As already mentionned, Taradellas et al. (2003) recommended that the discharge flows at
the ZIPLO outlet stay below 1 m3s−1 so as not to impact badly the L’Aire river. Four sus-
tainable stormwater management strategies were designed so as to limit discharge flows in the future (scenario “climate change and urbanization” of previous section). These stormwater management strategies are not exhaustive and should not be considered as ex- ecution plans; they are proposed as examples so as to illustrate the interest of our modeling approach for long-term planning of stormwater infrastructures. As the ZIPLO is located above a ground water table used for providing drinking water, infiltration infrastructures were not envisaged. The four stormwater management strategies are defined as follows:
1. Large retention basin: Taradellas et al. (2003) proposed the implementation,
downstream of the ZIPLO, of a retention basin with a storage capacity of 8000 m3to
limit discharge flow in the case of increased urbanization. Considering climate change impact in addition to increased urbanization, iterative runs of STORM suggested to
augment the basin capacity to 12500 m3 to achieve the same performance in flow
limitation as reported in Taradellas et al. (2003). The basin empties with a maximum
throttle discharge of 1 m3s−1 and when the maximum storage capacity is reached,
water discharges toward the outlet via an overflow weir. The basin is connected to pipe 1 and collects all stormwater from the ZIPLO.
2. Combined retention basins: Limiting more efficiently discharge flows at 1 m3s−1
would require an even larger retention capacity than the one proposed in strategy 1. Increasing the basin capacity would not be judicious due to constraints of town planning and of the shallow ground water table; however, the presence, downstream, of the football playing-fields of the Cherpines sporting zone (cf. Figure 5.2) offer an alternative to create a secondary retention. Indeed, a playing-field, whose size is typically about 70m large by 120m long, could easily be fitted up with 1m-height walls (or excavated) and act as a temporary retention basin with a capacity of
8400 m3 (this idea was inspired from an ongoing project in Vilarinho, Belo Horionte,
Brazil). Such a retention basin would not thwart town-planning plans, while the risk of the playing-field to be flooded from time to time remains acceptable. A second
basin of about 8400 m3 is thus implemented downstream of the basin proposed in
strategy 1 and collects overflow waters of the latter. This second basin empties with a maximal rate of 500 l/s, and potential overflow would discharge through a weir toward the outlet.
3. Green roofs and retention basin: In order to take advantage of the large flat roofs located in the ZIPLO area, several green roofs were implemented in addition to the retention basin of strategy 1. According to the availability of flat roofs in each sub-catchment, green roofs (GR) with a thickness of 12 cm were implemented as follows:
GR 1 GR 2 GR 3 GR 4 GR 5 GR 6 GR 7
Sub-catchment ID: 2 9 11 20 21 22 23
GR surface (m2): 4000 5000 5000 6000 5000 4000 5000
4. Decentralized infrastructures: As an alternative to strategy 2 where stormwater is collected in a centralized infrastructure, the benefits of an option with decentral- ized elements were evaluated. The infrastructures of strategy 3 were thus completed by the implementation of several small retention basins. Their locations were se-
lected according to the runoff contribution of individual sub-catchments and to the space availability required to their implementation. Iterative optimization runs of STORM suggested the following set of basins’ characteristics :
basin 1 basin 2 basin 3 basin 4 basin 5 basin 6
Connected sub-catchments: 3,5 9,10 17,19 6,7,12–14 18 20
Capacity (m3): 600 560 1800 2500 500 3500
Throttle discharge (l/s): 150 150 150 100 150 450
Performances of these four strategies in reducing peak discharge flow at the ZIPLO outlet
are presented in Figure 5.67. The large retention basin (strategy 1) is very effective and
allows to divide by two the peak discharge flow, while the ability of the green roofs (strategy 3) to limit peak flows is rather poor. The combined retention basins and decentralized infrastructures (strategies 2 and 4) improve significantly strategy 1, allowing to reach the
optimal value of 1 m3s−1.
Return periods (years)
Retur n le v els (m3/s) 5 10 15 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 10 20 30 50 Scenario without adaptation large retention basin combined retention basins green roofs and retention basin decentralized infrastructures
Figure 5.6: Ability of different stormwater management strategies to limit peak discharge flows at the ZIPLO outlet.
While the some proposed stormwater management strategies seem to be effective in con- trolling peak discharge flows, their value should be weighted against several other criteria including: building and maintenance costs, pollutant removal, social amenity, landscape enhancement and town planning.
7
The indicated values result from an empirical frequency analysis of the simulated maximum annual discharge data at the outlet, and not from a Gumbel distribution fitted on annual maxima as was done for Figure 5.5 in the sake of compatibility with the analyzes made by Taradellas et al. (2003)
For instance, in the present application, extensive green roofs 8 were implemented in the STORM model: their stormwater retention capacity is limited but they are rather inexpensive to build and offer at least four additional advantages: 1) they protect the buildings against summer overheats, thanks to the thermal capacity of the gravels and to plant transpiration, 2) their potential to enhance biodiversity is high as they constitute dry meadows ecosystems that are rarefying in Switzerland, 3) they have fairly no impact on town-planning and 4) their maintenance costs are low. Retention basins were found very effective to reduce peak flows, but because of their space consumption, they must be included in town planning. The large basin of strategy 1 is located downstream of the ZIPLO where there is available space. In addition to stormwater retention, this basin could be landscaped or could be permanently pounded to form a constructed wetland: this would enhance pollutant removal by plants uptake and sedimentation, as well as improving the landscape. In a preventive perspective, the small decentralized retention basins of strategy 4 could be equipped with emergency closing in order to retain, for example, contaminated
water used to extinguish building fire 9.