Environmental Engineering Reference
In-Depth Information
and Development Review department, the municipal department respon-
sible for stormwater management, describes the conditions in Austin:
What you get when you have urban development is that the creek system just starts
to unravel. What was once a naturally fl owing creek gets shut off in our climate.
The concrete and impervious cover doesn't allow for infi ltration and contribution
to groundwater fl ow and basefl ow. So our creeks are drying up, they are eroding
and the channels are widening because we have runoff at a higher velocity, we have
more fl ooding because of the impervious cover and we have lower water quality.
The forces of nature are having a huge effect.
11
The fl ashiness of the hydrologic regime in Austin is compounded by an
increasingly impervious landscape and a highly erosive soil horizon. The
fi rst street paving in Austin took place in 1905 on Congress Avenue, the
grand promenade that connects the Colorado River to the State Capitol.
Visitors are quick to recognize the large expanses of pavement in the city
center, not only on Congress Avenue but throughout the downtown and
adjacent neighborhoods, and particularly the transportation infrastructure
that serves automobile and truck traffi c. Today, the urban watersheds con-
sist of about 50 percent impervious cover from roofs and paved surfaces,
with some land usage (such as shopping malls and the central business
district) almost completely impervious.
12
The geology of the region exacerbates the highly impervious landscape
of the inner core. The city is situated on a geologic transition zone called
the Balcones Escarpment, which separates the Edwards Plateau to the
west from the Texas Blackland Prairie to the east (see fi gure 3.1).
13
Unlike
the Hill Country landscape, the Blackland Prairie to the east consists of
gently rolling terrain with shallower stream valleys and deep soils. The
large volumes of stormwater from impervious surfaces cut down the loose
soils and widen the receiving waterways. For example, Little Walnut Creek
in East Austin expanded from an average width of twenty feet wide to
eighty-fi ve feet wide between 1962 and 1997 (see fi gure 4.3).
14
The Memorial Day fl ood of 1981 served as a reminder of the threat
that urban waterfl ows pose to life and property. Responding to the fl ood
damage, the city implemented a drainage fee; between 1981 and 1984,
voters approved $75 million for capital improvement projects related to
fl ood protection.
15
The result has been a preponderance of armored creek
banks that prevent the creek channels from eroding, meandering, and
causing property damage. Early, informal methods of bank stabilization
involved dumping concrete onto the banks and letting it solidify. More
recently, this practice has been refi ned to include formed concrete walls,
gabion walls (cobble in wire cages), and geosynthetic fabrics that mimic
vegetated creek banks. Though still a structural strategy for fl ood and
Search WWH ::
Custom Search