The next page of our DSSP shows the grading plan. This is more about movement along the surface for the storm water. I have labelled the key elements of the plan in red type. Basically we are placing manholes at the corners of the property, and connecting them with a combination of pipe and surface swales. At the final manhole, we are going to create a concrete surface box that could act as a retention pond in the event of a flood. The pond would drain back into the last manhole, and finally out into the storm sewer.
The idea is that these catchment basins will slow the initial flood of water into the storm system, and delay its arrival that bow river which is downstream of this community. Eavestroughs must be pointed toward the manholes so that the water off the roof will not exit the site in an uncontrolled manner. In combination with soft surface landscaping, the new project should not have an impact on storm system capacity. It is fairly obvious that these systems are complex and expensive to install, and the City policy is what determines outflow rate at the end of the pipe. This is where it can get really difficult to comply.
This is the slightly edited grading page, where the flows are labeled with directional arrow and the surface pond is identified.