Amplification of the self-purification capacity of a small urban river using a natural solution to reduce the effect of urban stormwater degradation on its ecological status (France)

The NbS implementation project required several actions with stakeholders: river union, sanitation union, fishing federation:

Analysis of the technical and operational context of the NbS project

• Shared analysis of the degraded ecological state of the river on which the NbS is to be installed;
• Discussion on the different solutions implemented to reduce the impact of urban wet weather discharges:
  • Reduction of runoff at the source by infiltration and/or evaporation in rain gardens
  • Transitional storage of rainfall for 30 years and 1 hour with a flow of restitution to the natural environment at a rate of 5 liters per impervious hectare per second. Valid for new constructions.
  • Disconnection of urban runoff water from combined networks by putting it into separate networks. Treatment of rainwater by planted filters before restitution to waterways.
• Discussion on the question: "Why use an in-stream solution? Answer: "It complements all the actions taken upstream, which will take time to produce noticeable effects on the reduction of urban discharges during rainy weather.
• Explanation by the researchers of the functioning of the innovative NbS in the stream from the results obtained on a stream subjected to urban discharges during rainy periods (proof of concept); the NbS creates long sandy flats not very favorable to the diversity of habitats but it is a natural characteristic of the stream in its downstream zone.

Transition to the operational phase

- Discussion on the effective location of the NbS: The NbS is placed at the outlet of the watershed to intercept the urban pollution that is concentrated in the downstream part. It is also the location of the confluence with a watercourse of good ecological quality whose condition must be preserved.

- On the basis of the opinions and constraints, stated above, the study area is selected in the land owned by the river syndicate in the downstream zone. This makes it possible to intervene in the public domain without any condition of purchase or private right.

- Study of the possible locations of the NbS in the pre-selected study area: use of the topographic longitudinal profile of the river bed;

- Choice of two locations out of five identified because of the request of the fishing federation not to impact the diversity of the hydromorphological facies in the reproduction zone of the fario trout. The NbS would interfere with a flood risk reduction and restoration program located in the upstream part of the pre-selected study area.

- Simulation of the hydraulic effect of the NbS on the overflow of the watercourse and analysis of the associated risk; The hydraulic study allows to finalize the height of the NbS in the watercourse;

- Negotiation with the river syndicate for the modalities of realization of the NbS: marking of the precise places of establishment and market of subcontracting for intervention of the earthmoving machines with limited ecological impact to the maximum;

- Taking charge of the construction operation by the river syndicate, landowner of the places; piloting of the realization by the researchers to write a follow-up of building site for any future replication;

- Communication with specialized media to inform managers about the innovative NbS;

Elaboration of monitoring and information tools on the NbS

- Setting up of early indicators of non functioning for a follow-up by the river technicians;

- Setting up of a metrological device to evaluate the effect of the NbS on the physico-chemical parameters of the organic matter in the surface water and in the sandy sediment (hyporheic zone).  Drafting of a light protocol for a follow-up by the river technicians.

- Installation on the site of the NbS of a panel of information intended for general public and with the formation; links QR code on video explaining the phases of the project.