The Dilemma of Water in the City - Politics of Urban Runoff: Nature, Technology, and the Sustainable City - page 11

Rainfall Rainfall

Evapo-
transpiration

Evapo-
transpiration

Runoff

Runoff

Infiltration

Infiltration

Figure 1.2
An illustration of preurban (left) and urban (right) rainfall fl owpaths with urbanization
resulting in reduced evapotranspiration and infi ltration fl ows and increased surface runoff.
Illustration by Shawn Kavon.

an aesthetically pleasing look for streets through specifi cations for curb-
and-gutter design. 53 With these standards, the conveyance logic of the
nineteenth century became embedded in the codes and regulations of city
building and had both functional and cultural implications. Today, urban
development typically involves the removal of vegetation and topsoil to
allow for the construction of buildings, roads, and other infrastructure.
Whereas an undeveloped parcel of land may infi ltrate 90 percent of rain-
fall, urbanized areas have signifi cantly less infi ltrative capacity, resulting
in larger volumes of runoff that change the hydrologic cycle from a largely
vertical fl owpath to a largely horizontal one (see fi gure 1.2). 54 Drainage
networks are then installed to convey surface runoff to the nearest water-
way for discharge. Overall, the built environment of U.S. cities consists of
impervious surfaces that are about two-thirds transportation infrastruc-
ture and one-third building roofs. 55
The rationalization of the urban landscape for water conveyance also
had a number of implications for the biological and ecological function
of watersheds, none of which were understood or even considered during
the storied network-building period from the mid-nineteenth century to
the 1960s. The increase in impervious surfaces results in a “fl ashy” hydro-
logic regime, with higher peak fl ows in receiving waterbodies that increase
fl ooding and bank erosion processes, threaten habitat for vertebrate and
invertebrate populations, and lower basefl ows during dry periods due
to depleted groundwater volumes (see fi gure 1.3). Stormwater picks up