Case study:River Stour Habitat Enhancements at Great and Little Bradley

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Location: 52° 8' 56", 0° 27' 1"
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Project overview

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Status Complete
Project web site
Themes Fisheries, Habitat and biodiversity
Country England
Main contact forename Alex
Main contact surname Moore da Luz
Main contact user ID User:Dedhamvale
Contact organisation Dedham Vale AONB
Contact organisation web site http://www.dedhamvalestourvalley.org
Partner organisations Environment Agency
Parent multi-site project
This is a parent project
encompassing the following
projects
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Project summary

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The ecological value of this stretch is limited by three key factors: poor water quality due to agricultural run-off; barriers to longitudinal connectivity in the form of water-level control structures associated with the Ely-Ouse Transfer Scheme; and the periodic flushing of vast quantities of water through the reach due to the Ely-Ouse Transfer scheme. As a result large sections of the river here are dominated by epilithic and floating algae, which are prevalent in areas of slow-moving water. Several frogs were also seen in such areas, alongside invasive, non-native American signal crayfish.

The scheme focused upon maximising the habitat diversity of the river with the view that water quality and quantity issues can addressed at as later date, either as part of or separate to this project. In particular the scheme concentrated upon creating those habitats that are currently lacking within the river. In total 94 woody debris features (including flow deflectors, log jams and brushwood shelves) have been installed by EA and AONB Project staff and volunteers. Using large woody material had the dual benefit of instantaneously increasing oxygenation and flow diversity whilst simultaneously adding structural complexity and cover. Flow deflectors (large pieces of natural wood) were used to accentuate sinuosity in areas that already exhibited meander initiation and for encouraging bed scour in reaches that were comparatively straight. Flow deflectors were orientated downstream at an angle from the bank between 30° and 45°. The length of the deflectors varied but ideally each structure typically occupied between 50-75% of the channel width. Where possible, deflectors were installed on alternating sides of the river to increase channel sinuosity. The distance between deflectors averaged around 25m longitudinal river length, although this varied according to local conditions. Flow deflectors were placed upon the bed of the river and built to varying heights so that different deflectors are effective under different flow conditions. The reasons for using wooden flow deflectors are manifold. Most broadly, reintroducing woody-material to a river creates variations in flow velocity and depth. Diverse flows create diverse habitats, such as slow-flow pool areas where fish can rest when migrating upstream. Variations in flow also create variations in fluvial geomorphological processes, with flow deflectors often being the instigator in the development of erosion. The physical structure of flow deflector installations can also have benefits, providing an important refuge for fish and other in-stream organisms where they can avoid predation. Increased channel roughness and the resultant fluid turbulence through reaches containing flow deflectors often increases the concentration of dissolved oxygen in these sections, further improving fish habitat. More specifically, flow deflectors can be used to narrow the river channel and encourage the development of berms in river reaches that have been over-widened. Artificial log-jams perform a number of functions. Firstly, the localised backing-up effect caused by the jam is likely to force fine sediment to fall out of suspension and accumulate upstream of the structure, thereby reducing its entrainment downstream. Secondly, the water falling over the log-jam may generate a small scour pool from which gravel is excavated and transported downstream to create further features. This creates a habitat in itself, as well as facilitating bed scour and landform creation. Thirdly, the log-jam oxygenates water, provides a refuge for organisms and creates a permeable barrier between two distinct ecotones (the slow-flowing, backed-up upstream habitat and the fast-flowing, slightly derogated downstream habitat). All log-jams were constructed of wood that spanned the entirety of the channel. The maximum height of the log-jams was no more than 20cm above the existing bed level. Where possible the jams were constructed of a single piece of large wood, cut to the dimensions of the channel. All of the log-jams were secured in place using vertical stakes driven into the bed of the river using a post driver, mallet or digger-bucket tool. 4ft and 6ft oak were used construction, whilst some stakes were made from local tree branches. Vertical and horizontal pieces of wood were secured together using 100mm and 150mm timber screws. The sides of each log-jam were dug into the banks of the river where possible. All of the aforementioned measures were taken to ensure the maximum environmental benefit whilst still securing the structures.

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Catchment and subcatchment

Catchment

River basin district Anglian
River basin Combined Essex

Subcatchment

River name River Stour
Area category 10 - 100 km²
Area (km2)
Maximum altitude category 100 - 200 m
Maximum altitude (m) 135
135 m
0.135 km
13,500 cm
Dominant geology Calcareous
Ecoregion Great Britain
Dominant land cover Arable and Horticulture
Waterbody ID GB105036040980



Site

Name
WFD water body codes GB105036040980
WFD (national) typology
WFD water body name River Stour
Pre-project morphology
Reference morphology
Desired post project morphology
Heavily modified water body No
National/international site designation
Local/regional site designations
Protected species present No
Invasive species present No
Species of interest
Dominant hydrology
Dominant substrate
River corridor land use
Average bankfull channel width category
Average bankfull channel width (m)
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m3/s)
Average channel gradient category
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Average unit stream power (W/m2)


Project background

Reach length directly affected (m)
Project started 2015/09/01
Works started
Works completed
Project completed 2016/02/01
Total cost category
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Benefit to cost ratio
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Cost for project phases

Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname
Investigation and design
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Works and works supervision
Post-project management and maintenance
Monitoring



Reasons for river restoration

Mitigation of a pressure
Hydromorphology Addressing the physical modifications of the river to meet the requirements of the Water Framework Directive; enabling natural recovery; creating a self-sustaining river system that requires less intervention to maintain;
Biology improving biodiversity;
Physico-chemical improving dissolved oxygen levels;
Other reasons for the project


Measures

Structural measures
Bank/bed modifications Creation of wooden deflectors, Introducing large woody debris
Floodplain / River corridor
Planform / Channel pattern
Other
Non-structural measures
Management interventions
Social measures (incl. engagement)
Other


Monitoring

Hydromorphological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Biological quality elements

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Physico-chemical quality elements

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Any other monitoring, e.g. social, economic

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Monitoring documents



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Supplementary Information

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