07 - mcbride - exploring regional water...

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.


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


2


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.


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).


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


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.


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.


Table 2: Influence of physical and operational factors on the uncertainty of an assessment of net yield of a

transfer (HR Wallingford, 2018)

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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


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).

4 REFERENCES

Atkins (2018) Thames Water Stochastic Resource Modelling Stage 2&3 Report. Available at:

https://corporate.thameswater.co.uk/-/media/Site-Content/Thames-Water/Corporate/AboutUs/Our-

strategies-and-plans/Water-resources/Document-library/Water-reports/WRMP19--Stochastic-

Resource-Modelling-Stage-23-Report-Atkins-July-2018-DG04.pdf.

British Geological Survey (2019). BGS hydrogeology 625k. Available at:

https://www.bgs.ac.uk/products/hydrogeology/maps.html

Environment Agency (2015) Understanding the performance of water supply systems during mild to

extreme droughts SC120048/R.

Environment Agency (2017) Operating Rules for the River Severn Resource / Supply System Version

7.

Hough, M. N. and Jones, R. J. A. (1997) The United Kingdom Meteorological Office rainfall and

evaporation calculation system: MORECS version 2.0-an overview. Hydrol. Earth Syst. Sci., 1(2), pp.

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HR Wallingford, Centre for Ecology and Hydrology, British Geological Survey and Wheeler, A. F.

(2015) CCRA2: Updated projections for water availability for the UK MAR5343-RT002-R05-00.

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projections-for-water-availability-for-the-uk/.

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Moore, R.J., (2007). The PDM rainfall-runoff model. Hydrol.Earth Syst.Sci., 11(1), 483-499.

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water-resource-solutions.pdf.


Tanguy, M., Dixon, H., Prosdocimi, I., Morris, D. G. and Keller, V. D. J. (2015) Gridded estimates of

daily and monthly areal rainfall for the United Kingdom (1890-2014) [CEH-GEAR]. Available at:

https://doi.org/10.5285/f2856ee8-da6e-4b67-bedb-590520c77b3c

Thames Water (2018) Section 11 Preferred Plan. Available at: https://corporate.thameswater.co.uk/-

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Thames Water (2019a) Water Resources Technical Stakeholder Meeting. Available at:

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strategies-and-plans/Water-resources/Document-library/Past-meetings/28-May-2019/28-may-2019-

presentation.pdf

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WaterUK (2016) Water resources long term planning framework (2015-2065) Technical Report.

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