network . 22
10.2.1 Data requirements . 22
10.3 Network reviews and cost benefit analysis . 24
10.4 Socio-economic costs of not having hydrometric data . 25
11 Ensuring sustainability . 26
11.1 The sustainability of the network. 26
11.2 Carbon footprint and maintaining sustainability . 26
11.3 The impact of climate change and change in land use . 26
12 Decommissioning sites in a network . 27
Annex A (informative) Typical operating costs of a hydrometric network . 28
Bibliography . 29

European foreword
This document (CEN/TR 17798:2022) has been prepared by Technical Committee CEN/TC 318
“Hydrometry”, the secretariat of which is held by BSI.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
1 Scope
This document provides guidance to assist with the planning and design of hydrometric networks, to
ensure a better understanding of the water cycle and that any data are observed and collated in an
effective and appropriate manner. This document is intended for use when:
• a new network is being planned and designed;
• the nature, value and extent of an existing network is being reviewed;
• a redundant network is being decommissioned or modified.
This is to ensure that the impacts of these changes are considered objectively, and all changes are
adequately monitored and recorded.
Even though this document covers network design principles in general it focuses mainly on river
(streamflow) monitoring networks.
This document covers all aspects that are considered pertinent to the design of hydrometric networks.
The guidance is intended to be used to inform the decision-making process employed by the network’s
owners and operators. The objective nature of the review will ensure that all influential factors, both
beneficial and otherwise, are considered. This will ensure that primary and potential alternative uses of
the network are considered. It will also ensure compliance with any extant environmental legislation.
The intended audience for this document may include:
• Government, Non-Government Organizations (NGOs), agencies and other organisations which are
responsible for designing and developing hydrometric networks that provide data to support a
public service.
• Research and academic institutions that aim to develop a better understanding of the natural and
human influences on the hydrological cycle.
• Developers of the built environment seeking to comply with environmental legislation that requires
them to monitor those parts of the natural hydrological cycle that have been, or will be, impacted by
their activities.
• Any individual seeking a better understanding of the water cycle for private and personal reasons.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
4 Nomenclature
4.1 Categories
There are three categories of terms and nomenclature:
• those relating to the geophysical and hydraulic nature of the river basin;
• those relating to the need for data and information about the flow regime in the catchment;
• those relating to the processing and provision of the data and information.
4.2 Geophysical and other restraints originating from the river basin
1) Patterns of runoff and snow melt: where rainfall and snow melt occur in the catchment, producing
variations in runoff, including the long-term trends
2) Winter conditions: where permanent or semi-permanent snow fields exist in mountainous and polar
regions
3) Patterns of evapotranspiration: where rainfall totals are low and where effective rainfall can be
negligible due to high losses through evapotranspiration
4) Catchment morphology: the river pattern and nature of the catchment and its effect on average and
extreme values of discharge, stage (or river level), plant growth, temperature, water salinity and
turbidity
5) Accessibility of the catchment: particularly in remote areas and wilderness where road and transport
routes have not been developed
6) Environmental constraints: limitations on access due to environmental restrictions such as working
near or in SSSIs (Sites of Special Scientific Interest), Ramsar sites, World Heritage Sites, or more
generally at or near sites where protected fauna and flora are found
7) Legislative constraints: limitations on working due to temporary or permanent rulings, Acts, Orders
or Licences
4.3 The need for data and information
1) Existing and potential uses of the catchment: uses include water resource management, water
resource planning, strategic environmental assessments, drought contingency planning, flood
forecasting and warning, development of navigation
2) Hydrometric parameter management: maintained water level or stage, discharge regulation,
sediment management, water quality and water pollution management
3) Required data availability and data accuracy: requirements of data in terms of temporal and special
availability and accuracy
4) Available budget: justifiable and agreed expenditure on developing and managing the network
4.4 Technical and economic considerations due to data capture, data processing and
data archival
1) Existing related networks and established requirements: what hydrometric networks and databases
already exist, and what the original purpose for their development was
2) Length and quality of established data records: what periods of time do these databases cover and
how accurate and representative they are
3) Use of temporary networks to inform the design of specific engineering schemes: are there any existing
or abandoned temporary networks that were established for a specific survey for which useful data
exist on archive
4) Use of a permanent measuring site or structure: the existence of a measuring site that has a usable
flow record relevant to the current need
5) Choice of flow measuring method: whether to install a flow measuring structure to measure flow with
potentially greater accuracy of measurement, or the use of less expensive and potentially less
accurate open channel flow measuring techniques
6) Capital cost of the hydrometric network: the cost of construction and development of the network e.g.
design costs, civil engineering costs, mechanical and electrical costs, technological costs
7) Revenue cost: the ongoing cost of operating and maintaining a permanent network. In the case of a
survey for a specific project, this would be the budget allocated for the operation of the network over
the duration of the project. This may have been capitalized when the project was established
8) Data management requirements to be considered at design stage: how best to operate the network
given all logistical, technological and practical constraints of data capture, data processing and data
archival, over the expected life of the network
9) Use of surrogate gauged catchments to characterize ungauged catchments: the availability of data
from adjacent and similar catchments to provide the required information. This can avoid the cost of
a new network, but may incur reduced accuracy of data and information.
5 Strategic considerations
5.1 The need for hydrometric data
The good and effective management of water and its availability in a catchment is dependent on reliable
hydrometric measurements. The quality of these data is very dependent on the selection of the
monitoring sites and the measurement techniques deployed. These data can serve a number of purposes
that include but are not limited to:
a) water supply – assessment of current and future water availability, design and operation of water
supply schemes;
b) flood management – design and operation of flood mitigation schemes, flood forecasting;
c) drought monitoring, drought forecasting and drought management;
d) effluent discharge and pollution control, and the dilution of effluents;
e) monitoring of abstraction licences and discharge consents;
f) conservation and environmental protection, fisheries management, maintenance of compensation
flows, environmental impacts of major water related developments;
g) hydro-electric power production (HEP) management, assessment of the potential for HEP;
h) navigation and recreation;
i) hydrological research activities including monitoring the impacts of climate change.
Many gauging stations serve more than one purpose and the data from a site can be used for a number of
different purposes. It is essential that all the potential uses and relative value of hydrometric data are
considered when designing, reviewing and assessing networks.
5.2 Network requirements
A hydrometric measuring network is a system of monitoring stations that observe the major parameters
of the water cycle within a river basin. These may include river flow gauging stations, precipitation gauges
and groundwater observation boreholes. These monitoring stations provide the data needed for the
planning, design and management of the water resources of the river system. The data need to enable
accurate estimation of the principal characteristics of the hydrological regime of the river basin and the
impacts of any significant human interventions. The network requirement is greatly influenced by a
number of factors, including:
1) the precipitation pattern in the basin catchment;
2) drainage characteristics of the river basin;
3) the other physical characteristics of the catchment;
4) the purposes for which the data are required;
5) existing and potential uses of the river basin;
6) the availability of financial, skilled staff and other resources.
The selection of hydrological observation sites forming the basic network will be dependent on the above.
The main objective of hydrometric network design is to obtain the maximum amount of useful,
hydrological information for a given investment of time and money.
Hydrometric networks need to have the following characteristics:
a) be fit for the purposes they are intended;
b) be practical and if possible, relatively easy to implement and maintain;
c) be manageable in terms of whole life cost i.e. be possible to determine and hence optimize, the
balance between initial capital outlay and ongoing revenue expenditure of developing and running
the network;
d) be sustainable with as low a carbon footprint as can be achieved. This is of paramount importance.
5.3 Existing and potential users of the catchment data
In order to define the purposes and objectives of the Hydrometric network it is essential to have clearly
defined the existing and potential data users. All existing and potential users of Hydrometric data have to
be identified. The formation of the Hydrological Data Users Groups (HDUG) is a way of facilitating this,
and these have been established by some countries and organizations. Each member of the HDUG needs
to be consulted to establish their data requirements, both in terms of location and frequency. For
example, on a large river which responds slowly to rainfall events a 15-min recording frequency may be
adequate. However, on smaller, urban catchments where the prime purpose of the data might be for the
design of flood protection works, then a 5-min frequency of streamflow measurement might be required.
There is a tendency for the organization, department or group responsible for data collection to
concentrate on activities within their own sphere of interest. However, it is essential when revising or re-
designing the network that all existing and potential users of data are identified. This may require
forward projection of future needs. To assist with this process, it is important to study the basin(s) under
consideration in detail to identify the following factors which could influence the network design:
• proposed water resources development locations;
• major river diversions and off-takes or basin transfers;
• existing and potential Hydro-electric power facilities;
• areas earmarked for industrial development;
• areas of water supply shortages;
• waste disposal sites and areas of contaminated land;
• areas earmarked for de-forestation or re-forestation;
• conservation areas and areas of ecological interest;
• physical characteristics - changes in topography, geology, soils.
5.4 Water resources utilization and demand for water
The effective development of the water resources of a river catchment is heavily dependent on the
availability of flow data at key points within the catchment. Without this information the design of
infrastructure that utilizes and deploys the resource, and the procedures that are put in place to manage
the resource, will have the potential for misuse and the system will be at risk of failure, especially during
periods when droughts reduce the natural base flow in the watercourses. Furthermore, at these times,
serious environmental damage to the river can occur due to over abstraction of water from the rivers.
Typically, the utilization of the water resource of the river system consists of:
• the construction of impounding reservoirs and abstraction facilities to provide potable water for
public water supply;
• the construction of impounding reservoirs and abstraction facilities for hydro-electric power
generation, or cooling water at non-HEP power stations;
• abstraction facilities for other industrial use;
• abstraction facilities for the irrigation of food crops and food processing;
• river regulation to maintain adequate water depth in navigable channels.
Any of the above activities has an impact on the flow regime of a river, but where an impounding reservoir
is built to support this abstraction the impact on river flow can be significant. In all cases, the change to
the natural flow regime must be quantified. This is because the health of the river, and the biodiversity
that it supports, rely on an adequate range of flow over the seasons of the year. For example, the
maintenance of low-flow is essential to support the very basic ecology of the river, especially during hot
seasons when high river water temperature can prove damaging or even lethal to water based fauna and
flora. However, the occurrence of mid- and high-range flows is of equal importance as these conditions
help sustain and refresh faunal habitats and floral variety throughout the watercourse margin.
Additionally, where a watercourse receives discharges of treated effluent, adequate dilution of the
effluent is essential to prevent the damaging effects of chemical pollution and high biological oxygen
demand.
The safe and reliable development of the water resources in a region is therefore heavily dependent on
the availability of good quality flow data. This information needs to be in the form of daily mean discharge
at specific key locations along the river network. Moreover, to enable the determination of the likelihood
of occurrence of specific flow ranges during critical periods, the flow record needs to be of sufficient
length to enable key statistics on the flow distribution to be determined.
The length of the flow record needs to be greater than 10 years and include at least one period when low
flows occur. For example, Severn Trent Water, a utility has a flow database that has daily mean flow at
critical points on its resource rivers, that covers the period January 1920 to present. This period contains
three double season droughts during the years of 1933-4, 1975-6, 1995-6, and several severe single
season droughts, e.g. 1959. Environmental managers and legislators also use similar databases to model
the impacts of low flows to determine the minimum acceptable flow in a river system. This is sometimes
known variously as the ‘environmental flow factor’, the 95 % exceedance flow’ or ‘hands-off’ flow.
Critically, this will include the period of time when the flow is below this minimum flow threshold. As an
example, the minimum flow at a point on a river might be considered to be a set value. Whilst an isolated
day or two when the flow drops below this level might not give too much concern, a prolonged period i.e.
7 to 14 days or longer, might prove seriously detrimental to the river’s ecology. This measure therefore
helps determine the water available for abstraction without the risk of detriment to the riverine ecology.
All these aspects of the development of a river’s resource and its safe use therefore rely on an adequate
hydrometric network that monitors flows in the catchments, and provides a sufficient length of record to
allow meaningful statistical analyses to be made. Nevertheless, there are catchments, particularly in the
developing World, where no data exist, yet there is a need to develop the natural water resources of the
river system.
5.5 National and international needs
Stable long-term hydrometric networks providing data to national and international initiatives are
essential to meet many of the applications outlined in 5.1 and other requirements. National and
international initiatives hold or connect data collected by many different network operators. Through the
collation, integration and sharing of such data, it is often possible to generate more information about the
hydrology of an area/country/region which otherwise cannot be determined by considering local data
alone.
Where data are, or could be, contributing to such large-scale archives it is important that their
requirements are taken into consideration when designing or reviewing hydrometric networks. For
example, large-scale initiatives often prioritize data from monitoring sites which are established for a
long period of time to give information about the long-term hydrological variability of a particular
location.
5.6 Consideration of other hydrological monitoring networks
When designing or reviewing a hydrometric network, it is important to consider the monitoring of any
individual variable within the wider context of other hydrological observations. For example, the true
benefits of a river flow monitoring network may only be fully understood by taking into consideration
the parallel precipitation monitoring network, as more knowledge can be derived from a gauged
catchment where the precipitation inputs are also known. It is only through an integrated assessment of
hydrological monitoring that a full picture of the benefits of any individual observation point can be
understood.
6 Factors affecting hydrometric design
6.1 Patterns of runoff
One of the main objectives of a hydrometric network is to observe and understand the patterns of runoff
in the catchment. These patterns depend primarily on the nature of incident precipitation and snowmelt.
The variations in the runoff caused by these patterns must be clearly understood and quantified before
any development of the water resources of the catchment can take place.
In maritime regions, patterns of runoff may vary with, for example, the orographic rain shadow effect of
a mountain range. However, in more continental regions, rainfall is more commonly associated with
convective storm cells. Hence the impact of these variations must be observed before any development
of the water resources is made. Furthermore, to quantify the likelihood of extreme events affecting the
operation of any such development, a representative period of flow record is necessary to define the
statistical variation of flows within the catchment.
6.2 Catchment morphology
Streamflow gauging stations forming part of a hydrometric network may need to be established where a
significant change occurs in the morphology of the catchment. This will allow the impact of this change
on stream flow to be observed.
Similarly, the transition from an upland stream to a lowland river, could result in a change from a
watercourse that responds quickly to rainfall with little storage, to a mature one with extensive washland
storage. Therefore, the morphological changes will have a significant impact on the nature of stream flow.
Observing and managing river flow over this transition requires well sited observation points.
6.3 Winter conditions and snow melt
In regions influenced by near- or sub-zero temperatures, special measures are required to enable
hydrometric stations to operate to maintain the quality and consistency of flow measurements, and these
may influence the nature of the network. For example:
a) winter operation of float sensors can be jeopardized either by the formation of ice on the free water
surface or by freezing of the intake pipes connecting the stilling well to the stream channel;
b) submersible transducers can be exposed to freezing in shallow rivers, and anchor ice during the ice
formation period, and ice scour during periods of thaw;
c) leaks or blockages in the pressurized system of gas purge transducers can occur as a result of
differential contraction rates of materials at junctions within an instrument;
d) blockages can also occur in as a result of moisture accumulating within instrument units and the
buildings in which they are housed.
Careful planning in the selection of gauge sites can prevent loss of use of flow measuring sites, resulting
in streamflow records that are free of ice effect. The siting of flow gauges where water temperatures are
artificially enhanced needs to be considered. For example, downstream of discharges from wastewater
treatment plants, or dam gates where the deeper water in the reservoir is potentially warmer than the
ambient air temperatures.
It is usual for the normal relationship between discharge and water level in a freely flowing channel to be
changed when ice is present on the water surface. Additional under-ice discharge measurements may be
necessary to adjust level to discharge relationship in these circumstances and hence maintain the
accuracy of the data series. In some cases, it may be necessary to adjust the flows by reference to the flow
record from a nearby or surrogate gauge (see 6.4). Additionally, flow modelling can also be used to adjust
incorrect winter discharge data.
The utilization of the water resources in a catchment where snowfall is a big part of the annual amount
of precipitation can be very effective, especially in areas of higher elevation where the precipitation is
correspondingly higher, and the evapotranspiration is lower. The runoff patterns in such catchments
must be carefully investigated as a snow field can be regarded as a natural reservoir to be drawn on when
melting occurs. For example, many of the national and provincial hydrometric networks in the boreal
areas were developed to help manage waterpower production. It is therefore essential that an
observation network is capable of observing snowmelt as where and when it occurs.
Observations to prepare for snowmelt when it begins are important to ensure the effective use of the
reserves in the snowbank. Man-made reservoirs can be overdrawn in the knowledge that snow melt will
restock these reserves. Flow observations can be made at carefully sited hydrometric stations on adjacent
unregulated tributaries as input to hydrological forecasting models to inform management decisions on
the regulated watercourses. These stations can also indicate the start of floods after heavy rainfall.
6.4 Availability of surrogate gauged catchments
As noted in other clauses of this TR, the density of a river flow gauging station network depends in part,
on the hydro-geological complexity of the area of interest. Where hydrogeological and physio-geographic
areas are inhomogeneous, the station network needs to be of a higher density, and where possible,
measure river flow from smaller catchment areas. This will allow the use of the resulting hydrological
data to characterize the particular monitored streams, and hence provide the background data for
deriving the key hydrological parameters for similar unobserved catchments. The observed gauged
catchments therefore become surrogate to the unobserved locations.
Deriving key hydrological parameters for ungauged catchments is based on the hydrological similarities
of the hydrological regimen in gauged and ungauged catchments. Monitoring points therefore need to be
developed in a way to cover all main types of streams in the area of interest according to the hydrological
regime. This requires careful regionalization, based on analyses of physical, hydrographic, geological, and
climatic conditions. These include catchment size, river network density and pattern, altitudes, slopes,
precipitation normal and extremes, air temperatures, evapotranspiration, geology and soil types. All key
hydrological parameters need to be considered for inclusion.
Using the hydrological parameters in a suitable flow algorithm (or model), and the flow data series for
the gauged catchment, the flow data set for the ungauged catchment can be computed using what is
known as a deterministic model. This approach is discussed in 8.4.
However, in its simplest form, the calculation of a flow series can be undertaken by simple linear
regression between key statistics such as mean long-term discharges from the surrogate catchment:
Q
a, ungauged
k=
Q
a, gauged
where k is a constant that can be applied for a particular month or season. For example, the monthly
discharge in the ungauged profile can then be computed as:
Q = k*Q
m,ungauged m,gauged
More complex regression relationships using different k values for different months, or the inclusion of
precipitation inputs and expected losses due to evapotranspiration can be used, if sufficient data are
available for the gauged catchment to calibrate the relationship. This is known as stochastic modelling.
Complex processing of discharge data from water-gauging stations in GIS systems enables the generation
of digital maps of specific runoff values in raster form (e.g. in mm) for larger areas (country, river basin).
This also requires the discharge inputs from a suitable density of water-gauging stations, which are
representative of all main types of hydrogeological conditions. The maps, that can be of various time steps
e.g. yearly, monthly, daily, allow determination of the runoff data for each sub-catchment. However, the
shorter the time-step and smaller the catchment area that is selected, the higher will be the uncertainty
of such value.
6.5 Environmental and legislative constraints
Hydrometric network designers and planners need to be aware of all environmental and legislative
requirements that might influence the nature of the network under development. It is of particular
importance that any development is undertaken in a sustainable manner, and that all operational
activities necessary to keep the network functional is achievable in a sustainable way.
7 Technical considerations
7.1 Use of temporary networks
Temporary gauging stations are very useful for special investigations, and assist with the evaluation and
design of specific engineering schemes and research activities. Types of investigations include:
• monitoring the impacts of regional groundwater abstraction;
• extraction of gravels;
• pollution monitoring;
• flood studies;
• hydropower schemes;
• river restoration schemes.
These investigations might involve a single or a number of monitoring stations, and include other
measurements such as groundwater level, evaporation, soil moisture, precipitation and individual
discharge measurements to supplement the quasi-continuous gauging station data.
Temporary hydrometric networks must record specific characteristics that are of benefit to the scheme
or meet the research objectives. Examples include:
• time to peak of runoff within catchments to inform flood mitigation scheme design. This does not
require the development of a stage-discharge relationship as a level hydrograph also gives time to
peak;
• time of travel of flood waves along washlands to allow the design of flood storage systems to be
optimized, or the calibration of 3-dimensional hydraulic models;
• low flows to allow dilution rates of wastewater discharges to be observed and managed;
• whole range flows to be observed to allow ecological capacity and diversity of a river to be observed
and understood.
7.2 Choice of measuring technique
7.2.1 General
Where longer term flow measurement is justified, a permanent flow measuring site needs to be
considered. The choice is between an open channel site which could possibly include the use of velocity
sensors such as acoustic devices, or a flow measuring structure. The decision is largely driven by the
financial resources available, in that open channel measurement sites require less initial capital outlay
when compared to a measuring structure which will involve more expensive civil engineering works.
Therefore, the decision-making process will ideally involve a cost benefit analysis that compares the
value of the data and information that will be provided, set against the cost of installation and operation
of whatever measuring facility is installed. The environmental impacts may also have to be taken into
consideration.
7.2.2 Open channel flow measuring techniques
Flow can be measured in an open channel without having to build a structure. Several techniques are
available for use:
Spot flow gaugings – discharge can be measured using a suitable current meter.
The advantages are:
• low cost;
• limited environmental impact.
The disadvantages are:
• not continuous;
• not always able to access the river.
Spot flow gauging with level recorder
The advantages are:
• low cost;
• low environmental impact;
• continuous level record;
• continuous flow record if rating curve created.
The disadvantages are:
• additional costs will be incurred to create rating curve;
• may take a long time to build stable rating curve over the flow range (may be years).
Level and velocity measurement systems with velocity index rating
The advantages are:
• medium cost;
• continuous velocity and level measurement;
• continuous flow record if index rating created;
• measure through range with greater certainty;
• non-contact technologies are easier and safer to deploy – e.g. Radar, PIV.
The disadvantages are:
• cost of creating velocity index rating;
• infrastructure cost;
• only measure a proportion of velocity profile.
Time of flight acoustic systems
The advantages are:
• continuous velocity and level measurement;
• good quality flow data available throughout flow range;
• measure majority of the velocity profile (dependent on configuration of system).
The disadvantages are:
• infrastructure cost.
7.3 Permanent flow measuring structures
7.3.1 General
Flow measuring structures are permanent constructions in the water course. They are relatively
expensive to construct compared to open channel techniques and require a higher level of engineering
input. Added to this, the environmental impact of the structure can be significant which may not be
acceptable to the local environmental management agencies or planning authorities.
Set against this is the permanent nature and ability to produce high quality flow data as soon as
constructed, without the need to wait to develop a specific stage = discharge relationship or velocity
index rating.
ISO 8368 sets out guidance to the choice of flow measuring structure, and hence the reader is referred to
this for advice. In summary, the main points to consider are:
• The building of a permanent structure will, in most cases, involve significant civil engineering, hence
considerably greater cost. On some smaller water courses prefabricated flumes and weirs have been
installed without significant engineering overheads, but nevertheless, the use of such structures still
requires some degree of civil engineering.
• Environmental legislation may impose strict limitations on the design of any structure, particularly
if the location is on a fish migratory river.
• A permanent structure will have to be approved by the local or National Planning authorities. This
approval may prove difficult to obtain if the location is in a National Park or other planning sensitive
area.
On watercourses with active beds the presence of high silt loads may render a weir unacceptably
inaccurate or physically inoperable
7.3.2 Accessibility
Sites need to be readily and safely accessible under all but the most extreme conditions. Servicing of sites
that are difficult to access during high water levels may be problematic during floods, and alternative
locations may be preferred even if they are not in the ideal location in the river basin. Also, if sites are
difficult to access this may have a considerable impact on sustainability.
7.3.3 Length and quality of established data records
Long hydrological records are important. Just a few years of data are insufficient for most water
resources, flood, environmental and other river management purposes. Careful consideration needs to
be given to the value of continuing the recording of flow at an existing recording station so that valuable
long-term flow statistics can be compiled.
7.3.4 Distribution of gauging s
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