Case study:Reopening of Mjølløst stream

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Location: 59° 9' 34.98" N, 10° 15' 39.20" E

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Project overview

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Status	Planned
Project web site
Themes	Habitat and biodiversity, Land use management - agriculture, Monitoring
Country	Norway
Main contact forename	Miguel A.
Main contact surname	Segarra Valls
Main contact user ID
Contact organisation	Vannområde Horten-Larvik
Contact organisation web site	http://https://www.vannportalen.no/vannregioner/vestfold-og-telemark/vannomrader/horten---larvik/
Partner organisations	Sandefjord kommune, Sandefjord Forvaltningsråd for Anadrom Laksefisk
Parent multi-site project
This is a parent project encompassing the following projects	No

Project picture

Project summary

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Mjølløst stream is part of a 17 km long stream system. Mjølløst stream has since the 1960s had a 270 meter long stream closure that prevents sea trout from accessing a 1.4 km stream stretch. The concrete pipe is getting closer to the end of its lifetime, and landowner and local and regional authorities are positive to remove it and restore the original stream. This project got funding from Norwegian Environment Agency (Miljødirektoratet) i 2020.

Monitoring surveys and results

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Results electrofishing October 2020 (before reopening of the stream):

Station 1 (downstreams closure): 141 trout / 100 m2. Station 2 (upstreams closure): 1 trout / 100m2. Station 3 (200 m ustreams closuse): 57 trout / 100m2.

Fish from upstream stations is in principle stationary trout.

Lessons learnt

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1. Because of its proximity to the railway (< 30 m) it is required an approval from the railway authorities (Bane-Nor) in order to be able to daylight / deculvert the stream. 2. There can be quick clay (kvikkleire) according to NVE (Norwegian Water Resources and Energy Directorate). This makes necessary to explore the ground stability before digging the pipe out of the ground. This will increase considerably the costs of the project.

Image gallery

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Catchment and subcatchment Select a catchment/subcatchment Site Edit site Name WFD water body codes NO014-113-R WFD (national) typology REL2321 WFD water body name Unnebergsbekken bekkefelt Pre-project morphology Reference morphology Desired post project morphology Heavily modified water body No National/international site designation Local/regional site designations Unnebergsbekken, Frombekken, Mjølløstbekken Protected species present No Invasive species present Yes Species of interest Sea trout (Salmo trutta), European eel (Anguilla anguilla) Dominant hydrology Perennial Flashy (PF) 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) 0.030.03 m³/s <br />30 l/s <br /> Average channel gradient category Average channel gradient Average unit stream power (W/m2) Project background Edit project background Reach length directly affected (m) 270270 m <br />0.27 km <br />27,000 cm <br /> Project started 2020/03/01 Works started Works completed Project completed Total cost category Total cost (k€) Benefit to cost ratio Funding sources The Norwegian Environmental Agency (Miljødirektoratet) 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 Sea trout migration Hydromorphology Closure of stream Biology Fish Physico-chemical Not relevant Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Reopening of culverted river Floodplain / River corridor Planform / Channel pattern Other Non-structural measures Management interventions Social measures (incl. engagement) Planting riparian vegetation with school pupils. 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 Fish: Abundance Yes No No Yes No 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 Supplementary Information Edit Supplementary Information