Case study:Saving Chiswick Eyot

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Location: 51° 29' 15", -0° 14' 45"
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Project overview

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Status In progress
Project web site http://www.savingchiswickeyot.com
Themes Economic aspects, Environmental flows and water resources, Habitat and biodiversity, Hydromorphology, Land use management - forestry, Monitoring, Social benefits, Urban
Country England
Main contact forename Martin
Main contact surname Richardson
Main contact user ID User:Mobilitydream
Contact organisation Natural History Museum
Contact organisation web site http://www.nhm.ac.uk
Partner organisations Royal Holloway University of London, MoLA, University of Leicester, Thames21, Carillion, Port of London Authority, DHI
This is a parent project
encompassing the following
projects
Eyot erosion

Project summary

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Project team

Dr. Dave Morritt, Royal Holloway University of London. Dr. Paul Clark, Natural History Museum, London. Martin Richardson, PhD student RHUL & NHM.

Project

Chiswick Eyot (meaning island) is a small, historically important island in the Thames near Chiswick Mall that is subject to erosion caused in part by burrowing mitten crabs. Crab burrows occur in high densities at the site in excess of 30 per square metre. Individual burrows may be complex structures 70 cm in length and 5 cm in diameter so the amount of soil excavated is significant and the banks of the island are considerably weakened as a result thereby accelerating erosion. We have established an ‘ecosystem’ of partners to address the problem of maintaining the island employing best practice techniques. An initial 3D scan of the entire island was conducted in 2016 in association with the University of Leicester, the Museum of London Archaeology (MoLA), and Leica Geosystems. Subsequent, annual scans will permit calculation of the rate of erosion with analysis of the contribution made by mitten crab burrowing. A conventional topographic survey of part of one bank was undertaken by MoLA in 2000 and this will allow decadal scale assessment of the erosion rate caused by mitten crab burrows. High resolution aerial photographs and digitized historical Ordnance Survey maps are available dating back to at least the 1930s and these will be used to assess migration of the Eyot back and forth across the river, and permit analysis of changes to the island itself in terms of long term erosion. Additional remote sensing and environmental data sets are available and will be used to conduct a 3 dimensional analysis of the area around the Eyot within the context of the Thames catchment as a whole. Ecological changes will be inferred from analysis of the models and verified through species recording data from the NBN. We have engaged the local community and formed partnerships with other organisations including Thames 21, the River Restoration Centre, Leica Geosystems, DHI, MoLA, and Royal Holloway University of London. Additionally we plan to include international partners: the California department of fish and wildlife as well as several international researchers involved in invasive mitten crab work in Poland, Germany, China and elsewhere.

Monitoring surveys and results

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The first surveys have been completed. The island has decreased in size by over 40% since the 1930's and there is evidence of correlation with populations of Chinese Mitten crabs burrowing into the banks. The population dynamics of the crabs are characterized by large fluctuations related to environmental factors including temperature and rainfall. Larval development in the Thames requires a residency time of over a month which only occurs when flow rates are low. The burrows are long and wide which means that considerable quantities of sediment are mobilized during excavation but also the damage caused by burrowing is long-lived as the burrows continue to accelerate slumping for several years after the crabs have gone.

A second series of surveys will be conducted over the winter of 2017 and these will be used to assess the current rate of erosion and changes in the morphology of the island.

Lessons learnt

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Restoration work using traditional withes (willow saplings) has resulted in accretion to some areas of the bank with development of stabilizing vegetation. However, changes to the flow of the river caused by barriers made of withes have evidently caused some scouring in others, possibly because the barrier is too far from the bottom of bank at these points.


Image gallery


Tree and slumping bank
Detail of entrance to mitten crab burrows in the banks of Chiswick Eyot.
Using the Leica P40 scanner
Accretion behind restoration structures on the north bank of the Eyot
Chinese mitten crab burrows
Comparison 2002 to 2010 bottom of bank
Detail from a laser scan 2016
LiDAR data
Photogrammetry of part of the bank 2016
Photogrammetry point cloud of restored bank 2016
Photogrammetry model made this year of the East bank
More detail of the East bank. Photogrammetry 2017
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Catchment and subcatchment

Catchment

River basin district Thames
River basin London

Subcatchment

River name Thames Middle
Area category
Area (km2)
Maximum altitude category
Maximum altitude (m)
Dominant geology
Ecoregion Great Britain
Dominant land cover Urban
Waterbody ID GB530603911402



Other case studies in this subcatchment: Barking Creek near A13, Barking Creekmouth, Chambers Wharf, Cuckolds Haven Nature Area, Greenwich Peninsula, Lower River Roding Regeneration Project, Mill Pool, Wandsworth Riverside Quarter


Site

Name
WFD water body codes GB530603911402
WFD (national) typology Intertidal
WFD water body name Thames Middle
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 Yes
Species of interest Chinese mitten crab (Eriocheir sinensis)
Dominant hydrology
Dominant substrate
River corridor land use Urban
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
Average channel gradient
Average unit stream power (W/m2)


Project background

Reach length directly affected (m) 290
290 m
0.29 km
29,000 cm
Project started 2014/12/23
Works started 2015/06/01
Works completed
Project completed
Total cost category 50 - 100 k€
Total cost (k€)
Benefit to cost ratio
Funding sources Commercial partners. Local charities.

Cost for project phases

Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname
Investigation and design 1 - 10 k€ Natural History Museum Paul Clark
Stakeholder engagement and communication Less than 1 k€ Royal Holloway University of London Martin Richardson
Works and works supervision 100 - 500 k€ Natural History Museum Diana Catovan
Post-project management and maintenance 1 - 10 k€ Natural History Museum Paul Clark
Monitoring 1 - 10 k€ Natural History Museum Diana Catovon



Reasons for river restoration

Mitigation of a pressure Invasive species
Hydromorphology Structure & condition of intertidal zone
Biology
Physico-chemical
Other reasons for the project


Measures

Structural measures
Bank/bed modifications Bank stabilisation, Restoration of natural vegetation, Removal of invasive plants, Removal of invasive species
Floodplain / River corridor
Planform / Channel pattern Bank restoration
Other
Non-structural measures
Management interventions reducing bank side collapse (another main source of sediment)
Social measures (incl. engagement) Awareness raising, Community involvement, Citizen participation in the restoration project
Other


Monitoring

Hydromorphological quality elements

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

Biological quality elements

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

Physico-chemical quality elements

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

Any other monitoring, e.g. social, economic

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


Monitoring documents



Additional documents and videos


Additional links and references

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Supplementary Information

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