ANALYSIS OF HISTORICAL FLOODING INCIDENT IN AKURANA

Transcription

1 ANALYSIS OF HISTORICAL FLOODING INCIDENT IN AKURANA Foundation for Environment, Climate and Technology C/o Mahaweli Authority of Sri Lanka, Digana Village, Rajawella, Kandy, KY 218, Sri Lanka Citation Najimuddin, N, L. Zubair. (217). Analysis of Historical Flooding Incident in Akurana. FECT Technical Report Foundation for Environment, Climate and Technology, Digana.

2 Climate Change and Polgolla Barrage Did Not Cause Recent Floods in the Pinga Oya Tributary of the Mahaweli Nushrath Najimuddin 1, Lareef Zubair* 2, Ruchira Lokuhetti 3, Ashara Nijamdeen 4 Janan Visvanathan 5 1-5, Foundation for Environment, Climate and Technology, Digana Village, Rajawella. *Corresponding Author: lareefzubair@gmail.com Abstract: Since 21, frequent floods of hours to a day are faced by travelers on the Kandy to Jaffna road at Akurana along the Pinga Oya. The flooding is about 5 km upstream of its confluence with the Mahaweli at Katugastota. A common belief among some officials is that the Polgolla Barrage and/or climate change are implicated. Other causes could be narrowing of the river, dumping in the river,and increase in runoff due to land use change. In this paper, the possible causes are narrowed down by review of riverine surveys, bridge construction data, rainfall data, flood dates, remotely sensed images and field visits. Statistical tests were used to see whether there were changes in rainfall extremes. The analysis shows that (a) as the Polgolla barrage is about 1 meters below Akurana, it could not be the cause (b) the rainfall extremes before and after 21 are no different. (c) there has been construction and debris dumping into the river, its margins and flood plains over the last 4 decades. River cross-sections in critical locations have been narrowed and confined. (d) Siltation and debris dumping has filled up the stream bed but its impact on the river cross-section is modest except at the confluence of the tributary from Dunuwilla at Akurana. (e) The changes in land-use, paving of roads, drains and building construction is significant. The evidence so far points to the construction in the river, river margins and flood plains and land use change as the principal causes. Keywords: Floods, Ungauged Basins, Mahaweli, Pinga Oya, Land Use Change, Urbanization 1. Introduction A flood is defined as a temporary condition of surface water (river, lake, sea), in which the water level and/or discharge escape from their normal confines (Van Te Chow et al., 21). Population growth, development of infrastructure, commercial, industrial and other human activities spread towards the sensitive areas such as streams, stream corridors and flood plains (Dissanayake 22, Chandrasekara and Gunewardene 21). The Pinga Oya catchment has seen rapid population rise, land use change, intensification of land and resource use. There has been an increasing frequency of floods as well as dengue and other water borne diseases, water quality problems and decline the aesthetic values of streams (Dissanayake 213). The absence of appropriate management of stream corridors in urbanizing areas is one of the major issues in Sri Lanka and across Asia. Frequent floods over the last two decades in Akurana area is faced by the inhabitants and travelers on the A-9 trunk road in Akurana - this is the major vehicular artery between Jaffna and Kandy. Floods are a recent phenomenon in this catchment starting in 21 there have at least 13 floods since then. While there was a rapid rise in river level to near-flooding in November 23, 1978 (day of the Batticaloa cyclone with 263 mm of rainfall), there was no floods until 21. After 21, there have been at least 13 floods.

3 The challenges involved include limitations in availability of data records. Records of construction into the rivers were not accessible from the local government there are also illegal constructions. Hence when considering the recent rise in flooding frequency in Akurana, the possible reasons could be i. The construction of the Polgolla Dam in ii. Changes in rainfall characteristics. iii. Construction in to the river and in the stream corridors and flood plains. iv. Changes in Stream bed due to accelerated siltation v. The changes in land use so that there is reduced infiltration into the soil. In this paper, we shall quantify the likelihood of contribution from these causes with the available data and tools. 2. Methodology 2.1 Study area The total land area covered by Pinga Oya catchment (Figure 1) is 14,467 Ha. It consists of a number of sub tributaries, mainly Hunnan Oya, Owissa Oya, Balapitiya Oya, Kurugoda Oya and Wahagalla Oya. The main stream connecting Hunnan Oya, Owissa Oya and Kurugoda Oya, is named as the Pinga Oya and it flows to the Mahaweli river at Katugastota. About 75% of Pinga Oya catchment is covered by home gardens and paddy fields. The upper catchment is covered by tea lands and the Northwestern corner by scrub lands. 2.2 Data Figure-1. The Pinga Oya Catchment is shown shaded. Official floods records have not been kept and informal sources were mined for flood events. Streamflow and soil moisture measurements are not available in this catchment. There is one long rainfall record in Katugastota and shorter daily records in Akurana and Rabukela for last two years. Satellite rainfall data are available since 2 at a resolution of 1 km. Satellite land cover data at moderate resolution (1 km) and these are available from 23 episodically. Higher resolution data are available at a cost, but this too covers only the more recent period. We have used the available records of floods, meteorological data, remotely sensed images and secondary data. The following data sets were used in the analysis; a) Rain fall data at Katugastota obtained from the Sri Lanka Department of Meteorology and from the FECT instruments at Akurana from 1951 onwards.

4 1978/1/2 1978/1/6 1979/7/1 198/4/ /1/ /1/ /7/ /4/ /1/ /1/3 1985/8/3 1986/5/7 1987/2/8 1987/11/ /8/ /5/2 199/2/21 199/11/ /8/ /6/1 1993/3/5 1993/12/7 1994/9/1 1995/6/ /3/ /12/2 1997/9/ /6/ /3/31 2/1/2 2/1/6 21/7/1 22/4/13 23/1/15 23/1/19 24/7/22 25/4/25 26/1/27 26/1/31 27/8/4 28/5/7 29/7/7 21/5/16 211/2/18 211/11/28 212/8/31 213/6/4 214/3/9 214/12/12 215/9/15 Precipitation (mm) b) Satellite observed Rainfall Estimates (RFE) from NOAA/CPC were through the FECT / IRI Data Library for N, E c) The dates for the flood occurrence were estimated from recent reports in newspapers, Desinventar data base from DMC and social media reports from Akurana News (data available separately). d) The number of bridges were obtained from the images from google earth in the historical mode by counting the bridges along the Pinga Oya starting from the years of 21 to 216 at their recent near dates for the floods. Satellite images of the catchment at a resolution of 96 dpi were obtained from Google Earth. (Source: Digital Globe and CNES Airbus). Cadastral surveys obtained from the Akurana Divisional Secretary. e) Cross-sectional surveys and silt analysis of the river bed were reported by Dissanayake (213) and Mahees (213). 3. Results and Discussion 3. 1 Upstream Impact of The Polgolla Dam Some believe that the construction of the Polgolla barrage in 1972 is responsible for the recent flooding. Even if the water level in Polgolla barrage is maintained at maximum to full supply level, there is no possibility to cause flood at Akurana as its elevation is at least 1 meters above the spill away. The full supply level of Polgolla barrage (44.74 MSL) is same as the bed level of the stream at 2.7km, away from Katugastota bridge in Katugastota (near the Mag-Car Service Center) (Hilal, 213) A Rise in Extreme Rainfall since 21? If the rise floods in recent decades due to changes in rainfall, then there should be a rise in extreme rainfall (intensity-duration-frequency) in recent decades (Figure-2) Rainfall at Katugastota and flood events ( ) Date Figure-2. Daily Rainfall at Katugastota ( ). Flood events are shown as red boxes.

5 Count (No flood) Count (Flood) Count (No flood) Count (Flood) Flood threshold after 21 Flood threshold before Flood threshold (mm) Flood threshold (mm) No flood Flood Flood No flood (a) (b) Figure-3. The number of flood events happening for different rainfall thresholds is shown for the (a) and (b) periods. The blue lines indicate the number of times there were floods for that given threshold and the red indicate the number of occasions of the rainfall threshold being reached but that did not lead to floods. Before 21 there has been only one occurrence of flood recorded (1978), even though the rainfall was 162mm in 199 and 161mm in 1996, there were no floods recorded. After 21 even, rainfall of 68mm (214) and 63mm (26) caused flooding in Akurana (Figure-3). Figure-4. Box and Whisker plots of daily rainfall for years intervals ( ). The differences in extreme values are not significant based on statistical tests. The lowest rainfall, which has caused the flooding is on which is 63mm. In 1978 the threshold for flood was 263mm while in 21 it was 19mm. The reduction in flood threshold in between 1978 and 21 is 154mm. The frequent flood occurrences have been recorded in the last 15 years (21-216), even though there are no changes in the rainfall patterns (Figure-4). 3.3 Narrowal of the river through construction in to the river, stream corridors and flood plains. The initial changes due to constructions on the stream corridors by land proprietors, squatters, and the Road Development Authority (Dissanayake 213). Reduction in the width of the stream is due to construction of road bridges with short spans, retaining walls and buildings (Hilal, 213). Several large buildings have been built in the stream reservation, while some are on the stream bed (Zubair, 213).

6 Number of bridges across the Pinga oya Bridge constructions can be taken as a proxy for the extent of construction in the river margins. In 21, the bridges counted in satellite images were 28 while in 216 it is 43. The increase in the number of bridges across the river is coincident with flood rise. (Figure-5). The number of bridges upstream of Akurana was 26 in 21 and 4 in 216. Figure-5. Number of bridges across the Pinga Oya Changes in Stream bed due to accelerated siltation Filling up of the river bed of Pinga Oya due to siltation and waste dumping into the river leads to changes in the stream bed. Dumping of garbage/waste directly into the stream and indirectly everywhere throughout the catchment, blocks the free flow of water (Nawas and Nawas, 213). Disposal of demolition debris on to the upper part of the stream corridor has resulted in changes to the stream bed (Dissanayake 213) The changes in land use so that there is reduced infiltration into the soil. Changes in land use results in expanded flood recurrence due reduced infiltration. Close inspection of landsat satellite images show discernible increase in construction and paved areas (Figure-6). The Vegetation Transect Survey done by Dissanayake (213) reveals that the stream segment in Pinga Oya from rural area towards the urban, along the vegetation covers, as well as the vegetation diversity, are in poor condition. In this analysis the changing land use patterns could be a significant contributor, and we shall undertake quantitative analysis of these satellite images in the future Figure-6. Land use in 23, 21, 214 and 217

7 4. Conclusion The analysis shows that as the Polgolla barrage is about 1 meters below Akurana, it could not be the cause the rainfall extremes before and after 21 are no different. there has been construction and debris dumping into the river, its margins and flood plains over the last 4 decades. River cross-sections in critical locations have been narrowed and confined. Siltation and debris dumping has filled up the stream bed but its impact on the river crosssection is modest except at the confluence of the tributary from Dunuwilla at Akurana. The changes in land-use, paving of roads, drains and building construction is significant. The evidence so far points to the construction in the river, river margins and flood plains and land use change as the principal causes. We shall try to set up stream gauges, and try to use the satellite images to estimates to quantify the impacts of land use changes in the future.

8 References Chandrasekara, S.S.K. and Gunewardene, E.R.N. (211). Effectiveness of existing laws and Regulations to prevent encroachments of stream reservation. Tropical Agricultural Research Vol. 22(2): Chow, V.T., Maidment, D.R., and Mays, L.W., (21). Applied Hydrology. Tata McGraw Hill Education Private Limited, Delhi. Dissanayake, D.M.L. (2). Settlement growth, land use changes and its consequences in Akurana town, Sri Lanka. M.Phil. Thesis, Norwegian University of Science and Technology. Dissanayake, D.M.L. (213). Changes of Natural Functions on Upper Mahaweli Stream Corridors. In Zubair, L and Wickramagamage, P., Proceedings of the Pinga Symposium, Foundation for Environment, Climate and Technology, Kandy. Dissanayake, D.M.L., Breuste J., Robinson, A. (213). Health Impact of Solid Waste in Urban environments: Case of Akurana town, 28th international conference on Solid Waste Management, Winder University, Philadelphia, PA, U.S.A. Hilal, T. (213). Floods in Akurana & Pinga Oya. In Zubair, L and Wickramagamage, P., Proceedings of the Pinga Symposium, Foundation for Environment, Climate and Technology. Moore, R. J., S. J. Cole, Y. A. Bell & D. A. Jones. (26). Issues in flood forecasting: ungauged basins, extreme floods and uncertainty. Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX1 8BB, UK. IAHS Publ. 35 Munich-Re, (1997). Flooding and Insurance. Municher Ruckversicherungs-Gesellshaft. Munich, Germany. Nawas, M. F. and Nawas, N. F. (213). Review of research on water Quality and health of Pinga oya). In Zubair, L and Wickramagamage, P., Proceedings of the Pinga Symposium, Foundation for Environment, Climate and Technology, Kandy. Sivapalan M. (23). Prediction in ungauged basins: a grand challenge for theoretical hydrology. Hydrological Processes. 17: , doi: 1.12/hyp Wickramagamage, P. (213). Pinga oya catchment responses to human activity. In Zubair, L and Wickramagamage, P., Proceedings of the Pinga Symposium, Foundation for Environment, Climate and Technology, Kandy.