Pollution Prevention - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 456

TABLE 13.1
Urban Parcel Water Balance during Precipitation a

Component Paths Micro-Topography

Inflows
Precipitation

Atmosphere → biosphere Vegetation soil, pavement (driveways,

sidewalks, and road surfaces)

Atmosphere → lithosphere

Runoff Soil, grass surfaces, pavement, roofs,

gutters

Lithosphere → lithosphere

Outflows
Runoff

Lithosphere → lithosphere
→ hydrosphere

Soil, grass surfaces, pavement, storm
sewers

Evaporation Lithosphere → atmosphere Ground surface

Evapotranspiration Biosphere → atmosphere Vegetation

Storage
Interception

Atmosphere → biosphere Vegetation

Infiltration Atmosphere → lithosphere

→ hydrosphere b

Soil, vadose zone water

Surface depression storage Atmosphere → lithosphere Ground surface

a Including the road frontage.
b Interflow.

capacity, or providing basins for storage. An earth science approach looks at the scale of
the processes generating the runoff, the source location and direction of the flows, and the
landscape components affecting the transmission system at the relevant scales.
In Table 13.2, opportunities for stormwater storage or diversion at the micro-topographic
scale are shown for each landscape feature influencing stormwater flows. The objective
is to maximize storage at the site closest to the origin of runoff. At the micro-topographic
scale of the house lot or single parcel, this translates into keeping stormwater on the lot
and out of the storm sewers (Kaufman 1999). This is accomplished by directing water
downward, instead of horizontally.
As Table 13.2 shows, increasing interception and infiltration and minimizing overland
flow from impervious surfaces form the core components of a stormwater retrofitting strat-
egy in developed urban areas. Increasing the organic content of soils (by topfilling with
humus and cutting with a mulching mower), using available storage in surface depres-
sions, and the use of swales can provide large amounts of infiltration if the lot layout per-
mits their construction.
Flat roofs (such as those on parking lots) converted into vegetated green roof systems
can reduce stormwater volumes and delay its release, and also help reduce the urban heat
island effect. One study showed that green roofs not only reduced the amount of storm-
water runoff, but they also extended its duration over a period of time beyond the actual
rain event (van Woert et al. 2005). In another study, rain and roof runoff data collected
from seven rains during October and November 2002 showed that the green roofs delayed
the start of runoff by an average of 5.7 h. The green roofs retained an average of 45% of
the rain from the seven storms evaluated and delayed the peak runoff by 2 h. Roof tem-
perature data collected between April 2002 and February 2003 showed that the green roof