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NRA \(itu>nol Huvrs Authority RIVER DARENT LOW FLOW ALLEVIATION ANNEX I Darent Catchment Investigation (GDC, November 1991) Ki &A t o V National Rivers Authority (Southern Region) NRA Darent Catchment Investigation CWP/8709 En v ir o n m e n t A g e n c y re-Feasibility Report November 1991 NATIONAL LIBRARY & INFORMATION SERVICE SOUTHERN REGION Guildbourne House, Chatsworth Road, Worthing, West Sussex BN 11 1LD ENVIRONMENT AGENCY 054653 Groundwater Development Consultants Cambridge, UK REPORT DISTRIBUTION AND REVISION SHEET Project: Darent Catchment Investigation Project Code: 70223B01 Report Nr: 1 Report Title: Pre-feasibility Report Revision Date of Originator Checker Approver Scope of Letter Issue Revision 30.6.91 P Rippon T Jefferies B Misstear B 14.11.91 P Rippon A Wyness B Misstear Text changes and Figures and Tables indicated in italics in Contents (in Rev B only) 25.11.91 P Rippon A Wyness T Evans M inor text changes 1 ^ 1 RIVER DARENT INVESTIGATION PRE-FEASIBILITY REPORT CONTENTS Page Nr SUMMARY CHAPTER 1 INTRODUCTION 1.1 Background and Objectives 1-1 1.2 Catchment Description 1-2 1.3 Scope of Work 1-2 1.4 Methodology 1-3 1.5 Acknowledgments 1-4 CHAPTER 2 EXISTING INFORMATION 2.1 Introduction 2-1 J^ma ouua^ z- i 2.3 Review of Reports 2-2 2.3.1 Introduction 2-2 2.3.2 Water Management Study 2-2 2.3.3 Low Flow Alleviation Study 2-3 2.3.4 Environmentally Acceptable Flow Regime Study 2-6 2.3.5 Other Reports and Papers 2-8 2.4 Historical Development 2-8 2.5 Public Perception 2-10 CHAPTER 3 HYDROGEOLOGY 3.1 Data Availability 3-1 3.1.1 Geology 3-1 3.1.4 Groundwater Levels 3-1 3.1.3 Test Pumping 3-3 3.1.4 Groundwater Abstraction 3-4 70223 B01/C/D A R/wp (i) CONTENTS (cont) Page Nr CHAPTER 3 (cont) 3.2 Geology 3-4 3.3 Piezometry 3-5 3.4 Aquifer Properties 3-6 3.4.1 Data Quality 3-6 3.4.2 Data Analysis 3-6 3.5 Groundwater Abstraction 3-7 3.6 Water Quality 3-7 CHAPTER 4 SURFACE WATER HYDROLOGY AND WATER BALANCE 4.1 Introduction 4-1 4.2 Hydrological Data Evaluation 4-1 4.2.1 Rainfall 4-1 4.2.2 Evaporation 4-2 4.2.3 River Flow 4-2 4.3 Recharge Assessment 4-3 4.3.1 Approach 4-3 4.3.2 Data Preparation 4-4 4.3.3 Surface Water Modelling 4-4 4.3.4 Discussion 4-6 4.4 Catchment Water Balance 4-6 4.4.1 Introduction 4-6 4.4.2 Comparison of Recharge and Groundwater Abstraction 4-9 4.4.3 Conclusions 4-10 4.5 Environmentally Acceptable Flows 4-11 4.5.1 Introduction 4-11 4.5.2 Flow Deficits 4-12 70223B0t/C/DAR/wp (ii) CONTENTS (coot) Page Nr CHAPTER 5 ENGINEERING STUDIES 5.1 Introduction 5-1 5.2 Augmentation Flow Required 5-1 5.3 Water Related Infrastructure 5-4 5.3.1 General 5-4 5.3.2 Demand Growth 5-7 5.4 Review of Engineering Options 5-8 5.4.1 General 5-8 5.4.2 Groundwater Management 5-9 5.4.3 Surface Water Management 5-15 5.4.4 Short Term Options 5-23 5.5 Demand Management 5-25 5.6 Summary of Options 5-26 CHAPTER 6 CATCHMENT MODELLING c 1 muuui c 6.2 Data Assessment and Further Requirements 6-3 6.2.1 Surface Water and Recharge 6-3 6.2.2 Geology 6-4 6.2.3 Piezometry and Aquifer Properties 6-4 6.2.4 Groundwater Abstraction 6-5 6.3 Channel Survey 6-6 CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS 7.1 Conclusions 7-1 *> 7.2 Recommendations 7-3 REFERENCES APPENDIX QUARTERLY WATER BALANCES 70223B01/C/DAR/wp (iii) LIST OF TABLES Table Title Page Nr N r S-l Design Augmentation Requirements at Gauging Sites S-6 S-2 Summary of Options Considered S-7 S-3 Comparison of Structural Options S-9 3.1 Observation Borehole Details 3-2 3.2 Test Pumping Data 3-3 3.3 Analysis of Constant Discharge Tests 3-6 3.4 Analysis of Step Discharge Tests 3-7 3.5 Licensed Groundwater Abstraction Details 3-8 4.1 Gauging Stations and Flow Statistics 4-2 4.2 Calibration Parameters 4-5 4.3 Annual Water Balances 4-7 4.4 Mean Rainfall and Recharge 1984 to 1990 4-8 4.5 Long Term Recharge 4-8 4.6 Environmentally Acceptable Flow Regime 4-11 5.1 Sustained Deficit Periods 5-2 5.2 Augmentation Flows Required 5-3 5.3 Design Augmentation Requirements at Gauging Sites 5-3 5.4 Abstractions for Public Water Supply 5-5 5.5 Catchment Imports and Exports 5-6 5.6 Demand Projections 5-7 5.7 Demand Projections for Darent Catchment 5-8 5.8 Summary of Options Considered 5-27 5.9 Comparison of Structural Options 5-28 70223B01/C/DARArp (iv) LIST OF FIGURES Figure Title Following Nr Page Nr 1.1 Location Map 1-1 1.2 Solid Geology 1-1 2.1 Questionnaire Replies 2-9 3.1 Observation Well Locations 3-1 3.2 Availability of Observation Borehole Data 3-3 3.3 Location of Licensed Groundwater Abstractions 3-4 3.4 Location of Geological Cross-section 3-4 3.5 Geological Cross-section 3-4 3.6 Chalk Groundwater Level Hydrographs 3-6 3.7 Availability of Groundwater Abstraction Data 3-9 4.1 Location of Rain Gauges 4-1 4.2 Availability of Daily Rainfall Data 4-1 4.3 Subdivision of Catchment 4-2 4.4 Flowchart of Stanford Watershed Model 4-3 4.5 Simulation at Otford 4-5 4.6 Simulation at Hawley (with catchment subdivided at Otford) 4-5 *t.a n/ Siiiiuiaiiun Hi Hawley (without ctitcfinicnl subdivision) A.fs 4.8 Flow Duration (Lullingstone) 4-12 4.9 Annual Flow Deficits 4-12 4.10 Days of Deficit 4-12 5.1 Monthly Deficit Record 5-1 5.2 Seasonal Distribution of Deficit Days 5-2 5.3a Water Supply Operational Areas 5-4 5.3b Indicative Water Distribution 5-4 5.4 Public Water Supply Abstractions 5-5 5.5 Darent Valley Trunk Sewer 5-5 5.6 Kent Wells Conservation Area 5-7 5.7 Demand Growth Projections for Kent Wells Zone 5-7 5.8 Location of London Ring Main 5-10 5.9 Thames Water Supply Zones in South East London 5-10 5.10a TWU South East London: Schematic 1 5-11 70223B01/C/DARy«p (V) LIST OF FIGURES (cont) Figure Title Following Nr Page Nr 5.10b TWU South East London: Schematic 2 5-11 5.11 Potential Links in Trunk Main System 5-12 5.12 Surplus and Deficit Volumes at Lullingstone/Hawley 5-16 5.13 Existing Lakes 5-17 5.14 Summary of Options 5-26 6.1 Modelled Aquifer System 6-1 6.2 Catchment Model Area 6-2 6.3 River Darent Channel Characteristics 6-5 70223 BOI/C/DAR/wp (vi) SUMMARY Background and Objectives The River Darent has been identified by the National Rivers Authority as one of 40 over-abstracted catchments in England and Wales. The Darent catchment, located in north Kent, is regarded as being of particular importance, owing to the severity of river flow depletion. The river is fed by springs from the Lower Greensand aquifer lying along the northern rim of the Weald of Kent, and also from the Chalk aquifer of the North Downs. Both aquifers are sources of major groundwater abstraction for public water supply. In recent periods of low rainfall, particularly the early to mid 1970s and from 1989 to the present, the river has dried up completely in certain sections for durations of several months. The reach from Lullingstone to Hawley has been particularly severely affected in these periods. Groundwater Development Consultants (GDC) was appointed by the National Rivers Authority (Southern Region) in February 1991 to undertake an investigation of the River Darent catchment. The main objective of the investigation is to identify the most appropriate measures from a large number of engineering options to alleviate low flow conditions in the river. This objective is to be achieved by: an initial water infrastructure and water balance study to assess the feasibility of various potential engineering options to alleviate low flow conditions; a broad costing of options; computer-based, integrated catchment modelling with simulation of the interrelationship between the aquifers and the river, allowing detailed assessment of the effects of engineering options; recommendations based on benefits to the river, technical feasibility and comparison of costs. This report describes the first two activities listed above. A review of previous reports concerning the problem of low flows in the River Darent is also presented and an assessment made of available hydrogeological data. The integrated catchment model is discussed together with additional data requirements for modelling. The report has been produced at the end of Part 1 of the investigation. Many of the engineering options have been described in earlier reports. Detailed modelling of the catchment has not been carried out previously, however. The present investigation therefore provides the first opportunity to assess in detail the effects of options on river flow and aquifer conditions. Environmentally acceptable flow objectives for the Darent have been the subject of a separate study for the NRA (Southern Region) by WS Atkins.