Property:Lessons learnt
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R
This project had a range of key challenges that were overcome through the design process, including:
• The restoration of natural hydromorphological processes whilst minimising the potential for large scale channel migration. This was resolved through the design of a multi-stage channel which allows the low flow channel to adjust within a more permanent high flow bank line. Seeded gravels were also allowed to rework naturally to create a wide range of in-channel habitat niches. The two stage channel safely accommodates both the low dry weather flows and the high flood flows.
• Ensuring that the river could be re-meandered through contaminated land at the site without increasing remobilisation of in situ contaminated sediments and leachate. An uncontaminated capping layer using uncontaminated soils was used to protect the environment and park users from contaminated materials.
• Minimising the need for expensive off-site disposal of contaminated materials and the need to import uncontaminated soils for capping. Soils on the site were categorized as contaminated and less contaminated (i.e. with contaminant concentrations below hazardous levels), and the former were used as a base layer for landscape features within the park. These were capped with less contaminated soils from the site to prevent exposure to the wider environment. +
W
This project has benefited from detailed investigative work and consultations that were undertaken as part of the site selection, scheme design and impact assessment work as well as for the preparation of the monitoring programmes. However, the need to find such a large site that met all the requirements was a long-term and costly exercise. There has been a full time project manager with supporting teams including representatives from statutory and non-statutory authorities and these aspects have enabled problems to be foreseen and rapidly addressed. The team approach has also allowed innovative new design elements to be identified which should provide high value at low cost (e.g. the island features in the site and a new ‘heterogenic’ borrow-dyke design).
The initial monitoring is still ongoing, and the full results from the first five-year monitoring programme have not been reported yet. However, from the interim progress reports that have been produced by ABPmer and Jacobs up to the end of 2010 (i.e. up to four years after the breaching), the following general observations have been made:
Breaches and channels: The breaches and channels through them are very stable which confirms the effectiveness of the approaches that were taken to design them.
Accretion: Within the site, sediment accretion has occurred relatively consistently and evenly due to the slow flows and stable internal creek/channel configurations. In the first year after breaching (2006 to 2007), accretion was around 10cm (of which about 50% is considered internally relocated materials and 50% externally imported sediments). In each subsequent year, the annual accretion is considered to be predominantly related to imported sediments and amounted to 3 to 5cm on average (specifically 5cm in both 2008 and 2009, and 3cm in 2010).
Invertebrates: The accretion and the relatively stable and depositional nature of the environment has helped to promote rapid benthic invertebrate colonisation of the mudflat (approx 80ha in extent). After the first, year invertebrates abundance was at 20,000 organisms/m2 and has ranged between 10,000 and 20,000 organisms/m2 in each successive year. The benthic assemblages have been dominated by large numbers of mud snail (Hydrobia ulvae) and although the patterns of organism recruitment are clearly complex and variable, there is evidence that the assemblages are maturing over time. This is indicated by the bivalve species becoming an increasingly important component over time (representing 2%, 4%, 14% and 26% of the populations in each successive year).
Birds: The bird monitoring (by CJT Ecology) shows that over the course of the four completed years of the monitoring programme the site has been supporting very good numbers of waterbirds. This was the case even from the first winter survey (2006/07) when the site supported around 7,000 waterbirds and included good numbers (i.e relatively high in a national or international context) of many key species such as shelduck, dunlin, black-tailed godwit, ringed plover and golden plover. In the following two winters the value of the site continued to improve as the abundance of waterbirds increased to around 10,000 and then 12,000. For the most recent winter (2009/10) however the overall abundance levels declined slightly. These broad trends are strongly influenced by some major inter-annual changes in the abundance of certain species which, in turn, are likely to be influenced by the weather conditions (e.g. particularly bad weather conditions were experienced in 2009/10 when compared with other years) and/or a range of other factors that influence the natural dynamics of bird populations (e.g. breeding success, timing of migrations, national population trends and inter-annual or inter-generational changes in roosting/feeding site selection). It is of note however, that the numbers of birds using the site as a roost reduced in 2009/10 while the number of birds feeding increased. This may indicate that the habitat and the relationship with migratory birds is still maturing and developing or that under adverse weather
conditions the site’s value as a feeding site as opposed to a roosting site increases.
Saltmarsh: Saltmarsh coverage of elevated areas of the sites (approx 25ha in extent) has occurred relatively rapidly. On average plant coverage (i.e the amount of marsh plant compared to bare mud at any given location) has rapidly increased from less than 1% in 2007 to 6% in 2008, 60% in 2009 and finally at or around 100% in 2010.
E
Thus, the experimental flood release showed limited geomorphic
benefit, despite reaching (or even overtopping) the channel banks. Most
changes were related to the transport of fine sediments, but they did not
enhance habitat variation, or create new fluvial forms. Cebrián et al.
(2015) suggested that the results may have been associated with
extensive modification of the channel after six decades of dam management.
Bed incision or unbalanced river morphology could be hampering
the geomorphic influence of ordinary flows, requiring higher flows than
theorised to enhance sound morphological and ecological changes. +
Timing of the project - advice would have been more beneficial if delivered during 2015 earlier in the season. Also, the length of the project limits the scope of work which can be carried out. +
R
To be used as a reference in river restoration science on how to re-establish a natural flow regime in other European river systems near urban areas. +
U
To get maximum benefit from projects, in areas with less constraints, we needed to look wider than interventions focused just within the channel and consider the floodplain as well. Further learning for individual projects can be found in the project specific case studies. +
R
Too soon to say - this section will be updated when monitoring reults have been evaluated. +
Undertake a Level 2 archaeological survey prior to demolition
Ensure landowners and fishery owners are on board in the early stages
Evaluate likely channel impacts from proposed modifications prior to demolition so no significant supprises +
L
Upon completion of the project, the following results have been listed:
+ More than 10,000 fish are using the fish passages. 80% of fishes which try to use the vertical-slot fishway achieves it, 50% does it in less than half an hour.
+ First weir removal within the Segura Basin
+ Volunteer collaboration and the landstewardship scheme are essential for the achievement of a favorable development of the riparian vegetation. In case of volunteers, thanks to its implication in plantation and refered to landowners for their commitment to properly mantain riversides and to avoid the development of alien species spots as in the case of Arundo Donax.
+ Results obtained by the removal of Arundo Donax have been diverse, depending on the area and the technique applied.
+ Plantation has been more effective for willows (Salix sp), poplars (Populus sp.), elms (Ulmus minor), mastic trees (Pistacia lentiscus) and rosebay (Nerium oleander).
+ A proper implementation of the plantations, a good size of the holes, the location of each plant and the time of plantig are essential for the succes of the works.
+ One of the most succesful achievements has been giving visibility to the problem and to the project and the social awareness about it. +
R
Utilise the material within the channel where possible to reduce the costs and impacts of importing foreign material. +
Water board Drents Overijsselse Delta observes that it is important to plan and time maintenance well and to communicate well with water managers. Most notably, the developing vegetation caused the water to smell when water levels were high, so that weeds had to be cut along the entire stream. +
T
Water pollution is a difficult topic to engage and enthuse members of the public about. More people were engaged about the topic when we held stands at already existing events that people would be going to rather than hoping people would attend an event specifically held by this project. We found that making the events aimed at homeowners targeted to families meant that more people attended and we could discuss general water management and water pollution with the adults whilst the children did craft activities. Relating water pollution to wildlife on the estuary (which is a prominent feature locally) meant it was easier to engage members of the public about the topic.
The walkover surveys were a success in gathering information to inform future delivery projects and in engaging partner organisations in Water Framework Directive project delivery.
The quality of the Pollution Prevention Pack was highly regarded by partners including stakeholders from other Catchment Partnerships. Enthusiasm for the packs has come from stakeholders in Devon, Gloucestershire, Northumberland, Derbyshire, Sussex, Shropshire and Bedfordshire. We know of two organisations from neighbouring Catchment Partnerships that are making minor amendments to the electronic version so they can use them locally. +
H
We can provide data to NE to support and inform restoration works. We will continue to work with partners to improve WFD status. +
W
We did not encounter any major issues. Yes there were small hiccups during the delivery side of things (wrong gravel delivered, last-minute change of contractor), but not something that truly endangered the project.
The approach we took - project delivery mainly by volunteers - was so successful that we will endeavour to make that an important part from all future projects. +
S
We have learned a number of lessons:
1. Importance of Bottom up approach and community support for Natural Flood Management. It is impossible to undertake this work on the scale needed to make a difference to both natural processes or flood risk if widespread community support is not given.
2. We think it is important to start work quickly to create momentum and inspire communities that the approach is feasible rather than theoretical.
3. Even small interventions can make a difference quickly in a degraded stream system, creating habitat complexity, refugia, and allowing light into a heavily shaded area.
4. Use local contractors to increase buy-in and capacity for undertaking works.
5. Start at the most upstream location feasible and work your way downstream. Working on smaller water courses has less risk and less requirement for modelling and feasibility work. +
D
We have yet to review the lessons learnt. +
R
What we have learned so far is the importance of having reliable discharge data, and also the need to target monitoring to measure any specific restoration work being carried out. +
M
When adding gravels to the river bed to previously dredged sectiosn be aware ofg the imapct this may have on flood connection during higher flows. Follwing the addition of gravel the two following winters have experienced very high flwos whith significant prolonged out of bank flows for severals months each winter. In this case thelandowner was ok with this but in other projects this may have become a significant issue. +
B
When planning a large-scale project of this nature, the first consideration should be what is the lowest cost and least disruptive way of achieving the objectives. Previous habitat works have focussed on lowering the floodplain to meet the water surface. This is often costly with excess spoil to move and spread and has a high initial impact on site both ecologically and aesthetically. It can also be very effective to work up projects in several phases over several years to allow interventions to be in and establish e.g channel narrowing followed by gravel introduction. +
W
While the project was successful at stabilizing the bank where it was most at risk (i.e. steep and close to public footpath), the bank poaching caused by dogs has just moved to another location. This should be considered in any similar project that seeks to restore bank that is deteriorating as a result of livestock or recreational activity. Consider where dogs or livestock will next enter the river, and consider whether the river at that point will be sensitive to the pressure. Are there any other unstable banks or important habitats such as spawning gravels that may be impacted? Consider whether fencing or formal drinking bathing points may be required. +