Mapping flood hazards in Relation to agriculture in Nilwala upper basi

Mapping of Flood Hazards in Relation to Agriculture in Nilwala Upper Basin : 

Mapping of Flood Hazards in Relation to Agriculture in Nilwala Upper Basin D.L.W.S. Pushpakumara1, Champa M. Navaratne1, Chandana P.Gunasena2, 1Department of Agricultural Engineering, Faculty of Agriculture, 2Department of Geography, Faculty of Humanities and Social Sciences, University of Ruhuna,

Introduction : 

Introduction Sri lanka have103 river basins . Among these major rivers, Kalu, Kelani, Gin, Nilwala and Mahaweli are vulnerable to floods. Flood was identified as most common and hazards natural event in Sri Lanka Flood area mapping in the Nilwala river basin became a top necessity with frequent floods in Nilwala River. Structural measures and non structural measures have to be improved against floods as there are serious damages to the Agriculture in area. 2 National Geography Conference-2010

Objectives : 

Objectives 1. Study of hydrological Behavior of the Nilwala upper catchment corresponding to the flood. 2. Mapping the Flood inundation of the lower catchment area with reference to the major floods in 2003 and 2008. 3. Mapping the effect of damages to agriculture with reference to the major floods. 3 National Geography Conference-2010

Materials and methods : 

Materials and methods 4 National Geography Conference-2010

Study area : 

Study area 5 National Geography Conference-2010

Data : 

Data 6 National Geography Conference-2010

Slide 7: 

Basin average rainfall calculated for the study area based on using the Thiessen polygon method. Arc view GIS software is used to calculate and drawing the Thiessen polygon and mapping 3 year moving averages are used in the analysis of hydrologic data to minimize the effect of inter-annual variations. Co-relation analyses were carried out using discharge and rainfall of four rain gauge station. 7 National Geography Conference-2010

Slide 8: 

Statistical methods are used to get the flood discharges of the predetermined return periods. As such annual peak discharges at the Pitabaddara gauging station for 30 years were used in flood frequency analysis to determine the peak discharge. 8 National Geography Conference-2010

Mapping the Flood inundation and damage : 

Mapping the Flood inundation and damage Field survey was conducted to the estimate the damage mainly in the agriculture in selected GN divisions in the Akurassa and Aturaliya area. 9 National Geography Conference-2010

GPS data point map : 

GPS data point map 10 National Geography Conference-2010

Result and discussion : 

Result and discussion 11 National Geography Conference-2010

Calculate the average Rain fall in the basin(Thiessen polygon) : 

According to Thiessen polygon Basin Average annual Rainfall is 3386.83 mm Calculate the average Rain fall in the basin(Thiessen polygon) 12 National Geography Conference-2010

Co- relation between rainfall and discharge : 

Co- relation between rainfall and discharge 13 National Geography Conference-2010

Rainfall variation in the catchment : 

Rainfall variation in the catchment Mean Annual Rainfall varying 2550mm-3980mm 14 National Geography Conference-2010

Total Annual Rainfall and Runoff relation. : 

Total Annual Rainfall and Runoff relation. During 1969/70 to 1998/99 are shown an increasing trend of rainfall while runoff and runoff to rainfall ratio are decreasing. Changes in catchment’s characters and land use changes were affected. 15 National Geography Conference-2010

Short term rainfall-runoff relations : 

Short term rainfall-runoff relations In short term there is linear relationship between rainfall and runoff . However, there are substantial differences in rates and directions of short-term deviations. 16 National Geography Conference-2010

Annual Peak discharge variation : 

Annual Peak discharge variation Annual Peak discharge is decreasing mainly due to changes in catchment characteristics and base flow reduction 17 National Geography Conference-2010

Mean Monthly water level(2000-2008) : 

Mean Monthly water level(2000-2008) Highest mean monthly water level in the May (High flow Period) value is higher than spill level Spill level 18 National Geography Conference-2010

Flood Hydrograph-2008 flood in Bopagoda : 

Flood Hydrograph-2008 flood in Bopagoda There was the flood condition during 31st May to 3rd June 19 National Geography Conference-2010

Flood frequency analysis : 

Flood frequency analysis 20 National Geography Conference-2010

Flood routing : 

Divitura Welihena Howpe Pahala Aturaliya Flood routing 21 National Geography Conference-2010

Slide 22: 

Agriculture in the study area 22 National Geography Conference-2010

Slide 23: 

Flood inundation map 23 National Geography Conference-2010

Slide 24: 

24 Flood inundation map National Geography Conference-2010

Flood affected areas : 

Flood affected areas 25 National Geography Conference-2010

Affected Population : 

Affected Population 26 National Geography Conference-2010

Paddy cultivation : 

Paddy cultivation Paddy cultivation is distributed in the all low lands in the Aturaliya and Akuressa area. Paddy lands in the Higgoda , Diyalape area are mainly affected due to near the Degali oya and Nilwala main stream. Thanahalagama west and Maramba north are highly paddy cultivated areas but damage is low due to small hilly topography of the area. 27 National Geography Conference-2010

Paddy cultivation : 

28 Paddy cultivation

Slide 29: 

Tea is mainly in Aturaliya area than Akuressa. Howpe is the most tea cultivated GN division . But, damage is low due to most tea lands in the small hilly topography. only boundaries of the tea lands are affected . Main damage due to Flood in the transportation ways to transport the harvest and difficult to access the harvesting. Tea cultivation 29

Tea cultivation : 

Tea cultivation 30 National Geography Conference-2010

Rubber cultivation : 

Rubber cultivation 31 National Geography Conference-2010

Cinnamon Cultivation Distribution : 

Cinnamon Cultivation Distribution Cinnamon cultivation mainly distributed in Higgoda, Godapitiya and Welihena areas. But Higgoda area is mainly affected in flood due to low land and near to Degali oya and main Nilwala river. Loss of Quality of matured cinnamon is a big problem 32 National Geography Conference-2010

Conclusions : 

Conclusions Increasing trend of annual rainfall while runoff and are decreasing. Decreasing the base flow shown in same study area Flood is main affected to paddy in the study area. Based Frequency analysis , Frequency of flash flood is between 25 to 30 years return periods . 33 National Geography Conference-2010

Slide 34: 

. Damage to Tea is low due to most tea lands in the small hilly topography. only boundaries of the tea lands are affected Damage to cinnamon Higgoda area is mainly affected in flood due to low land and near to Degali oya and main Nilwala river. Future mitigation measures must focus the routing high intensity storm runoff, and should be adjusted the cultivation practices based to rainfall pattern. 34

References : 

References W. K. B. Elkaduwa and R. Sakthivadivel. 1998. Use of historical data as a decision support tool in watershed management: A case study of the Upper Nilwala basin in Sri Lanka. Research Report 26. Colombo, Sri Lanka: International Water Management Institute. P. Deturckl, K.D.N. Weerasinghe, D.A.B.N. Gunarathna,J.P. Lexa and K.Vlassak.,Rice production on acid sulphate soils of Sri Lanka.,KU Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee.,University of Ruhuna, Faculty of Agriculture, Mapalana,Kamburupitiya, Sri Lanka. I. P. Ajith Gunasekara.,Flood Hazard Mapping in Lower Reach of Kelani River., ENGINEER - Vol. XXXXI, No. 05, pp. 149-154, 2008,The Institution of Engineers, Sri Lanka D.N. Mungai, C.K. Ong,, B. Kiteme , W. Elkaduwa , R. Sakthivadivel ., Lessons from two long-term hydrological studies in Kenya and SriLanka., Agriculture, Ecosystems and Environment 104 (2004) 135–143 Ranjana. U.K. Piyadasa ,Champa.M.Naverathna ., River Sand Mining in Southern Sri Lanka and its Effect on Environment.,River symposium,Sri lanka. Nilwala Ganga Flood Protection Scheme -First stage of Work (Flood Warning system.,(1984).,pg:8-10.,Ministry of Land and Land Development 35 National Geography Conference-2010

Thank You ! : 

Thank You ! 36 National Geography Conference-2010