New Forest LIFE-III Monitoring Report

Transcription

1 New Forest LIFE-III Monitoring Report The Geomorphic and Hydrological Response of New Forest streams to river restoration David A. Sear Duncan Kitts Cath Millington School of Geography University of Southampton Highfields Southampton SO17 1BJ i

2 Executive Summary This report details the methods and results of a 3 year monitoring programme centred around the restoration of river channel and floodplains in the New Forest SSSI/SAC supported by the EU LIFE-3 programme. The results include details of the geomorphological, hydrological and hydraulic processes and characteristics of the Highland Water and Blackwater study catchments. In addition the report details some of the specific characteristics of floodplain forest within this specific ecosystem. Comments are made on the nature of monitoring restoration projects and new methods for undertaking monitoring of wooded channel and floodplain habitats are presented. The results broadly show that the restoration has had detectable impacts across the hierarchical scales of riverine ecology from catchment to feature and patch. However, the precise nature of the impact varies in its expression with scale. Hydrological impacts are detectable at catchment scale, but some morphological changes are no larger than background natural variability. Specific results of the restoration monitoring are: 1) From a hydrological perspective the restoration has been a success. Objectives set at the beginning of the project have been met and the monitoring has shown this and is presented in this report. In particular, the data show that the restoration has had no negative affects on downstream flooding. Rather it has increased retention of flood water within the catchment. 2) The restoration has modified in-channel habitats relative to the channelised condition primarily by providing more pool habitat while reducing riffle habitat. The effectiveness of the restoration in reproducing semi-natural channel morphology is variable between restoration treatments, but generally has not yet attained the same frequency of habitats. 3) The restoration has met the objective of increasing floodplain connectivity and restoring geomorphic processes on the floodplain characteristic of seminatural reaches. Restoration by addition of woody debris to channelised reaches has failed to restore floodplain connectivity. 4) Further restoration of channelised reaches should seek to re-occupy old channels in order to achieve wet woodland targets. Woody debris within these re-occupied or re-created channels is a key part of the process of floodplain connectivity and aids retention of wood and sediment within the river system. 5) Fine sediment loads have increased downstream of the restoration works, particularly associated with the import of bed material used in raising bed elevations. Noting that the subsequent year was unusually dry, these loads fell but remained higher than those prior to the restoration. It is recommended that specific aspects of the monitoring are maintained as a matter of urgency largely because of the short time post-restoration and the unusually dry conditions in the year following the works. A monitoring strategy to build on the work to date and to feed into the design and management of subsequent restoration projects should be developed. ii

3 Table of Contents Table of Contents...iii Table of Figures...vii Table of Tables...x Table of Plates...xiii Acknowledgments...xiii 1.0 Introduction The basis of monitoring River Restoration Projects LIFE-3 Geomorphology and hydrology monitoring project Objectives of the Monitoring Programme Indicators to monitor for assessment of Objectives Techniques for monitoring indicators Baseline Data Summary Field Sites Geology Topography, topology and drainage network Soils and Landuse Conservation status River Restoration Changes in Channel morphology, sediments and woody debris Changes in channel plan form The affects of river restoration on Large Woody Debris and pool-riffle habitat at the catchment scale Introduction Methods Detecting a restoration effect Results Catchment-scale affects of the Restoration on channel habitat features Reach Scale Affects of the Restoration on channel habitat features The effect of a reach-scale channel restoration on pool-riffle morphology Introduction Defining Pool-riffle sequences Methods Results Conclusions Bed material changes in response to restoration Introduction & Methods Results Conclusions Hydraulic and hydrological affects of restoration Introduction Aim Field Sites...52 iii

4 4.4 Hydrology of the restored reaches Monitoring Results Floodplain Inundation Frequency Restored Site Floodplain Inundation Duration Blackwater Rhinefield Site Water Surface Elevation Required for Inundation Floodplain Inundation Patterns Catchment Flood Hydrology Upstream Hydrology Downstream Hydrology Attenuation of Flood Peak Timing Flood Peak Attenuation River Blackwater Flood Peak Travel Times Large Woody Debris Dam Resistance Field Data Comparison with other data Wood Volume and Blockage Ratios Variation with Discharge Discussion Lessons Learnt and Future Research Geomorphic processes Introduction Geomorphological features and processes that are expected to develop on the floodplain post-restoration: their identification, catchment and reach scale controls on their distribution, their morphology and their formation Identification of geomorphological features and processes found on semi-natural floodplains Results Catchment scale distribution of floodplain channels Results Interpretation Conclusion Reach scale distribution of floodplain channels Method Results Interpretation Reach-scale morphology of floodplain channels Results Floodplain channel formation Restoration and dynamic floodplain geomorphology Patterns of sediment deposition across the floodplain upstream and downstream of debris dams before and after restoration Method Location of traps before restoration Location of traps after restoration Procedure for changing mats Laboratory analyses Results Before restoration iv

5 6.2.2 After restoration Sediment deposition Grainsize Organics Overbank sediment deposition and floodplain vegetation Method Results Grainsize Discussion Implications Floodplain erosion Method Measurement precision Location of erosion pins Results Summary The impact of restoration on woody debris retention Introduction Measuring woody debris residence times Woody debris movement Method Results Interpretation Reach retention Method Data sources Results Principal Component Analysis Interpretation Limitations Implications Conclusions Limitations applying findings to natural woody debris The influence of restoration on fine sediment transport Method Calibration Turbidity Stage Results Summary Coarse Gravel Movement Methods Results Interpretation Wider-scale implications Conclusions v

6 9.0 Conclusions Further work Report Conclusions Recommendations for further research :0 References vi

7 Table of Figures Figure 1: Hierarchical scales adopted for the LIFE-3 geomorphic and hydrological monitoring programme (after Frissell et al., 1986)...19 Figure 2: LIFE-3 project catchments showing the locations of channel/floodplain restoration and the main monitoring reaches. 1 = Semi-natural Control, 2 = Channelised Remeandered Reach, 3 = Channelised LWD Reach, 4 = Rhinefield Remeandered reach. Image based on Memory-Map & OS data. Vertical Exaggeration is 2.5x...21 Figure 3: Solid and drift geology for the Lymington River catchment...22 Figure 4: Drainage network of Highland Water, Blackwater and Ober water basins. 23 Figure 5:Soils map (for inclosures only)...25 Figure 6: Reach-scale planform changes arising from the restoration. Red is the 2006 post-restoration planform, Blue is the 2002 pre-restoration planform Figure 7: Radius of bend curvature frequency distributions for semi-natural control and restored reaches of he Highland Water and Blackwater catchments. Decreasing radius of curvature equates to tighter curved bends...30 Figure 8: Handheld PDA with ARCPAD loaded base maps, permits direct digitalmapping of features Figure 9: Definition of pools and riffles using linear regression (Milne, 1982)...42 Figure 10: Definition diagram for residual pool dimensions (Lisle & Hilton 1992)...43 Figure 11: Long profile for pre (black) and post restoration (green) showing the net increase in bed elevation of 0.81m, and the positive and negative residuals around polynomial regression lines that define pool and riffle sequences according to Milne (1982)...44 Figure 12: Identification of pool-riffle sequences using residual pool method (after Lisle, 1987). The ponding due to the presence of active debris dams is shown in the post-restoration reach...47 Figure 13: Shows the change in surface grainsize populations for the three monitoring sites on the Highland Water whilst table 17 summarizes the percentile grainsize data for each of the particle size curves...49 Figure 14: Change in surface grainsize population for the re-meandered restoration site on the Highland Water for three dates illustrating the progressive accumulation of fine sediment...51 Figure 15: Location of the reach-scale monitoring sites on the Highland Water...53 vii

8 Figure 16: Location of the reach-scale monitoring site at Rhinefield on the River Blackwater Figure 17: Digital Elevation Models of the restored-remeandered reach on the Highland Water pre and post restoration Figure 18: Duration of floodplain inundation recorded at the three reach-scale monitoring sites on the Highland Water Figure 19: The discharge required to initiate floodplain inundation at each crest gauge cross-section...61 Figure 20: Overbank deposition at crest gauges 6 and 7 with an overbank flood channel in the foreground and the main channel in the background. Rhinefield Site, River Blackwater Figure 21: Inundation patterns at the restored planform site for two similarly sized flood events (0.37m 3 s -1 and 0.33 ) before and after restoration...63 Figure 22: Linear relationship between floodplain inundation extent and discharge for the restored (re-meandered) site on the Highland Water...64 Figure 23: The variation in inundation extent with increasing discharge postrestoration, at the restored re-meandered site on the Highland Water...65 Figure 24: Location of the gauging stations used in the hydrological monitoring programme...66 Figure 25: Flow duration curve for the semi-natural Control reach upstream of the restoration works on the Highland Water Figure 26: Flow duration curve for the Restored-LWD reach, Highland Water...69 Figure 27: Flow duration curve for Millyford bridge Gauging station, Highland Water...70 Figure 28: The reach over which the flood peak travel time was measured...71 Figure 29: Flood peak travel time vs discharge. Additional data from Gregory (1992) Figure 30: Flood attenuation for a similar event pre and post restoration...73 Figure 31: Location of the stream gauges on the River Blackwater...74 Figure 32: Flood peak travel time versus discharge for the River Blackwater...75 Figure 33: The measured resistance of artificial and natural LWD dams on the Highland Water...77 Figure 34: Variation in Measured Darcy-Wisbach friction factor with channel slope viii

9 Figure 35a: Variation in Darcy-Weisbach friction factor with blockage ratio; b) Residual Darcy-Weisbach friction factor vs. blockage ratio; c) Residual error between measured and predicted resistance in a range of fluvial environments Figure 36: Photographs of an active natural debris dam that causes a step in the water surface profile Figure 37: The variation of Darcy-Weisbach friction factor over a range of discharges for four different LWD dams in the Highland Water Figure 38: Calculation of floodprone width...88 Figure 39: Explanation of how survey accuracy was calculated...89 Figure 40: Distribution of reaches with and without floodplain channels...90 Figure 41: Tape and offset map of Millyford...95 Figure 42: Tape and offset map of a reach on the Black Water...96 Figure 43: Tape and offset map of a reach on the Ober Water...97 Figure 44:Typology of reach scale distribution of floodplain channels...98 Figure 45: Examples of reach scale morphology of floodplain channels in relation to the main channel...99 Figure 46 Floodplain channel planform types Figure 47: Conceptual model of floodplain channel formation Figure 48: Site locations Figure 49: Distribution of mats at (a) SNCT, (b) RSRM and (c) RSDD for the flood season of 2003/04 (before restoration) looking downstream Figure 50: Distribution of mats at (a) Millyford and (b) RSRM for the flood season of 2004/05, looking downstream Figure 51: Boxplots for winter 2004/05 showing: Figure 52: Flow sheltering on the floodplain close to the channel banks immediately downstream of debris dams Figure 53: Distribution of vegetation mats Figure 54: Boxplots showing: (a) Total sediment deposition (kg.m -2 ) and mat type 124 Figure 55: Location of erosion pins Figure 56: Graphs showing the amount of erosion and deposition experienced by each erosion pin for different periods at SNCT, RSRM and Millyford Figure 57: Mean doweling travel distance for the different sites over the three flood seasons monitored ix

10 Figure 58: Relationship between mean distance travelled and % of doweling retrieved Figure 59: Doweling length and percentage retrieved from the different sites Figure 60: Doweling diameter and percentage retrieved from different sites Figure 61: Doweling length and percentage retrieved from the different sites Figure 62: Doweling diameter and percentage retrieved from different sites Figure 63: Eigen vectors for RI, ω, and doweling mean travel distance Figure 64: PCA case scores Figure 65: Relationship between retention index and channel sinuosity Figure 66: Relationship between retention index and sinuosity excluding the value for RSDD post the addition of debris dams Figure 67: Turbidity probe calibration for (A) RSRM, (B) RSLWD and (C) SNCT157 Figure 68: Calibration of discharge from stage Figure 69: Examples of SS concentration and discharge data from the three sites Figure 70: Relationship between cumulative water volume and cumulative SS for years 1, 2 and 3 at (a) RSRM, (b) SNCT and (c) RSDD Figure 71: Tracer movement data for the Highland Water illustrating the LIFE-3 data in relation to previous experiments on a channelised reach 200m downstream of the LWD-restoration reach. Also shown are the relative movement of gravel tracers pre and post-restoration, showing the effectiveness of re-meandering relative to LWD.167 Figure 72: The distribution of sediment transfer reaches before and after the restoration Table of Tables Table 1: Levels of monitoring and associated uncertainty in the results, after Rutherford et al., (2002). The position of the LIFE-3 hydrological and monitoring programme are highlighted in green...17 Table 2: Summary of the monitoring programme...20 Table 3: Catchment characteristics...22 Table 4: Statutory changes affecting landuse in the Forest (Countryside Commission, 1984) x

11 Table 5: Summary of channel / floodplain restoration works undertaken during the LIFE-3 project...27 Table 6: Channel planform data for catchment and reach-scales resulting from the restoration. Values for planform sinuosity are given for replicate semi-natural control reaches for comparison Table 7: Summary statistics for bed radii of curvature in semi-natural and restored reaches...29 Table 8: Summary data for woody debris supply measured under different woodland types in the study catchments Table 9: Catchment-scale Before and After changes in selected channel features...34 Table 10: Catchment scale changes in habitat features arising from the restoration relative to expected were they to remain as channelised Table 11: Catchment scale comparison between observed and predicted numbers of features assuming semi-natural feature frequencies Table 12: Reach-scale Before and After changes in restored channels...37 Table 13: Reach-scale changes in habitat features arising from the restoration relative to what would be expected were they to have remained as channelised reaches Table 14: Reach-scale comparison between observed and predicted numbers of features assuming semi-natural feature frequencies Table 15: Reach summary data for pre and post restoration long profile surveys Table 16: Summary morphological data for residual pools and riffles within the restored reach and in comparison with an upstream semi-natural control...46 Table 17: Summary grainsize data for each site, before and after restoration. All values are in millimetres. Grain size truncated at 10mm...49 Table 18: Summary grainsize data for the re-meandered restored site. All values are in millimetres. Grain size is truncated at <1 mm Table 19: Summary characteristics of the reach-scale monitoring sites...54 Table 20: Average flood duration times for the three reach-scale monitoring sites on the Highland Water...59 Table 21: Hydrological flow characteristics for the semi-natural Control reach, Highland Water...67 Table 22: Hydrological summary statistics for the Restored-LWD reach, Highland Water...69 Table 23: The timing of the restoration works on the Blackwater...74 xi

12 Table 24: Channel characteristics from where the LWD data was obtained...78 Table 25: Calculation of survey error...89 Table 26: Summary statistics for reaches with and without floodplain channels...91 Table 27: Student s t-test results for the different variables(ho = no significant difference; H 1 = significant difference)...92 Table 28: Characteristics of sites Table 29: Amount of sediment deposited on mats at SNCT during winter 2003/ Table 30: Average discharges required for overbank flow at the different sites Table 31: Total amount of sediment deposited in the upstream and downstream transects for the winter of 2004/ Table 32: Number and dates mats processed from Millyford Table 33: Number and dates mats processed from RSRM Table 34: Sediment deposition on each mat type Table 35: ANOVA test results from the different mat types at the 0.05 level Table 36: Tukey HSD post Hoc test results Table 37: Homogenous sub-sets of mat type based on the Tukey test Table 38: Average percentages of different grain size classes and mat type Table 39: Deposition on different surfaces from the Highland Water (this study) and from the River Cole (Briggs, 1999) Table 40: Grain sizes of deposits on different surface from the Highland Water and from the River Cole Table 41: Average erosion and deposition at each site Table 42: 16 th, 50 th, and 84 th percentiles of the distribution of sizes of woody debris measured at SNCT Table 43: Density of doweling before and after soaking Table 44: Dates of doweling experiments Table 45: Summary statistics of distance moved (m) of in-channel doweling Table 46: Summary statistics of distance moved (m) of floodplain doweling Table 47: Total water volumes passing through the reaches during the periods when the doweling was in the field Table 48: Retrieval percentages Table 49: Percentage of doweling retrieved in different locations Table 50: Weightings of locations where doweling was retrieved Table 51: Score table xii

13 Table 52: Data sources Table 53: Calculation of retention indexes Table 54: Raw and transformed data used in PCA Table 55: Eigen values and percentage of the variance in the data accounted for by each axis Table 56: Component loadings used to make up the Eigen vectors Table 57: PCA case scores Table 58: SS loads for winter periods for years 1, 2 and 3 for each site Table 59: Details of coarse gravel tracer deployments Table 60: Summary hydraulic and movement data for the gravel tracer injection sites Table of Plates Plate 1: Dry floodplain channel,approx. 0.5m wide and 0.1m deep...86 Plate 2: Floodplain channel with water...86 Plate 3. Floodplain debris dam...86 Plate 4. Floodplain scour hollow...87 Plate 5. Area of intense dissection...87 Plate 6 (a) and (b) Tape and offset method for mapping reaches...94 Plate 7. Astroturf sediment trap partially submerged Plate 8: Vegetation traps Plate 9: Juncus sp on the floodplain of RSRM represented by long plastic grass Plate 10: Distribution of vegetation mats Plate 11: Erosion pins exposed on the floodplain surface Plate 12: Doweling entered into channel at RSLWD Acknowledgments The authors gratefully acknowledge the support of all the LIFE-III project partners throughout the research process. In particular we would like to thank the Environment Agency for funding and supporting the work, and the Forestry Commission for xiii

14 supporting the research process on the ground. The GeoData Institute and in particular Chris Hill and Duncan Hornby are thanked for their involvement in analysis of woody debris. xiv

15 1.0 Introduction River restoration has expended rapidly within the last two decades to become a multibillion pound global industry (Palmer et al., 2005; Sear et al.,in press). This is despite difficulties in defining precisely what restoration is (see Brookes et al., 1996, Sear, 1994) and despite relatively low levels of direct government funding (Bruce-Burgess, 2004). At the same time as this expansion has taken place, there has been a growing recognition among both scientific and practitioner communities that the effectiveness of the investment has not been monitored sufficiently (Bruce-Burgess, 2004, Skinner et al., in press, Sear et al., 1998). In fact recent reviews of river restoration have drawn distinctions between ad-hoc projects (by far the most numerous) and those that have a scientific, demonstration and experimental value (Skinner et al., in press). The latter are among the few that have rigorous monitoring programmes from which information and knowledge may be derived to the benefit of others. As river restoration evolves, projects are becoming more expensive, complex and technically difficult, with lifetimes now extending over geomorphologically relevant time scales (Newson, 2002; Sear and Arnell, 2006). With increasing sophistication comes additional risks in terms of setting and meeting realistic project targets and communicating complex models of river environments to stakeholders. Indeed, the results from recent monitoring programmes are beginning to cast doubt on the ability of restoration projects, as currently practiced, to deliver some of these targets (Harrison et al., in press; Williams et al., 2004). At the heart of a river restoration project lies the desire to improve the ecological state of that river. Typically this is referenced to a target state (a concept enshrined within the Water Framework Directive), be it historical or represented by an existing undisturbed reach or river. The problem with this approach stems from the fact that we often do not have sufficient knowledge on which to base decision making during the design of restoration projects. This effectively renders each project the status of an uncontrolled experiment. If we accept this situation then logically one of the most important aspects of the river restoration project is the monitoring programme. 1.1 The basis of monitoring River Restoration Projects Palmer et al., (2005) propose five criteria for measuring the success of river restoration projects with an emphasis on an ecological perspective: 1) The design of an ecological river restoration project should be based on a specified guiding image of a more dynamic, healthy river that could exist at the site. 2) The river's ecological condition must be measurably improved. 3) The river system must be more self-sustaining and resilient to external perturbations so that only minimal follow-up maintenance is needed. 4) During the construction phase, no lasting harm should be inflicted on the ecosystem. 15

16 5) Both pre- and post-assessment must be completed and data made publicly available. Central to their implementation is the ability to define these criteria, and furthermore to be able to monitor changes resulting from a restoration. Some key elements to monitoring river restoration projects include: A comprehensive baseline survey that quantifies the variables to be monitored prior to the restoration Clear and unambiguous project objectives and related targets against which to monitor success Sufficient budget to permit the level of monitoring necessary to achieve the objectives of the monitoring programme The technical expertise necessary and consistently within the project throughout the monitoring period Monitoring at appropriate frequency and over sufficiently long timescales to meet the objectives of the project and monitoring programme targets. Downes et al., (2002) highlight the need to design monitoring programmes that are capable of discriminating between natural variability within a river system and that resulting from river restoration. The standard (yet infrequently applied at least to physical habitat studies) approach is the Before-After-Control-Impact or BACI framework. The essential elements are a Before survey including a Control site that is unaffected by the restoration, and a repeat or After survey including the control and Impacted site. One further aspect of monitoring needs to be considered and that is replication. Replication is necessary to offset the idiosyncrasies that may arise from using single controls or single before and after surveys. In principle replication should permit the discrimination of natural variability both spatially and temporally. The combination of controls and replication within monitoring programmes directly influences our ability to make scientific statements about the impacts of a restoration project. Table 1 from Rutherford et al., (2000) explicitly links the levels of uncertainty within a monitoring programme with the degree of control and replication awarding Gold medals for those that include both replication and control within a BACI framework. 16

17 Evaluation Level Description Example Uncertainty in the results LEVEL 1 Plastic Medal No replication No Control Anecdotal observation We saw lots of fish in the reach VERY HIGH LEVEL 2 Tin Medal No replication No control Sampling after There was a gradual increase in fish numbers 2 years after work HIGH LEVEL 3 Bronze Medal NO replication No / Some Control Sampling before & after After the project there were more fish compared to a control. MODERATE Level 4 Silver Medal Un-replicated Controlled Sampling before & after The numbers of fish increased after the project but not in the control. LOW Level 5 Gold Medal Replicated Controlled Sampling before & after The increase in fish after the project was greater than in any control. VERY LOW Table 1: Levels of monitoring and associated uncertainty in the results, after Rutherford et al., (2000). The position of the LIFE-3 hydrological and monitoring programme are highlighted in green. 1.2 LIFE-3 Geomorphology and hydrology monitoring project The Monitoring project or hydrology and geomorphology was initiated prior to the submission of the project programme to the European Union. This ensured that a budget and timescale was built in to the overall project proposal. Futhermore, as part of the project brief it was necessary to define the monitoring objectives and targets. This process was useful in planning the works, and enabling personnel and equipment to be deployed within the first year before works were undertaken. One of the disadvantages was in the tight timescales within which the funding and works was required to be undertaken. This led to relatively short post-project monitoring, that forced the project to adopt a more streamlined monitoring programme where works were scheduled for completion less than a year before the project end Objectives of the Monitoring Programme The objectives of the monitoring programme as set out in the early stages of the project were as follows. 1. To demonstrate a change in hydrological regime within the channel and floodplain suitable for development of wet woodland and analogous with those found in other wet woodland sites within the catchment. 2. To demonstrate a change in geomorphological processes on the floodplain analogous with those found in other wet woodland sites within the catchment 17

18 3. To demonstrate no net increase in downstream flooding arising from the restoration work. 4. To demonstrate the design and application of techniques appropriate for restoring favourable hydromorphological conditions in support of wet woodland. Each objective was ascribed an indicators that was to form the basis of the monitoring study. These are set out as follows: Indicators to monitor for assessment of Objectives ) Increase in the frequency, extent and duration of floodplain inundation following restoration (Objective 1) 2) Development of diverse erosional and depositional habitats on the floodplain typical of analogue reaches within the catchments (Objective 2) 3) No change or decrease in downstream flood elevation, no change or increased time of travel along restored reach, no change or increase in time to peak of flood flows. (Objective 3) 4) No net increase in channel adjustment over or above those found within other analogue reaches (Objective 4) Each indicator required a method of quantification. These lead to the development of a specific suite of monitoring techniques designed to meet the monitoring objectives. The initial suite o techniques is given as follows Techniques for monitoring indicators Measurement of input discharge and output discharge to demonstration reach across flow range including over bank floods (Pressure transducer and logger, with calibrated (rated) sections up to high-overbank floods). (Indicators 1 & 2, Objectives 1 and 3) Measurement of water level elevation along channel at 6 locations using Pressure transducers (2) and crest Gauges (4). (Indicator 1, Objective 1) Field survey of channel/floodplain physical habitats (6 transects across FP, long section up channel) to include Physical biotopes (Runs, Riffles, pools) and floodplain features (debris, sand shadows, erosional channels). (Indicator 2, Objective 2) Topographic survey of 6 floodplain cross-sections (associated with water level recorders and crest gauges) including channel cross-sections, Long profile of bed and bank elevations along current and restored channel 10 m centres (Indicators 1, 2, 4, Objectives 1, 2, 4) 18

19 The original intention was to undertake this monitoring across all of the restored reaches within the project. In the end the logistical scale of the work, coupled with the timescale of the project implementation (which in some cases led to works on-going at the time of reporting), necessitated a more focussed plan. The basis of the monitoring plan was to undertake appropriate measurements across a range of scales that mimicked those identified in the scientific literature as defining the structure of riverine ecosystems (Frissell et al., 1986). These scales are represented graphically in Figure 1, whilst Table 2 shows the details of monitoring programme in relation to these scales. Feature Reach Segment Patch Catchment Figure 1: Hierarchical scales adopted for the LIFE-3 geomorphic and hydrological monitoring programme (after Frissell et al., 1986). Variable Monitored Scale BACI Catchment Dates Geomorphology Riffle Locations Catchment - Feature BACI HW/BW 2002 / 2006 Debris dam locations Catchment - Feature BACI HW/BW 2002 / 2006 Pool Locations Catchment - Feature BACI* HW/BW 2006 Long Profile Segment - Feature BACI HW 2004 / 2005 Channel Cross-sections Reach BACI HW/BW 2004 / 2005 Channel Planform Segment - Reach BACI HW/BW 2004 / 2006 LWD frequency Segment - Feature BACI HW 2004 / 2006 Floodplain geomorphology Catchment Patch BAI HW/BW/OW 2004 / 2005 Physical habitat Reach BA BW 2005 Geomorphic Processes Fine Sediment transport Catchment - Patch BACI HW Fine sediment deposition Reach - Patch BACI HW Coarse gravel transport Reach BACI HW Woody debris transport Segment - feature BACI HW Erosion of the floodplain Reach - patch BACI HW Erosivity of floodplain Patch N/A HW 2005 Hydrology / hydraulics 19

20 Rainfall Catchment BA HW/BW Discharge / Stage Catchment - Reach BACI HW/BW Water surface slope Reach - Feature BACI HW/BW Floodplain inundation Reach / Feature BACI HW/BW Flood travel time Segment/Catchment BAI HW/BW Flood attenuation Segment/Catchment BAI HW/BW Floodplain hydraulics Reach-Patch BAI HW Flow Velocity (channel & Reach-Feature BAI HW floodplain Debris Dam hydraulics Feature N/A HW/BW/Flume HW = Highland Water, BW = Blackwater, OW = Ober Water. Flume is an experimental channel facility. BACI Before-After /Control-Impact monitoring. * Pool data from 1996/7 Table 2: Summary of the monitoring programme 1.3 Baseline Data The LIFE-3 project has among the most comprehensive baseline datasets available to a restoration. This arises from the coincidence of the project with a long-term research catchment run by academics from the School of Geography at the University of Southampton. The presence of baseline data and scientific understanding helped in the selection of appropriate variables for monitoring. For example a series of segment-scale maps of woody debris and riffle spacing existed from (Gregory et al., 1994, Gurnell & Sweet, 1998). These were supplemented by a Before survey in 2002 and After survey in The resulting dataset enables the project to quantify natural variability in woody debris and riffle frequency within control reaches of the river and thus to identify the impacts of the restoration. In addition to the existing and on-going monitoring associated with the scientific research programme, a specific baseline survey was commissioned as part of the restoration design process. In 2002/2003 a fluvial audit of the Highland Water and Blackwater main river channels was undertaken by the GeoData institute (GeoData 2003). This audit recorded data on the channel and floodplain morphology prior to the restoration works. In addition the report mapped the location and type of channel modifications made throughout the two catchments together with the type of woodland management on the adjacent floodplain. 1.4 Summary The monitoring programme therefore represents among the most comprehensive undertaken on a river restoration project. Best practice features of the monitoring programme include: The extensive and relatively long-term nature of baseline data the hierarchical scale at which many of the variables have been measured; the integrated measurements of morphology, hydrology, hydraulics and processes; the wide range of variables measured over the period of the restoration; the clear statement of monitoring objectives and targets linked explicitly to specific measurements. 20

21 2.0 Field Sites The Highland water and Blackwater are two adjacent headwater sub-catchments of the Lymington River, that flows south into the Solent. They lie centrally within the New Forest National Park, and drain a total of km 2. Details for each catchment are given in Table 3. Blackwater 1 2 Highland Water N 1 km 3 4 LWD restoration Re-meandering restoration Figure 2: LIFE-3 project catchments showing the locations of channel/floodplain restoration and the main monitoring reaches. 1 = Semi-natural Control, 2 = Channelised Remeandered Reach, 3 = Channelised LWD Reach, 4 = Rhinefield Re-meandered reach. Image based on Memory-Map & OS data. Vertical Exaggeration is 2.5x. To the north, the head of each catchment is cut by the A31 and towards the south by the A35. The headwater of the Highland water upstreamof the A31 is semi-natural woodland and heath in contrast to the Blackwater whose headwaters are in a coniferous plantation. Characteristic Highland Water Blackwater Catchment area (km 2 ) Total stream length (km) Drainage density (km/km 2 ) Length of main stream (km) Relief (max to min) 97 (105 to 15) 80 (95 to 15) Slope (m/m)

22 Solid geology Barton clay and sand Barton clay and sand Drift geology Alluvial silt and gravels Alluvial silt and gravels Valley soils Wet al.,luvial brown earth Wet al.,luvial brown earth Land cover Forest/heathland Forest/heathland Land management Foresty/commoning Forestry/commoning Table 3: Catchment characteristics 2.1 Geology The Highland Water and the Blackwater both rise in incised valleys within Barton Clays (of marine origin) and Barton Sands (sand with local loam) (Figure3). The Highland Water crosses into the sand boundary at SU 26952, and the Blackwater at SU 25371, There is slightly more Clay underlying geology within the Blackwater catchment, but this is not expressed in the drainage network. Figure 3: Solid and drift geology for the Lymington River catchment The drift geology overlaying both catchments is alluvial silt. However, the Highland Water flows for a short distance across sand and gravel (undifferentiated river terrace deposits) between SU 25031, and SU 25856, The Blackwater flows across drift sand and gravel from SU 27378, to SU 28627, The drift geology highlights areas of alluvial sediment accumulation notably around Millyford Bridge and at SU 28129, 05102; both probably represent an area where the valley gradient is lower, and both are currently subject to flood inundation (Figure 3). 2.2 Topography, topology and drainage network Although both rivers have the same catchment area (just over 25 km 2 ), the Highland Water has a higher drainage density (Based on 1:10,000 scale land line data 2000) and is appreciably steeper than the Blackwater (Table 3). The Blackwater is around 20% longer 22

23 than the Highland Water, but has around 30% less stream length. Both drainage density and stream length are positively correlated with sediment yield and flood hydrology (Gregory & Walling, 1976). The present drainage pattern reflects Pleistocene reworking of the solid and drift geology s, with river captures of the upper part of both rivers made by the Lymington river as it eroded north (Tubbs, 1986). The valley sizes probably reflect a period of intense fluvial activity when material was reworked during the early Holocene at the end of the last glaciation, resulting in relatively wide valleys within the middle and downstream parts of the Highland Water and Blackwater catchments. The contemporary floodplains are composed of fluvially deposited clays, silts, sands and gravels. Some evidence suggests sea level change occurred that resulted in a period of downcutting beginning years ago, leaving relict floodplains in some areas (see discussion in Tubbs, 1986). Terraces of former floodplain can be observed next to some parts of the Highland Water. The present drainage network within channelised sections of the river has more recently been expanded by drainage ditches cut into the former floodplain surface. These significantly increase the drainage density of both rivers (Figure 4). Figure 4: Drainage network of Highland Water, Blackwater and Ober water basins. 23

24 Both rivers join with the Ober Water just north west of Brockenhurst at SU to become the Lymington river and flow into the Solent. Four tributaries join the Highland Water (Long Brook at SU248099, Bagshot Gutter at SU , White shoot at SU and the Warwickslade Cutting at SU282053). The Blackwater begins life as the Bratley Water in the Slufters inclosure (SU232091) and becomes Blackensford Brook where it is joined by a right-bank tributary flowing from Stinking Edge wood (SU237066), and is marked as the Blackwater as it flows out of Burley Lodge and through Dames Slough inclosure, after which point it becomes known as Fletchers Water at SU Soils and Landuse The New Forest is underlain by mesotrophic soils, and this lack of fertility has prevented agriculture from dominating landuse. Instead, a complex system of landuse currently exists in the New Forest. Floodplain soils within the inclosures of both catchments are characterised by Clay and Gleyed (poorly drained) pedology. Drainage of these soils is typically via overland flow routes (Figure 5). 24

25 Figure 5:Soils map (for inclosures only) Two landuse practices dominate: commoning and inclosures (for forestry). Commoner s animals graze the whole of the open forest, and this grazing pressure maintains pasture woodland and lawns. Commoning rights apparently predate any statutory legislation, although they were not ratified until the New Forest Act of 1698, which defined the Open Forest as an entity that the Commoners had rights to. A series of statutory Acts apply to the use of commoning today, and these Acts have also affected the number of deer and the area of inclosures within the New Forest (Table 4). Period Landuse Bronze age Partly cleared for arable cultivation and pastoral agriculture - poor soil was never very fertile Ca Forest law imposed by the Crown: commoners no longer had total rights to graze their stock 1698 New Forest Act - enclosed 6,000 acres of land for inclosures 1851 The Deer Removal Act (deer were no longer hunted because woodland became more economically viable than hunting) included another of woodland inclosures 25

26 1851 to 1871 Inclosure between these dates amounted to 5,037 acres under the 1851 Act, and 4,228 acres under the 1698 Act of the total 67,000 acres of Open Forest 1877 New Forest Act - allowed the verderer's court to exercise the power of common rights, thus eroding the interests of the crown. Also reduced the power of inclosure for forestry, which meant that a maximum of 16,000 acres was to be enclosed at any one time of the 67,000 total forest acres (total forest acreage is 90,000 acres) This Act allowed a further 5,000 acres to be inclosed for forestry, although only 2,000 acres were actually affected; these 'verderer's inclosures' require the verderers agreement and are only allowed to exist for an agreed term of years before reverting to open forest. Table 4: Statutory changes affecting landuse in the Forest (Countryside Commission, 1984). 2.4 Conservation status The whole of the New Forest has SSSI, SPA and csac status The SSSI is also a Ramsar site due to the plant and invertebrate species associated with the wetland areas. 2.5 River Restoration The LIFE 3 project initiated a set of restoration measures aimed at securing and increasing the extend of wet woodland and bog woodland within the forest. The River restoration was undertaken within the Blackwater and Highland Water catchments. The specific methods adopted were essentially four-fold: 1) Re-occupation of the former meandering course and the infilling of the channelised reach. 2) Creation of a new sinuous course where former channels had been destroyed 3) Bed-level raising using locally sourced clay and gravels 4) Re-introduction of Large Woody Debris (LWD) into channelised reaches where some degree of natural bed level raising had occurred. In addition to the channel works, some of the floodplains had been planted with conifers within the forest inclosures. These conifers were harvested from the floodplain prior to channel works taking place, leaving natural broadleaved deciduous trees where they stood. This activity resulted in increased light and temperature within the newly restored channels, relative to the conditions experienced under the closed conifer canopy. Details of the restoration works are given in Table 5. 26

27 Site Restoration dates (inclusive of site set-up) Restoration type NGR Rhinefield 28/7/03 12/8/03 Highland Water 17/8/05 2/9/ /6/06 13/8/04 20/10/ /6/05 1&2/12/ /5/06 Blackensford 27/6/05 16/8/05 30/11/ /5/ /6/06 Dames Slough 9/5/05 3/6/ /6/06 22/6/06 Reconnection plus debris work at up-stream end Snagging plus reconnection of snail meander to confluence d/s of bridge Snagging plus bed level raising down channelised section just upstream of bridge to downstream end Bed level raising on Open Forest, reconnection within Inclosure plus debris work in Holmhill Snagging plus reconnection for approx 350m at down-stream end Revetment work Snagging Bed level raising on Open Forest, reconnection within Inclosure Debris work within Inclosure Snagging Snagging Reconnection Snagging Snagging Debris work from SU to SU Reconnection (plus some bed-level raising towards downstream end) from SU to SU Bed level raising from SU to SU Bed level raising on Open Forest from SU to SU Reconnection (plus some bed level raising & new channel cutting) from SU to SU Debris work from SU to SU Bed-level raising on OF to Inclosure boundary Western trib: from SU to SU Eastern trib: from SU to SU Reconnection & debris work from Inclosure boundary on 2 tribs to SU Reconnection (minimal bed-level raising) from SU to SU Table 5: Summary of channel / floodplain restoration works undertaken during the LIFE-3 project. 27

28 3.0 Changes in Channel morphology, sediments and woody debris The following section describes changes in the geomorphology of the river channels with a specific emphasis on features that support habitat or have hydraulic significance such as woody debris dams. Two main scale of analysis are presented catchment level analysis and reach-based analysis. Changes arising from the restoration are wherever possible related to semi-natural control sections. 3.1 Changes in channel plan form Changes in channel planform arising from the restoration were mapped by the Environment Agency in summer 2006 following completion of the works (Figure 6). Wherever possible the previous line of the existing remnants of the old watercourse was occupied, but in some reaches this was not possible due to either lack of evidence of the location of the former channel course, or where other constraints demanded a change in location. A total of 1435m (6.4%) of additional channel within the two catchments have been added to the river network as a result of re-occupation and recreation of channels. To put this in to context, the total length of artificially abandoned channel within the Highland Water is 3419m and 4535m in the Blackwater, giving a total of 7954m. The LIFE-3 restoration has re-occupied 1435m or 18% of this total. This work has marginally increased the catchment scale planform sinuosity by 4% in the Highland Water and 8% in the Blackwater. Thus at the catchment scale the effects of the restoration are modest but detectable (Table 6). River Valley Distance Channel length Pre-Restoration (2002) Channel length Post-Restoration (2006) Difference Sinuosity (2002) Table 6: Channel planform data for catchment and reach-scales resulting from the restoration. Values for planform sinuosity are given for replicate semi-natural control reaches for comparison. Sinuosity (2006) CATCHMENT SCALE Highland Water ,124 11, BlackWater ,103 11, REACH SCALE Highland Water Blackwater Dames Slough Blackwater Rhinefield Blackensford Brook HW Control1# N/A HW Control # N/A BW Control # N/A BW Control # N/A At the reach scale, planform modifications have altered the length of channel by 35% on the Highland Water channelised re-meandered reach, 30.5% at Dames Slough, 42.5% at Rhinefield and 7.4% at Blackensford Bottom. However the latter is a crude estimate of total channel length mapped using handheld GPS and is likely to be an underestimate. Sinuosity values a measure of the increase in channel length relative to down valley length, have all increased, and are similar to those recorded at seminatural control sites within both catchments. Thus at the reach scale, the restoration 28