Our project site is a landfill area which was used as a parking during the construction of a tunnel located nearby. The site consists on very low permeable soil (mainly clay). There is a pumping station located between the project site and the tunnel which is used to drainage the tunnel area (lower the groundwater level). In the area next to the project site is planned a flooding park with capacity to manage torrential rain from this part of the city. Our project consists on an office building with a green roof (partially). The buildings area is aprox. 70% of the site, which means that the space for rainwater management is quite limited.
We first approached the rainwater management looking at option 1 (98th percentile), but the runoff volume to be managed is extremely high. We are now looking at option 2 to manage the delta pre vs post for the 95th percentile. The measures to reduce the runoff includes a green roof, some green areas with trees (nowadays there are no trees in the area) and an underground detention tank. The detention tank is planned to be a gravel-filled tank, to best replicate the natural hydrology process. The tank will have capacity to storage the delta pre vs. post. The water will then be discharged after the rain event in a natural flow. We are even studying the possibility to reuse part of the volume to watering of the vegetation.
- Which approach do you think is better in this case, option 1 or option 2, considering the soil conditions?
- Does the use of a detention tank with a natural flow discharge help meet the credit definition (is it acceptable to delay and discharge in a natural way or it is just reuse the only option)? Due to infiltration is not feasible in this case, we are looking at storage and release to the downstream flooding park in a natural flow after the rain event.
- Does it help the fact that the tank will be a gravel-filled-tank, which replicates in a better way a natural retention?
- Can we consider the flooding park downstream the project site (located in the adjacent plot) to manage rainwater and obtain points in this credit?