Democratic Socialist Republic of Sri Lanka
Ministry of Agriculture Development & Agrarian Services
Contract DSWRPP-1/CS/QCBS/02
Dam Safety and Water Resources Planning Project
(DSWRPP)
DAM BREAK SIMULATIONS
MAHAWELI CASCADE
DSWRPP-NCH-REP-POY-081-rev0
September 2010
Dam Safety and Water Resources Planning Project (DSWRPP)
Mahaweli Cascade – Dam Break Simulations
i
Contact
Pöyry Energy AG
DSWRPP Project Office
11 Mahaweli Authority
Jawatte Road
Colombo 5, Sri Lanka
Tel. +94 11 258 8184
Pöyry Energy AG
Hardturmstrasse 161, P.O. Box
CH-8037 Zurich/Switzerland
Tel. +41 44 355 5554
Fax +41 44 355 5556
http://www.poyry.com
J. Lockwood
Team Leader
A. Sorgenfrei
Dam Engineer
Dam Safety and Water Resources Planning Project (DSWRPP)
Mahaweli Cascade – Dam Break Simulations
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Contents
1
INTRODUCTION .................................................................................................................. 1
1.1
1.2
1.2.1
1.2.2
Main Salient features of Randenigala Reservoir...................................................................... 2
Dam and River Channel Characteristics Pertinent to Dam Break Analysis ............................ 2
Dam and Reservoir.................................................................. Error! Bookmark not defined.
Stream Channel ....................................................................... Error! Bookmark not defined.
2
REFERENCES ....................................................................................................................... 5
3
DATA COLLECTION .......................................................................................................... 6
3.1
3.2
3.3
Topographic Data ..................................................................................................................... 6
Field Reconnaissance ............................................................................................................... 6
Rainfall data ............................................................................................................................. 6
4
HYDROLOGIC MODELING .............................................................................................. 6
4.1
4.2
General Methodology .............................................................................................................. 6
Development of Probable Maximum Precipitation and Probable Maximum Flood ...... Error!
Bookmark not defined.
5
HYDRAULIC MODEL DEVELOPMENT......................................................................... 6
5.1
5.2
5.3
5.4
5.5
5.6
General Methodology – Objective of the Model ..................................................................... 6
Geometry .................................................................................................................................. 7
Cross Sections ........................................................................................................................ 10
Structures ............................................................................................................................... 10
Roughness Values .................................................................................................................. 10
External Boundary Conditions ............................................................................................... 10
6
DAM BREACH ANALYSIS ............................................................................................... 12
6.1
6.2
6.3
Dam Failure Scenarios ........................................................................................................... 12
Failure Characteristics............................................................................................................ 13
Determination of Breach Parameters ..................................................................................... 13
7
RESULTS OF DAM BREACH SIMULATIONS ............................................................. 14
7.1
7.2
7.3
7.4
Model Calibration .................................................................................................................. 14
Results of Simulation ............................................................................................................. 14
Predicted Inundation .............................................................................................................. 16
Evacuation ............................................................................... Error! Bookmark not defined.
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Mahaweli Cascade – Dam Break Simulations
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1
INTRODUCTION
Largest reservoirs in Sri Lanka are located in the Mahaweli river forming a cascade.
Most upstream reservoir is the Kothmale Reservoir. Downstream of this Polgolla
Diversion is located. The Victoria reservoir is the next reservoir in the cascade and
downstream of this, Randenigala reservoir is located. A run-of-river generation
Rantambe Dam is the next and last dam of the cascade.
Kothmale Reservoir is the largest uppermost reservoir in the Mahaweli cascade
reservoir system holding more than 174 mcm. The major diversion at Polgolla
downstream of this reservoir takes water to North Central area of the country for
irrigation. The reservoir is hence, centrally important to the country because of its
storage for irrigation water, space for flood control of Gampola, Peradeniya and
Katugastota townships and power generation. Power plant of the Kothmale Dam is
situated about 13km downstream. About 42km downstream of the dam the Polgolla
diversion weir is located. This diversion takes water to irrigate North Central area of the
country. In the flatter flood plains of the river very populated townships of Gampola,
Gelioya, Peradeniya, Getambe and Katugastota are located. The towns are the suburbs
of main central country capital, Kandy.
Polgolla dam is a run-of-river diversion sending Mahaweli river water to North-central
province of Sri Lanka predominantly for irrigation. Hydropower generation is an
additional benefit on the water course of this diversion. The dam is nationally important
as the irrigation area under it is significant to influence the national economy. The dam
is built across Mahaweli River at Polgolla. The dam is equipped with double leafed
gates which has its sill levels close to the river bed. At fully open position they represent
the unobstructed river cross section except for the piers. The downstream reach from
Polgolla dam up to the entrance to Victoria Reservoir is inhibited. These people have
experienced several large flood discharges, however has not experienced flood
discharges near magnitudes corresponding to PMF or dam break discharges. Therefore
it is important to know the effect in downstream due to a catastrophic failure of the dam.
The Victoria Dam is the only concrete arch dam in the country. It is the highest dam
creating one of the largest reservoirs. Main purposes of the reservoir are hydropower
generation and irrigation. The reservoir is the hub of both hydro electric network and
the largest irrigation scheme. Part of the irrigation areas are located further down the
river and downstream of Randenigala reservoir which is the next reservoir in the
cascade. The area below Randenigala reservoir is populated with Mahiyangana Town
located closer to the river. The reach between the Randenigala Reservoir and the
Victoria reservoir is a river gorge and is not inhibited except for the hydro power plant
of Victoria Reservoir.
Randenigala Reservoir is the largest reservoir in Sri Lanka holding more than 860
mcm. The reservoir is centrally important to the country because of its storage for
irrigation water, space for flood control and power generation. The irrigation area
extends to the Easters plains of the country. Just below the Randenigala Dam a run of
the river power plant called Rantambe is situated. About 4km downstream of this
Minipe diversion weir is located. This diversion takes water to irrigate both left bank
and right bank of the river. Further down the river is the Mahiyangana Township which
is the largest inland town in the eastern plains of the country. The town is the social,
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economic and agricultural hub coordinating and providing for all needs of the
agriculture community of the irrigated lands.
Rantambe Reservoir is the last reservoir in Mahaweli cascade system. It has a low
capacity of less than 11.2mcm. Regulating the discharge is done by upstream
Randenigala Reservoir and this dam serves as run-of-the river type power generation
after picking up the discharge from Uma Oya. The reservoir is centrally important to the
country because of its power generation and its location upstream of the Mahiyangana
city that attracts pilgrims. About 4km downstream of this dam, Minipe diversion weir is
located. This diversion takes water to irrigate both left bank and right bank of the river.
The irrigation area extends to the Easters plains of the country. Further down the river is
the Mahiyangana Township which is the largest inland town in the eastern plains of the
country. The town is the social, economic and agricultural hub coordinating and
providing for all needs of the agriculture community of the irrigated lands.
One major component on which the Emergency Action Plan (EAP) for the reservoir
system is based on is the Inundation Maps. Such maps are extremely important to
recognize and demarcate the inundation area, decide on evacuation centers and escape
routes and to develop a notification flow chart. This document reports the procedure,
parameters and results of inundation mapping during a dam breach event.
A dam breach scenario is therefore analyzed for Mahaweli Cascade considering the
effect of breach of upstream dam on the immediate downstream dam. The inflow during
the breach is considered as the Probable Maximum Flood (PMF) inflow to the
reservoirs. The PMF derived for spillway adequacy analysis is used in this study and it
is described in detail in the ‘Detail Design Report’ of dam safety review. The PMF
design is based on Probable Maximum Precipitation (PMP) derived using the methods
specified in the World Metrological Organizations Manual.
Breach parameters for the dam were estimated using several equations that described
such situation and most appropriate values were selected as described in the in dam
breach reports of the individual dams. Analysis conducted with the HEC-RAS software
package, performing unsteady flood routing along the channel stream.
1.1
Main Salient features of Kothmale Reservoir
Pertinent information of the Kothmale Reservoir is summarized as follows:

Dam Type
=
Rock fill with upstream concrete apron

Dam Crest Elevation
=
706.5 m MSL

Dam Crest Length
=
600 m

Maximum Dam height
=
87 m

Top width
=
10 m

Storage at FSL
=
174 mcm

Spillway type
=
Radial gated open chute
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3

Effective spill crest elevation =
688 m MSL

Crest Length
=
3 x 14 m

Gate size
=
16.75 m (H) x 14 m (W)

Flood discharge
=
4668 m3/s (at PMF)

Afflux at PMF
=
16.24 m
Main Salient features of Polgolla Reservoir
Pertinent information of the Polgolla Reservoir is summarized as follows:
1.3

Dam Type
=
Gated Barrage (Commissioned in 1977)

Dam Crest Elevation
=
447.45 m MSL

Dam Crest Length
=
143.86 m

Maximum Dam height
=
14.63 m

Top width
=
10.36 m

Storage at FSL
=
4.1 mcm

Spillway type
=
double leaf vertical gates, sill on river
bed

Effective spill crest elevation =
434.34 m MSL

Crest Length
=
12.19 x 10 m

Gate size
=
6.4 m (H) x 12.19 m (W)

Flood discharge
=
4272m3/s (at design flood, water level at
443.66 m msl))

Afflux at design flood
=
9.32 m
Main Salient features of Victoria Reservoir
Pertinent information of the Victoria Reservoir is summarized as follows:

Dam Type
=
Concrete Arch (Commissioned in 1985)

Dam Crest Elevation
=
442.5 m MSL

Dam Crest Length
=
520 m

Number of blocks
=
35
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
Maximum Dam height
=
122 m

Top width
=
6m

Full supply level
=
438.0

Storage at FSL
=
722 mcm

Spillway type
=
Ogee & eight radial gates

Effective spill crest elevation =
430.0 m MSL

Crest Length
=
12.5 x 8 m

Gate size
=
8.5 m (H) x 12.5 m (W)

Sill Level of gates
=
430.0 m MSL

Flood discharge
=
4920 m3/s (at design flood, water level at
438.82m msl))

Afflux at design flood
=
8.82 m
Main Salient features of Randenigala Reservoir
Pertinent information of the Randenigala Reservoir is summarized as follows:

Dam Type
=
Rock fill with central clay core

Dam Crest Elevation
=
239 m MSL

Dam Crest Length
=
485 m

Maximum Dam height
=
91 m

Top width
=
10 m

Storage at FSL
=
860 mcm

Spillway type
=
Radial gated open chute

Effective spill crest elevation =
218 m MSL

Crest Length
=
3 x 16.3 m

Gate size
=
16.7 m (H) x 16.3 m (W)

Flood discharge
=
8334 m3/s (at design flood)

Afflux at design flood
=
19.27 m
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5
Main Salient features of Rantambe Reservoir
Pertinent information of the Rantambe Reservoir is summarized as follows:

Dam Type
=
Concrete gravity

Dam Crest Elevation
=
155.5 m MSL

Dam Crest Length
=
420 m

Maximum Dam height
=
41.5 m

Top width
=
10 m

Storage at FSL
=
11.2 mcm

Spillway type
=
Radial gated ogee

Effective spill crest elevation =
137 m MSL

Crest Length
=
4 x 16 m

Gate size
=
16.4 m (H) x 16 m (W)

Flood discharge
=
5551 m3/s (at design flood)

Afflux at design flood
=
15.37 m
1.6
Dam and River Channel Characteristics Pertinent to Dam Break Analysis
2
REFERENCES
[1] Activity 11: Assessment of Adequacy of Spillway and Outlet Works Capacity and
Flooding Impacts, Final Report, January 2004, Jacobs Gibb (Part A: Hydrology and
Flood Routing)
[2] Operational Hydrology Report No.1, (1986); Manual for Estimation of Probable
Maximum Precipitation, World Meteorological Organization, WMO No. 332.
[3] S Arumugam (1960) “The Floods of December 1957 and their Impact on Water
Conservation Works”, Shanmugam Arumugam Commememoration Volume,
Water for People and Nature; Edited by Badra Kamaladasa, D.L.O. Mendis.
Department of Irrigation, Colombo.
[4] National Engineering Handbook; Part 630 Hydrology, (1997), Natural Resources
Conservation Service, United States Department of Agriculture.
[5] T.L.Wahl (1998) “Prediction of Embankment Dam Breach Parameters”, Dam
safety office, Water Resources Research Laboratory, Bureau of Reclamation, US
Department of Interior.
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DATA COLLECTION
3.1
Topographic Data
6
One meter contour data was derived by the Department of Surveys based on their
ground data points for making 1:10,000 maps, engineering surveys and city maps.
These data were then used to get the required river cross sections and also to derive the
Digital Elevation Model for inundation mapping. The maps use Kandawala Datum of
Sri Lanka. The extent of the maps are from Kothmale dam up to Mahaweli Flood plains
downstream of Mahiyangana.
3.2
Field Reconnaissance
A field reconnaissance surveys were conducted by the Consultants with respect to each
dam in the cascade for the preparation of dam break EAP reports. Field observations
were made of the river channel and overbank of Kothmale Oya and Mahaweli River.
River channel and overbank roughness characteristics were noted.
3.3
Rainfall data
Annual maximums of the daily rainfalls at NuwaraEliya and Katugastota rainfall
gauging station was collected for last hundred years from the Department of
Meteorology.
4
HYDROLOGIC MODELING
4.1
General Methodology
Hydrologic flood routing of the Kothmale reservoir, Polgolla Reservoir, Victoria
Reservoir, Rantambe Reservoir and Randenigala Reservoir showed that probable
maximum flood can be managed satisfactorily and the spillways are adequate to pass
the flood. Breaching of the dams under a PMF is considered is simulated in this model.
The dams were set to breach when the water levels reached the dam top levels. HECRAS version 4.1 is used for the unsteady simulation of the flood wave routing with a
dam breach. Method of calculation and flood hydrographs of PMF are given in the
hydrology report of each dam.
5
HYDRAULIC MODEL DEVELOPMENT
5.1
General Methodology – Objective of the Model
HEC-RAS version 4.1 was used to conduct the dam breach analysis for Mahaweli
Cascade. It is a one-dimensional unsteady flow routing model capable of integrating
complex channels and structures under dynamic hydrologic conditions. HEC-RAS also
has the capability to model dam breach events under a wide range of scenarios. A 1-D
model such as HEC-RAS is appropriate for modelling inundation of Mahaweli casca in
an event of dam break.
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Cross sections, stream centrelines, and other geometric features of the stream were
extracted from available topographic data using HECGeoRAS. Arc GIS was used to
derive the Digital Elevation model to be used in the HEC Geo RAS.
5.2
Geometry
The HEC-RAS model of the Mahaweli cascade does not consists of any storage areas.
Major structures like bridges were incorporated in the model. The reservoir upstream of
the dam was modelled with a series of cross sections giving the same topographic
geometry and area-capacity curve. HEC-GeoRAS, Version 4.2.93, was used as an
extension to ArcGIS to generate the Stream Centreline and cross sections for the HECRAS model of this study area. A plan view of the HEC-RAS Model is shown in Figure
1.
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Figure 1a: Plan view of the Kothmale Reservoir study area as modelled in HEC
RAS
Figure 1b: Plan view of the Polgolla - Victoria Reservoirs study area as modelled in
HEC RAS
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Figure 1c: Plan view of the Randenigala Reservoir and study area as modelled in
HEC RAS
Figure 1d: Plan view of the Rantambe Reservoir study area as modelled in HEC
RAS
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5.3
10
Cross Sections
Cross sections are used to define the shape of the stream channel, adjacent floodplain
and characteristics such as roughness, flow expansion and contraction, and ineffective
flow areas. The cross sections were extracted from the DEMs using HEC-GeoRAS
supplemented with topographic maps for the channel geometry.
The cross sections were located to adequately describe geometric features such as
hydraulic roughness changes, grade breaks, and flow expansion and contraction. The
cross sections are generally oriented perpendicular to the expected flow lines of the
maximum flood wave.
5.4
Structures
The major structures present in the downstream reach of the Kothmale Dam up to
Mahiyangana which could withstand or impede a flood wave caused by a dam breach
were considered. Altogether there were eight bridges and one weir. All such structures,
were defined as in-line structures. The HEC-RAS model dam geometry is derived from
field survey data, as well as data obtained from the Mahaweli Authority of Sri Lanka
and Jacob GIBB’s reports on respective Dams. Data for components such as outlet
works, embankment side slopes, coefficients etc. were entered directly into the HECRAS model.
5.5
Roughness Values
Manning’s n-value ranged from 0.03 to 0.045 for the main channel and 0.04 to 0.055 for
overbank areas. To provide numerical stability to the hydraulic model Manning’s nvalues were based on published values for similar conditions, and on engineering
judgment and experience. Observed water depths at Peradeniya in 1978 were used to
calibrate the river roughness values.
5.6
External Boundary Conditions
For the unsteady flow model the upstream boundary conditions are input as discharge
hydrographs. The input hydrographs for the Kothmale dam breach model represent the
PMF flood event. The PMF inflow hydrograph is shown in Figure 2. The downstream
boundary condition was set to inline structure which is the Polgolla dam. The Polgolla –
Victoria model uses the PMF inflow to Polgolla and Victoria reservoirs combined with
the Resulting flood hydrograph of Kothmale dam breach. Similarly the Randenigala
Rantambe system has the respective PMF inflows and resulting flood hydrograph from
Victoria dam breach. The Victoria and Randenigala dam breachs are considered here
only if the flood levels in the respective reservoirs exceed the dam top levels. Also for
Randenigala – Rantambe system 40% PMF is taken as the inflows as the high rains in
Kothmale – Polgolla - Victoria catchments do not coincide with high rains in
Randenigala – Rantambe reservoirs.
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Figure 2a: The PMF inflow hydrograph used as upstream boundary condition of
Kothmale reservoir
Figure 2b: The combined PMF and Kothmale dam break hydrographs used as
upstream boundary condition of Polgolla - Victoria reservoir
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Figure 2c: The combined 40% PMF and Victoria dam break hydrographs used as
upstream boundary condition of Randenigala reservoir
Figure 2d: The combined 40% PMF and Randenigala dam break hydrographs
used as upstream boundary condition of Rantambe reservoir
6
DAM BREACH ANALYSIS
6.1
Dam Failure Scenarios
The breach of the upstream most reservoir, the Kothmale reservoir, is taken to be
triggered by the PMF inflow to the reservoir. The resulting dam breach flood wave will
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result overflow of the downstream reservoirs and as the water level at the dam top level
is taken as the dam breach triggering event for the downstream.
6.2
Failure Characteristics
Because dam failure is a hypothetical event, the actual breach size, location, and timing
are unknown and must be estimated. A range of potential breach sizes and formation
times, location of the breach and the breach initiation were considered as explained in
the reports on individual dams.
6.3
Breach Parameters
Table 1 shows the selected dam breach parameters of each reservoir.
Table 3a: Selected Breach Parameters of Kothmale Dam
Dam height (m)
Full formation time (hours)
Side slope (H:V)
Bottom of breach width (m)
87
2.77
0.5:1
58
Table 3b: Selected Breach Parameters of Polgolla Dam
Dam height (m)
Full formation time (hours)
Side slope (H:V)
Bottom width of breach (m)
14.63
0.25
0:1
28
Table 3c: Selected Breach Parameters of Victoria Dam - Abutment Failure
Dam height (m)
Full formation time (hours)
Side slope (H:V) – Left Bank
Side slope (H:V) – Right Bank
Bottom of breach width (m)
122
0.5
1.2:1
0:1
15
Table 3d: Selected Breach Parameters of Randenigala Dam
Dam height (m)
Full formation time (hours)
Side slope (H:V)
Bottom of breach width (m)
91
3.00
0.5:1
140
Table 3e: Selected Breach Parameters of Rantambe Dam
Dam height (m)
Full formation time (hours)
Side slope (H:V)
Bottom of breach width (m)
41.5
0.5
0:1
50
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RESULTS OF DAM BREACH SIMULATIONS
7.1
Model Calibration
14
Model calibration was performed in this case using the flood of 1978. Depth at the
Peradeniya gauging station and field observations were used for calibration. The field
observations according to personal memories of inundation levels were also used for
calibration. The river roughness was changed to get the near coincidence of the
inundation levels. In the areas where no such details were available, standard roughness
parameters were used.
7.2
Results of Simulation
The resulting flood propagation and its attenuation from the Dam Breach simulations
with HEC-RAS for the reach from Kothmale to Polgolla is shown in Figure 3a. The
reach from Rantambe to end of simulation is shown in Figure 3b. For Kothmale
reservoir the highest discharge at the dam is 21,358 m3/s just after the breach. This peak
is attenuated along the river to a peak of 13,275 m3/s when it reaches Polgolla. After
cascade breach the peak discharge at Rantambe 64525 m3/s which get attenuated to a
peak of 48125 m3/s at the end of considered simulation extent.
Figure 4 shows the flood water depth distribution. The flood water depth in the each
from Kothmale to Polgolla reach a maximum of 35m in the reach Gampola – Gelioya
while it reaches a maximum of about 24m in Gohagoda – Polgolla reach. The Rantambe
downstream reach, though carries a larger discharge will have a depth between 5 to
25m. The reach close Mahiyangana will have a depth between 5 to 15m.
A three hour delay is observed in flood wave propagation from Kothmale to Polgolla.
Storage routing in Victoria and Randenigala reservoirs delay this peak by another four
hours. Further two hours of delay is seen for flood wave propagation from Rantambe to
Mahiyangana.
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Mahaweli Cascade – Dam Break Simulations
Figure 3a:
15
Dam breach flood wave progression with time (Dam at 4454m,
Confluence with Mahaweli at 10805m)
Figure 3b: Dam breach flood wave progression with time for Rantambe
Reservoir
Figure 4a: Maximum water depths in river channel from Kothmale to Polgolla
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Figure 4a: Maximum water depths in river channel from Rantambe reservoir
7.3
Predicted Inundation
The inundation shows that all main roads connecting Kandy and Gampola with other
cities will be inundated due to an catastrophic failure of the Kothmale Dam. Most of the
populated areas of the towns of Gampola, Katugastota, Gelioya and Penideniya will be
inundated. Gampola, the upstream most town will receive the flood wave within two
hours from the initiation of the breach. The inundation shows that the KandyMahiyangana roadway across the Mahaweli River running along downstream banks of
the dam will be inundated. Mahiyangana Township, roads to Dehiattakandiya and
Hasalaka will be inundated.
The extent of maximum inundation is shown in Figure A1.
Only about 25km out of 52km is covered with measured cross sections. The other cross
sections were derived from 1m contours obtained from survey Department which have
lower accuracy. In order to improve the accuracy it is recommended to re-model using
measured river cross sections.
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Figure A1-1: Maximum inundation due to dam breach flood wave Kothmale to
Polgolla
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Figure A1-2: Maximum inundation due to dam breach flood wave Kothmale to
Polgolla
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Figure A1-3: Maximum inundation due to dam breach flood wave Kothmale to
Polgolla
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Figure A1-4: Maximum inundation due to dam breach flood wave Kothmale to
Polgolla
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Figure A1-5: Maximum inundation due to dam breach flood wave Kothmale to
Polgolla
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Figure A1-6: Maximum inundation due to dam breach flood wave Kothmale to
Polgolla
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Figure A1-7: Maximum inundation due to dam breach flood wave Kothmale to
Polgolla
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Figure A1-8: Maximum inundation due to dam breach flood wave Kothmale to
Polgolla
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Figure A1-9: Maximum inundation due to dam breach flood wave Victoria to
Randenigala
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Figure A1-10: Maximum inundation due to dam breach flood wave Randenigala
to Rantambe
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Figure A1-11: Maximum inundation due to dam breach flood wave Rantambe
downstream
Dam Safety and Water Resources Planning Project (DSWRPP)
Mahaweli Cascade – Dam Break Simulations
28
Figure A1-12: Maximum inundation due to dam breach flood wave Rantambe
downstream
Dam Safety and Water Resources Planning Project (DSWRPP)
Mahaweli Cascade – Dam Break Simulations
Kothmale - Polgolla - DB
29
Plan: Plan_01 2/27/2011
750
Legend
WS Max WS
700
Ground
Elevation (m)
650
600
550
500
450
400
0
10000
20000
30000
40000
50000
60000
Main Channel Distance (m)
Figure A2-1 Maximum water depths from Kothmale to Polgolla
Rantambe_4
Plan: Plan 01 3/13/2011
180
Legend
WS Max WS
160
Ground
Elevation (m)
140
120
100
80
60
0
10000
20000
Main Channel Distance (m)
Figure A2-1 Maximum water depths from Randenigala
30000
40000