Surface Water Management

SKAGIT RIVER FLOOD DAMAGE REDUCTION FEASIBILITY STUDY HYDRAULICS TECHNICAL DOCUMENTATION
Includes:

I. Background

A. General
B. Purpose of Documentation
C. Study Area
D. Skagit River Basin

II. Hydraulic Analysis Methodology

A. Model Extent
B. Study Approach
C. Floods Studied
D. Description of Hydraulic Models

1. UNET Model Development

a. Purpose of Model
b. Procedures and Process
c. Boundary Conditions
d. Basic Assumptions and Limitations

2. FLO 2-D Model Development

a. Purpose of Model
b. Procedures and Process
c. Boundary Conditions
d. Basic Assumptions and Limitations

3. Structures Affecting Flow

a. Levees
b. Bridges
c. Diversion/Impoundment Structures

4. Levee Breach Methodology

III. Model Calibration

A. Sources of Data
B. UNET Calibration
C. FLO-2D Calibration

IV. UNET/FLO-2D Results and Output

A. Index Points
B. Flow/Stage/Frequency Relationships
C. Floodplain Delineations 15 D. Qualification of Base Condition Results

V. The Use of Hydraulic Results for Developing Project Outputs

A. Risk-Based Analysis
B. Overview of RBA in Flood Management Studies
C. Flood Damage Reduction Analysis Model
D. Uncertainties Specific to the Skagit River Study
E. Expected Annual Damages
F. Expected Annual Exceedance Probability
G. Long-Term Risk
H. Conditional Probability of Non-Exceedance
I. FEMA Certification

References

LIST OF TABLES

Table 1 Skagit River Cross Section Comparison (1975-99) - Summary Table 4A
Table 2 FLO-2D Dynamic Wave Coefficients 7
Table 3 Modeled Bridges on the Skagit River 9
Table 4 Levee Failure Points in UNET 10A
Table 5 December 1975 Simulated vs. Observed High Water Marks 12B
Table 6 UNET Roughness Ranges 12
Table 7 November/December 1995 Simulated vs. Observed High Water Marks 13
Table 8 FLO-2D Floodplain Roughness Values 13
Table 9 Variation of Roughness Values for Rating Curves 14A
Table 10 Rating Curves with Uncertainty 14B
Table 11 5, 50, and 95% Flow Peaks at Each Index Point 14C

LIST OF FIGURES

Figure 1 Damage Reaches - Skagit River Valley, WA 10B
Figure 2 December 1975 Simulated and Observed Hydrograph at Mount Vernon 12A
Figure 3 Observed Versus Simulated November/December 1995 Event at Mount Vernon 13A

PLATES

Plate 1 Study Area Map with Locations of FLO-2D Modeling, UNET Cross Sections, and Levees

LIST OF APPENDICES

Appendix A Background Hydrology Reports
Appendix B Water Surface Profiles With Levee Failure Points and Varying Debris Loading Runs
Appendix C 10- to 500-year Floodplain Maps:

10 Year Flood Map
25 Year Flood Map
50 Year Flood Map
75 Year Flood Map
100 Year Flood Map
250 Year Flood Map
500 Year Flood Map (50% Levee Failure)
500 Year Flood Map (Overtopping)