Case study:Burn of Mosset, Forres

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Location: 57° 35' 43.20" N, 3° 35' 30.93" W

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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.

Since construction in 2008 the scheme is effectively storing large wood and sediment, with a large sediment outwash fan forming at the upstream breach. A small amount of adaptive management has been required at the upstream breach due to erosional events interacting with woody material in the channel. Measures were taken to ensure that a further breach of the old channel did not occur, as this could have led to flooding of a significant area of agricultural land.

In 2010 the scheme was awarded an "environmentally sustainable construction" commendation by the Institute of Civil Engineers. 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

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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)

Catchment and subcatchment Select a catchment/subcatchment Edit the catchment and subcatchment details (affects all case studies in this subcatchment) Catchment River basin district Scotland RBD River basin Findhorn Subcatchment River name Burn of Mosset Area category Area (km2) 13001,300 km² <br />130,000 ha <br /> Maximum altitude category Maximum altitude (m) Dominant geology Organic Ecoregion Great Britain Dominant land cover Moorland/heathland, Woodland Waterbody ID Site Edit 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 Edit project background Reach length directly affected (m) 500500 m <br />0.5 km <br />50,000 cm <br /> Project started 2008/01/01 Works started Works completed Project completed 2008/12/31 Total cost category Total cost (k€) 116116 k€ <br />116,000 € <br /> 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 Edit reasons for restoration Mitigation of a pressure Flood risk management Hydromorphology Biology Physico-chemical Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Create breaches in the bank Floodplain / River corridor Floodplain reconnection, Riparian tree planting Planform / Channel pattern Allow future natural channel changes to occur Other Non-structural measures Management interventions Social measures (incl. engagement) Other Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Physico-chemical quality elements Edit 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 Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents Additional links and references Edit 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 Edit Supplementary Information