We are planning to follow option 2 (Natural Land Cover Conditions). The area consists mainly in clay with very low infiltration capacity. The rainfall data are 78,50 mm (100 percentile) and 18,40 mm (95th percentile) if calculated with runoff coefficients for clay of 0,2, the runoff from the area before developtment is 12 m3 (using the 95th percentile as rainfall event). The runoff coiefficent after development is 0,76, which results in a runoff of 44 m3.
Moreover, there is a pumping station to drainage a train tunnel located nearby the area.
The municipality is planning a flooding park just downstream the building for managing stormwater from this part of the city. Due to infiltration is not a feasible option in this case, we are working with the alternative of installing an underground detention tank. The tank will have capacity for the 95th percentile and just a natural flow will leave the area (the volume that is not required to be managed according to the credit). When the rainfall ends, the stormwater will be discharged from the tank as a natural flow and will not be reused as graywater for flushing toilets or watering plants. In this way, 95th percentile is being retained on site and discharged in a natural way. Does this approach meet the credit definition? Does it help the fact that the tank will be a gravel-filled-tank, which replicates in a better way a natural retention? Do you have any recommendation to a better approach in this case to meet the credit requirements? Is the 95 percentile the right rainfall value to be used in the calculations according option 2?
Would it for example be ok if we retained the water and then discharged it to the flooding park that is designed in order to handle some of the runoff volumes from the area and to also show and display LID measures to the public.