Case study:Burn of Mosset, Forres

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Location: 57° 35' 43", -3° 35' 31"
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Project overview

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Status Complete
Project web site
Themes Fisheries, Flood risk management, Habitat and biodiversity, Hydromorphology
Country Scotland
Main contact forename Paul
Main contact surname Winfield
Main contact user ID
Contact organisation Royal HaskoningDHV
Contact organisation web site
Partner organisations
Parent multi-site project
This is a parent project
encompassing the following
projects
No
Burn of Mosset- sustainable sediment and flood management by reconnecting the river with its floodplain and working with natural processes

Project summary

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The Burn of Mosset is a small gravel bed stream draining an area of 49km2. A Tributary of the River Findhorn, it flows north through the town of Forres before entering Findhorn bay. The town of Forres has a long history of flooding from the burn, with six flood events causing serious damage to property or disruption in the last fifty years. The new Flood Alleviation Scheme (FAS) took two years to complete. It included the construction of an earth-filled embankment dam designed to allow for discharges up to 8.5 m3/s to flow through Forres, with excess floodwater temporarily stored behind the dam.

In addition to this, the upstream storage area has been designed to create an extensive natural sediment accretion zone (for sand, gravel and large wood). This will reduce the risk of sediment or other debris blocking the dam control structure. The scheme also aimed to create a mosaic of river and floodplain habitats, by working with natural processes to develop a multi-thread (anabranched) system. This was achieved by breaching the existing embanked channel which ran around a field boundary at two points, allowing flow to spill out across the open field, before re-joining the original channel just upstream of the dam. Tress were planted across the site to create a wet woodland habitat.

This project is part of the UK River Restoration Center's manual of river restoration techniques (update due to be published in late 2013).

Monitoring surveys and results

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Approximately one year after the banks were breached in September 2009, the channel experienced an estimated 30m3/s flood flow (of the order of a 1 in 10 year event). The stone protection at the upstream breach was partially washed out, as anticipated. The breach enlarged such that the majority of the flow was diverted along the new route after the flow subsided. The result was rapid development of river features including the formation of an outwash fan. Some ecological degradation has occurred in the short term as the old channel is now dry except during very high flow events.

Small on-going adaptive management is predicted to be necessary in the short to medium term until this modified river system becomes better established.

Lessons learnt

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Overall this scheme illustrates what can be achieved when working with natural sediment transport processes in flood storage zones. In 2010, the Saltire Society of Scotland in association with the Institution of Civil Engineers awarded the Forres Flood Alleviation Scheme its 'Environmental Sustainable Construction' commendation.


Image gallery


Chapelton Dam control structure (2013)
The Burn management works (2013)
Woody material and sediment is now effectively being stored in the floodplain
Chapelton Dam (2013)
Chapelton Dam flood storage area (2013)
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Catchment and subcatchment

Catchment

River basin district Scotland RBD
River basin Findhorn

Subcatchment

River name Burn of Mosset
Area category
Area (km2) 1300
1,300 km²
130,000 ha
Maximum altitude category
Maximum altitude (m)
Dominant geology Organic
Ecoregion Great Britain
Dominant land cover Moorland/heathland, Woodland
Waterbody ID



Site

Name Forres Flood Alleviation Scheme
WFD water body codes 23021
WFD (national) typology Lowland, Small, Siliceous
WFD water body name Mosset Burn Altyre to Forres
Pre-project morphology Straightened, Embanked, Over-widened
Reference morphology Anastomosing, Pool-riffle
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 Quick run-off
Dominant substrate Gravel, Cobble
River corridor land use Improved/semi-improved grassland/pasture, Moorland/heathland, Woodland
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) 500
500 m
0.5 km
50,000 cm
Project started 2008/01/01
Works started
Works completed
Project completed 2008/12/31
Total cost category
Total cost (k€) 116
116 k€
116,000 €
Benefit to cost ratio
Funding sources

Cost for project phases

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



Reasons for river restoration

Mitigation of a pressure Flood risk management
Hydromorphology
Biology
Physico-chemical
Other reasons for the project


Measures

Structural measures
Bank/bed modifications Create breaches in the bank
Floodplain / River corridor Floodplain reconnection, Riparian planting
Planform / Channel pattern Channel naturalisation
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

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

Link Description
http://www.royalhaskoning.co.uk/en-gb/Publication/Documents/projects/forres-flood-alleviation-scheme.pdf Royal Haskoning- Forres Flood Alleviation Scheme
http://www.moray.gov.uk/downloads/file86456.pdf Moray council- Forres FAS

Supplementary Information

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