Flood Modelling in the Mahaweli River Reach from Kotmale to Polgolla
S. B. WEERAKOON, K.D.W. NANDALAL, UDITHA RATHNAYAKE & UPAKA RATHNAYAKE
sbweera@pdn.ac.lk

Department of Civil Engineering, University of Peradeniya, Peradeniya, Sri Lanka

ABSTRACTThe occurrence of floods and inundation of the low lands adjacent to the Mahaweli River reachfrom Gampola to Polgolla in Central Sri Lanka were very frequent prior to last two decades. It iswith the completion of the Kotmale reservoir project in mid 1980s, the regulation of flow by theKotmale reservoir has reduced the inundation risk of these lands which were vulnerable to frequent
flooding. Since then, the lands are developed at an increasing rate and many people have started tolive in the fertile river valley. This development accompanies an overlooked risk to the community
and alarming signal to the authorities as the damages that might be caused due to an extreme flood
event could be significant. It is therefore of paramount importance that flood modeling andinundation analysis of the Mahaweli River reach between Kotmale and Polgolla is carried out. This
paper presents the flood modeling in the 42.2 km long Mahaweli river reach from the Kotmale dam
to Polgolla barrage using the HECRAS model. The model is set up to estimate the water stages
along the river reach for the floods of different return periods.
Key words: extreme flood, inundation modeling, Mahaweli River, HECRASINTRODUCTION
The Mahaweli River; the longest river in Sri Lanka originates from the central hills of the country
and flows over a length of about 335 km to discharge to the Indian Ocean at North-Eastern city ofTrincomalee. It drains a basin area of 10,025 km2 receiving rainfall from 1500 mm -5500 mm,elevations extending up to 2524 m. The Mahaweli River is the most important source for the water
resources development in the country. There are several multi-purpose reservoirs impounded by theconstruction of dams, barrages and weirs in the river to facilitate the 6 major hydropower plants,
and number of inter and transbasin diversions for irrigated agriculture and water supply.
Figure 1 shows the location of the Mahaweli River basin, and its upper basin of 1573 km2 ranging
from 434 m to 2524 m. elevation up to the Polgolla barrage of the river. The basin receives highrainfall during the monsoon and inter-monsoon seasons a large runoff is generated. Several major
cities including Kandy which is the hill capital are located in the valleys of this sub basin of theriver.
The occurrence of floods and inundation of the low lands adjacent to the Mahaweli River reachfrom Gampola to Polgolla were very frequent prior to mid 1980s. The Kotmale reservoir project
was completed in 1985 as part of the massive water resources development project centering theMahawelir River. The Kotmale dam is a rock fill dam of about 87 meters height constructedintercepting the flow of the Kotmale Oya which the uppermost major tributary of the Mahaweli
River. The Kotmale reservoir with a capacity of 174 million m3 was impounded with a 200 MW
hydropower plant. During last two decades, the regulation of flow by the reservoir has reduced theinundation risk of these lands, which were vulnerable to frequent flooding. As a result, these lands
1are developed at an increasing rate with significant investment on housing and infrastructuredevelopment. Many people have started to live in the fertile valley with pleasant weather and close
to major city of Kandy. This fact gives an alarming signal to the authorities, as the damage thatmight be caused due to an extreme
flood event could be significant. The
major flood experienced by the regionin 1978 before the Kotmale project,which was estimated to carry about3050 m3/s dam, inundated a large
extent causing severe damages.
Though the Kotmale reservoir acts as aflood control reservoir for floods of
medium return periods, it becomesineffective to reduce the flood levels inthe downstream flood plains due tofloods of high return periods. TheKotmale dam is equipped with aspillway which could release adischarge 5500 m3/s. It would
disastrous if a flood of similar
magnitude occurs at present condition.
Therefore, flood modeling andinundation
analysis
inregion
downstream of the Kotmale reservoir
is extremely important to plan theflood mitigations measures.
Hunan
Oya

Pinga
Oya

Nanu
Oya

Geli
Oya

NilambeOya

1

Mahaweli Lower
Catchment
Atabage
Oya

Mahaweli
Ganga

Kothmale Reservoir

Gootook Kothmala
Oya
OyaLower
ReachPuna
Oya

Mahaweli UpperCatchment

Figure 1 shows the stream network anddetails of the catchments for Kotmale
dam and Polgolla barrage. The Upper
Mahaweli River joins the river atseveral kilometers downstream of theKotmale dam. There are also several
small tributaries joining the river asshown in Fig. 1 and the areas are given
in Table 1.

Kothmala
Oya

Pundalu
Oya

Nanu
Oya
KothmaleCatchment

Dombagascolawa
Oya
Agra
Oya

Fig. 1 The Mahaweli River basin above Polgolla barrage
MODEL APPLICATIONThe HEC-RAS model of US-Army
Corps Engineers was set up to estimate
the water surface elevation levels in the42.2 km reach of the Mahaweli River
from the downstream of the Kotmale
dam up to the Polgolla barrage. Steady,one-dimensional water surface profile
computation in the HEC-RAS (Release4.0) model is based on the solution of
one-dimensionalenergyequation.
Energy losses due to friction and changes

Table 1. Details of the sub basins
No1234567892

Sub basin nameAtabage Oya
Nilambe Oya
Geli oyaNanu Oya
Hunan/Pinga Oya
Mahaweli Lower
Mahaweli Upper
Kothmala Oya
OtherArea / (km2)

94.4
71.3
15.981.7
75.2
434.6
215.0
535.2
49.6

in cross sections are taken into account by distributed roughness coefficients and expansion/contraction coefficients. Momentum equation is used at the locations where the water surface
profile is rapidly varied such as at hydraulic jumps, stream junctions and controls.
River cross sections at every 200 m intervals along the 42.2 km river reach were taken from the1:10000 scale maps of the Survey Department of Sri Lanka as the first step. A reach length of 1.2
km was used for the Mahaweli Upper tributary (Fig. 2). The upstream boundary conditions were set
up as the normal slope conditions and the downstream boundary condition was the specified water
surface elevation at the Polgolla barrage using the weir formula.
For the calibration of the model, the only available data for extreme flows which is the flood
discharge of 3050 m3/s with a water depth of 15 m at Peradeniya Railway Bridge was used. It wasin 1978 before the Kotmale reservoir. Peradeniya Railway Bridge is located at about 14 km
upstream of the Polgolla barrage. The daily rainfalls recorded at the gauging stations in the basinvary from 60 mm to 143 mm. Simulation was carried out under an approximated rainstorm of 3hour duration with an intensity of 29 mm/hr in the basin except for the sub basin above theupstream boundary of the Kotmale Oya. To match the flood discharge of 3050 m3/s at Peradeniya
Railway Bridge, an inflow of 1338 m3/s at the upstream boundary of the Kotmale Oya was used.
The corresponding total discharge at Polgolla was 3305 m3/s with the contributions from the subbasins at the downstream of Peradeniya. The water depth at Peradeniya Railway Bridge due to 1978flood was computed as 14.6 m which compares satisfactorily with the observation. The roughness
coefficients used in the HECRAS model are given in Table 2. The computed water surface profiles
is given Figs 2.
Table 2 Roughness coefficients in different reaches
Reach
Length from D/S (km)Kothmale34.8 - 42.2
Mahaweli Upper Tributary
0-1.2
Mahaweli10.0 - 34.8
Mahaweli
7.0 - 10.0
Mahaweli0 - 7.0

LOB
0.12
0.05
0.05
0.17
0.05

Manning's n (SI Units)
Channel
ROB
0.07
0.12
0.03
0.05
0.03
0.05
0.1
0.170.03
0.05

LOB:Left over bank; ROB: Right over bank
The available hydrological data for the basin are the daily rainfall at 15 stations and daily flow dataat three locations in the river. The maximum rainfalls of different return periods in the basin were
estimated by Gumbel distribution fitted to the maximum annual basin rainfalls. Daily rainfall with
200-year return period was estimated as 175 mm. In the absence frequency-duration-intensity
curves, rainstorms with estimated total rainfall but with different durations are considered for
extreme flood analysis. Then, runoff from each sub basin is estimated using SCS Dimensionless
Triangular Unit Hydrograph Method for each of the events. The release from the Kotmale reservoir
defines the inflow to the reach. The Kotmale dam has a spillway equipped with radial gates torelease a design discharge of 5500 m3/s . Different spillway discharge scenarios are considered as
upstream inflow boundary conditions to the reach.
The resulting large discharges in the river reach accompany larger depths. Thus, computation
essentially require wide river cross sections extending to high elevations.. The trial simulations
carried out showed the need for establishment of wide river cross sections as the inundations
become excessive under large discharges that 4000 m3/s in the reach with mild slope present below
Gampola. It is with the acquisition geometric data by field surveying, which is ongoing, thecomputations of water stage and inundation extents due to estimated floods of high return periods
are carried out.
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CONCLUDING REMARKSThe HECRAS model was set up to compute the water surface elevations along the Mahaweli Riverreach of 42.2 km from the Kotmale reservoir to the Polgolla Barrage under extreme flood events.
The model is useful to estimate inundation extents under extreme floods of different return periods
once the refine geometrical data covering wider river cross sections are made available. The model
would be a useful tool for forecasting flood levels and inundation extents and thereby for planningdisaster reduction strategies.

650

WS Elevation
Bed Level

Elevation / m MSL

600

550

500

450.2
38.2
534.4
32.4
30.4
28.4
26.4
24.4
22.4
20.4
18.4
16.4
14.4
12.4
10.4
8.
6
6.
64.
62.
60.
6.2
4036.242400

Length from Polgolla barrage/ (km)Fig 2. Water surface elevations along the river reachReferences
1. US Army Corps of Engineers, http://www.hec.usace.army.mil/software/hec-ras/ , 2008 Sept.
2. Kotmale Hydropower Project Report, Mahaweli Authority of Sri Lanka, Colombo, 1985.

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