FLOOD03719

• County: Adams
• Community: Thornton
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Floodplain Information Report
• Date: 4/15/1988
• Prepared For: Thornton
• Prepared By: McLaughlin Water Engineers, Ltd.
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION n
HYDROLOGIC DATA

INTRODUCTION
Big Dry Creek is in the northern portion of the Denver metropolitan area and has a
total drainage area of about 86.3 square miles at its confluence with the South Platte
River. The drainage basin is in Townships 1 and 2 South, Ranges 67, 68, 69, and 70
West of the Sixth Principal Meridian.

The area of interest on the Big Dry Creek for this study lies between 1-25 and 136th
Avenue in the City of Thornton. At 1-25, the drainage area is about 54.5 square miles
including the area contributing to Standley Lake and the Great Western Reservoir. There
are 15.3 square miles tributary to Standley Lake and 4.6 square miles tributary to Great
Western Reservoir.

Historically, the Big Dry Creek floodplain has remained undeveloped; hence, damage due
to extreme storm events has been minimal. Two 100-year flows are examined in this
study, one flow is for fully developed conditions upstream (Q=l1,190 cfs) and the other
for existing conditions (Q:8,839 cfs). The UDFCD uses developed conditions for its
studies and FEMA uses existing conditions.

Storm water drainage, golf course fairway subdrainage, irrigation water supply, tributary
flow, and restroom facilities for the golf course are addressed in a separate report
entitled, "Thornton Municipal Golf Course - Utilities Engineering Report." The planned
channels for Quail Creek and Tanglewood Creek are also addressed in that report.

DESIGN STORM FLOWS
Urban Drainage and Flood Control District
The design storm flows used in this study are from the most recent UDFCD study, Big
Dry Creek (ADCO) Flood Hazard Delineation (FHAD) (Muller Engineering Company, Inc.,
Nov. 1986). At 1-25, the 100-year event is 11,190 cfs, and the lO-year event is 5,620
cfs. In the Muller Engineering study (Nov. 1986), the 100-year flow decreases from
11,190 at 1-25, to 11,120 at 136th Avenue, but for this study 11,190 cfs was used
throughout the reach. The 2-year event flow of 3,650 cfs is from Big Dry Creek (ADCO)
and Tributaries Outfall Study (Muller Engineering Company, Inc., Aug. 1987). It should
be noted that another study used a flow of 11,480 cfs at 1-25 for the 100-year event