Summary
Summary
The Odense is a river in southern Denmark. It was channelized and deepened in the late 1940s to improve agriculture. The NWRM consists of a series of measures to restore floodplain connectivity along a 17 km section of the river. The measures will prevent flooding in downstream towns and cities. This will have a number of effects on the pressures relevant for the WFD. Reduced risk for flooding of urban environment reduces the risk for storm overflows from sewers as well as diffuse pollution resulting from flooding in general. Re-meandering reduces the pressure from previous physical alterations for flood protection and agricultural purposes. It will also potentially have a positive effect on ground water recharge in temporarily flooded areas.
Since 2004, ~12 km of Odense River have been remeandered and 350 ha of wetlands restored under LIFE REGAIN. Recent actions include removal of migration barriers, and mussel habitat restoration.
In 2024, two artificial stone reefs (4500 tonnes) were established in Odense Fjord to enhance marine biodiversity. The project was led by SDU in partnership with Odense and Nordfyn municipalities and Aage V. Jensen Naturfond. The reefs aim to improve conditions for fish fry, seagrass, and aquatic invertebrates, and contribute to the ecological recovery of the fjord.
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Overview
Year of issue
2025
Project
NWRM
Implementation status
Completed
Contact
Mats Ivarsson, Enveco
Location of the project
The Odense is a river in southern Denmark. The project consists of a series of measures to restore floodplain connectivity along a 17 km section of the river.
Country
Denmark
NUTS2 Code
Syddanmark
Sponge measures implemented in the case study
Wetland restoration and management
Floodplain restoration and management
Re-meandering
Stream bed re-naturalisation
Objectives
Regulation of the chemical status of freshwater
Flood control and flood risk mitigation
Biodiversity and gene-pool conservation in riparian areas
Flood control and flood risk mitigation
Biodiversity and gene-pool conservation in riparian areas
Involved partners
| Authority type | Authority name | Authority role | Comments | Authority responsibility |
|---|---|---|---|---|
| Regional / sub-national water authority | Fyn County | Implementation | The former Fyn County actually carried out the project. After the county decommissioning in 2006 the municipality took over. | Responsible |
| Local water authority | Faaborg-Midtfyn Municipality | Implementation | Faaborg-Midtfyn Municipality is the current responsible authority for the project. | Responsible |
Site Information
Climate Zone
Cool temperate moist
Precipitation
727.00 mm
Temperature (°C)
8.20 °C
Runoff
316.00 mm
Elevation range
26
Slope range
very gentle (1-2%)
Soil type
Fluvisols
WFD Ecological status
Good
WFD Chemical status
Unknown
Design and Implementation
Project scale
Large
Project scale specifications
The area affected by flooding during extreme precipitation events was for the investigated river stretch 43,8 ha.
Project area
43,8
Project area specifications
The area affected by flooding during extreme precipitation events was for the investigated river stretch 43,8 ha. If the remaining 95% of restored river stretch behaves in the same manner, the figure only represents 5% of the entire area affected. The entire area affected by the measure consists of 78 ha.
Area subject to Land use change or Management/Practice change (ha)
125.00 ha
Basis of design
The area affected by flooding during extreme precipitation events was for the investigated river stretch 43,8 ha. If the remaining 95% of restored river stretch behaves in the same manner, the figure only represents 5% of the entire area affected.
Constraints
The land use in affected areas have to be transformed permanently from e.g. intensively cultivated land to grazed meadows. This will induce a cost to the farmer which requires compensation, either by an annual compensation based on market values for crops or some other metric, or by land aquisition. Re-meandering and creation of wetlands might also be in conflict with infrastructure and residential or commersial housing.
Favourable preconditions
The evident need to mitigate the effects of the re-occuring flooding events in the downstream towns and villages in the catchment area, together with the availability of land for the purpose of floodplain restauration and wetland creation.
Financing
Total cost
635976.00 €
Total cost information
14,520 €/ha.
The whole LIFE REGAIN project had a total eligible budget of 3,852,367 €. The more recent UC LIFE Denmark project (for mussel habitat restoration) has a total eligible budget of 2,077,261 €
The whole LIFE REGAIN project had a total eligible budget of 3,852,367 €. The more recent UC LIFE Denmark project (for mussel habitat restoration) has a total eligible budget of 2,077,261 €
Costs capital information
3 120 €/ha
Costs land acquisition
11400.00
Costs land acquisition unit
€/ha
Costs land acquisition information
In 2014 prices
Costs operation maintenance information
Not available
Economic costs, actual income loss
The average decrease in income from farm operations due to the conversion from arable land to grazed meadows of land adjacent to the river has been assessed to €11 400/ha.
Policy and Governance
Policy description
The measure will prevent flooding in downtream towns and cities. This will have a number of effects on the pressures relevant for the WFD. Reduced risk for flooding of urban environment reduces the risk for storm overflows from sewers ( PN 1.1) aswell as diffuse pollution resulting from flooding in general (PN 2.1). Re-meandering reduces the pressure from previous physical alterations for flood protection and agricultural purposes (PN 4.1.1 and 4.1.2). It will also potentially have a positive effect on ground water recharge in temporarily flooded areas (PN 6.1).
Land ownership
Over 70 landowners involved, with grazing agreements for 150 ha.
Policy targets
Improved Biodiversity
Pollutants Removal
Peak-flow reduction
Policy pressure
| Directive | Relevant pressure |
|---|---|
| WFD identified pressure | 4.1.2 Physical alteration for agriculture |
| Floods Directive identified pressure | Natural Exceedance |
Policy impact
| Directive | Relevant impact |
|---|---|
| WFD identified pressure | Altered habitat due to hydrological change |
| Floods Directive identified pressure | Waterbody status |
Requirement directive
| Requirement directive |
|---|
| WFD-achievement of good ecological status |
| Floods Directive-mitigating Flood Risk |
Part of a wider plan
Yes
Wider plan
| Wider plan type | Wider plan focus | Name | Comments |
|---|---|---|---|
| National | Environment & Biodiversity | National action plan II for the aquatic environment | 1998 plan. The objective was to restore 16.000 ha of wetlands in Denmark to retain nitrogen. The application was finaced by a state program under the Danish Nature Agency. |
Public consultation
No
Has contractual arrangements
Yes
Contractual arrangements
| Name | Role | Comments |
|---|---|---|
| Grazing agreements. | Land management based on voluntary agreements. | Signed for ~150 ha. |
Impacts and Monitoring
Monitoring System
Monitoring parameters
Ongoing monitoring of flows and riparian inundation could help to validate the modelling results used to justify this measure.
Maintenance
Maintenance requirements
No maintenance required
Impact Assessment Methodology
Performance impact assessment method
Laboratory
Performance impact estimation information
Exploration of the linkages between floodplain hydraulics and in situ observed sedimentationpatterns by: (i) performing a validation of a dynamic 2D river andfloodplain model based on in situ measurements of water depth and flow velocity for a section of a re-meandered 5th order Danishriver channel and floodplain; (ii) compare floodplain flow velocities with measured net deposition of sediment, phosphorus and organicmatter; and (iii) test if simple empirical relationships between floodplain hydraulics and sediment deposition could be developed.
Biophysical impact assessment method
Biophysical impacts were assessed by a combination of modelling and observational studies. Modelling was performed to assess the flow patterns in the floodplains while observational studies were performed to assess the amount of sediment and nutrients deposited.
Cost assessment method
Cost information were provided by Claus Paludan at the municipality of Faaborg-Midtfyn No information was available on the assessment of cost effectiveness of measures.
Socio-economic Impacts
Direct benefits information
There are no monetary valuation of the direct benefits accruing from the re-meandering and wetlands available. Qualitatively, the direct benefits can be described as reduced risk for flooding in down stream villages, towns and land adjacent to the Odense Å during events of extreme precipitation.
Ancillary benefits information
The aditional benefits from this measure are connected to the reduction of nutrient leakage and consequent eutrophication in the Odense Å and in the coastal water receiving the river discharge. The riverbed restoration also bring about a strengthened biodiversity as migrating fish and benthos return to the stream. Positive effects can also be anticipated on cultural ecosystem services such as recreation in the affected areas.
Biophysical impacts: Hydrology
Hydrological effects
Wetland restoration (~860 ha total) increased water retention and restored floodplain dynamics. Overbank flooding is re-enabled in remeandered sections.
Information on retained water
At maximum water detention, approximately 3650 m3 are detained in the investigated area (%5 of the restored river stretch). Assuming that the remaing 95% of the restored river stretch behave in the same manner, the correct figure would be 72 960 m3.
Information on increased water storage
Since the storage is done in temporary wetlands formed during events of extreme precipitation and consequent flooding of grazing meadows (previously cultivated land) the entire volume can be considered to be an increase compared to the previous situation.
Information on peak flow reduction
The measure has led to a reduction in peak flow during events of extreme precipitation.
Information on Restoring hydraulic connection
Barrier removal reconnected 3.1 km of river, improving fish and Unio crassus migration. Longitudinal continuity was partially restored.
Information on peak delay effects
No overall attenuation in runoff is expected, but the timing of runoff, and hence the height of the flood peak, is altered.
Biophysical impacts: Biodiversity
Improved biodiversity
Yes
Information on improved biodiversity
Restoration of riverbeds and re-meandering are themselves measures that strengthen biodiversity in terms of improved biotopes for stream benthos. The periodical flooding of wetlands in connection to the restored river enhances this effect for a number of species, both flora and fauna.
Biophysical impacts: Water & Soil Quality
Water quality overall improvements
Positive impact-WQ improvement
Information on water quality overall
The measurzeaffected water quality insofar as it led to increased deposition of sediment, phosphorus and organic matter on the floodplain as well as removal of nitrogen in the wetlands along the re-meandered river stretch.
Water quality improvements - Nitrogen (N)
39.00
Water quality improvements - Nitrogen (N) - Unit
% reduction pf pollutant
Information on water quality Improvements (N)
An estimate of 235 kg N/ha/yr is removed in the wetlands along the re-meandered river stretch. Restored wetlands reduced nitrogen exports by ~124 t/year. Overall, a 39% N-load reduction (–377 t/year) was measured between 2000–2015.
Soil quality overall improvements
N/A info
Biophysical impacts: Others
Ecosystem impact climate regulation
Reduced energy use (farm operations)
Information on ecosystem impact climate regulation
The conversion from arable land to grazed meadows of land adjacent to the river will result in decreased energy use in farm operations.
Lessons Learned
Key lessons
Water detention through temporary flooding of wetlands can play a significant role in cutting of flow peaks, and thereby decreasing water levels and floodrisk in downstream towns and villages during incidents of extreme precipitation.
Drivers
| Driver type | Driver role | Comments |
|---|---|---|
| Other | Main driver | The financial support of the Danish Nature Agency through the national plan for the aquatic environment together with the initiative from the former Danish Counties played a cruicial part for enabling the project. |
Success factors
| Success factor type | Success factor role | Comments |
|---|---|---|
| Specific incentives for stakeholder involvement | Main factor | The "toolbox" offered contained several measures (including land consolidation). The project succeded in establishing voluntary agreements with and among the landowners in this project. |
Cost effectiveness
No “traditional / structural” methods are available to achieve the floodplain reconnectivity accomplished by the Odense measures, thus it is very difficult to make this comparison.
Flexibility / Adaptability
There may be a potential to adapt the measure to changing baseline conditions either through additional remeandering or further work on the channel bottom.
Transferability
Some elements of the measures implemented here are transferrable to many agricultural rivers throughout north and central Europe. In any place where historical channelization has disconnected rivers from floodplains, a potential may exist for measures which reconnect the river and its floodplain.
Sources
Sources