RIVER DARENT LOW FLOW ALLEVIATION
MAIN REPORT NRA
RIVER DARENT LOW FLOW ALLEVIATION
National Information Centre The Environment Agency Rio House Waterside Drive Aztec West BRISTOL BSi 2 4UD
Due for return
MAIN REPORT
National Rivers Authority Southern Region [Nc+!onal Rivers A uthority f '' nation Centre July 1994 s O f f i c e
iv !o __ ENVIRONMENT AGENCY
| ~ '3Slon No . B O f f ...... 099858 RIVER DARENT LOW FLOW ALLEVIATION
CONTENTS
CHAPTER 1 INTRODUCTION
1.1 The Problem 1.2 History of Events
CHAPTER 2 REVIEW OF PAST REPORTS
2.1 Introduction 2.2 Water Management Study (Dossor, 1978) 2.3 Study of Alleviation of Low River Flows Resulting from Groundwater Abstractions, Interim Report Annex A (May 1987) Volume 1 Main Report and Volume 2 - Darent Case Study (Halcrow, 1988) 2.4 Environmentally Acceptable Flow Regime Study (Atkins, 1991) 2.5 Darent Catchment Investigation (GDC, 1991) 2.6 Groundwater Study of Blue Circle Industries Northfleet Works (GDC, 1992) 2.7 A Strategy for the Enhancement of the River Darent, Kent (Mott MacDonald, August 1992)
CHAPTER 3 GROUNDWATER AND SURFACE WATER DATA
3.1 Data Requirements and Sources 3.2 Catchment Description 3.3 Geology 3.4 Rainfall and Evaporation 3.5 River Flow - 3.6 Groundwater Levels 3.7 Aquifer Properties 3.8 Groundwater Abstractions
7O223B10VWRP\B\RDER-Jol 94\«rp ( 0 CONTENTS (cont) Page Nr
CHAPTER 4 INTEGRATED GROUNDWATER/SURFACE WATER MODELLING
4.1 Description of Model 4-1 4.2 Calibration 4-2 4.3 Calibrated Flows 4-2 4.4 Catchment Water Balance 4-3 4.5 River Losses 4-4 4.6 Naturalised Flows 4-7 4.7 Target Flows 4-8 4.8 Options for Restoring River Flows 4-10 4.8.1 Introduction 4-10 4.8.2 Abstraction at Full Licence Capacity 4-11 4.8.3 Reductions in Abstraction 4-11 4.8.4 Conjunctive Use of Groundwater and Thames Water 4-13 4.8.5 Augmentation of River Flows 4-14 4.8.6 Conclusions 4-15
CHAPTER 5 ECOLOGICALLY AND ENVIRONMENTALLY ACCEPTABLE FLOWS
5.1 Introduction 5-1 5.2 Early River Darent Studies (Dossor, 1978 and Halcrow 1987,1988,1989) 5-1 5.3 Environmentally Acceptable Flow Regimes (W S Atkins, 1990) 5-2 5.4 Ecologically Acceptable Flows (IH, April 1993) 5-3 5.4.1 Assessment of Instream Flow Incremental Methodology (IFIM), (IH/IFE, April 1993) 5-3 5.4.2 Flow Regime Requirements, River Darent (IH/IFE, Unpublished 1993) 5-6
5.5 Conclusions 5-8
70223B1CNWRPVB\RDER-Jal 94\wp (H) CONTENTS (cont)
CHAPTER 6 ENGINEERING OPTIONS INVESTIGATED
6.1 Introduction 6.2 Flow Augmentation 6.3 Water Supply System 6.4 Demand Growth 6.5 Review of Engineering Options 6.5.1 General 6.5.2 Long-term Options 6.5.3 Short-term Options 6.5.4 Darent Action Plan (TWUL/NRA, 1992)
CHAPTER 7 PLAN FOR THE DARENT (NRA/TWUL, 1992)
7.1 Introduction 7.2 Statutory Responsibilities 7.3 Water Efficiency and Management 7.3.1 Introduction 7.3.2 Demand Management 7.3.3 Leakage 7.3.4 Water Consumption 7.3.5 Metering 7.3.6 Summary
7.4 Options Considered 7.4.1 Introduction 7.4.2 Sewage Effluent 7.4.3 Artificial Springs 7.4.4 Demand Management 7.4.5 Reduction in Groundwater Abstraction 7.4.6 Blue Circle Industries (BCI) Quarry Dewatering
70223B]G\WRP\B\RDER-Jnl 94\wp (iii) CONTENTS (cont)
Page Nr
CHAPTER 7 (cont)
7.5 The Recommended Scheme 7-9 7.5.1 Reduced Groundwater Abstractions and Conjunctive Use Operating Agreements 7-9 7.5.2 Low Flow River Augmentation 7-12 7.5.3 Review of River and Public Water Supply Needs 7-14
7.6 Summary of Strategy 7-15 7.7 Summary and Recommendation (after TWUL/NRA 1993) 7-16 7.7.1 Summary of Results 7-16 7.7.2 Recommendations 7-18
CHAPTER 8 COST-BENEFIT ANALYSIS
8.1 Introduction 8-1 8.2 Engineering Costs 8-2 8.2.1 NRA Engineering Costs 8-2 8.2.2 TWUL Costs of Developing Alternative Sources 8-3 8.2.3 Revocation of Licences 8-4
8.3 Economic and Environmental Benefits 8-6 8.3.1 Summary of Impact of Low Flow Alleviation 8-6 8.3.2 Approaches to Benefit Assessment 8-9 8.3.3 Results of the Contingent Valuation Survey 8-11 8.3.4 Results of Market Price Valuations 8-16 8.3.5 Summary 8-17
8.4 Net Present Value Calculations 8-17 8.4.1 Introduction 8-17 8.4.2 Principal Results 8-18 8.4.3 Incremental Net Benefits of the Action Plan 8-19
8.5 Conclusions 8-21
70223B10NWRP\B\RDER-Jnl 94\wp (iv) CONTENTS (cont)
Page Nr
CHAPTER 9 CONCLUSIONS AND RECOMMENDATIONS
9.1 Conclusions 9-1 9.2 Recommendations 9-4
REFERENCES
APPENDICES
APPENDIX A Relevant NRA and TWUL Legislation A-1
APPENDIX B Programme of Work, Leading to decision on BCI Pipeline B-1
ANNEXES
ANNEX I Darent Catchment Investigation, (GDC, November 1991)
ANNEX II Darent Catchment Investigation, Model Calibration Results, (GDC, November 1993)
ANNEX III ‘Plan for the Darent’, Report of the Joint .NRA/TWUL Project Team, (NRA/TWUL, November 1992)
ANNEX IV An Economic Analysis of the Benefits derived from the Alleviation of Low Flows in the Darent, (ERM, October 1993)
70223BlftWRP\B\RDER-Jn) 94Wp (V) LIST OF TABLES
Table Title Page Nr Nr
1.1 All TWUL Sources in Darent Catchment 1-3
3.1 Daily Rainfall Stations 3-4 3.2 Gauging Station Details 3-5 3.3 Licenced Groundwater Abstraction Details 3-7
4.1 Mean River Flows (1970 to 1989) 4-3 4.2 Simulated Steady State Water Balances for the Darent Catchment 4-3 4.3 Mean Monthly Flow Difference between Lullingstone and Hawley Gauging Station 4-6 4.4 Mean Simulated River Flows (1970 - 1989) 4-7 4.5 Target Flows for the Plan for the Darent 4-9 4.6 Proposed Monthly Target Flows 4-9
5.1 Environmentally Acceptable Flow Regime in the River Darent (after W S Atkins, 1991) 5-3 5.2 Preference Curves of Selected Organisms in the River Darent 5-7
6.1 Environmentally Acceptable Flow Regime 6-2 6.2 Design Augmentation at Gauging Sites 6-2 6.3 Abstraction for Public Water Supply 6-3 6.4 Catchment Imports and Exports 6-4 6.5 Demand Projections for Darent Catchment (Ml/d) 6-5 6.6 Summary of Options Considered 6-7 6.7 Comparison of Structural Options 6-9 6.8 Comparison between TWUL/NRA (1992 Short-listed Options) 6-12
7.1 Agreed Reductions in TWUL Darent Abstractions 7-10
8.1 Engineering Costs Associated with Darent Valley Action Plan (£ million 1993 prices) 8-2 8.2 Annual Average Abstractions in the Darent Valley under the Three Scenarios (Ml/d) 8-3 8.3 TWUL Costs of Water Supply for the Three Selected Scenarios 8-5 8.4 Costs of Licence Revocation 8-6 8.5 Environmental Consequences of Flow Regimes: Westerham to Thames 8-7 8.6 Method of Extraction of Benefits and Disbenefits (Costs) 8-12
70223B10\WRP\B\RDER-Jul 94\vrp (vi) LIST OF TABLES (cont)
Table Title Nr
8.7 WTP of Residents, Visitors and General Public for Low Flow Alleviation (£ per household per year) 8.8 Aggregate Benefits of Low Flow Alleviation in the River Darent (£’000 per year) 8.9 Discounted Benefits and Disbenefits of Avoiding 100% Abstraction Scenario - Summary Table (£ million, 1993 prices) 8.10 Key Assumption in Cost/Benefit Evaluation 8.11 Costs and Benefits of Implementing the Action Plan 8.12 Costs and Benefits of Securing 70% Abstraction 8.13 Incremental Costs and Benefits of the Action Plan
APPENDICES
B.l Programme for Decision Making for BCI Pipeline
70223B10VWRP\B\RDER-Jttl 94\wp (vii) LIST OF FIGURES
%
Figure Title Following Nr Page Nr ft 1.1 Location Maps 1-1 1.2 Increase in Abstractions from TWUL Boreholes in the Mid and Upper Catchment of the River Darent 1-1 P 1.3 Comparison Between Naturalised and Recorded Flow and Hawley (1970-1988) 1-2
3.1 Geology 3-2 3.2 Geological Cross Sections 3-2 ® 3.3 Hydrometric Stations and Subcatchments 3-3 3.4 Observation Wells 3-5
4.1 Catchment Model 4-1 ^ 4.2 Modelled Aquifer System 4-1 4.3 Simulated Flows at Oxford 4-2 4.4 Simulated Flows at Lullingstone 4-2 4.5 Simulated Flows at Hawley 4-2 4.6 Simulation of Naturalised Flows at Oxford 4-7 ® 4.7 Simulation of Naturalised Flows at Hawley 4-7 4.8 Naturalised Flow Accretion Profiles 4-7 4.9 Frequency Distributions (GEVI) for Annual Minimum Naturalised Flows 4-7 ^ 4.10 Frequency Distributions for Naturalised August Flows 4-7 4.11 1 in 20 year Monthly Variations in Naturalised Flows 4-8 4.12 1 in 20 year Flow Accretion Profiles 4-8 ^ 4.13 Variation in Groundwater Catchment 4-11 4.14 Comparison of Reduction in Abstraction from Chalk and Lower Greensand 4-12 4.15 Effect of location of Chalk Abstractions 4-13 0 4.16 Operation of River Support 4-14 4.17 Example Augmentation Requirements 4-14
5.1 Effect of Options on Flow Accretion, August 1976 (after Halcrow 1988) 5-1 5.2 Comparison of Estimated Natural EAFR and Observed Mean Flows and Q95’s (after Atkins, 1990) 5-2
70223 B10\WRPVfl\RDER-Ja] 94\wp (viii) LIST OF FIGURES (cont)
Figure Title Following Nr Page Nr
5.3 Flow Duration Curves fro River Darent (after Atkins, 1990) 5-2 5.4 River Darent Channel Characteristics (after Atkins, 1990) 5-6 5.5 Frequency Distribution of Annual Minimum Flow: River Darent at Hawley 5-8
6.1 Monthly Deficit Records 6-2 6.2 Water Supply Operational Records 6-3 6.3 Indicative Water Distribution 6-3 6.4 Public Water Supply Abstractions 6-3
7.1 The Recommended Scheme 7-9 7.2 Proposed Thames Reservoir/River Darent Abstractions: Conjunctive Use Scheme-WRMSl 7-11
8.1 Comparative Low Flow Profiles in the River Darent, 1 in 20 year Drought 8-1 8.2 Range of NPV Benefits for Action Plan and 70% Abstraction Options. 8-21
APPENDICES
B.l Programme of Investigations leading to Decision on Pipeline B.2 Programme of Implementation of River Darent Action Plan
70223BK71WRPVB\RDER-Jb1 94\wp (ix) CHAPTER 1
INTRODUCTION
1.1 The Problem
The National Rivers Authority has ranked the Darent catchment as the most over-abstracted river in studies listing 40 of the worst cases in the UK.
Groundwater abstractions from the mid Darent catchment began from the Chalk aquifer in the late 1800s but were below 10 Ml/d until the 1930s, by which time they had doubled. However, it was in the late 1950s and early 1960s that abstractions increased rapidly, due mainly to wells sunk by the Metropolitan Water Board, which have now transferred to the Thames Water Utilities Ltd (TWUL). Currently TWUL holds 70% of the total 155 Ml/d licences issued for the Darent catchment.
FIGURE 1.2
Increase in Abstractions from TWUL Boreholes in the Mid and Upper Catchment of the River Darent
70223B1&WRP\BVRDER-Jn] 94Wp 1-1 Development of Upper Greensands was more gradual but again in the 1950s and 1960s the capacity of Metropolitan Water Board abstractions at Sundridge and Brasted was increased several fold.
Figure 1.2 shows the development of the abstractions for the six TWUL wells at Brasted, Sundridge, Lullingstone, Eynsford, Horton Kirby and Darenth, which are shown later in this report to hold the key to re-establishing a more satisfactory flow regime in the River Darent.
The graph demonstrates clearly the dramatic increase in abstraction of groundwater which occurred during the 1950s. The problem of over-abstraction is therefore a relatively recent phenomenon and is one which should have been recognised and remedied well before it reached such an acute stage.
The condition of the river before the excessive abstractions took place is familiar in the memory of many of the local residents and visitors who enjoyed the natural beauty of this outstanding Chalk river. The difference between the conditions then and now are best highlighted by comparing recent flows with those which would have occurred had no groundwater been abstracted from the Darent catchment. Figure 1.3 shows hydrographs of recorded and naturalised flows for the river at Hawley. The significant differences between low flows during summer months are particularly noticable. Certainly there were never any dry river beds.
FIGURE 1.3
Comparison Between Naturalised and Recorded Flows at Hawley (1970 - 1988)
A list of the TWUL abstractions is given in Table 1.1. The two Lower Greensand wells in the upper catchment at Brasted and Sundridge have licences to abstract 18.2 Ml/d which represents 17% of the total licence capacity of TWUL. These two wells are close to the river as are six Chalk boreholes at
70223BI (A.WRP\B\RDE R-Jol 94\wp 1-2 Figure 1.1
Location
Wootwtch G R E A T E R Erfth LONDON
Crayford < Swanscombe EMiam Weitern
DA X OR B lukCe LakeL**v I | J /eserp W'mtngton SidCUp y y rl Hswtoy / \ \ (W » ------/ V Sooth Darenth
Hartley Bromley Vrit >" i j Horton Kirby M25 / / Fammgham
Naw Ash / Fawkham Eynsford Qraen Graan
>AJ—nw* < « Shoraham Kingadowr
Otfcwd
Kemsing Biggin J M2 S
WMt#rham Sundridga s e v e n o a k s Braetad
]
Grid Squara : TQ
Legend Motorway or Main Trunk Road Catchment Boundary
River Blue Circle Cement Quarries
Lake Scale ) Urban Area 4 6 8 10km
70223B01\GDC\2\A\DCI FOOTMlbfiS lEWtO EJQSTWa ALD£X
EXISTING LOWM2-EA w h ic h couu> &e su rr^g iy CAKA/Eb OUT AS A KAY "TO PROVIDE AN Afc-EAOF A ttA LM.CitL'/ AtCA Of -MPfcfc- MA*SH7 HA&lTiAtT ovEJfoeatih) uitTH smiMGS NETTIES COULO It OfiM ? IMMM& t l EXCAVAIIONANt> AU. EXGAW EfcM ATEe^L VJ»U. &£ OF USE TD AtTEJ£Et> MAMGEMENtT CtEATE. SPrT5 ANt> IStAM&S WITHIN T H E MAIM LAKES WHICH IKI TUtM WILL EASE THE WMtt LEVEL.
RDOiTPftTH TO LAKES NOT TO SCALE. T he ceeation of n e w habitats woult> B£_Av valuable enhancement to the wCCC*l4C. IUV/EIL(DeeCX7lt AS IT PASSES THED06H
As the river passes through Sevenoaks Wildfowl Reserve valuable enhancements could include creating bays and using the excavated material to make more interesting margins on the main lakes. 70233BOVUWO !/• Lullingstone, Eynsford, Horton Kirby, Darenth, Wilmington and Dartford, which contribute 83.2 Ml/d or 54% of the total. It is these wells which have been mainly responsible for the drastic reduction in River Darent flows illustrated in Figure 1.3.
TABLE 1.1
All TWUL Sources in Darent Catchment
Licensed abstractions Source Aquifer Million m3/annum Ml/d (average)
Westerham CK and UGS 0.35 0.97 Bras ted LGS 1.66 4.56 Sundridge LGS 4.98 13.64 Lullingstone CK 3.32 9.09 Eynsford CK 6.14 16.82 Horton Kirby CK 4.98 13.64 Green St Green CK 1.66 4.55 Darenth CK 7.63 20.91 Wilmington CK 6.97 19.09 Dartford CK 1.33 3.64
Total 39.02 106.91
Both the National Rivers Authority (NRA) and Thames Water Utilities Ltd (TWUL) have duties and obligations under the Water Resources Act 1991 and the Water Industry Act 1991 respectively which have a direct bearing on the present condition of the River Darent.
In essence the NRA has a duty to conserve, augment, redistribute and secure proper use of water resources. In addition the NRA has a general duty to have particular regard to the duties imposed on water undertakers. These duties and powers do not relieve any water undertaker of its statutory responsibility to develop water resources in line with its general duty to develop and maintain a water supply system. The water undertaker has a primary duty to make water available to persons for domestic and industrial use as demanded. Both NRA and TWUL have duties to enhance the environment whilst carrying out primary functions in relation to water resources and water supply.
The principal relevant responsibilities and duties are summarised in Appendix A.
70223 B10VWRP\B\RDER-Jal 94\wp 1-3 1.2 History of Events
The droughts of 1972/73, 1976/77,1981/82 and 1989/93 have all highlighted increasingly the growing problem of abstraction on Darent flows. Until 1974 the catchment was the responsibility of the Kent River Authority, who issued the key Licences of Right in 1968.
In 1974 the water resources responsibilities for the Darent were transferred from the Southern Water Authority to Thames Water Authority (TWA), so that the development and protection of the water resources lay within the same organisation. Clearly in such a situation the seeds are sown for a conflict of interests.
Following the 1976 and 1981/82 droughts, local pressure was mobilised with the formation of the Darent River Improvement Protection Society (DRIPS). The Society, whose aim is to safeguard the river for future generations, has a membership of about 200. It represents the Parish Councils, and where applicable, 16 local towns and villages as well as the Darent Valley Anglers* Consultative Association and its member clubs. Since its formation, five public meetings have been held jointly with Thames Water Authority and subsequently with the NRA Southern Region. Meetings have frequently attracted between 200 and 250 participants. The local knowledge of members of the society has been actively directed into supporting improvement projects and studies associated with the river. It is a well motivated organisation and serves as a focal point for all connected with the river who want to prevent further decay in one of the country’s best known chalk streams with its unique variety of flora and fauna. At the time of the formation of DRIPS the Water Resources Directorate of Thames Water Authority recognised that there was a low flow problem in six heavily abstracted Chalk stream catchments surrounding the London Basin, including the Darent.
In 1985 TWA’s Board was appraised of the serious situation on the Darent and they agreed to a voluntary restriction in use of six riverside boreholes in the Chalk in the Darent catchment to 70% of their licensed quantities.
Over the period 1986 to 1989 Sir William Halcrow & Partners was engaged to carry out investigations and to report on and propose solutions.
In 1988, during the run up to privatisation of the Water Industry, the CPRE published their critique ‘Liquid Assets’ which, inter alia, drew attention to the national problem of over-abstracted rivers. In 1989 Thames Region NRA raised the question of action on rivers affected by abstraction and a national survey was carried out to examine the extent of the problem. Twenty rivers, including the Darent, were identified as first priority, with another twenty to be investigated in following years. A national research contract was let to develop a consistent methodology for comparing the severity of the low flows and a ranking. The Darent is the most severely affected of the forty rivers.
In April 1990 the responsibility for the Darent in water resources and fisheries was transferred to Southern Region NRA, returning to the pre-1974 position with the river catchment managed on an integrated basis for resources, water quality, flood defence, fisheries and conservation. Responsibility for solving the Darent problems passed to Southern Region NRA.
70223B1C\WRP\B\RDER-Jol 94\wp 1-4 Early discussions between interested parties, including DRIPS, established that the bed lining solution proposed in the Halcrow report of 1985 was not acceptable. A more fundamental approach was required and a contract was let to Groundwater Development Consultants (GDC) to investigate all possible options, including replacing the Darent abstractions from elsewhere and returning effluent to the river. The start of investigations was announced at a stormy public meeting in November 1990, by which time the 1989 to 1993 drought had begun to take effect. At the same time W S Atkins was commissioned to carry out an ecological survey during the drought condtions of 1990 and to suggest an environmentally acceptable flow regime.
In 1991 GDC produced its pre-feasibility report with the co-operation of TWUL, particularly in the description of the distribution system in south-east London. A second public meeting was held and TWUL sent a representative to speak alongside NRA speakers and their consultants.
Formal correspondence with TWUL on solutions for the Darent was opened in autumn 1991. NRA proposed a variation of licences to the 70% level and a phased plan to reduce abstractions further to half their authorised quantities.
The NRA programme for improving the condition of the River Darent involved three phases:
By October 1992 apply a downward variation of TWUL licences by 30% of licensed annual total;
By April 1993 secure agreement with TWUL for a further reduction of 15 Ml/d to be introduced by end of 1995 at latest.
If agreement with TWUL was not reached then Darent licences would be replaced by time- limited licences of sufficient duration to allow TWUL to introduce alternative supplies.
Further correspondence and meetings over six months had resulted in little firm progress by May 1992. In June 1992 the NRA Board decided that the Darent licences would be revoked unilaterally by the NRA if TWUL did not enter into meaningful negotiations. This resulted in an acrimonious exchange of letters between NRA and TWUL in the media, with the NRA Chairman making public the Authority’s intention to formalise the reduction in licensed abstractions to 70%. There was considerable support nationally for the NRA’s stance. Preparations were put in hand for licence revocations, and a draft notice of revocation for the London Gazette was sent to DoE in September 1992. If TWUL appealed against the revocation of licences then the application would be likely to be referred to a public inquiry. If their appeal was upheld then they would proceed to a claim for compensation for the cost of replacing the lost capacity, under the jurisdiction of the Lands Tribunal.
Before the downward variation in licences was implemented TWUL and NRA agreed in October 1992 to set up a Joint Project Team to develop a cost-effective plan for the recovery of the River Darent by 1998. A Memorandum of Understanding included as its main aim, to ‘return flow to the River
70223BI0VWRP\B\RDER-Jnl 94\wp 1-5 Darent whilst safeguarding supplies to Thames Water’s Customers’. The team was tasked to produce its report by 30 November 1992 with a view to TWUL and NRA agreeing a solution by the end of 1992. A joint press release was agreed.
Meanwhile NRA’s consultants GDC had developed and successfully calibrated a groundwater model which was to prove the key to testing the efficacy of alternative options. The JPT met once or twice a week for the six weeks to 30 November. The JPT comprised three members of TWUL, two from NRA Southern Region, one from NRA Thames Region and the Consultant’s Director responsible for the GDC Darent work.
The first step was to satisfy TWUL of the reliability of the groundwater model and then the model was used to test alternative options for restoring flows. As a result a technically sophisticated package of measures was devised which included the closing of Greensand sources, reductions and increases to Chalk sources and replacement of Darent water with River Thames water imported to South-East London via the London Water Ring Main. The overall package of measures was described as an Action Plan and included two water resource management schemes under section 20 of the Water Resources Act 1991 (see Annex III). The Action Plan for the Darent catchment was prepared by 30 November 1992. The scheme in essence contained four integrated components:
(i) discontinuing groundwater abstractions from the Greensand wells at Brasted and Sundridge and reducing abstractions from mid-catchment wells in the Chalk and a small increase in the lower catchment;
(ii) in association with a reduction in abstractions, changing the public water supply infrastructure to use Thames water to compensate for the reduced groundwater supply;
(iii) to facilitate (i) and (ii) operate a Section 20 water resource management scheme for the conjunctive use of River Thames reservoirs and River Darent groundwater sources;
(iv) augment the River Darent in drought years in its middle and lower reaches with groundwater drawn from installed purpose-built wells and with water imported by pipeline from Blue Circle Industries (BCI) quarries at Northfleet.
The technical and financial proposals were considered by the TWUL and NRA Boards in December 1992 and January 1993. A joint press statement announcing agreement on the Action Plan was released at the end of January.
Following the agreement reached between TWUL and NRA on the way forward to alleviate the problem of low flows in the River Darent, additional studies have been undertaken which are brought together in this report. These include an economic evaluation of the benefits derived from alleviation of low flows in the River Darent (ERM, October 1993), a report on the calibration of the integrated surface and groundwater model used to assess the effect of varying abstractions (GDC, November
70223B1 The aim of the present report therefore, is to bring together material and conclusions from both these new and the past studies in summary form and in particular to see how they relate to the proposed strategy as agreed by the NRA and TWUL. In addition the report was required to contain sufficient technical material to satisfy an appraisal by the Department of the Environment. In order to achieve both objectives Annexes have been included which contain some of the more important reports and provide additional technical detail. These include the ‘Plan for the Darent’ (Joint NRA/TWUL Project Team, November 1992), the economic analyses (ERM, October 1993), ‘Catchment Model Calibration Report’ (GDC, November 1993) and ‘Darent Catchment Investigation’ (GDC, 1991). Since the submission of the ‘draft’ River Darent Low Flow Alleviation Report in December 1993, the ‘final’ report has been amended and updated to include for DoE requirements. 70223B10\WRP\B\RDER-JttJ 94\wp 1-7 CHAPTER 2 REVIEW OF PAST REPORTS 2.1 Introduction Before the 1960s the monitoring of river flows and the licensing of borehole abstractions was not undertaken on a systematic basis. The rapid growth in groundwater abstractions from within the Darent catchment occurred in the 1950s before the hydrometric and groundwater monitoring networks were in place. From the 1960s there has been growing public concern over the detrimental effects of over-abstraction on river flows and river ecology. This concern was focused in the formation of the Darent River Preservation Society (DRIPS) which was established in 1985. The poor condition of the River Darent in drought years of 1972 and 1973 followed by 1976, which has a return period of 20 years, resulted in the Dossor (1978) study followed by Halcrow (1987, 1989). The recent drought from mid-1988 to 1992 has further highlighted the problem and added to public concern. As a result a number of new studies have been made in recent years with their aims directed toward defining the problem in environmental and ecological terms and, through groundwater/surface water modelling of the catchment, assessing the means of enhancing and re-establishing a satisfactory flow regime. The main reports prepared to reach this objective are listed below: Water Management Study (Dossor, 1978); Low Flow Alleviation Study (Halcrow 1987, 1988, 1989); Environmentally Acceptable Flow Regime Study (W S Atkins 1991); Darent Catchment Investigation (GDC 1991); Groundwater Study of Blue Circle Cement Northfleet Works (GDC 1992); A Strategy for Enhancement of the River Darent (Mott MacDonald 1992); Report of NRA/TWUL Joint Project Team (1992); Ecologically Acceptable Flow in the River Darent (IH, 1993); An economic analysis of the benefits derived from the alleviation of low flows in the River Darent (ERM 1993); Darent Catchment Investigations - Model Calibration Results (GDC, 1993). A brief summary of the contents and conclusions of reports prepared up to, and relating to, the River Darent Action Plan are given below. The reports GDC (1991), NRA/TWUL (1992), GDC (1993) and ERM (1993), are included in this report as Annexes I, II, III, and IV respectively, and their contents described and reproduced in the relevant chapters. 70223B10\WRP\B\RDER-Jnl 94\wp 2-1 2.2 Water Management Study (Dossor, 1978) The brief for this study was to review and put forward recommendations regarding possible future river basin management of the River Darent. This related mainly to minimum desirable river flows but included consideration of water quality and also flood control. The report includes a review of land use, hydrogeology, river flows and hydrochemistry. A mean annual water balance was developed. Land drainage, water supply and sewage disposal are discussed. As a separate issue (recognised as having no direct bearing on the problems of low flow) the report puts forward recommendations for improvements to the Valley Trunk Sewer System. After costing various options for alleviating low flows, Dossor favoured the following: sewage treatment at Sevenoaks with discharge of high quality effluent (after lagoon settlement) to the River Darent; pumping of lagoon-matured, high quality effluent from the existing Long Reach sewage works at Dartford back into the Darent catchment for discharge to the river; utilisation of a possible 2.3 m drawdown in the main lake at Longford Lakes to provide river regulation. These three options have all been taken up in later reports and were considered in the ‘Plan for the Darent’ (November 1992). The first two were aimed at trying to redress the damage caused by the export of drainage water from the Darent catchment through the Valley Trunk Sewer System to Long Reach sewage treatment works. The main problem with these solutions was costs, particularly for treatment of water to a satisfactory standard for augmenting low flows in the River Darent (JPT, 1992). The use of storage at Longford Lakes to regulate flow has since been found to be unsatisfactory on both technical (insufficient storage) and environmental grounds (GDC, 1991). 2.3 Study of Alleviation of Low River Flows Resulting from Groundwater Abstractions, Interim Report Annex A (May 1987) Volume 1 Main Report and Volume 2 - Darent Case Study (Halcrow, April 1988) Between 1987 and 1989 Sir William Halcrow and Partners produced a series of reports on low flow alleviation, as part of a wider study of six catchments in the region. The study assessed the problems of low flows and put forward recommended solutions, giving costings and outline designs. In the final report (Halcrow 1989), a programme for implementation of recommended solutions was set out in detail. The Darent was considered as one of three catchments with major problems of depleted river flows. The reporting sequence for the study was as follows: 70223B1CN'WRF\B\RDBR-Jol 94\wp 2-2 May 1987 Interim Report comprising main report summary for the six catchments with details for each catchment in separate annexes. April 1988 Final Report with Volume 1 a main report summary for the six catchments, with separate case study reports giving details for individual catchments. February 1989 A single report on implementation of schemes. The objectives of the study were to identify the areas where flows had been depleted by groundwater abstraction, to determine the extent to which improvement of river flows would be desirable, and to evaluate the technical feasibility, costs and environmental consequences of various options for improvement. For the Darent, the brief was to consider improvement to flows in the reach within the North Downs from Otford to Hawley, ie where the river flows through the Chalk outcrop. Proposed solutions, however, included schemes located outside this area, in particular in the subcatchment upstream of Otford. The Interim Report, Annex A - River Darent Case Study, dated May 1987, provides a general assessment of river flows, hydrogeology and water balance. The ecology, landscape, historical setting and history of abstraction are described comprehensively in Annex A for the Darent catchment. In particular it provides an excellent historical survey, mentioning known records of river flows, fishing, water mills, watercress farms and remains of water meadows, and then gives a general account of river ecology. It is hard to improve on this survey, as Halcrow covered the whole of the river within the background study, partly because many quoted references to the river and its fisheries apply to the whole of the Darent. In addition, it specifically covers the eight miles upstream of Otford in the catalogue of water mills (starting at Westerham) and in its general description of the catchment. For the reach of the Darent upstream of Otford, fed both from the Lower Greensand and the Chalk, there is plenty of literature to support its past importance as a stream and to validate the need to sustain this upstream reach as part of the overall strategy for the river. An ecological database was presented as an appendix to Annex A. The main report proposes water quality and flow objectives. A preliminary minimum flow target from Otford to Hawley is set as 10 Ml/d increasing downstream to 40 Ml/d (this range of flows is also compatible with those recommended in the River Darent Plan (November 1992)). Various options for alleviation were costed. The following options were recommended for further consideration in later stages of the study: river regulation using the main Longford Lake, utilising a possible 3 m drawdown of lake levels; ceasing abstraction at Lullingstone and Eynsford with replacement by supplies brought in via a proposed London Ring Main; 70223B1CNWRP\B\RDER-Jal 94\wp 2-3 redistribution of abstraction within the catchment, away from the immediate vicinity of the river, in periods of low flow. Augmentation of flows using Chalk groundwater was dismissed on the grounds that this would place further demands on an already highly developed aquifer. Furthermore it would represent poor resource management since the water would be lost downstream of Hawley. Aquifer recharge using surplus flows in winter and river support, with high quality effluent pumped into the catchment from Dartford, were dismissed on grounds of excessive cost. River water recirculation was shown not to be technically feasible as the river loses water progressively downstream below Lullingstone. In the Final Report (April 1988), the Main Report (Volume 1) provides a general discussion of the types of solutions considered for all catchments included in the study. The report redefines flow objectives, following an improved assessment, as being maintenance of a target flow of 30 Ml/d throughout the reach from Otford to Hawley, reducing to 20 Ml/d in severe drought conditions. It is argued that a target flow profile following a presumed natural pre-abstraction increase in flow in a downstream direction in this reach could only be achieved by cessation of groundwater abstraction. The report recognises as objectives a need to restore some of the previous wetland in the valley bottom where possible and the opportunity afforded by study implementation to engineer river cross- sections to suit flows, and to restore mill ponds. In considering options there is reiteration of the arguments concerning schemes already discussed at the interim stage. The reliability of a surface water supply source with normal 7 day bankside storage located in the Dartford area is considered. The report concludes that reliability would be much lower in drought conditions than that of the groundwater sources within the catchment which it would be intended to replace. Reliability of supply could be increased by provision of a reservoir but this would increase the cost enormously. The option was not considered further. Redistribution of abstraction away from the immediate vicinity of the river in the summer was considered favourably at the interim stage. However in the Final Report, the option is not considered for detailed costing owing to uncertainties in the consequent resurfacing of springs along the river. Halcrow also considered that the relocation of sources would have a greater benefit for the river in winter. This option, however, needed to be modelled in detail to assess the seasonality of benefits to flow. An additional option was introduced with river augmentation using groundwater from the Folkestone Beds. The possibility of using river bed lining to prevent bed leakage between Lullingstone and Hawley was included in options for costing. Options for which detailed costings are given include one or more of the following elements: flow preservation through river bed lining; augmentation using Folkestone Beds groundwater; river regulation using main Longford Lake storage; cessation of pumping at three wells above Hawley with replacement by water from the 70223B10VWRP\B\RDER-Jol 94\«rp 2-4 London Ring Main, necessitating development of links within the London water supply network. It was assumed that any existing abstraction not subject to closure as part of the option, would be maintained at the rate of abstraction in 1986, ie at about 70% of full licensed rate. The cost of alternative means of making up supply to the full licensed rate (on the assumption of increase in demand) was included in all options. Costs attached to alternative sources of supply were found to be very expensive in relation to river regulation, augmentation and flow preservation. A scheme involving the first three of these later options was assessed as meeting target flows. It was preferred on the basis of lower cost to options involving cessation of pumping as cessation of pumping required provision of even larger alternative supplies at a very high cost. The study involves a lengthy discussion of effects of implementation of options on the riparian environment, fisheries, landscape and amenity value. Opportunities for enhancement, with public accessibility, are included together with sketches of landscaping features. Preliminary designs for engineering components of the recommended scheme are also given. The implementation report (February 1989) recognises that in the case of the River Darent ‘bed lining is proposed as a means of partially restoring the ecological and landscape value’ and that ‘hydraulic design should allow for weed growth in the river bed and bank at low flows’. The section of the report concludes that the River Allen project has shown that a stream bed can be sealed against leakage; lining work could be combined with vegetation and habitat diversity if taken into account during detailed design. Each of these three schemes has been rejected in later studies; river bed lining of the River Darent has been rejected on environmental and ecological grounds. The bed lining option may have uses in specific cases but for a natural river with significant ecological and environmental value such an extreme solution is unlikely to be treated seriously in the future except over very short lengths of river; the Halcrow reports did not have the benefits of a groundwater model. The study (NRA/TWUL, 1992) using the GDC model has demonstrated that the Lower Greensand aquifer has a major role to play in maintaining flow in the Darent. Rather than sinking augmentation wells in the Folkestone Bed to supplement the river it is preferable to reduce abstractions in the Lower Greensands Bed as such reductions will quickly appear as river flow and benefit the whole river. Reduced pumping from Chalk Beds would not be as effective; river regulation of Longford Lake has serious environmental drawbacks and is not capable of restoring flow to the River Darent in its lower reaches (GDC 1991). 70223BIOVWRP\B\RDBR-Jnl 94\wp 2-5 2.4 Environmentally Acceptable Flow Regime Study (W S Atkins, 1991) The report defines the environmentally acceptable flow regime (EAFR) as ‘flows that will maintain the biological and general environmental integrity of the river under other than natural discharge conditions’. An EAFR was derived using a methodology involving: a survey of macroinvertebrates, site characteristics, flow and water quality at nine sites between Westerham and Wilmington in October 1990; the River Invertebrate Prediction and Classification System (RIVPACS) to predict the type of invertebrate community which would be expected at the sites under natural flow conditions for comparison with survey results; statistical analysis of flows and broad assessment of sub-catchment areas contributing recharge to the Chalk groundwater system. Following the field survey and RIVPACS analysis a survey site at Shoreham was identified as being the most downstream location at which the macroinvertebrate community remained unaffected despite flow depletion. Further downstream communities were severely affected. The location at Shoreham was then used as an index site assuming that the flow regime was just managing to maintain biological and general environmental integrity at that site. A flow duration curve was established for Shoreham based on flow characteristics at gauging stations up and downstream at Otford and Lullingstone and the flows measured on the day of the survey. Mean flow and the flow exceeded 95% of the time at Shoreham were determined from the flow duration curve. These flows were established as mean and minimum values for the EAFR at the site. An assessment of river channel widths using field survey data indicated that, contrary to expectation for a natural river channel, widths decreased downstream of Shoreham. A possible explanation was considered to be that the channel has adjusted to accommodate substantially depleted flow. One approach to establishing an EAFR for the catchment using channel width characteristics was therefore rejected. It was considered that such an approach would produce an EAFR in part reflecting the abstraction pattern in the catchment. The mean value of EAFR at the other survey sites was determined using mean effective rainfall at Shoreham and contributing surface water catchment area at each site. With a mean environmentally acceptable flow defined at each site, ninety-five percentile flows were determined from Otford or Lullingstone type flow duration curves appropriate for each individual site. The catchment EAFR was hence defined by mean and ninety-five percentile (minimum) flows at the survey sites between Westerham and Wilmington. The limited survey work carried out over a short period in 1990, did not provide data from which seasonality could be built into the catchment EAFR. Seasonality of flows is discussed however, with the comment that seasonal variation in EAFR should ‘mimic’ the pattern of seasonal variation in the 70223 B1(AWRP\8VRDER-Ja] 94\wp 2-6 natural flow regime. The report discusses the general difficulties of defining an EAFR and the early stage of development of this approach to catchment management. The need for further investigations was recognised. Although this study has its limitations it does provide a basis for determing a minimum acceptable flow on ecological grounds. The 95% EAFR values of 10 Ml/d at Otford rising to 23 Ml/d at Hawley were used (NRA/TWUL, 1992) as a guide to determining target flows. 2.5 Darent Catchment Investigation (GDC 1991) In February 1991 Groundwater Development Consultants (GDC) were appointed to identify the most appropriate measures from a large number of engineering options to alleviate low flow conditions in the river. The study was to be in three parts: Part 1 - include a study of water supply infrastructure, hydrological and groundwater studies and feasibility of engineering options to alleviate low flow conditions; Part 2 - set up an integrated distributed catchment model for surface flows and groundwater; Part 3 - run the model to evaluate the benefits of selected low flow alleviation options. Part 1 report was submitted in November 1991 and is included as Annex 1 of this report. Parts 2 and 3 were scheduled for completion in October 1992. However, events overtook the studies for Parts 2 and 3 and the model was used as a tool for evaluating the various options investigated during the NRA/TWUL Joint Project Team negotiations (discussed later). The model and its calibration are included as Annex II of this report (GDC, 1993) and results of production runs, Part 3 of the original study are summarised here in Chapter 4 (and fully reported, in GDC (April 1994), Options for Restoring Flows, Catchment Modelling Assessment Report). The report (GDC, 1991) brought together all the work contained in the past studies and significantly advanced the investigations in several directions; most notably by setting up an integrated surface water (Stanford model) and groundwater model (GDC). Without a catchment model of this type it is impossible to evaluate the effect of different levels of abstractions on river flows. Moreover, the hydrometric network was established only in the 1960s and the naturalised flow regime was unknown. Although the model had not been fully developed in the first phase of GDC’s studies (GDC, 1991), results of recharge from the surface water model demonstrated the serious over-abstraction. Mean recharge in the Chalk exceeded actual abstractions by only 15% with the result that in a 1 -in-5 year dry year, abstractions exceeded recharge by 30%. Actual abstractions over the period 1985 to 1989 averaged 75% of licensed totals for Thames Water’s ten boreholes. 70223Bl(AWRPM!\RDER'Jal 94\wp 2-7 Extensive engineering studies were also undertaken and the water supply infrastructure investigated in detail. A wide range of options was investigated and a methodology developed for comparing costs and effectiveness of options. Preliminary costs were estimated for full or partial revocation of selected abstraction licences generated by the necessity to import water from the London Ring Main. Ten different option types were developed: river augmentation from wells in undeveloped areas of the catchment; use of sewage effluent; aquifer recharge; seasonal storage in existing lakes; replace groundwater abstraction with surface water abstraction at Dartford; conjunctive use of surface water from Thames reservoirs and groundwater; recirculation; bed lining of critical sections; development of bankside wells; demand management. Capital costs, discounted costs and unit costs (£/m3) were calculated for 22 cases. However, most of the cases investigated were not independent and this inter-dependency could only be investigated using the integrated catchment model to determine optimum solutions. Recommendations for a definitive solution were deferred until production runs of the model had evaluated the technical effectiveness of the different options. It was recommended that none of the options should be rejected at that stage. As mentioned previously, events overtook Parts 2 and 3 of the GDC study with calibration and production runs being undertaken during the joint TWUL/NRA (1992) negotiations. The results of these studies are contained in Chapters 4 and 7 and Annex II. 2.6 Groundwater Study of Blue Circle Industries Northfleet Works (GDC, 1992) Blue Circle Industries (BC1) operates a cement manufacturing works with two large Chalk quarries in the Ebbsfleet catchment to the north-east of the River Darent, close to the River Thames. Chalk extraction from the Western Quarry is due to finish by 1994 but extraction from the Eastern Quarry will continue for several decades. BCI plans to seek permission to deepen significantly the maximum permitted excavation level of the Eastern Quarry. Estimates suggest that present dewatering necessary for excavation amounts to approximately 30 Ml/d. BCI also dewaters an estimated additional 21 Ml/d from another site in the area, the Blue Lake. Most of the water pumped by BCI eventually discharges to the Thames. The NRA has expressed concerns that such large quantities of groundwater are lost from the Chalk aquifer in an area where available water resources are scarce. Not all the water is of good quality, however. There is evidence for some poor quality water around the quarries and problems with saline intrusion and industrial contamination in Blue Lake. 70223B10\WRPV8\RDER~Jti] 94\wp 2-8 Following initial discussions with the NRA, BCI appointed Groundwater Development Consultants (GDC) to undertake a groundwater investigation of the Northfleet area. The report by GDC covers the following topics: data collection and analysis and a historical review of abstraction and development in the Northfleet area; a review of previous reports for the area; a water audit to establish quantities of water abstracted and used within the works; preliminary steady state calibration of a groundwater model for pre-development and 1992 abstraction conditions and prediction of future dewatering requirements. It was established in the water audit that there are few accurate records of dewatering pump discharge quantities and quality. Dewatering is not metered. The BCI groundwater model was developed to be compatible with the Darent catchment model. The preliminary calibration work indicated that losses from the River Darent are all intercepted by abstraction sources along the river, and the river is little affected by quarry dewatering. Dewatering induces some inflow from the River Thames giving rise to saline intrusion, but this was modelled as only 10% of total pumping. For planning purposes, dewatering requirements for excavating to greater depth were assessed at 47 Ml/d. Dewatering at Blue Lake might be reduced to about 6 Ml/d, however. Recommendations were given for extensive field investigations and further detailed modelling work. Monitoring of groundwater levels and water quality, metering of pipelines, construction of new observation wells and test pumping of trenches were included in the recommendations. 2.7 A Strategy for the Enhancement of the River Darent, Kent (Mott MacDonald, August 1992) In 1992, National Rivers Authority (Southern Region) commissioned the landscape/ecological team at Mott MacDonald to liaise with residents and pressure groups in the valley, produce a broad corridor survey of the river from source to mouth and identify potential areas where landscape and habitat could be restored or enhanced. This report is parallel to detailed hydrological investigations also being prepared on the Darent by Groundwater Development Consultants of the Mott MacDonald Group. Against this background, the enhancements proposed in this report are not a substitute for water restoration but a sensible part of the overall wise management of the river. The brief for this study was to identify possible enhancement measures within the river corridor in order to modify the effects of low flows, and to improve landscape and ecological quality which has been impoverished by past drainage and other forms of management. Justification for the proposed enhancements can be summarised as follows: (i) Starting in the winter of 1992/93 with four sites where the river environment is especially degraded, the proposed enhancement will give ‘on the ground’ results 70223BlftWRP\B\RDER-Jul 94\wp 2-9 ahead of full-scale restoration of river flows. Priority sites such as that at Horton Kirby, may also have some influence over the setting of targets for reduced abstraction on specific reaches of the river. (ii) Once the low flows problem in the Darent has been resolved, the enhancements will remain as an additional bonus, created by taking the opportunity at the time when there was a focus of interest on the Darent and its problems. (iii) The environmental degradation of the River Darent is not confined to that of low flows. Heavy land drainage management in the past, intensive farming up to its margins, vandalism and neglect of historical structures, inadequate footpath access in a few places, unsympathetic engineering structures and ugly barbed-wire fencing all contribute to a reduction in landscape and ecological quality. The NRA has a duty to enhance the river environment and therefore the upgrading of the River Darent landscape can be seen as a legitimate part of its brief in addition to the restoration of water levels. (iv) It is a good time to assess the larger scale sites under consideration such as the Dartford Marshes and the Shoreham water meadows, in relation to wider planning issues which might usefully be dovetailed with good river management and also strategies which may include water storage as part of the restoration of flows. It is also possible that such proposals may be the springboard for more ambitious long term schemes eligible for grant aid, as part of overall conservation strategies for the valley, part of which is already within the North Downs Area of Outstanding Natural Beauty and may qualify for Countryside Stewardship payments under the category of waterside landscapes. (v) All the enhancements proposed will still cost considerably less than the major proposals recommended by Halcrow and yet will not involve the difficulties inherent in such large-scale artificial solutions as extensive bed-lining or the pumping of dilute sewage effluent. 7O223B101WRP\B\RDER~Jb) 94\wp 2-10 « /. i;, * . \- V ^*r£'4 ,:/l f The relationship between the Darent and its historic structures and buildings is important. Mill weirs (above) and water meadow structures still survive. Provided flows are maintained, the river sets off to perfection the shingled spires of its village churches just as it did in Samuel Palmer’s day (below). 70223B02/BSD/01/B CHAPTER 3 GROUNDWATER AND SURFACE WATER DATA 3.1 Data Requirements and Sources The main data requirements for hydrological assessment and catchment modelling are ' - for hydrological analysis and recharge estimation: rainfall (precipitation); potential evapotranspiration; river flows; groundwater and surface water abstraction; river support discharges; surface water catchment topographical characteristics; land use. - for catchment modelling: geological information to define the geometry of the aquifer system; observation well water level data within the catchment to provide aquifer piezometry for omparison with model simulation results; water levels from wells in surrounding catchments for definition of model boundary conditions; test pumping and well logging information to provide an indication of aquifer properties; abstraction and river support discharges; river flows for comparison with catchment model surface water output. Much of the existing hydrometeorological, hydrological and groundwater data are available in NRA archives. Potential evapotranspiration data were obtained from the Met Office for two MORECS grid squares covering the catchment area. Water companies operating in the catchment provided some groundwater abstraction data as well as information relating to borehole lithology, geophysical logging and test pumping. Hydrogeological information was available from the British Geological Survey who also publish a hydrogeological map for Kent. A programme of test pumping of existing public water supply wells was undertaken at selected TWUL wells in 1992 to obtain additional data on aquifer properties. The following sections describe the catchment and catchment geology and the hydrological and hydrogeological data available. 70223BHJVWRP\B\RDER-Jal 94\wp 3-1 3.2 Catchment Description The River Darent rises near Westerham about 10 km west of Sevenoaks (Figure 1.1). The upper reaches of the river are fed by springs both from the dip slope of the (Lower) Greensand to the south and the scarp slope of the Chalk of the North Downs to the north. Both the Greensand and Chalk outcrops attain heights of more than 200 m AOD in the vicinity. The river first flows in an easterly direction towards Sevenoaks. Disused sand quarries just to the west of Sevenoaks have given rise to lakes (Longford Lakes and others) which are connected with the river system. The river then follows a major valley cutting through the North Downs towards the Thames basin. Within the valley the river receives further contributions to flow from Chalk springs or seepages. A series of flooded gravel pits is located close to the northern end of the river valley. The river emerges on to the Thames floodplain about 20 km north of Sevenoaks where it is joined by a major tributary, the River Cray which rises within the Chalk of the North Downs to the west of the Darent. The Darent surface water catchment to the confluence with the Cray has an area of 250 km2. Long term mean flows in the Darent between gauging stations at Otford and Hawley are in the range 0.5 to 0.7 m3/s. The Darent is tidal to approximately 3 km above the point of discharge into the Thames. Mean annual rainfall varies from less than 500 mm along the Thames floodplain to more than 700 mm in some upland areas of the North Downs and Greensand Ridge. Land use in the Darent catchment is a mixture of pasture and arable with extensive areas of woodland on both the Chalk and Greensand outcrop areas. There are some water meadows in the valley bottom. The main urban area in the upper Darent catchment is at Sevenoaks. The lower 6 km of the catchment down to the confluence with the Thames is dominated by urban and industrial development in and around Dartford. The Ebbsfleet catchment to the north-east of the Darent has been extensively altered from its natural state as a result of massive chalk quarrying operations by Blue Circle Cement, major industrial development and urbanisation at Northfleet and Gravesend. The original source of the river is about 3 km from the Thames, to the south-west of Gravesend. No flow records exist for the river. 3.3 Geology The Darent catchment is located on the southern side of the London Basin. Formations dip gently to the north with successively younger formations overlying older formations from south to north. A geological map showing outcrop areas is shown in Figure 3.1 and geological cross sections and succession are given in Figure 3.2. 70223B1AWRP\B\RDER-Jol 94\wp 3-2 Figure 3.1 Geology GREATER LONDON FORD Legend Lower Greensand Folkestone Beds Recent ( Gravel/Alluvium) Sandgate Beds Tertiary Deposits Hythe Beds Chalk Atherfield Clay Gault Clay 2 4 6 10km S cale 70223B01/GDC/2/A/DCI Figure 3.2 Geological Cross Sections Section A - A’ Thames Darent Location of Section B - B ’ Darent 200 200 100 I 100 o J 0 ■ - 100 - -100 -200 -200 300 -300 Legend Section B - B’ Recent/Tertiary Gravels/Alluvium Location of Section A - A' Tertiary Deposits C)arent Chalk 200 200 Upper ✓ J Chalk Rock „ 100 100 u Si 'j Middle M f 0 0 Melbourn Rock I Lower ® -100 -• -100 Gault Clay 200 200 Lower Greensand 300 -300 Folkestone Beds Sandgate Beds N W SE Hythe Beds Atherfield Clay Scale 0 2 4 kin ______1 I_I 70223B01/GDC/2/A/DCI The oldest formation present at outcrop, the Lower Greensand, is found along the southern boundary of the Darent catchment. Lower Greensand comprises a basal clay (the Atherfield Clay) with two sandstone aquifers above, the Hythe Beds and Folkestone Beds. These are separated by an aquitard of silts and clays, the Sandgate Beds. The Lower Greensand is overlain by the impermeable Gault Clay which forms an important aquitard between the aquifers of the Lower Greensand and the Chalk. The outcrop of the Chalk in the central part of the Darent catchment forms the main geological feature in the study area. It gives rise to a well defined escarpment in the southern part of the Cray catchment and to the east of the M20, with the Chalk dipping gently to the north. The chalk is characterised by three distinct units, the Upper, Middle and Lower Chalk. The Chalk lithology comprises a soft microporous, white limestone becoming increasingly grey and marly in the lower units. The bases of the Upper and Middle Chalk are marked by hard, nodular chalk beds, the Chalk Rock and the Melbourn Rock, indicated on Figure 3.2. The Northern part of the catchment is characterised by the outcrop of Tertiary formations, mainly silty sands of the Thanet Beds but overlain successively in some areas by clays of the Woolwich Beds and fine sands of the Blackheath Beds. Superficial deposits include gravels and alluvium. Gravels flank the river valleys and also cap some high ground. Alluvium of varying extent and thickness is deposited in the main river valleys. 3.4 Rainfall and Evaporation Daily rainfall data for 19 stations and short lengths of autographic records for four stations were used in recharge.calculation. Details of daily rainfall stations are given in Table 3.1 and the locations of all stations are shown on Figure 3.3. Met Office MORECS tables were used to provide potential evapotranspiration information required for recharge calculation. The MORECs grid squares covering the area are also indicated on Figure 3.3. The information is available for all years from 1961 onwards. 7O223B10VWRP\B\RDER-Jul 94\wp 3-3 TABLE 3.1 Dally Rainfall Stations Station Name NRA Gauge Nr Grid Reference Mean Annual Data Available (Square TQ) Rainfall (mm) Bayleys Hill 3410001 519521 704 1952 - 1990 Christchurch Road 1211001 531740 531 1957 - 1990 Colgates Farm 1243001 495617 669 1969 - 1990 Cramptons RDPS 1343001 530570 680 1942 - 1990 Danson Park 1221001 468754 474 1935 - 1990 Eltham High School 0000018 433745 505 1970 - 1990 Eynsford PS 1320002 535655 594 1931 - 1990 Hartley 1314001 617664 617 1944 - 1990 Holwood 1241003 422636 645 1972 - 1990 Horton Kirby 1320003 561685 557 1934 - 1990 Kemsing PS 1341002 569576 692 1930 - 1990 Knockholt WW 1351001 466583 729 1930 - 1990 Orpington 1231004 462768 - 1987 - 1990 Southfleet WW 1413001 610724 512 1930 - 1990 Sundridge PS 1350001 489556 684 1930 - 1990 Trottiscliffe 2111001 640594 652 1930 - 1990 Westwood PS 1354003 425541 711 1960 - 1990 Wilmington WW 1310001 543728 509 1930 - 1990 Westerham PS 1354001 429558 734 1930 - 1990 3.5 River Flow There are three permanent gauging stations on the River Darent at Otford, Lullingstone and Hawley. Gauging station details are indicated in Table 3.2 and locations and surface water subcatchments are shown on Figure 3.3. The location of a permanent gauging station on the River Cray is also shown. The mean flows given in Table 3.2 show clearly that losses through river bed leakage occur in the lower reaches of the catchment in most conditions of flow. 70223 B1(KWRP\B\RDER-Jnl 94Wp 3-4 Figure 3.3 Hydrometric Stations and Subcatchments Legend Scale River 4 6 10km Lake or Gravel Pit Catchment Boundary River Flow Gauging Station + Autographic Rainfall Station Subcatchment Boundary # Rainfall Station - daily data -j rj'y M ORECS Grid Square 70223B01/GDC/2/A/DCI TABLE 3.2 Gauging Station Details Gauging Grid Surface Sub Catchment Start of Mean flow Station Reference Catchment Surface Geology Record Area (km2) (m3/s) Ml/d Otford TQ 525 584 100.5 Lower Greensand, 1969 0.55 48 Gault Clay and Chalk Lullingstone TQ 530 643 118.4 Chalk 1968 0.65 56 Hawley TQ 551 718 193.4 Chalk 1963 0.57 49 3.6 Groundwater Levels Groundwater level data are essential for comparing with simulations of levels produced during catchment model calibration. About 30 observation wells with long-term records up to the present day are located in the Darent and Cray catchments. They average about 25 years of level measurements but for many there are gaps in the record. Ground levels have been measured in about 25 other wells at various times in the last 35 years. Records for these additional wells generally extend over less than ten years, many are for periods before 1980 and some pre-date 1970. Nonetheless they provide useful additional data for catchment modelling. The locations of all observation wells are shown on Figure 3.4. The overall number of observation wells is fairly reasonable for a catchment the size of the Darent. However, there is limited coverage in some areas. These include much of the north-west and south east of the Cray catchment and the Lower Greensand in the southern part of the Darent catchment. There are also no observation wells located away from abstraction sources to monitor Chalk water levels close to the River Darent where it cuts through the North Downs. These deficiencies in the network should be rectified by the proposed construction of new observation wells. 3.7 Aquifer Properties Aquifer properties can be determined from the results of controlled discharge pumping tests on wells and boreholes. Transmissivity controls the rate of groundwater flow through an aquifer and can be determined from water level data for a pumped well. To determine aquifer storage coefficients, changes in water level are required for observation wells generally located close to the pumped well. 70223B1ftWRPVB\RDER'Jnl 94\wp 3-5 Values have lo be assigned to aquifer properties as part of the input data for a catchment model. If test data are available for the aquifers in various areas of the catchment, these can provide a useful initial indication of suitable aquifer property values for modelling. During the pre-feasibility study (GDC, November 1991) very few well testing data were found which could be used to define aquifer transmissivity. No data from observation wells were available to allow calculation of storage coefficients for any of the aquifers. However, during 1992 well testing was carried out at five Thames Water Utilities Chalk groundwater sources in the Darent arid Cray catchments. In addition, the water company provided comprehensive reports for recent testing at one Chalk and one Lower Greensand groundwater source together with data for testing carried out in the 1950s and 1960s at a further Lower Greensand source. West Kent Water Company also provided data from the testing of a new Lower Greensand well. Finally IGS (1975) gave information on more than 100 Chalk wells in the Dartford area, some of which can be used to determine aquifer properties. The data available therefore provided a reasonable starting point for setting up the catchment model. 3.8 Groundwater Abstractions Abstraction data have been processed mainly from paper copies of licensee returns and from microfiche archive records. The data were available in various forms from daily to annual abstractions with very variable units (metric and imperial). Results have been summarised in Table 3.3. In order to simplify data processing, abstractions with an annual licence of less than 50 Ml/year were not included in modelling. These small licences have a total of less than 300 Ml/year (<1 Ml/d) which is less than 0.5% of total groundwater abstraction in the catchment model area. The total annual pumping permitted for licences considered in this investigation is as follows: Lower Greensand Aquifer 50 Ml/d Chalk Aquifer 123 Ml/d Total 173 Ml/d These licences are for the Darent catchment only. Licences for the Cray and Ebbsfleet catchments were also included in catchment modelling. Of the abstraction in the Darent 155 Ml/d are held in licences by the five water companies operating in the catchment. Thames Water Utilities hold licences to abstract 107 Ml/d at 10 sites. In the period 1985 to 1990 between 70 the 75% of total licenced quantities was actually abstracted from both aquifers. There are some gaps with no data available in the records but most licences have some information from 1974/75 onwards. This allowed reasonable infilling of the records where required. 70223B1(71WRPVB\RDER-Ja] 94\wp 3-6 Figure 3.4 Observation Wells 501 * Qrld Squar* : TQ Legend Observation Well Reference Nrs TW A standard reference eg TQ56/25 abbreviated Monitored currently to 56/25 on the figure Monitoring previously by NRA standard reference eg 441 346 001 abbreviated to 346/1 Thames Water Authority (TWA) Scale 8 10km 70223801 \G DC\2\A\DCI TABLE 3.3 dwater Abstraction Details TTTSEY ESTATE CO. T0424537 FOLKESTONE BEDS IGW 70918 i»oa®G» WIGGINS TEAPE TQ535752 CHALK 1 2000240 PAPERS LTD TQ530748 ■a -akw'jLL. j ^.^^••^':^x-:<;':^v'-::>-:-> EAST SURREY WATER CO. TQ424541 LOWER GREENSAND WESTWOOD PS. PWS 2045700 TQ425542 HORTON KIRBY T0S62B94 CHALK 1 300038 PAPER MILLS T0563604 WELLCOME FOUNDATION LTD T0543744 CHALK nc 2273000 T0548745 T0545748 A VINSON LTD T0496897 CHALK A 66585 COATES LO«LLEUX T0468880 CHALK IGW 65817 CLUBBS WASHED T0551727 GRAVEL IGW 1S2609 GRAVEL CO. LTD WELLCOME FOUNDATION LTD 105*4745 CHALK 1 1513818 TOSS4748 BEXLEY SAND* T0500735 GRAVEL IGW 205480 BALLAST CO. LTD SCHWEPPES (HOME) LTD T047Z707 CHALK IflC 412788 WILLIAM NASH LTD T0472897 CHALK 10) 120014 LUUJNGSTONE WATER LTD T0531642 CHALK PWU THAMES WATER UTUJTTES T0543728 CHALK POWERMU1 LANE. PWS 6960018 WILMINGTON P.S. THAMES WATER UTILITIES 10429556 CHALK A PILGRIMS WAY, PWS 364688 UPPER GREENSAND WESTERHAM P.S. THAMES WATER UTJUTTES 70573700 CHALK GREEN STREET PWS 1850200 GREEN, DARE NTH THAMES WATER UTTUTIES TQSS8880 CHALK HORTON KIRBY PS. PWS 4877870 THAMES WATER UTtUTlES TOS2S629 CHALK LULLINGSTONE P.S. ZfcS*S PWS 3318580 THAMES WATER UTILITIES T04B0557 LOWER GREENSAND SUNDR1DGE PH. PWS 4877870 .... THAMES WATER UTILITIES T0464673 CHALK ORPINGTON PWS 3318680 i I t THAMES WATER UTUJT1ES TQ53SB65 CHALK EYNSFORDP.S. PWS 6130373 THAMES WATER UnUTIES T0501741 CHALK WANSUNT^EXLEY PWS 4877870 T0507746 THAMES WATER l/TUTIES T0464673 CHALK ORPINGTON PWS 4045040 T0450653 THAMES WATER UTUJTIES T0610704 CHALK PWS C%v <4.t*t t1|>_ M 820046 "THAMES WATER UTTLTTIES TOS13743 CHALK CRAYFORD PWS 4877870 TQS15748 THAMES WATER LTmiTIES T0482728 CHALK y- ''"','{<,, _J BEXLEY PWS 11508504 imwy1&tt;r-f , s ^ J THAMES WATER UTUITIES T0470557 LOWER GREENSAND BRASTEDPA ms 1683838 *" $ w/‘ **i 'S'* *&*“ ^ y 4 ■$&&&&' '"/'I * +'\ THAMES WATER UTUJTIES T0548741 CHALK OVERY STidARY, PWS 1327432 70541728 DARTFORD P.S. if0iaafOft^v;- „ THAMES WATER UTUiTIES TQ558710 CHALK HAWLEY ROAD. PWS 7632734 :-^ :^»:^>;-:->>»: MID KENT WATER C a TQB16804 LOWER GREENSANO HARTLEY* ASH- PWS 827372 CUM-RIDLEYP& 1/6lS0ft*R '„??/'' "i SOUTHERN WATER T0507687 CHALK PAWKHAM PA. ^ ^ -*v^■ PWS 2401208 irtieo^w SAMAS VICKERS LTD TOS48727 CHALK FAWKHAM P.S. 1 08103 'H'iKt £ MID KENT WATER C a T0614640 CHALK RIDLEY PWS 3730540 w satto't^lz THE NATIONAL GRID T0473720 CHALK FOOTSCRAYLANE 1C 058370 S>-.h{^ ^ ~JXh. >' CO. LTD COOLING STATION THE NATIONAL GRID TQ4Q6728 CHALK VICARAGE ROAD, c 068370 K'f" y THE NATIONAL GRID T0438731 CHALK BLANMERLE ROAO. 1C 708641 <$••*.<}, >,-M/^.vJ' CO. LTD NEWELTHAM. v/f-X COOUNG STATION CROSSWAYS PARK TQSA47S2 GRAVELS CROSSWAYS PARK, SI £0008 V ^C''"*' DARTFORD DARTFORD TRADE :ilf PARK SOUTHERN WATER T0620713 CHALK HAZEUSP^. * 177221264 PURPOSE DoDOMESTlC AGRICULTURE PWU-PRIVATE WATER UNDERTAKING H-HORT1 CULTURE SI-SPRAY IRRIGATION UNOUSTOAL SGW-AGGREGATE WASHING IC-COOUNG IQoOTHER USE ______FF-BSH FARM PWS-WBUC WATER SUPPlV WC-WATERCWESS * Aggregate with 9 other sites outside catchment. CHAPTER 4 INTEGRATED GROUNDWATER/SURFACE WATER MODELLING 4.1 D escription of M odel The model used for this investigation is an integrated groundwater and surface water catchment model. This simulates flows in aquifers and river systems, and the interaction between surface water and groundwater flow components. The groundwater component uses the integrated finite difference method and incorporates an iterative solution technique based on a backward difference approximation. The model comprises a network of irregular polygons varying in size, shape and orientation to fit local details and physical characteristics of the surface water and groundwater systems. Rivers are incorporated as additional elements located between adjacent groundwater polygons. The model grid network is shown in Figure 4.1. It covers the catchments of the Rivers Darent, Cray and Ebbsfleet. The model grid covering the Darent catchment comprises a network of 619 polygons produced by subdivision of a regular mesh of squares. The grid area is 527 km2 with polygon sizes ranging from 0.27 to 4.4 km2. Groundwater conditions can be represented more accurately by increased subdivision of polygons giving a dense network of small polygons in specific areas of particular interest. The highest density of polygons occurs along river valleys and in areas of high groundwater abstraction. The river system is represented by a set of 99 river elements which are located along the interfaces between specified polygons. The elements range from 0.5 to 1.0 km in length depending on location. Representation of lakes can also be fully integrated within the model. Inflow and outflow to lakes are balanced by lake bed leakage. External boundary conditions are specified as either fixed head, for example along the northern boundary formed by the River Thames, or with a fixed groundwater gradient. Chalk fissure systems can be simulated using a depth-dependent permeability function. The principal aquifers of the Darent catchment are the Chalk and Lower Greensand, separated by impermeable Gault Clay. Six geological layers are included in the catchment model as indicated in Figure 4.2. The Chalk is represented as a single layer whilst the Lower Greensand is divided into three layers with Hythe Beds and Folkstone Beds aquifers separated by the Sandgate Beds aquitard. There is an additional aquifer modelled within Tertiary deposits and alluvium in the Cray catchment river valleys and northern-most parts of the model area. Aquifer recharge which forms the main contribution to river baseflow is estimated using a separate lumped-parameter hydrological model (a modified version of the Stanford Watershed Model). This also provides surface runoff and interflow components. Hydrological models of three gauged 70223 BmWRPVBVRDER-Jol 94\wp 4-1 subcatchments were calibrated against river flow data principally through adjustment of soil moisture parameters. Monthly recharge estimates were then extrapolated to cover the entire model area and transferred to the catchment model taking into account the effects of variation in outcrop geology, rainfall and urbanisation. 4.2 Calibration Catchment model calibration comprises two stages. The first is steady state calibration in which mean groundwater conditions, often inferred from scattered historical evidence, are simulated for either pre-development periods in which natural hydrological conditions prevailed, or periods in which abstraction was well established and remained reasonably constant. No changes in groundwater storage are simulated for steady state conditions, hence the number of model calibration parameters is reduced, simplifying the initial stage of calibration. Steady state calibration is used mainly to establish aquifer transmissivity and leakage parameters controlling flows between aquifers. For the Darent, reasonably uniform transmissivities averaging about 400 m2/d were found appropriate for both the Lower Greensand aquifers. For the Chalk, transmissivities specified during steady state calibration ranged between 20 to 300 m2/d in relatively impermeable Chalk beneath upland interfluves and 500 to 2 000 mVd in highly fissured zones in valley areas. The second stage of catchment model calibration is for the transient state. Variations in the groundwater and surface water systems with time were simulated for the period 1970 to 1990 using a monthly timestep. Variations in river flows are controlled by the recharge pattern, river bed leakage and aquifer storage properties. Good simulations of flow at the three permanent gauging stations on the River Darent were obtained by applying unconfined storage coefficients of 3.5% and 10% for the Hythe and Folkestone Beds respectively, and 2% for the Chalk. Groundwater levels at two observation well sites with long-term records for the Lower Greensand were simulated reasonably accurately. For the Chalk, simulated variations in groundwater levels were initially much greater than observed. Considerable improvement was achieved in simulation by specifying depth dependent permeability to model fissure flow in a zone corresponding to the normal range of Chalk groundwater level fluctuations. 4.3 Calibrated Flows 4.3.1 Summary of Results Mean simulated and observed river flows for the three permanent gauging stations on the River Darent at Otford, Lullingstone and Hawley are given in Table 4.1. Monthly simulated and observed flows for the three stations are plotted in Figures 4.3 to 4.5. 70223B10VWRP\B\RDER-Jn] 94\wp 4-2 F igure 4.1 Catchment Model Le ge nd Model Polygon River Lake or Gravel Pit A/ Model River Elements Catchment Boundary Sca le 4 6 8 10km Blue Cirde Cement Quarries 70223\B10\GDC\1\A Figure 4.2 Modelled Aquifer System Layer Name Flow Function number direction Alluvium/ River Gravels/ A quifer 6 Tertiary Deposits f - Leakance interface 5 Chalk A quifer 4 Gault Clay ^ Aquitard 3 Folkestone Beds A quifer 2 Sandgate Beds I A quitard ^ |538322§> 1 Hythe Beds A quifer Impermeable base Atherfield Clay 7D22381AQOCiaAtDCI vuvsaomscKu Q A W T V ^ E N K 3 E M U J K k D A R E H T \ T E R H Y \ R 9 - 4 0 O E M b y ( U 0 E 2 1 3 © 2 1 90 91 92 93 94 95 96 97 98 99 90 91 92 93 94 95 96 97 98 99 1990 1989 1988 1987 1986 1985 1984 1983 1982 1981 1980 1979 1978 1977 1976 1975 1974 1973 1972 1971 1970 iuae Fos t Otford at Flows Simulated iue 4.3 Figure Figure 4.4 Simulated Flows at Lullingstone ■l CO M (soauino) M oy 7oaaaBiAaoc\i\A vuvsaovoigcnw iuae Fos t Hawley at Flows Simulated iue 4.5 Figure TABLE 4.1 Mean River Flows (1970 to 1989) Gauging station Observed Simulated % (m3/s) (m3/s) Difference Otford 0.556 0.532 -4.3 Lullingstone 0.664 0.676 -1.8 Hawley 0.584 0.578 -1.0 Long-term observed flows were simulated to an accuracy of 4% at Otford, 2% at Lullingstone and 1% at Hawley. As gauging station measurements themselves are generally considered acceptable if accurate to 5%, the overall accuracy of the simulations is considered to be very good. Figures 4.3 to 4.5 indicate good simulation throughout the period of record although there is some oversimulation of winter peaks in some years. However, more importantly for this investigation, accurate simulation of low flows in summers and over extended drought periods has been achieved with considerable precision. 4.4 Catchment Water Balance Water balances are presented in Table 4.2 for the Lower Greensand and Chalk/Tertiary deposits aquifer systems. The balances are for steady state model simulations for pre-development conditions (ie without abstraction) and abstraction at maximum historical level. The area of the Chalk/Tertiary deposits aquifer system corresponds approximately to the groundwater catchment contributing to the River Darent upstream of the Cray/Darent confluence under pre-development conditions. TABLE 4.2 Simulated Steady State Water Balances for the Darent Catchment Water balance Lower Greensand Chalk/Tertiary Deposits component Pre-development With abstraction Pre-development With abstraction (Ml/d) (Ml/d) (Ml/d) (Ml/d) Recharge 59.0 59.0 77.4 77.4 Abstraction - -30.9 - -108.9 Inflow/outflow -8.3 -6.8 21.0 42.9 Discharge to river -29.0 -9.3 -71.7 -1.0 Spring flow -19.9 -10.5 -27.2 -10.2 Lake leakage -1.6 -2.0 - - Note: A positive values indicates a gain to the aquifer system in the catchment - a negative value indicates a loss. Minor imbalances not shown. 70223BlCf\WRP\B\RDER-.Jnl 94\wp 4-3 Abstraction from the Lower Greensand aquifers has simply reduced the net baseflow discharge to the river. Abstraction has not caused an increase in the contributing groundwater catchment area. There is therefore no significant change in boundary outflow from the aquifers. With abstraction the total contribution to the river, comprising baseflow discharge and spring flows, is less than 40% of the contribution in pre-development conditions. In preparing Table 4.2 the same catchment area was considered for pre-development conditions and conditions with abstraction. However, in the Chalk, as abstraction increases the positions of the groundwater divides between catchments change. The catchment area contributing to the Darent water balance increases at the expense of adjacent catchments. This is reflected in the doubling in net cross boundary inflow from 21 to 43 Ml/d for the fixed catchment area considered. The additional inflow originates as recharge over areas which, with abstraction, fall within the expanded Darent catchment. As a result of abstraction the baseflow from the Chalk is reduced to a point where total leakage from the river system in areas of high abstraction almost equals the baseflow contributions in the few remaining gaining reaches. Abstraction has also caused the drying up of many Chalk springs and reduced the groundwater discharge to the Thames. Overall good simulations of hydrological conditions have been obtained throughout the catchment for a 20 year period of simulation. This particularly applies to river flows at the three historical gauging stations on the Darent where low flow periods are simulated well. As a result there is high confidence in using the model to predict the effects of water resources management strategies. 4.5 River Losses The River Darent flow generally decreases below Shoreham (midway between the gauging stations at Otford and Lullingstone). The main cause for this decrease is leakage through the river bed into the underlying Chalk aquifer. Over-abstractions from the Chalk aquifer have resulted in the watertable falling well below the bed of the river foT extended periods of time. As a result the river changes from being influent, as would be the case under naturalised flow conditions, to being effluent; the river becomes perched over much of its course between Shoreham and Hawley and thence to its confluence with the River Thames. The effect of these perched river conditions on flows can best be seen by reference to Figure 4.14 and Table 4.3. During winter months and also in early spring and late autumn, soil moisture levels over the catchment are generally high: surface runoff occurs, base flow rises and localised springs discharge. Runoff from the catchment between Lullingstone and Hawley (73 km2) will therefore mask the losses and accurate estimates will not be possible during these seasons. In addition, the higher flows recorded in these months will lead to larger errors (ie differences between larger numbers). In contrast during summer months flow differences between Lullingstone and Hawley will more accurately represent losses over this stretch of the river. They will also be more applicable to the drought periods when augmentation of flow will be of more value. Errors could also arise during 70223B10\WRPVB\RDSR~Jti] W\wp 4-4 extreme low flows when the river dries out completely at Hawley. During this time it is possible that the difference in flow will underestimate the potential loss. Flows in these months have been excluded therefore from the analysis of flows in Table 4.3. Between June and September the mean loss ranges from about 12 Ml/d in June to 9 Ml/d in July, with an average over the four months from June to September of 10 Ml/d. Most of the very high values can be explained by anomalies in the data. For example, the flow difference of 450 1/s in June 1986 appears to be due to a change in rating or a series of spurious water levels recorded at Lullingstone. Therefore the apparent or net loss is about 10 Ml/d. However, Horton Kirby Paper Mill abstracts 0.9 Ml/d from a local borehole and discharges directly to the river. Dewatering of quarries also takes place below Hawley during the day at a rate of 0.5 Ml/d which is added to the river flow. Therefore, the gross loss could be 11.4 Ml/d. The river length between Lullingstone and Hawley is about 10.7 km which gives a net loss per kilometre of 0.9 Ml/d per kilometre and gross loss of up to 1.1 Ml/d per kilometre. However, a spot current metering programme undertaken by NRA indicates that there is a wide variation in flows measured over the reach between Lullingstone and Hawley. Apparent losses are found to be greater between Lullingstone and Horton Kirby than between Horton Kirby and Hawley. Investigations are continuing to evaluate whether these variations are real or represent unmeasured subsurface flow. A knowledge of the variations in losses along the river will be a factor in determining the location and amount of augmentation flow. During drought periods topping up of fishing lakes between Horton Kirkby and Hawley will affect instantaneous flows and could lead to an overestimate of losses over shorter time periods. However, any such errors will in the long term be averaged out. The losses measured between Lullingstone and Hawley will contain both bed leakage from the river and lake beds to the underlying aquifer and also evaporation losses. The surface area of the lakes and the river between Lullingstone and Hawley is approximately 0.3 km2. During summer months a typical maximum open-water evaporation is 100 mm/month, which equates to a loss of 1 Ml/d or 0.12 Ml/d per kilometre. Therefore, of the gross losses of 11.4 Ml/d over this reach, about 9% will be through evaporation loss and 91% through bed leakage. The integrated surface/groundwater model of the River Darent Catchment estimates both evaporation and bed leakage losses extremely well. Simulated base flow recessions at all gauging stations matched closely recorded flows over the whole of the 20 year period for which flows were calculated by the model. Details are given in Annex II where the calibration of the model is described. Simulated flows for both Lullingstone and Otford are reproduced in Figures 4.3 and 4.4. If the recorded river losses are modelled accurately, then the losses estimated with increased flow through augmentation wells and reduced borehole abstractions, should be so also: the physical processes involved in producing the river losses are all simulated in the model. However, to check the model, trial augmentation wells are currently being installed and will be operated to confirm the results. It is intended that the trial wells will be incorporated in the final augmentation scheme. 70223BiavWRP\B\RDER-Jul 94\wp 4-5 TABLE 4.3 Mean Monthly Flow Difference (1/s) Between Lullingstone and Hawley Gauging Stations (Lullingstone-Hawley) Jan J F MA MJJ A S 0 N D 1970 71 50 17 72 123 198 120 115 118 99 107 58 1971 37 33 113 87 97 82 126 112 133 101 1972 117 110 ■ 70 55 80 103 98 86 113 119 115 273 1973 118 110 97 132 170 134 118 81* 141** 121 100 220 1974 297 207 90 99 118 108 125* 162* 420** 207 155 36 1975 22 27 89 83 108 1976 4 68 46 125 145 101* 58* 57* 128* 353* 563** 101 1977 6 26 55 88 129 131 44 125 137 106 1978 106 51 79 113 102 101 198 1979 132 48 42 67 91 114 113 78 191 1980 9 63 152 131 145 137 168 98 124 1981 70 47 105 133 88 91 109 122 218 175 156 125 1982 5 60 78 107 235 98 1983 1 10 51 48 66 202 105 1984 8 47 39 88 69 141 193 140 57 1985 51 22 44 107 3 91 164 88 93 129 297 1986 390 163 144 167 266 450 169 108 129 178 254 167 1987 240 22 14 66 119 99 104 123 97 240 1988 21 163 175 140 159 102 1989 101 172 157 54 100 117 136 123 111 151 206 363 1990 269 80 260 359 171 95 212 192 252 1991 461 304 255 212 196 212 184 104 80 98 215 143 1992 173 161 228 317 175 70 128 163 114 64 1993 29 72 199 264 361 182 140 154 334 M ean difference (1/s) -- * 136 101 111 115 -- - Mean difference (Ml/d) - ** 11.8 8.7 9.6 9.9 * - - Notes: * Zero flow at Hawley recorded in month (not included in mean loss calculation) ** Zero flow in month followed by surface runoff (not included in mean loss calculation) Blank spaces indicate that either missing data at one of the stations or that the Hawley flow exceeds that recorded at Lullingstone. Flows in bold print >200 1/s. ?0223B10VWRPVB\RDER-Jol 94\wp 4-6 4.6 Naturalised Flows Following calibration, the catchment model was used to simulate naturalised catchment and river conditions by operating with all abstractions set to zero. Simulated naturalised monthly flows at Otford and Hawley are compared with simulated historical flows in Figures 4.6 and 4.7 and Table 4.4. Naturalised and historical flow accretion profiles for May and August of 1984 (a year of near average recharge conditions), and August of 1976 (a year of severe drought), are shown in Figure 4.8. TABLE 4.4 Mean Simulated River Flows (1970 - 1989) With abstraction Naturalised Gauging Station m3/s Ml/d m3/s Ml/d Otford 0.532 46.0 0.931 80.4 Hawley 0.578 49.9 1.516 131.1 The table and all three figures demonstrate the extreme effect that abstraction from the Lower Greensand and the Chalk has had on river flows. Abstraction from the Lower Greensand reduces average river flows at Otford by more than 40%. At Hawley flows are reduced by more than 60% as a result of abstraction from the Lower Greensand and the Chalk, In summers, naturalised flows are very much larger than historical flows. As a comparison, whilst mean monthly flows of less than 8 Ml/d (0.1 m3/s) are simulated for historical conditions in 40% of years at Hawley, the minimum naturalised flow simulated at Hawley throughout the period is greater than 52 Ml/d (0.6 m3/s). The three flow accretion profiles in Figure 4.8 indicate that in natural conditions the river gains flow from groundwater throughout its length. The very significant increase in flow just upstream of Otford results from flow contributed from the Honeypot Stream. In historical conditions very large losses in flow occur by leakage through the river bed in the Chalk stream reaches downstream of Shoreham. Losses increase as the summer progresses as indicated for 1984. In August 1976 there was virtually no flow downstream of Horton Kirby. In contrast simulated naturalised flows increase from 56 to 69 Ml/d (0.65 to 0.80 m3/s) between Horton Kirby and the confluence of the Darent with the River Cray. The simulated naturalised flow data were analysed to produce various low flow statistics. Frequency distributions of annual minimum series for flows at Otford and Hawley are shown in Figure 4.9. These indicate low flows with various return periods. Frequency distributions for simulated flows in August only are given in Figure 4.10 for five key sites between Otford and the Darent/Cray confluence. Naturalised flows for August in selected drought years (1973, 1976, 1989 and 1990) and years of more normal recharge (1984 and 1985) are also shown. 70223 BICTVWRPVBXRDER-Jiil $4\wp 4-7 Frequency distributions for each month, similar to those shown in Figure 4.10, were used to derive the l-in-20 year monthly variations in flow shown in Figure 4.11 and the approximate l-in-20 year flow accretion profiles for May and August in Figure 4.12. Distributions in Figure 4.11 show only small seasonal variations in flow. This is to be expected as low flows in all seasons comprise mainly baseflow from the Lower Greensand and Chalk. The aquifers provide a steady flow contribution buffered by aquifer storage. At Hawley the 1 in 20 year monthly flows range from 61 Ml/d (0.70 m3/s) in late summer to 91 Ml/d (1.05 m3/s) in late winter. An approximate 1 in 20 year profile derived from the annual minimum series for the five key sites between Otford and the Darent/Cray catchment is also shown on Figure 4.12. Although August is quite frequently the month with lowest flow in a particular year, the profile derived from the annual minimum series has slightly lower flows than the August profile. The annual minimum series includes data for other months in those years when flows are not at the minimum in August. The approximate profile based on the annual minimum series for five sites was also used to derive a more detailed 1 in 20 year flow profile. The l-in-20 year flows for the annual minimum series are of similar order to the naturalised flows for August 1976. 0ne-in-20 year low flows were therefore calculated for each model river element as the product of the August 1976 flow for the element and the ratio of the 1 in 20 year low flow to the August 1976 flow for the nearest of the five key sites. This detailed profile is also shown on Figure 4.12. 4.7 Target Flows Target flows for the river were required before proceeding to model options for restoring flows. It was considered that the target flows should: (i) be related to naturalised river flows and should provide a natural flow profile along the river; (ii) satisfy a minimum ecologically acceptable flow which was taken as that necessary to sustain a brown trout fishery; (iii) be reasonably achievable in the foreseeable future. Flows comprising 50% of the 1 in 20 year annual minimum series profile were found to be of similar order to restored flows proposed by Atkins (1991). Subsequent application of PHABSIM simulation system by the Institute of Hydrology (I/H), Institute of Freshwater Ecology (IFE) (see Chapter 5) have confirmed the suitability of the target flows selected. Furthermore it appeared that these flows, which increase up to 0.38 m3/s (32 Ml/d) at the confluence of the Darent with the River Cray might be achievable in all years by a realistic reduction in abstraction, possibly with river support pumping to top up flows in drought conditions. The joint project team therefore adopted 50% of the l-in-20 year flows as the target for the Plaii for the Darent. The target flows for the five key sites are given in Table 4.5. 70223B10\WRP\BVRDER‘inl 94\wp 4-8 Figure 4.6 Simulation of Naturalised Flows at Otford • • • • • • • • • • • iuain f auaie Fos t Hawley at Flows Naturalised of Simulation iue 4.7 Figure lOEM By 9 A C . 02B10\ODC\1 \fc70223B 11 tit auaie Fo crto Profiles Accretion Flow Naturalised 11 s t t uut 1976 August August 1984 August May1984 6 o « >. o Figure 4.8 Figure I * QEM By SAC. 70223BlO\ODCUlA JO . H 0.5 1 n n b € 15 JZ © E 1.0 E c E W u (0 S> © E w E 3 0.0 1.5 0.0 — 0.5 1.0 1.5 — eun eid (Years) Period Return Return Period (Years) Period Return 5 10 20 50 100 100 50 20 10 5 1 2 5 100 50 20 10 5 o MW) W (M Row fc) ’ {m How years) ReturnPeriod( o MW) W (M Row o ) m ( Row eunPro yeara) ( ReturnPeriod for Annual Minimum Naturalised Flows Naturalised Minimum Annual for I I I I I i r i— i — rqec itiuin E ) I GEV ( Distributions Frequency .407 0.70 0.74 0.64 0.52 36 61 64 73 2 541 45 2 .704 0.43 0.45 0.47 5 02 50 20 10 5 02 50 20 10 938 39 .7062 0.67 854 58 Hawley Otford 0.41 36 60 — Figure 4.9 Figure 100 130 110 120 130 100 110 120 80 20 30 60 if 90 0 | 40 “ 50 70 I 10 20 30 "I o 50 0 | 70 80 90 10 0 0 3 o | laOEMBySAC. 70223B10iaDCl1lA Frequency Distribution for Naturalised August Flows August Naturalised for Distribution Frequency S/eUJ)MO|d (S ( B t o A | ) / w o u £ CL .o “8 s Figure 4.10 Figure Figure 4.11 1:20 Year Monthly Variations in Naturalised Flows a (FVW)w»u o 3* < c 3 S' W 2 *8 - 5 o* (S/eUl)/AOjd ToaaaBio\ooc\ivA rU\DGD\OlBC«M Q\WRP\£W,Q£MJJ(«\Afl£N1\TEBRY\F10-*-1 2. GEM By C.SA 1 1 1 1 _____ i ____ o o o Z i i 2 Ya Fo Acein Profiles Accretion Flow Year 20 in 1 Flows corresponding to 50% for the l-in-20 year flows for each month were also calculated. They are shown in Table 4.6. Although these flows were not used by the joint project team in assessing options, it is proposed to use them as individual monthly target levels at a later stage of flow restoration in order to obtain natural seasonal variability. Some basis for comparison of effectiveness of flow restoration options was also required at this stage. Naturalised flows for two months, August 1976 and August 1984, indicate that these months are representative of extreme drought and average late summer flow conditions. Flow profiles for these two years were therefore considered when comparing options for restoration. TABLE 4.5 Annual Minimum Target Flows for the Plan for tbe Darent Target flow Key site (m3/s) (Ml/d) Otford 0.215 18.5 Lullingstone 0.285 24.5 Horton Kirby 0.310 27.0 Hawley 0.335 29.0 Cray Confluence 0.375 32.5 TABLE 4.6 Proposed Monthly Target Flows Month Otford Lullingstone Horton Kirby Hawley Cray Confluence January 29 40 44 45 51 February 29 33 43 45 51 March 24 36 41 43 47 April 25 34 40 41 48 May 23 29 34 38 44 June 22 29 34 35 37 July 22 28 30 34 35 August 21 26 28 30 35 September 20 26 30 33 35 October 20 26 32 33 37 November 22 28 32 34 39 December 28 37 41 45 49 Note: All units Ml/d. 70223B1(^WRt'Vfi\RDER*Jal 94\wp 4-9 4.8 Options for Restoring River Flows 4.8.1 Introduction A wide range of options involving variations in abstraction was considered in preparing the Plan for the Darent. Variations in abstraction at particular sites were modelled with other abstractions at historical levels or at full licence capacity. Seasonal variations in abstraction and peak pumping requirements were also simulated for some changes to abstraction. Once the main abstraction changes needed for the plan had been established, flow augmentation using river support boreholes to meet target flow requirements in drought periods was also included in simulations. In all, more than 30 separate simulations were carried out using the calibrated catchment model. The worst affected reaches of the river are known to be downstream of Lullingstone. Initially therefore the effects of varying Chalk well licences in this area of the catchment were assessed. An assessment was also made at an early stage of the worst-case effect of all major licences in the catchment abstracting at full licensed rate. Simulations showed that major reductions in Chalk well licences alone could not produce the required levels of river flow restoration. The effects of reducing abstractions from the Lower Greensand in improving flows along the entire length of the river were therefore considered. Naturalised flow accretion profiles had indicated the importance of flow contributions from the Lower Greensand upstream of Otford, both directly to the river and via the Honeypot Stream tributary. Having established the approximate relative levels of reduction in Lower Greensand and Chalk abstraction to produce the target restored flows, a number of simulations were carried out to determine at which Chalk well sites reductions would have greatest benefit to the river. Conjunctive use of groundwater with Thames Water was also considered with simulation of the effects of operation of TWUL abstraction control rules in drought periods. In these periods a sharp increase in abstraction was modelled to compensate for an overall restriction in surface water supplies. As indicated above, river flow augmentation was also considered with a number of options. The most important results of this work were discussed and illustrated in Appendix 2 to the Plan for the Darent. Summaries are given in the following sections with discussion of some important points not fully dealt with in the Plan. More recently a further report ‘Options for Restoring River Flows, Catchment Modelling Assessment Report’ (GDC, April 1994) has been published which deals more comprehensively with the studies associated with the TWUL/NRA Joint Team negotiations. The standard unit of abstraction used for this work was Ml/d. For consistency, river flows are also expressed in Ml/d rather than the more generally applicable unit of m3/s as included in discussing the model calibration work. 70223BH7lWRP'3\RDER-Ju] 94Wp 4-10 4.8.2 Abstraction at Full Licence Capacity Over the past 20 years total pumping from the Lower Greensand and the Chalk within the Darent catchment has been operated generally at 70 to 75% of the sum total of annual licences but there is a theoretical possibility in future years that total pumping may increase to 100% of total annual licences (although 90% is the probable maximum that could practically be abstracted). The model was used to demonstrate the consequences for river flow of the worse possible 100% case scenario. In general terms the additional depletion caused to river flows is very severe. In an average year the river would dry out by late summer in all reaches except for those between Otford and Horton Kirby. The maximum flow in August in any part of the river would be about 8 Ml/d, occurring in the vicinity of Shoreham. This contrasts sharply with a corresponding flow of 21 Ml/d for the simulation with historical abstractions. In a drought year the maximum flow in August would be a mere 3 Ml/d with no flow at all in any reach downstream of Lullingstone. 4.8.3 Reductions in Abstraction (i) Chalk Wells Several simulations were carried out initially to assess the effect of various levels of abstraction from zero to full licensed quantity at six Chalk well sites. The sites were Lullingstone, Eynsford, Horton Kirby, Darent, Wilmington and Dartford, all close to the river. The river is often perched over the middle and lower Chalk reaches of the catchment with the groundwater table lying well below the river bed. In such conditions there is leakage from the river bed which on average between Lullingstone and Hawley is of the order of 1.1 Ml/d per km. Reduction in abstraction from the Chalk aquifer has relatively little effect on river flows during drought periods. The groundwater table has first to recover to ground level before leakage from the river is reversed. In addition, the model also indicates that if the Chalk abstractions are reduced substantially then the groundwater catchment reduces significantly as a result of the influence of abstractions in adjacent catchments. This is illustrated in Figure 4.13. The reduction in groundwater catchment with reduced abstraction is clearly shown. From simulation it was found that a reduction of 15 Ml/d in Chalk abstraction is likely to result in only a 10 Ml/d increase in mean river flow at Hawley. (ii) Lower Greensand Wells The next set of simulations weighted reductions in abstraction equally between the Chalk sources and TWUL Lower Greensand sources at Sundridge and Brasted. Whilst the size of the Chalk groundwater catchment was found to vary considerably with abstraction, model calibration had already indicated that the Lower Greensand catchment remained virtually constant in area under pre-development and historical abstraction conditions. Changes in abstraction produced virtually no change in boundary 70223B1 outflow from the Lower Greensand. It appeared that any additional groundwater made available by a reduction in abstraction from the Lower Greensand would therefore contribute to an increase in river flow. The effects of reducing Lower Greensand abstractions still further whilst maintaining an overall level of reduction equivalent to just over 50% of the licensed abstraction for the original six Chalk sources was assessed. In further simulations abstractions at Brasted and Sundridge were eliminated completely. The balance of reduction in abstraction was spread over the four Chalk sources at Lullingstone, Eynsford, Horton Kirby and Darenth. Results from two simulations shown in Figure 4.14 demonstrate the benefits to the river of greater reductions in abstractions from the Lower Greensand. In both simulations (reference PR 15 and PR 16) total reductions in abstractions are equal. By concentrating the reduction in abstraction on the Lower Greensand sources (as in simulation reference PR16) the river benefits by an additional 7.5 to 8.5 Ml/d over its length from a point midway between Westerham and Otford down to Lullingstone within the main Chalk valley. Below Lullingstone there is a gradual, slight reduction in benefit reducing to about 6.0 Ml/d just upstream of the Cray confluence. The greater abstraction from the Chalk in simulation PR 16 has very little effect on the length of river which is perched above the watertable. Very little additional loss in flow therefore results from greater Chalk abstraction whilst the contribution of additional groundwater from the Lower Greensand to the river is, in comparison, very large. The slight convergence in benefits to river flow from the two strategies below Lullingstone results from greater leakage to groundwater with the higher flows in simulation PR16. (iii) Lower Greensand Response Time Having established the benefits to be gained from maximising the reduction in Lower Greensand abstractions, the timing of the response to major reductions was investigated. The river above Otford is almost always influent in character with springs feeding river flows. Some immediate impact on flows resulting from a reduction in abstraction would therefore be expected. The model was run with a repeated annual cycle of mean monthly recharge values over a period of 19 years. Abstraction at Brasted and Sundridge was kept constant at 70% of licensed rate but reduced to zero for two separate periods of four years. The simulation indicated that 50% of the abstraction returned to the river in one month following shutdown. All the abstraction had appeared as additional river flow at the end of a five-month period. The rapid response results from low confined storage characteristics in the Hythe Beds and the relatively small volume of Folkestone Beds aquifer dewatered as a result of abstraction. 70223B1ftWRP\B\RDER-Jnl 94\wp 4-12 Q:\WRP\EN\GEM\UK\DARENTYTERRY\FKi-4-13.GEM d n e g e L ae nSmltdWtrLvl o uut 1976 forAugust Levels Water Simulated on Based = = = = = With historical Abstraction Abstraction historical With = = = = = ______ih oAsrcin at Abstraction no With acmn o h Chalk the for Catchment Groundwater Darent of Simulations With 100 % Abstraction at Abstraction % 100 With 6 T W U Abstraction Sites Abstraction U W T 6 6 7 W U Abstraction Sites Abstraction U W 7 6 Variation in Groundwater Catchment Groundwater in Variation Catchment Boundary Boundary Catchment aeo rvl Pit orGravel Lake River le a c S Figure 4.13 10km • • • • • • • • • • • • • • • t f » VUttOOtOlBCTCM • ' • :wpOJ»iuaA* \n\i 4.1 .4 T\Dnw\Fio a:\wnp\ONJ«»Biiu»aoAA*M 20 60 70 40 50 30 10 4 » i . a hl ; Chalk Lower Greensand Lower Westertiam Eynsford Kirby Horton Sundridge Darenth Lullingstone Brasted VMM 1 R5PR16 PR15 R5PR16 PR15 3514.7 13.5 . 0.0 0.0 0.5 2.3 4.6 . 11.8 8.4 8 6 6.8 PWS Abstractions (Ml/d) Abstractions PWS 9.6 6.4 0 0 ------1------1 Chainage (km downstream ofdownstream source) (km Chainage 0 25 20 ------1 1n 20 Year Low Flow Flow 1nLow Year 1 20 Ags 1976) (August PR16- GREENSAND REDUCEDBY 100% § O z o Ul I 8 s •TARGET FLOW •TARGET 35 oprsn f euto i Asrcin rm hl & oe Greensand Lower & Chalk from Abstraction in Reduction of Comparison iue 4.14 Figure (iv) Preferred Locations for Reductions in Cbalk Abstractions Having established the benefits of discontinuing abstraction from the Lower Greensand, the next set of simulations was used to assess the effects of varying the locations at which reductions in Chalk abstraction could be made. The model indicated that in general the location of Chalk abstraction would make little difference in drought years. However in years of near average or above average flow there would be additional benefits to river flow by concentrating abstraction at the downstream end of the catchment. At these times the length of the river perched above the watertable is considerably reduced. This is demonstrated in Figure 4.15. Simulations PR25 and PR26 included a total abstraction under normal operating conditions of about 40% of the current Lower Greensand and Chalk licences considered in Figure 4.15. In PR25 the reduction in abstraction is distributed evenly over four Chalk sources in normal conditions. In PR26 there is a greater reduction in abstraction from the three upstream sources atLullingstone, Eynsford, and Horton Kirby with much higher abstraction maintained at Darenth. The river flow profiles shown are for August 1984 in a period of normal operation in which naturalised flows in the summer months are all close to the median naturalised values for those months. The additional benefits to river flow from concentrating abstraction further downstream are evident from a point about 4 km upstream of Lullingstone. The difference in flows reaches a maximum of 5 to 6 Ml/d between Horton Kirby and Hawley. Much of this improvement is maintained downstream to the confluence with the Cray. 4.8.4 Conjunctive Use of Groundwater and Thames Water Having established the preferred locations and quantities for Lower Greensand and Chalk abstraction reductions, the catchment model was used to assess the effects of reduced abstractions overall, especially during winter, but allowing for increased abstractions to peak levels in drought conditions. These peak abstractions are required by TWUL as part of an operating agreement (WRMS1, see Chapter 7). The additional groundwater replaces surface water supplies from the Thames which are restricted in droughts. Two peak drought operating regimes assessed are indicated in Figure 4.14. The regimes do not affect the shutdown of Lower Greensand well sites. A further regime considered was similar to simulation PR25 but with an additional peak abstraction at Bexley of 40 Ml/d. Drought periods of peak pumping specified in simulation were April to October 1976, September 1984, August to December 1989 and July to September 1990 inclusive. The peak pumping was found to have little effect on river flows as, in extended drought periods, the river is already perched over much of its length downstream of Shoreham. It could, however, have an impact on recovery of flows following the drought. In the cases of both extended droughts (1976 and 1989), periods of very high rainfall followed which restore simulated river flows to reasonably high levels. As simulation finished at September 1990 the effect of peak pumping in 1990 could not be fully assessed. 70223BtftWRP\B\RDER-Jal 94\wp 4-13 Undoubtedly switching some of the peak pumping to Bexley in the Cray catchment has advantages for flows in the Darent. However it probably also has a detrimental impact on flows in the River Cray. 4.8.5 Augmentation of River Flows The concept of bankside river support wells pumping to the river with groundwater introduced as artificial springs was discussed in the Prefeasibility Report (GDC, 1991). None of the reductions in abstraction considered was sufficient alone to restore flows to target levels at all times. The catchment model was therefore adapted to simulate pumping from bankside wells. Fixed rate pumping is triggered at individual well sites in months when flows at the well site decline below the target. Pumping at a site is shut down once the augmented flow at the site returns above a level equivalent to the target flow plus augmentation. Simulation runs were carried out to determine the frequency of operation and total volumes abstracted from the Chalk for various options for reduction in abstraction. Suitable fixed rates of pumping and numbers and locations of wells were determined from an assessment of river bed leakage losses in drought conditions. Six sites were specified between Shoreham and Dartford each with potential simulated pumping rates of 4 Ml/d. Operation of the scheme for one of the early options shutting down Lower Greensand abstraction is illustrated in Figure 4.16. Five sources are in operation pumping a total of 20 Ml/d to the river. Analysis of results for one of the strategies involving the TWUL conjunctive use scheme is presented in Figure 4.17. This was the preferred TWUL option. Abstractions from Lullingstone, Eynsford, Horton Kirby and Darenth are reduced in normal years to 31.4 Ml/d with abstractions weighted in favour of downstream sources. As previously, abstractions from Sundridge and Brasted were set at zero with all other abstractions at historical levels. Peak pumping conditions are shown on Figure 4.17. The abstractions for the river during periods of support would amount to an average of only 2 Ml/d pumping continuously over the period 1970 to 1990. This level of abstraction would have negligible effect on aquifer levels. The maximum pumping rates would be 24 Ml/d though over short durations of time. The river support scheme would be operated in 11 years out of 21. Augmentation requirements would, however, increase if the monthly target flows indicated in Table 4.6 were implemented or if, in future, abstraction from any existing licensed well sites not included in the scheme increased above historical levels. Augmentation requirements were also determined for other strategies. For example, augmentation was included with one of the earlier strategies, PR 16, which was run to demonstrate the advantages of reduction in Lower Greensand abstraction. The strategy is shown on Figure 4.14. With a total 70223 BHnWRPVBVRDER'Jttl 94\wp 4-14 VUNOOCMUl QAWRP\DHA3EMVllK\DARENTyDRW\FI3-4-15.DRW 2 f 70 60 20 30 40 50 10 (H=historical) (Firstfigure figurenormal,inbracketspeak) oe resn pR25 LowerGreensand Chalk Wilmington Sundridge Darenth Lullingsione Eynsford Horton KirbyHorton Brasted Bexley PWS Abstractions (Ml/d) PWSAbstractions 092.)19.4(22.7) 10.9(22.7) . (.)2.8(12.7) (9.1) 4.7 8.7(18.2) . (82 . (12.7) 4.1 (18.2) 7.1 (H) (H) H H R5PR26 PR25 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 . (6.4) 5.1 (H+7.5) H (H+4.2) H 0.0 (0.0) 0.0 PR26 ------1------1 hiae k dwsra of downstream (km source) Chainage 0 25 20 ------Median Year Low Flow Flow Low Year Median Ags 1984) (August ...... T ' OW LO T'F E G R 'TA RUN PR25RUN U PR26 RUN 35 fet f oain f hl Abstractions Chalk of Location of Effect iue 4.15 Figure VUVOCJSNOll QAWRP\DNJV3aftLH<«AROrronWKM-ie.DRW £ (NOT USE) IN hiae(m ontem of downstreamsource)(km Chainage 11 RIVER SUPPORT POINTS SUPPORT RIVER 11 I prto o Rvr Support River of Operation iue 4.16 Figure 7Q223B10V3OCMVV IQ-4-17. DRW cc 73 > CO .n £ DC .1 E 2000 H tr a> O’ C o> E a> o c O E $ O co 3 3 E a> 3 a. a. o W btatosAsmd (Ml/d): Abstractions Assumed PWS 3000 4000 1000 Sundridge otnKry6.4 Bexley Darenth Lullingstone Eynsford Kirby Horton Brasted 90 1972 1970 9017 1974 1972 1970 itrcl40.00 Historical 94 1976 1974 omlPeak Normal 20.0 . 9.09 18.18 0.0 0.0 5.0 0.0 . 0.0 0.0 Estimated Volume of River Support Required Support River of Volume Estimated 9617 9016 941966 1984 1962 1980 1978 1976 umnainoeae 1 oto 1 years outof 1121 operated Augmentation Average Annual River Support = 1.9 Ml/d 1.9 = Support River Annual Average Maximum Annual Abstraction = 8.4 MVd 8.4 = Abstraction Annual Maximum 96 1980 1976 22.73 18.18 Total Volume = 14500 Ml 14500 = Volume Total xml umnain Requirements ExampleAugmentation 92 94 96 98 1990 1988 1986 1964 1982 Periods of Peak Abstraction Peak of Periods u 19- 1990 p e 1990-S Jul 1989 c e -D 1989 Jul May 1976-O c t 1976 t c 1976-O May e 1984 Sep to Lower Catchment Lower to Note. Chalk abstractions weighted Chalk Note. 1986 Figure 4.17 Figure 1990 abstraction of 42.5 Ml/d from Lullingstone, Eynsford, Horton Kirby and Darenth, but no peak drought pumping requirements, augmentation is necessary in 17 out of 21 years with average annual pumping of 3.2 Ml/d. 4.8.6 Conclusions The following conclusions were drawn from modelling more than 30 abstraction strategies. (i) The level of total pumping was generally 70 to 75% of the total for all groundwater licences in the catchment over the 20 years prior to 1990. Increasing total pumping to 100% of the total for all licences would have a very severe effect in causing further depletions to river flows. (ii) Reducing abstractions from the Lower Greensand in the upper catchment is more effective in restoring river flows than is reducing Chalk abstractions in mid and lower catchment. (iii) Variation in the locations for reducing Chalk abstractions has little impact on river flows in drought years as much of the river is perched. For years with average river flows, it is better to make these reductions at those sites which are located furthest upstream. Here groundwater levels could still be at or above ground level with groundwater contributing baseflow to the river. (iv) Peak pumping from the Chalk in line with the TWUL conjunctive use requirements has little additional impact on the river at the time of pumping. The effects in extending perched reaches of the river are not significant. However, peak pumping might have an impact if the drought is followed by a period of normal rainfall and recharge conditions giving rise to a slow recovery in flows. (v) None of the abstraction reduction strategies provides the target flow requirements throughout this period of historical simulation. (vi) The additional augmentation requirement for August ranges from 35 Ml/d with no reduced abstractions to 24 Ml/d if Sundridge, Brasted and Westerham are set to zero. (vii) Augmentation requirement increases as: abstractions from non-TWUL boreholes increase above historical levels; and separate monthly flow targets are used. (viii) Augmentation would be required at some time in about 50% of years. 70223BHAWRf'\B\RDER-Jul 94\wp 4-15 CHAPTER 5 ECOLOGICALLY AND ENVIRONMENTALLY ACCEPTABLE FLOWS 5.1 Introduction The complex ecology of a particular riverine system is dependent upon many variables. Reduced discharges and lowered velocities affect water dependent communities but the optimum and critical values for specific species are not well understood or adequately documented. Over the past three years research in the UK has been undertaken to assist in determining Ecologically or Environmentally Acceptable Flows (EAFs). In short, the methodology applied is to survey the existing communities in the river, and compare the findings with theoretically perfect communities so that deficiencies can be identified. Those particularly sensitive species of fauna and flora are selected as indicators, which can then be used to quantify the effect of low flows. Using the indicative species the velocities and depths required to obtain and maintain that desired level of ecology should be predictable. However, it is this prediction which has often proved difficult to achieve; primarily because of the complexity of communities and the differing habitats offered over any one reach of a watercourse, let alone transferring the results to a different river. 5.2 Early River Darent Studies, (Dossor, 1978 and Halcrow 1987, 1988, 1989) Early investigations into problems of low flows in the River Darent did not address ecological concepts. Nevertheless solutions were proposed to improve the condition of the river by increasing low flows. The desk study, ‘The Water Management of the Darenth Valley’ by John Dossor and Partners (1978) recommended augmentation sufficient to ‘ensure water flows in the river at all times'. A minimum discharge of 17.3 Ml/d was selected but was not linked to an environmental need. Preventing the river from drying up was seen as the primary task and ecological benefits were assumed to be generated as a matter of course. The Halcrow Study (1987) included an inventory of the flora and fauna in the river at that time. The report concluded that riverflow augmentation was required and set a minimum target flow of 30 Ml/d throughout the length of the river from Otford to Dartford (see Figure 5.1). 70223BlftWRP\B\RDEROn] 94\wp 5-1 Environmental needs were discussed, primarily based on fisheries requirements, with the principal target being to reinstate the stream as a reasonable trout fishery between Chipstead lakes and Eynsford. However, the flow requirements for such a fishery were not explored. Preventing bed losses by bed lining was another major conclusion and because of the disturbance this would cause the reinstatement of bank margins was explored, but no prediction or projection of instream species propagation was considered. 5.3 Environmentally Acceptable Flow Regimes (W S Atkins, 1991) In October 1990, during the worst period of the recent drought, Atkins carried out an invertebrate survey for use with RIVPACS (River Invertebrate Prediction and Classification System) which predicts the community of invertebrates that should occur at undisturbed sites. When comparing results from samples taken in the river against the expected scores from RIVPACS, one site, Shoreham, appeared to have survived the worst ravages of low flows. The remainder of the river fell far short of those scores predicted and it was concluded that low flows were responsible for the poor communities. The Shoreham site was used as an index to determine acceptable flows regimes for the rest of the river based on hydrological parameters. These parameters were then taken to be the minimum Environmentally Acceptable Flow Regime (EAFR) for sustaining the remainder of the river. Accepting that Shoreham represented a regime capable of supporting an undamaged ecosystem, an artificial flow duration curve was generated dependent upon contributing surface area of catchment. Using contributing surface area the minimum flow found at Shoreham was translated throughout the entire catchment. The calculated and measured flows are presented in Figure 5.2. Atkins noted a low diversity of macrophytes (reeds) which correlated with impoverished macro invertebrate scores. Macrophytes present the Ephemeroptera (mayflies) with a suitable habitat, which in turn are a major food source for young trout. With a lack of macrophytes Crustacea (snails) become dominant, displacing the trout's food source and hence reducing the acceptability of the habitat. In slow moving water Oligochaeta (worms) tend to dominate, feeding on deposited detritus. Again this displaces the food source of the trout. Comparison of the flow regimes in Figure 5.2 illustrates that the EAFR proposed by Atkins represents substantial increases in low and particularly in mean flows in the lower half of the catchment and a slight improvement in the upper half. It is also important to note that the need was recognised for a profile of increasing flow. This is distinct advance over theHalcrow target flow which recommended a constant 30 Ml/d from Otford to Dartford. The major concern with this work is whether Shoreham was experiencing the lowest flows capable of supporting diverse macro invertebrate communities or whether more diverse communities would be generated with increased flow. In other words could Shoreham be considered as a valid location. 70223BIOVWRP\B\RDER-Jol 94\wp 5-2 Figure 5.1 Effect of Options on Flow Accretion, August 1976 ( After Halcrow, 1988) Flow M l\d T02 a »B 1 0 .QEMbyEN Comparison of Estimated Natural, EAFR, and Observed Mean Flows and Q95’s and Flows Mean Observed and EAFR, Natural, Estimated of Comparison CO o o ▼- CVJ o o o O) o 00 o lw( /s ) m ( Flow lw Mld ) l/d M ( Flow r*. o OIHVSIIOX ^VWVVfNVlEVNJtCOAfl ENTlTE RRYNBQ-M OEM by EN o’ tn ft to 5 e O 2 * fTm Fo Ecee ( oml rbblt Sae ) Scale Probability Normal ( Exceeded Flow Time of 5 - - - 50 - - 500 - 1 10 100 a 3 lw uain uvs o Rvr aet atr tis 19 ) 1991 Atkins, after ( Darent River for Curves Duration Flow Atkins conclude that the proposed EAFR is assumed to be: ‘capable of maintaining an abundance and diverse invertebrate and plant community and a healthy trout population because it does so at Shoreham and Otford’. The mean EAFR for Hawley was estimated as 97 Ml/d, although seasonality would vary it between approximately 150% and 50% giving 145 Ml/d and 48 Ml/d, respectively. The impact of seasonality upon species was not explored in depth. The recommended EAFR for mean and 95 percentile flows are given in Table 5.1. TABLE 5.1 Environmentally Acceptable Flow Regime in the River Darent (after W S Atkins, 1991) Site Catchment Present flow Recommended EAFR area (km2) Mean 95% Mean 95% (Ml/d) (Ml/d) (Ml/d) (Ml/d) Westerham 7.1 - - 3.5 0.7 Brasted 22.7 -- 11 2.3 Otford 101 48 10 51 10 Shoreham 111 - - 56 13 Lullingstone (g.s) 118 57 12 60 14 Lullingstone (Rv) 130 - - 66 15 Farningham 151 - - 76 17 Horton Kirby (Lakes) 160 - - 81 19 Horton Kirby (Mill) 172 - - 87 20 Hawley (g.s) 191 55 3 97 23 Atkins noted that water quality along the whole watercourse was excellent due largely to the trunk sewer which removes effluent out of the catchment. This lack of returned effluent reduces flows further, but has possibly allowed the invertebrate communities to survive the extreme low flows experienced during the 1980s and in recent years. 5.4 Ecologically Acceptable Flows, (IH, April 1993) 5.4.1 Assessment of Instream Flow Incremental Methodology (IFIM), (IH/IFE, April 1993) This research project undertaken by the Institute of Hydrology. (IH), and the Institute of Freshwater Ecology (IFE) carried out an assessment of the application of the Instream Flow Incremental Methodology (IFIM) using the Physical Habitat Simulation System (PHABSIM), a suite of computer programs developed by the US Fish and Wildlife Service. 70223BlftWRP£\RDER-Jnl 94\wp 5-3 IFIM gives a quantitative measure of the ecological value of river flows for specific aquatic species. On the basis of limited field observations the model may be calibrated to give predictive estimates of species success for the complete range of discharges experienced. The damaging effect of low flows in some UK rivers was highlighted during the recent drought of 1991/92. This coupled with the possible need to set Minimum Annual Flows (MAFs) under water resources legislation prompted the need to develop operational tools for managing aquatic communities in the UK. IFIM has been widely used for this purpose in other countries (USA, New Zealand) but application in this country has been limited to: a flood defence channel in NRA Thames Region; two sites on the River Allen; a chalk stream in the former Wessex Region of NRA, named in the Top 40 ALF list. The R & D project went on to examine ten rivers with a variety of habitat types and demonstrated its potential as a powerful tool in the assessment of flow requirements of aquatic species. At the heart of the methodology is the development of habitat suitability curves for particular indicator species. In the report curves were constructed for: (1) Fish - Trout Roach Dace (2) Macrophytes - Ranunculus (Water crowfoot) Nasturtium (Watercress) (3) In v e rte b ra te s - 10 species Behind the selection of species for chalk streams lies specific ecological relationships typical of that habitat. For example: Whilst salmonids such as trout enjoy fast flowing, clear water of a constant temperature, dace and roach will benefit from slower velocities and higher water temperatures, conditions which are associated with low flows. Water crowfoot and watercress are characteristic of chalk streams, but the relationship between them is complex and is best illustrated by the following from Giles, Phillips and Barnard (1991). ‘The characteristic plant of a high flow rate in chalk streams is water crowfoot * (Ranunculus spp.) The long, undulating tresses of thin, dark green leaves begin growth early in the year to fill the stream by mid-summer and flower profusely before dying back in the autumn. Water crowfoot is often interspersed with water starw ort (Callitriche spp). Several other species, also adapted to high flow rates, may be present. The reduction in the current caused by the weed beds results in a 70223B1C\WRPVB\RDER-inl 94\«p 5-4 Riverside Gravel Pits at Lullingstone Castle (above) and Bradbourne Lake (below) are typical of the wealth of offsite water bodies in the Darent Valley. These lakes have become a kind of wetland substitute for the river itself, both as a resource for wildlife and as a sink for some of the surviving valley springs. 7 0323 BOJ/BSD /01/B build up of silt around the roots. Water cress (Nasturtium officinale) develops in the resulting slower-flowing conditions. The current caused by the weeds scours the river bed downstream creating the mosaic of different habitats which is typical of chalk streams. This range of habitats provides homes for a great diversity of animal life’. The balance of invertebrate communities is closely related to that of the macrophytes. Again Giles, Phillips and Barnard (1991) can be quoted: ‘The invertebrates that live on stones and on the leaves of plants in fast flowing water cannot survive in slow, muddy-bottomed rivers. Silt fills in the spaces between the stones, reducing the hiding places available to small creatures. There will also be a reduction in dissolved oxygen concentrations present. It is not only the oxygen supply that influences the communities. In lower flows it has been found that there are often fewer species that feed by catching items drifting by, and more predators and crustaceans feeding on detritus. In general, as the average flow decreases stoneflies, mayflies and caddisflies become fewer whilst various worms and crustaceans become more plentiful. This is true for both numbers of species and for the size of the populations'. For the selection of target species the R & D study concludes that: ‘choice of target species is a critical issue’. The difficulty is selecting a species that is ‘sensitive enough to changes of flow regime but at the same time occurs in sufficiently high numbers to allow the gathering of adequate habitat suitability data'. In further IFIM work on the River Allen the project team selected trout as the critical species and developed the habitat curves into flow requirements which could be compared with existing discharge records. From this it was concluded that the: ‘historic abstraction regime has had significant effect in reducing the availability of habitat for trout fry. The extent of this impact would appear to be directly proportional to the level of abstraction and confined almost entirely to the summer period’. However, no flow regime or MAF was defined using IFIM, stating only that trout fry would require a minimum discharge of 0.35 m3/s (30.24 Ml/d). One criticism which can be levelled at the study is that the Allen is a managed ‘put and take’ fishery rather than a natural chalk stream. Bank margins or cover and in-channel substrate, both measured indices in IFIM, are manipulated to produce an optimum habitat for game fishing. Whether the resulting data captured from site represents a natural occurrence of indicator species remains to be addressed. If the ecosystem to be improved or maintained resembles these base data then the results of the River Allen Study should be applicable. 70223B10\WRPVB\RDHR-Jal 94Wp 5-5 5.4.2 Flow Regime Requirements, River Darent (IH/IFE, Unpublished 1993) Part of the Institute of Freshwater Ecology (IFE) and Institute of Hydrology (IH) team which carried out the R & D project on IFIM were asked to apply some of the IFIM methodology to the River Darent. After visiting the river and surveying twelve locations, suitability curves for specific species were obtained (see Table 5.2). Hydrology, geomorphology and macrophyte communities of the River Darent were also commented upon. The report suggests that the River Darent because it is not wholly a Chalk river, as one third of its catchment is covered by Greensand, Gault clay, Tertiary or Recent deposits, should have larger differences between maximum and minimum discharges than should a typical Chalk river. Ratios of 1 : 15/20 are quoted rather than typical Chalk values of 1 : 5/10. The records at Hawley give 95% and 5% percentile flows of 0.05 m3/s and 1.8 m3/s or a ratio of 1 : 36. Higher up the catchment at Lullingstone the ratio decreases to 1 : 9. If the IH/IFE ratio of 1 : 15 is used with the 5% maximum flow of 1.8 m3/s, then minimum flows of 120 1/s (10.3 Ml/d) to 90 1/s (7.6 Ml/d) are obtained. Using naturalised flows (GDC, 1993), the ratios of the 5% and 95% percentile flows at Hawley give a ratio of 1 : 4 suggesting that the river acts.more like a typical Chalk river than the IH/IFE (1993) study team believed. This ratio applied to a 5% maximum flow of 1.8 m3/s would give 95% flows of 450 1/s (39 Ml/d): a value closer to the EAF which they later recommended in the report. The study concludes that the current channel geometry is certainly smaller than that which existed under the unabstracted flow regime. This confirms an assertion from W S Atkins (1990) report that channel morphology changes as the influence of abstraction reduces flows (see Figure 5.4) such that: ‘the channel downstream of Lullingstone no longer represents natural width but represents a channel that has adjusted itself to cope more efficiently with the substantially reduced flow regime it has experienced over the last thirty years’. During the survey, all the species tabulated were recorded except for trout and grayling. Some of the life stages of these fish are particularly sensitive to velocity and discharge conditions and these two species may have suffered as a result of reduced discharges. The survey found an: ‘almost total absence of aquatic macrophytes for many reaches’ although ‘most sites on the Darent are capable of supporting rich and varied aquatic flora’ As has been shown earlier the absence of macrophytes indicates unsuitable low flow conditions. This loss of appropriate habitat will depress the species numbers of Ephemerophera (mayfly), food source of trout and so lead to a downward spiral in the ecological condition of the river. 70223B10VWRP\B\RDER-Ja] 94\wp 5-6 TABLE 5.2 Preference Curves of Selected Organisms in the River Darent River Darent pref. Curve Season Depth (m) Velocity (m/s) Sediment Cover Brown trout spawning Oct - Dec 0.1 - 0.3 0.2 - 0.4 Gravel Brown trout fry Mar - Aug 0.05 - 0.5 0.1 - 0.3 Gravel Brown trout juvenile Whole year 0.2 - 0.6 0.05 - 0.4 Gravel 50% oh 50% instream Brown trout adult Whole year 0.2 - 1.2 0.1 - 0.6 Sand-Cobble 50% oh 50% instream Grayling spawning Mar - May 0.1 - 0.3 0.2 - 0.4 Fine gravel Grayling fry Jun - Aug 0.05 - 0.5 0.05 - 0.2 Silt-Sand Grayling juvenile All year 0.3 - 1.0 0.1 - 0.5 Gravel-Sand 10% instream Grayling adult All year 0.3 - 1.0 0.1 - 0.5 Gravel-Sand 10% instream Ranunculus overwinter Nov - Feb 0.5 - 2.0 0.5 - 1.5 Gravel Ranunculus growing Mar - Jun 0.1 - 2.0 0.2 - 0.7 Gravel-Sand Ranunculus flowering May - Jun 0.1 - 2.0 0.2 - 0.7 Gravel-Sand Ranunculus rip out Sept * Nov Ephemera oviposition May - Jun Bankside cover from wind Ephemera young larvae Jun * May 0.3 - 1.0 0.2 - 0.5 Sand-Gravel Ephemera larger larvae All year 0.3 - 1.0 0.2 - 0.5 Sand-Gravel Not relevant Ephemera emergence May - Jus Bankside cover from wind Ephemera adult May - Jun Bankside cover from wind Gammarus juvenile Mar - Oct 0.05 - 0.2 0.0 - 0.3 Organic sediment Emergent plants Gammarus adult All year 0.1 - 1.0 0.0 - 0.7 Cobbles-Sand Ranunculus - Emergents Baetis muticus ovipos Mar - Jan Baetis muticus larvae All year 0.1 - 0.5 0.1 - 1.0 Gravel Baetis muticus adult Apr - Sept Elmis aenea ovipos Apr * Jul Stones/Moss Stones/Moss Elmis aenea larva All year 0.1 - 1.0 0.1 - 1.0 Stones/Moss Stones/Moss Elmis aenea pupae Jul - Aug Elmis aenea adults Aug - Mar .0.1 - 1.0 0.1 - 1.0 Stones/Moss Stones/Moss Simulium ornatum ovipos Mar - Apr 0.5 - 1.5 Bankside grasses/ reeds Simulium ornatum larvae Jun 0.1 - 2.0 0.5 - 1.5 Ranunculus/ Ranunculus/ submergents submergents Simulium ornatum pupae Aug 0.1 - 2.0 0.5 - 1.5 Ranunculus/ Ranunculus/ submergent! submergent! Simulium ornatum adults Oct Bankside vegetation 70223BJ0\WRP\B\RDER*Jnl 94\wp 5-7 The report concludes that: ‘substantial parts of the channel should sustain velocities in excess of 0.5 metres per second and depths in excess of 0.3 metres even in low discharge conditions’. The report goes on to say that in view of: ‘the requirement of current loving species such as Ranunculus an appropriate flow regime for the lower reaches (of the Darent) might range from greater than 2 m3/s (with spikes to perhaps 3-5 m3/s) to an absolute minimum value of 0.33 m3/s (29 Ml/d) in drought conditions’. Currently, Hawley gauge exceeds 2 m3/s during 3% of the record (1963 to 1990) and ‘spikes of 3-5 m3/s’ have been recorded for less than 1% of the record (some 98 days) (see Figure 5.3). The peak flows therefore meet the IH/IFE (1993) recommendations, though the frequency with which they do so is probably lower than the IH/IFE report would recommend; however, no recommendations on the frequency of flood spates were made. Possibly of more importance was the recommendation that 0.3 m3/s (27 Ml/d) be set as a MAF or target flow. Flows below this have occurred for 30% of the historical record. More critically, the reach at Hawley has dried out completely and this occurs for about 4% of the record (approximately 394 days). The minimum flow to be maintained at Hawley under the Action Plan is 0.33 m3/s (29 Ml/d). This compares with Atkins minimum flow under the EAFR (derived from invertebrate studies at Shoreham) of 0.26 m3/s or 23 Ml/d and the IH/IFE minimum value of 0.3 m3/s or 26 Ml/d. 5.5 Conclusions Uniform target flows such as those recommended by Dossor and Halcrow are too simplistic and fail to address the influence of seasonal changes upon the ecology of the river. However, Halcrow did suggest augmentation flows of 30 Ml/d and it is this quantity which is targeted as the minimum maintained flow at Hawley under the Action Plan. This quantity also only slightly exceeds the minimum acceptable flow stated in the IFE/IH (1993) study. That abstraction does impact adversely upon the river is illustrated by comparing present day flows with naturalised flows derived from computer modelling. The naturalised flows far exceed any minimum environmental*flow regime artificially imposed upon the watercourse (see Figure 5.5). The proposed Action Plan target flows satisfy an accretion profile as would be found under a natural regime. The flows will be capable of supporting a trout fishery (one of Halcrow’s and NRA’s targets) and maintain flow at all times (Dossor’s target). In addition they slightly exceed the EAF values proposed by Atkins (1991). 70223BKKWRPVB\RDER-Jn] 94\wp 5-8 1\FKUJRE5\BEMVTERRY\FK15-4.QEM &1 EN Figure 5.4 Figure otmcrco* • :WnNOMUVAtNVdt\1--.& yB —.. . .— N B. ky Q:\WRnENVOEM\UKV0AftENTV1dUtY\F10-3-).a&M & 2 © 2 * • • • • • • • • • • Return Period () Years Period Return rqec Dsrbto o Ana iiu Fo: ie Drn a Hawley at Darent River Flow: Minimum Annual of Distribution Frequency Target flows prescribed by the Action Plan do maintain or slightly exceed the seasonal flow and species requirements, defined by the IFIM work. Recent work by the Flood Hazard Research Centre (Middlesex University) into benefits of low flow improvement schemes (unpublished R & D project 1993) has established that the public values not just adequate flows but improvement over the whole of the river corridor. Thus, any EAFR must consider not just the aquatic zone but the margins as well. IFIM moves towards that goal and is complemented by the conservation work being carried out by the North West Kent Countryside Project. The project concentrates on specific conservation tasks such as planting and pond construction, preparing the river for the awaited return of flows. A study, Mott MacDonald (August 1992) has established a ‘Strategy for Enhancement of the River Darent’, which should accompany the proposed restoration of a satisfactory flow regime. The method of introducing augmentation water from bankside wells and the pipeline has to be carefully considered, as establishment of trout depends upon a silt free river bed. Confirmation of this need is illustrated by the NRA gravel raking undertaken on the Hampshire chalk streams. Returning the river to a satisfactory flow regime will provide the potential for an increase in species in a balanced habitat. However, some assistance may be required such as replanting macrophytes to return the variety and density expected in such a watercourse. 70223B1 G\WRP\B\RDER-Jttl 94Wp 5-9 CHAPTER 6 ENGINEERING OPTIONS INVESTIGATED 6.1 Introduction Since the alleviation of low flows began to be investigated in the 1970s a wide range of solutions has been put forward. Some of the main recommendations have been discussed already in Chapter 2. The report, GDC (1991), brings together a comprehensive list of possible solutions which are costed and evaluated in terms of technical and environmental benefits or disbenefits. However, at the time of submission in November 1991, the catchment model had not been developed fully and comparison between the options and their effects on groundwater and river flows could not be evaluated. The alternative solutions were in most cases not independent stand-alone schemes but could be integrated. In such cases the optimum solution could not be evaluated without the use of a model through which the various combinations could be tested. In July 1992, a groundwater study (GDC, 1992) for Blue Circle Industries Quarries at Northfleet, indicated that a quantity of groundwater might be made available from current and future dewatering operations. By September 1992 the groundwater/surface water model of the catchment had been set-up and preliminary calibrations made. During the six week period to 30 November 1992, the NRA and TWUL met twice weekly as a Joint Project Team (JPT) in an attempt to agree a technical solution. During the meetings of the JPT the catchment model was used to assess a wide range of options. This chapter summarises the various solutions which have been investigated and explains the choice of the schemes contained in the Action Plan which was jointly agreed beteen TWUL and the NRA. The Plan is the outcome of prolonged detailed and thorough studies and not a solution devised hurriedly during six weeks of intensive negotiations between TWUL and the NRA. 6.2 Flow Augmentation The strategies for alleviating low river flows can be broadly categorised as abstraction reduction/relocation, flow augmentation, flow conservation, recirculation, demand management or some combination of these. Options within these strategies may involve changes in the management of existing groundwater and surface water resources, development of new groundwater sources, use of storages within the catchment, artificial recharge, use of sewage effluent, control of demand, etc. Any options which involve a reduction in current abstractions will involve some assessment of the practicality of finding replacement supplies. To compare options GDC (1991) accepted Atkins (1991) Environmentally Acceptable Flow Regime (EAFR) discharge values (see Table 6.1). 70223BlftWRP\B\RDER-Jol 94Wp 6-1 TABLE 6.1 Environmentally Preliminary Acceptable Flow Regime (after W S Atkins, 1991) e a f r *5 e a f r ^ Gauging Station m3/s Mid m3/s Mid Otford 0.12 10.4 0.59 51.0 Lullingstone 0.16 13.8 0.69 59.6 Hawley 0.26 22.5 1.12 96.8 The EAFR95 is assumed to be the lowest flow required to maintain the abundance and diversity of macroinvertebrates expected in a theoretically pristine Chalk stream at the location. The EAFR^^ is the mean flow desirable from an environmental viewpoint at the location. The deficits at Otford and Lullingstone were found to occur relatively infrequently (see Figure 6.1) occurring only in the very dry years, and of moderate duration and volume. The problem at Hawley was clearly of a different order of magnitude and likely to require a strategic solution whereas the problem in the Otford/Lullingstone reach may be amenable to local solutions. Any augmentation solution proposed needs to be able to provide flows for lengthy periods and deliver large volumes of water, particularly at Hawley. The suggested design requirements for augmentation flows at the three gauging stations are summarised in Table 6.2. It has been recognised in all studies that to return the river to satisfactory conditions, it is essential to re-establish a normal accretion pattern with river flows increasing as they move down river. TA BLE 6.2 Design Augmentation Requirements at Gauging Sites Station Maximum Duration Volume capacity (Mid) (months) (Ml) O tford 2 3 150 Lullingstone 5 5 700 Hawley 22 . 9 5 000 To compare the various options a maximum augmentation flow of 40 Ml/d was selected. This flow is close to the target flow of 35 Ml/d chosen for evaluation of options in the River Darent Action Plan (TWUL/NRA, 1992) and consistent with EAFRs calculated subsequently. 70223B1G\WRP\B\RDER-Jal 94\wp 6-2 O:\WRreN\OOrf\UK\OAItEMTYtEIUmnO-M.OEM ky & NiI s. 9 i £Z £ § § « I £ -4.0 §* -10.0 *6.0 -9.0 - -7.0 -3.0 - - -24 -11 -10 -22 -20 *14 -13 -12 •16 -16 -14 •12 -10 8.0 2.0 0.0 1.0 -9 -8 -7 •6 -4 -5 -4 -3 -2 -2 -1 ■fl •6 o 0 Environmentally AcceptableRegime Row Environmentally AcceptableFlow Regime EnvironmentallyAcceptable Flow Regime Dry RiverDry RiverDry Dry RiverDry 1964 ^9S 97 98 81 89 99 87 99 91 93 83 97 1988 1987 18*3 1983 1981 1979 1877 1979 1879 1871 1998 1987 ^^H98S isea 8 0 93 94 96 0* 90 92 94 98 98 1980 1988 1988 1964 1982 1900 107* 1976 1974 1973 1870 19M_ 1988 I T ( | si i9>3 | is>i M 19 T V i> I iafr7 I is>5 I 0 1 1972 1974 1970 1978 I I 1980 I | I I i*9 i*9 I | igei | 1983 T (rotative of 22.5MI4) to a fixedEAFR 1882 (relative of14Mld) to fixed a EAFF^ (relative of10Mld) to a fixed EAFR ifs | isfcs | 10*4 1 i Monthly Deficit Records Deficit Monthly DoficJtLulilngetone at 1868 1*67 1988 l980 1 I Deficit «t Hawley Deficit «t Daficit at Otford DaficitOtford at | aa | iaea T Figure 6.1 Figure 6.3 Water Supply System The major water supply infrastructure and operational zones in and adjacent to the Darent catchment are illustrated in Figure 6.2, which shows public supply wells and reservoirs, their ownership and the areas served by the relevant supply companies. The area is served by four water companies, namely, Thames Water Utilities and West Kent, Mid Kent and East Surrey Water Companies. Substantial quantities of groundwater abstracted from the Darent catchment by the water companies are exported to supply other areas in the region. About 30% of the water supplied to south-east London by TWUL is derived from surface water sources in the Thames Valley and is transferred via the south-west trunk main system to the Honor Oak complex, from where it is distributed. The remainder is derived from 22 groundwater sources, 10 of which are located in the Darent catchment. An indication of the links between wells, reservoirs and supply zones and the associated distribution of abstracted water is provided in Figure 6.3. The size of groundwater abstractions for public water supply is indicated in Table 6.3. TABLE 6.3 Abstractions for Public Water Supply Abstractor Number Licensed average Average of abstractions (Ml/d) abstractions for sources 1985 to 1989 (Ml/d) Thames Water 10 106.9 81.0 West Kent Water 3 23.1 18.8 Mid Kent Water 3 12.5 5.4 Southern Water 1 6.8 6.1 East Surrey Water 1 5.6 1.9 Total 18 154.9 113.2 Notes: (1) Licensed average abstraction is the licensed annual abstraction divided by 365 days. (2) Source: TWUL Database and water companies’ data. It is clear that the focus of any assessment should be with the first two abstractors shown in Table 6.3 which account for 80% to 90% of abstractions. An approximate assessment of the exports from and imports to the catchment is given in Table 6.4. The table provides some indication of how abstractions from the catchment are distributed. Although there are few data on flows across the catchment boundaries, it is estimated from the data provided that about 50 Ml/d (40% to 50%) of the water abstracted in the catchment is exported to adjacent supply zones, compared to imports of about 3 Ml/d. 70223 BlCfvWRPVBNRDER-Jol 9«Wp 6-3 TABLE 6.4 Catchment Imports and Exports (after GDC, 1991) A verage Status in Supply zone(s) Darent source(s) daily flow relation to at source Darent (M l/d ) catchm ent Thames Water Westerham (SLE2) Westerham Hill 0.9 35% exported Eynsford (SLE10) Lullingstone 5.5 20% exported Blackfen/Bexley/Dartford' (SLE 11/12/13) Dare nth/Dart ford/Wilmington 3 5 .0 30% exported Sidcup/Cray/Swanley (SLE 17/18) Eynsford/Horton Kirby 2 1 .6 . 60% exported North Downs (SLE19) B ras ted/S u nd ri dg e 13.8 70% exported South Darenth2 (SLE33) Green St Green 4.3 10% exported T otal 81.1 West Kent Water Co S e v e n o a k s/ Cramptons Road/Kemsing/ 18.8 45% exported Tonbridge/Tunbridge W ells Oak Lane Mid Kent Water Co Stan sted Hartley/Ridley/S tan sled 5.4 Internal Exedown (part)3 N one 3.0 Imported East Surrey Water Co Kent Hatch/Crockham Hill W estw ood 1.9 Exported Southern Water Medway Area Fawkham 6.1 Exported Sources: Thames Water Utilities; Mid Kent Water Co, West Kent Water Co, Southern Water, East Surrey Water Co. The major sewerage infrastructure of interest is the Darent Valley trunk sewer which serves the main population centres in the Darent catchment and discharges to the Long Reach sewage works in Dartford. The tTunk sewer was originally constructed during the last century, and comprised a 24 inch stoneware pipe running from Westerham to Dartford. A parallel sewer of 36 inch diameter was added between Farningham and Wilmington to provide additional capacity which has been further uprated in more recent years. The flow in the sewer of about 14 Ml/d, is a potential resource currently lost to the catchment. The potential for using this resource depends on the quantity and quality of effluent. 6.4 Demand Growth Demand projections have been estimated for the Darent Catchment (GDC, 1991). 70223 BKNWRPVB\RD£R-JuJ 94\wp 6-4 Figure 6.2 Water Supply Operational Areas Legend Operational Area Groundwater Pumping N h r ? Boundary ^ Station Zone Boundary Seal* n , »------s------*------I 1 i Darent Catchment Boundary Figure 6.3 Indicative Water Distribution THAMES WATER 0*i»m W o o d fcltham D w rtfor d ( • /C r w f o r d * Wilmington s <§> S o u th fla a t i BeliTr1 D a re n th ' Ingham •th Orplngl H o rto n K ir b y a, E y m fo rd Lulllngatone Farnbor< Chalaflald I m m Ik e n t WATEM K nockholt Kamatng T roaley K am atng S u o d rl Borough Green Oak B a n k (•; Brastad 9f Liwrtnct w«»t ond River Hll! E A S T £J UltliEY WEST KENT WATER y/j-'i'Eh Legend Groundwater Pumping Operational Area Boundary Station Reservoir Zone Boundary 5c a • Schematic Darent Catchment Boundary Water Supply Distribution Figure 6.4 Public Water Supply Abstractions 80 Dortford (TW) Hortlsy (UK) s LEGEND TW Thorn#* Wot«r 1.9 MK WMt Kant Water ■ Avorog* Abstraction ( Mid ) MK MM Kmt Wot*- 1 1985 - 1989 SCALE o SW Southern WaUr ES Eoot S u m y WoW 1.0 UofftMd Afextroctioru ( M!d } * Combtood UloanM tar MortJoy, 1 Rldtoy and StonrUod. © Crown Copyright TABLE 6.5 Demand Projections for Darent Catchment (Ml/d) (after GDC, 1991) Year 1990 1995 2000 2010 Exported 51 53 56 62 Internal use 62 65 68 76 Total 113 119 125 138 Clearly, while framing solutions to the current problems, the above projections need to be considered. It is evident that these long term requirements could not be met from the Darent catchment, and there is thus a need to consider how this growth can be met from sources outside the catchment. It would be feasible to divert some of the current exports to meet internal demands, but wider solutions are required for the adjacent supply zones in any event. If catchment abstractions are sustained at current levels, additional resources of about 25 Ml/d would need to be found elsewhere by the end of the above planning horizon. If catchment abstractions are reduced to ensure minimum river flow criteria are met, resources of 50 to 60 Ml/d would need to be found from outside the catchment by 2010. The NRA believes that the GDC (1991) growth in demand of 1% per annum is much too high. However, should abstractions rise to their licensed totals then additional augmentation will be required to meet target flows. 6.5 Review of Engineering Options 6.5.1 General Solutions to the low river flow problem may involve groundwater or surface water management or new development options, or some combination of these. Options considered were: Groundwater Management revoke licences or reduce licensed abstractions by agreement, importing water to the catchment to replace these supplies; develop additional wells away from the river for use as summer sources or for direct river augmentation; bankside wells (artificial springs). 7O223B10\WRP\B\RDER-Iol 94\wp 6-5 Surface Water Management groundwater recharge using surplus (winter) surface flows; seasonal storage of flows in existing lakes; bed lining of critical channel sections to limit seepage losses; recirculation of river flows from downstream; river intake, bankside storage and treatment works at Dartford (to replace existing groundwater abstractions); use of sewage effluent to augment natural river flows. Demand Management limiting demand through quotas, domestic metering and associated tariff setting pricing strategies; differential pricing of water from different sources; leakage control. These options are discussed in more detail in Chapter 5 of Annex I and summarised below. 6.5.2 Long Term Options Some options are only capable of addressing the problems between Otford and Lullingstone, and would have little impact on flows in the reach between Lullingstone and Hawley. Others have potential for providing longer term solutions, while also addressing current problems. Some of the options are only used on an as-needed basis, whereas others depend on full time operation. It is thus difficult to compare options directly in terms of performance, and two benchmarks have been developed. The first benchmark is the ability of the solution to meet the ‘design’ deficit period (9 months) at Hawley. In cases where the available volume is limited, performance is expressed in terms of average sustainable flow over the design deficit period. Where volume is not limited, the nominal capacity of the scheme is used. The second benchmark is the unit cost of the solution, expressed in £/m3. Evaluation of this is complicated by the fact that some solutions have to be run full time, whereas others would only be used on an as-needed basis. In order to allow for this, the average annual deficit period for Hawley, calculated as 100 days per annum over the period of record, has been used. If an option provides a solution which has to be run full-time, this is still only considered to be effective in dealing with the low flow problems in the river over the 100 days of deficit each year. This means that ‘full-time’ solutions appear expensive. 70223B10VWRP\B\RDER-Jn) 94\wp 6-6 TABLE 6.6 Summary of Options Considered Option Comment Construction of additional well* for Relies on utilising groundwater storage in undeveloped parts of the catchment, altering direct river augmentation or for the seasonal pattern of groundwater flow. Water is drawn from storage in dry periods, groundwater use as summer water supply reducing potential groundwater flow to river at other times. With direct river sources augmentation has the advantage that it only needs to be used when required, thus keeping operating costs low. Effectiveness in doubt on (he basis of the water balance assessment and needs to be tested using the catchment model. Re-use of sewage effluent Makes use of resource currently lost to the catchment. Resource options considered include use of sewage arising within the catchment (partial solution to the low flow problems) and pumping of effluent from the Long Reach treatment works (full solution feasible). Treatment options considered for both resources include tertiary standard for direct return to river or land treatment for indirect return via aquifer. Latter appears most cost effective. Recharge of aquifer using surplus Pumps surplus water from river to recharge aquifer, relying on natural seepage and/or (winter) flows subsequent abstraction to augment river flows or provide a summer source. Diversion at Lullingstone provides only a partial (10%) solution in average years, and would fail in extreme years such as 1972/73. Pumping from downstream of Hawley would be a possibility but is expensive. Would need to be used each winter in anticipation of a dry summer and is thus somewhat inefficient. Seasonal storage in existing lakes Provides only a partial (5 to 10%) solution to the problems at Hawley, but would overcome the lesser problems at Otford. Interest-group pressures would make implementation difficult. Surface water abstraction and treatment Relies on closure of selected wells on the assumption that river flow will recover at Dartford sufficiently to allow surface water abstraction at Dartford. Uncertain how many wells will need to be closed without modelling and reliability of river flows in dry periods may be a drawback. Expensive. Increased conjunctive use of surface Has long term potential for meeting growth, as well as providing solution to current water from Thames Valley and local problems. Links between surface water supplied and groundwater supplied reservoirs groundwater would allow over-stretched wells to be rested, allowing aquifer to recover. Impacts of reducing abstractions at selected locations need to be determined using the catchment model. Effectiveness of links in transferring water from west to east can be examined in the network model being developed by TWUL. Recirculation No potential alone but may have potential in combination with other options. Bed lining of critical channel sections Would be effective in conjunction with another augmentation option, but difficult to implement. Problems with ecological disruption and land ownership, plus the technical uncertainty over long term performance and the need to carry out trials to establish tech nical feasibility weigh against this solution. Bankside wells provide a direct alternative. Development of bankside wells Recirculates flows lost to ground in reaches of river where losses are high. Cheap to implement and run. Requires site trials, but risk low. Impact of wells can be tested using the model. Will probably need to be used in conjunction with other augmentation options. Alternative to bed lining. Demand management Involves use of financial incentives to encourage abstractors to prefer alternative sources and for consumers to be more economical in their use. May not be implementable immediately, but should be pursued as a matter of policy to deal with long term resource problems. The comparison of options is presented in Table 6.7. These are the engineering or ‘structural’ options and exclude demand management which at this stage is a concept which cannot be realistically costed for comparison. The options are listed under four category headings broadly in the same order as summarised in Table 6.6. Use of winter flow surpluses for artificial recharge has potential application for direct river augmentation and also for reduction of existing abstractions in the river valley through relocation of abstraction to areas of recharge. These two options are costed separately and included in separate categories. 70223B1G\WRP\B\RDBR-Jnl 94\wp 6-7 The results indicate that options such as construction of a water treatment works at Dartford or a sewage treatment works at Otford would be expensive. The use of the Longford Lakes appears attractive in terms of unit cost but provides a very limited solution in terms of meeting the deficit in the Lullingstone to Hawley reach. The problems likely to arise given the user interests makes this option unattractive, except perhaps as a means of dealing with the problems in the river down to Lullingstone. Augmentation using groundwater has a low relative cost, but from the catchment water balance appears to have little potential unless linked to artificial recharge. Recharge of winter surplus would only be technically feasible with pumping from downstream of Hawley, which makes it fairly expensive. Bankside wells would appear to be more attractive than bed lining for conserving river flows in the Lullingstone to Hawley reach, and some combination of augmentation, recharge and conservation might have potential. With a view to demand growth, the preferred options which make available additional resources (rather than seasonal redistribution of existing resources) are the use of sewage effluent from Long Reach (although doubts over quality would need to be satisfied if land treatment is to be adopted) and conjunctive use of off-peak water from the Thames Valley surface water system. 6.5.3 Short Term Options River support pumping from wells at Brasted pumping station and Horton Kirby paper mill were arranged by NRA (Southern Region) during the summer and autumn of 1990. Amenity value was also improved at certain locations when flow was available by use of temporary sandbag weirs which increased the depth of water. In addition to measures implemented in 1990 further schemes which could be implemented at relatively low cost and in the short-term include: additional flow augmentation schemes utilising Chalk groundwater; utilising drawdown in Longford Lake; bankside wells (artificial springs). In view of the conclusions of the water balance assessment, there would seem limited opportunities for implementation of flow augmentation schemes. It might be possible to construct wells in the Chalk in the upper Darent (Otford subcatchment) with pumping which could benefit the river perhaps as far downstream as Shoreham. Benefits might be extended further downstream if combined with a scheme of bankside wells. A water balance assessment of the adjoining Cray catchment to the north of Westerham would have to be carried out before proceeding with an augmentation scheme. 702238l TABLE 6.1 Comparison of Structural Options Option category Option Sub-option Effective capacity Capital cost Unit cost River Augmentation Augmentation Wells Chalk (Lower Darent) 24 2.1 0.09 Chalk (Upper Darent) 8 0.6 0.08 Greensand with Iron Removal 8 1.0 0.12 Reuse of Sewage Effluent Primary STW at Otford + Land Treatment 12 4.9 0.48 Tertiary STW at Otford 12 12.4 0.95 Ex. STW at Long Reach + Land Treatment 40 6.0 0.13 Ex. STW at Long Reach + Tertiary Treatment 40 13.3 0.23 Use Winter Surplus and Recharge (pumped from Land Spreading 30 5.2 0.20 Dartford area) Direct Injection 30 8.0 0.27 Use Storage in Existing Lakes Longford Gravity Scheme 1 0.3 0.07 Pumped Longford plus Upstream Lakes 4 0.5 0.05 Revocation/Relocation/Variation of Summer Sources Chalk 24 2.1 0.09 Existing Licences Greensand with Iron Removal 8 -- Dartford Treatment Works With river regulation 35 14.2 0.41 50 19.4 0.38 With storage of winter surplus at Dartford 35 32.5 0.76 50 44.3 0.72 Use Existing Off-peak Capacity Conjunctive Use/Recharge 30 6.5 0.18 London Ring Main Conjuntive Use/Recharge (inc off-peak) 60 __ New Resource Development 60 - - Other Catchments Cray 7 _ Medway 7 - - Swanscombe area ? - - Use Winter Surplus and Recharge as Summer Land Spreading 30 5.9 0.22 Source (pumped from Dartford area) Direct Injection 30 8.6 0.29 Recirculation Using Stored Winter Surplus 40 14.2 0.26 Conservation Bed Lining Lullingstone to Hawley 16 2.8 0.15 Artificial Springs Lullingstone to Hawley 16 0.8 0.08 Notes: (I) Effective capacity = average sustainable flow over design deficit period (9 months) (2) Capital cost in £m illion. unit cost in £ /m \ capacities in Ml/d (3) Unit costs based on average annual number of deficit days (100) at Hawley 70223B10\\VRPVA\RDER-Nov93\»p Use of the Longford Lake would be strongly opposed and be very unpopular with users of this amenity. Benefits to the river would also be very limited, although these might be extended by combination with bankside wells. Bankside wells could provide the means to maintain amenity and ecological value of the river both upstream and downstream of Lullingstone, on a reduced scale in drought conditions. There is little risk that recirculation of water by this means could deplete the Chalk groundwater system further to a significant degree and risk failure of existing abstractions or springs. 6.5.4 Darent Action Plan (TWUL/NRA, 1992) Since the GDC (1991) report was completed further studies were undertaken which indicated that dewatering of the Blue Circle Cement Quarries at Northfleet, just outside the catchment and adjacent to the River Thames, had potential for providing a possible 25 Ml/d either for use as an additional source to be fed, after treatment, into the public water supply or to augment River Darent flows. The negotiations and studies undertaken by the Joint Project Team (JPT) during October/November 1992 used the options prepared by GDC (1991) as a starting point for negotiating an optimum technical solution. It inevitably involved some compromise from both parties due to the competing interests of satisfying public water supply demands and meeting the demands for halting the destruction of one of Britain's most famous chalk streams. It was also clear at the outset that the reduction in abstraction was the obvious solution although what effect reductions in specific wells would have on river flows was not known at this time. A short-list was drawn up to which was added the Blue Circle Industries (BCI) chalk dewatering option. Other options were rejected as being either unfeasible or too costly. The agreed short-list was as follows: (i) Sewage Effluent - River augmentation from a new sewage treatment works (STW) at Otford using treated ‘upstream’ (Sevenoaks) effluent; - River augmentation at Otford using appropriately treated effluent from Long Reach STW. (ii) River Augmentation - From a series of riverside boreholes referred to as ‘artificial springs’. (iii) Demand Management - Leakage Control; - Metering. 7O223B10VWRPVB\RDER-J&]94\wp 6-10 (iv) Abstraction Reduction - Licence Variation; - Conjunctive Use Operating Agreement. (v) BCI Chalk Dewatering Water - Direct Public Water Supply; - River Augmentation; - River Augmentation with downstream surface water/groundwater abstraction in lower catchment. The five short-listed options were evaluated against the following nine criteria: security of river flows (using 35 Ml/d as a yardstick); conservation of ecology; effect on water utility (split into water resources and operations); cost estimates (capex/opex/combined NPV); elapsed time; public acceptability; risk of failure to implement; operational security; flexibility of options. Details of the five options investigated are discussed in Chapter 7. The full report ‘Plan for the Darent’, (TWUL/NRA, 1992) is attached as Annex III of this report. The results are summarised in Table 6.8. The principal conclusions of the JPT were as follows: (i) No single option scores sufficiently highly across all nine key criteria as to provide a convincing robust, flexible and cost effective overall solution. (ii) The options rank in the following descending order of overall merit: - artificial springs - abstraction reduction - river augmentation using BCI water - BCI water - direct supply - sewage effluent - STW at Otford - sewage effluent - using Long Reach 70223B1^WRP\B\RDBR-Jnl94\wp 6-11 security Other II b comments forward infrastructure Brixton-Konor improvement* in STAGE II in II STAGE 2 ROMS bringing Oak Oak by year* 3 and Up to 20 Ml/d included requites could be could be available ^’Copper ^’Copper removal not "’Assume* effluent suitably treated for requirement* To be assessed prtyosajs. ice Appendix 5 (item 3) 0,F greater, !UWBEP~ Potential for further surface water ecological '"Could be require* evaluation abstraction (sec Option E3 under 5.3) • • • supply Flexibility degree to scenarios Measure Measure of nccas/water or cope with which which option at at design stage changing river can be adjusted • • • ♦ » ♦ • • • • • «•« security work work in practice Operttiontl confidence option Lhsi Metsare of **tll tlvftys**tll ♦ • • • • • • 9m • « • « • esctliting M enare of permission to to implement eg Planning to to iraplemeot Risk of fidare risk of fiilure costs refDSed, • » • *« *• »» « • « • • •• • ** acceptability 1-2 2-5 (Year*) Elipsed Public 23 2-3 ----- 26% 37* * COMMENT 16 16 -4 6 * * 1’ 9 9 - 20 20 75 IS'^NG - <43% TT TT IS1" IS1" - 4 3 * 7 7 r r 7T Secure river flow '35 Ml/d yirdsliiV 1 Option contribution Ml/d 1 1 >35 • 100* Assume quantity requirement far river it35 Ml/d during extreme Net Gain) low low now* periods (NG ■ Key Key criteria Effluent Al.STW at Otford A2.Long Reach antmented it Otlord D3.Licence C2. Metering C2. Cl.Leakage Control reducing to )0« SEE Variation/ Conjunctive U*c Operating Agreement El.Direct Supply E2.Rivei Augmentation A. Sewage Effuem Option* B. Licence Viriition Artificial Spring* C. A Conjuctive aie D. E. BCI Definition* 6-12 Because of the widely differing views of the scale of benefit which could accrue from demand management, it was not possible to give this option a strict ranking. However, the Team agreed that a Demand Management Programme should form part of the final overall package. (iii) From (ii), it was the Team’s view that the overaN solution should comprise the following components: - reduced abstraction from Upper Greensand and mid-catchment chalk wells; - river augmentation using artificial springs with back up from river augmentation using BCI water; - demand management with increased use of Thames reservoir, London Ring Main supply to underpin abstraction reduction and river augmentation. More details on these options are given in Chapter 4 and Chapter 7. 70223B1(AWRP\B\RDER-Jol94\wp 6-13 CHAPTER 7 PLAN FOR THE DARENT (NRA/TWUL, 1992) 7.1 Introduction Thames Water Authority (TWA) in the mid 1980s acknowledged that over-abstraction of groundwater from within the Darent Catchment was responsible for the reduced river flow and the drying up of the river within its middle and lower reaches; particularly between Lullingstone and Hawley. It also planned to take action to alleviate the problem and as a provisional measure, agreed a voluntary restriction in the use of six riverside boreholes to 70% of their licensed quantities. TWA, NRA Southern Region and TWUL have all held meetings with interested parties including local councils, and the Darent River Protection Society (DRIPS) to discuss what could be done to resolve the problem. Formal correspondence with TWUL on solutions for the Darent was opened in Autumn 1991. NRA proposed a variation of licences to 70% and a phased plan to reduce abstractions further to half their authorised quantities. Further correspondence and meetings over six months had resulted in little firm progress by May 1992. In June 1992 the NRA Board decided that the Darent licences would be revoked unilaterally if TWUL did not enter into meaningful negotiations. This resulted in an exchange of letters between NRA and TWUL in the media, with considerable support nationally for the NRA’s stance. Preparations were put in hand for licence revocations, and draft notices for the London Gazette were sent to DoE in September 1992. The NRA and TWUL, finally met for negotiations in early October. Although little change in the position of the two parties was achieved, it was agreed that a Joint Project Team would be convened to recommend a technical solution which could be implemented by 1996 and a joint press release was agreed. A Joint Project Team (JPT), comprising two managers from TWUL and two from NRA South Region and one from NRA Thames Region, together with a Director from the Consultants responsible for the GDC Darent Studies, was set up in mid October and met once or twice a week until 30 November when an agreed report was produced. The finding of this report, called the Action Plan, forms the basis for the recommendations submitted here. Other chapters in this report supply supplementary information in support of the recommended Action Plan. A copy of the Plan for the Darent is included as Annex III. 70223B1(KWRP\B\RDER-Jol 94\»p 7-1 7.2 Statutory Responsibilities Both the National Rivers Authority (NRA) and Thames Water Utilities Ltd (TWUL) have duties and obligations under the Water Resources Act 1991 and the Water Industry Act 1991 respectively which have direct bearing on the River Darent situation. In essence the NRA has a general duty to conserve, augment, redistribute and secure proper use of water resources. In addition the NRA has a general duty to have particular regard to the duties imposed on water undertakers. These duties and powers do not relieve any water undertaker of its statutory responsibility to develop water resources in line with its general duty to develop and maintain a water supply system. The water undertaker has a primary duty to make water available to persons for domestic and industrial use as demanded. Both NRA and TWUL have duties to enhance the environment whilst carrying out primary functions in relation to water resources and water supply. The NRA may enter into a water resources management scheme with a water undertaker under section 20 of the Act. The principal relevant responsibilities and duties are summarised in Appendix A. 7.3 Water Efficiency and Management 7.3.1 Introduction A significant amount of groundwater is abstracted from the Darent catchment, predominantly for public water supply. Abstractions grew in the 1950s and 1960s with increasing demand, and the Darent wells have in the past supplied a substantial amount of water to South East London. Existing actual abstractions are close to the long-term mean recharge (85% for Chalk; 80% for Greensand). TWUL has average annual authorised abtractions of 106.9 Ml/d from 10 sources or 78.6 Ml/d from the 6 river bank sources. Over recent years the trend has been towards greater use being made of River Thames reservoir supplies. 7.3.2 Demand Management The Action Plan includes a water resources management scheme agreement to pursue demand management and both Thames Water and the NRA recognise the need for effective and prudent management of water. The recent drought has resulted in increased attention being paid nationally to leakage control and demand management. Nationally, initiatives and studies are underway to establish appropriate strategies and targets, sponsored by Regulators and Government and supported by the water industry. These studies will clarify substantial issues on metering, demand forecasting and leakage control and allow the benefits of such approaches to be built into future water resource strategies. At present there are widely differing views of the scale of benefit which could accrue. It should be possible to give to the Darent the appropriate benefits of demand management gains that are achieved. 70223BlftWRP\B\RDER-Jal 94\wp 7-2 Given the uncertainty which surrounds the projected benefits, a staged approach to the solution for the River Darent was adopted. An initial stage seeks to benefit the River Darent from the existing situation and does not assume any specific contribution from demand management. A second stage allows for benefits of water efficiency to be taken into account in determining the package of measures necessary to protect both the River Darent and public water supplies. The introduction of this second phase will be after the results of national studies and debates have been completed, and thus be able to reflect national policies, agreed best practice and expectations. 7.3.3 Leakage Both Thames Water and NRA are committed to water efficiency. Leakage control programmes in TWUL have reduced local leakage from 11.5 to 7.5 litres/property/hour. The programme continues with an overall local leakage aim of 4 1/p/h, which matches the NRA ‘good housekeeping’ target for local leakage which generally equates to 16-18% of total supply. Achievement of this target would make a significant contribution to the future water resource situation in the Darent. It would however, be unwise to develop strategies for the Darent which are dependent on savings which are yet to be demonstrated as achievable and sustainable or cost effective. The phased approach proposed for the Darent allows for full account to be taken of water efficiency savings if their certainty is demonstrated. TWUL is committed to achieving the lowest practicable leakage in South East London. The move towards metering will provide benefits for leakage control. 7.3.4 Water Consumption A critical feature of water resource planning is the current level of water consumption and projected changes in future demand. Thames Water has been assessing water consumption through a domestic water use study on selected properties across the ACORN range since 1976/7. The results of the study provide evidence for a current average per capita consumption of water at 155 1/h/d in the Thames Water region. This figure includes proper allowance for night consumpton. Studies by Southern NRA and Southern Region Water Companies have identified consumption in the Southern Region of 143 1/h/d and NRA Southern Region claim a similar figure should apply to the Thames Region. None of the figures include supply pipe leakage. Forecast growth in demand for water in the Thames Region is 0.5% per annum, with total demand rising by 12.5% over the 25 year planning period. In the short term, reductions in total supply are projected taking into account leakage control, recession etc. The overall prediction takes into account the 1989 update of projections of populations and households issued by the Office of Population Census, independent econometric forecasts of industrial growth, the effects of changes in water using appliance ownership and the achievement and maintenance of a local leakage rate of 6.28 1/property/h by 1996. Forecasts are reviewed annually to take into account the latest available information on the components making up the forecast. 70223B10\WRP\B\RDER-Jol 94\wp 7-3 7.3.5 Metering Metering remains a substantial issue. Commercial metering proceeds apace in the Thames Region. In addition to normal industrial customers all pubs, hotels and restaurants are now metered. Doctors, dentists and hairdressers are currently part of the commercial metering programme. AU new domestic and commercial properties are metered. TWUL has metered 14000 commercial properties annually for the last two years. These programmes will continue to their economic limit. For the present Thames Water has decided against compulsory metering of all domestic customers. It has not decided how to replace the rateable value system as a basis for charging and whether domestic metering should be extended. National surveys suggest that up to two thirds of people questioned have indicated a preference for metering and this appears to be an approach the Government is supporting. There are, however, high costs and practical difficulties associated with metering in highly urbanised areas. Thames Water has introduced trial metering areas and the River Darent catchment would be a potential candidate for such an exercise. 7.3.6 Summary Both TWUL and NRA are committed to water efficiency measures. The extent of the benefits remains to be quantified, but whatever savings are achieved and can be reliably predicted to be achieved in the future will be accounted for in the second phase of the River Darent project. The debates and discussions on demand management and leakage control will continue at both national and regional level. During 1993 it is anticipated that national policies and practices will be introduced which encourage or require levels of action to achieve specific targets. It is considered essential that any strategy proposed for the River Darent is sufficiently flexible to accommodate the results of these actions and that investment programmes should be capable of taking them into account. 7.4 Options Considered 7.4.1 Introduction Screening of options discussed in Chapter 6 was undertaken early on in the study. Five possible options were selected based on new costings and on the results from the catchment model, which had been set up and calibrated shortly before the study began. These five options are discussed below. 70223B10VWRP\B\RDER-Jal 94\wp 7-4 7.4.2 Sewage Effluent There has never been a modern sewage treatment works in the Darent Valley; sewage has always been directed towards Long Reach STW on the Thames Estuary. One option would be to intercept all the sewage in the existing trunk main which runs down the Darent Valley to Long Reach and would involve the construction of a new works, probably just downstream of Otford for Sevenoaks. The present dry weather flow (DWF) in the trunk main at Otford is estimated at 8 Ml/d; allowing for future growth, this figure is unlikely to exceed 10 Ml/d over the next 10 years. The DWF at Long Reach is 170 Ml/d and therefore has ample flow to meet the maximum estimated river augmentation requirement of 35 Ml/d. The second option would consist of tertiary treatment of 35 Ml/d, effluent pumping from Long Reach to a discharge point at Otford via a pipeline. Maintaining the very high river quality standards would be a primary consideration with respect to these options. Critical parameters would be ammoniacal nitrogen, copper (found in domestic sewage) and organic substances (Long Reach has a high proportion of trade effluent). A high level of treatment would be required for both options. For the purposes of costing it has been assumed that a new sewage works at Sevenoaks would require tertiary treatment and Long Reach effluent would require both tertiary treatment and advanced treatment. 7.4.3 Artificial Springs This option provides for groundwater u> be pumped from a series of riverside boreholes thereby augmenting river flow in a manner which would simulate natural discrete spring discharges. The main features are as follows: (i) It assumes that streamflow at Shoreham would be at or above the target flow. Model simulation shows that the most effective way of achieving this is by minimising Lower Greensand abstractions. (ii) Augmentation boreholes would be constructed between Shoreham and Hawley with the aim of obtaining the desired target flow accretion profile between Shoreham and Hawley. For costing purposes it has been assumed that five or six boreholes each yielding 4 to 5 Ml/d will be adequate when combined with reduced groundwater abstractions. Field trials will enable the actual performance of this option to be properly evaluated. (iii) Augmentation would be controlled by NRA and would vary seasonally. Abstractions would be dependent on actual river flows. Preliminary computer runs suggest that wells would be operated one year in two and abstracted annual volumes would range from 0 to 3 000 Ml/annum over a maximum period of seven months. Because of the intermittent operation of the wells the 70223B1(AWRPVB\RDER-Jq1 94Wp 7-5 volume abstracted expressed as a continuous discharge would only amount to 2 Ml/d although peak abstractions would be around 24 Ml/d. The operation would be most satisfactorily implemented as part of a Water Resources Management Scheme (under Section 20 of the Water Resources Act 1991). 7.4.4 Demand Management Significant reductions in demand would, of course, mean that groundwater abstraction could be similarly reduced thereby enabling more groundwater storage to be available for baseflow. Unfortunately, at present there are widely differing views of the scale of benefit from leakage control and metering. What is not in doubt is that some benefit will result. Pressure on existing and future water resources in the area mean it is essential to manage water efficiently. 7.4.5 Reduction in Groundwater Abstraction A possible package of possible radical changes to the current groundwater abstraction regime in the Darent catchment has been examined against the benefits which accrue in terms of improved river flow, whilst still securing public water supplies. Two approaches were taken namely licence variations alone and licence variations involving a conjunctive use operating agreement. However, the option which has been put forward is a combination of the two. The latter incorporates the most attractive aspects of each. (i) Variation in Groundwater Licences Fundamental to any variation of the existing abstraction regime is its effectiveness in improving river flow. The GDC model (see Chapter 4) has demonstrated that of the six Darent sources under consideration reductions from Brasted and Sundridge in the Lower Greensand (see Figure 7.1) produce the greatest improvement in river flow. The chalk sources of Lullingstone and Eynsford in the southern-most of the upper part of the chalk catchment are next most effective, followed by Horton Kirby and Darenth respectively. Furthermore, significant increases in abstraction levels in the Lower Cray and Lower Darent (including the Darenth source) are shown by the model to produce little reduction in river flow, especially during the critical low flow periods. Therefore, the picture that has emerged from the modelling work is that a redisposition of abstraction from the upper catchment (primarily Brasted and Sundridge, but also the upper three Chalk sources - Lullingstone, Eynsford and Horton Kirby) down to the lower catchment (Lower Darent, including the Darenth source, and Lower Cray) could provide maximum benefit to the river whilst maintaining the same water resources capability. 70223B10lWRPV8\RD£R‘Jal 94\wp 7-6 On this basis consideration was given to varying licences downwards in the upper catchment and making an equivalent variation upwards in the lower catchment to off-set the shortfall. Unfortunately, there are two difficulties with achieving this to the full extent: one is the high infrastructure expenditure, and the other is the significant risk of producing saline intrusion from the Thames estuary. Nevertheless, the Team has concluded that some moderate increases in the lower catchment, especially if such increases are aimed at meeting periods of overall resource shortage and not permanent long-term abstraction, would be entirely consistent with sustainable and environmentally acceptable water resources management. Conjunctive Use Operating Agreement It has been recognised for some time that there is scope for application of a modern conjunctive use water management scheme between the Darent Valley groundwater sources and the Lower Thames surface water system. The trunk distribution capacity required to enable such a scheme to be implemented is currently being enhanced by the construction of the London Water Ring Main due for completion in 1995. Some considerable relief has already been provided to the Darent catchment from operation of the first phase of the Main. An eastward extension to Honor Oak is proposed to support increasing demand in South East London. The introduction of conjunctive use on this scale implies some advancement of this tunnel. There are also increased operating costs. A conjunctive use operating agreement would take the form of a Water Resources Management Scheme (under Section 20 of the Water Resources Act 1991). In essence it would seek to minimise groundwater abstraction from the most sensitive Darent sources by replacing any shortfall with River Thames derived supplies. TWUL has actually been voluntarily limiting abstractions from the Darent Valley by partial conjunctive use for some months, demonstrating the general approach. Its systematic adoption would mean that for most of the time Darent sources would be being partially ‘rested’ compared with historic abstractions, with benefits to groundwater levels and hence to groundwater resources and river flows. However, when a ‘Thames drought’ saw reservoir levels dropping significantly in the Thames Valley, the extra demand put on the Thames surface water sources would increase, leaving the Darent sources to operate to the revised licences. Historically, ‘Thames droughts’ have been substantially less frequent than groundwater droughts affecting the River Darent. (iii) Combination Licence Variation/Conjunctive Use Operating Agreement Proposal The Team has devised a hybrid of (ii) and (iii) which seeks to combine the most attractive features of both options. The two principal benefits of the proposal are: substantial abstraction reduction in the upper and middle catchment achieved by licence variations and preservation of resource neutrality and reducing infrastructure costs by a conjunctive use operating agreement and upward variations in licensed abstraction in the Lower Cray and Lower Darent catchments. Thus a hybrid scheme has been evaluated as the most practical, cost-effective approach which combines licence variation with conjunctive use of groundwater and surface water sources. 7.4.6 Blue Circle Industries (BCI) Quarry Dewatering The chalk quarries at Northfleet, are said to be the largest excavations in Europe. Presently some 30 Ml/d of chalk groundwater are pumped from the Western and Eastern quarries to keep them from flooding. This water is either channelled to the Swanscombe salt marshes on the estuary or used to augment the Ebbsfleet stream. Being a dewatering operation BCI has no need of an abstraction licence although the NRA is requiring BCI to obtain discharge consents for the discharges to surface waters. BCI has put forward plans to further develop the Eastern Quarry by excavating it to deeper levels, although immediate plans have been postponed as a result of the economic recession. The preliminary GDC report indicates that the dewatering requirements would be little different from the present day total although for planning purposes 47 Ml/d has been allowed. It has been assumed, conservatively, that between 15 and 20 Ml/d of reasonable quality chalk groundwater could be available, for direct supply or river augmentation. Further groundwater modelling will however be required to confirm what abstractions are feasible and do not result in saline intrusion from the River Thames or interfere with other abstraction sources, particularly as proposals involve increasing abstraction from the lower Darent catchment. Further investigations of water quality are also required. The following have been considered: (i) Direct Public Supply For costing purposes it has been assumed that pesticide removal and high turbidity should be addressed; new local infrastructure and pumping equipment are also included. 70223B1C\WRP'\B\RDER-Jul 94\wp 7-8 (ii) River Darent Augmentation Scheme The basic scheme would be a pumping station at Northfleet to boost raw chalk groundwater up to Eynsford or Lullingstone via a pipeline running up the Darent valley. (iii) River Augmentation followed by Public Supply The BCI resource has further potential as a combined river augmentation and water supply scheme in which a surface water or groundwater abstraction at the lower end of Darent would pick up the augmentation water put in over the middle reaches. 7.5 Tbe Recommended Scheme 7.5.1 Reduced Groundwater Abstractions and Conjuctive Use Operating Agreements (i) Reduced Abstractions It has been demonstrated conclusively by use of the catchment model that reductions in abstractions from the Lower Greensand aquifer will produce significant increases and rapid responses to the flow in the upper Darent. Moreover, this increase in flow in the upper catchment will supplement flow throughout the length of the River Darent; although bed losses do occur they are estimated to amount to around 1.1 Ml/d per km. Such losses are not excessive and the modelling has shown that they do not increase proportionately with flow but at a much slower rate. In contrast, reduction in chalk abstractions from the lower catchment have little effect on river flows. The reason being that the river is perched for most summer months with the watertable well below bed level. The modelling also demonstrated reduced abstraction from the Chalk were most beneficial in the upper catchment close to the interface between the Lower Greensand and Chalk and less so the lower in the river they were made. Agreement was reached between TWUL and the NRA to close down abstractions in the two main TWUL boreholes in the Lower Greensand at Brasted and Sundridge. This represents a major break through and for the first time this century the River Darent may see a reversal in the relentless increase in abstractions and reductions of river flows. By combining these reductions in abstractions with conjunctive use of Thames reservoirs supplied via the London Ring Main, further reductions in abstractions from the four mid-catchment TWUL wells at Lullingstone, Eynsford, Horton Kirby and Darenth were agreed. A summary of the agreed variations in licences is given in Table 7.1. 7022JB10\WRP\B\RDER-Jnl 94\wp 7-9 TABLE 7.1 Agreed Variations in TWUL Darent Licences Licence-Average Annual Rate (Ml/d) Licence - Peak Rate (Ml/d) Aquifer Borehole Proposed Proposed L ocations Existing Existing Immediate Stage 1 Immediate Stage 1 Upper Greensand Brasted 4.56 0 0 6.82 0 0 (UG) Sundridge 13.60 0 0 18.20 0 0 Sub total 18.16 0 0 25.02 0 0 Chalk M id Lullingstone 9.09 4.50 4.54 9.89 6.14 6.36 catchment (CM) Eynsford 16.82 11.60 8.39 18.18 18.18 12.73 Horton Kirby 13.64 13.60 6.80 18.18 18.18 12.73 Darenth 20.91 20.90 20.90 22.73 22.73 22.73 Sub total 60.46 50.60 40.63 68.18 65.23 54.55 (UG + MC) Total 78.62 50.60 40.63 93.20 65.23 54.55 % of existing 100% 65% 52% 100% 70% 58% licen ce Chalk-Lower Increase in 4.4 14.4 13.4 24.08 Catchment (CL) peak rates in Lower Darent and Lower Cray (UG+CM+CL) Grand Total 55.0 55.0 78.63 78.63 % of existing * 70% 70% * 84% 84% licen ce Note: (i) Lower Darent: Sources are Darenth, Wilmingham and Dart lord Lower Cray: Sources are Bexley, Crayford and Wansunt (ii) The proposed licence variations represent the maximum permitted entitlements. A fundamental part of the licence variation proposals is that the adoption of the conjunctive use operating agreement (WRMS1) will mean in practice that the maximum annual abstraction rates will be used no more frequently than one year in ten on average ie during a ‘Thames drought’. For most of the time abstraction levels will be substantially lower at about 32 Ml/d (40%) from the Middle Catchment ‘4 \ Furthermore, the weighting applied to the ‘4’ during these ‘normal’ periods will be biased down-catchment, with the Darenth source taking over 60% of that abstraction. (iii) The resource neutral position (ie London’s present deficit is not increased) for the proposed conjunctive use mode of operation is for a maximum take during an extreme ‘Thames drought’ of 55 Ml/d (70%) average annual and a peak capability of 78.6 Ml/d. This has been achieved by off-setting the proposed range of downward variations in the upper and middle catchments by upward variations in the lower catchment. (iv) The average annual mid-chalk abstraction of 50.6 Ml/d and peak of 65.2 Ml/d were used to evaluate TWUL estimated costs for this option. An early requirement within Stage I will be for the NRA to review the benefits to river flow of moving a greater proportion of ‘Thames drought’ additional abstractions to the lower catchment. In order for the cost-benefit to be assessed, TWUL will review the infrastructure costs needed to provide for an increase of lower catchment abstractions. It can be seen that the licences for mid-catchment sources from the Chalk are scheduled to be reduced from 60.5 Ml/d to 50.6 Ml/d with immediate effect. This reduction represents TWUL’s best estimate of the maximum upward variation in the lower catchment which could be made without incurring significant additional costs above those estimated for this option. During Stage 1 an evaluation will 70223BIAWRPVB\RDER-Jn] 94\wp 7-10 Figure 7.1 The Recommended Scheme Woolwich GREATER LONDON Nortbfleet Lullingstone, Eynsford, Horton Kirby and South Darenth to be operated DARTFOR normally at 32 Ml/d (Existing licence value 60 Ml/d), (60% of 32 Ml/d abstracted from Darenth ) 5 or 6 Augmentation \ HmUy Darent C a p a c ity BCI Augmentation Pipeline Distribution System changed to accept London Ring Main Thames Water KffTWHQ / M2B Sundridge Licence reduced from S E V E N O A K S 13.6 Ml/d to 0 Ml/d Brasted Licence reduced from 4.4 Ml/d to 0 Ml/d L e g e n d Motorway or Main Trunk Road A Groundwater Abstraction Site River Lake or Gravel Pit S c a le 0 2 4 ' o 8 10km Catchment Boundary 70 2 2 3 \B 10 be made to see if it is possible to reduce the mid-chalk wells licence to 40.6 Ml/d and to evaluate any cost implications. However, because of the proposed conjunctive use of Thames reservoirs water and groundwater, operational procedures will be introduced immediately which should reduce the average abstractions from the four Chalk mid-catchment boreholes to 32 Ml/d; a reduction of 60% below the licensed value. Moreover, it is hoped that it will be possible to take 19.2 Ml/d of this amount from the Darenth borehole thereby significantly reducing abstractions from the three Chalk boreholes uppermost in the catchment and further increasing River Darent Flow. These reductions from the Chalk and Greensand aquifers will have quite a dramatic effect on low flows in the River Darent, although the low flow profile would still be one of diminishing flows from the upper to lower catchment: an anomaly which would need to be remedied by augmentation to satisfy the ecologically acceptable flow requirement. (ii) Conjunctive Use of Thames Surface Water and Darent Groundwater Abstractions (WRMS 1) A conjunctive use scheme is proposed under Section 20 Water Resources Management Schemes (WRMSs) to serve the conjunctive use of River Thames reservoirs and Darent groundwater sources together with supplementry augmentation of the River Darent during drought conditions. This scheme will secure commitments to conjunctive use, as follows: It will commit TWUL to reduce abstraction from the Darent/Kent sources when Thames reservoir storage is maintained at normal levels while permitting TWUL to increase abstraction from the Darent when Thames reservoir storage is below normal. It will specify three modes of operation, following the definitions contained in the Section 20 WRMS registered in the ‘Privatisation* Transfer Scheme as Agreement A2, ‘Lower Thames Abstraction Scheme’. This Agreement includes an Operating Strategy Diagram (OSD) which relates water-saving measures (such as hosepipe bans) and protected residual flows in the River Thames at Teddington, to storage in the London reservoirs and to time of year. Figure 7.2 shows an adapted version of this diagram for use with the Darent. Usually storage can be kept full or nearly full (Condition X). Only in a significant drought does it fall far enough to invoke two specific triggers identified in the proposed WRMS1. The triggers are: (Y) when River Thames target residual flows are dropped to 600 Ml/d; (Z) when hosepipe bans are to be imposed to reduce demand or when the OSD allows for them to be imposed, whichever is the later. TWUL retains freedom to make decisions, with the OSD as ‘guidance’; so WRMS1 allows for some conjunctive use to continue, if TWUL delays hosepipe bans until the reservoir situation is recognised by TWUL as sufficiently serious. 70223Bl The scheme will indicate that when a severe drought, with Thames storage in Zone Z, occurs, the full annual average licensed quantities in the Darent will apply until Zone X is regained. However, whilst in Zone X and sometimes Zone Y, abstractions will be restricted to follow the amended figures for annual average given below, whilst Thames reservoir storage levels remain the relevant zone: 4 Middle Catchment Chalk Sources Zone X Ml/d Y Ml/d Z Ml/d L + E + HK 19.6 23.1 29.7 Darenth 20.9 20.9 20.9 TOTAL 40.6 44.0 50.6 L = Lullingstone. E = Eynsford, HK = Horton Kirby Peak day abstractions will be unaffected. WRMS1 will also require TWUL to use ‘best endeavours’ to keep winter abstractions from October to March inclusive down to less than 32 Ml/d, from the 4 middle chalk sources combined whilst Thames reservoirs remain in Zone X. It is recognised that there is a practical need to abstract much less than the annual average allowed, from October to March, to allow for possible peak demands later in the year. Minimising winter abstraction has the added benefit of significantly increasing chalk aquifer storage and reducing the frequency of river augmentation. (iii) Costs TWUL costs for reducing licences at Sundridge and Brasted and implementing the conjunctive use scheme are summarised in Table 6.8. The capital costs involved in infrastructure changes to the water supply distribution system in addition to advancing the extension of the London Ring Main have been estimated by TWUL at £3.6 million. Additional annual operating costs of £100 000 have also been calculated, which give a combined net present value of £4.3 million. 7.5.2 Low Flow River Augmentation (i) Augmentation Wells WRMS1 will commit the NRA to ensuring by providing monitoring and augmentation, that target river flow regimes at different locations, as determined during the TWUL/NRA study throughout the 70223B1(KWRP\B\RDER-Jol 94\wp 7-12 kTEMtYlFia-T-J OEM k; B- K >ni> C/5 £ H 06 ** 0>(A V u © U o u eg 0£ V o « ► a Fb a Ar a Jn u Ag e Ot o Dec Nov Oct Sep Aug Jul Jun May Apr Mar Feb Jan Proposed Thames Reservoirs / River Darent Abstractions : Abstractions Darent River / Reservoirs Thames Proposed Teddington Target Flow Target Teddington l I 400 tcmd tcmd 400 I tcmd 600 I l f E T Z l 200 tcmd tcmd 200 l Z T E A ~ r r = = Level 1 ( Hosepipe ban) Hosepipe ( 1 Level = = Months 800 tcmd tcmd 800 i i i i Conjunctive Use Scheme - WRMS I - WRMS Use Scheme Conjunctive ...... r ...... i" Full- Figure 7.2 Figure £ fa* <» ■+* O ► I* a o u ea Qfi year, are met. The monthly river flow targets will be specified in the scheme, as recommended by GDC consultants and agreed by the Team. The scheme will also include management consultation arrangements, to seek to resolve where possible any differences of opinion or shortcomings by either side. The scheme has been designed provisionally to consist of two elements. In the first element five or six augmentation wells with capacities of 4 to 5 Ml/d located on or close to the river and spaced between Lullingstone and Dartford would ensure that monthly target flows are maintained. Peak abstraction would amount to about 24 Ml/d. On average augmentation from one or more of the wells would be required only one year in two and the average annual abstraction expressed as a continuous discharge has been estimated to amount to only 750 to 1 100 Ml. The well augmentation water would be introduced to the river to simulate as far as possible natural spring flow. Their siting would obviously be chosen to ensure that any localised perched spring water was not adversely affected. Trial wells are currently being installed to confirm the effectiveness of this element of the augmentation scheme. More detailed information on the augmentation wells are included in Chapter 4. The capital cost for the six wells is estimated to be about £1 million with operating costs of £100 000 per annum and a net present value of £1.7 million. (ii) Blue Circle Cement (BCI) Augmentation Pipeline As discussed previously, preliminary studies have indicated that there may be available up to 30 Ml/d from BCI Northfleet quarries which could be used to augment river flows in the middle reaches of the River Darent above Shoreham. The pipeline will be required should the augmentation wells prove to be inadequate or have unsatisfactory influences eg: insufficient yield: unacceptable interference with local springs; unacceptable influences in local Chalk aquifer levels; unacceptable draw down in the Chalk aquifer or reductions in surface water due to increased TWUL water supply abstractions in the lower Darent Catchment. These qualifications on the augmentation wells remain to be resolved by field trials accompanied by additional catchment model runs undertaken during the implementation of the Action Plan. A pre-feasibility study has recently been awarded to consultants to determine the routing of the proposed pipeline together with the preparation of preliminary engineering drawings and costs. However, further studies are required to evaluate the effect of dewatering on adjacent wells and saline intrusion from the River Thames. More information is also required on the quality of the BCI quarry water. Discussions are being held between BCI and NRA on the possibility of a joint study into the potential development of quarry de-watering into a useful source of water supply. 70223B10lWRPVB\RDBR-Jiil 94\wp 7-13 One area where the augmentation pipeline has a distinct advantage over the augmentation wells is that it introduces new water into the catchment. This will obviously be beneficial generally but may be crucial if the heavier abstractions proposed in the lower catchment, as part of the conjunctive scheme, WRMS1, prove difficult to sustain and other companies’ abstractions increase to full licence levels. The introduction of new water into the catchment and increased recharge through the river bed will also assist in maintaining groundwater quality below Dartford and help counteract possible saline intrusion resulting from increased abstraction. The capital costs for developing the augmentation pipeline are estimated to be £5 million with annual operating costs of £80 000. The combined net present value is £5.3 million. In Stage II it is proposed that TWUL applies for a licence for public water supply which would permit re-abstraction of augmentation water in the lower Darent, or take water direct from BCI quarries. The licence would include the condition that utilisation of authorised quantities will be dependent on TWUL demonstrating need according to apre-determined methodology contained within the licence. If abstraction from BCI quarries goes ahead then the total quantity taken, together with augmentation on an annual basis is not expected to exceed 20 Ml/d. However, additional studies may indicate that the full augmentation requirement of 35 Ml/d is available. This flow is of course only intermittent and required generally over short periods of time. The issue of the licence would allocate a resource, but its utilisation would depend on demonstrating need and securing the proper use of water resources. 7.5.3 Review of River and Public Water Supply Needs This scheme will commit NRA and TWUL to collaboration and actions to enable review both of river needs and of public water supply needs in the future. It will commit the NRA to collecting data, and refining the catchment model, which will ensure confidence about the relationship between the River Darent and various associated groundwater levels and abstractions. The NRA will also be committed to assessing the overall health of the river under Stage I arrangements and considering the adequacy of the target flow profile and the acceptable frequency of augmentation. It will commit TWUL to a continuing programme of leakage control and appropriate demand management measures with a view to demonstrating future needs for water in the area. It will commit both bodies to a joint review to establish the future need for water from the proposed conditional BCI licence, and to establish further steps (should they prove necessary) to safeguard the Darent. 70223 B1 G\WR P\BVR DER-Jn) 94\wp 7-14 The joint review should be carried out as far as possible by 1998, in the context of AMP3 proposals, but in any case should be completed by June 2000. 7.6 Summary of Strategy In broad terms there are three elements to the strategy: (i) Reduce Key Abstractions Licensed quantities in the relevant six boreholes would reduce to 70%, normal annual abstractions would be reduced to 50% of current licences, abstraction balance would migrate from key Lower Greensand and middle catchment sources to lower catchment sources, with increased licences where necessary, and for most winters, abstraction rates would be down to 40% of current annual licensed rates. The West Kent Water Company licence at Crampton Road would also be varied down by 24%. (ii) Augment River Flows Flows in the upper reaches will be much higher, as a result of (i); this will often but not always be true of middle reaches. To ensure target flows are maintained from Shoreham downstream, five or six Chalk augmentation boreholes (‘artificial springs’) will be drilled, tested, and operated. Plans will be developed to supplement this with water from BCI, unless early experience of artificial springs makes this water unnecessary. (iii) Arrange Suitable Resources Beyond 1998 Subject to the statutory application and advertisement process, a new licence will be granted to TWUL for review by the year 2000 against need for extra resources. In the meantime TWUL will continue development of monitoring, leakage control and demand management to provide a secure base for forecasting of future demands. By the year 2000 both public water supply needs and river health will be reviewed jointly. The strategy has two stages, in seeking to develop towards efficient water management practices which better balance public water supply needs with those of the river environment. Stage I will set out to meet the target river flows by 1996, by combinations of 7,6 (i) and (ii) above. Uncertainty about BCI augmentation could delay full achievement slightly. Stage II will comprise the leakage control and demand management activity, so that a review of licence needs and perhaps balance of existing abstractions at the time, can be carried out jointly. This review should be carried out as far as possible by 1998, in the context of AMP III proposals, but in any case should be completed by June 2000, with a view to deciding on the future of the new licence and on any other adjustments needed. 70223 B10\WRP\B\RDBR-Jol 94\wp 7-15 7.7 Summary and Recommendations of Joint Project Team (after TWUL/NRA, 1991) 7.7.1 Summary of Results Quote: 1. ‘An intensive period of work by the members of the Joint Project Team, supported by vigorous scrutiny of options and their effects and costs by supporting staff, has led to a proposed plan which would secure flows and river ecology in the Darent while safeguarding medium-term and long-term water supplies. 2. Inevitably some details need exploring and ‘honing’ further, but the Team is confident that its proposals are both practical and cost-effective. 3. The technical conclusions of the Team, on the path to identifying a strategy, include the following statements: (i) the upstream Lower Greensand sources at Brasted and Sundridge are more harmful to river flows than are the chalk abstractions; and of the chalk ones, Lullingstone, Eynsford and Horton Kirby have more critical effect on low flows, than do those sources further down the catchment; (ii) securing a flow regime down the river system at half the level of a l-in-20-year natural regime, appears likely to be adequate to protect the ecology of the river as well as providing a secure amenity; (iii) provision of a sewage treatment works at Otford, or further treatment and transfer of Long Reach sewage effluent upstream, would not make an economical contribution to solving the flow problem; (iv) it is feasible to develop a new source at (or near) the BCI site at Northfleet; it could produce between 15 and 20 Ml/d of reasonable water. It could be used to augment the Darent, or for treatment and supply, or one followed by the other. (v) development of artificial springs close to the river should be practical, should produce good ‘gain’ to the river, and should not affect groundwater levels or river recovery unduly. (vi) reconfiguration of the supply system in South East London, securing normal, drought, peak and emergency supplies for a modified pattern of source use, is practicable and its cost has been estimaed. 70223B1(KWRP'\B\RDER-Ju) 94\wp 7-16 (vii) there appears to be scope for controlling present and future demands in the area, by increasing the leakage control activity and seeking effective demand management through metering of commercial and some domestic properties; although it is uncertain at present how much demand-saving would result. Treating the Darent area as a priority would potentially help TWUL, align well with policy statements by the DoE and the two (OFWAT, NRA) regulators, and help limit demands for future abstractions in the area. 4. The Team’s proposals provide for early progress to secure flows in the Darent, while allowing flexibility for and review of future arrangements, which will develop in the context of knowledge and information being acquired over the next 8 years. 5. Features of the plan are set out in Section 7.6. In outline, it provides for (i) water supply infrastructure changes, as a basis for a more ‘river friendly’ configuration of abstractions; (ii) a ‘resource-neutral’ plan for changing licences, and both normal and drought abstractions, to secure water supplies while contributing to better natural river flows; (iii) two river augmentation schemes, one using water from the Blue Circle Industries site at Northfleet, the other providing local 'topping up’ artificial springs; (iv) reservation until 2000 of a licence capacity for a new source at Northfleet, to be available to meet future demands if the need is proven after committed work on leakage control and demand management in the area; (v) agreement to collaboration on water management issues in the area, culminating in a review by the year 2000 and appropriate subsequent steps. 6. Implementation of the immediate Plan will require significant capital expenditure, and some extra operating costs at times. Detailed estimates will only be available after more consideration, but it appears likely that extra costs to solve the Darent problems will be of the order of (i) for infrastructure changes, any new boreholes, and altered operations £3.6 m capital £100 K p.a extra operating cost £4.6 m combined NPV 70223810\WRP\B\RDER-Jul 94\wp 7-17 (ii) for artificial springs £1 m capital £80 K p.a operating cost on average £1.7 m combined NPV (iii) for new BCI source and pipeline to augment at 15 Ml/d £5 m capital £80 K p.a operating cost on average £5.3 m combined NPV 7. In return for this investment, flows in the Darent in a l-in-20-year drought could be sustained from west of Sevenoaks, down to Dartford, at flows and quality sufficient to maintain amenity and a healthy ecology. The flexibility of the scheme means that flows in worse droughts could also be significantly helped. 8. The Joint Project Team was asked to produce a plan to return flow to the Darent while safeguarding drinking water supplies to Thames Water’s customers. The report was to include - proposals for varying the six Darent licences a comprehensive technical solution - the provision of appropriate replacement resources We believe the information in this report achieves the aims, and meets these three stipulations. 7.7.2 Recommendations This report is written for the Directors and Boards of the NRA and of TWUL. In summary, we recommend acceptance and joint implementation of this plan as the most appropriate means of alleviating the low flow problems in the Darent before and by 1998. In specific terms we recommend the following: A. Agreement to the integrated strategy proposed here, and commitment to the challenge of implementing it; 70223Bl(KWRP\B\RDER-]o.l 94\wp 7“ 1 8 B. Discussion leading to the agreement of the NRA and TWUL as to how the plan will be financed; C. Appointment of a joint steering group to provide a detailed project plan, to arrange and monitor management of relevant aspects of the development, and to coordinate communications with local people including DRIPS, the Darent River Presevation Society. D. Announcement as soon as possible, on a joint basis, of decisions about the Plan. In practice a proposed Joint Steering Group will have much work to do, and will need to be empowered to overcome obstacles. We recommend an appropriate level of seniority be considered (such as Regional General Manager/Director of NRA, Director of TWUL) while day-to-day work and negotiations can be carried out at an executive level reporting to the Steering Group. The Project Team wishes to acknowledge the freedom with which it has been allowed to work, and considers the resulting Plan provides a sound and acceptable basis with which to address and solve the current problems of the River Darent’, 70223B1^WRP\B\RDER-Jnl 94\wp 7-19 CHAPTER 8 COST-BENEFIT ANALYSIS 8.1 Introduction This chapter contains in summary form the results of the economic analysis by ERM, (November 1993). The analysis is concerned with the evaluation of the three scenarios - the proposed Plan for the Darent (NRA/TWUL, 1992), called the Action Plan, the current situation or status quo, and the 100% abstraction scenario - and the comparison of the associated costs and benefits (CBA). These three main scenarios are summarised in Figure 8.1 and below: The 100% abstraction scenario whereby abstractions could increase to 100% of the permitted amount resulting in a flow regime of 1% of the natural flow volume in a 1-in-20 year drought. This is at present the legally permitted situation; Formalisation of the current operational situation, whereby abstractions in the Darent catchment would be restricted to 70% of the current maximum licensed volume, but no further augmentation works would be undertaken nor would Greensand abstraction be terminated. This would result in a flow regime of 5% of natural drought flow volume; Implementation of the Action Plan involving cessation of abstractions from the Brasted and Sundridge boreholes, flow augmentation and conjunctive use of water resources. This would result in an ecologically acceptable target flow at roughly 50% of natural drought flow. In addition to the above scenarios, the Department of the Environment (DoE) requested subsequent to the field surveys that the appraisal indicate the incremental costs and benefits of the measures comprising the Action Plan. It has not been possible to provide such estimates owing to the heavy reliance of the CBA on figures derived using the contingent valuation methodology (CVM). The surveys undertaken were designed to elicit information concerning the above three scenarios and the Results cannot be attributed therefore to individual measures comprising a scenario. The CVM survey is too coarse an instrument to distinguish between different levels of augmentation provided by reduced abstractions, augmentation wells and the BC1 pipeline link. Such a CVM survey based on these criteria is unlikely to have produced meaningful results. The approach to the analysis has been to compare the total costs and benefits of the Action Plan with the other two scenarios. Comparison between the three scenarios requires that the full resource costs and benefits associated with maintaining an adequate water supply and improving low flow conditions 70223B1<*WRP\B\RDER-Jq] 94\wp 8 - 1 in the Darent should be analysed. The following types of cost and benefit have been taken into account: costs of the engineering measures required for flow augmentation; costs of providing alternative water sources, ie costs of alternative distribution systems, pumping station, water mains etc; economic and environmental benefits and disbenefits resulting from increased flows in the River Darent. The 1% drought flow/100% abstraction scenario has been assumed to be the baseline as it is the current legal situation. Permanent maintenance of the current operational situation of 70% abstraction implies some costs associated with providing alternative water supply sources, and environmental benefits resulting from avoidance of further abstraction. Implementation of the Action Plan implies further benefits in addition to those associated with the second scenario and significant engineering and water resource costs. 8.2 Engineering Costs 8.2.1 NRA Engineering Costs The NRA would only incur engineering costs under the Action Plan scenario. The engineering works associated with the Action Plan, comprise: provision of augmentation boreholes; and construction and operation of the pipeline from Blue Circle Industries (BCI) Quarries of Northfleet. The costs of these works are summarised in Table 8.1. TABLE 8.1 Engineering Costs Associated with Darent Action Plan (£ million, 1993 prices) Item Capital Annual Programme of implementation costs operating costs Costs incurred by NRA in implementation of Action Plan Augmentation springs 1.00 0.08 Capital costs incurred in 1994, 1995 BCI pipeline 5.00 0.08 Capital costs incurred in 1995 and 1996 Total 6.00 0.16 70223B1CrvWRP\B\RDER*Ju] 94\wp 8 - 2 OlSVCtCOJ :Wc\rE\KDRrTM\1- laM7E u m E. N l.aeMk7 O:\WfcF\BraEM\UKlDARQrnTCMY\F10-f Chainage( kmDownstream of Source) oprtv Lw lw rfls n h Rvr aet 1 n 0 er Drought Year 20 in 1 Darent, River the in Profiles Flow Low Comparative 8.2.2 TWUL Costs of Developing Alternative Sources Under the 70% abstraction scenario and under the Action Plan, TWUL will incur costs of obtaining water supply from alternative sources, with the relevant volume being 23.62 Ml/d in both cases. The revised abstractions associated with the three scenarios analysed are given in Table 8.2. TABLE 8.2 Annual Average Abstractions in the Darent Valley under the Three Scenarios (Ml/d) Borehole 100% Securing 70% Action Plan Action Plan abstraction abstraction drought year normal year drought year drought year River flow % of 1% 5% 54% 72% drought flow Brasted and 18.16 12.71 0 0 Sundridge Chalk-middle ‘4’ 60.46 42.30 50.6 32.0 Chalk lower 0 0 4.4 4.4 increase Total 78.62 55.00 55.0 36.4 licensed/agreed (100%) (70%) (CJU)' Total demand 78.62 78.62 78.62 60.02 Shortfall met from 0 0 23.62 " '2 3 .6 2 London Ring Main transfer Shortfall met from 0 23.62 0 0 alternative sources Source: National Rivers Authority, Proposed Licence Variations (TWUL) Notes: Volume of abstractions based on annual average values 1 Conjunctive use agreement ensures that abstraction from Darent six boreholes does not exceed 32 Ml/d in all normal years to rest Chalk storage. 2 Normal year demand assumed to be 25% below that of a drought year. 3 Action Plan will achieve 54% river restoration as closing down two Greensand sources results in a flow slightly higher than target. 70223B10\WRP\B\RDER-Jal 94\»p 8-3 Under the Action Plan, extensions will be made to the London Ring Main so that water can be imported from the River Thames Reservoirs. Additional water supply will be obtained also from leakage control and from efficiency improvements in the South London distribution system. Such improvements to the supply infrastructure will increase the availability of yield from existing xresources without involving the need for developing new sources of supply. The unit capital costs will therefore be relatively low at about £0.3 million/Ml per day in comparison to those associated with development of new sources which can range from £0.5 million/Ml per day for groundwater to £2.5 million/Ml per day for storage reservoirs. A detailed breakdown of capital expenditure is not available but TWUL has indicated that the net present value of expenditure attributable to the Darent scheme on leakage control, pumping stations, redistributions, and extension to the London Ring Main amounts to £7.2 million (discounted at 8%). This cost is a revised up-to-date TWUL estimate of their Action Plan costs given in Chapter 7. While this expenditure will continue up to 2011, the major part will be incurred between 1993 and 2001. Incremental operating costs will be £100 000 per year. Under the 70% abstraction scenario, TWUL would incur some capital costs to make a permanent redistribution of remaining sources. A further allowance has to be made to bring available resources up to the full 78.62 Ml/day which is the total licensed abstraction. The costs of meeting the shortfall will depend critically on the type of supply option chosen. The least cost option is likely to involve similar measures to the Action Plan and we have therefore taken the costs of these measures as those occurring under this scenario. Table 8.3 shows the capital and operating costs of providing the total licensed amount of water under the three scenarios. This is based on the costs of meeting demand in a drought year. 8.2.3 Revocation of Licences An alternative option to the Action Plan is for the NRA to revoke TWUL’s present abstraction licences under Section 52 of the Water Resources Act 1991. This would involve a series of legal costs as well as compensation for TWUL. The total costs involved depend on how the compensation is calculated. If it is calculated on the basis of the replacement costs of the water, it could range from £7.2 to £50 million depending on the assumptions made about the nature of the new supply source. If development of new surface water sources is required costs could be at the upper end of this range. If instead compensation is paid on the basis of loss of land value as outlined in ‘Using Water Wisely (1992)’, it is likely to be significantly lower1. TWUL would still however, incur the costs of replacing supply. Table 8.4 shows the different components of cost in the case of revocation. The public enquiry costs are based on previous enquiries that NRA has been involved in. ’Department of Environment. Using Water Wisely (1992) 7Q223B1 (NWRP\B\RDER-JoS 94\wp 8-4 TABLE 8.3 TWUL Costs of Water Supply for the Three Selected Scenarios Scenario Ml/d Unit Total Discounted operating operating capital cost(n per costs costs Ml/d per (£k/year) (£m) annum (£k) 1 100% abstraction Use of groundwater 78.62 2 160 Use of surface water 0 4 0 Total cost of supply 78.62 Ml/d 160 2 70% Abstraction Use of groundwater 55 2 110 Shortfall to come from other sources 23.62 2-6 47 -142 Total cost of supplying 78.62 Ml/d 157-252(3) 7.2-12(2) 3 Action Plan Use of groundwater 55 2 110 Shortfall to come from surface water 23.62 6 142 Total cost of supplying 78.62 Ml/d 252 7.2 Source: NRA, TWUL Notes: (1) Unit operating costs of groundwater and surface water based on 5p and 10 to 15p/m3 respectively. (2) Unit capital cost £0.3-0.5 million/Ml per day. Lower end of the range refers to supply from the London Ring Main, upper end refers to development of new groundwater sources. (3) Lower end of the range refers to operation of new groundwater sources, higher end refers to supply from London Ring Main. 70223B1^WRP\B\RDER-Jal 94\wp 8-5 TABLE 8.4 Costs of Licence Revocation Cost components Department of TWUL NRA Total Environment (£ million) (£ million) (£ million) (£ million) Public inquiry 0.05 0.25 0.25 0.55 Lands Tribunal 0.05 0.10 0.05 0.20 0.75 Compensation to TWUL (i) Replacement cost 7.2-50 (ii) Loss of land value <1 8.3 Economic and Environmental Benefits 8.3.1 Summary of Impact of Low Flow Alleviation Changes in the flow in the River Darent will have an impact on the environmental quality of the river. In terms of enjoyment of the river by visitors and residents, the different water levels and flows will affect the general attractiveness of the river, as well as the terrestrial and aquatic ecology. There will also be an effect on specific activities associated with the river, in particular angling. The main environmental characteristics associated with the three scenarios are summarised in Table 8.5 for the four stretches of the river, under the headings: banks and vegetation fisheries terrestrial ecology aquatic ecology. The changes in each of the areas will bring benefits (and possibly some disbenefits) to residents and visitors to the region. The types of benefits and the appropriate methods for evaluating them are discussed below. 70223 B1(AWRP\B\RDER-Jnl 94\wp 8-6 TABLE 8.5 Environmental Consequences of Flow Regimes : Westerham to the Thames Westerham to Otford Current Situation 100% Abstraction Scenario Action Plan (70% Abstraction) Issue : Banks and Vegetation Indicator : Walks and patbs Access to river generally poor. River flows Encroachment of terrestrial vegetation Increased flow and changes in predominantly through farmland, with earth banks. into main channel. riverain and bank vegetation. Issue : Fisheries Indicator : Brown trout, coarse fishing lakes Grayling and brown trout present, limited Permanent loss of trout. Possible Restoration of natural brown trout. breeding around Otford. Limited fishing in the detrimental effect on coarse fishing river, augmented by coarse fishing in lakes and lakes due to reduced water levels and artificial ponding of trout fisheries. flow. Issue : Terrestrial Ecology Indicator : SSSI Predominantly arable land and improved Possible drawdown on gravel pits at Improves spring flow in upper grassland, of comparatively little wildlife interest. Sevenoaks SSSI, with adverse effects catchment with consequent increase Sevenoaks SSSI is important to woodland and on aquatic biota. in wetland habitat water birds, as well as insects amphibians, reptiles and small mammals. Possible enhancement of feeding area for waterfowl due to exposure of mud. Issue : Aquatic Ecology Indicator : Water crowfoot, watercress, crayfish Upper reaches and spring-fed streams important as Loss of remaining water crowfoot beds, Probably no significant effect. habitat for locally rare species, especially water with a knock on effect on aquatic Recolonisation of main channel by crowfoot and its associated fauna. Both watercress insects and trout populations. Further water crowfoot with subsequent and water crowfoot are present in tbe river. damage to watercress beds. Probable enhancement of environment for - Crayfish believed by some to inhabit tributary loss of remaining crayfish population- game fish. Possible recolonisation by streams. crayfish. Otford to Farningham Current Situation 100% Abstraction Scenario Action Plan (70% Abstraction) Issue : Banks and Vegetation Indicator : Walks and paths Access to river generally good. River flows Encroachment of vegetation in channel, and shift Enhancement of scenic value. through farmland and grassland with earth of bank vegetation to more terrestrial species. and wooded banks. Potential damage to banks and bed by walkers. Loss of paths due to disuse and overgrowth. Issue : Fisheries . ----- Indicator : Brown trout Grayling and brown trout present, limited Potential damage to both river and lake fisheries. Reestablishment of sustainable breeding around Otford. Limited fishing in brown trout and coarse fisheries. the river, though coarse fish are present at Lullingstone lake. Issue : Terrestrial Ecology Indicator : Otford water meadows, Lulllngstone woods and marsh Predominantly arable land and improved Probable damage to or loss of remaining water Potential to increase frequency grassland. The water meadows near Otford meadows at Otford and marsh at Lullingstone. of or extent of inundation of are important for lapwing and snipe. Mature Drawdown of Lullingstone lake. water meadows. woodland and marshy areas near Lullingstone lake support a diverse fauna and flora. Issue : Aquatic Ecology Indicator : Water crowfoot, watercress Watercress beds are present, though water Encroachment of terrestrial vegetation into river Possible recolonisation of the crowfoot has declined. River flows are channel. Further damage to watercress beds. river by water crowfoot, with generally good, thus the biota of the river is associated benefits for aquatic diverse. insects and troat. Source ERM (1993) 70223BI^WRP\B\RDER-Jnl 94\wp 8-7 TABLE 8.5 (cont) Farningham to Dartford Current Situation 100% Abstraction Scenario Action Plan (70% Abstraction) Issue : Banks and Vegetation Indicator : Walks and paths Access to river good. River flow Colonisation and spread of Reduction of channel overgrowth. through amenity grassland and semi vegetation into channel. Potential Potential enhancement of paths improved neutral grassland, with earth damage to riverbanks and bed by due to increased use. and wooded banks. Low flows result in walkers. Loss of paths due to disuse overgrowth of channel. and overgrowth. Issue : Fisheries Indicator : Coarse fish Low flows and dry-bed conditions have Potential loss of coarse fishery at Reinstatement of sustainable adversely affected fish. Important coarse Horton Kirby lakes. brown trout and coarse fisheries. fishery at Horton Kirby lakes. Issue : Terrestrial Ecology Indicator : No specific Indicator Predominantly grassland, with wooded Probable loss of amphibians and Unlikely to be significantly areas near lakes. insects if Horton Kirby lakes lost. affected. Issue : Aquatic Ecology Indicator : Horton Kirby lakes, watercress Low flows result in regular dry-bed Potential loss of Horton Kirby lakes. Removal of in-channel terrestrial conditions. The channel is partly Increased plant growth in mam vegetation. Recolonisation by overgrown by rushes in some areas. channel. Damage to remaining aquatic plants. Increased growth Horton Kirby lakes are the most watercress beds. of watercress. important aquatic habitat along this stretch. Dartford to the Thames Current Situation 100% Abstraction Scenario Action Plan (70% Abstraction) Issue : Banks and Vegetation Indicator : Dartford Parks Banks largely built-up, with good access Possible loss of some marsh and Enhancement of riverside in park areas. Fresh and salt marsh marginal aquatic vegetation in parks. dominant on Hast bank. parkland. Issue : Fisheries Indicator : Coarse Fish General coarse fishery. Estuarine species Probable loss of fishery. Improvement in coarse fishery. present downstream of tidal weir in Dartford. Issue : Terrestrial Ecology Indicator : Marshland Wildlife The marshes support a wide variety of No effect on tidal reaches. No effect on tidal reaches. plants and animals. Issue : Aquatic Ecology Indicator : Dartford Freshwater Marshes and Saltmarsh Marshes largely sustained by water from Potential damage to marshes- Unlikely to be a significant the Cray and the Thames. effect. 70223B1 CAWRP\B\KDER-Jol 94\wp 8-8 Two major areas of wetland landscape beside the Darent deserve special studies of their own: the Dartford marshes (above) and the Shoreham Watermeadows (below). 70223B02/BSD/01/B 8.3.2 Approaches to Benefit Assessment Economic and environmental benefits and costs fall into two broad categories: Benefits from changes in non-marketed environmental assets’, these include improved fisheries and recreational facilities, improved amenities, general improvement in environmental quality. Benefits measurable in market prices. These include gains arising from induced economic activity, employment generation, regional income impacts; changes in land use which may offer benefits or costs. For example, enhancement of low flow in Dartford may improve development prospects in the town, with possibilities of higher rental income and wages through higher quality buildings and developments. Increased numbers of visitors to the region, attracted by the enhanced beauty of the Darent or quality of the fishing, may lead to net income gains in the area and associated multiplier effects. In addition, there may be other direct or indirect benefits associated with improved water flows (eg savings in security costs for ‘moated’ premises in Dartford). These categories are important in that they influence the choice of estimation method. Benefits measurable in market prices can be evaluated through direct estimation of changes in income. For non-marketed environmental assets other methods such as contingent valuations and surrogate market approaches are required. (i) Types of Value Use values of conventional commodities can be measured by observing or estimating changes in market prices and volumes of goods sold, that is from the demand curve for the goods. Where no price is charged, as in the case of open access recreation along the Darent, then value can be inferred from how much people are prepared to pay to gain access to the river with an EAFR compared to the present flow regime. Non-use values refer to the utility people obtain from an amenity such as an EAFR for various reasons other than from visiting it. Option value measures how much individuals are willing to pay for the option to visit the Darent with an EAFR in the future, given their current uncertainty about their demand. Existence values arise from the satisfaction that individuals gain under an EAFR from knowing that the river is protected and exists as a natural habitat for plants and wildlife. Bequest values exist when a person benefits from the knowledge that an environmental amenity will be available for future generations to enjoy. Quasi-option value is used to measure the value of an irreversible change such as the extinction of a species or the irreversible loss of a habitat: however, the latter value does not appear to be applicable to the Darent. 70223B10V,WRP\BVRDER-I«l 94\wp 8-9 (ii) Techniques for Valuation The majority of non-monetary environmental effects were valued using the contingent valuation method (CVM). This technique was used as it has the following advantages: it can be used to estimate directly the benefits of change in river flow (without the need for comparison to other similar rivers); it covers all groups benefiting from an EAFR: users (visitors and residents) and non-users (rest of the general public). No other single valuation method can do this. it can estimate the values of different types of benefits: consumer surplus attached to use; option value; amenity value; existence and bequest values. it can measure marginal benefits and costs for changes in water quantity and quality from an EAFR. The use of CVM has been endorsed in the US by an expert panel in a report to the Natural Oceanic and Atmospheric Administration (NOAA) and its guidelines were followed by the consultants. (iii) Valuation of Market Priced Benefits and Costs not Covered by Survey Methods However, a number of potential benefits or costs could not be directly captured by the CVM and some additional valuation methods were used; these were: increase in commercial property value resulting from the improved development potential; in practice, development is restricted within the Green Belt, and improvement in the river is likely only to affect commercial properties in Dartford. The impact has been assessed qualitatively through discussions with local estate agents; profits from increased use of local tourist and recreational facilities, as a measure of local benefits of increased visitor numbers; estimates of changes in visitor numbers under the two scenarios of change were derived from the CVM survey; effect on the number and quality of educational visits to the river; increased congestion or visitor management costs, based on estimates made by the English Tourist Board and other organisations; increased flood risk to commercial, residential and agricultural property; effects on existing local industries, in particular cress farming. 70223B10V.WRP\B\RDER-Jnl 94\wp 8-10 In addition, there are two cases where it was thought that estimation in parallel with CVM could be useful to provide a cross-check on CVM estimates: residential amenity where property price approaches may provide a cross-check on the CVM estimates; improved fisheries where estimates of income from additional licences or increased licence fees can provide information on the revenue implications of the alleviation of low flow (ALF) scheme. Table 8.6 summarises the types of benefits and costs which may be associated with restoration of EAFR, and notes the approaches which have been used to estimate their values. 8.3.3 Results of the Contingent Valuation Survey (i) Introduction A detailed description of the CVM survey and analysis is presented in full in Annex IV. This section presents the key points of the methodology, and summarises the main conclusions. The CVM survey was constructed to elicit willingness to pay (WTP) values for the two scenarios noted in Section 8.1 compared to the baseline scenario of 100% abstraction: maintenance of the current situation, (with abstraction of 70% of the licensed abstraction volume); and improvement in the river to give an ecologically acceptable flow regime (ie the regime which would result from implementation of the Action Plan). The WTP to pay in each case reflects the value to residents, visitors and the general public. In constructing the survey two WTP questions were used: firstly to assess the WTP for low flow alleviation in all 40 affected rivers in England and Wales; secondly to elicit WTP for the Darent specifically. This is important since recent work has demonstrated the dangers of the ‘embedding’ or ‘part whole effect’, whereby individuals find it difficult to distinguish between what they would pay for a single river and what they would pay for all rivers. By asking for total WTP for all low flow rivers, the parameters of the question are set, and the respondent will take account of the initial value given in assessing the WTP to pay for a specific river. In both cases, the payment vehicle, ie the way in which it was suggested that the money would be paid, was water rates, as it is important to specify a realistic means of paying. 70223BI0\WRP\B\RDER-Jnl 94\wp 8-11 TABLE 8.6 Method of Estimation of Benefits and Disbenefits (Costs) Impact Benefit or Cost Affected Valuation Method B enefits Archaeological sites Preservation value Of small value Changes in ecology Biodiversity value Not quantified Residential amenity Value placed by residents on improved Darent or Increased residential property CVM results cross-checked by discussions with estate agents (HPM not considered appropriate) Development potential Increased commercial property Estimation based on discussions with value estate agents Recreational amenity Value placed by visitors on CVM (visitors survey) improved Darent and Value placed by non-visitors on CVM (general public survey) improved Darent (option value) Improved fisheries Value placed on improved fishing CVM (visitors survey) by recreational anglers Additional income from fishing licences Insignificant as most fishing clubs operate on a cost recovery basis Local economy Profits from increased use of local Estimates based on number of tourist and recreational facilities additional visitors and expenditure ■nd Local multiplier effects Not included Educational value Value placed by parents on Estimate based on number of visits and improved educational attraction of parents* contributions the Darent Other non-use benefits Existence value CVM (general public survey) C osts Risk of flooding and subsistence for Change in property value Not considered significant relative to domestic, commercial and agricultural market changes. property or Expected cost of damage Estimates based on flood damage tables Construction impacts Various local environmental cosu Fish spawning grounds Biodiversity value Not quantified Development potential Environment impacts of increased Included in CVM (residents survey) development - land use, ecology, noise Recreational disamenity Congestion, noise Included in CVM (residents survey) Source ERM (November 1993) 70223Blfl\WRP\B\RDER-Jnl 94\wp 8-12 In conjunction with the questionnaire, respondents were presented with booklets giving general information on the problem of low flow rivers in England and Wales and specific information on the River Darent. The various policy options open to the NRA were spelt out: do nothing and allow increases in abstractions to the licensed values and so reduce flows further; continue with current abstraction levels; reduce abstractions so that flows in the Darent increased. Each scenario was depicted by a series of photographs and text, illustrating the effect of the three different flow levels on the appearance of the river, its ecology, and recreational opportunities. Maps of the River Darent showed the effect of the different flow regimes along the course of the Darent, in terms of flow and periods over which flow occurred. (ii) The Sample The survey sought WTP from three distinct populations reflecting three broad interest groups: Local residents (325 interviews), defined as those whose utility and lives are affected by the Darent on a daily basis. Operationally, residents were defined as those living within 2 km and between 2 and 4 km of the Darent. The 2 km range encompasses the immediate valley of the Darent, although in fact the river is not visible over the whole of this area. The sampling frame was mainly concentrated in the 2 km radius but also included a small number of locations in the band 2 to 4 km from the river. Visitors (335 interviews) to the Darent based on households who visited the river and engaged in some activity connected with the river (eg angling, picnics, enjoying riverside pubs and villages or using the Darent Valley Path) in the period late June to early August covering school holidays and term time. The general public (758 interviews); these interviews covered those people who had not visited the Darent during the preceding 12 months, but who might in fact be willing to pay for improvement in its flow. This provides an estimate of non-use values. The general public sample was drawn from 44 locations throughout south-east England, up to a distance of 60 km from the Darent. The locations were selected to provide a wide cross-section of settlement types and sizes, in an area from as far north as Braintree and Luton; west to Slough and Farnborough; south to Brighton; and east to Canterbury. In total 1418 personal interviews were successfully completed. 70223BI<7VWRP\B\RDER-Jal 94\wp 8-13 (iii) The Results The survey yielded considerable information about utilisation and preferences for activities along the river and which is presented in full in Annex IV. The survey revealed that the majority of the population (72.3% of households) visit a river at some time during the year, and walking, visiting riverside villages and observing wildlife are the most popular recreational activities. In addition, awareness of the problems of low flows in general among the general public was high (92% claimed to be conscious of the issue); not surprisingly, awareness of the problems of the Darent was highest among residents (97%), with a lower percentage of visitors (72%) claiming to be aware of the problem. The survey results show that a large proportion of the population, whether residents, visitors, or the general public, are willing to pay through increased water rates to maintain current flows in rivers and a slightly smaller proportion are willing to pay to increase flows from current levels to an EAFR. They are also willing to assign a part of this WTP for maintenance or improvement of flows in all rivers to the River Darent. The main results are summarised in Table 8.7 which shows the mean WTP for each category of respondent for all rivers and for the Darent alone. TABLE 8.7 WTP of Residents, Visitors and General Public for Low Flow Alleviation (£ per household per year) Residents Visitors General public (£) (£) (£) Maintaining current flows in all 40 rivers: Mean W TP 18.45 15.06 17.18 Standard deviation 24.37 20.37 23.38 Increasing flows in all rivers: Mean W TP 12.32 9.76 12.92 Standard deviation 18.07 13.99 19.60 Maintaining current flows in the Darent: M ean W TP 10.19 7.16 3.85 Standard deviation 19.80 16.81 8.19 Increasing flows in the Darent: Mean 6.25 4.85 3.00 Standard deviation 12.40 11.19 8.45 70223B1^WRP\B\RDER-Jiil 94\wp 8-14 The mean annual WTP values per household were then aggregated across all households in the respective populations from which the samples were drawn. These aggregate estimates are given in Table 8.8 and are based on the following assumptions concerning number of households in each population. The total number of residents was estimated from enumeration district data from the decennial census of population produced by the Office of Population Censuses and Surveys. The total number of households within 2 km distance from the river was estimated at 35 444. Although the sample included some households in the 2 km to 4 km band the total number in the 2 km band was adopted as the tighter and more conservative definition. Neither local authorities nor other agencies in Kent had undertaken surveys of visitors to the Darent. For this reason, total visitor numbers were derived from the random sample survey of the general public in which respondents were asked if they had visited the Darent during the preceding 12 months. This enabled estimates to be made of the number of households in each distance zone from the Darent that visited the river. The annual number of households visiting the Darent each year from distances up to 60 km was estimated at 318 456. The total number of general public households was derived from the numbers resident in the zones from 2 km to 10 km up to 60 km distance from the river and was estimated to be over 3 million. This procedure ensured that a conservative estimate of aggregate non-use benefits was obtained, avoiding the greater uncertainty about the number of households in the rest of England and Wales who might have a non-use value for tow flow alleviation in the Darent. The estimates of WTP for this group of non-users are those expressed by the general public who indicated that they had not visited the Darent within the last 12 months. From Table 8.8 it can be seen that: aggregate WTP for maintenance of the current flow is higher than for improvement of the flow regime; for both scenarios, WTP of the general public far exceeds that of visitors and residents combined; total mean resident and visitor benefits resulting from the Action Plan exceed £2 million per year. 70223B1(AWRP\BVRDER-Jal 94W p 8-15 TABLE 8.8 Aggregate Benefits of Low Flow Alleviation in tbe River Darent (£’000 per year) Policy to maintain Policy to improve Total benefits current flow current flow (mean) (mean) (mean) Residents 256.4 148.4 404.8 Visitors 1 049.1 569.2 1 618.4 General public 11 588.6 8 747.9 20 336.5 Total 22 359.7 8.3.4 Results of Market Price Valuations As previously discussed in Section 8.3.2, there are a number of potential effects which could not be captured by the CVM. These, assessed using market place based approaches, in general, are very small in relation to the benefits estimated by means of the CVM, with the resulting valuations being as follows: Improvements in the flow regime will result in an increase of commercial property rental value in Dartford of roughly £13 500 per year with a corresponding decrease in the case of increased abstraction. Profits earned by local businesses would decrease by £37 500 per year under the 100% abstraction scenario and would increase by £18 750 under the Action Plan scenario. Loss of educational value would be £5 600 to £10 000 per year under increased abstraction. The benefits of improved flow have not been evaluated as it is not possible to predict whether the number of educational visits would increase as a result. The annual average additional flood damage resulting from improvement in the flow regime would be in the range £2 000 to £30 000, although these values are believed by the NRA to be conservatively high. Other types of impact were examined but were found to be insignificant. These are as follows: impacts on residential property values; increased income to fishing societies; impact on local cress farmers. 70223B1 Table 8.9 summarises the estimated benefits and disbenefits associated with the two alternative scenarios, taking the 100% abstraction scenario as the baseline. These estimates are in present value terms, having been discounted at 6% over 20 years with an initial four year phasing-in period. TABLE 8.9 Discounted Benefits and Disbenefits of Avoiding 100% Abstraction Scenario - Summary Table (£ million, 1993 prices) Securing Securing 70% abstraction Action Plan (£ million) (£ million) Residents 2.6 4.1 Visitors (recreational benefits) 10.6 16.4 Educational 0.06 0.06 Commercial property 0.1 0.1 Other - net revenues to local business 0.4 0.6 Flood damage (0.09) Total benefits - use values 13.8 21.2 General public - non-use values 117.1 205.6 Non-use values refer to existence and option values. 8.4 Net Present Value Calculations 8.4.1 Introduction Taking the 100% abstraction scenario as the baseline, the costs and benefits have been estimated as: formalising 70% abstraction through legalisation of the current voluntary situation; implementing the Action Plan The main assumptions used in the analysis are summarised in Table 8.10. 70223B10VWRPVB\RDER-Jn) 94\wp 8-17 TABLE 8.10 Key Assumptions in Cost/Benefit Evaluation Item Values used in base case Discount rate 6% Volume of water displaced 23.6 Ml/d Evaluation period 20 years Timing of benefits Initial phase in period of 4 years in which benefits increase by 25% each year. Evaluated over 20 years Phasing of capital costs NRA costs phased over 4 years TWUL costs (discounted total provided by TWUL) Phasing of operating costs Initial phase in period of 4 years in which both NRA and TWUL costs increase by 25% each year Prices All prices expressed in constant 1993 terms 8.4.2 Principal Results The costs and benefits of the 70% abstraction scenario and the Action Plan are presented in Tables 8.11 and 8.12 respectively. These show the full costs and benefits of increasing river flow from the 100% abstraction scenario. From these tables it can be seen that: both scenarios are economically justifiable on cost-benefit grounds on the basis of use values alone; the inclusion of existence values significantly increases the net benefits of both scenarios. 70223B10VWRPVB\RDER-Jal 94\wp 8-18 TABLE 8.11 Costs and Benefits of Implementing the Action Plan Item Excluding non-use(1) Including non-use values values (£ million) (£ million) Total discounted costs 15.0 15.0 Total discounted benefits 21.2 226.8 Benefit cost ratio 1.4 15.2 Net present value 6.3 211.8 Note : (1) Non-use values refer to existence and option values. TABLE 8.12 Costs and Benefits of Securing 70% Abstraction Item Excluding non-use(1) Including non-use values values (£ million) (£ million) Total discounted costs 8.2 8.2 Total discounted benefits 13.8 130.9 Benefit cost ratio 1.7 15.9 Net present value 5.6 122.7 Note : (1) Non-use values refer to existence and option values. 8.4.3 Incremental Net Benefits of the Action Plan The incremental costs and benefits of moving from the 70% abstraction scenario to the Action Plan scenario can be calculated from Tables 8.11 and 8.12 and are shown in Table 8.13. The net present value (NPV) of this incremental stage is positive both with and without non-use values. While a high proportion of benefits are associated with the formalisation of the 70% abstraction scenario, the additional measures involved in the Action Plan are still justifiable on cost benefit grounds. 7022361U\WRP\B\RDER-Jnl 94Wp 8-19 TABLE 8.13 Incremental Costs and Benefits of the Action PIan(1> Excluding non-use Including non-use Item values values (£ million) (£ million) Incremental discounted costs 6.7 6.7 Incremental discounted benefits 7.4 95.8 Benefit cost ratio 1.1 14.2 Net present value 0.7 89.1 Note: (1) Incremental costs and benefits of moving from 70% abstraction scenario to full implementation of the Action Plan. Sensitivity analysis was undertaken to determine the implications of changes in key assumptions on the analysis results. The key findings are as follows: Estimated non-use values are considerably higher than use values, even through they represent conservative estimates given that they correspond to the population within a 60 km radius from the Darent rather than all of England and Wales. Inclusion of only a small percentage of aggregate non-use benefits increases the net present value of both scenarios significantly, with there being substantial incremental benefits of moving to the Action Plan. For example, inclusion of only 10% of non-use values raises the net present value of the Action Plan from £6.3 million for use values alone to £26.8 million. Capital costs to TWUL would have to increase by significantly more than 20% for the economic viability of either the 70% abstraction scenario or the Action Plan to be affected. As capital costs to TWUL are assumed to be the same under both scenarios, the incremental NPV associated with the Action Plan are not affected. The results are quite sensitive to a 20% increase or decrease in NRA capital costs, with the incremental NPV excluding non-use benefits becoming negative under the higher cost assumption and doubling under the lower cost assumption. Although it has not been possible to consider the incremental costs and benefits with the individual measures comprising the Action Plan, the implications of deferring the BCI augmentation pipeline have been considered. Deferring the pipeline by four years reduces the discounted costs of the Action Plan by roughly 7%. 70223BKAWRP\B\RDER-Ja! 94\wp 8-20 8.5 Conclusions The above analysis has shown that: implementing the Action Plan is likely to yield net benefits; securing of the 70% abstraction option would result in new benefits considerably lower than that of the Action Plan if non-use values are included - if these are excluded the differential is lower; analysis of the incremental costs and benefits of the Action Plan shows that there are net benefits to be gained in going further than the securing of 70% abstraction and taking the additional measures contemplated in the Action Plan; sensitivity analysis has shown that these estimates are based on conservative assumptions. Two factors are crucial in the interpretation of the results: non-use values; the assumptions made about the measures which TWUL would take to make up the resource shortfall in the 70% abstraction scenario. The non-use values have proved to be considerably higher than the use values even though they represent a conservative estimate given that aggregation has been over a 60 km radius of the Darent rather than over the whole of England and Wales. Even if only a small percentage of the aggregate non-use benefits are included, the net present value of the Action Plan in both absolute and incremental terms increases dramatically. With 10% of these values included the net present value of the Action Plan rises from £6.3 million in the base case to £26.8 million. In moving from the 100% abstraction to the 70% abstraction it has been assumed that TWUL would take similar actions to make up the shortfall as in the Action Plan. Sensitivity analysis has shown that if TWUL opts instead for development of new groundwater sources, the incremental benefits of the Action Plan will be considerably higher. Figure 8.2 has been included to illustrate the range and change of net benefits involved in moving from the 100% abstraction scenario to the Action Plan via the 70% abstraction scenario. What is not immediately evident is that the 70% scenario does not improve on the critical situation which presently exists in the River Darent. In other words the 70% option does not solve the problem. It only maintains the status quo. 70223BI G\WRP\B\RDER-Jal 94\wp 8-21 Figure 8.2 Range of NPV Benefits for Action Plan and 70% Abstraction Options CHAPTER 9 CONCLUSIONS AND RECOMMENDATIONS 9.1 Conclusions The Problem (i) The cause of the reductions in flows in the River Darent is substantial over-abstraction of the Chalk and Lower Greensand aquifers which provide the base flow to the river. (ii) Thames Water Utilities (TWUL) holds the majority of the 155 Ml/d licences issued for the Darent catchment and it is the six wells sited closest to the River Darent which are mainly responsible for the rapid depletion which occurred in the 1950s and 1960s. (iii) Up to 1950 the abstractions were not excessive. The problem is therefore a recent phenomenon and not one of long standing. (iv) Water balance studies have shown that in a dry year, which would occur once on average every five years, existing abstractions exceed recharge by 30%. (v) Existing TWUL abstractions from the six riverside wells amount to 70% of those authorised. Overall existing abstractions in the catchment amount to 75% of those authorised. (vi) Should water companies abstract at a rate of 100% of their licence values then in an extreme drought such as occurred in 1976 the river would contain only 1% of the flow which would have occurred without abstraction and the river would be dry for 75% of its length. The Hydrology (vii) An integrated model of the groundwater and surface water sections has been set up which accurately simulates the main elements of the hydrological processes active in the catchment. Of particular importance is the excellent calibration obtained in reproducing low flows and base flow recessions throughout the 20 years of simulation and over the different seasons. (viii) There are two main aquifers: the Chalk and Lower Greensand. The former underlies the mid and upper catchment of the River Darent, the latter the upper catchment. The base flows arc less affected by groundwater abstraction in the lower catchment. Above the Chalk the river is often perched with the watertable well below the bed of the river. 70223B10\WRP\B\RDER-Jul 94\wp 9-1 (ix) The perched river conditions, which occur below Lullingstone because of over-abstraction of groundwater, results in the river losing water at a gross rate of around 1.1 Ml/d per kilometre, the losses between Lullingstone and Hawley are around 11 Ml/d during summer months. (x) The model has simulated natural flows assuming no groundwater abstractions. Under these conditions the flow at Hawley would never have fallen below 60 Ml/d. With historic abstractions the mean flow at Hawley was 9 Ml/d and the river was effectively dry on over 900 days. Environmentally and Ecologically Acceptable Flows (EAFs) (xi) A series of studies has been made to determine the acceptable minimum flows in the River Darent: Dossor (1978) - 17.3 Ml/d (not linked to ecology) Halcrow (1988) - 30 Ml/d (throughout length) W S Atkins (1990) - 23 Ml/d (EAFR 95 RIVPACS method) IH/IFE (1993) - 27 Ml/d (absolute minimum PHABBSIM) The above flows are for Hawley in the lower reaches. (xii) The ecological studies recommend that the natural flow accretion profile should be maintained throughout the river’s length. The model enables naturalised flow to be determined. (xiii) During the TWUL/NRA (1992) studies a ‘target minimum flow’ was established based on naturalised flow, which met EAF standards and maintained a natural flow accretion profile. The selected minimum flow is half the 1 in 20 year natural low flow and amounts to 30 Ml/d at Hawley. This flow not only meets the ecological standards but is achievable by abstraction reductions and flow augmentation. Means of Alleviating Low Flows (xiv) Over 13 different options were investigated, Halcrow (1986, 1987, 1988) and GDC (1991), ranging from short-term measures to long-term solutions including augmentation wells, re use of sewage effluent, seasonal storage in lakes, replacing groundwater with surface water abstraction at Dartford, conjunctive use of Thames valley water with groundwater, bed lining, bankside wells and demand management. All options were costed but recommendations deferred until the catchment model was developed (GDC, 1991) (xv) The catchment model (TWUL/NRA, 1992) showed that reduced abstractions from the Lower Greensand produced an immediate and equivalent increase in river flow which could not be matched by similar reductions in Chalk abstractions. The model also demonstrated that the 70223B1(AWRP\B\RDER-J«1 94\wp 9-2 immediate benefits of reducing abstraction from the Chalk decreased, moving towards the lower reaches of the catchment. (xvi) None of the abstraction reduction strategies provided the full target flow requirements throughout the period of historical simulation. An additional augmentation scheme capable of supplying at least 24 Ml/d was therefore included. The Action Plan allowed for up to 31 Ml/d for planning purposes to compensate for increased abstractions within the catchment by other water companies and uncertainties in estimating the EAFR requirements and returned flow from reduced abstractions. The Action Plan (TWU/NRA, 1992) (xvii) A sophisticated package of measures was prepared to return the river to an acceptable flow regime by meeting a minimum Target Flow. Based on GDC (1991) studies, five options were shortlisted which included re-use of sewage effluent at Long Reach Dartford, riverside augmentation boreholes, demand management, reduced abstractions and use of BCI Quarry dewatering water. The recommended Action Plan included: reduced abstractions from Lower Greensand and mid-catchment Chalk boreholes; transferring Chalk abstractions from mid to lower catchment; river augmentation in low flow years using artificial springs and BCI water (pipeline link); conjunctive use of Thames reservoir supplies, transferred by the London Ring Main to reduce groundwater abstraction in normal years; demand management. (xviii) The Action Plan provides the least cost solution for returning the River Darent to an ‘Environmentally and Ecologically Acceptable Flow Regime’. NRA Capital Costs to implement the Action Plan are estimated to be: artificial springs, £1 million; BCI pipeline, £5 million. TWUL Capital Costs for replacing lost borehole capacity with new source supply and infrastructure changes are estimated to be: source redeployment, £3.6 million. 70223B10\WRP\B\RDER-Ju) 94\wp 9-3 Cost Benefit Analysis (xix) A contingent valuation survey (CVM) based on willingness to pay (WTP) gave aggregate benefits of £2.0 million/annum to prevent further deterioration of the river and improvement to the Action Plan least cost solution. If the general public non-use WTP benefits are included then the £2.0 million/annum would increase by a further £20.4 million/annum. Discounted net present value benefits for use and non-use were estimated to be £21.2 million and £206 million respectively. Benefit cost ratios for the use, combined use and non-use benefits are estimated to be 1.4 and 15.2 respectively. (xx) Conclusions from the cost benefit analysis are: the Action Plan will yield net benefits; sensitivity analysis shows that benefits are based on conservative assumptions. 9.2 Recommendations The Action Plan has been shown to be the least cost solution for returning the River Darent to an environmentally and ecologically acceptable low flow regime. It has been shown also to be economically viable on the basis of a contingent valuation survey and a cost benefit analysis. In depth technical studies, including a sophisticated integrated surface water and groundwater model of the catchment, have demonstrated that the Action Plan represents the most effective technical solution to alleviating low flows. The need for the BCI pipeline in Stage II of the Action Plan, depends on an evaluation of the measures implemented during Stage I: reduced, and relocated, ground water abstractions and installed bankside augmentation boreholes. The investigations needed to determine if additional supplies are required are described in Appendix B. Fundamental to the success of the recommended plan has been the agreement between the NRA and TWUL to formalise a conjunctive use scheme, to integrate the operation of Thames reservoir water with groundwater abstractions. Agreement has also been reached on setting up a ‘Water Resource Management Scheme’ for catchment and demand management. The way forward for the River Darent depends on co-operation between TWUL and the NRA. This has now been successfully achieved and formalised in the River Darent Action Plan. It is strongly recommended, without reservation, that the River Darent Action Plan should be implemented. 70223B10\WRP\B\RDER-Jul 94\wp 9-4 REFERENCES W S Atkins Consultants Ltd 1991 River Darent: 1990 Drought Ecology Survey (WSA) and Environmentally Acceptable Flow Regime Study, NRA Southern Region John Dossor & Partners (with June A Desk Study for the Water Management of Associated Engineering 1978 the Darent Valley Consultants) Environmental Resources Nov An Economic Analysis of the Benefits Management (ERM) 1993 Derived from Alleviation of Low flows in the River Darent, NRA Southern Region. Giles N, Phillips V E, Barnard S 1991 Ecological Effects of Low Flows on Chalk Streams Groundwater Development Nov Darent Catchment Investigation. Consultants Ltd (GDC) 1991 Pre-Feasibility Report, NRA Southern Region Groundwater Development July Groundwater Study at Blue Circle Cement Consultants Ltd (GDC) 1992 Northfleet Works, BCI Groundwater Development Nov Darent Catchment Investigation. Catchment Consultants Ltd (GDC) 1993 Model Calibration Report, NRA Southern Region Groundwater Development April Darent Catchment Investigation. Options for Consultants Ltd(GDC) 1994 Restoring River Flows - Catchment Modelling Assessment Report, NRA Southern Region. Groundwater Development March Darent Catchment Investigation. Visualising Consultants Ltd (GDC) 1974 Restored Flows - Completion Report, NRA Southern Region Sir William Halcrow & Partners Study of Alleviation of Low River Flows Resulting from Groundwater Abstraction May Interim Report (Main Report) 1987 Interim Report (Annex A, River Darent Case Study) April Final Report Volume 1 - Main Report 1988 Final Report Volume 2 - Darent Case Study Feb Report on Implementation of Schemes 1989 Institute of Freshwater Ecology 1993 Ecologically Acceptable Flow in the River (IFE) and Institute of Hydrology Darent, NRA Southern Region (IH) Institute of Freshwater Ecology 1993 Ecologically Acceptable Flows. Assessment (IFE) and Institute of Hydrology of Instream Flow Incremental Methodology, (IH) NRA R&D Report 282/1/WX Institute of Geological Sciences 1975 Records of wells in the area around Dartford: (IGS) inventory for one-inch geological sheet 291. Mott MacDonald (MM) Oct A Strategy for the Enhancement of the River 1992 Darent - Final Report, NRA Southern Region TWUL/NRA Nov Plan for the Darent 1992 70223B10\WRP\B\RDER-Jul 94\wp APPENDIX A APPENDIX A RELEVANT NRA AND TWUL LEGISLATION The relevant legislation pertaining to the Darent issues are summarised below: (i) NRA Water Resources Act 1991. Section 19(l)a conserving, redistributing or otherwise augmenting water resources in England and Wales. Section 19(l)b securing the proper use of water resources in England and Wales. Section 15(1) to have particular regard to the duties imposed on water undertakers under the Water Industry Act 1991. Section 16(1) to further the conservation and enhancement of natural beauty, flora, fauna and geological or physiographical features of special interest. Section 20(l)a enter into and maintain arrangements with water undertakers for securing proper management or operation of waters which are available. Section 52(1) powers to revoke or vary abstraction licences. Section 61(1) provides for compensation to be paid in the event of losses sustained by water undertakers where the abstraction licence is modified by the Secretary of State following the NRA’s proposals. 70223Bl Water Industry Act 1991. Section 3(2) to further the conservation and enhancement of natural beauty, flora, fauna, and geological and physiographical features of special interest. Section 37 developing and maintaining an efficient and economical water supply to persons on demand. 7022381 OVWRP\B\RDER-Jol 94\wp A-2 APPENDIX B APPENDIX B PROGRAMME OF INVESTIGATION LEADING TO DECISION ON BCI PIPELINE B.l Introduction The implementation of the restoration of ecologically and environmentally acceptable flows in the River Darent, the Action Plan, is to be undertaken in two stages. During Stage I reductions and relocation of licensed abstractions will be put in place by TWUL, together with the installation of augmentation bankside boreholes by the NRA. Until trials are undertaken, however, during and after the construction of the augmentation boreholes, and supplemented by computer runs and recalibration of the existing integrated catchment model, a decision on the need for additional augmentation water imported from outside the catchment through the BCI pipeline cannot be taken. To provide an indication of the additional work required to arrive at a decision, an outline programme has been prepared. This is presented in this appendix. Since the publication of the draft ‘River Darent Low Flow Report (NRA, December 1993), a project manager has been recruited by the NRA, who will have responsibility to oversee this work programme. Also specialist consultants will be employed to integrate the field investigations, well testing flow measurements and groundwater level data collection etc, and to use the data in the integrated catchment model and to present firm conclusions. The NRA will have responsibility for the field investigations. The programme presented here is inevitably provisional and will change as the implementation of the Action Plan progresses. It does, however, give the framework and timetable of the probable programme and indicates the integrated role of the different authorities. It also demonstrates the complex nature of the work involved. B.2 Computer Modelling (i) Target Flow Regime A series of model runs is required to supplement those undertaken during the Action Plan negotiations. These supplementary runs are summarised below: Additional computer programming to enable real time operation of the augmentation wells to meet target flows. 7O223B10VWRP\B\RDER-Jnl 94\wp B-1 Comparison between the Action Plan scenario with all other wells operating at historical abstractions of 90% or 100% of licensed abstractions; 90% probably the most realistic with 100% as the worst possible case. Comparison between the effect on augmentation requirements for meeting a single annual series 1 in 20 year minimum target flow or 12 monthly target flows. Comparison between 6 x 4 Ml/d, 6 x 5 Ml/d and 8x5 Ml/d outputs from augmentation wells. Comparison between full augmentation supplied by bankside wells or by BCI pipeline or 15 Ml/d and 20 Ml/d from BCI pipeline with the remainder from wells. Optimising the location of BCI pipeline outlets. This work would probably require 20 to 30 model runs. (ii) Assessment of Greensand Recovery The response time of the Lower Greensand aquifer to the closure of the Sundridge borehole is important to the rate of recovery of flows in the River Darent. The recalibrated model will be run to compare simulated results with the trial data for the effect on both river flows and groundwater levels. Because of the heavy recharge during the autumn of 1993 and the winter period, it is possible that the effect of closure and start up of the Sundridge borehole will be masked by high river flows and significant groundwater recharge. From May 1994 year it may be necessary to increase abstractions by up to 18 Ml/d. It has been recommended to raise the increase in two steps: 1 Ml/d to 6/8 Ml/d 6/8 Ml/d to 18 Ml/d with as long a period between the two steps as possible. (iii) West Kent Abstractions The Action Plan assumed that West Kent boreholes at Cramptons Road and Kemsing would be restricted to historic levels. It is possible that the AMP 2 negotiations may lead to a reduction from these boreholes to 70% of licensed values. The catchment model will be used to determine the implications to river flows and river augmentation of West Kent abstractions. An agreement is being sought to limit Kemsing abstractions to 70% of licensed value for the next five years. 70223B1CNWRP\B\HDER-Jn] 94\wp B-2 (iv) Lower Darent and BCI Investigations The Action Plan Stage I proposes increasing groundwater abstractions in the lower Darent catchment whilst reducing abstractions in the upper catchment. Dewatering of BCI quarries and transfer of additional supplies by pipeline to the upper catchment to augment River Darent flows may also deplete groundwater in the lower catchment. Both these proposals, therefore, may have detrimental side effects: reducing yields of existing boreholes; limiting the proposed increased yields of TWUL wells; causing intrusion of saline and/or polluted water; reducing surface flow in the lower catchment. Field studies are required to refine the lower Darent (including BCI area) groundwater model and to recalibrate it. This will include: calibration of the model in transient mode; production runs to investigate the effect on the hydrological regime of dewatering requirements; investigations of the possible intrusion of saline water from the Thames and pollution transported from the Blue Lake area; production runs to evaluate the revised abstraction regime on piezometry in the lower Darent catchment. A preliminary review of the effect of dewatering during the deepening of BCI quarries will be undertaken during earlier model runs covered in Section B.2. B.3 Assessment of Lower Greensands Recovery (i) Scope of Work Data requirements for the assessment include: daily rainfall for the Sundridge gauge; mean daily flows for the Otford gauge; 70223B1^WRP\B\RDER-Jnl 94\wp B-3 mean daily water levels for the two temporary gauging stations at Park Farm and Sundridge together with stage discharge relationships; monthly (or more frequent if available) water level measurements for the long-term observation wells at Foxwold (441 355 001) and Riverhead (TQ55/1); data from the data loggers set up at approximately 11 observation well sites during the shutdown; data from loggers set up in wells at Brasted and Sundridge by TWUL during and, possibly in the case of Brasted, for a period prior to the shutdown; monthly abstraction data for pumping stations at Westwood, Brasted, Sundridge, Oak Lane, Cramptons Road and Kemsing; at present consultants have a long-term record for rainfall, river flows, observation wells and abstraction to 1990. This needs to be brought up-to-date. Analysis of the data would comprise the following: (ii) Analysis (a) River Flows The river flow recession observed at Lullingstone, Otford and Hawley after about 16 October 1993 and starting about two weeks after the shutdown would be compared with previous recessions with flows at approximately the same level and/or occurring in the autumn. The purpose would be to determine whether a recovery or reduction in flows due to the shutdown or start-up are evident, on the flow recession in late 1993 and summer 1994. Flow recession and other flow characteristics would be compared for the two temporary gauging stations and the Otford gauging station. (b) Groundwater Hydrographs The responses to the shutdown would be compared at observation wells in different areas (and aquifers) in the Lower Greensand. Long term records for Foxwold and Riverhead would be examined to see if there were any significant changes in groundwater level response following the shutdown. (c) Computer Runs Computer simulations would be undertaken over the periods of shut down and start up to compare simulated river flows and groundwater levels with recorded data. 70223BKAWRP\3\RDER-Jal 94\wp B-4 B.4 Bankside Wells for Augmentation (i) Testing and Monitoring of Two Existing Wells The two wells and their approximate yields are as follows: Eynsford School 50 to 60 1/s Farningham 30 to 40 1/s Yields need to be confirmed from analysis of test data or by undertaking new tests. The sites are about 1 km apart. The proposal is to carry out augmentation trials in 1994 or 1995 as a pilot for the full scheme. The outfall from the two wells will be constructed as a prototype to demonstrate how the augmentation water from the wells or pipeline would be introduced into the river. The following detailed work item/programme would apply: (a) Pump sizing and design of pipelines/outfalls, power source (mains or diesel generator with noise reduction) production of tender documents, tendering, evaluation and award of contract. (b) Construction of new observation chalk boreholes to monitor the effects of pumping on groundwater levels are advised. Suggested locations are: close to the river between Eynsford and Farningham and just downstream of the M20: on outcrop chalk on either side of the valley between Eynsford and Farningham. These sites may present difficulties of land negotiation and may not be possible before late 1994 or early 1995. (c) Fit transducers to all boreholes/wells including augmentation wells. (d) Construction of temporary weir to supplement the main gauging stations. Suggested locations are: Eynsford near existing temporary structures; near Eynsford Mill weir; between Eynsford and Farmingham; Frank’s Bridge; Westminster Mills, Horton Kirby. 70223B10\WRP\B\RDER-Jol 94\wp B-5 (e) The hydrological situation could be simplified if TWUL could pump from Eynsford and Horton Kirby pumpstations at constant rates (even if a high level) from, say July 1994 onwards. (f) The first few days of augmentation pumping should be monitored as for a constant discharge test. Thereafter water levels could be monitored four times daily by transducer in the augmentation wells. Well yield should be kept constant unless a change is advised. (g) The trials should be run in the period July to September and extended as necessary to coincide with the start of the autumn recharge. Eynsford should be started up in advance of Farningham. Farningham would be started once the effects of Eynsford pumping were established, possibly after a month. To make the trials useful, flows at Lullingstone gauging station need to be of the order of 0.3 m/s or less, for augmentation to form a reasonable, measurable proportion of flow. With the high winter rainfall extending into the spring and summer of 1994 these low flows may not be obtainable. (h) Results could be utilised in refining the catchment model in 1995. (ii) Siting of Five New Wells and Construction These will be installed in 1995 on completion of the field trials of the two augmentation wells and modelling calibration. Additional observation boreholes are needed to give detailed monitoring of the effects of augmentation pumping, as very extensive periods of pumping may arise during extreme drought periods. The observation boreholes would preferably be constructed with the augmentation wells and certainly in advance of the full field trials. Initial tests of new wells would be short yield-proving tests of a few days duration at each site. (iii) Full Field Trials Experience gained from the two existing wells would be applied in planning the trials. This would include determining the level of monitoring of river flows, order of start-up at various well sites and possibly flow levels at which useful results could be obtained. Although dependent on hydrological/weather conditions the trial is not dependent on having extreme drought conditions. Monitoring around the fishery lakes would have to be carefully planned. An appraisal of water level data, hopefully collected following the establishment of lake stage boards, would be required prior to the full field trials. B.5 Lower Darent and BCI Investigations As part of the decision making process on the need or otherwise for a pipeline from the BCI quarries, it is necessary to establish the quantities of water potentially available for augmentation from this source. It is also important to consider the long-term impacts of various quarry dewatering/water use 70223B1QNWRPVS\RDER-Jtt) 94\wp B-6 options on water levels and water quality in the Northfleet area. A detailed hydrogeological investigation was commissioned in early 1992 by BCI, after liaison with the NRA, which led to the preparation of a groundwater model of the Northfleet area as a sub-model within the regional Darent model grid. However, owing to lack of funds and uncertainties over future quarrying options, the investigation was suspended in late 1992, before full calibration of the sub-model was completed. It is now necessary to continue with the modelling and associated investigations in the BCI area. Moreover, it is proposed that this work should be carried out at the same time as the proposed investigations of the adjacent lower Darent catchment, the objectives of which are to establish the impacts on water levels and water quality of possible increases in abstraction in this area (such increases were considered in the Action Plan as a means of partly offsetting the planned reductions in abstraction in the upper catchment). Both the lower Darent and BCI investigations would require construction of monitoring boreholes to look at piezometry near the Thames, and also the modification of the existing modelling to simulate the movement of saline waters. The full scope of the investigations would need to be discussed with BCI but could include: further data collection in the Northfleet area and lower Darent catchment, including water level and water quality data; construction of monitoring boreholes, including multi-level piezometers close to the Thames to investigate the degree of hydraulic connection between the river and aquifer; review of BCI’s future quarrying and dewatering proposals; carrying out quarry dewatering trials (provisional, depending on future excavation/dewatering proposals); confirmation of suitability of water quality for augmentation; refine calibration of regional model, calibrate sub-model transient model; using the groundwater models, investigate the long-term implications of various quarry dewatering and abstraction options on groundwater level and quality in the Northfleet and lower Darent areas. B.6 Identify Flow Losses in River Bed (i) Background The integrated catchment model was calibrated on hydrometric station data recorded at Otford, Lullingstone and Hawley. Good calibration was obtained with long-term means simulated to within 4% of the observed data. Minimum monthly flows were closely simulated. 70223B10\WRP\B\Jf DER-Jnl 94\wp B-7 However, as a result of a spot flow measurement programme undertaken on 22 days at up to 35 locations between February 1991 and October 1993, a wide variation in flows within reaches and between reaches was obtained. For example, the average loss between Lullington and Hawley gauging stations (10.7 km) was about 1 Ml/d per kilometre, whilst from Frank’s Bridge to Westminster Mill (2.2 km) the measured loss on average was 3 Ml/d per kilometre. As a result of these differences it may be necessary to increase the augmentation flow over certain reaches whilst decreasing augmentation over others. (ii) Programme flow records need to be analysed to eliminate measurements which may have been affected by surface runoff; a flow measurement programme should also be set up to re-calibrate the low flow range of the main gauging stations. In particular field work should be undertaken to ensure that low flows do not by-pass gauging station structures; water balance studies on selected fishing lakes; a short report should be prepared to show the sensitivity of the model to the level of bed losses. Consideration should be given to re-calibrating the model to simulate measured losses between Lullingston and Hawley if the measured losses can be shown to be true losses and not indicative of subsurface flow variations along the river; finally, the effect of augmentation wells on river flows should be investigated with a programme of flow measurements, groundwater levels in gravel and chalk and test pumping of augmentation wells. These studies are important to demonstrate that augmentation will be effective. B.7 Feasibility Study of BCI Pipeline A pre-feasibility study of the BCI Pipeline has been awarded to consultants. This study is limited in scope to the following items: identification of most suitable method of abstracting augmentation water from BCI quarries; defining route of pipeline taking account of environmental, financial and technical factors, and land ownership; preparation of outline designs for the pipeline; 70223 B10\WRP\B\RDER -J«l 9*\w p B-8 review outfall arrangements with specific emphasis on river bed outlets preliminary designs to be adequate to construct a typical outfall for an augmentation well in the summer of 1994; assess water quality implications of introducing BCI Water into the River Darent. This study will not evaluate the yield from the BCI quarry or its effect on groundwater levels and groundwater quality. This will be undertaken separately. It will however provide preliminary capital cost estimates and indicate the technical feasibility of transferring water from the lower to upper Darent catchment. B.8 Other Studies In addition to the foregoing work, further studies will be required to ensure that the augmentation wells provide an acceptable solution to the River Darent low flow problem. A major environmental difficulty could arise if the proposed augmentation wells deplete local springs, particularly where they feed existing lakes or water cress beds. The careful location of the wells and an evaluation of their influence on the local hydrological regimes will be required to satisfy local landowners and fishery interests. These studies will be combined with river bed loss studies discussed in Section B.6. During the ongoing studies all new research which assists in determining target flows to meet ecological and environmental objectives will be reviewed for their applicability to the River Darent. B.9 Programme An indication of the work required during Stage I of the River Darent Action Plan before a decision can be made on the need for the BCI pipeline, is given in Table B-l. A tentative programme leading up to this decision is provided in Figure B-l. The monitoring work and field studies will generally be undertaken by NRA staff with the consultants utilising the field data in the catchment model to evaluate and report on the effectiveness of Stage 1 of the Action Plan. The requirement of an additional supply from the BCI pipeline to supplement Stage I improvements will be determined as part of these investigations. Delays in recruitment of staff and appointment of consultants will probably lead to slight delays in completing some of these items of work. Figure B-2, prepared for the implementation of the River Darent Action Plan Consultants Brief, provides a more up-to-date assessment of the consultants role. However, both programmes of work will inevitably be modified once the consultants’ proposals have been received and once the implementation of the Action Plan commences. They should therefore be treated as a guide only of the future work programme and of the phasing of the individual items of work. 70223BHAWRP\B\RDER-JoJ 94\*p B-9 RIVER PARENT ACTION PLAN - PROTOCOL FOR DECIDING NERD FOR PIPELINE FHAIUU d e c i s i o s w o a DVBSHGMI€E KKTBOD TXMBSCALS OOTCOHB HDZCAXXBG SPRJUGS OflUT OR PIPSLDD 1. Target flow regime Augmentation required to meet 12 Re-run model to reproduce AP, AP By March 1994 Augmentation volumes significantly in excess monthly flows along river length. + 1001 abstractions and AP + of AP or significant drawdown in groundwater 1001 abstractions and 12 monthly levels will indicate pipeline. target flows to determine shortfall. I 2. Target flow regime. Adequacy of target flow for Application of latest monitoring By end 1996 If agreed flow targets inadequate, pipeline environmental objectives. and research to the agreed may be needed. targets flow profiles. e o Dcso Mkn o Ne fr C Pipeline BCI for Need on Making Decision forie 3. Greensand recovery - Ret gain in baseflow aa a result of Field trials of cut back with By end 1994 Model indicated 901 recovery in river of cut TWUL cutting back Sundridge and Brasted flow and groundwater monitoring. back abstraction. Each 1 Ml/d short will have to ba made up by axtra springs or pipeline flow. 4. Greensand recovery - AP assumed West Kent would agree to Outcome of AMP 2 negotiations By Sept 1994 If West Kent do not agree to hold West Rent 701. holding present Greensand and submission and OFWAT abstraction at 70| an additional 8 Ml/d abstractions to 70% of licences. decisions. augmentation will be required. 3. Gronnannd recovery - West Kent asked to cut back 4 Ml/d Outcome of AMP 2 negotiations By Sept 1994 If West Kent do agree to a further 4 Ml/d West Kent 4 Kl/d below existing abstraction. and submissions and OFWAT cut back this will reduce need for decision. augmentation over that assumed in AP by 4 Ml/d. Total springs yield dependent on Location/negotiation of sites By end 1994. AP assumed 5/6 sites would ba available - if 6. Number of springs. number of suitable borehole sites for boreholes. not possible to find need for pipeline acquired. increases. 7. Springs Yields. AP assumed yields up to 4 Ml/d per Teat pumping each spring as By end 1995. If individual yields found to ba graatar, spring. construct*d. than need for pipeline reduced correspondingly. 8. springe derogation. AP modelling indicated no apparent Test pumping each spring as By end 1995. Adverse effects on local protected rights effect on protected rights, constructed. will require relocation. Failure to find including fisheries lakes. suitable sites will reduce available yield and increase need for pipeline. 9. Bed losses. Actual losses depart from those (i) Recalibration of low By end 1994. If gauging stations low flow ratings are derived for model. flow ratings at gauging confirmed then model simulation of bed stations. losses satisfactory and AP conclusions supported. (ii) Monitor incremental By end 1995 Greater local loaaes may have to be losses over shorter reaches. augmentation. (iii) Monitor losses d/a each Stage I mid If average losses below springs are more artificial spring during 1995. Stage II than aaaumed then extra springs or pipeline test pumping. by end 1997(?) will be needed. • • • • • • • • • • • • FEATURE DECISION AREA INVESTIGATION METHOD TIMESCALE OUTCOME INDICATING SPRINGS 0NLT OR PIPELINE 10. Lover catchment chalk Piezometry in Chalk below Dartford U) Additional observation By end 1994 If not possible to obtain adequate groundwater levels. needs further definition. boreholes. doubt regarding springs. (ii) Improve groundwater By mid 1995 Better definition and calibration for this level calibration is part of the model is essential for springs model. only option. (iii) Test pumping and By end 1996 Significant reductions in groundwater level modelling with TWUL to may require extra TWUL expenditure on quantify the effect of boreholes, which would not bo required with augmentation wells, pipeline. pipeline and increased lower catchment abstractions. 11. Lower Chalk A? modelling did not include (i) Investigate implications By Feb 1994 Water Quality data will indicate the Groundwater pollution. salinity or pollution. for salinity and potential dangers of saline or contaminated increased pollution. water intrusion. (ii) with 100| abstraction By end 1995 Increased pollution risk may threaten and all springs existing abstractions. Even relatively low augmentation use model pollution risk will indicate pipeline to identify intrusion of preferences. aaline or contaminated water into the aquifer. (iii) TWUL test pumping. By end 1996 Undesirable effects of test pumping will increase nsed for pipeline. 12. Low flow testing NRA Board proposals include full U) Low flaw conditions need July 1994 Specification of severe low flow conditions trials in drought before committal to be established fran for testing may delay decision making by to pipeline construction. groundwater levels, many years. rainfall and model etc (ii) Trials and modelling in 1994-98 If in practice Springs cannot meet target repreaentative low flow and/or loeal derogation occurs, pipeline conditions following will be needed. completion of springe - 13. BCI dewatering. Further quarry dewatering nay Modelling and monitoring. By end 1996. If Darent sourcea are affected by BCI affect Darent groundwater levels dewatering, which NRA cannot control, then and hence quantity and quality of BCI should contribute to pipeline cost as sources. remedial measure. 14. BCI Water Quality. BCI water as delivered must be of Check an source protection By end 1996. Poor quality would rule out pipeline option. suitable quality. pollution risks (eg pesticides) and temperature on delivery. 15. Fishery Lake level* Artificial aprings and increased More detailed local modelling By end 1996. Local effects on fisheries lakes may rule groundwater abstractions by TWUL and monitoring. out some artificial springs. may lower lake levels. 16. Feasibility Study of Technical feasibility of pipeline Consultancy feasibility study. July 1994 Should technical problems be indicated, Pipeline. and costs. environmental problems with route arise or costs increase, then the pipeline would be less viable. (revised 15/12/93) misc028.pwh(22.3.94 1.GEM Programme of Investigations Leading to Decision on Pipeline on Decision Leadingto Investigations of Programme FigureB-1 Q:\EWRVPROJECTSWZn9A01VlGURESVGEM\nGBZ.GEM by E. Nei U03*\AUIUEWn\00 ia Report Final Monitoring Environmental ntl Osrain Boreholes Observation Install C ae Sources Water BCI ae Blne o Rvr aet/Md acmn Lakes Catchment Mid / Darent River of Balances Water Lower Catchment TWUL Borehole Testing Borehole TWUL Catchment Lower eue Gcnad Abstractions Grccnsand Reduced 3 Stage acmn Modelling Catchment tg 2 Stage tg 1 Stage Augmentation Wells Augmentation augmentation and results of operation trials of augmentation walls augmentation of trials operation of results and augmentation Submit final report; including comprehensive review of studies for addtlonal addtlonal for studies of review comprehensive including report; final Submit NRA by outlined as Darant for plan enhancement /flow A £ Monitoring weOs on fishing takas fishing weOson (IH) L U / TW NRA with ooUabarationm tests Yield Prepare contract documents and supervise Installation Installation supervise and documents contract Prepare Reporting groundwater polluted and intrusion saline of Investigation development Northfteet BCI of Review Reporting collection Data model Calibrate NRA monitoring of river and lakes and river of monitoring NRA Refinement for lower catchment model catchment lower for Refinement collection Data Reporting tnducfing revised riverbed losses and Impact of augmentation augmentation of Impact and losses riverbed revised tnducfing Reporting calibrate and areas lake of sub-model Prepare model catchment integrated Setup Reporting Greensand simulate to runs Model levels groundwater and flow river of monitoring NRA Reporting runs production Model Reporting Construct well outlets Monitor new weDs new Monitor Mods! stage 1 and 3 resuits 3 and 1 stage Mods! losses bed estimated inducting trials ett R weOoutlets ol Construction Reporting Reporting trials Rett II) { weds existing two Monitoring Reporting Reporting of testing and construction supervise wells and additions] documents contract Prepare Model stage i results and setect sites for additional weHs additional for sites setect and results i stage Model ( I ) augmentation w&fls ( 7nr. chalk, 8nr. grave)) 8nr. chalk, 7nr. w&fls( augmentation ) I ( lower Darent (S n r. chalk) r. n (S Darent gravel) lower Bnr. chalk, r. n (4 mid-catchmentlakes Programme for Implementation of River Darent Action Plan Action Darent River of Implementation for Programme 1994 V d 1995 V V Year m 1996 Figure B.2 Figure 1997