Mapping Flood Hazards in Relation to Agriculture in Nilwala Upper Basin_Pushpakumara Et Al

8/8/2019 Mapping Flood Hazards in Relation to Agriculture in Nilwala Upper Basin_Pushpakumara Et Al

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apping of Flood Hazards apping of Flood Hazards n Relation to n Relation to

griculture in griculture in i l w a l a i l w a l a

pper Basinpper Basin

D.L.W.S. PushpakumaraD.L.W.S. Pushpakumara 11 , Champa M. Navaratne, Champa M. Navaratne 11 , Chandana, ChandanaP.GunasenaP.Gunasena 22 ,,

11 Department of Agricultural Engineering, Faculty of Agriculture,Department of Agricultural Engineering, Faculty of Agriculture,22 Department of Geography, Faculty of Humanities and SocialDepartment of Geography, Faculty of Humanities and Social

Sciences,Sciences,University of RuhunaUniversity of Ruhuna,


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Objectives 1. Study of hydrological Behavior of the

Nilwala upper catchment corresponding to the flood.

2. Mapping the Flood inundation of the lower catchmentarea with reference to the major floods in 2003 and2008.

3. Mapping the effect of damages to agriculture withreference to the major floods.

National Geography Conference-2010


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Materials and methods



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Study

area



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Data



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Basin average rainfall calculated for the study area basedon using the Thiessen polygon method.

Arc view GIS software is used to calculate and drawingthe Thiessen polygon and mapping

3 year moving averages are used in the analysis of hydrologic data to minimize the effect of inter-annualvariations.

Co-relation analyses were carried out using dischargeand rainfall of four rain gauge station.



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Statistical methods are used to get the flood dischargesof the predetermined return periods.

As such annual peak discharges at the Pitabaddaragauging station for 30 years were used in floodfrequency analysis to determine the peak discharge .



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Mapping the Flood inundation and

damageField survey was conducted to the estimate the damage

mainly in the agriculture in selected GN divisions in the Akurassa and Aturaliya area.



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GPS data pointmap



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Result and discussion



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According to Thiessen polygon Basin Averageannual Rainfall is 3386.83 mm

STATION % Area (ha)

Paninkanda 10.80 4097.31

Aninkanda 21.88 8303.70

Mawarala 42.57 16156.74

Arpthop 24.75 9392.23

Total100.00 37950.00

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



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Co- relation between rainfall anddischarge

Aninkanda Panilkanda Mawarala Arpthop

Day 0.212 0.201 0.263 0.156

Before 1day

0.524 0.341 0.612 0.499

Before 2 days 0.293 0.177 0.345 0.366

Before 3 days 0.202 0.173 0.253 0.308



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Rainfall variation in the catchment

Mean Annual Rainfall varying 2550mm-3980mm



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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 catchments characters and land use changes were

affected. National Geography Conference-2010


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Short term rainfall-runoff relations

In short term there is linear relationship between rainfall andrunoff .However, there are substantial differences in rates anddirections of short-term deviations.



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Annual Peak discharge variation

Annual Peak discharge is decreasing mainly due tochanges in catchment characteristics and base flow

reduction National Geography Conference-2010


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Flood Hydrograph-2008 flood inBopagoda

There was the flood condition during 31 st May to 3 rd June



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Flood frequency analysis

y = 534.98Ln(x) - 178.81R 2 = 0.6708

0

500

1000

1500

2000

2500

3000

3500

1 10 100Return per iod (years)

P e a

k d i s c

h a r g e

( c u m e c

)



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Divitura

Welihena

Howpe

Pahala Aturaliya

Flo o d ro u tin g



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Agriculture in thestudy area



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Flood inundation map



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Flood inundation map



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Flood affected areas

Flooded Area(Ac)

Percentage (%)

Yearly 585 4.32

Flood 2008 1993 14.72

Flood 2003 6730 49.71

Total Area 13537.84



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Affected Population



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Paddycultivation

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.National Geography Conference-2010


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Paddy cultivation


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Tea is mainly in Aturaliya area than Akuressa . Howpe is the most teacultivated GN division . But, damage is low due to most tea landsin the small hilly topography. only boundaries of the tea lands areaffected .

Main damage due to Flood in the transportation ways to transport theharvest and difficult to access the harvesting.

Teacultivation


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Teacultivation



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Rubbercultivation



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Cinnamon CultivationDistribution

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



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ConclusionsIncreasing 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 isbetween 25 to 30 years return periods .



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.

Damage to Tea is low due to most tea lands in the smallhilly topography. only boundaries of the tea lands areaffected

Damage to cinnamon Higgoda area is mainly affected inflood due to low land and near to Degali oya and mainNilwala river.

Future mitigation measures must focus the routing highintensity storm runoff, and should be adjusted thecultivation practices based to rainfall pattern.


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ReferencesW. 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 andK.Vlassak., Rice production on acid sulphate soils of Sri Lanka., KULeuven, 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)135143

Ranjana. U.K. Piyadasa ,Champa.M.Naverathna ., River Sand Miningin Southern Sri Lanka and its Effect on Environment., Riversymposium,Sri lanka.

Nilwala Ganga Flood Protection Scheme -First stage of Work (FloodWarning system.,(1984).,pg:8-10.,Ministry of Land and LandNational Geography Conference-2010


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Thank You !National Geography Conference-2010

Mapping Flood Hazards in Relation to Agriculture in Nilwala Upper Basin_Pushpakumara Et Al

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