FLOOD02086

• County: Gilpin
• Community: Black Hawk/ Central City
• Title: Black Hawk/Central City Colorado Radar Rainfall Reconstruction
• Date: 7/30/1998
• Prepared For: Gilpin County
• Prepared By: HDR Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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1.0 Introduction

HDR Engineering Inc, was contracted by the Colorado Water Conservation Board to produce a
WSR-88D Doppler radar rainfall reconstruction for a precipitation event in and around Black Hawk
and Central City, Colorado, The precipitation event consisted of two thunderstorm cells that
moved through the area during the early afternoon of July 3D, 1998. Rainfall from the first
thunderstorm cell lasted approximately 36 minutes and rainfall from the second thunderstorm cell
lasted approximately 12 minutes, Approximately ninety percent (90%) of the rainfall that was
observed was associated with the first thunderstorm cell.

According to a field investigation report produced by the Colorado Water Conservation Board
titled 'Blackhawk, Colorado Flood of July 30,1998', rainfall from this event initiated flooding on an
unnamed gulch above Miner's Mesa Street and the Golden Gate Casino. This rainfall
reconstruction will focus on the unnamed gulch basin, which will be referred to as Unnamed Gulch
throughout the remainder of this report,

The heavy rain production from the thunderstorms was a result of a very moist air mass that was
in place east of the Continental Divide combined with the presence of a weak atmospheric
disturbance, The thunderstorms developed along the Continental Divide and moved to the
east/northeast at approximately 13 MPH as the disturbance crossed the region.

Base reflectivity observations from the National Weather Service WSR-88D Doppler radar located
near Watkins Colorado was used in the reconstruction process, The radar is located
approximately 47 miles to the east/southeast of the rain estimation location. The radar rainfall
estimation process will be discussed in detail, later in this report, along with factors that can have
an effect on the relationship,

An "atmospheric reality check" is performed by comparing the observed rainfall at nearby rain
gauges to radar-estimated rainfall at the gauge location. This reality check is known a ground
truthing, Unfortunately the closest rain gauges are in the Jefferson and Boulder county foothills
and were not considered representative of the rainfall/radar relationship within the rain estimation
location,

Rainfall estimates for this event were discussed in the field investigation report mentioned above
and were made by Steve Ferris, Black Hawk Building Inspector and Larry Dezmond, of Futura
Engineering. Phone calls were placed to these two individuals to get a better quantification of
rainfall depth/duration and location to possibly utilize the information in the ground truth process.
The phone conversations revealed the following information: Steve Ferris estimated 2 to 2.5" of
rainfall within the city of Black Hawk in approximately 25 minutes, but stated that this was a visual
estimate and did not measure the rainfall with a rain gauge, Larry Dezmond stated that he
conducted a "rain bucker survey the day after the event. This was accomplished by placing a
tape measure in a 5-gallon bucket situated at the top of Miners Mesa, Two and a half inches of
liquid was measured in the bucket, but the time distribution was unknown due to the fact that it
was not known when the bucket was placed there or if it contained any liquid in it before the
precipitation event on July 30, 1998.

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