FLOOD07750

• Designation Number: 448
• County: Arapahoe
• Community: Unincorporated Arapahoe County
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Big Dry Creek & Tributaries
• Date: 11/1/1996
• Designation Date: 3/1/1997
• 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

TABLE OF CONTENTS

LIST OF TABLES

Page N'.

Letter of Transmittal
Table of Contents
List of Tables
List of Figures
List of Appendices
List of Drawings

i
ii
ii
ii
iii

Pal!e NO.

Table III-I Two-hour Design Storm for Basins Between Five and Ten Square Miles. III-5

Table III-2 CUHP Sub-Watershed Parameters III-6 - 111-7

Table III-3 SWMM Routing Element Parameters III-8 - III-II

I. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-I
A. General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-I
B. Purpose and Scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-I
C. Mapping and Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
D. Information Procurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
E. Analysis and Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
F. Project Advisory Committee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3

Table III-4 Peak Runoff Comparisons III- 15

Table III-5 Peak Runoff Comparisons - No Detention III-16

Table III-6 Sub-Watershed Peak Flow Summary III-16 - III-17

Table III-7 Summary of Peak Flows III-18 - III-20

II.

Study Area Description . . . . . . . . . . . . . . .

II-I

Table V-I Floodplain and Floodway Data V-2 - V-1O

III. Hydrologic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-I

A. General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-I
B. Design Rainfall. . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-I
C. Colorado Urban Hydrograph Procedure Parameters .................... III-I
D. Storm Water Management Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-5
E. Modelling Results ....................................... III-15

LIST OF FIGURES

V. Flood Hazard Area Delineation .....................................

A. General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Floodplain/Floodway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Comparison with 1975 FHAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Description of Channel Reaches and Floodplains . . . . . . . . . . . . . . . . . . . . . .

Figure I-I Study Area I-I
IV-I
IV-I Figure III-I(A-C) Watershed Map III-2 - 111-3
IV-I
IV-2
IV-2 Figure III-2(A-C) SWMM Routing Elements III-12 - ITI-14
IV-2
IV-2
Figure III-3(A-D) Typical Runoff Hydrographs I1I-21 - III-22
V-I
V-I
V-I Figure III-4(A-C) Peak Flow Diagrams III-23 - 111-25
V-I
V-2
Figure I11-5(A-F) SWMM Routing Schematics 111-26 - III-31

IV. Hydraulic Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Hydraulic Parameters ......................................
C. Analyzed Flood Frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Water Surface Frome Determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E. Shallow Flooding/Ineffective Flow Areas ..........................
F. Special Floodplain Analysis Areas ..............................

References

Tables
Figures
Appendices
Drawings

LIST OF APPENDICES

Appendix A. Detention Pond Storage Discharge Relationships

ii