FLOOD10420

• County: Adams
• County: Broomfield
• County: Weld
• Community: Broomfield, Westminster and Thornton
• Stream Name: Lower Big Dry Creek
• Title: Lower Big Dry Creek Hydrology Study
• Date: 6/1/2005
• Prepared For: Big Dry Creek Watershed Association
• Prepared By: Wright Water Engineers, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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TABLE OF CONTENTS

EXECUTIVE SUMMAR~Y .....................................,...................................................................... 1

Overview of Analysis Approach........................................................................................................2
Findings of Hydrologic Analysis........................................................................................................3
Recommendations to Improve and Address Drainage Problems in the Lower Watershed .............6

1.0 INTRODUCTION................... ......... .........."... ................................................................. 1 0

2.0 BACKGROUND ON THE BIG DRY CREEK WATERSHED ASSOCIATION ................11

3.0 OVERVIEW OF WATERSHED CHARACTERISTICS................................................... 12

3.1 Location ..................................................................... .................... ................................... .12

3.2 Hydrology..... .................. ............. ................ ............................................. ......................... .12
3.3 Stream Characteristics.. ...................................... .......... ........ ........................................... .15
3.4 Extent of Current Development and Urban Growth Boundaries........................................16

4.0 SUMMARY OF EROSION AND FLOODING RELATED FINDINGS FROM PREVIOUS

STU DI ES ........ ................. ........... ........ .....".. ...... ............ ................................................20

5.0 OVERVIEW OF DRAINAGE AND FLOOD PLANNING AND POLICIES IN THE BIG

DRY CREEK 'VII' A TERSH ED ... ...................... ................................................................ 25

5.1 Current Approach to Stormwater Management, Policies and Criteria in

Upstreanl Cities ................................. ............................................................................... .25
5.2 General Legal Principles Governing Drainage Planning ...................................................30
5.3 Summary of Drainage and Planning Studies Completed in the Urbanized

Portion of the Watershed................................................................................................... 31
5.3.1 Big Dry Creek, Adams County and Tributaries Master Drainage Plan ............33
5.3.2 Broomfield and Vicinity Outfall Systems Planning Studies..............................36
5.3.3 Outfall Systems Plan North Area Tributaries (ADCO).....................................36
5.3.4 Flood Hazard Area Delineations. ......................... ....... ................ .................... .37
5.4 FEMA Floodplain Mapping..."........... ............. .................................................................... .38
5.5 Weld County Drainage Ordinances, Programs and Policies .............................................40

6.0 PROBLEM CHARACTERIZATION: FIELD VISITS AND INTERVIEWS ......................40

6.1 Howard Property................... .............................. ............. ................................................ ..44

6.2 Wright Property....... .......... .... ... ....... ..... .... ....... .......... ......... ..... ..... ... ........ ....... .................. ..45
6.3 Chikuma Property. ...... ........ ............ .................. ........................... ..................................... .45
6.4 Rosenbrock Property....... ................................................................................................. .45

6.5 Wagner Property... .... .... ....... ..... ........ ..... ... ..... ...... ..... ...... ............... ................... ................ .45

6.6 Ira Property..... .................................. .."............................................................................. .46

6.7 Green Property..... ........................... .................. ................................................................47
6.8 Weld County Bridges...... ................. ..".......... ............ ........ ............................................... ..48
6.9 Oil and Gas Companies......................"..............................................................................48
6.10 Water Commissioner ..... ............... ..... "..................................... ....................................... ...49

7.0 CASE STUDIES AND LITERATURE REVIEW .............................................................49
7.1 Current State of the Practice.............................................. ............................................. ..49
7.1.1 Urban Drainage and Flood Control District Storm Drainage Criteria...............49
7.1.2 Engineering Manuals of Practice and Texts ....................................................50
7.2 Case Studies in Colorado ..................................................................................................52
7.2.1 Fountain Creek Watershed ............................ ............................ .....................53
7.2.2 Bear Creek and Chatfield Reservoir Watersheds ...........................................55

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