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 context is that of a peri-urban area of a large city (Lyon, France, 1.4 million inhabitants). This area develops rapidly in a low mountainous relief (altitude from 400m to 800m) drained by numerous small streams with a rainfall regime. During low water periods, urban runoff from summer storms is a major source of water quality degradation in the entire headwater system. The NbS is located at the outlet of a 30 km2 peri-urban watershed, occupied by 16% cropland, 25% forest, 27% grassland and 30% built-up areas. The influence of the urbanized area is reflected in the increasing number of overflow devices (CSOs) for urban effluents that pollute the river during rainy weather.

Ecohydrological engineering is used here to enhance the self-purification capacity of a small stream polluted by urban rainfall discharges. The demonstration site, realized in collaboration with the stakeholders, shows the positive effect on the chemical quality of the stream

This case study is one of the 3 that are part of the ATENAS “To Ally Technology, Nature and Society for integrated urban water management” - an international project of JPI Water financed under the framework of the WaterW

National financing:

National Research Agency (France)

https://anr.fr/Projet-ANR-18-WTW7-0005

orks 2017 Programme.

• Improvement of the ecological quality of the river (chemical, biological, hydrogeomorphological)
• Low investment cost
• Self-sustaining operation by the natural flow regime (on average)
• Early indicators of dysfunction (for low cost remediation)
• Educational support for awareness and replication

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.

The river union associated with the ATENAS project has expressed interest in replicating this solution on other sites in its territory of action if it proves to be successful.  The solution tested on the demonstration site has proven its effectiveness. The site will serve as an educational platform for schools, the general public and developers. An information panel is being created before being installed on the site. The study of replication sites is being discussed with the sanitation union

The realization of a NbS implies shared information and understanding by the stakeholders. Construction and maintenance costs as well as monitoring indicators are important criteria for the decision, as well as the absence of additional costs and ecological consequences in case of failure.

National Research Agency, and local operational actors for water related-issues: Yzeron river union (Sagyrc) and the upper Yzeron valley sanitation union (SIAHVY).

INRAE/RIVERLY-Hybv team & INRAE-ARA center communication team

Pascal BREIL, French partner scientific coordinator

National Research Institute for Agriculture, Food and Environment / Auvergne-Rhône-Alpes centre

CS 20244, 69625, 5 Rue de la Doua, 69100 Villeurbanne, France

Phone: +33 04 72 20 87 81



pascal.breil@inrae.fr

https://www.inrae.fr/en/about-us

https://www.inrae.fr/centres/lyon-grenoble-auvergne-rhone-alpes