<br />Table 1. Metropolitan areas included in nationwide urban flood-frequency study-Continued
<br />
<br />State
<br />
<br />Metropolitan area
<br />
<br />Number
<br />of
<br />sites
<br />
<br />City
<br />skew
<br />
<br />Source of equivalent rural discharge
<br />(see references)
<br />
<br />Texas
<br />Texas
<br />Washington
<br />Washington
<br />
<br />Houston
<br />San Antonio
<br />Portland-Vancouver
<br />Seattle-Tacoma
<br />
<br />21
<br />5
<br />3
<br />6
<br />
<br />-.3
<br />-.6
<br />.1
<br />.0
<br />
<br />Liscum and Massey (1980)
<br />Schroeder and Massey (1977)
<br />CUmntans and others (1975)
<br />Do.
<br />
<br />total, an average, a percentage, or an index for the total
<br />drainage basin. A few of the variables were computed
<br />for "thirds" of the basin in an attempt to define some
<br />variables further and to provide locations of basin devel-
<br />opment. For this study, some basins were divided into
<br />upper, middle, and lower thirds on a drainage map with
<br />the drainage divide delineated. Each third contains
<br />approximately one-third of the contributing drainage
<br />area and drains the upper, middle, or lower reaches of
<br />the basin. Because travel time or flow time was consid-
<br />ered in drawing the lines separating the basin thirds
<br />distances along main streams and tributaries were
<br />marked to help locate the boundaries of the thirds. This
<br />drawing of the boundaries means not that all thirds of
<br />the basin bave equal travel distances but that within
<br />each third the travel distances of two or more streams
<br />are about equal. Since precise definition of the lines
<br />dividing the basin into thirds was not considered neces-
<br />sary for the variables that utilize this concept, the lines
<br />can generally be drawn on the drainage map by eye,
<br />without precise measurements. Figure 2 shows sche-
<br />matics of three typical basin shapes and their division
<br />into thirds. Complex basin shapes and drainage patterns
<br />are sometimes encountered; they require more judgment
<br />in subdividing.
<br />
<br />Topographic and Climatic Variables
<br />
<br />The physical and climatic conditions existing in
<br />each basin are described by a selected set of topographic
<br />and climatic variables. Parameters of physical charac-
<br />teristics include drainage-area size, channel length, valley
<br />length, stream slope, storage, Soil Conservation Service
<br />(SCS) soil classification, SCS soil-cover-complex curve
<br />number, and SCS index of potential infiltration. Each
<br />basin is divided into thirds, as previously described, and
<br />dominant soil classifications are given for the upper
<br />third, middle third, and lower third of the basin. The
<br />percentage of the total basin covered by each soil type is
<br />included. The channel and drainage system efficiency is
<br />described by a coefficient estimated according to proce-
<br />dures dermed by Espey and Winslow (1974). Bankfull
<br />discharge at each gaging station is included, and each
<br />
<br />basin that has significant in-channe1 storage is identified.
<br />In-channel storage, distinguished from basin storage, is
<br />dermed as temporary storage created by detention ponds
<br />or ponding at roadway embankments. Climatic variables
<br />include mean annual precipitation, rainfall intensity of
<br />the 2-hour-duration 2-year recurrence interval, and
<br />rainfall intensity of the 2-hour-duration lOO-year recur-
<br />rence interval.
<br />
<br />Land-Use Variables
<br />
<br />Land use within each drainage basin is described
<br />with two sets of land-use variables. Each set is derived
<br />from an independent source, and although similar results
<br />were obtained for most stations, there are some stations
<br />for which the two data sources yielded quite dissimilar
<br />results. No attempt was made to resolve the differences
<br />nor to indicate which was more nearly correct. Land use
<br />was not significant in the final results of this study.
<br />The first set of land-use data was obtained from
<br />I :250,000 land-use maps compiled by the Geological
<br />Survey from recent high-altitude photography. Because
<br />maps are not available for all cities, these data are not
<br />available for some basins. Classifications of land use
<br />follow the standard system for remote sensing described
<br />by Anderson and others (1976) and include percentages
<br />of the basin occupied by residential areas, commercial
<br />areas, industrial areas, transportation facilities, mixed
<br />urban areas, cropland, forests, lakes and reservoirs,
<br />wetlands, rangelands, and a few other miscellaneous
<br />types of land use. Dates of the maps used are given in
<br />the data base.
<br />The second set of land-use data was compiled from
<br />recent maps and field surveys by the Soil Conservation
<br />Service. Again, because of a lack of suitable maps for
<br />some cities, these data were not determined for some
<br />stations. Categories of land use follow the SCS classifi-
<br />cation system and include residential areas (percentages
<br />of the basin having lot sizes of :la, :la, Y., Y., and I acre are
<br />provided), paved areas, streets, industrial areas, com-
<br />mercial areas, forests, meadows, pasture and rangelands,
<br />cultivated lands, and open spaces.
<br />
<br />Data Base 5
<br />
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