Restoration of the Odense river, Denmark | Natural Water Retention Measures

General

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.

Light or indepth?

In-depth

The in-depth description of the case study

cs-dk-01-final_version.pdf

NUTS Code

Syddanmark

RBD code

DK1

Transboundary

Data provider

Mats Ivarsson, Enveco

Source(s)

Linking floodplain hydraulics and sedimentation patterns along arestored river channel: River Odense, Denmark

Vandtillbageholdelse i vådområder: Odense Å case område

Odense Å ved Brobyværk

NWRM(s) implemented in the case study

Wetland restoration and management

Floodplain restoration and management

Re-meandering

Riverbed material renaturalization

Longitude

10.2824

Latitude

55.2196

Site information

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

Monitoring maintenance

Monitoring impacts effects

Performance

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.

Design & implementations

Application scale

Field Scale

Age

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 contractual arrangement

Arrangement type	Responsibility	Role	Comments	Name

Design consultation activity

Activity stage	Key issues	Name	Comments

Design land use change

Land use change type

Design authority

Authority type	Role	Responsibility	Name	Comments
Other	Implementation		Fyn County	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.

Lessons, risks, implications...

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	Success factor role	Comments
Specific incentives for stakeholder involvement	main factor	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	Comments
National funds	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	Barrier role	Comments
Other	main barrier	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	Driver role	Comments
Other	main driver	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).

Financing share

Financing share type	Share	Comments

Policy, general governance and design targets

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

Policy target

Target purpose
Peak-flow reduction

Policy pressure

Pressure directive	Relevant pressure

Policy area

Policy area type	Policy area focus	Name	Comments

Policy impact

Impact directive	Relevant impact

Policy wider plan

Wider plan type	Wider plan focus	Name	Comments

Policy requirement directive

Requirement directive	Specification

Socio-economic

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

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

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.

Biophysical impacts

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