Attaining Sustainable Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 509

Roof Downspout Driveway Road surface Curbs

Storm sewer pipes

Outfall (local water body)

FIGURE 16.1
Paths of stormwater in an urbanized watershed.

water to the next component in the chain horizontally. Although precipitation reaching the
ground surface can go in three possible directions (up with evaporation, down with infil-
tration, and horizontally with surface runoff), the human-use system has heavily favored
the horizontal flow path. This routing bypasses the soil as a flow path and groundwa-
ter recharge declines. Instead, stormwater is delivered rapidly to the nearest water body
where it discharges its load of contaminants.
Source control intervention for stormwater must occur before contaminants reach the
environment. Since intervention in the atmosphere is impossible, the appropriate location
occurs at the individual land parcel, for once water has left a lot it becomes part of the
human-engineered horizontal transmission system. As discussed in Chapter 13, discon-
necting downspouts is an example of source control intervention at the parcel scale. The
significance of when to intervene with source control is illustrated by Figures 16.2 and 16.3.
Figure 16.2 shows the grid pattern typical of most urban areas. In Figure 16.3, this
human-engineered pattern of development and drainage infrastructure is overlaid on
the dendritic drainage patterns characteristic of the sedimentary environments found in
many large urban watersheds. The result is a maze of water pathways that develops dur-
ing wet weather, and these routes differ considerably from the natural paths, serving to
increase the volume, velocity, and contamination of stormwater (Figure 12.8).
Source control for stormwater routes downspout flow vertically toward its natural path
into the soil. In highly developed urban watersheds, this redirection of the flows mini-
mizes the mismatch between human infrastructure and natural drainage patterns. The

FIGURE 16.2
Grid pattern of streets, with storm sewer location
shown as a dashed line.

FIGURE 16.3
Overlay of infrastructure on dendritic drainage pat-
tern (density reduced).