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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7.2.3 Rock Creek, Boulder, Colorado.......................................................................56
7.3 New Approaches to Improve Current Stormwater Management Practice.........................56
7.3.1 Designing Stormwater Facilities to Better Match Flow Frequency Curve........57
7.3.2 Full Spectrum Detention Concept ...................................................................60
7.3.3 Using Concepts of Work to Evaluate Hydromodification Impacts in Santa
Clara Valley, California..... .............. ................................................................ .62
7.3.4 Advanced Stormwater Management Strategies in Place in Other
Parts of the Country............ .............................................................................65
7.4 Summary of Literature Review Findings............................................................................66

HYDROLOGIC ANALYSIS ... .......... ...... ..."......... .................. ................ .............. ...........67

8.1 Analysis of USGS Gage Data ............................................................................................67
8.2 Characterization of Relative Contribution and Role of Base Flows ...................................73
8.2.1 Wastewater Discharges ......... ...... ............ ............... ........................................74
8.2.2 USGS Baseflow Data ......................................................................................77
8.2.3 Irrigation Return Flows....... ....................... .................. ................ ...... ......... ..... 78
8.3 Aerial Photo Review............ .......... ......................................................... ............................78
8.4 Characterization of Summer 2004 Rainfall Events............................................................79
8.5 Analysis of Existing and Proposed Detention ....................................................................85
8.6 Expected Future Hydrologic Conditions ......................... ..................... ............................. .93

STRATEGIES TO ADDRESS DRAINAGE AND FLOODING PROBLEMS................... 93

9.1 Detention/Storage............................................................................................................. . 94
9.2 Channel Stabilization ..........................." ............... ................ ...................................... ....... .95
9.3 Targeted Infrastructure Improvements... .................... ............. ....................................... ...95
9.4 Upstream Policies and Planning Strategies.......................................................................98
9.5 Floodplain Acquisition. ............. ..... ......... .................... ......................................................100
9.6 Public Education-Flood Protection.. ......... ............... ...... ....................... ............ .............104
9.7 Public Education-Land Management ............................................................................105

10.0

CONCLUSIONS AND RECOMMENDATIONS ...........................................................107
10.1 Hydrologic Analysis .......... ........... ....... "........... ...... ............................................................107
10.2 Recommendations to Improve and Address Drainage Problems
in the LOINer Watershed..... ................ ......................... .................................. ................... 11 0

11.0

REFERENCES................... ........... ........ ............ ............................................. ........ ..... 114

Paae

TABLES

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Estimates of Development in Big Dry Creek Watershed ................................................................19
DRCOG (2005) Population and Employment Forecast for Big Dry Creek Watershed................... 19
Portions of Big Dry Creek Subject to Erosion (from WWE 2000)...................................................23
Detention Requir1ements for Developments................................................................................... .28
Allowable Release Rates (cfs/acre) of Detention Ponds: Values Used for
Cities of Broomfield, Westminster, Thornton and Adams County..................................................29
Summary of Drainage Studies Along Big Dry Creek ......................................................................32
USGS Gages on Big Dry Creek.. ......... ........ .................. ........................................................... ......69
Summary of USGS Field Measurements at Fort Lupton Gage ......................................................77
Summer 2004 Rainfall for Big Dry Creek Flooding Events.............................................................79
Peak Flow Rates from Big Dry Creek Master Plans.......................................................................82
Summary of Rough Estimates of Required and Actual Detention in Big Dry Creek Watershed ....86
Existing and Proposed Storage Facilities in Big Dry Creek Master Plans ......................................88

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