Post on 28-Jun-2020
Irish National Hydrology Conference 2019 McBride
07 - EXPLORING REGIONAL WATER TRANSFERS TO SECURE FUTURE
WATER SUPPLY: A CASE STUDY FROM THE UK
A McBride1, M Durant1, C Counsell1, A Ball1, C Lambert2, P Blair2 1 Flood and Water Management, HR Wallingford 2 Thames Water
Abstract
Water companies in the UK are required to produce long-term plans of water resources for their
supply area every five years, detailing how they will maintain secure and sustainable supplies, taking
account of social and environmental impacts as well as economic costs. As a result, the water
environment is highly regulated to ensure competing demands are satisfied. On a national scale, there
are regions of water security and regions forecast to face water stress over the coming decades. As a
result, regional transfers of water from donor catchments to receiving catchments are being explored
by several water companies. One such scheme is a transfer of water from the River Severn to the
River Thames, to secure the water supply of the south east of the UK. Extensive hydrological and
water resources modelling, analysis of historical droughts and droughts beyond the historical records,
and identification of the key factors which may influence the transfer are presented in this paper.
1 INTRODUCTION
Approximately a quarter of the UK population lives in the south-east of the country where the
population is projected to grow at a rate exceeding the national average. Some areas are predicted to
face water supply deficits in the near future. Should no action be taken, the demand for water is
forecast to increase whilst the availability of water resources decreases due to climate change and the
reduction of some licences to improve the freshwater environment (HR Wallingford et al., 2015).
A recent government funded study to understand the future challenges of drought resilience for the
water industry and identify potential solutions concluded that large-scale inter-regional transfers of
water could offer good value for money (WaterUK, 2016). Strategic schemes which transfer water
from areas of projected water security to those of projected water scarcity are actively being explored.
The financial regulator of the UK water industry, Ofwat, expects a fully informed decision to be made
on a selected scheme by 2022 (Ofwat, 2019). A Regulators’ Alliance for Progressing Infrastructure
Development (RAPID) has been created to develop a regulatory framework which is suitable for
future schemes and ensure that strategic infrastructure is developed in a timely and co-ordinated
manner.
As the major supplier of public water in the south-east, Thames Water has set out how it plans to
maintain its supply demand balance in their supply area until 2100. A regional transfer of water from
the River Severn to the River Thames was identified as one of the supply options to maintain this
balance (Thames Water, 2018). A schematic diagram of the scheme is provided in Figure 1. The key
operational questions this scheme poses are when should a release be made, how much water should
be released, and how much water will be available for abstraction? To answer these questions, we
present the development of a hydrological and water resources model, analysis of gauge uncertainty
and the likelihood of drought coincidence, and an assessment of the key factors which could impact
the overall net yield of the scheme.
Irish National Hydrology Conference 2019 McBride
Figure 1: Schematic diagram of the Severn Thames Transfer (Thames Water, 2019a)
2 A REGIONAL WATER TRANSFER
2.1 River Severn catchment overview
The headwaters of the River Severn rise in the Welsh uplands flowing down into Shropshire, Worcestershire
and Gloucestershire, as shown in Figure
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2
Irish National Hydrology Conference 2019 McBride
Figure 2. The River Severn is regulated to maintain minimum five-day average river flows at
Bewdley of 850 megalitres/day (Ml/d) by releasing water from the upstream reservoirs of Llyn
Clywedog and Lake Vyrnwy. This regulation is required to maintain river flows primarily during the
summer months and ensure there is enough available water for the Gloucester and Sharpness Canal
for the purposes of both navigation and water supply for the City of Bristol. The regulation of river
flows can be maintained further through releases to the River Severn by the Shropshire Groundwater
Scheme (SGS) during periods of very low river flows. The order in which the three sources are used
to maintain the regulated Bewdley flow is based on a forecast of the risk of regulation failure on an
annual basis.
The River Severn is used by Severn Trent Water Ltd and South Staffordshire Water to provide much
of the public water supply to the West Midlands with significant abstractions from the River Avon.
Llyn Clywedog reservoir is owned and operated by Severn Trent Water with the sole purpose of
regulating River Severn flows. Lake Vyrnwy is owned by Severn Trent Water but used by United
Utilities to provide public water supply to Liverpool.
2.2 Baseline hydrological and water resources modelling
Integrated hydrological and water resources modelling of the River Severn catchment was carried out
using HR Wallingford’s in-house modelling suite Kestrel.
A probability distributed rainfall-runoff model (Moore, 2007) of the catchment was developed. Such
models include a ‘mass-balance’ probability distributed soil moisture accounting component, with
resulting direct runoff and recharge routed via ‘slow’ and ‘fast’ pathways to the basin outlet. A Pareto
distribution was used to describe the distribution of the storage capacity across a catchment, with the
distribution shape altered to reflect different proportions of deep or shallow stores. If the storage
capacity at a point is exceeded, direct runoff occurs, otherwise water remains in storage with losses to
evaporation and via recharge to the groundwater store.
Irish National Hydrology Conference 2019 McBride
Figure 2: Overview of the River Severn catchment
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The hydrological model uses daily time series of gridded precipitation (Tanguy et al, 2015) and
MORECS potential evapotranspiration (PET) (Hough et al, 1997). The 1 km x 1 km gridded model
area is parameterised based on hydrogeological data (British Geological Survey, 2019) and
information on the location of significant urban areas. Soil moisture stores are also gridded and flows
are routed between assessment points using a Muskingum routing scheme in which reach storage is a
linear function of a weighted combination of the reach inflow and outflow.
Hydrological model calibration prioritised reproducing low to medium flows and overall flow
volumes using naturalised records and observed records held by the National River Flow Archive
(2019). The hydrological model was calibrated against naturalised flows and the water resources
model was calibrated against observed flows. Flow duration curves for the simulated flows and the
observed record of the River Severn at Bewdley and River Severn at Deerhurst are provided in Figure
3. The Nash Sutcliffe model efficiency coefficients at these assessment points is 0.93 and 0.90
respectively.
Figure 3: Simulated and observed flow duration curves at two assessment points
The Kestrel water resources model uses a node and link system to represent the key water resource
system components. Model nodes represent system components such as river abstraction points,
reservoirs, and demand centres which all operate to rules for their specific node type. The model
nodes can then be joined by links which represent interactions between the nodes, for example a
reservoir is linked to a downstream river node to enable its releases to be routed appropriately.
The key model component is the representation of the Lake Vyrnwy and Llyn Clywedog reservoirs.
Reservoir nodes receive inflows from an upstream river node in the hydrological model. Llyn
Clywedog is the main resource for river flow regulation with releases up to 500 Ml/d subject to the
ordering rule of sources. An additional constraint on the regulation volume available from Llyn
Clywedog is for regulation to decrease to 300 Ml/d if reservoir storage enters the “Apply Drought
Order” band. Vyrnwy’s primary purpose is to provide public water supply abstraction to United
Utilities at an assumed rate of 205 Ml/d. It does, however, provide regulation to the River Severn
through the use of a “Vyrnwy Bank” process. The bank has a maximum volume of 5,000 Ml which is
carried over between years and its volume is protected at 725 Ml in April and May and from October
to December 15th. The bank balance is also reduced when the reservoir overtops due to high capacity
(spill) and releases for flood control (Environment Agency, 2017).
Irish National Hydrology Conference 2019 McBride
The reservoirs make releases based on compensation release requirements, flood control curve release
requirements and then any regulation release requirements over and above the former releases. The
releases from a reservoir are then routed to a downstream river node.
The water resources model will not exactly reproduce the gauged record on a day to day basis due to
the cascade of uncertainties inherent in the modelling process from differences in climate inputs
which influence the natural hydrological model calibration, differences in the artificial influences that
are assumed to occur, and simplification in the simulated regulation process. It should be noted that
the water resources model adheres to the strict rules that it is given whereas in reality the regulation is
operated using expert knowledge of the local conditions at the time.
The hydrological and water resources modelling undertaken provided a more robust flow sequence for
the River Severn at Deerhurst. The calibrated model was subsequently hindcast to cover a simulated
period from 1910 to 2012 on a daily time step.
2.3 Drought analysis
Due to the regional nature of this scheme, a key question to understand is whether or not water would
be available in the River Severn when a transfer to the River Thames might be called upon. Several
droughts over the past century in the historical record provide a limited dataset on which to base a
decision. Droughts beyond the historical record were therefore explored using both stochastic and
synthetic drought libraries. The computational efficiency of the Kestrel modelling suite enabled the
simulation of these time series for further drought analysis, though sub-catchment average rainfall and
PET time series were used rather than gridded inputs.
For this study Thames Water provided a stochastic drought library of 15,600 years of generated
weather replicating the climate of the 20th century (Atkins, 2018). This library contained equally
plausible droughts to those in the historical record, which were spatially coherent across both the
Severn and the Thames catchments. Analysis of stochastic data was carried out to identify droughts in
the Thames catchment, and quantify the likelihood of coincident drought in the Severn catchment.
Stochastic droughts that were similar to historical drought events in the River Thames were identified
based on their rainfall characteristics. For each historical drought template, the weighted root mean
square error (RMSE) between the template ± 20% for the first 18 months of drought was calculated,
providing a bounding envelope. The RMSE between the template and stochastics over 18 months was
calculated, and those with a lower weighted RMSE than the bounding envelope were identified as
matches. For each template, the weighted RMSE between all stochastic droughts and the template
over 18 months was calculated. For each template, the patterns had a lower weighted RMSE than the
upper bound calculated above, and therefore are deemed to match the template. A summary of the
templates explored and the number of matches is provided in Table 1.
Analysis of the Severn data corresponding to the stochastic droughts matched in the Thames
highlighted that both catchments experience droughts of a similar pattern, though the severity of the
rainfall deficit is greater in the Thames and the range of drought severity in the Severn is larger than
that in the Thames (HR Wallingford, 2016).
A summary of the likelihood of stochastic droughts having a greater impact than the historical
template is provided in Table 1. The severity of the 1975/76 October historical drought is evident, as
only 16 % of stochastic droughts have a greater impact on flows at Deerhurst. The historical event
where the stochastic droughts demonstrate the greatest increase in the range of impacts compared with
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the baseline is 1920/21 which is exceeded by 96% of the equivalent stochastic template drought
events. These results demonstrate the added value of using stochastically generated data compared
with just using the historical record alone.
Understanding the sensitivity of the Severn system to drought is important due to the reliance on river
regulation to maintain lower flow periods. The drought sensitivity of the River Severn was assessed
using a ‘bottom–up’ framework of synthetic droughts (Environment Agency, 2016). A library of
spatially coherent rainfall and PET time series for synthetic droughts varying in duration and severity
was developed. Drought duration ranged from 6 to 60 months, at 6 month intervals. Drought severity
ranged from 95% to10% long term average (LTA, i.e. 1/1/1960 to 31/12/1989) rainfall, at intervals of
5 %. The library contained two versions of each unique combination of drought duration and severity,
the first beginning in October and the second in April, the start and midpoint of the hydrological year.
Each drought has a minimum 5 year warm up and cool down period of LTA climate. This amounts to
a library of 361 synthetic droughts, including a synthetic baseline with a constant LTA profile. To
systematically quantify the impact of drought on the River Severn catchment, the number of days per
year below a Hands Off Flow (HOF) at Deerhurst was calculated for each drought using the
hydrological and water resources model. The HOF is the river flow level at which abstraction from
the river for a transfer would not be permitted. The drought response surface, or colour flood, shown
in Figure 4 is the result of this set of model runs.
Table 1: Summary of identified stochastic droughts and associated impacts (HR Wallingford, 2016)
Historical
drought*
First month of
drought
HOF breaches in
the first 12 months
Number of stochastic
matches
Likelihood of a match
having a greater impact
1920/21 April 31 days 294 96 %
1933/34 October 62 days 311 62 %
1943/44 October 50 days 336 62 %
1975/76 October 119 days 217 16 %
1989/90 April 81 days 299 57 %
1995/96 April 66 days 289 45 %
* droughts listed in chronological order
In order to interpret the stochastic drought events in the context of the synthetic droughts, the
probability distribution of the rainfall deficits of the stochastic drought events at each synthetic
drought duration were calculated. From these distributions it is possible to derive the exceedance
probability and the associated rainfall deficits. These were overlaid on the drought response surface as
probability contours in Figure 4. The contours describe the probability of a given rainfall deficit in the
River Severn not being exceeded and the associated impact on river flows at the Deerhurst HOF for
periods of time when the River Thames is in drought.
Irish National Hydrology Conference 2019 McBride
Figure 4: Response surface with the probability of exceedance for stochastic droughts in the Severn (HR
Wallingford, 2016)
2.4 Assessment of factors impacting the net yield of a transfer
The feasibility of a transfer is reliant on the availability of water at the point of abstraction when it is
required. The proposed scheme relies on a release of water from a reservoir in Wales reaching an
abstraction point approximately 200 km downstream at Deerhurst. There are several physical and
operational factors which may impact the amount of water available (net yield). In recognition of the
uncertainty in quantifying the significance of each potential factor comprising net yield, a method of
scoring uncertainty was derived prior to any analysis. This method was based on data availability,
methodology, and the significance of the loss estimated, and a value of low, medium, or high
uncertainty assigned to three river reaches.
Dividing the quantification of factors impacting the net yield into separate components was necessary
in order to isolate influences, however there are a range of interdependencies between the various
components. It is also apparent that the key driver of uncertainty is not the physical processes
governing the River Severn (e.g. evaporation), but the certainty associated with measurements of the
system, river flow in particular. A summary of the findings of this assessment is provided in Table 2.
Abstraction and discharge data made available by regulators and water companies were found to be a
large source of uncertainty in assessing the net yield of a transfer. As the catchment area covers
approximately 10,000 km2, lies within the areas of two regulators (the Environment Agency and
Natural Resources Wales), and is a source of water for three water companies, this activity required
extensive stakeholder liaison. The uncertainty was founded in the spatial and temporal resolution used
by different bodies when recording the data which meant that daily analysis of anthropogenic
influences in the catchment at specific locations was not possible. Ongoing work in collaboration with
regulators is being undertaken to resolve these challenges.
Irish National Hydrology Conference 2019 McBride
Table 2: Influence of physical and operational factors on the uncertainty of an assessment of net yield of a
transfer (HR Wallingford, 2018)
Reach*
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River Severn upstream of
Bewdley Low High Medium Medium High Low
River Severn from
Bewdley to Saxons Lode Medium High Low Medium High Low
River Severn from Saxons
Lode to Deerhurst High High Low Low High Low
* Refer to Figure 2 for location of reaches assessed
The ability to gauge flows along the River Severn with confidence was identified as an influential
factor, and the key factor we focus on in this paper. This factor is also intrinsically linked to the
uncertainty associated with the assessment of flow attenuation and conveyance. Quality flag
comments in the observed flow records were analysed for instances of observed uncertainty that could
potentially influence the gauged data and its interpretation. The number of comments and the
distribution of these both during the year and between years was assumed as a proxy for data issues,
and while this does not necessarily mean that these data are inaccurate, the presence of a comment
indicates some concern about the measurement or data that could indicate greater uncertainty
surrounding the gauge. The results shown in Figure 5 show a trend for data uncertainty increasing at
Deerhurst during the summer months, but not at Bewdley. This pattern is of concern to the feasibility
of a transfer, as a reduction in net yield of the scheme is realised at the gauging station which controls
the abstraction, in this case Deerhurst. The Environment Agency is currently systematically reviewing
the rating of flow gauges in the River Severn, which will reduce this uncertainty.
Figure 5: Data quality flag histograms at the regulation gauge (Bewdley) and abstraction location gauge
(Deerhurst) (HR Wallingford, 2018)
3 CONCLUSIONS AND FUTURE WORK
To assess the feasibility of a regional transfer, a thorough understanding of the existing water
availability is required. We developed a distributed integrated hydrological and water resources model
Irish National Hydrology Conference 2019 McBride
to simulate historical flows from 1910 to present and enable the analysis of droughts beyond the
historical records using stochastic and synthetic drought libraries. Analysis of droughts which are
spatially coherent for both the donor and the receiving catchment can inform the likelihood of
coincident droughts and of plausible droughts having a greater impact than experienced previously.
A quantification of the net yield of the scheme once operated is influenced by several factors, both
physical and operational. Our assessment highlighted that operational factors such as gauging station
accuracy and data collection were greater sources of uncertainty than physical processes such as
evaporation. Where donor and the receiving catchments cross regulatory boundaries and involve
several water companies, extensive stakeholder engagement is needed to collate the data and
regulatory information required to assess these factors.
The next phase of work in assessing the feasibility of this scheme will be to incorporate water quality
and hydroecological assessments to the water quantity work presented in this paper. A scoping phase
of work is currently being planned with regulators across the two catchments, water companies, and
stakeholders to physically test the scheme as outlined by Thames Water in its recent water resources
plan (Thames Water, 2019b).
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Environment Agency (2015) Understanding the performance of water supply systems during mild to
extreme droughts SC120048/R.
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Hough, M. N. and Jones, R. J. A. (1997) The United Kingdom Meteorological Office rainfall and
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Tanguy, M., Dixon, H., Prosdocimi, I., Morris, D. G. and Keller, V. D. J. (2015) Gridded estimates of
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