On densely developed sites the surface area of the facility may be constrained, in this case the required water storage reservoir depth of the bioretention cell or infiltration trench, d, can be calculated assuming three dimensional drainage as follows: | On densely developed sites the surface area of the facility may be constrained, in this case the required water storage reservoir depth of the bioretention cell or infiltration trench, d, can be calculated assuming three dimensional drainage as follows: |