This research was taken place along the lower Mekong river reach part in Cambodia. The purpose of the study is to evaluate the HEC-RAS performance whether it eligible for Cambodia flood studies or not.

1. Flood Studies in Lower Mekong River Cambodia via
HEC-RAS and ArcGIS
By: KIM Lengthong
Advisor: Dr. LY Sarann
2015-2016
Institute of Technology of Cambodia
Department of Rural Engineering

2. I. Introduction
II. Materials and Methods
III. Results and Discussions
IV. Conclusions and Recommendations
2 Flood Studies by using HEC-RAS

3. Introduction
Materials and
Methods
Results and
Discussions
Conclusion sand
Recommendations
Why flood studies?
3
49%
38%
6%
5% 2%
0%
Flood Lightning Fire Storm Epidemic Other
Source: National Committee of Disaster Management, 2015
Objectives of the study
• Evaluate HEC-RAS model performance
• Produce flood maps from 2000 to 2013 at
peak flow
• Produce 10-years flood return period map
• Assess the critical result from map
Flood Studies by using HEC-RAS

4. Introduction
Materials and
Methods
Results and
Discussions
Conclusions and
Recommendations
4
Location
From Chbar ampov to
Korkir Thom commune,
Kean Svay(approximate
50 km)
Data collection
• DEM 30m cell size from STRM
• 18 cross sections (ADCP measurement on 24th May 2015)
• Manning roughness (n)
Study Area
Flood Studies by using HEC-RAS

5. Introduction
Materials and
Methods
Results and
Discussions
Conclusions and
Recommendations
Discharge
5
0
5000
10000
15000
20000
25000
30000
35000
2000 2001 2002 2003 2004 2005 2006 2007 2008 2011 2013
Q(m3/s)
Year
Maximum Discharge
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
2 4 6 8 10 12
Q(m3/s)
H (m)
Rating Curve
Water level in 2009, 2010 and 2012
were missing.
Flood Studies by using HEC-RAS
Lower Mekong River at
Koh Norea
Q = 976.147(H-0.428)1.494
Available discharge and
water level at Koh Norea
Station

6. 6
Pre-Process Post-Process
HEC-RAS 4.1
RAS-Geometry Water Surface Profile
1.Edit and Enter
Manning Value
2. Input river
flow data
3.Steady flow with
subcritical RUN
ArcGIS
Model Framework
Flood Studies by using HEC-RAS

7. Introduction
Materials and
Methods
Results and
Discussions
Conclusions and
Recommendations
• Results of inundated area were
evaluated by fit values: F1 and
F2 (V. Moya Quiroga et al.,
2016)
- A is the total flooded area that predicted
correctly by model
- B is the total area that over predicted
by the model
- C is the total area that is under predict
by the model
• Water Level was evaluated by
index error (Moriasi et al.,2007)
7
Model Performance Evaluation
F1=
A
A+B+C
F2=
A−B
A+B+C
Satisfactory criteria:
• NSE > 0.50 and
• RSR < 0.60,
• PBIAS = [-25%;25%]
The satisfactory value:
• F1 and F2 > 0.5
Flood Studies by using HEC-RAS

8. Introduction
Materials and
Methods
Results and
Discussions
Conclusions and
Recommendations
Model Simulation Performance Rating
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Simulated output on 24th August 2008 Observed flood on 24th August 2008
Source: UNOSAT,2008
0
100
200
300
400
500
600
700
800
900
Area(ha)
Communes
Affected Area
Simulation result Observed result
F1= 0.86
F2= 0.74
1st Note: Affected area from simulation is greater
except in Sambour commune.
Flood Studies by using HEC-RAS

9. Introduction
Materials and
Methods
Results and
Discussions
Conclusions and
Recommendations
9
8.5
9
9.5
10
10.5
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11.5
2000 2001 2002 2003 2004 2005 2006 2007 2008 2011 2013
WaterLevel(m)
Year
Water Level
Sim WL Obs WL
NSE = 0.992101
PBIAS = 0.234619
RSR = 0.088878
2nd Note: The water level
from simulation is less than
observation except 2005,
2006 and 2007 .
The result from HEC-RAS flow
came out of bank earlier than
actual and the water surface
elevation is a bit lower than in
real situation (Goodell & Warren,
2006).
Flood Studies by using HEC-RAS

10. Introduction
Materials and
Methods
Results and
Discussions
Conclusions and
Recommendations
10
Results from 2000-2013
Flood Studies by using HEC-RAS
Video Playback

11. Introduction
Materials and
Methods
Results and
Discussions
Conclusions and
Recommendations
11
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8.5
9
9.5
10
10.5
11
11.5
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
2000 2001 2002 2003 2004 2005 2006 2007 2008 2011 2013
Waterlevel(m)
FloodExtent(ha)
Year
Summary Results
Water level Flood extent
From 2000 until 2013, the affect area
ranged from 1,400 to 8,000 hectares
(excluding river area 4600 ha), while water
level fluctuated from year to year.
Flood Studies by using HEC-RAS

12. Introduction
Materials and
Methods
Results and
Discussions
Conclusions and
Recommendations
12
10-years return period flood map
The inundated area was about 8000 hectares and flood depth range
from 0.05 mm to 11 m (at the bottom of river). The 10-years return
period inundation map is really important for planning, development
project, and industries.
Flood Studies by using HEC-RAS

13. Introduction
Materials and
Methods
Results and
Discussions
Conclusions and
Recommendations
13
Conclusions
• Based on results of model evaluation, HEC-RAS model is capable
flood studies for Mekong river and produce flood map
• The affected area for the both sides of river was ranged from 1,400 to
8,000 hectares while flood depth varied from year to year.
Recommendations
• High resolution of DEM is recommended
• At place water level and more surveying cross-sections are needed
• The study should account with islands along the river
• HEC-HMS or SWAT should be used to simulate surface runoff
• HEC-RAS version 5.1 is recommended for future study
Flood Studies by using HEC-RAS

14. Thank you
for your attention
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