Case studies: Restoration streams of the Råån

Restoration of streams

The Lussebäcken stream, a small tributary to the river Råån, has historically been straightened and deepened, like almost all water courses in the agricultural landscape in the south of Sweden.

Since the end of the 1990s, the municipality of Helsingborg performed various nature-based measures in the small catchment. The aim was to remediate base flows and reduce peak flows, and at same time reduce nutrient transport to the sea and increase biodiversity. Approximately 20 ponds and wetlands have been constructed. A few stretches of the stream have also been rehabilitated to a two-stage channel design.

Now a comprehensive study has been conducted to find out about the effect of these measures.

Method

• Hydraulic modelling of the flood reducing effects ofstormwater ponds, constructed wetlandsand two - stage ditchesin the sub- catchment of Lussebäcken.
• Analysis of two-stage channel hydraulic performance.
• Evaluation on biodiversityof the Lussebäcken two-stage channelsand analyse benthic and fish monitoring data to assess the effect of implemented restoration measures in the Rååncatchment.

Results

The cumulative effects of measures in the upper part of the catchment have a positive effect on both flooding extent and peak flows for the studied scenarios. However, by adjusting the outlet capacity from ponds and wetland, either by new weir designs or smaller outlet diameters, several thousands of additional cubic meters can be further retained.

The two-stage channels can reduce the water velocity significantly and also, the water depth at high flows. This leads to an increase in hydraulic residence time which results in reduced erosion and increased retention of sediments and nutrients. Hence, they can also significantly contribute to sediment trapping and nutrient retention during overbank flows.

However, the study highlights the importance to promote early overbank flow but also to impede dense emergent in-channel vegetation, otherwise the hydraulic residence time will decrease.

Early tree planting is therefore part of ensuring the safeguard of the two-stage channel hydraulic performance.

Improvement of the biodiversity of benthic invertebrates was observed for the mid and lower reach sites of Råån. The data could however not prove if the improvement was caused by the measures.

Local context

The Lussebäcken stream falls into river Råån in the city of Helsingborg (pop. 108 000). During the last 50 years there has been a comprehensive urbanisation in the small catchment. The need to retain peakflow has therefore increased significant, as well as the need of recreation areas for the residents.

By applying building with nature techniques for water management, a wide range of ecosystem services are provided:reduced risk of flooding, improved water quality and enhanced biological diversity.

Today, the Lussebäcken stream provides important spawning habitats for anadromous brown trout, whichprovides the basis for recreational fishing, taking place in the river Råån as well as along the coast.

Future research directions

Building on the developed joint field-modelling approach, improve design-support and performance-assessment of two-stage channel restoration by:

• further refining the approach and expanding the use of conservative trace data in model calibration/validation process to include in-channel vegetation seasonal effects;
• simultaneously assessing the effect of interventions maximising floodplain overflow on both hydraulic and nutrient/sediment retention performances;
• studying the effect of restoration length on performance;
• studying the balance between flooding and riparian biodiversity.

Documents

All documents of the BwN project can be found at publications,  below you find the documents belonging to this specific case study:

• Flood analysis Lussebäcken, Helsingborg - Hydraulic impact of urbanization and effect of implemented water management measures (pdf, 4.2 MB)
• Lansstyrelsen Skane Catchment Policy brief (pdf, 1.8 MB)

Upcoming reports:

• Lussebäcken, Helsingborg Municipality: Evaluation of the hydraulic performance of two-stage channels through comparative analysis – joint field – modelling approach.
• Råån, Helsingborg Municipality: Field evaluation of two-stage channels impact on local biodiversity and nutrient retention potential