FLOOD02473

• County: Weld
• County: Logan
• Basin: South Platte
• Title: A Post Evaluation of Rainfall Reports Associated with Pawnee Creek Flood of July 29-30, 1997 in Eastern Weld and Western Logan Countie in Northeastern Colorado
• Date: 8/1/1998
• Prepared For: Weld County/Logan County
• Prepared By: CSU
• Floodplain - Doc Type: Flood Documentation Report

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The area of the Pawnee Creek watershed is very sparsely populated. However, a
remarkably large percentage of the residents of this ranching and dryland fanning area
were equipped with rain gauges and attempted to measure the rainfall. This evaluation of
the original rainfall reports gathered after the storm found that most reports and .
observations in the areas of heaviest rainfall were estimates as opposed to accurate gauge
measurements since most rain gauges overflowed. Some of these estimates were derived
from stock tank levels and water depths in 5-gallon buckets. A few rainfall reports near
the storm center likely included rain from previous days or from splash from surface
water, thus placing in question some of the largest reports. Nevertheless, there was
convincing evidence that rainfall totals for the evening of July 29 into the early morning of
July 30, 1997 exceeded 12 inches and locally approached 14 inches over an area of
perhaps 17 square miles.

The 1998 survey uncovered several more observations that apparently were not obtained
for the original analysis. These additional data suggest that the original survey may have
underestimated rainfall in some areas, particularly over eastern Weld County. A re-
analysis shows that rainfall totals for the evening may have exceeded six inches over on
area of approximately 270 square miles. The areas of greatest uncertainty in this analysis
include portions of western Logan county just east of the storm's center and a narrow
band extending southwestward to just northwest of New Raymer where there are solid
indications of extreme rainfall but few confident observations.

In addition to ground measurements of rainfall, the National Weather Service radar data
were collected and are available to help estimate storm rainfall for areas without gauge
measurements, These radar data proved very helpful in defining storm locations, duration,
and areas of maximum rainfall over Morgan and Weld counties, but were less helpful over
Logan County which is farther away from the radar transceiver.

In conclusion, despite the low population density of the area, a reasonably confident
depiction of storm rainfall has been assembled suitable for hydrologic and engineering
applications. The Pawnee Creek storm of July 29-30, 1997 was, indeed, an extreme event
in terms of total rainfall depth, area and duration. It will stand out for many years to come
as one of just a handful of extreme rainfall events in eastern Colorado's recorded history
with rainfall totals exceeding 12 inches in a duration ofless than 12 hours.

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