In this lecture, Rachel Gaddis, EIT detailed the hydrologic and hydraulic analysis and dam breach modeling process using ArcGIS, HEC-HMS, and HEC-RAS.

1. Not Just Another Dam Safety Lecture
Partners for a Better Quality of Life
Rachel Gaddis, EIT
June 6, 2019

2. About the Speaker: Rachel Gaddis, EIT
 Graduated LeTourneau University, May 2018
 CP&Y Water Practice Group
– Dam Hydrologic & Hydraulic (H&H) analysis
– Texas Department of Transportation (TxDOT)
Off-System Bridge Analysis
– Reservoir Planning
– Plant Permitting
– Drainage
– Site Planning

3. Not Just Another Dam Safety Lecture?
 How is this presentation different?
– Few Rules
– Analysis and Process Focused
– Conversational Dam Safety

4. Structure Elevation (ft) Width (ft)
Dam 310 8000
Service Spillway 300 300
Emergency Spillway 305 1000
Water Surface 300 -
EL. 310 EL. 305
EL. 300
The Problem
You need to analysis the hydrology and hydraulics of Lake Gaddis
dam. You must use the following information to determine if the
dam is tall enough and the spillways are wide enough. You must
also determine which residences and roadways are at risk
downstream, should the dam fail.

5. How much rain
is coming to
this location?
Can the dam
structures
handle the
amount?
If failure
happens, who
and what is at
risk?
The Process
1. Arc-GIS
2. Watershed
3. PMP
4. Soil Types
5. Curve Number
1. HEC-RAS Model
2. Inundation Area
3. Time to Reach
Downstream
Structures/Roads
4. EAP
1. HEC-HMS Model
2. PMF
3. Max. Water Surface
Elevation
Hydrologic Analysis Hydraulic Analysis

6. The Approach: Part 1
 Hydrology
– ArcGIS
 Watershed Delineation
 Determine the Probable Maximum
Precipitation (PMP)
– Temporal Distribution
 PMP Value
 Unit Hydrograph

8. The Approach: Part 1
 Hydrology
– ArcGIS
 Watershed Delineation
 Determine the Probable Maximum
Precipitation (PMP)
– Temporal Distribution
 PMP Value
 Unit Hydrograph

9. Duration (hr) x (%) y (%) PMP time x time y
1 33 50 12.28 0.33 6.14
2 33 60 21.4 0.66 12.84
3 33 50 23.89 0.99 11.95
6 33 80 28.6 1.98 22.88
12 33 85 36.5 3.96 31.03
24 33 85 43.8 7.92 37.23
48 33 85 45.6 15.84 38.76
72 33 85 45.6 23.76 38.76
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
0:00:00
1:45:00
3:30:00
5:15:00
7:00:00
8:45:00
10:30:00
12:15:00
14:00:00
15:45:00
17:30:00
19:15:00
21:00:00
22:45:00
0:30:00
2:15:00
4:00:00
5:45:00
7:30:00
9:15:00
11:00:00
12:45:00
14:30:00
16:15:00
18:00:00
19:45:00
21:30:00
23:15:00
1:00:00
2:45:00
4:30:00
6:15:00
8:00:00
9:45:00
11:30:00
13:15:00
15:00:00
16:45:00
18:30:00
20:15:00
22:00:00
23:45:00
CumulativePrecipitation(in)
Storm Duration
1 HR 2 HR 3 HR 6 HR 12 HR 24 HR 48 HR 72 HR
Temporal Distribution of PMPs

10. 0.000
0.050
0.100
0.150
0.200
0.250
0.300
0:00
0.031
0.063
0.094
0.125
0.156
0.188
0.219
0.250
0.281
0.313
0.344
0.375
0.406
0.438
0.469
0.500
0.531
0.563
0.594
0.625
0.656
0.688
0.719
0.750
0.781
0.813
0.844
0.875
0.906
0.938
0.969
1.000
1 hr 2 hr 3 hr 6 hr 12 hr 24 hr 48 hr 72 hr
Duration (hr) x (%) y (%) PMP P, x time y
1 50 50 12.28 0.5 6.14
2 50 60 21.40 1 12.84
3 33 50 23.89 0.99 11.95
6 33 60 28.60 1.98 17.16
12 33 70 36.50 3.96 25.55
24 33 80 43.80 7.92 35.04
48 33 85 45.60 15.84 38.76
72 33 85 45.60 23.76 38.76
Temporal Distribution of Unit Hydrographs

11. 4708.34
6208.34
5280.23
5102.24
4213.03
2908.62
1635.67
1084.77
4652.79
6104.08
5422.82
5008.5
4002.38
2932.07
1640.8
1032.63
491
491.5
492
492.5
493
493.5
494
494.5
495
0
1000
2000
3000
4000
5000
6000
7000
1HR 2HR 3HR 6HR 12HR 24HR 48HR 72HR 1HR 2HR 3HR 6HR 12HR 24HR 48HR 72HR
ReservoirElevation(ftAMSL)
Outflow(cfs)
STORM DURATIONS
OUTFLOW ELEVATION
Temporal Distribution of PMPs
Temporal Distribution of Unit
Hydrographs times the PMP
TOP OF DAM ELEV. 492.75 FT
PMF Determination

12. The Approach: Part 1
 Where to begin?
 Hydrology:
– HEC-HMS
 Create basin
– Enter hydrologic data
– Specify outflow structures

13. The Approach: Part 2
 HEC-HMS
– Meteorological Model
– Control Specifications
– Time-Series Data
– HEC-HMS Paired Data
 Elevation Discharge
 Function
 Elevation-Storage Function
 Create a Simulation and
Run, Run, Run!

14. Reservoir Results for Run “100%”

15. Summary Results for Reservoir

16. The Approach: Part 3
 HEC-HMS Output
– What is it telling you?
– Which PMP creates Probable
Maximum Flood (PMF)?
– Is dam tall enough?
 The water surface elevation is below allowable freeboard limit
during 100% PMF of an existing dam that meets Texas Commission
on Environmental Quality (TCEQ) requirements.
– Is spillway wide enough?
 Calculate using Weir Equation

17. Let’s Summarize Our Progress

18. Let’s Summarize Our Progress

19. Let’s Summarize Our Progress

20. The Approach: Part 4
 HEC-RAS
– ArcGIS
 HEC-GeoRAS
 Import Geometric Data
– Specify Manning’s n value for
inundation area

21. Geometric Data

22. The Approach: Part 4
 Import Geometric Data
 Create Inline Structure
 Import Hydrograph Data
 Set Criteria for Dam
Breach Scenarios
– Sunny Day
– Design Breach
– Overtopping
 Run Simulation

24. Cross Section

25. Profile Pilot

26. The Approach: Part 5
 HEC-RAS Output
– Inundation Area
– Hydraulic Cross-Sections
– Time to Peak
 Information Use
– Create an EAP
– Inform Improvement
Planning
– Meet TCEQ Dam Safety
Requirements

27. Why does it matter?
American Society of Civil Engineers, Code of Ethics

28. Partners for a Better Quality of Life