Where rain goes after it hits a city street

Watch a summer shower from a window and two routes for the water appear. Rain falling on a garden may soak into the soil or collect in a low spot. Water landing on a roof can run into a gutter, down a pipe and across the pavement. A street drain may carry it away quickly. The surfaces and connections around a city change both how much water runs off and how soon it arrives downstream.

Begin with the surface

The U.S. Geological Survey explains that surface runoff is the part of precipitation that flows over the land. Whether water soaks in depends on the surface, the soil, its existing moisture and the storm. Roofs and ordinary pavement are examples of impervious cover. They prevent much of the rain falling on them from soaking in where it lands.

For scale, imagine 10 millimetres of rain falling evenly on a 100-square-metre roof. One centimetre is 0.01 metres, so the volume landing on the roof is one cubic metre, or 1,000 litres. That is the amount arriving on the surface. To estimate what reaches a drain, you would also need to know how much was held back or diverted along the way.

The route matters as much as the surface

Two identical roofs can send water in different directions. A downpipe that empties onto a planted area gives some water a chance to soak in. A pipe connected directly to a storm drain can move it into the network much more quickly. When many connected surfaces drain in a short period, downstream pipes and channels have to carry that flow together.

Water can pick up sediment, litter and other pollutants along the way. The U.S. EPA describes rain gardens and other forms of green infrastructure that collect, slow or filter runoff. How a project works depends on the location, soil, local rainfall and the drainage arrangement around it.

Read a project description by tracing the flow

Look for the size of the area that drains to a project, the route water takes into it and where any overflow goes. A small planted area could be designed to handle runoff from a particular roof, for example. The amount it can hold and the speed at which water can soak away tell you more about the project than its appearance.

Compare proposed and existing routes. Finding the place where the paths diverge can show exactly what a project is meant to change.