Restoration of the Odense river, Denmark

National Id
Denmark_01
Site name
Odense river catchment
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.
The in-depth description of the case study
NUTS Code
Syddanmark
RBD code
DK1
Transboundary
0
Data provider
Mats Ivarsson, Enveco
Longitude
10.2824
Latitude
55.2196
Climate zone
cool temperate moist
Mean rainfall
727
Mean rainfall unit
mm/year
Average temperature
8,19999980926514
Mean runoff
316
Mean runoff unit
450 - 600 mm
Average runoff coefficient
0,430000007152557
Type
Actual Test Site
Light or indepth?
In-depth
Monitoring impacts effects
1
Performance impact estimation 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.
Application scale
Field Scale
Age
10
Performance timescale
5 - 10 years
Area (ha)
43,7999992370605
Area subject to Land use change or Management/Practice change (ha)
125
Design capacity description
Maximum water detention capacity for the investigated part of the restored river stretch (5%): 3648 m3. If it is assumed thar the remaining 95% of the restored river stretch behave in the same manner, this figure thus represents 5% of the total water volume that can be detained.
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.
Design authority
Authority type
Other
Role
Implementation
Name
Fyn County
Comments
Faaborg-Midtfyn Municipality is the responsible authority for the project. However it was the former Fyn County who actually carried out the project. After the county decommissioning in 2006 the municipality took over.
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.
Success factor(s)
Success factor type
Specific incentives for stakeholder involvement
Success factor role
main factor
Comments

Two main factors: a) the project was fulfilled without the use of compulsatory means. Thus the project was fulfilled based on voluntary agreements. b) Nitrogen retention equivalent to approx. 200 kg N / ha/ yr.

Financing
Financing type
National funds
Comments
Basically it was finaced by a state program under the Danish Nature Agency. However, as time went by, the land prices became higher and higher and in the end the County had to hold the remaining finacing (approx. 50 %).
Barrier
Barrier type
Other
Barrier role
main barrier
Comments
The main barriere was the willingness of the landowners to participate in the project, However - the "toolbox" contained several measures (including land consolidation) to overcome this problem. Therefore project succeded in reaching voluntary agreements among the landowners in this project.
Driver
Driver type
Other
Driver role
main driver
Comments
The project was part of the national action plan II for the aquatic environment from 1998. The objective was to restore 16.000 ha of wetlands in Denmark to retain nitrogen. The main drivers of the project were the former Danish Counties (they were decommisoned in 2006).
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).
Part of wider plan
1
Policy target
Target purpose
Peak-flow reduction
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.
Costs investment
3120
Costs investment information
The figure represents the investment cost per hectare in (in 2014 prices).
Costs land acquisition
11400
Costs land acquisition unit
€/ha
Costs land acquisition information
In 2014 prices
Costs operation maintenance
Not available
Costs total
14520
Costs total information
The figure represents the investment cost per hectare including cost for land acquisition but exkluding maintenance costs.
Ecosystem improved biodiversity
1
Information on Ecosystem 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.
Ecosystem provisioning services
0
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.
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 runoff reduction
Null
Water quality overall improvements
Positive impact-WQ improvement
Information on Water quality overall improvements
The removal of nitrogen in the wetlands along the re-meandered river stretch is estimated to 235 kg N/ha/yr
Soil quality overall soil improvements
N/A info

Partners

Logos of all partners of NWRM project