Title: Constructed Wetlands and Links with Sustainable Drainage Systems
Author: J B Ellis
Author: R B E Shutes
Author: M D Revitt
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_1500, Representation ID: 523, Object ID: 2647
(ix) Acknowledgement (x) R and D TECHNICAL REPORT P2-159/TR1 iv List of Tables Table 1.1 Wetlands in UK Urban Surface Drainage Systems 3 Table 1.2 Scottish SuDS Database 3 Table 1.3a Wetland Pollutant Removal Mechanisms and their Major Controlling Factors 14 Table 1.3b Relative Importance of Wetland Pollutant Removal Mechanisms Table 2.1 14 Percentage Metal Removal Efficiencies from Acid Mine Drainage by Natural Wetlands in Wales 23 Table 2.2 Metal Levels in a Natural Wetland Receiving Highway Runoff 24 Table 2.3 Reductions in Hydrocarbon Concentrations within a Semi-natural Wetland; the Welsh Harp Basin, NW London 24 Percentage Pollutant Removals for Domestic Wastewater and Artificial Stormwater Wetland Systems in the UK 25 Wetland Metal removal Efficiencies for Natural and Artificial Wetlands in the UK 27 Table 2.6 SuDS Pollutant Removal and Flow Attenuation Capacities 32 Table 2.7 Reported Removal Rates for US Stormwater Constructed Wetlands 31 Table 2.8 Removal rates for US Stormwater SSF and SF Wetlands 33 Table 2.9 Wetland and Dry/Wet Storage Basin Indicators 34 Table 2.4 Table 2.5 Table 2.10 Evaluation of Wetland Effectiveness Potential 35 Table 2.11 Distribution of Wetland Capital Costs 37 Table 2.12 Stormwater Wetlands in the Environment Agency Midlands Region 38 Table 2.13 Capital and Maintenance Costs for Highway Treatment Systems 39 Table 3.1 Kinetic Model Parameters for Wetland Design 47 Table 3.2 Plant Species Commonly Used in Constructed Wetlands 54 Table 3.3 Summary of the Main Methods Used to Establish the Common Reed (Phragmites australis) 55 Table 5.1 Conservation Value of Urban Wetlands 74 Table 6.1 Other Bodies with Important Roles in the WFD 82 R and D TECHNICAL REPORT P2-159/TR1 v Table 6.2 Types of Water Management Plans (England and Wales) 83 Table 6.3 Possible Management Strategies 84 Table 7.1 Sustainability Criteria and Indicators for Urban Wetlands and SuDS 96 Table 7.2a Technical and Scientific Benchmark Indicators 100 Table 7.2b Environmental Impacts Benchmark Indicators 100 Table 7.2c 101 Social and Urban Community Benefits Benchmark Indicators Table 7.2d Economic Costings Benchmark Indicators Table 7.3a Technical and Scientific Benchmark Indicators for the A34 Newbury Bypass Constructed Wetland (Pond F/G) 101 103 Table 7.3b Environmental Impacts Benchmark Indicators for the A34 Newbury Bypass Constructed Wetland (Pond F/G) 104 Table 7.4a 105 SuDS Technology Evaluation Matrix Table 7.4b SuDS Stormwater Control Evaluation Matrix Table 7.4c 106 SuDS Environmental and Urban Community Amenities Evaluation Matrix 107 Table 7.5 SuDS Restrictions Evaluation Matrix 108 Table 8.1 Exclusion Criteria for Selecting Stormwater Control Structures in France 118 Table 8.2 Stormwater Selection Criteria and Measurement Units 120 Table 8.3 Results of Multi-Criteria for the Paris Area 123 Table 8.4 Multi-Criteria Analysis for the Study of Four Sites for the Construction of a Retention Basin at Blanc-Mesnil, St Denise, Paris 125 R and D TECHNICAL REPORT P2-159/TR1 vi Table Appendix A1 Solid Sizes and Settling Velocities 141 Table Appendix A2 Pollutant Load Fractions Attached to Stormwater Solids 142 Table Appendix C1 Standards for Intermittent Discharges 149 Table Appendix C2 Sediment Quality Standards 151 Table Appendix D1 Proposed SEQ Standards in France 156 Table Appendix D2 Pollution Standards for Retention and Wetland Basin Sediments in France 157 List of Figures Figure 1.1a SF Constructed Wetland design 8 Figure 1.1b SF Constructed Wetland Illustrative Cross-section 8 Figure 1.2a A SSF Constructed Wetland 9 Figure 1.2b A SSF Constructed wetland Illustrative Cross-section 10 Figure 1.3 Nitrogen Transformation in a Wetland System 12 Figure 1.4 Solids Retention under Differing discharge and Volume Considerations 16 Figure 1.5 Solids Time Retention Curves for Three Wetland Sites 17 Figure 2.1 Performance Comparison of Constructed and Natural Wetlands 28 Figure 2.2 Log EMC Plots for Wetland TSS Data 30 Figure 2.3 SSF Wetland Performance Costs 36 Figure 3.1 Linkages and Interactions between Wetland Design Elements 42 Figure 3.2 Wetland Treatment Storage Volumes 46 Figure 3.3 Section through a Sub-surface Constructed Wetland 50 Figure 3.4 Idealised layout of a Constructed Wetland 57 Figure 4.1 Original On-stream Wet retention Balancing Pond before Retrofitting 60 Figure 4.2 Flood Balancing Pond Following Retrofitting to Incorporate a Constructed Wetland 61 Figure 4.3 Section Through Retrofitted Constructed Wetland 62 Figure 4.4 Predicting Sediment Removal Maintenance Requirement Time 65 R and D TECHNICAL REPORT P2-159/TR1 vii Figure 5.1 Schematic Landscaping for a Wet Retention and Wetland Basin 71 Figure 5.2 The Great Notley Garden Village Wetland 72 Figure 6.1 RBMP Programme of Measures 84 Figure 7.1 Multi-Criteria Analysis for the Evaluation of Urban Runoff Control and Treatment Options 98 Design of Constructed Wetland Pond F/G Adjacent to A34 Newbury Bypass 103 Figure 7.3 SuDS Selection Process 110 Figure 7.4 Process Diagram for the Design of Constructed Wetlands 111 Figure 8.1 Simplified Diagram of the Decision-making Process for Urban Drainage in France 115 Figure 8.2 Design Process for the Selection of Stormwater Source Control Structures in France 117 Figure 8.3 Design Parameters 119 Figure 9.1 Possible National Database Structure 127 Figure 7.2 Figure Appendix B1 Bacterial Removal Efficiency and Hydraulic Loading Rate 146 Figure Appendix C1 Water Quality Assessment Schemes 152 R and D TECHNICAL REPORT P2-159/TR1 viii EXECUTIVE SUMMARY The report provides a brief background to Environment Agency strategic policy development for sustainable drainage systems (SuDS) in urban catchments and defines the various types, nature and designs of wetland systems found in the UK. The information and data available from wetland systems used to treat domestic wastewater are, however, not directly applicable to stormwater wetlands due to the fundamental differences in inflow regimes and pollutant loading characteristics. A review of wetland processes includes application of plug-flow modelling to wetland pollutant removal rates. Basic performance and efficiency rate indicators are developed together with costing data and a full review is given of wetland design parameters and planting considerations together with examples of kinetic approaches for wetland sizing. Wetland retrofitting, operation and maintenance are considered as well as the role of wetlands in amenity and wildlife provision. Issues of SuDS implementation and future catchment planning are reviewed in the context of future Agency approaches to the EU Water Framework Directive and partnerships with key stakeholders. Generic decisionsupport approaches for constructed wetland and SuDS design and selection procedure and for the design development of urban stormwater wetland treatment systems are also developed. A chapter on equivalent decision-support approaches for the design and selection of urban stormwater runoff systems in France is also included together with a final chapter identifying priority areas and themes for future research and development. Pollutant removal efficiencies of constructed wetlands clearly perform better than natural systems and there is considerable evidence that toxic substances (metals, hydrocarbons, bacteria etc.) in both aqueous and sediment-associated phases are reduced in urban stormwater wetlands. However, negative efficiencies are not uncommon especially for organic and nutrient parameters and/or when inflow concentrations are low. Excessive outflow loadings are normally related to (re-)mobilisation of sediment-associated contaminants which can be flushed out during intense stormflow activity. Urban stormwater wetland design should be capable of treating storms with a minimum return period of 10 years and the system should be capable of treating the polluted first-flush of any storm event. A by-pass is recommended to direct higher storm flows away from the wetland to avoid disturbance of contaminated sediment. Hydraulic conductivity is one of the most important determinants in pollutant removal efficiency, and is especially important in sub-surface flow (SSF) constructed wetland systems where purification processes are largely confined to the root zone. It is clear that regular and systematic wetland maintenance is critical in order to ensure the basic performance and longevity of urban wetlands, and over a 25 a 30 year lifetime the full maintenance and operational costs could well be roughly equivalent to initial construction costs. Adopting and managing authorities therefore need to fully and carefully evaluate how long-term, future maintenance costs can be covered. A simple diagnostic methodology is provided for predicting sediment removal maintenance requirement time. It is important that the designer, developer and regulator establish what the general and/or specific objectives are before selecting a particular wetland or other alternative SuDS device. After establishing what the flood control, water quality and amenity objectives are, an analysis is then required of what is feasible on a particular site given the characteristic meteorological, physical, economic and institutional constraints. R and D TECHNICAL REPORT P2-159/TR1 ix A decision support approach to evaluate the relative sustainability of SuDS structures, as well as conventional pipe systems, has been developed utilising simple multicriteria analysis. The developed methodology identifies primary generic criteria based on technical/scientific performance, environmental impacts, social/urban community benefits and economic costings. A range of secondary sub-criteria and benchmark astandardsa are also identified against which specific wetland or other SuDS structures and drainage options can be compared. A similar multicriteria decision support approach used by the Agence de LaEau Seine-Normandie in France is also presented. It is clear that substantial land value enhancement can be achieved through the provision of well designed and landscaped wetland facilities on urban development sites which can offer major community benefits as well as offsetting total investment costs. Waterfront sites can increase unit process/rentals by 3% to 13% on average with some ground rents on commercial awetland parka developments increasing by two to three times. Urban social/community benefits will only accrue if they are considered early on in the design and planning process as they are frequently difficult and costly to retrofit into existing structures. Their success and long term community benefit is also essentially dependent on adoption agreements and continued, positive management either by public or private agencies. There have been widespread concerns expressed over the provision of openwater bodies such as wet retention (flood storage) ponds and wetlands in urban areas. However, given an extensive plant cover, restricted access to deep water and contaminated sediment areas can be safeguarded by barrier planting schemes and thus such concerns are much less appropriate in the case of constructed wetlands. It is clear that constructed wetlands systems offer considerable potential as sustainable SuDS options for the control and treatment of urban stormwater runoff even given that their design and operational criteria is still an emerging engineering science. The review of current information has enabled a summary of the potential capabilities, performance and range of social/urban community benefits that can accrue from their implementation within integrated urban catchment planning, and provides generic end-user approaches for the quantification and evaluation of urban stormwater wetland sustainability. Acknowledgement Thanks is given to all those Environment Agency staff who provided freely of their time and expertise during the completion of this report. However, the views expressed are entirely the responsibility of the authors and do not reflect future Environment Agency policy intentions. The work of Professor J-C Deutsch and Dr. J-M Mouchel of CEREVE, Ecole Nationale des Ponts et Chaussees, Paris in providing the draft material for the chapter relating to French stormwater systems and practices is gratefully acknowledged. The sections on French practice were considerably aided by the very helpful discussions with staff of the Agence de LaEau Seine-Normandie (AESN), with the comments of Mme. Nadine Aires being of particular value and insight. R and D TECHNICAL REPORT P2-159/TR1 x 1.
Publisher: Environment Agency
Subject Keywords: Wetlandsss; Urban stormwater runoff; Urban runoff quality control; Sustainable drainage systems (suds); Urban catchment management; Decision support systems; Multicriteria assessment.
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