Severn Trent Water AMP6 Low Flows Programme: River Noe and River Ashop Low Flow Distribution Assessment

Severn Trent Wa ter AMP6 Low Flows Progra mme: River Noe a nd River Ashop Low Flow Distribution Assessment

Official Sensitive

(Sensitive information redacted)

13 February 2020

Severn Trent Wa ter AMP6 Low Flows Progra mme: River Noe a nd River Ashop Low Flow Distribution Assessment

Prepared for: Severn Trent Water Ltd 2 St John’s Street Coventry CV1 2LZ

Report reference: 64116AB R36rev1, February 2020

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Report status: Final rev1

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Severn Trent Water AMP6 Low Flows Programme: River Noe and River Ashop Low Flow Distribution Assessment

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OFFICIAL SENSITIVE

This document is classified by Severn Trent Water Ltd (STWL) as Official Sensitive and the information contained within is sensitive. Distribution of this document must be restricted and managed within organisations given access to it. If in doubt, please seek STWL’s permission before this document is shared with third parties.

This report has been prepared by Stantec UK Ltd (Stantec) in its professional capacity as environmental specialists, with reasonable skill, care and diligence within the agreed scope and terms of contract and taking account of the manpower and resources devoted to it by agreement with its client and is provided by Stantec solely for the internal use of its client.

The advice and opinions in this report should be read and relied on only in the context of the report as a whole, taking account of the terms of reference agreed with the client. The findings are based on the information made available to Stantec at the date of the report (and will have been assumed to be correct) and on current UK standards, codes, technology and practices as at that time. They do not purport to include any manner of legal advice or opinion. New information or changes in conditions and regulatory requirements may occur in future, which will change the conclusions presented here.

This report is confidential to the client. The client may submit the report to regulatory bodies, where appropriate. Should the client wish to release this report to any other third party for that party’s reliance, Stantec may, by prior written agreement, agree to such release, provided that it is acknowledged that Stantec accepts no responsibility of any nature to any third party to whom this report or any part thereof is made known. Stantec accepts no responsibility for any loss or damage incurred as a result, and the third party does not acquire any rights whatsoever, contractual or otherwise, against Stantec except as expressly agreed with Stantec in writing.

Report Reference: 64116AB R36rev1

Report Status: Final rev1

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Name

Adam Sutcliffe, Thomas Brook
Rick Hayes

Signature

Author

APEM

Checked by

Dan Cadman Paul Daily
Reviewed by

Revision record:

Reference

Date

Status

Comment

Author

Checker

Reviewer

Issued to

64116AB R36D1

19/03/2019

Draft

First draft

AS/TB

RH

DC/PD

STWL*

STWL/EA** STWL/EA STWL/EA
64116AB R36D2
Second draft
23/05/2019 07/06/2019 04/10/2019
Draft Draft Draft
AS/TB AS/TB AS/TB
RH RH RH
DC/PD DC/PD DC/PD
64116AB R36D3
Third draft Final draft
64116AB R36D4

Final for client checks
64116AB
R36

19/11/2019

Final

AS/TB

AS/TB
RH RH
DC/PD DC/PD
STWL
64116AB R36rev1

20/12/2019 Final rev1

Final

STWL/EA

*Severn Trent Water Limited. **Environment Agency

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Exec utive Summa ry

The River Ashop, River Noe and Jaggers Clough transfers are important sources of water to the Derwent Valley Reservoirs. Abstraction at the three structures must cease when inflows reach predetermined conditions, stipulated in the associated abstraction licence. Impact Assessment and Options Appraisal undertaken during AMP5 and AMP6 resulted in recommendations to change conditions on these abstraction licences under normal operation. This report complements those studies, recommending how compensation flows at the structures should be operated under natural circumstances when inflows cannot simultaneously maintain recommended flows in the River Ashop, River Noe and Jaggers Clough. This report does not

recommend changes to the operation of these structures under typical climatic conditions.

Naturally arising circumstances during which inflows cannot simultaneously maintain recommended flows on the River Ashop, the River Noe and the Jaggers Clough can occur in dry years between late spring and midautumn. There is no storage at any of the structures, meaning that during such circumstances, trade-offs may be needed in maintaining the ecology of the three rivers. Historically, support has been provided to the Jaggers Clough at the expense of flows in the River Noe or, less often, the River Ashop. This support has maintained flows in the Jaggers Clough at rates several times greater than those naturally expected in this stream during dry periods and can leave very small flows on the main stem of the Rivers Ashop and Noe.

The Jaggers Clough is valued as a brown trout nursery, and as such its importance to the River Noe fishery may be greater than suggested by its size. Even so, analysis of habitat and experience during the very dry summer of 2018 do not suggest long-lasting impacts, and in any case, it is not standard practice to transfer raw water between catchments to support ecology in a recipient waterbody. Moreover, the historic practice of reallocating flows risks of deterioration in Water Framework Directive (WFD) status in the waterbodies downstream. The WFD makes provision for effects of naturally arising low flows, and in the absence of intervention, temporary impacts caused by natural low flows do not constitute a deterioration in status. This may not be the case were STWL to reallocate flows, regardless of the intention behind the reallocation.

At the Ashop impoundment, the requirement to cease abstraction into the diversion channel is also problematic operationally. Any water transferred to the Jaggers Clough must also use this offtake. Under natural circumstances in which compensation flows on the Jaggers Clough cannot be met from the Noe and Jaggers combined inflow, the requirement to cease abstraction into the diversion channel can therefore be at odds with that to maintain the compensation on the Jaggers Clough.

This report therefore recommends that support to the Jaggers Clough during naturally arising circumstances of low inflow any diversion of flow from the River Ashop to the Jaggers Clough should cease and support to the Jaggers Clough should be sourced from the River Noe in reduced quantity. To avoid rapid changes in flow, reductions should be preceded by a preparatory change implemented when there is sufficient flow in the River Noe to prevent rapid changes in both watercourses. Recommendations for operation of the compensation during naturally arising circumstances of low flows make provision for future changes as prescribed by the Notional Solution at these sites. However, under current provisions:

•

Once combined baseflows to the River Noe impoundment and Jaggers Clough compensation fall to 18 Ml/d, support to the Jaggers Clough can be reduced to maintain flows on the Jaggers Clough from 10 Ml/d to 6 Ml/d. If this results in temporary baseflow increases above 10 Ml/d on the River Noe these should be passed at the impoundment and not abstracted.

•

Once flows to the River Noe impoundment and Jaggers Clough fall to 13 Ml/d, support from the River Noe can be reduced to maintain flows on the Jaggers Clough at 2 Ml/d.

The effects of any such changes should also be monitored.

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C ontents

1

INTRODUCTION

1

11
1.1 1.2
This report Previous studies

2

OPERATIONAL CONSIDERATIONS

Abstractions on the River Ashop, River Noe and Jaggers Clough Current licence requirements
22234
2.1 2.2 2.3 2.4
Recommended licence changes Limitations on operational flexibility

3

ECOLOGICAL CONSIDERATIONS

Potential effects on ecology Character of reaches
5567
3.1 3.2

3.3

Water Framework Directive

4

EFFECTS OF THE 2018 DRY PERIOD

9

4.1

4.2 4.3
Flows Macroinvertebrates Fish
9
11

5

SUMMARY AND RECOMMENDATIONS

Summary
15 15 15 15 16 17 18 18 19
5.1 5.1.1 5.1.2 5.1.3 5.2
Normal operation Operation during natural circumstances of low flow Potential impacts Recommendations
5.2.1 5.2.2 5.2.3
Phasing of flow changes – current licence conditions Phasing of flow changes post implementation of the Notional Solution Simulation of recommendations during 2018

REFERENCES

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ILLUSTRATIONS

Illustration 2.1: Schematic of Ashop-Noe-Jaggers system Illustration 4.1 Macroinvertebrate data for the Ashop and Noe.
3
10 12 13 13 14 20
Illustration 4.2. Standardised fish density (per 100 m²), Ashop d/s Alport confluence (E3-06) Illustration 4.3. Standardised fish density (per 100 m²), Noe d/s impoundment (E2-03) Illustration 4.4. Standardised fish density (per 100 m²) Noe d/s Jaggers Clough confluence (E2-07). Illustration 4.5. Standardised fish density (per 100 m²), Jaggers Clough d/s compensation (E2-06) Illustration 5.1 Simulated effect of low flow allocation on the Ashop for the 2018 dry period Illustration 5.2 Simulated effect of low flow allocation on the Noe and Jaggers Clough for the 2018 dry period.
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Illustration A.5.3 Measured inflows; River Ashop (E3-01), River Noe (E2-02), Jaggers Clough (E2-05a) Illustration A.5.4 Double mass plots; Ashop vs Noe and Jaggers Clough inflows, Jan 2017 to Feb 2019 Illustration A.5.5 Flows lower than operational thresholds: River Noe (top) and River Ashop (bottom)
25 25 26

TABLES

Table 3.1 Sensitivities of fish species and lifestages late May to early October

5

APPENDICES

Appendix A Appendix B Appendix C
Inflows Predicted habitat changes in downstream reaches Operation during the 2018 dry period

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1 Introduc tion

1.1 This report

The River Ashop, River Noe and Jaggers Clough PWS transfers are important sources of water to the Derwent Valley Reservoirs, water from the Ashop being diverted to Derwent Reservoir and water from the Noe being transferred to Ladybower.

Impact Assessment and Options Appraisal undertaken during AMP5 and AMP6 resulted in recommendations to change licence conditions for Severn Trent Water Limited’s (STWL’s) abstraction licence for the River Ashop impoundment, the River Noe impoundment and Jaggers Clough compensation under normal operation.

This report complements those studies, recommending how compensation flows at the structures should be operated under natural circumstances when inflows cannot simultaneously maintain recommended flows in the River Ashop, River Noe and Jaggers Clough, and when, therefore, trade-offs may be needed between ecological receptors in the different streams. It sets out current and proposed licence requirements, operational constraints, the potential effects of low flows and recent experience of low flows in the 2018 dry period, drawing on the findings of previous reports as well as on new data collected during the exceptionally low flows experienced in the summer and autumn of 2018:

••

Section 1 – this section - introduces the aims of the report, with reference to previous studies. Section 2 describes the operation of the Ashop and Noe water transfer system and sets out operational considerations of relevance to recommended flow allocations.

•••

Section 3 describes the reaches downstream of the Ashop and Noe impoundments and the Jaggers Clough compensation, and sets out ecological considerations of relevance to flow allocation, including consideration of potential effects on Water Framework Directive (WFD) status.

Section 4 describes recent experience during the 2018 dry period, including flows into and out of the Ashop, Noe and Jaggers Clough and setting biological data collected during the period in the context of baseline variability from previous years.

Section 5 provides a summary and sets out recommendations for the allocation of flows under natural low flow conditions.

1.2 Previous studies

Previous studies have assessed the long-term impact of flow changes on the Rivers Ashop, Noe and Jaggers Clough. Results are presented in the following reports:

••••

The AMP5 Impact Assessment report (ESI and APEM 2013), The AMP6 compensation release trial reports (ESI and APEM 2016a; ESI and APEM, 2016b) The AMP6 Compensation Review reports (APEM and ESI, 2017; APEM and ESI, 2018a) The AMP6 Options Appraisal reports (ESI and APEM, 2018b; ESI and APEM, 2018c).
Prior to this, the Environment Agency had also undertaken a PHABSIM assessment when compensation flows were first implemented at the Ashop and Noe impoundments (EA, 1999a/b).

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2 Opera tiona l c onsidera tions

2.1 Abstra c tions on the River Ashop, River Noe a nd Ja ggers C lough

The site details and operation of the River Ashop impoundment, River Noe impoundment and Jaggers Clough are provided in ESI & APEM (2017) and ESI & APEM (2018a).

The River Noe is impounded at Netherbooth, approximately 2 km upstream of its confluence with Jaggers Clough. Flow diverted by the impoundment of the River Noe enters the ‘Noe to Jaggers Clough’ section of the gravity diversion transfer tunnel to Ladybower. As this crosses Jaggers Clough at a second STWL structure, ‘Top House’ (SK 16 86), water abstracted from the Noe is supplemented by all flow in Jaggers Clough upstream. Under normal operation, a regulated compensation release is made from the Ladybower tunnel back into Jaggers Clough at Top House. Under normal circumstances (i.e. when natural flows are able to support the full compensation volume at all structures) the volume of water discharged to the Jaggers Clough shall be no less than 10 Ml/d and no greater than 17 Ml/d. Any remaining water in the tunnel in excess of this volume can be transferred to Ladybower Reservoir.

The Ashop impoundment comprises a weir across the River Ashop, with an intake to divert flow into an open channel and thence a tunnel to Derwent Reservoir. The transfer to the Jaggers Clough is from an offtake off the channel/ tunnel and is only made to meet compensation requirements during periods of low flow when the River Noe alone cannot support the Jaggers Clough. This was originally required when the Jaggers Clough compensation was set at 17 Ml/d¹; however, this is now only rarely needed, and in any case relies upon water being available in the River Ashop.

2.2 C urrent lic enc e requirements

The current licensed compensation flow release at the Ashop impoundment is 5 Ml/d. Under the abstraction licence this flow need not be maintained should natural inflows fall below this figure, but the gates to the diversion tunnel must be closed to prevent further abstraction.

Current compensation flow requirements for the River Noe are for a minimum release of 10 Ml/d at both the Noe impoundment and at Top House on Jaggers Clough. Under the abstraction licence flow on the River Noe need not be maintained should natural inflows fall below this figure.

Under current licence conditions, STWL are, however, required to divert water from the Noe and, if necessary, the Ashop impoundment to maintain the Jaggers Clough compensation.

Taken together, the required total inflow required to meet the compensation requirement of 10 Ml/d on the Jaggers Clough and to pass 10 Ml/d at the Noe impoundment and 5 Ml/d at the Ashop impoundment and is 25 Ml/d.

¹The original compensation was set in the 1940s for a minimum of 17 Ml/d, which was reduced to a minimum of 10 Ml/d in 2004.

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Illustration 2.1: Schematic of Ashop-Noe-Jaggers system

2.3 Rec ommended lic enc e c ha nges

Through the AMP5 and AMP6 investigations, amendments to the licensed requirements for compensation flows on the three watercourses have been proposed as the notional solution for the Ashop and Noe sites:

APEM & ESI (2017) recommended that the licence condition for the Ashop impoundment should be expressed as a requirement for a mean monthly minimum flow of 8.5 Ml/d, with a minimum permissible flow of 5.5 Ml/d, when inflows allowed. This recommendation reflected operational experience in recent years, which demonstrated that average flows of 8.5 Ml/d were sufficient to support GES, but also allowed short term reductions in flow to 5.5 Ml/d, due to measurement error and/or the level of control possible at the Ashop impoundment structure. Operational experience also demonstrated this to be compatible with supporting GES.

For the Noe impoundment and Jaggers Clough compensation, APEM & ESI (2018a) recommended a phasing of flow changes. The compensation flow on the Jaggers Clough will initially be reduced to 8 Ml/d and the required flow at which abstraction at the Noe impoundment must cease will be increased to 12 Ml/d. If this change is demonstrated not to cause impacts during adaptive management, a subsequent increase on the Noe to 14 Ml/d and reduction on Jaggers Clough to 6 Ml/d will be implemented. This phasing maintained aggregate flows downstream of the River Noe/ Jaggers Clough confluence at 20 Ml/d.

Taken together, the total of these flow requirements is 25.5 Ml/d as a minimum instantaneous flow, and 28.5 Ml/d as a minimum monthly average.

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2.4 Limita tions on opera tiona l flexibility

Natural circumstances when inflows cannot simultaneously maintain recommended flows in the River Ashop, River Noe and Jaggers Clough have been examined using HYSIM simulated inflows and are assessed in further detail in Appendix A. Flows below current or proposed licence thresholds are more frequent on the Noe than the Ashop, and as might be expected, occur predominantly in the late spring to early autumn period. However, combined inflows to the River Noe and Jaggers Clough may occasionally fall below 20 Ml/d in any month.

During these natural circumstances, current licence requirements present operational problems. The structures were built primarily to ensure sufficient head to divert water from the streams to the Derwent and Ladybower Reservoirs (Ashop and Noe impoundments respectively). There is little storage behind any of the structures and this lack of storage means that the existing infrastructure cannot maintain downstream flows above natural inflows simultaneously in the River Ashop, River Noe and Jaggers Clough. Given the lack of storage, the licenced requirement to maintain compensation flows to the Jaggers Clough may leave very small flows on the main stem of the rivers Ashop and Noe downstream of the respective impoundments.