Aquatic Habitats In Sustainable Urban Water Management

Urban aquatic habitats: characteristics and functioning 21
Schitthelm, 2006) and, in particular, what remedial measures must be planned. A
detailed listing and discussion of various possible measures is beyond the scope of this
book, but one can propose actions that preserve or enhance the resilience capacity and
prevent significantly exceeding the LOUE. Some (but not all) possible management
measures are highlighted in Figure 2.3.
2.3.3 Selecting preservation or rehabilitation measures
In this section, the seven main measures for enhancing the aquatic ecosystem resilience
capacity are defined as follows:
(1) preserve or rehabilitate the geomorphic corridor of aquatic ecosystems and diversity
of aquatic habitats (realistic in suburban areas)
(2) preserve or restore the hydrologic connectivity and the dynamics of hydrologic
exchanges between surface water and groundwater
(3) prevent pollutant storage in the ecosystem, either in the hyporheic sub-system or
in fine sediments, storage which constitutes a time-bomb when environmental
conditions become favourable for pollutant release
(4) avoid excessive pollution discharges with respect to the size of the receiving aquatic
habitat
(5) where preserved areas exist (mainly in suburban areas), provide hydrological connections
between those areas
(6) permit pollution inputs only if the resilience domain is preserved and the LOUE is
not significantly exceeded
(7) check the efficiency of remediation measures every year by specific indices, and
every two to three years by integrative methods.
The importance of stream corridors (see Chapters 4, 5 and 9) for stream water quality and
biological integrity is well known. The dynamics of hydrologic exchanges between surface
water and groundwater greatly stimulate the nutrient cycling (Jones and Mulholland,
2000; Boulton and Hancock, 2006; Breil et al., 2007), but only if the stream-bed is permeable.
When the stream-bed is impervious (artificial concrete bed), the transport of pollutants
prevails, but pollutants may be stored in downstream areas as soon as conditions
favourable for pollutant settling occur. In particular, pollution discharges excessive to the
size of the receiving aquatic habitat induce downwelling of surface polluted water and storage
of pollutants in the hyporheic layer of gravel streams (Lafont et al., 2006). The reduction
of flow velocities in streams or rivers, for example by a dam reservoir or a pond,
causes the deposition of polluted sediments originating from upstream reaches.
Management practices have to account for this pollutant storage, which is a time-bomb
triggered off by the occurrence of environmental conditions favourable for pollutant
release (a decrease of the redox potential in fine sediments, upwelling of polluted hyporheic
waters to the surface, etc.). Furthermore, remedial actions have to be subject to social and
economic considerations. In general, remedial actions are easier to implement in suburban
rather than urban aquatic habitats, and in new cities rather than in the older ones.
One final point requires clarification. There is a risk of misinterpretation of the
resilience/ED capacity factor. It is not the intention of the remedial action plan to
transform receiving ecosystems into wastewater treatment plants, which would not be