Does the ancient Yoda Ela in Sri Lanka represent a technical innovation of hydraulic engineers? A geoenvironmental appraisal

Abstract

Yoda Ela (YE) located in the northern dry zone of Sri Lanka has been an irrigation marvel sustainably existed for over 1500 years without any break except for the recent politicized decisions abandoning its functions. A study was conducted on its geoenvironmental regime and compared with modern New Jaya Ganga (NJG) which replaced YE with rapid and high discharge straight channel. The examination of aerial photographs, Google Earth Pro and Digital Elevation Models (DEM) followed by field investigations assess the respective environmental changes. Thirty-one water samples were also collected from both YE and NJG at different discharges aiming at measuring turbidity variations. A theoretical model based on the application of the function of sinuosity developed by Langbein and Leopold in 1966 was applied for 12 meander segments from YE. The slope, sinuosity [K], angle[ω], wavelength [λ], amplitude [A] and the radius at curvatures [ ] of respective meanders were then measured. The output of these meander sections tracks a sinusoidal fitting to a modified sine curve may be resultant due to the construction of a headwater canal. The DEM indicated that the ancient YE follows the topographic contours when conveying water from Kala wewa to Tissa wewa but NJG is not. The positioning of the single embankment of YE has a direct relation to the drainage divide of the basins. Turbidity gradually decreases along its path however current inadequate flow may inherit algal boom at places causing the local increase. Meandering produces a sustainable environmental flow along with the ancient YE. A further analysis on flow parameter calculations, sediment analysis, landforms analysis with DEM, Google Earth, air photo and drone studies are expecting.

Full text

 Abstract—Yoda Ela (YE) located in the northern dry zone of Sri
Lanka has been an irrigation marvel sustainably existed for over 1500
years without any break except for the recent politicized decisions
abandoning its functions. A study was conducted on its
geoenvironmental regime and compared with modern New Jaya Ganga
(NJG) which replaced YE with rapid and high discharge straight
channel. The examination of aerial photographs, Google Earth Pro and
Digital Elevation Models (DEM) followed by field investigations
assess the respective environmental changes. Thirty-one water samples
were also collected from both YE and NJG at different discharges
aiming at measuring turbidity variations. A theoretical model based on
the application of the function of sinuosity developed by Langbein and
Leopold in 1966 was applied for 12 meander segments from YE. The
slope, sinuosity [K], angle[ω], wavelength [λ], amplitude [A] and the
radius at curvatures [

] of respective meanders were then measured.
The output of these meander sections tracks a sinusoidal fitting to a
modified sine curve may be resultant due to the construction of a
headwater canal. The DEM indicated that the ancient YE follows the
topographic contours when conveying water from Kala wewa to Tissa
wewa but NJG is not. The positioning of the single embankment of YE
has a direct relation to the drainage divide of the basins. Turbidity
gradually decreases along its path however current inadequate flow
may inherit algal boom at places causing the local increase.
Meandering produces a sustainable environmental flow along with the
ancient YE. A further analysis on flow parameter calculations,
sediment analysis, landforms analysis with DEM, Google Earth, air
photo and drone studies are expecting.
Keywords—Geoenvironment, meandering, sustainability, Yoda
Ela
I. INTRODUCTION
RRIGATION water supply to dry areas is a challenge not
only at present but even in the ancient historical periods [1]-
[3]. Many civilizations have adequately domesticated the water
and the majority was dependent upon sophisticated systems of
water management [4]. The Indus valley, Egypt and
Mesopotamia excelled with cultural upbringing mainly
supported by sophisticated hydraulic engineering achievements
and systematic water management skills [5]-[7].
Implementation of different adjustments and strategies
depending on the environmental, geological, climatological and
sociological conditions had interacted with such irrigation
systems either to avoid negative impacts or to enhance the
positive outcomes for the benefit of the people [8], [9]. Even
H. A. H. Jayasena is with the Department of Geology, University of
Peradeniya, Sri Lanka (corresponding author, phone: +94-727-435-161; fax:
+94-812-388-018; e-mail: cjayasena@ pdn.ac.lk).
community watershed management at the landscape has also
been used as a growth engine for sustainable development in
the dry regions through efficient rainwater management [10],
[11].
The YE in the northern dry zone of Sri Lanka which supports
sustainable irrigation over 1500 years has been selected for this
study. As a tropical island in the Indian Ocean, Sri Lanka has
three climatic zones viz; the dry, the wet, and the intermediate
zones defined by the average annual rainfall [ARF] [Fig. 1]. The
dry zone receives ARF < 1750 mm and is generally confined to
a topographically flat area, where paddy and other crops
bearing lands are being cultivated from the ancient period
onward [12]. Such cultivation could be implemented in the dry
zone due to a large number of cascaded manmade water bodies
[tanks] which store and harvest rainwater. Artificial water
channels are used for conveying and distributing water from
such tanks while supporting the croplands in between. Those
tanks and water conveying channels have connected as a
network for a sustainable water supply mainly to the flood
irrigated paddy cultivations [13]. As a result, a flourishing
civilization with manageable agriculture has appeared
supported by a historical water management system known as
Tank Cascade System [TCS]. A ‘cascade’ is defined as a
connected series of village irrigation tanks organized within a
micro- catchment of the dry zone landscape, storing, conveying
and utilizing water from an ephemeral rivulet [14].
II. GENERAL BACKGROUND
A. Ancient Yoda Ela [YE]
Yoda Ela [YE] is a magnificent creation of the ancient
irrigation engineers which was built during the reign of King
Dhatusena [459-477 AD] to convey water from the Kala Wewa
in the Kala Oya basin to Tissa Wewa in the Malwathu Oya
basin. Several episodes of reconstructions in the history were
recorded [15] until the beginning of the “Mahaweli”
development project in the year 1975 which abandoned a
segment of the flow path [16]. Literature and maps from the
earlier periods indicate the presence of similar YE’s in many
parts of the country which may indicate that the system was an
engineering concept implemented to convey water to the
country’s lowland [15], [17]. The entire length of the flow
R. D. D. P. Rathnayake was with the Department of Geology, University of
Peradeniya, Sri Lanka (e-mail: rddprathnayake@gmail.com).
Does the ancient Yoda Ela in Sri Lanka represent a
technical innovation of hydraulic engineers? A
geoenvironmental appraisal
R. D. D. P. Rathnayake, H. A. H. Jayasena
r
I

Fig. 1 (a) A map of Sri Lanka showing the climatic boundaries; (b) Kala Oya, Modaragam Aru and Malwathu Oya drainage basins; (c) YE
fed by headward streams from the respective basins as indicated by arrows, and (d) the enlarged section of the area around headwater of
Modaragam Aru indicating (by an arrow) how embankment on LHS change into RHS of the YE
d
)
0
a
)
0
b
)
0
c
)
0

path of the YE is about 87 km [54 miles] and consist of the
delicately worked meandering route. Although it has a low
flow rate, water is radially accelerating along with meanders
while being subjected to deflect and deformed along the whole
stream giving steady but sluggish flow. YE has been built to
follow outstanding gradients vary from 10-20 centimetres per
kilometre. It is collecting water from 66 mini catchments
whereas it fed water to about 120 small lakes along its length
[15]. In many respect, it is akin to an elongated reservoir
because of the single embankment. This single embankment
supports lateral push of waters towards the upslope while
reducing the pressure on the bank so that free flooding on the
opposite side of the embankment is maintained. The existence
of a single embankment throughout the entire length is
depending on the slope and headwater valleys.
B. New Jaya Ganga [NJG] and its effects on YE
The ambitious Mahaweli Development program provided an
additional quantity of water, so that, the YE was fed by not only
from the Kala Oya catchment but also from the Mahaweli
diversion. The authorities expected to irrigate maximum
possible command area under the Mahaweli waters and
increase the livelihood of the community in the dry zone,
however, which resulted in neglecting the sustainable aspects
of the ancient irrigation technology.
The NJG has been constructed parallel to the YE for
distributing the additional water receiving from the Mahaweli.
As a result, the existing 1500-year-old water conveying
mechanism of YE has been obliterated. The flow path of the YE
was separated into 3 segments with the middle segment
completely being abandoned. At present, the ancient YE mainly
provide water to paddy cultivations up to Mahailuppallama.
The meanders in the middle segment were straightened, which
resulted in an adverse outcome to already time-tested ancient
hydraulic regime. There was no need to implement such
controlling methods across the ancient YE because its water
was conveying along the routes defined by nature itself.
Moreover, the main purpose of the NJG is to accommodate high
discharge of Mahaweli water and quickly carrying it over to
irrigate areas without concerning the geoenvironmental
repercussions.
C. A comparison of YE functions with NJG
The comparison of irrigation with the ancient YE and from
the modern NJG shows some pertinent issues at present [Fig.
2]. Deterioration of water quality parameters such as increasing
salinity and hardness have been reported [18]. The groundwater
flow has been obstructed by concreting the embankments on
either side and deepening [19]. Stream embankment and bed
erosion have been augmented after the introduction of a
straightened channel with increasing discharge. With this
modern irrigation system, effects of backwaters on
destabilization along the banks were evident causing erosion
and excessive sediment accumulation in the bed. Moreover,
when comparing with ancient and modern irrigation practices,
the analysis demonstrates the importance of evaluating the
efficiency of proper hydrological practices against politically-
driven “ad hoc” modern water management implementation
strategies [20] [Fig. 3].
Fig. 2 Active and abandoned segments of YE, the NJG with points of
water samples
Fig. 3 Water quality changes due to groundwater encroachment to
NJG compared to surface water in YE
III. OBJECTIVES
Comparison of ancient and modern irrigation technologies
and their impacts on environmental sustainability is the most
pertinent object for this study in addition to assessing the
persistence of each on future diversity. In this respect, the study
was designed to unravel the geometry of the YE supported by
its channel parameters, sinusoidal distribution, meander
patterns, drainage divides and turbidity variations. In respect to
the ancient YE, the NJG with modern ad hoc construction
developments are assessing and comparing for a more
meaningful outcome.

IV. MATERIALS AND METHODS
The literature on ancient irrigation networks and water
distributing technologies related to YE, its renovations and
construction records, command areas between Kala Wewa and
Tissa Wewa, and recent discoveries were initially compiled.
The construction details of NJG by the Mahaweli development
project and recorded subsequent positive and negative effects
were also compiled. The rainfall, physical parameters and
agricultural extent, besides the YE, were collected from the
Mahaweli Authority. The 1:10000 soft dataset of sheet 36 and
1:50000 sheet 31 were bought from the Department of Survey,
Sri Lanka.
Water samples were collected into 100 and 500 ml
polyethylene bottles as two batches; the first in January during
the growing season and the second in February 2020 during
yielding period, and measure turbidity under different discharge
conditions. Initially, eight samples were collected four each
from the NJG and YE up to Mahailuppallama. The second
batch, six samples from the sluice in Kala Wewa up to
Mahailuppallama [Segment I along the YE], nine samples from
the sluice up to Kiralogama [along the NJG], and eight samples
from Batuwatta to Srawasthipura [Segment II along the YE]
was collected. Locations were chosen to avoid anicuts and
weirs to prevent backwater effects which can influence the flow
and turbidity [Fig. 3]. The turbidity of water samples was
measured with standard procedures using TN-100/T-100
turbidimeter after calibrating through 800 NTU, 100 NTU, 20.0
NTU, 0.02 NTU standards.
An aerial view of the flow paths including remnant parts of
YE and NJG, the surrounding topography, channel morphology
and the positioning of embankments, were extracted using
Google Earth pro. The digital elevation model [DEM] covering
the study area was downloaded [21] and the watershed was
delineated using spatial analyst and hydrology tools in the
ArcMap 10.4.
Lengths of channel segments and elevation [extracted from
the DEM] were initially calculated using ArcGIS and respective
sinuosity and slopes were then determined. The equations
developed by Langbein and Leopold [22] for meandering
functions of streams were used as a sinusoidal distribution
through the function of sinuosity [K] which as a dependent
factor varying with the angle at which the curve crosses at the
X-axis [ in degrees], wavelength [λ], amplitude [A] and the
radius of curvature [Rc] [Fig. 4]
Fig. 4 Symbolizations describing meander shape variables
(Sinuosity [K] = 1.5)
The relevant equations are,
    
 (1)
    
 (2)

    (3)
Twelve meanders distributed along the flow path of the
ancient YE were chosen [Fig. 5] and the above parameters were
measured using Arc Map 10.4.
Fig. 5 Meander sections from A to L in YE used for the experimental
data collection
V. RESULTS AND DISCUSSION
A. Evolutionary Characteristics
The evolutionary characteristics of YE and NJG [23] with
evidence supported by the data from the Mahaweli authority are
analyzed for its geoenvironmental sustainability.
B. Flow path following contours
According to the DEM, the exit at the sluice from Kala Wewa
to the YE has an elevation of 131 m above mean sea level
[MSL] and reaching 95 m MSL at the entrance to Tissa Wewa.
The YE maintains an elegant single embankment channel
following the contours [Fig. 6] and running through the parched
lowlands. The average slopes of 0.200, 0.204 and 0.863 mkm-
1are respectively maintained within the initial, abandoned and

the final segments. Even in Machu Picchu some of those
irrigation canals maintained a slope of 1 to 4.8% [24] which is
far higher than the slopes of YE which vary from 0.019-
0.020%. When considering the entire flow path, the slope of
0.420 mkm-1 was maintained by the YE while the NJG has
maintained a slope of 0.603 mkm-1 up to Thalawa.
Fig 6 YE flow path extracted on to DEM
Meandering nature of the YE follows natural morphology as
appeared in the contouring patterns and the DEM. Some loops
can be observed with exceeding sinuosity of five that they had
a long-distance instead of passing high elevated spurs with a
deep cut. However, the NJG is not following the topographic
contours; instead, it simply passes the flat rolling topography of
the area with straight segments having deep cuts. At places,
some of those cuts are extended approximately 18 m [~60 feet].
The YE is a trans-basin canal that conveys water from the
Kala Oya to the Malwathu Oya basins with a short distance
running through Modaragam Aru basin in between [Fig. 1]. The
slopes of the canal segments for each basin are; Kala Oya 0.266,
Modaragam Aru 0.155 and Malwathu Oya 1.003 mkm-1
respectively. The tributaries of these basins are contributing
headwater flow and overland flow to the meandering path of the
YE so that only one embankment was constructed akin to
represents its functions as an elongated reservoir [Fig. 1]. The
YE was supplied with overland flow including contributions of
headwater tributaries through command areas of 95.1, 15.6 and
15.7 km2 via Kala Oya, Modaragam Aru, and the Malwathu
Oya respectively.
Especially, the side of the single embankment has been
designed corresponding to the flow direction of natural
tributaries. The tributaries of Kala Oya and Modaragam Aru
flow towards west and NNW direction so that the YE
embankment was constructed in the left-hand side [LHS]
however the tributaries of Malwathu Oya flow towards the east
so that the embankment was constructed in the right-hand side
[RHS] [Fig. 1]. It revealed that the interchange of the respective
sides of the embankment is located near Pahalawewa when the
YE passing the drainage divide between Modaragam Aru and
Malwathu Oya. At Pahalawewa area, the channel [bearing]
perpendicularly crosses the drainage divide by passing
approximately 3 km within a deep cut when compared with
other areas. Paddy fields and homelands mostly were located
downslope below the embankment of YE while the forest
covers the upslope above it. A similar phenomenon has been
used to augment water supply to Minneriya and Giritale tanks
through Elaehera-Minneriya Yoda Ela, which bifurcates after
34 km at Diyabeduma [25].
C. Discharge to the Channels
Irrigation water is issued during the cultivating stages so that
more water is issued during the ‘Yala’ season [May to end of
August] than the ‘Maha’ season [September to March]. The
rainfall is less during southwest monsoons compared with the
northeast monsoon dominate during Maha season in the
northern dry zone of Sri Lanka [Fig. 7]. Moreover, regularly 10
cusecs are daily discharged to maintain the sustainability of the
environment including drinking water to animals while
preventing the weed growth along the channel.
Fig. 7 Average Rainfall over study area for the year 2019
D. Turbidity variations along the channels
Turbidity of the discharged water through sluices of Kala
Wewa depends on the slope and the local inflow along the flow
path. We examined its variations up to Mahailuppallama about
25 km relative to the entire flow path. The theoretical bases
were verified by testing the turbidity of the water samples
collected.
Suspended sediments are added to the channels from the Kala
Wewa via sluice, in addition to contributions from the
tributaries of the natural headwater drainage network and
through bank erosion. Among these factors, only the bank
erosion is triggered by the discharge of the channels. Though
the NJG get sediments from the croplands as the wastewater, no
0
100
200
300
400
500
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Rainfall (mm)

water is passing to the ancient YE from the farmlands. Turbidity
values have been plotted vs. the distance along the channels
separately. In the first sampling season, sluice discharges were
90 and 800 cusecs from the YE and the NJG respectively and
in the second period, they were 125 and 400 cusecs.
Fig. 8 Turbidities of water samples collected in the first term
Fig. 9 Turbidities of water samples collected in the second term
The figures illustrate that the turbidity near the sluice of the
YE is relatively higher than the values near the sluice of the
NJG [Figs. 8 and 9]. This is because the water discharges to the
sluice of the YE is located at a deeper level of the Kala Wewa
when compared to that of the NJG so that more suspended
sediments tend to leave through the deeper conduit. It is a
question whether such a mechanism associated with the YE is
representing a technical artifact of the ancient hydraulic
engineers because it removes more sediments from the tank.
The YE up to Mahailuppallama [segment I] shows a gradual
decrease of suspended sediments mainly due to the sluggish
flow [Figs. 8 and 9]. The last data point acts as an outlier from
the above-mentioned pattern. The NJG shows a gradual
increase in suspended sediments because of the flow generated
bank erosion and sediments lifting from the bedload giving an
increment with the faster discharge. There is a sudden drop in
turbidity at 26 km [Fig. 8] which could be due to sediment load
abruptly settled at the Kon Wewa. However, increasing
turbidity may be partly supported by the algal growth due to the
backwater effect from the Kiralogama Wewa.
The segment of YE from Batuwatta up to Tissa Wewa
[segment II] shows an irregular pattern of turbidity variations
which may be due to more modern construction implants
causing backwater effects or algal growth. At some places, the
YE acts as a natural stream with areas having local rapids due
to roots of Kumbuk (Terminalia Arjuna) trees and boulders on
the constructed channel bed. Besides, this segment is fed by
spill water with unregulated discharge coming from the nearby
tanks [e.g. Divul Wewa] and some return flow from the paddy
fields, so that the turbidity varies from place to place.
Fig. 10 Satellite image of the Kiralogama Wewa showing abandoned
YE and NJG. Note clearing of forests adjacent to the “thaulla” area
It is interesting to show that the ancient YE behave in a very
delicate controlled manner when discharging waters to the
nearby tanks. Aerial photographs and the satellite images
provide the remnant traces of abandoned YE segment where a

different mechanism had been used to supply additional water
to the nearby tanks. In the past the YE did not directly discharge
water to the tanks; instead, hydraulically feed through the
“Thaulla” area [Fig. 10]. Such technically adaptable sustainable
water distributing system observed in the past has been
obliterated by the NJG which uses direct discharge of waters to
the tanks causing an increase in the sediment load to the
reservoirs in one hand and backwater effects along with the
upstream aiding for excessive detachments of sediments along
the banks.
E. Sediment deposition
Along the channel path of the YE with frequent meanders,
sediments mostly deposited along the point bars as well as a
part of the sediments slowly settle down in the flood areas
adjacent to the free boundary of the channel without the
embankment. The sediments bringing by the sequence of
natural tributaries in the headwater regions also tend to settle
down in flood areas but in connection with NJG, this process is
completely absent instead tributaries directly dump to the
deeper channel flow aiding embankment erosion.
F. Analysis of Sinusoidal Behavior with Meandering
ArcGIS was used to calculate the sinuosity indices on several
segments along the YE. The YE provides a meandering channel
corresponding to the overall calculated sinuosity index of 2.20
while NJG as a fairly straight channel with a value of 1.25.
However, the segment I and II are still maintaining the original
YE display the same sinuosity indices around 1.91. The
abandoned segment, however, has the most sinuous reach with
a sinuosity index of 2.28.
Corresponding to the meandering sections of the YE, the
measured values of sinuosity [K], the angles at which the curve
crosses at X-axis [ in degrees], the wavelength [λ], amplitude
[A] and the radius of curvature [Rc] are calculated and following
graphs of     
 vs. K were developed [Figs. 11 to
13]. It could be identified that the experimental equations (4)
and (5) given below are following more or less similar patterns
with the theoretical equations (1) and (3). However, K vs. Rc/λ
is giving a different scenario, where a mirror image of the
theoretical equation (2) is expected.
     
  (4)

    (5)
The  calculated for the meanders of YE show negative
deviations but could approximate with the theoretical values for
the sinuosity indices of the chosen meanders as shown in the
above equations. The experimental ratio of the amplitudes to
wavelengths and the radius of curvatures show somewhat a
similar relationship with the theoretical data. The exaggerated
meanders have significant sinuosity indices which deviate from
the  value of the sine generated curve.
Therefore, the experimental data support for a worked
example of a modified flow path after satisfying the landform
parameters. The sine generated meanders show a stable
meander pattern in natural streams with minimum bank erosion
at the mature stage; however, YE falls mostly in the young stage
artificially maintain stable meanders due to flow control.
Fig. 11 The graph of  vs K
Fig.12 The graph of Rc/ vs K
Fig. 13 The graph of A/ vs K
VI. SUMMARY
The overall study can be summarized as follows
1) The YE construction maintained an elegant meandering
channel following the contours with a drop of 36 m for the
entire length of 87 km. Besides, segment I and the

abandoned middle segment shows an approximate slope of
0.20 mkm-1 but the NJG passes over the rolling topography
with straight segments and deep cuts.
2) The YE is passing over three drainage basins and received
contributions from headwater tributaries through command
areas of 95.1 km2, 15.6 km2, and 15.7 km2 via Kala Oya,
Modaragam Aru, and the Malwathu Oya respectively. The
single embankment changes from right-hand to left-hand
after the drainage divide between Malwathu Oya and
Modaragam Aru near Pahalawewa
3) The process of controlled sedimentation along with the
point bars and flood-prone areas as observed in the YE is
absent in the straight NJG. The latter has excessive erosion
and deposition causing uncontrolled sedimentation
coupled with braided sand bars. In general, the channel bed
of the YE consists of poorly sorted medium sand while
well-sorted fine sand is present in the flood areas indicating
such controlled deposition.
4) The YE flow path does not correspond to the pure sine
generated curve as of stable meanders in the mature stage;
however, artificially generated meanders crossing such
headwater regions of the trans-basin canal sustained a
controlled flow.
5) Overall, there are many uncertainties associated with the
outcome of this result. Several aspects such as landscape
and discharge factors, including the behavior of the
groundwater in respective areas represent wide deviations.
The mathematical approach to solve the meander
generation needs in-depth analysis. Therefore, further
controlled and detailed scientific study based on the
outcome of the present work needs to be conducted to
assure the quality of the research output.
VII. CONCLUSION
It is clear that the ancient YE has been constructed with
detailed technical information controlling natural slopes,
drainage divides, sedimentation, flow properties and
sustainability. The method is properly drafted with unknown
technical maneuverability. The modern engineering practices
completely obliterated the natural flow mechanism embanking
the flow both on the surface and within the subsurface.
Considering other areas consist of similar YE remnants may be
indicative of serving the same purpose so that YE is a technical
mechanism established in the ancient hydraulic regime to
supply sustainable water satisfying geoenvironmental
conditions in the dry flat areas of Sri Lanka. A further analysis
on flow parameter calculations, sediment analysis, landforms
analysis with DEM, Google Earth, air photo and drone studies
are expecting.
ACKNOWLEDGMENT
The authors acknowledge the supports rendered by the
Mahaweli Development Authority offices in
Thambuththegama and Mahailluppallama for supplying data.
Prof. W. B. Daundasekera of the Department of Mathematics,
Mr Dinura Hettiarachchi a colleague and the community around
the YE are mentioned with gratitude. Supports extended by the
Ministry of Science and Technology for the field component is
appreciated.
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