FLOOD04145

• County: Adams
• Community: Thornton, Adams County
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 10/1/1980
• Prepared For: Thornton, Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

I.

MAJOR DRAINAGEWAY PLANNING
DIRECT FLOW AREA 0054

Introduction

North of Denver, in Adams County and the City of Thornton, lies a small un-named
drainageway which flows directly into the South Platte River, identified by the Denver
Regional Council of Governments as Direct Flow Area 0054. Historically, the land within
this drainage basin has been used for agricultural purposes and drainage was not a concern
for the local residents and farmers. In a relatively short period of time, mostly within the
last decade, however, the City of Thornton has experienced rapid growth and the trend is
toward fully developed conditions in the uplands of the drainage basin within a few years.
The increased residential development has caused an increase in the drainage flows from
the upland areas which has been the cause of numerous problems in the lower areas of the
basin still involved in agricul tural activities.

As a result of the conflicts which have been growing over the increased runoff in
Direct Flow Area 0054, the City of Thornton and Adams County together with the Urban
Drainage and Flood Control District have contracted with Merrick &: Company, Consulting
Engineers, to compile information, evaluate, study and to recommend design solutions to
the drainage problems on the creek which are in the best interest of the involved parties
and agencies. The results of the study are to be published in three volumes:

I. Flood Hazard Area Delineation - Direct Flow Area 0054
n. Major Drainageway Planning - Phase A, Direct Flow Area 0054
III. Major Drainageway Planning - Phase B, Direct Flow Area 0054

Volume I, Flood Hazard Area Delineation - Direct Flow Area 0054, was completed
in October of 1979. This volume of the study presented information on the characteristics
of the drainage basin and channel, summarized historic flooding problems, and estimated
the nature and extent of probable future floods under existing channel conditions.

Volume II, Major Drainageway Planning - Phase A, Direct Flow Area, 0054 was
completed in November of 1979. This volume summarized the development and evaluation
of feasible alternative flood control plans and made specific recommendations to alleviate

tlood hazards. Upon reviewing the study findings and recommendations presented in the
Phase A report, the Urban Drainage District, Adams County, and the City of Thornton
selected an alternative plan for implementation which would most satisfy local interests
and remain consistent with their planning endeavors. The selected plan, detailed in a
Directive Memorandum reproduced in Appendix A of this report, served as the guideline
for the preparation of the final volume of the study.

This final report, Major Drainageway Planning - Phase B, Direct Flow Area 0054,
presents the selected plan and culminates the Direct Flow Area 0054 Master Plan Study.
The focal point of the report is the preliminary design of the proposed drainageway
improvements, presented in the form of conceptual drawings that appear in the body of
this report. The report also includes a description of final hydrologic and hydraulic studies
conducted during the preliminary design, describes existing flood hazards, and gives a
narrative description of the proposed improvements and their estimated costs. Plan and
Profile Maps are provided which define the 100-year flood plain for existing and improved
channel conditions.

Existing Flood Hazards

Before this study was initiated, drainageway planning and improvements in the
basin had been incremental and haphazard, resulting in an overall incompatible
drainageway and often times increased flood hazards. Floods as minor as the 2-year event
in magnitude have occurred as recently as May of 1979, causing damage to roadways,
embankments, utilities, crop and pastureland, and creating various other problems
associated with soil erosion and sediment deposition. In the upper stretches of the basin,
residential development has obliterated all traces of a historic channel. A storm sewer
system, generally adequate to convey only frequent events, has been constructed in this
upper stretch. Except for the vicinity of East 112th Way cul-de-sac, however, flood
problems remain minimal as excess flows are confined to street and front yards.

From Cherry Drive to East 112th Avenue, the property remains yet undeveloped.
However, no adequate means of conveying runoff from storms of large magnitude exist
causing flow to continue down East 112th Avenue. As storm runoff follows 112th,
overflow is distributed as "sheet flow" to the south, exposing large areas to shallow
flooding.

H