Stream bio-assimilation capacity is assessed in a developing peri-urban area on the west side of the Lyon city (France). The total population of Lyon is 1.6 M (2000) and the climate is mild continental with Mediterranean influences. Perennial and intermittent small streams with porous sediment beds and high stormwater/groundwater exchange capacity represent ecosystems very sensitive to pollution inputs. Threats are poor water quality, physical habitat degradation and flood hazard. The study objective is to adapt Combined Sewer Overflows (CSOs) to the bio-assimilation capacity of the receiving streams. Major action consists of identification of factors controlling the bio-assimilation capacity, and exploitation of the enhancement of these factors for improving the state of water resources in the catchment.

Yzeron River, Combined Sewer Overflows (Photo credit: P. Breil).

Background

The city of Lyon is facing rapid urban development. Its west-side catchment is a hilly land drained by a dense network of streams and seasonal tributaries. The total water demand is greater than the natural water input to the catchment, and the natural water resources are unable to fully support the needs of the present population. Complementary supply of potable water comes from the alluvial plain of the Rhône and Saône rivers, on whose confluence the old Lyon city centre was built. A similar situation is often observed in peri-urban areas of large cities of the word.

During the last 25 years the urbanised area of the catchment studied (Yzeron) has grown from 20% to 25% of the total area (150 km²) and is expected to reach 50% in 2030, with serious impacts on water resources. For this ultimate development, hydrological simulations revealed that the peak flows of small floods will be about 10 times higher than in the present situation.

The annual volume of water flowing in the main sewer pipe at the outlet of the watershed is presently composed of 44% of domestic wastewaters, 14% of stormwater and 42% of groundwater. Given the amount of water flowing in the Yzeron river at the outlet, it means that the combined sewer network conveys about a quarter of the natural rainfall and groundwater flow from this watershed. The urban development is then expected to deliver more pollution to the stream during rainfall events and to withdraw good quality water that feeds the river during dry weather periods.

Seepage of good quality water into the sewer pipes results from the degradation and neglect of maintenance of this 30-year old sewer network. Rehabilitation of such an infrastructure is however costly and the replacement option seems to reach its sustainable limits when addressing the future peri-urban development. At the same time the limited flow capacity of the old sewer network also increases the number of CSOs. Thus, best management practices need to be proposed and integrated in the urban planning (see Chapter 6).

Key aquatic habitat issues in urban water management

Aquatic ecosystems consist of some perennial but mostly intermittent streams. Running waters are oligotrophic as a result of a granite bedrock covered by a thin layer of acid soil. Stream bottom sediment is dominated by sand, with thickness reaching more than one metre depth in some places, but it greatly varies along the water course. Prevailing hyporheic and benthic interstitial fauna are present in the bed sediment. They are composed of species tolerating well dry periods and disconnections between surface water and the otherwise connected groundwater. Porous sediments offer a high capacity for exchanging surface and ground waters, resulting in a very active ecosystem which is able to bio-assimilate nutrients (Breil et al., 2007), while being at the same time highly sensitive to pollution inputs. Presently there are well preserved stream corridors extending from the upper rural part to the lower urbanised part of the watershed. Such a layout avoids direct pollution inputs from agricultural (cultivated) areas and from direct urban runoff. However, at the same time about forty combined sewer overflows are located along the main watercourses, including some on tributaries. Preservation of sensitive stream corridors and high resilience of the aquatic ecosystems supported by these habitats, with respect to coping with pollution, is a key issue in managing CSO impacts in this catchment.

Objectives of the Case Study

Streams and rivers use self-purification processes to improve water quality, if the habitats and biota are properly managed, without impairing their natural bio-assimilation processes (Zalewski, 2000). This is a natural potential which can be quite reactive and intense in sandy bottom streams like in the Yzeron catchment. Although so far these stream properties are not yet used extensively enough, a lot of research studies demonstrated their existence and usefulness (e.g., Jones & Mullholland, 2000; Hancock, 2002; Hancock et al. 2005; Lafont et al. 2006a,b).

Considering the water management issues in the study area, and the potential benefits resulting from the understanding and proper management of the aquatic habitats quality, the objectives of this Case Study were:

• risk identification based on comparing CSOs loads and bio-assimilation capacity of the receiving streams;
• defining better locations for CSO outfalls with respect to lessening the stress on aquatic habitats;
• developing local intervention measures for artificial enhancement of bio-assimilation capacity in stream corridors (Kasahara & Hill, 2006);
• assessment of the stream bio-assimilation capacity contributing to the overall increase of the reliability and resilience of the urban water cycle, with respect to increased water demands and associated threats, and increased variability of water resources as expected from global climate change.

Stakeholders

The Yzeron River contract contains a series of actions to be implemented by a number of stakeholders directly involved in this process. These actions were agreed upon and planned during a four-year study as follows: list main sources of impairment, define corrective actions, and estimate financial budget. State, regional and local administrations contribute to the project funding. The first objective of the river contract is to rehabilitate and protect the water quality in streams. The research central role is to set up a series of performance indicators and design a field observatory to monitor the effect of the implemented measures. The objective is to get a feedback on the efficiency of these actions regarding their initial objectives: water quality improvement, flood control, and drought control.

See References