FLOOD02735

• County: Statewide
• Basin: Statewide
• Title: Comparison of Nine Uncalibrated Runoff Models to Observed Flows in Two Ssmall Urban Watersheds
• Date: 3/25/1999
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

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MODEL DATA DESCRIPTION
Watershed Characteristics: Two small urban watersheds were selected for model simulations that differed in size,
climate, and drainage characteristic. Information ~bout the watershed that is typically readily available to the engi-
neering community was provided to each modeler; this included a base map and tabular data on topography, build-
ings, roads, soils, and drainage characteristics.

Surrey Downs watershed (fig. IA) is a 0.14 mi2 area of mostly single-family residential land-use. The basin is about
2.5 times as long as wide and is drained entirely by a closed-pipe storm-sewer system. Base flow was given as 0.02
ftJ/s. Most streets have curb and gutter that drain to the storm-sewer system. Reliefis about 150 feet, with slopes
upwards of20 percent. Impervious area is about 30 percent of the watershed, with about 20 percent being effective
impervious area. Modelers were provided with details on the types of impervious areas, but not by subbasin. Figure
IA does not show details of the storm-sewer system including pipe diameter,length, invert elevations; slope, mate-
rial, and soil characteristics that were provided to the modelers. Soils are gravelly-sandy loam to fine-sandy loam.
Additional details on the watershed are reported by Ebbert and others (1985).

Harvard Gulch watershed (fig. IB) is a 3.10 mi2 area of mixed urban land-use. The basin is about twIce as long as
wide and is drained by a combination of storm sewers and open channels. Base flow ranges from 2.1 .md 5.6 &/s.-
Relief is about ISO feet with slopes generally ranging from about 2 to 0.5 percent. Impervious area covers about 38
percent of the watershed. Modelers were provided delineations, area, and percent inJpervious area for 33 subbasins,
but were required to determine the effective impervious area where appropriate to the model. This task was made
somewhat more difficult by a map of poor reproductive quality; a problem not uncommon in the real world. Soils are
SCS Group B. (Ben Urbonas, Urban Drainage and Flood Control, Denver, C.O., written commun., 1990)

,- Storm Characteristics: Harvard Gulch and Surrey Downs watersheds are in distinctly different clinJatic regions.
Harvard Gulch is in a semi-arid region with typically dry antecedent conditions and storms that are usually of short
~.' duration and high intensity. Surrey Downs is in the coastal Pacific Northwest, were antecedent conditions are typi-
cally wet and storms are usually oflow intensity and long duration.

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Table 2. Summary of storm characteristics used in runoff model test [All values are in inches].
Duration Volume . Maximum Intensity (in)t Antecedent Precipitation (in)
Storm (hr:min) (in)
,5 min 15min 60min I day 3 day 7 day 14 day
Harvard Gulch Watershed
1981/5/8 00:50 1.04 0.63 0.99 1.56
1983/6/5 02:10 0.65 0.32 0.77 1.00
1985/5/17 01:05 0.39 0~26 0.32 0.62
1985/7/19 01:35 1.05 0~35 0.67 1.25
1987/6/9 01:00 0.27 0.11 0.24 OAO
1987/6/30 02:40 0.54 0.08 0.20 0.54
Surrey Downs Watershed
1980/4/18 20:00 0.54 0.05 0.07 0.12 0.00 0.00 0.30 1.39
1980/8/17 08:00 0.58 0.08 0.18 0.34 0~03 0.Q3 0.Q3 0.03
1981/6/30 09:00 0.25 0.03 0.08 0.18 0.00 0.00 0.00 0.20
1981/9/20 09:00 0.12 0.03 0.06 0.09 0.04 OA8 OA8 OAO
1981/12/1 20:00 0.55 0.06 0.10 0.12 0.12 0.15 0.15 2.03
1981/1/15 24:00 0.97 0.02 0.04 0.14 0.00 0.05 0.36 0,36
"'. Volumes are the average for all rain gages.
t. Maximum intensiEy reported among all rain gages.

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