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Title: Management of Vegetation on Raised Embankments
Author: S Runham
Author: F Kirkham
Author: A Sherwood
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_511, Representation ID: 174, Object ID: 1850
Background The remit- of the project The approach to the project The main findings from the project (Stage I) Recommendations for management in the future Choice of seeding-mix 1 1 2 3. KEYwoRDs 4 2: 2.1 RECOMMENDATIONS. Recommendation for trials work 5 5 3. 3.1 3.2 SlmMMARY OF DECISIONS A FLOOD EMBANKMENT:: Choice of seeding mix : Management choices 4. 4.1 4.2 4.3 4.4 4.5 4.6 4.7 INTRODUCTION Flood defence in England and Wales Best environmental practiceMaintenance of flood embankments .. Vegetation on-raised embankments. General features of grass-covered embankments Establishing vegetation on embankments. Choice.of seeding mix 10 10 10 10 10 11 11 14 5. 5.1 5.2 THE OBJECTIVES OF THE-PROJECT. The overall objective The specific objectives of phase 1 15 15 15 6. 6.1 METHODOLOGY : The range of vegetation types 16 16 7. SURVEY RESULTS 17 8. 8.1 8.2 8.3 8.4 8.5 8.6 8.7 IDENTIFICATION:OF FACTORS AFFECTING Salinity, Age of the embankment Source and fertility of soil Slope : Orientation Adjacent land use Seed mix 9. EFFECT OF TKE MANAGEMENT R and D Technical Report MI133 FOR CHOICE OF h4ANAGEMENT OF 7 7 8 REGIMES THE VEGETATION 23 23 23 23 24 24 ._. 24 a 24 26, 26 26 26 27 9.1 9.2 9.3 9.4 General findings Management by mowing Frequency of mowing Management by grazing 10. 10.1 ,10.2 10.3 THE EFFECTIVENESS General findings Grazing Mowing 11. 11.1 11.2 11.3 11.4 11.5 11.6 DISCUSSION AND CONCLUSIONS The characteristics of embankment vegetation Scope for modification of management Variations in management at a site Changes in management Sowing seed Development of seed mixes 30 30 30 32 32 33 34 12. 12.1 12.2 SUMMARY OF RECOMMENDATIONS Recommendations for changes to existing management Recommendation for trials work 37 37 37 OF THE MANAGEMENT REGIME 28 28 28 28 REFERENCES 38 APPENDIX APPENDIX APPENDIX APPENDIX -APPENDIX APPENDIX 41 43 46 66 69 APPENDIX APPENDIX APPENDIX 1 - QUESTIONNAIRE 2 - INFORMATION SPREAD SHEET 3 - CASE STUDIES 4 - EXAMPLES OF SPECIES MIZS 5 - COMPOSITION OF RECOMMENDED SEED MIXTURES 6 - WILDFLOWER AND GRASS SEED POLICY FOR THE ENVIRONMENT AGENCY 7 - CURRENT TRIALS SITES ON RAISED EIvBANKMENTS 8 - WATERCOURSE MAINTENANCE SPECIFICATION FOR THE MIDLAND REGION 9 - COMMON NAMES AND SCIENTIFIC NAMES OF PLANTS IN REPORT R and D Technical Report W 133 71 78 73 75 1. EXECUTIVE 1.1. Background SUMMARY to the project The Environment Agency has a commitment to theymanagement of vegetation on both fluvial and tidal embankments. The Agency aims to ensure that the management regimes implemented. are operationally, financially and environmentally sound. The. Environment Agency maintains 36,000. km-of main river defences nationally:and 800 km of sea defences, protecting over two million people and over 850,000 properties from flooding.. The protection is provided by earth embankments in the main:.. 1.2. The remit.of the project The overall project aims are to,identify the significance of vegetation type and growing environment on the effectiveness of vegetation management. on raised embankments in both fluvial and tidal situations.- The project is approached in two stages, the second dependent on the findings from the first. stage. This report.covers the findings. of the first stage only... In :the: first instance, the range of vegetation established on raised embankments, The including both designed grass mixtures and -natural generation, was identified. range- of vegetation was- .related to information about. the site and management practices in order. to confirm the- likely response of the. vegetation in place to the management regimes currently practised. The remit included an assessment ,of changes that could be instigated to increase the,effectiveness of the management and to lead to beneficial variations in plant species. The scope for developing a modified standardseed mix for use with new and repaired raised embankments was also considered. If the first- stage .of the project identified that there are likely .to be benefits from modifications to existing management practice the project would proceed to a second stage comprising trials to evaluate particular. management operations. 1.3. The approach to the project Information.was sought from managers in the form of a questionnaire (Appe.ndix 1). This helped to determine the choice of sites visited (Appendix 2). The aim was to visit both fluvialand tidal-locations, sown with astandard mixesa and with other mixes, and to assess the effects of the method of control on the vegetation. Individual studies of,. species present are shown at Appendix..;. At each location, a species iist was compiled. A record of relative abundance according to a DAFOR.scale was made. 1.4. The main findings from theapryoject(Stage-1) ,I... The project showed that there are variations in the botanical composition of species on raised embankments, from 8 to 40. species identified : on individual sites. Most embankments were seeded with a astandard mixa which was considered by managers to be perennial ryegrass and clover, but in fact there have been several broadly similar standard mixes used in EA regions for many years, see section 4.6: ,.The uniformity-of the vegetation on the embankment has become ameliorated with time. Only 3 out of 25 embankment sections observed had fewer than 10 species. Six embankment. sections had over 30 species recorded. These differences did .not relate simply to. management but to the interaction of the non-management and .management factors present at each location. R and D Technical-Report W133 I The project showed that the botanical composition of the vegetation was very dependent on the location, topography and materials used in the construction of the raised embankment. The choice of seeding mixture or use of natural regeneration affected the range of species on the embankment. Several sites had been sown to low maintenance mixes and these mixes, in combination with lack of fertiliser or lowfertility soils, had allowed a range of species to flourish. There are wide variations in the methods used to manage the vegetation. These ranged from apparently no maintenance (in a nature reserve) to very frequent maintenance (up to 5 mowing cuts per year). The most common was four to five mowings per year. This achieved the standards of the flood defence requirement (100 per cent green cover, a tightly knit root network for bank stability, ease of visual inspection for rapid anticipation of problems to the flood defence). From the project, it was ascertained that the same high standard of maintenance could be achieved from fewer mowings but that this depended on the natural fertility of the soil used to construct the embankment and on seed mix used. In some locations, very few mowings and no mowings were the operational practice. The vegetation had become long and rank in places and this low maintenance approach may compromise the standards of flood defence service required. It was noted that grazing was used as the vegetation control method in many locations visited. Although this is not favoured by all managers nationally there did not appear to be any compromise to the standards of service required. This method of control was beneficial in terms of biodiversity of plants on the embankment. The management affected greatly the changes in species composition with time. On sites where the species composition was considered aspecies rich,a or ainterestinga it was concluded that past management had influenced this. 1.5. Recommendations for management in the future The project showed that changes in vegetation composition have occurred over time from the time of establishment and these can now influence the chosen method of management. There are recommendations for .modifications to existing management which include a reduction in frequency of mowing on sites (omitting some cuts) where wildflowers have developed, varying the timing from year to year of the cuts, and any omitted cuts, to allow for the different flowering and seeding times of wildflower species. There is scope to tailor frequency of cut to the fertility of individual sites. Some sites Other sites, where low are more fertile and require more frequent cutting. maintenance mixes have been seeded and which have low soil fertility, require fewer cuts per season, and the application of these cuts could be varied to increase diversity of species. The choice of soil for construction of an embankment will affect the subsequent management requirement. Fertile loam for topsoil, over a fertile subsoil will allow rapid establishment of a green cover, but there is a need to match this with a suitable seeding mix (perhaps a low-maintenance mix) that does not call for intensive management subsequently. R and D Technical Report W 133 2 It was concluded that future management should take into,. account the botanical composition of sites, leading to amanagement.. prescriptionsa for individual sites, particularly those .which have become valuable from the conservation point. of view. Some diverse sites were visited in the course of Stage 1, but there are other sites which should be considered for siting trials in Stage 2 (for example, the grass.seed trials at Beckingham, the Ouse Washes barrier banks seed trials,- and the wildflower trials on .L the Lower Clone,. from personal communication, with V Halt, .A Bullivant and.-A Driver respectively). In terms of operational efficiency, there may be scope to reduce the frequency of cut so reducing costs. It is suggested that this be the light of investigationsinto a) the effects of reduced frequency of cut on damage to vegetation.when cuttings are left in situ, b) the-effects on soil fertility of different cutting intervals and. c) the. effectof removal of cut vegetation when compared with leaving-it in situ. Management. by grazing was an effective regime in most cases,- except where some (unpalatable) species were left which .produced scrub vegetation which could hinderclose inspection of, the floodbank. Such sites could benefit from the occasional mowing to control scrub vegetation growth. There was little evidence of excessive poaching from stock. Severe grazin, 0 .from -rabbits-. was noted on one site, but the problem.was controlled before the visit. Many floodbanks were used by the public (even .where public access was denied). This caused excess wear and tear at the top of the bank with poor.vegetation cover and. some signs of soil erosion and lowering.of the bank. The use of,different seed mixes (or turfed grass) on and sides of the floodbank was not observed in the study, but could be considered for future evaluation. In some cases;there was minimal management of the embankments,- and in these cases it seemed appropriate to instigate a greater degree .of management. The choice of management to re-instate the embankment is- important. The first cut would prove a problem as there would be considerable vegetation causing a litter problem.a. It may be appropriate to graze the vegetation first, to mow twice in rapid succession to shred the vegetation, or to consider removal of the vegetation from-the first cut off the site. Although the move towards site-specific management will require additional costs at the outset, once a database of the soil type, fertility,seeding mix and past management has been set up, there will be savings in terms of management- in the medium and long term. 1.6 .-Choke of seeding mix The standard seed. mixes used achieved the. vegetation cover required. Some suggestions are made- to include additional herbs which will increase -the diversity and .. palatability without reducing its success in terms of rapid and reliable establishment. There is.scope to increase-the use-of low maintenance mixes, but their ability to ensure good cover in all situations requires further investigation. Some of the sites observed had developed a wildflower landscape by chance, but -it is known there are. other sites which. have been sown to wildflower mixes. R and D Technical Report. W 13 3 3 A short-list of preferred mixes to firlfil criteria for specific locations is available from EA conservation officers. The aim is to ensure reliable I?111 cover and to serve specific requirements for landowners (for example, for grazing). There has been a move towards targeting the mix to reduce the long-term maintenance load. This may be combined with the opportunity to maximise the conservation value of embankments. It is important to take care when procuring seed. There is a National Seed Contract which is awarded to a company on an annual basis. The company must adhere to certain criteria for sourcing its seed(Copas, 1996-Appendix 6). Environment Agency engineers should ensure that they use seed sources outside the National Contract for a good reason. The effects of management regimes on these should be investigated as part of Stage 2. It is likely that at the end of the Stage 2 investigation recommendations towards achieving the abest environmentally raised embankments. there will be clearer practicable optiona for
Publisher: Environment Agency
Subject Keywords: Embankments; Grazing; Flood embankments; Floodbank; Vegetation management; Mowing; Grass embankment; Seeding mixes; Grass seeds; Wildflowers
Extent: 89
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