Although cities afford untold opportunities to the human race, the development that goes with them tends to cause problems when it comes to runoff management. Concrete is filled with impervious surfaces that prevent water from infiltrating/percolating naturally into the ground. Managing the flow of excess water through urban environments can be a complex challenge.
When precipitation falls on an impervious surface in an urban environment, rather than soaking into the soil, it flows along the surface wherever the grade or slope forces it to go (runoff). In a well-planned system, the water is funneled into drainage ditches and storm drains before it can cause flooding. Even then, water that is funneled into creeks and rivers can cause them to swell faster and higher than their capacity will allow, leading to flooding locally and further downstream.
Therefore, when new construction is underway, one of the items that must be addressed is how this construction will impact the flow of water both above and below the surface of the ground when precipitation occurs, especially heavy precipitation. Fortunately, urban environments can use many methods to gather and manage rainwater runoff.
Bad drainage decisions not only cause issues initially, they also can create problems for someone else further down the line. This is why most large cities require drainage reviews as part of the building permit process, making sure that new construction adequately manages runoff and flooding. Excess water, no matter its origin, must have a place to go to prevent property damage and loss of life.
When new buildings are constructed, whether they are commercial buildings or new homes, the lots where they are to be built are graded with a highpoint for the building and the surrounding soil has a slope that flows downhill from the structure, thereby forcing any precipitation to flow away from the building and into the road. Streets are typically built with a crown at their center, forcing water to flow down to the sides and into its gutters; allowing the road to remain flood-free for vehicle safety. The water is continually funneled towards the next level of collection, flowing downhill in the gutter until such a time that a natural low point is reached. Subtle sloping is designed to force the runoff towards centralized drains and basins intended to capture the water.
At one point in time, stormwater runoff was allowed to flow into the city’s sanitary sewer system. It was believed to be a viable solution because it was already in place and had a well-developed system to carry water out of the city using the flow of gravity.
This solution, however, created problems for its final destination – the wastewater treatment plant – which had not been designed to process the city’s sewage, as well as whatever rainfall mother nature felt inclined to deliver. When the intake of water (sewage combined with rainwater) was beyond the capacity of the treatment plant’s ability to process or store, wastewater was released directly into waterways which flowed untreated into rivers, streams and lakes.
This unfortunate turn of events led to the creation of separate storm sewer systems (usually abbreviated as MS4s) which include networks of:
These all work together to move runoff from the urban environment, separate from the sanitary sewer system, to natural waterways that ultimately carry the water away. This water, which does not pass through the wastewater treatment plant, flows to natural waterways such as streams, rivers and lakes as it is; untreated.