Water Resources Management Plan 2014 - South Staffs … · the 2013 Draft Water Resources Management Plan (dWRMP) have been produced. ... deployable output • The impact of levels

Water ResourcesManagement Plan 2014Appendix B: Calculation of Deployable Output

1 Content

This appendix explains how South Staffs Water's deployable output calculations for the 2013 Draft Water Resources Management Plan (dWRMP) have been produced.

This report includes:-

• The regulatory requirements for this assessment • A description of the Company supply system, its constraints and data used

in the evaluation • The background to the choice of deployable output assessment framework • The use of the framework to assess climate change risks • Hydrological and hydrogeological techniques used and approaches to

extension of flow and level records • Results of the internal groundwater sourceworks review carried out in 2012 • The key components of the Aquator conjunctive use model • Details of changes to the modelling approach since PR09 • The key model outputs and the dry year annual average and peak week

deployable output • The impact of levels of service on deployable output • The key modelling assumptions and model outputs

The Aquator modelling work and the supporting hydrological information in this appendix were provided by Hydro-Logic Ltd, and the appendix has been prepared by South Staffs Water using their technical outputs.

Final Water Resources Management Plan 2014: Appendix B (Deployable Output) Page 2

2 Regulatory Requirements

2.1 PR09 Methodological Approach

The use of the water resources model for PR09 followed the industry best practice approach at that time, as outlined in:

• A Methodology for the Determination of Outputs of Groundwater Sources (UKWIR, 1995)

• Reassessment of water company yields (Environment Agency, 1997); • Surface Water Yield Assessment (National Rivers Authority, 1995); and • A Unified Methodology for the Determination of Deployable Output from

Water Sources (UKWIR and Environment Agency, 2000). The Company used a water resources modelling package called WRAPSIM to calculate deployable output. The model calculated the water resources that would be available to the Company, given a repeat of the climatic conditions of the 83 year period between 1921 and 2004. The hydrological constraints (simulated river flows and reservoir inflows) were modelled in combination with abstraction licence, infrastructure and asset constraints (including groundwater yields independently assessed), given a range of demands. This enabled the conjunctive use benefits of the Company’s resource system to be examined.

This approach was recently assessed by the Environment Agency in 2011 (Review of Existing Industry Approaches to Deployable Output Assessment). The review concluded that the Company approach represented Good Practice, and was in line with 46% of water companies (12% of water company approaches were assessed as Good to Best Practice and 42% Adequate). The EA review concluded in general that approaches with Good Practice had shortcomings relative to Best Practice due to:

• inconsistencies in treatment of levels of service (LoS) between groundwater and conjunctive use deployable output assessments; and

• the length of data record used.

These factors have been further evaluated with respect to the Company’s approach in PR14.

2.2 Criteria for Review

No instructions were received from Defra after the PR09 FWRMP to review DO, nor is the Company proposing changes to its levels of service or major investment in new resources. Nevertheless a new assessment of deployable output has been carried for the PR14 plan to reflect changes in data and requirements, the principal criteria for this review are as follows:

• Significant modifications to the operation and/or treatment of some existing groundwater sources had been made since the last assessment

• New data on mains infrastructure and capacity and demand patterns obtained from the Company’s hydraulic modelling and new insights on drought behaviour from UKWIR studies required new work

• Operational experience from the 2010/12 Drought highlighted refinements in operational practice at Blithfield Reservoir and provided additional data on the performance of groundwater sources

• A reassessment of surface water treatment works licences and operation implied changes to peak capacities and treatment losses


• An extension of the River Blithe simulated flow series to 2010 and use of updated flow series for the Rivers Severn and Trent from the STWL regional resource model was required

• Further and more detailed work was required to determine the impact of different levels of service on deployable output.

These criteria are summarised in Table 1.

Table 1: Criteria for updating deployable output (DO) assessment

Driver Ground-

water Sources

Blithfield Reservoir System

River Severn System

Instruction from Defra No No No Implementation of new, or

significant change to, resource system

Yes No No

New work on existing source Yes Yes No Change to planned level of

service No No No

Revision of data used in previous assessments Yes Yes Yes

Recent experience of drought Yes Yes No Reappraisal of DO prior to

proposal of resource development in WSZ

No No No

Change of assessment method to determine impacts of levels of

service on DO Yes Yes Yes

A decision was made at the end of AMP4 to carry out a major review of the existing water resources allocation model used by the Company to model conjunctive use of its surface water and groundwater sources since 2000. This has led the Company to adopt updated software on a new platform (Aquator). The review has been carried out in a systematic way with an initial detailed audit and documentation by the Consultant of the previous model schematic, parameters and control rules. This was followed by a validation exercise of all components by an internal Company cross-disciplinary technical group using Company data, models and systems as well as third party models and datasets. All changes were tested by the Consultant in a series of sensitivity runs and used to determine both the baseline deployable output model parameters as well the criteria for evaluating changes in levels of service and future climate change scenarios.

2.3 PR14 Methodological Approach

The methods used for calculating deployable output have been reviewed in a joint UKWIR and Environment Agency study (Water Resources Planning Tools 2012), otherwise referred to as WR27. The Company has used this work to establish the framework to determine the level of analysis required to assess deployable output in proportion with the nature of the supply system and the risk to both supplies and the environment.

The revised approach is risk based and consists of five steps as follows:

Step 1 - Choose a DO assessment Framework Step 2 - Assess Vulnerability to Climate Change


Step 3 - Establish DO assessment dataset Step 4 - Calculate DO with a Confidence Label Step 5 - Report DO assessment

This approach has been followed to validate the Company’s existing behavioural approach to assessing deployable output on a conjunctive basis and to explore ways in which further refinements are required and proportionate.


3 Choice of Assessment Framework

3.1 Background

Deployable output (DO) is a building block in determining water supplies available for use by a company and is defined as:

The output for specified conditions and demands of a commissioned source, group of sources or water resources system as constrained by;

• hydrological yield; • licensed quantities; • environment (represented through licence constraints); • pumping plant and/or well/aquifer properties; • raw water mains and/or aqueducts; • transfer and/or output main; • treatment; • water quality; • levels of service.

3.2 Description of the supply system

Two surface water sources provide approximately 50% of the Company’s water resources in the critical dry year.

There are 26 groundwater sources, which typically supply directly into the network. These sources provide approximately 50% of the Company’s water resources in the critical dry year.

The two principal treatment works are linked by a strategic treated water spine main which passes through the key population areas of the Black Country (e.g. Dudley, West Bromwich and Walsall). There are additional branches to demand centres at Tamworth, Burton and Cannock. Other residential areas are supplied by connections off this strategic network and from groundwater sources.

The supply system can be classified as a conjunctive use system as surface water storage and groundwater are mixed and operated together to increase DO.

3.3 Planning Scenario

The Company has a single resource zone and reports both a dry year annual average and peak week deployable output using the water resources allocation model and employing behavioural analysis.


Figure 1: South Staffs Water Supply System 3.4 Levels of Service

Despite the hot summers experienced in 1995 and 2006, and drought conditions in 1996, the Company has not imposed a hosepipe (temporary use) ban since the record drought on the River Severn in 1976. This has been used in previous water resources plans (2004 and 2009) as evidence for the stated Company levels of service. This sets the frequency of water use restrictions for DO calculations with respect to the Company resource zone for a temporary use ban every 40 years on average.

As part of its Drought Plan the Company has laid out the circumstances under which different types of customer restrictions will be applied. The Company however does not have a stated level of service for these measures (appeals for voluntary restraint, and non-essential use bans or ordinary drought orders).

The conjunctive use model assesses the impacts of customer levels of service by applying restricted demand patterns triggered by control curves which conserve


resources, allowing a higher deployable output to be obtained on average. These measures apply over the planning scenario (dry year average and peak). The frequency of any restriction is closely linked to DO and is determined by the highest average demand that can be sustained over the period of record (in the Company’s case this is 89 years) before a further year with restrictions is required. For example, a 1 in 30 year level of service for a specified restriction would require that it occurs in no more than 3 years within this period.

The way in which this process has been carried out is described further in Section 6.2 This has permitted the Company to investigate how deployable output relates to levels of service and to provide information to customers and other stakeholders on the implications for risk and investment should these change.

The levels of service scenarios investigated by the company meet with those recommended by Reassessment of Yields (1997) and current water resource planning guidelines, and are outlined in Table 2. Results are presented in Section 9.

Table 2: Levels of service scenarios Scenario Description

1 No Restrictions No customer restrictions or other drought actions applied

2 Water Company LoS System operated to meet water company LoS to include applications of demand restrictions and other measures identified by the company.

3 Reference LoS

Temporary use restrictions of 1 in 10 years and non-essential use restrictions of 1 in 40 years on average. No rota bans or standpipes should be used in the period of record.

3.5 Description of constraints

Individual groundwater sources are constrained by pump depth and capacity, treatment capacity, operational and treatment losses, abstraction licence limitations (daily and annual average), deepest advisable water level (DAPWL), depth or rate related water quality issues, and interactions with other on-site wells. Sources have also been screened for other infrastructure limits (such as mains capacity and blending requirements), raw water supply obligations and environmental constraints (such as prescribed river flows that affect licence volumes).

Additional constraints on groundwater abstraction arise from group licences which cover 24 of the 27 groundwater sources. These abstraction licences variously constrain daily, annual and 10-yearly aggregate volumes and sometimes include sub groups. 10-yearly licences are considered not to affect DO as elevated abstraction during a two to three dry high demand period can be readily compensated by reduced abstraction in subsequent wetter years when demands are lower.

Individual surface water sources are constrained by river intake capacities, gravity and pumped transfer main capacities, abstraction licence limitations, available storage, treatment works capacity, operational and treatment losses, and other infrastructure limits (such as treated water pump capacity).

Additional constraints on surface water sources are the availability of natural inflow to reservoirs, the capacity of any pumped transfers of raw surface water and raw groundwater into reservoirs, abstraction licence limitations of these pumped transfers and the limitations on these imposed by other abstractors, environmental “Hands Off Flow” limits for rivers affected and other operational rules on regulated rivers.


In addition to these individual source constraints, conjunctive use is constrained by the limitation of transfer capacity between the different source types and water demand centres, treated water capacity and the spatial distribution and characteristics of demand at these centres.

The framework under which future ecological needs may be identified has been captured in the Data Collection process (see Section 5 below). These include Water Framework Directive (WFD), Catchment Abstraction Management Strategy (CAMS), National Environment Programme (NEP) and Habitats Regulation Assessment (HRA) requirements and measures. Ecological needs identified to date have been addressed through abstraction licence conditions (e.g. prescribed river flows, HOF flows, reductions in annual volumes and imposition of lower 10-yearly volumes). Future ecological needs have all been addressed with source specific actions and are discussed under Section HH Sustainability reductions.

Accordingly, the degree of constraints on outputs has been assessed as:

Medium to High: Single source or several sources with simple to complex constraints, conjunctive use with complex constraints on outputs

3.6 Choice of assessment framework

A water resources zone (conjunctive use system) assessment framework has been selected for the following reasons:

• previous use in PR09 and availability of data and intelligence from previous model building and refinement studies

• a medium to high degree of constraints on outputs • requirement to evaluate existing Company LoS and options for alternative

LoS A catchment/aquifer assessment framework is not required in PR14 to assess ecological needs further to these requirements. Nevertheless the model has the capability to carry out this task if required in future.


4 Data

4.1 Hydrological Data

4.1.1 Background Hydrological data on inflows is required for the assessment of DO for the two surface water sources in the River Severn and River Trent catchments. The data that affect these sources are:

Blithfield Reservoir 1. River flow from the Upper Blithe catchment into the impounded Blithfield

reservoir: This comprises river flow from the upper River Blithe, Tad Brook and overland flow direct into the reservoir. Flows in the upper River Blithe have been impacted by groundwater abstraction in the catchment headwaters since the 19th Century.

2. Pumped transfer flows from the Nethertown (River Blithe) intake: The volume of this transfer is controlled by infrastructure (pump and main capacity), storage levels in Blithfield Reservoir and compensation releases, inflows from tributaries in the lower Blithe catchment, and the Hands Off Flow status of the River Trent at Newark (North Muskham).

3. River Trent flows at Newark (North Muskham): This comprises natural runoff from the upper catchment and its tributaries and discharges of effluent from sewage treatment works. It is affected by impoundments, river abstraction and compensation releases associated with surface water sources operated by water companies (South Staffs Water and Severn Trent Water) in the upper catchment and its tributaries.

River Severn 1. Storage levels in Chelmarsh Reservoir: This is affected primarily by discharge

of river water abstracted at HL and transfers to HL treatment works. There is some minor flow direct into the reservoir and compensation releases to the Chelmarsh Brook.

2. Abstraction rates: The river abstraction at HL is limited to abstraction rate bands set up by the regulatory regime. It is not constrained by flows in the River Severn.

3. The regulatory regime of the upper River Severn: This is largely determined by reference to storage levels in Clywedog Reservoir in the headwaters of the River Severn. These are controlled by natural inflows to this impounding reservoir, reservoir releases (for compensation, flood control and river support), releases from the Shropshire Groundwater Scheme and public water supply run of river abstractions operated by South Staffs Water and Severn Trent Water. It is also affected by operation of Lake Vyrnwy for public water supply in the Upper Severn by United Utilities, natural inflows from downstream tributaries and effluent discharges.

4.1.2 Hydrological Derivation techniques Flows in the River Blithe catchment have been determined by rainfall runoff modeling of naturalised flows at the catchment flow gauging station (Hamstall Ridware). The gauging station lies downstream of Blithfield Reservoir and was installed in 1937 before construction (1947 - 1953). The naturalised flow sequence is split into two using area factors: the Upper Blithe, representing inflow from the Upper Blithe and Tad Brook; and the Lower Blithe representing inflow from the lower tributaries (Ash


Brook and Pur Brook). These two inflow sequences are used respectively to determine inflows to the Blithfield reservoir and, in conjunction with reservoir releases, to determine constraints on pumped transfer flows from the Nethertown (River Blithe) intake.

Other inflow sequences have been determined by behavioural analysis using the Severn Trent Water strategic regional model and have been made available to South Staffs Water. This large Aquator model makes use of rainfall runoff models in 79 sub-catchments and simulates impoundment reservoirs, supply systems and discharges across the region including a simplified simulation of the South Staffs Water supply system. As a “full river” model, this is able to simulate river flows at key locations on the River Trent and Severn as well as their major tributaries. Inflow sequences used by the Company are River Trent flows at Newark (North Muskham) and the regulatory resource state of the Upper Severn at Bewdley (derived from Clywedog Reservoir control curves).

The remaining flow sequences (Chelmarsh storage, river abstraction at HL and pumped transfers at Nethertown) are determined by behavioural analysis using the Company Aquator model.

4.1.3 Extension of Flow records The principal components of the company supply system that are sensitive to drought are its surface water sources. These were constructed between 1947 and 1967 and have since undergone significant works particularly in the late 1990’s. It is therefore recognised that the present supply system needs to be re-evaluated against the droughts experienced in 1975/6 and 1995/6. The system also needs to be evaluated against droughts that occurred earlier in the 20th Century that might be reasonably expected to re-occur.

Flow records have therefore been extended using climatic data (Table 3) to enable behavioural analysis from 1921 to the recent present (December 2010). This includes the significant drought in 1933/34 and secondary events in 1921, 1929 and 1938.

The Company has made use of extended data back to 1920 in its water resources modeling since the late 1990’s. This is aligned with hydrological analysis by the Environment Agency and Severn Trent Water which makes use of measured river flow data which started in the 1920’s at Bewdley. A further extension of records to the 1890s would be onerous and is not currently justified at a regional or company level.

As regional datasets are only available to December 2010 it has not been possible to fully analyse the low flow conditions experienced in 2011/2. This period was characterized by exceptionally low rainfall in the supply area and low river flows and groundwater levels across the River Trent catchment and in tributaries of the River Severn requiring implementation of drought monitoring, maximization of groundwater sources and conservation measures for Blithfield Reservoir. Reservoir levels and river flows in the Upper Severn however were more robust and prevented implementation of more severe drought measures including customer restrictions. Operational experience during this period has been heavily relied upon in this assessment.

4.1.4 Calibration of derived sequences A variety of data sets have been used to calibrate the rainfall runoff models. These include: natural flows measured prior to reservoir construction; measured flows unaffected by abstraction/impoundments; naturalised downstream flow series


corrected for reservoir impoundment and/or abstraction, and; reservoir inflows determined by mass balance calculations.

Calibration has been based on flow duration curves and comparison of recession characteristics during low flow periods.

As part of the PR14 assessment the HYSIM rainfall-runoff model and previous flow outputs were reviewed by Hydro-Logic Ltd. The objective of this work was to extend previous flow records for deployable output assessment and to provide a continuous simulated flow series over the 1921-2010 period for the purpose of evaluating climate change scenarios.

The River Blithe inflow series used for deployable output modelling is a composite dataset Blithe_HYSIM115_rf935SimFlow_hybrid comprising:

1921- 1990: Mott MacDonald HYSIM model outputs using storage rainfall gauge data (R3, R5, R7) and regional evaporation datasets (PE3). The model parameters are calibrated to River Blithe measured flows before dam construction 1937-47 (F4). 1990-2006: Entec Ltd HYSIM model outputs using storage rainfall data and area-level evaporation datasets (PE1). Model parameters calibrated to recent Blithfield Reservoir inflows (F3). 2006-2010: HydroLogic Ltd HYSIM model outputs using tipping bucket and storage rainfall data and area-level evaporation datasets (PE2). Minor changes to rainfall factors and impermeable area parameters were made to allow calibration to previous simulated inflows (1963-89; 1991-2006).

This inflow series was used for all baseline deployable output runs in Aquator as described in Section 7.

The River Blithe inflow series HL_Oct12 used for evaluating climate change scenarios is based on HydroLogic Ltd HYSIM modeling using all datasets over the full historic record (1921- 2010). This is because the climate change assessment for PR14 requires that changes in rainfall and PE be assessed for each scenario, requiring use of a rainfall-runoff model. In previous climate change assessments for PR09, a composite series could be used as only changes to simulated flows were required. As part of production of the baseline inflow series HL_Oct12, the model parameters were further changed and reservoir evaporation calculations made to allow a calibration to recent Blithfield Reservoir inflows (F2, F3 and F5).

To consider different climate change scenarios, the inflow series HL_Oct12 was perturbed by re-running the HYSIM model with the same model parameters but with different rainfall and potential evaporation datasets.


Table 3: Summary of Hydrological Data for Blithfield Reservoir Rainfall Provider Type Period of Data Comments

R1 Cresswell (SJ973395) EA Storage 01/01/1961 01/01/1984 R2 Tipping Bucket 28/04/2000 22/04/2012 R3 Hanch Reservoir

(SK104136) EA Storage 01/01/1918 31/03/2008 R4 Storage 01/03/2008 30/06/2011 R5 Blithfield (SK075233) EA Storage 01/01/1961 30/04/2009 R6 Tipping Bucket 30/06/1974 22/04/2012 R7 Meir (SJ937421) SSW Unknown 01/07/1974 01/01/1995 R8 Wall Grange (SJ96595360) EA Storage 01/01/1882 01/04/2008 R9 Storage 01/04/2008 01/05/2012 Potential Evaporation Provider Time Step

PE1 MORECS 115 Met Office Weekly 27/12/1989 17/10/2006 PE2 MORECS 115 Met Office Monthly 01/01/1961 30/06/2011 PE3 Upper Trent Met Office Monthly 01/01/1918 31/05/1997 Determined from data at regional

climatic stations River Flows Provider

F1 BLIT8293.XLS EA Daily 01/01/1982 31/12/1993

Blithfield storage used in computation of daily flow at dam. Flows factored by 1.26 to obtain flow at HR and 1.28 to obtain flow at Trent confluence. No

adjustment for evaporation.

F2 Blithe nat_90-07_v2.xls SSW Daily 01/04/1996 28/02/2009 Blithfield storage – no flow naturalisation.

F3 Mass Balance flows at Blithfield Reservoir Dam SSW Daily 01/01/1990 31/12/2007 Continuation of the EA spreadsheet with

same factors.

F4 Naturalised flows at Hamstall Ridware EA Daily 1937 1992 Datasets used in original 1997 modelling

F5 Measured Flows at Newton Bridge SSW 15 minute 15/2/2012 ongoing

Blithfield Reservoir inflows from River Blithe calculated from river stage

measurements


4.2 Hydrogeological data and Groundwater DO assessment

4.2.1 Operational Data Data on the operation of groundwater sources have been collated and reviewed for PR14 (see Figures 2 and 3). This comprises:

• Site outputs, operational borehole outputs and manual pumping water levels for the drought periods over the last 20 years (1991/2, 1995/7, 2006 and 2010/11).

• Monthly water levels at observation boreholes (1967 – 2011) • Records of historic pumping tests and rest water levels. • Sourceworks constraints documented following experience of groundwater

source maximisation in 2010/12.

4.2.2 Groundwater DO evaluation techniques The assessment of DO is based on A Methodology for the Determination of Outputs of Groundwater Sources (UKWIR, 1995). It uses Option C with use of an assessment form and a summary diagram. The assessment form has been combined with a datasheet in a sourceworks proforma (Figure A1).

The summary diagram shows the relationship between total outputs (accumulated volume per month, expressed as Ml/d) and corresponding water levels at the borehole. Included on the diagram are all known depth-dependent features, constraints on output and test results (the latter plotted as actual pumping rates against levels). It uses the relationship between total outputs and water levels shown on the summary diagram to define a drought curve. The curve is a lower envelope through the data, extrapolated where necessary for outputs higher than those attained during operation. The deployable output corresponds to the smallest constraining output on each summary diagram and is shown both graphically and on the assessment form.

Option C has been implemented by the Company as follows:

• As the majority of sites are operated as base load a continuous pumping type curve is assumed on all summary diagrams

• Where available, step test data are used to define the yield curve, otherwise regional aquifer characteristics are used analytically. In both cases a drought curve is generated by adjusting this yield curve to match operational data.

• Summary diagrams are generated for each borehole/well on site and deployable outputs produced by appropriate summation of duty boreholes.

• Operational data from all previous drought periods are shown. However, drought curves are only matched to data representative of current drought site operation. At sites where multiple boreholes/wells are operated simultaneously, drought curves are matched to data points where appropriate interference effects can be demonstrated.

• Interpretation of summary diagrams has been carried out in close consultation with operational staff following experience of attempts to maximise groundwater output in 2011. This helped define in many cases additional water quality related constraints to yield and provided confidence in the assessment. Further close analysis of time series operational data over the normal/dry period 2006-2012 was used to determine normal year reliable yields.


• The assessment concluded that a peak demand condition assessment was not appropriate. This is because of the need to operate boreholes at continuous rates to ensure stable and reliable water quality and prevent outages. Groundwater outputs however have the potential to vary over the year because of the difference between daily and annual average licence volumes and this is explored in behavioural analysis.

The results of the Option C assessment are summarised on Table 4. These results were then tabulated against daily and annual licences (site and group level). Behavioural modelling assumed peak DO was available and considered potential constraints of all relevant peak and annual licences.

4.2.3 Validation and extension of groundwater levels The groundwater sources operated by the Company are generally much less sensitive to drought than its surface water sources. This is because groundwater levels within the high storage Permian and Triassic aquifers do not vary greatly (a few metres) when compared to pumped drawdowns (many tens of metres) and hence borehole yields are generally stable. Further checks are nevertheless required for the following reasons:

• To ensure that droughts for which operational data is available are representative of those used in behavioural analysis

• To ensure that operational data used to determine DO represents drought conditions and the way the source is used under these conditions

• To evaluate the sensitivity of individual groundwater sources to historic climate changes (drought) and those that may occur in the future

• To ensure that any seasonal variability in DO is understood and is used to advise the behavioural analysis.

Conjunctive use droughts were observed in 1933/34, 1975/76 and 1995/96. The period 2010/11 is not covered by the present model but is also regarded as a drought as groundwater levels were unusually low and these coincided with a period of low inflows to Blithfield Reservoir.

Groundwater droughts determined by the regional groundwater observation borehole at Heathlanes are similar and occurred in 1976, 1992, 1997/8 and 2011/12 (Figure 4). In each of these years groundwater levels fell below a one in 10 year minimum (90th percentile) and minimum levels were within a 0.5 m range of each other. This suggests impacts of individual droughts on groundwater yields are likely to be equivalent and can be readily applied in the conjunctive use model. There is a noticeable offset between the minimum of the mid 1990’s drought seen in the river flow series (1996) versus that in groundwater (1997-98). However there is much better correspondence between river and groundwater minima in 1976, and as this is the key period of concern, application of minimum groundwater levels from 1997/98 as representative of 1976 is a valid approach. By a similar process use of 1992, 2006 and 2011 minima are appropriate to assess the response to a 1976 event.

Data from 18 further observation boreholes local to South Staffs groundwater sites were also reviewed. These showed that, when data was available, the 1976 drought was regional and the 1992 drought was seen in most localities but that lowest (90th percentile) groundwater levels were often seen in 1997 or 1998 rather than 1996. In some localities low groundwater levels were observed in 2006. Autumn 2011 (when groundwater supply was maximized by South Staffs) is seen as a period of receding groundwater levels in all observation boreholes but is not always equivalent to the deepest levels observed. Nevertheless a similar equivalence between droughts as observed at Heathlanes can be observed in these boreholes.


Operational data at groundwater sources postdates the 1976 drought but covers the periods of low groundwater levels observed in local observation boreholes. The following was observed in conclusion of the preliminary Groundwater source DO assessment (Figures 2 and 3, Table 4):

• At some sites the lowest pumping water levels used to define the drought curves occurs in years before or after the drought year. In these cases drought curves have the potential to underestimate DO values during conjunctive use droughts.

• At two sites, Moors Gorse and Slitting Mill, operational data from earlier droughts cannot be used to define the drought curve because of changes in source operation.

• At two sites, Slade Heath and Hinksford, changes in source yield has been attributed to short term (5 – 20 year) declines in borehole specific capacity and drought impacts cannot be reliably differentiated.

• At two sites, Seedy Mill and Cookley, capital works have been carried out in 2011/12 and the design output has been used as DO. Sites where new/refurbished boreholes are to be commissioned into supply or are to be drilled/refurbished under the Company’s Borehole Asset Maintenance programme have been considered separately to this report.

• At the majority of sites DO is constrained by infrastructure (treatment or pump capacity) or by rate related water quality issues (e.g. sand)

• At only one groundwater source is there is clear relationship between drought and DO in one of the site boreholes (Moors Gorse well). This represents a difference in DO of 2.7 Ml/d out of 180Ml/d (1.5%) between normal and drought year yields and insignificant between peak week (June) and those determining ADO (Nov)

• At a further three sites, at least one borehole has a DO apparently limited by the pump intake. These do not constrain average DO values but may limit outputs relative to normal year peak week by 1.2 Ml/d.

Accordingly it has been concluded that:

• The groundwater drought of 1976 is comparable to the conjunctive use event at that time

• Groundwater droughts in 1992, 1997/8, 2006 and 2011/12 are locally equivalent in severity to the 1976 and operational dataset (1990 – 2012) is sufficient to assess this.

• The vulnerability of groundwater DO to seasonal and climatic variability is extremely low (1.5%).

The Moors Gorse source is located in a catchment poorly served by observation borehole data and is not covered by groundwater modeling and consequently this would present challenges for any behavioural analysis following the WR27 approach. Given its low contribution to DO full behavioural analysis of groundwater cannot be justified. The uncertainties in yield are recognised and have been accounted for in a more analytical way under headroom.


Figure 2: Observation Boreholes and record of groundwater droughts Data record shown by stippled (green) area. Groundwater minima shown by dark (red) squares

Observation Borehole Period of Record 1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

Heathlanes 1972 - 2012

Checkhills 1978 - 2012

Nuttalls Farm 1974 - 2011

Footherley Deep 2007-2011

Sherbrook Valley 1989 -2011

Dry Pits 1987 - 2011

Grangewood 1967-2011

Beechtree Lane 1977 - 2011

Wootton Lodge 1972-2011

Whittington Heath 1981-2011

Weeford Flats Deep 1994-2011

Kinver Old Waterworks 1976-2011

Swinford Common 1977-1997

Wiltell 1969-2011

Pipe Green 1992-2011

Stubbers Green 1976-2011

Roundhills 1992-2011

Four Crosses 1969-2011

Cat and Kittens 1998-2011


Figure 3 Groundwater sources, related observation boreholes and drought periods defining drought curves Period of operational pumping record shown by stippled area. Pumping water level minima shown by dark (red) squares.

Groundwater Source 1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

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2011

2012

Ashw oodBourne ValeBrindley BankChilcoteChurchillCookley

Crumpw ood

Fradley

Hagley

Hinksford

Hopw as

Hulme Springs

Kinver

Little Hay

Maple Brook

Mayfield

Moors Gorse

Pipe Hill

Prestw ood

Sandhills

Seedy Mill

Shenstone

Slade Heath

Slitting Mill

Somerford

Trent Valley

Beechtree Lane

CheckhillsNuttalls Farm and Footherley Deep

Sherborook Valley, Dry PitsGrangew ood

Beechtree Lane

Whittington Heath

n/a

Checkhills

Whittington Heath

Wootton Lodge

Four Crosses, Cat and Kittens

Whittington Heath, Wiltell

Sw inford Common, Roundhills

n/a

Wiltell

Wiltell, Footherley Deep, Weeford Flats

Four Crosses

Sherborook Valley, Dry Pits

Kinver Old Waterw orks, Sw inford Common

Weeford Flats

Wiltell, Pipe Green

Wootton Lodge

Sherborook Valley, Dry Pits

Weeford Flats, Footherley Deep, Stubbers Green

Wootton Lodge


Figure 4: Groundwater Levels and Droughts


Table 4: Results of groundwater source DO assessment

Site

PR14 Average

Deployable Output (Ml/d)

Reported for

FWRMP 09 (Ml/d) Difference

Peak Deployable

Output (Ml/d)

Normal Operational

Output (Ml/d)

Peak Operational

Output (Ml/d) Primary Constraints on DO

Ashwood 18.0 18.20 -0.2 Same as ADO 18.00 18.00 Pump Capacity

Bourne Vale 4.8 4.80 0 Same as ADO 4.80 4.80 Blend (controlled by demand in Sutton Coldfield)

Brindley Bank 01 1.00 0 Same as ADO 0.0 - 1.7 2.00 Pump intake Chilcote 6.9 7.00 -0.1 Same as ADO 6.90 6.90 Annual average license Churchill 10.0 10.00 0 Same as ADO 10.00 10.00 Pump capacity

Cookley 18.02 13.20 4.8 Same as ADO 15.00 18.00 Pump capacity & contact tank capacity

Crumpwood 6.1 7.00 0.9 Same as ADO 6.10 6.10 Sand

Fradley 10.0 10.00 0 Same as ADO 10.00 11.00 FY1 = pump capacity, FY2 = pump intake

Hagley 0 Hinksford 5.6 5.60 0 Same as ADO 5.60 5.60 Contact tank capacity Hopwas 2.45 2.45 0 Same as ADO 2.45 2.45 Annual average license turbidity

Hulme Springs 0

Kinver 12.823 11.80 1.02 13.5 13.50 13.50 Sub group licence/Pump capacity & blend

Little Hay 5.3 5.00 0.3 Same as ADO 0 - 5.3 5.30 Treatment

Maple Brook 6.2 6.90 -0.7 Same as ADO 6.80 6.80 MB1&4 = pump capacity, MB2&3 = sand

Mayfield 0.52 0.52 0 Same as ADO 0.52 0.52 Annual Average License & pump capacity

Moors Gorse 4.7 5.80 -1.1 Same as ADO 7.50 7.50 MG_well = adit, MG3 = pump capacity

Pipe Hill 11.6 12.64 -1.04 Same as ADO 11.60 11.60 Pump capacity & treatment Prestwood 19.03 20.00 -1 20.0 20.00 20.00 Sub group licence/Sand


Site

PR14 Average

Deployable Output (Ml/d)

Reported for

FWRMP 09 (Ml/d) Difference

Peak Deployable

Output (Ml/d)

Normal Operational

Output (Ml/d)

Peak Operational

Output (Ml/d) Primary Constraints on DO

Sandhills 0 Seedy Mill 6.02 5.40 0.6 Same as ADO 6.00 6.00 Water quality Shenstone 6.0 5.00 1 Same as ADO 0.0 - 6.0 6.00 Treatment

Slade Heath 4.18 3.90 0.7 4.6 4.60 4.80 Pump Capacity Slitting Mill 4.5 4.50 0 Same as ADO 4.40 5.00 Sand

Somerford 2.36 2.10 0.5 2.6 1.8 - 2.6 2.60 Annual average license, blend and pump intake

Trent Valley 12.9 13.70 -0.8 Same as ADO 12.90 12.90 TV1 = pump design head, TV3&TV4 = pump capacity

Totals 179.59 176.51 +3.08 181.97 187.47 Note: 1 Brindley Bank treated as a raw water transfer to Blithfield reservoir at reliable yield of 1.7Ml/d and included in conjunctive use DO

2 Design value as site refurbished in 2011/12

3 Constrained by sub-group annual average aggregate volume of Stourbridge Groundwater Unit under licence 18/54/6/140/G (Provision 5i)


5 Demand Condition and demand profiles

5.1 Demand profiles

The Company supply area is divided into twenty water supply zones (WSZ) for network management. For the purposes of behavioural modeling, where there are no in-zone sources these have been combined and a total of 15 zones are modelled along with selected treated water reservoirs. Calibration and peak demand data from the 20 WSZ hydraulic models used for network management have been interrogated. This has been normalized to 2006 and used to establish baseline (winter) demand, zonal leakage, and peak to average demand ratios for individual WSZs.

A company-level drought year demand profile was derived from company demand data spanning the period 1988-2012. The company-level demand profile was then disaggregated into supply zone demand profiles.

Daily demand data was summarised at weekly intervals to generate a weekly demand profile for each year of the record. It was evident that there were relatively few years with significant summer peaks, and also that there was substantial variation in the base demand due to a general decline in demand over the past 20 years.

Weekly demand profiles were therefore normalised with respect to the annual average demands of the respective years and used to better define the summer peaks. The highest peak week demands occurred in 1995 and 2006. The 1995 peak demand spanned the months of June to August, whereas the 2006 peak was confined to July. Other years of note include 1991, where there was a late summer peak extending into September, and 2004 when peaks were recorded in May and early June.

Within the dataset 1995 best represents the critical dry year planning case for the company and it was used to define demand profiles as shown in Figure 5.

5.2 Impact of demand restrictions

The way in which demand savings are modelled has been reviewed for PR14 for the following reasons:

• The previous modelled approach was not truly behavioural (i.e. it did not use repeatable rules over time) or spatially consistent, therefore it is potentially unreliable in determining Company DO and Levels of Service

• More evidence has emerged on the impact of restrictions on demand through an UKWIR Study (07/WR/02/3). This is based on experience of water companies in SE England during the Summer of 2006.

• South Staffs Water has reviewed its drought triggers and demand management actions for its revised Drought Plan in early 2012.


Figure 5: Disaggregated and normalised profiles for demand zones (1995)

5.2.1 Implications of UKWIR demand savings studies South Staffs has only implemented customer restrictions once since the second world war, in the summer of 1976. In line with most of the UK, it experienced hot weather in 1975, a dry winter and then a second hot summer in 1976 resulting in high customer demand and an acute supply shortage in both the Severn and Trent catchments. Measures were taken to reduce consumption through a waste reduction campaign, an intensive publicity campaign to save water and after 29th July 1976, a hosepipe ban. These restrictions continued until withdrawn following heavy rainfall in September and October. Company board papers state savings by comparison of 1976 consumption and that from 1975. They state savings of 12.65% for the three months to 30th September 1976 (approximating to the period of the hosepipe ban) and of 2.26% for the three months ended 30th June 1976 (approximating to a period of waste reduction and publicity campaigns without a hosepipe ban). The UKWIR report studied the impact of company campaigns and customer restrictions in the South East England in 2005 and 2006 when supplies were affected by low rainfall and in 2006 by hot summer weather increasing demand (the latter also affecting the Midlands). The impact of the various measures imposed were calculated by first modelling unrestricted demand as predicted by various climatic factors (sunshine hours, temperature, rainfall etc.) in unaffected years; then by comparing modelled to outturn demand for those periods when various demand savings measures and/or restrictions were imposed on customers. Savings were estimated in terms of percentage reductions in consumption (here defined as ‘distribution input minus leakage’) by calendar month by resource zone studied. A total of 6 resource zones were studied and the results showed:

• Summer demand April to October could be adequately predicted at daily to monthly frequency (correlations generally ‘good’)

• Savings were calculated for various measures including appeals for restraint, unattended hosepipe bans, hosepipe bans and non-essential use bans as well as the impact of bans in neighbouring companies

Disaggregated, Normalised 1995 WSZ Demand Profiles

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

01/01 01/02 01/03 01/04 01/05 01/06 01/07 01/08 01/09 01/10 01/11 01/12 01/01

BARCHPCLPCASCAWGLAHANHAYHOPOUTRUGSEDSHASPRSUTUTTWALWEDWESWINTotal


• Data was limited to certain periods owing to the nature of the dry weather event – there was relatively little data on appeals for restraint (March & April 2006) as most companies imposed hosepipe bans thereafter. Also there were no hosepipe ban savings data for September-October as all bans were lifted after August.

• Data for hosepipe ban savings showed an escalating pattern with savings increasing between April and July but lacked symmetry as there was much less data for August and none for September or October.

• Savings were calculated as cumulative values such that hosepipe bans savings included those from appeals for restraint which generally continued; non-essential use bans from both appeals for restraint and hosepipe bans, etc.

5.2.2 Company Approach The UKWIR findings have been used to advise revised savings profiles for SSW as follows:

• Median values for hosepipe bans have been adopted (rounded to the nearest 0.5%) for the months May to August

• Hosepipe ban savings of 5.0% have been projected for September based on the declining trend observed between July and August.

• A reduced hosepipe ban saving of 2.0% has been used for April in comparison to the UKWIR median of 3.3% as Company demands are usually normal in the month, even in dry years. The same value has been adopted for October.

• The UKWIR data for Appeals for restraint (~4.2%) has only been used to set an outlying upper band. The profile has principally been based on previous model estimates, 1976 outturn, knowledge of Company demands and the savings profile demonstrated for hosepipe bans. The previous profile has been changed by firstly extending the profile to April and assuming a 0.5% saving, and secondly by reducing the August, September and October savings by 0.5% to 2.5%, 1.5% and 0.5% thus matching the observed reduction in unrestricted summer demands after July.

• The isolated but limited data on cumulative savings from non-essential use bans derives from a single WRZ with higher than average savings. Total savings have been applied at a slightly reduced level during the spring and summer months and a low level savings value applied over the winter. This is consistent with a higher degree of domestic savings accompanying what is a non-household measure (due to the accompanying publicity) and, in the winter, to some persistent non-seasonal savings e.g. from reduction in mechanical car washing and cistern use.

• The data for hosepipe ban and non-essential use ban savings derived from UKWIR data is expressed as a percentage reduction in consumption. This has been corrected to a percentage reduction in DI using a recent mean leakage value of 74 Ml/d and an average annual demand of 380 Ml/d (approximating to Company DO)

• The “appeals for restraint” profile has not been adjusted as these are comparable to 1976 savings observed against DI.

• All savings profiles have been correlated to standard weeks (1 – 52) • All profiles apply uniformly to all demand zones in all years and replace the

selective application temporally to 1976 and spatially to Sedgley zone. The resultant change is more credible and transparent.


Table 5: Model assumptions for demand savings from customer restrictions

(after UKWIR 2007)

5.2.3 Operational Control Rules The Blithfield Reservoir-Nethertown-Seedy Mill supply system in a dry year is managed to conserve storage in the reservoir so that resources are available at times of peak customer demand. Control curves have been established for some time to guide operational management and are published within the Company Drought Plan. They act as triggers both for conjunctive use of surface water and groundwater supplies and for demand management and customer restrictions as outlined in Section 1.6.2.

UKWIR

SSW AFR

profile

Unattnedd

h/pipe ban

H/pipe ban

Non essnt

use banH/pipe

banH/pipe

banH/pipe

ban

Median H/pipe

ban effect

SSW HPB

Profile (DI-

Leakage)

SSW NEUB profile

(DI - leakage)

Study Zone B E A A A B D FJan-05Feb-05Mar-05Apr-05 -3.3%

May-05 -3.3%Jun-05 -4.9%Jul-05 -5.5%

Aug-05 -5.9% -11.6%Sep-05 -5.2% -10.4%Oct-05Nov-05Dec-05Jan-06 0.5%Feb-06 0.5%Mar-06 -4.10% 1.0%Apr-06 0.5% -3.5% -2.9% -4.2% -3.1% -3.3% 2.0% 4.0%

May-06 -4.30% 1.0% -7.3% -5.9% -8.6% -4.1% -6.6% 6.5% 10.0%Jun-06 -4.30% 2.0% -17.0% -8.9% -9.4% -4.5% -8.9% 9.0% 15.0%Jul-06 3.0% -21.3% -11.3% -11.5% -5.1% -11.3% 11.0% 18.0%

Aug-06 2.5% -9.5% -5.6% -7.6% 7.5% 12.0%Sep-06 1.5% 5.0% 8.0%Oct-06 0.5% 2.0% 4.0%Nov-06 1.0%Dec-06 0.5%

Appeals for restraint (local

media campaign)


Figure 6: Drought Triggers in Final 2012 Plan Table 6: Blithfield Control Curves

Storage (%)

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1 Drought Monitoring 81.6 85.3 90.0 86.0 81.0 76.3 71.0 68.0 66.0 66.0 69.0 76.0

3a Appeals for Restraint 67.0 70.8 75.5 71.5 66.5 61.8 56.5 52.5 51.5 51.5 54.5 61.5

4 Apply for Drought Permit

61.5 65.3 70.0 66.0 61.0 56.3 51.0 47.0 46.0 46.0 49.0 56.0

4a Temporary Use Ban 57.0 60.8 65.5 61.5 56.5 51.8 46.5 42.5 41.5 41.5 44.5 51.5

5 Implement Drought Permit

52.5 56.3 61.0 57.0 52.0 47.3 42.0 38.0 37.0 37.0 40.0 47.0

5a Non-Essential Use Ban

48 51.8 56.5 52.5 47.5 42.8 37.5 33.5 32.9 32.9 35.5 42.5

6 Emergency Storage 28.8 28.8 28.8 28.8 28.8 28.8 28.8 28.8 28.8 28.8 28.8 28.8

Flows in the River Severn are regulated by releases from Clywedog Reservoir, Lake Vyrnwy and the Shropshire Groundwater Scheme. Abstractions in the Upper River Severn are further controlled so as to maintain a minimum flow in the River Severn at


Bewdley to protect users downstream to the Severn Estuary. Under the current control rules the Environment Agency is required to maintain a flow at Bewdley of at least 850 Ml/d (as a 5 day average), with a minimum daily flow of 650 Ml/d.

The main conditions affecting the Company are a 5% reduction in abstraction licence at HL when the drought order in force trigger line is crossed at Clywedog reservoir. The key factors which control the Company’s abstraction licence at HL are the flow on the River Severn at Bewdley, the river Regulation status, and the storage level at Clywedog reservoir at the head of the river.

The flow factors are summarised in Table 7 below.

Table 7: Operational Rules for abstraction from River Severn at HL

Model Flow Band Condition Max abstraction from River Severn (Ml/d)

0 Bewdley Flow> 1,100 Ml/d *320 Apr-Oct *400 Nov-March

1 Bewdley Flow< 1,100 Ml/d and no River Regulation 280

2 River Regulation 212

3 Maximum Regulation 212

4 EA Drought Order in Force 201

* Abstraction rates are further constrained by pump capacity at HL and joint licence conditions with Trimpley WTW. These factors tend to slow refill times after peak demand periods when Chelmarsh is drawn down. Chelmarsh Reservoir provides bankside storage for the HL river abstraction. In dry years it also permits peak demand to be met when abstraction is limited by regulation. This is controlled in the model by the curves shown in Table 8.

Table 8: Chelmarsh Control Curves

Control Line Storage (%)

1 Full 100

2 n/a 65

3 Penalty Cost 55

4 Dead Storage 10

5.3 Treatment Losses

Losses and operational use at treatment works were not included within the DO modelling assessment in PR09. For the PR14 assessment of DO treatment losses have been included explicitly in DO.

Losses at surface water works are largely rate dependant and daily treated volumes vary seasonally and during drought periods. Volumes lost in critical periods and their impact on DO can therefore be predicted. This has the additional benefit in that the volumes of water simulated as conveyed through the network is lower and more realistic. Raw water operational use, losses and transfers are not explicitly Final Water Resources Management Plan 2014: Appendix B (Deployable Output) Page 27

considered but are evaluated in the DO assessment at individual source level (see above).

Waste water losses at sourceworks were evaluated for PR14. This review consisted of a review to identify all sourceworks processes, monitoring and discharge points as well as intra-stage metering. Spot measurements were made on site of all sample and monitor flows. All identified process losses have been either calculated from operational design criteria (i.e. rate, frequency, duration and/or volumes), or in the case of the larger works from effluent meter records. In the case of groundwater sites losses are fixed but in the case of surface water works the variable loss rate has been calculated from average raw water works input rates over the period of measurement. The results of the 2012 review are summarised in the table below.

Table 9: Treatment loss functions used in DO model

5.4 Emergency Storage

The total storage volume in Blithfield Reservoir is 18172 Ml. Approximately 25% of this (i.e. 4543 Ml) is considered as ‘dead storage’ due to the position of the lowest draw-off and water quality constraints.

The requirements for emergency storage (EA, 1997) mean that a further 30 days at the minimum treatment capacity rate of 23 Ml/d (690 Ml) has to be reserved for emergency storage over and above ‘dead storage’. In total therefore, 5233 Ml or 28.8% of the total volume of Blithfield should be maintained as a minimum storage in the yield assessment.

Site Works Losses Type

HL Up to 8.24 Ml/d Variable 3.8% of raw water

SM Up to 9.24 Ml/d Variable 7.7% of raw water

Chilcote 0.08 Ml/d Constant

Crumpwood 0.02 Ml/d Constant

Fradley 0.35 Ml/d Constant

Little Hay 0.21 Ml/d Constant

Moors Gorse 0.19 Ml/d Constant

Pipe Hill 0.29 Ml/d Constant

Shenstone 0.17 Ml/d Constant

Slade Heath 0.06 Ml/d Constant

Slitting Mill 0.00 Ml/d Constant


6 Assessment of Average and Peak Deployable Output

As discussed in Section 4.2.3 the preliminary source output assessment of groundwater sites in 2012 concluded that there was little evidence for seasonally variable groundwater DO values at site level. Nevertheless the peak DO values are constrained by licence conditions both at group and sub-group level and this has been explicitly modeled.

The supply system has been modeled to investigate the benefits of conjunctive use of surface water and groundwater sources. At peak periods the model allows greater use of raw water storage in Blithfield reservoir and, to a lesser extent, raw water from Chelmarsh and treated water storage in service reservoirs. Outside peak demand periods, storage is preserved and /or recovers with greater use of groundwater and pumped water transfers.

In critical drought years as Blithfield Reservoir storage levels fall, a sequence of customer demand restrictions are implemented. Similar supply measures are also implemented but these are limited to operation of the Brindley Bank raw groundwater source and Nethertown pump-back scheme within normal licence constraints and do not include operation of any drought permits.

Annual average deployable outputs are represented by the highest annual average demand that can be met before the levels of service criteria are not met, or that use of emergency storage is required. Peak deployable output is the demand met in peak week of the same model run.

Developments in the conjunctive use model since PR09 are summarized in Table 11. This summarises the model changes discussed in previous sections and includes an estimate of the impact of the model changes on deployable output in terms of whether it increased DO (+), decreased DO (-) or had negligible impact (neg).


Table 10: Results of Baseline Deployable Output Scenarios Baseline Reference

LoS DO Baseline Company

LoS DO Baseline No

Restrictions DO DYAA DO Scaled 376.5 368.3 342.4

DYAA DO Scaled and Fixed Export 377.9 369.7 343.8

DYAA DO Scaled and Fixed incl STW 418.5 410.3 384.4

DYCP DO Scaled 466.9 456.7 424.6

DYCP DO Scaled and Fixed Export 468.3 458.1 426.0

DYCP DO Scaled and Fixed incl STW 516.3 506.1 474.0

Failure Mode Includes 5 Level 4 events (1921, 1929, 1934, 1976, 1996) and two Level 5 events (1934 and 1976). Failure triggered by Blithfield Reservoir reaching emergency storage (1976).

Includes 2 Level 4 events (1934, 1976) and two Level 5 events (1934 and 1976). Failure triggered by third Level 4 event (1996).

Fails when Blithfield Reservoir reaches emergency storage (1976).

Note:1 DYAA DO Scaled Dry Year Annual Average demand from Company demand centres met by supply.

2 DYAA DO Scale Fixed additional includes minor bulk transfers with fixed volumes. This is the Deployable Output value used in the Company water balance.

3 DYAA DO Scale Fixed incl. STW includes the Wolverhampton entitlement of 40 Ml/d average. This volume is not reported as HL is a shared resource with STWL.

4 DYCP Dry Year Critical Period (peak week) equivalent volumes. Note STW Wolverhampton entitlement is 48 Ml/d at peak.

5 Level 4 restrictions equivalent to Temporary Use Ban. Level 5 restrictions equivalent to non-essential use ban.


Figure 7: Impact of Levels of Service on Deployable Output


Table 11: Developments in approach since PR09

Item Description Comments Impact DYAA

DO

Software Platform change from WRAPSIM to Aquator

Previous model parameters, rules and datasets audited and documented and outputs verified

-

Model schematic and links

Demand zone review

Number of demand zones modelled increased from 7 to 15. Based on water supply zones with hydraulic mains models. Criteria for aggregation based on presence/absence of sources in WSZ and peak demand characteristics.

neg

Transfer mains The number of links increased in model to reflect trunk main network in hydraulic model. neg

STWL links Additional links added to model schematic to facilitate modelling of water trading scenarios with STWL

neg

Service reservoirs

Key treated water reservoirs added to model. Net capacity assumptions based on diurnal use pattern. Introduced to simulate peak week use of treated water storage under reference LoS and climate change scenarios.

+

Demand profile

Hydraulic modelling data

Demand split between demand zones changed based on a review of hydraulic models data. Peak/average ratios determined on zone-by-zone basis, whereas previously fixed ratio used.

-

Dry year profile

Profile amended to reflect decline in average demand over time and reduction in leakage and non-household use. Fixed winter baseline demand set based on 2011 data. Summer profile based on excess consumption observed in 1995.

This resulted in increase in peak/average ratio from 1.18 to 1.24.

-

(DYCP ++)

Impact of customer

restrictions

Demand savings assumptions

Demand savings for appeals for restraint, temporary use bans and non essential use bans reviewed against and aligned to UKWIR (2007) guidance. Seasonal profile retained. Applied uniformly to all demand zones (previously bias to Sedgely zone) and using control curves (previously savings fixed in 1976).

+

Control curves

Principal control curves retained but secondary curves established to match changes to drought management triggers in Drought Plan in 2012. Separate curves established for Appeals for restraint, TUB's and non essential use bans.

-



DO

Blithfield- Nethertown-Seedy Mill

supply system

River Trent HOF

Flow series changed from naturalised flows at Colwick with HOF of 2500 Ml/d to simulated flows at North Muskham with HOF of 2650 Ml/d

neg

Nethertown operation rules

Rules amended to reflect operational practice in 2010/12 drought. Triggered by Level 1 curve (drought monitoring) whereas previously used secondary lower curves. Transfer capacity increased to 28 Ml/d subject to availability of water in lower Blithe.

+

Peak capacity Raw water capacity reviewed and amended to meet original design capacity (increase in raw capacity from 120 Ml/d to 125 Ml/d)

+

HYSIM modelling

Rainfall and PE data collated for River Blithe catchment. Recalibration of HYSIM model for use to extend existing inflow series (2006 – 2010) and to act as baseline for CC scenarios (2021- 2010).

-/+

HL

Licence under river regulation

No changes to control rules. Put and take licence included in model increasing permitted abstraction by up to 11 Ml/d

+

Chelmarsh Reservoir

Compensation release to River Severn (0.227 Ml/d) added neg

Peak capacity Raw water capacity of 216 Ml/d not changed. n/a

West Bromwich Booster

Transfer capacity reviewed following operational performance in 2010/11 and new profile adopted.

+

Raw water storage

Overdraw facilities

Facility to draw down Blithfield and Chelmarsh Reservoir at maximum rates during peak week retained

n/a

Emergency storage Emergency storage assumptions retained n/a

Groundwater sources

DO review 2012 Individual source deployable outputs reviewed and amended. +

Licence constraints and blending

Peak and aggregate licence volumes treated as constraints and explicitly modelled. Previously fixed volumes or profiles used. All blending arrangements are based on fixed volumes or included within site DO value, so no additional modelling requirement.

neg

Peak DO

Change from fixed profile to behavioural modelling of groundwater DO values. Note seasonal changes in DO are driven by licence only, not changes in yield so full behavioural analysis not required.

+

Treatment Losses 2012 update Review of waste water losses carried out

based on 2010/11 data. n/a



DO

Modelling approach

Treatment losses modelled explicitly within model and included within DO assessment whereas previously reported in WRP tables. Fixed losses assumed at all groundwater sources and rate-dependant losses at surface water works.

+

Interface with STWL

Bulk transfers

Additional links added to model schematic to facilitate future modelling of water trading options with STWL. Review of existing transfer volumes but flat profile left unchanged for planning purposes.

n/a

Wolverhampton Demand profile reviewed but left unchanged as reflects SWL entitlement and 1995 consumptions.

n/a

Use of regional model flows

In addition to simulated resource states of River Severn previously provided by STWL, new model uses River Trent flows at North Muskham. Models make use of current STWL outputs equivalent to purpose of prediction outputs (i.e. baseline DO, wet climate change scenario, dry climate change scenario, etc.)

neg

Simulation of SSW system

Additional data provided to STWL concerning works capacities, DO and licence as well as control curves and demand savings assumptions. STWL have updated simplified schematic and used this to produce an improved simulation of river flows and resource states of Rivers Trent and Severn.

n/a


Figure 8 Comparison of predicted Blithfield reservoir storage levels under different levels of service


Appendix D: Calculation of Deployable output - draft

7 Confidence Labelling

WR27 proposes a system for confidence labeling for DO assessments. This depends on two factors namely:

• The availability of long records of continuous and homogeneous hydrological/hydrogeological data sets for a robust DO assessment, and

• Consistency of constraints data, by reference to the agreed definition of DO

The supply system has been assessed as a conjunctive use system with surface water and groundwater sources, with intra-zonal transfers and with complex constraints on outputs. The degree of constraints has been assessed as High/Medium (see Section 3.5).

Constraints include treatment works capacity, abstraction licence limits, pump capacity and intake structure capacity.

• Availability of constraints data is assessed as A (available and of consistent quality).

Ninety years of hydrological data are used. There has been some infilling using hindcasting techniques. Twenty years of good hydrogeological data have been correlated to regional datasets of 45 years.

• This data has been assessed as C (<70years).

Accordingly the Confidence Label is determined as: AC

As the Company has a large supply surplus this confidence grade is regarded as appropriate for the purposes of water resources planning and no remedial work is required.

SSW dWRMP Page 36 of 39


8 DO Assessment results

Three levels of service scenarios have been assessed as follows:

• Company level of service, which is for a temporary use ban every 40 years on average.

• Reference level of service, which is for a temporary use ban every 10 years on average, or a non-essential use ban every 40 years on average.

• Unrestricted level of service, which is for no temporary use ban to be required within the period of the model duration.

Table 10 lists the results of the deployable output assessment for three baseline scenarios and these are illustrated in Figure 7. The impact of these scenarios on storage levels at Blithfield Reservoir in the 1975/76 drought period is shown in Figure 8.

The reference levels of service scenario fails due to more than 2 non-essential use bans occurring in the period of record (1 in 40 years). The temporary use ban level of service for this return period is however greater at no more than 5 failures (around 1 in 20 years). However the increase in DO is not significant at less than 10 Ml/d.

The No Restrictions scenario implies a DO of more than 25 Ml/d lower than the Company LoS. The Company currently has a large surplus in the base year and it is likely that its effective LoS is somewhere between this level and 1 in 60 years. However at such high return periods these values are difficult to estimate with a flow record of 89 years and there is inadequate justification in considering a move to a No Restrictions LoS.



Figure A1: Groundwater Sourceworks Data Proforma

South Staffs Water – Deployable Output Assessments 2011/2012 Groundwater Sourceworks

Source Name Data Source

Maximo Asset Tag 1

General Information

Source Description

2, 3, 4, 5

2

Aquifer Unit 6, 7

Normal operation 3, 8,9

Datum 5, 10

Lowest groundwater 11

Nearest EA OBH 12

Drought month (hydrograph) 12

Rest WL & Pumping WL 11

Constraints

Annual licence

• 13

Peak day licence

Pump details 14, 15

Pump inlet level

15, 16 Safe pumping level (+3.0m)

Duty capacity

Duty head

Treatment details & losses

17, 18

Environmental 13

Turbidity/sand 8

Air 8

Adit details 4, 5

DAPWL 11

Data SRO Discharge Data 10

SRO PWL data 10

Drought Curves 10

Uncertainty



Figure A1: Groundwater Sourceworks Data Proforma

Deployable Output

Drought Deployable Output (Ml/d) Constrained by Drought Potential

Yield (Ml/d)1 Constrained by

GWS1 0.0

From SRO analysis

GWSW2 0.0 GWSW3 9.0

GWSW4 9.0 GWSW5 Not used

GWSW6 Not used Total site (two BHs pumping) 18.0

Normal Operational Output (Ml/d) Constrained by Peak Operational

Output (Ml/d) Constrained by

GWSW1 0.0

0.0

Pump capacity Production Information

GWSW2 0.0 0.0 GWSW3 9.0

9.0

GWSW4 9.0 9.0 GWSW5 Not used

Not used

GWSW6 Not used Not used Total site (two BHs pumping) 18.0 18.0

Notes

FWRMP DYAA DO, 2009 (Ml/d) FWRMP DO, 2009 (Ml/d)

Reassessed DO (Ml/d) Reason

Recommendations/Actions

Actions

Telemetry •

Investigations •

Proposed Future Works .

AMP, Year AMP5 or AMP6, 2014/15

Revision No. Date: Initials Brief Details of Changes Checked by:

1 Potential Yield is a theoretical yield based on DAPWL (deepest advisable pumping water level) and is not related to operational practice.

References

1.


Documents

Water Resources Management Plan 2014 - South Staffs … · the 2013 Draft Water Resources Management Plan (dWRMP) have been produced. ... deployable output • The impact of levels