RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 9

Table 1-3 Water Balance by Region and River Basin

Region and River Basin Basin Mean

Rainfall (in) Runoff

Percent and Reference Size Discharge

(m 2 ) (ft 3 /sec)

of Area City Alteration City Basin (in.)

NE 60% Delaware

Philadelphia,
PA

12,757 14,902 NYC

diversion

42 42.5 25.6

NW 63% Willamette
Portland, OR

11,478 32,384 Dams/lakes 37 60.2 38.2

SE 23% St. Johns

Jacksonville,
FL

8,702 7,840 Lakes 52 51.6 11.8

SW 2.3% Santa Anna
Los Angeles,
CA

2,438 60 Diversions 15 13.4 0.3

Source: Derived from [9].

Whatever the average annual rainfall or variability of this volume in a given
location, the design of structures or systems to convey or mitigate the impacts
of this volume (and flow rate) of surface runoff have always focused on individ-
ual storm events. These “design storms” are events during which the intensity,
duration, and amount of rainfall produce the most severe impacts.
We remember the most extreme rainfall events, especially when they are the
result of cyclonic storm patterns produced in both the Atlantic and Pacific oceans
that approach the mainland in the form of hurricanes or cyclones. We even
identify them by name when they reach a given magnitude or anticipated wind
speed, assigning a category of intensity that can change during the approach.
Most recent memory cannot help to identify hurricane Katrina (Figure 1-7), which
devastated the Gulf coast in September 2005, but other names and memories are
shared by communities throughout the country. Most periods of prolonged rainfall
do not receive this recognition or nomenclature, but have produced dramatic
flooding impacts in large and small watersheds.
The statistic of rainfall that has the most common usage in defining severe
rainfall events is the 100-year storm, which is the rainfall that occurs during
a 24-hour period with a frequency of once in 100 years. This figure cannot,
however, convey the full impact on a local watershed of more severe and intense
rainfalls. For example, in July 2004 the Rancocas Creek in southern New Jersey
was visited by a rainfall pattern [10] that dumped some 13 in. in some portions of
this small (250 m 2 ) watershed (which has a 100-year rainfall frequency of 7.2 in.),
in a pattern that was anything but uniform. The net result was the destruction of
some 22 small earthen dams, built for various purposes, and significant property
damage (but no loss of life).
This type of localized event can be visited on any portion of the country,
regardless of our statistics and classification of storms, and is repeated all too
frequently all across the globe. While the total rainfall is a given period and