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 m
FLOODPLAIN HYDRAULICS

INTRODUCTION

The City of Thornton is proposing to build six fairways (fairways 10, 11, 15, 16, 17, and
18) of an IS-hole municipal golf course in Big Dry Creek's 100-year floodplain. The
proposed topographic contours will change the floodplain delineation between 136th
Avenue and 1-25.

No floodplain modifications will be done during golf course construction downstream
(north) of 136th Avenue. However, new roadway grading at 136th Avenue and
Washington Street, recently completed by the Northern Metropolitan District, has altered
the floodplain and upstream water surface elevations. The starting point for all water
surface profiles is downstream (east) of Washington Street. Future possible
improvements downstream (north) of 136th Avenue were analyzed for their impacts on
conditions upstream. The analysis is included herein.

The floodway through the proposed golf course will be redefined for both UDFCD and
FEMA 100-year flows. The floodway, loosely defined, is a zone within the 100-year
floodplain with high-velocity flows where habitable structures are not allowed by
regulating agencies. Because no habitable building will be built in the floodplain on the
golf course, this study (the City) proposes to declare the floodway as the 100-year
floodplain in the reach from 136th A venue to 1-25.

The floodplain will follow along the golf course property line in some areas. Although
golf course construction will occur ahead of surrounding development, local developers
have agreed with the City to fill in their property, where necessary, to remove the land
from the floodplain. Hence, the analysis in this study assumes future development fill
conditions along the property line.

The golf course floodplain modifications will encounter high-velocity zones both in the
channel and on some fairways during major storm events (similar to existing conditions).
Protection in these zones is discussed later in this report in detail and shown with
typical sections. Future improvement to the 1-25 crossing was also examined because it
affects the golf course and upstream water surface elevations.