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Application of GIS and HEC-RAS in Flood Plain Analysis and Risk Assessment of Lothar Khola : 

1 Application of GIS and HEC-RAS in Flood Plain Analysis and Risk Assessment of Lothar Khola Presenter Bikram Manandhar Institute of Forestry, Pokhara Nepal June, 2010

Slide 2: 

Background Objective Study area Materials and Methods Result and Discussion Conclusion Recommendation Limitation Outline of the presentation 2

Background : 

Background 3 River and their adjacent floodplain corridors fulfill a variety of functions. High relief, steep slopes, complex geological structures with active tectonic processes and climate characterized by great seasonality in rainfall, all combine to make Nepal natural disasters prone area. Low lying areas inundated and sediments deposits every year due to floods during the monsoon. The high level of poverty and rate of population growth has further increased the vulnerability to flood disasters. (Osti, 2004)

Objective : 

Objective 4 To assess the flood frequency for the quantitative assessment of flood problem in the area To analyze the floodplain by using the one-dimensional steady flow model To prepare flood hazard, vulnerability and risk map

Study Area : 

Study Area 5 Lat: 27o 33’ N -27o 35’ N Long: 84o 41’ E - 84o 50’ E Elevation: 450m to1650m Location: Chitwan and Makwanpur Area: 170 sq km Avg. Discharge: 10.17 m3/s Avg. annual ppt.: 1944mm Tectonic structure: MBT and MT

Materials and Software : 

Materials and Software 6 Stream flow data, Department of Hydrology and Meteorology Digital Topographical map (scale 1:25000), Department of Survey, Nepal Computer ArcGIS 9.3, ArcView 3.2a, HEC-RAS 4.0 and HEC-GeoRAS 3.1.1 as interface between Arc View and HEC-RAS

Slide 7: 

7 HEC-RAS HEC-GeoRAS Selected Models/Tools & Linkage (Interface Method)

Methodology for Objective: 1 : 

Methodology for Objective: 1 8 Flood Frequency Analysis for 2, 10, 50, 100 and 200 Yrs Return Period was calculated using Rank Plot Software Developed by Water Resources and Environment Studies ,ITC. 1.Based on Maximum Instantaneous Flow (Yadav, 2002) Gumbel’s Method Log Normal Method Log Pearson III Method The highest values obtained among these methods was used for Hydrological modeling in HEC-RAS.

Methodology for objective 2: : 

Methodology for objective 2: 9 Preparation of TIN Prepared by digital contour layer in ArcView 3.2a Preparation of Land use Map Land use map of the Lothar Basin is derived from the 1992 topo-sheet along with field verification, GPS used for collecting training data. Photographs was taken for the visual interpretation of the landuse/ landcover of the study area.

Slide 10: 

10 Conti… Digitized Data (Survey Department) One-dimensional Floodplain Analysis Using HEC-RAS, GIS and HEC- GeoRAS ( Awal, 2003)

Slide 11: 

11 Conti… Triangulated Irregular Network (TIN)

Slide 12: 

12 Conti…

Methodology for Objective: 3 : 

13 Methodology for Objective: 3 The methodology adopted for flood risk assessment follows the approach developed by Gilard (1996). The flood risk analysis includes the combination of the results of the both the vulnerability analysis and the hazard analysis. The flood risk maps was prepared by overlaying the flood depth grids with the land use map.

Conti… : 

14 Conti… Methodology for Hazard Analysis The spatial coexistence model was used for the hazard assessment, reclassifying the floodwater depth. The area bounded by the flood polygon with flood depth intervals was calculated to make assessment of the flood hazard level. Flood hazard maps was prepared by overlaying flood depth grids with TIN.

Conti… : 

Conti… 15 Methodology for Vulnerability Analysis The vulnerability assessment was done using binary model. Based on the presence or absence of a flood of particular intensity in a particular land use type. The flood vulnerability maps was prepared by intersecting the land use map of the floodplain with the flood area polygon for each of the flood event being modeled.

Results : 

Results c

Conti… : 

Conti… c

Conti… : 

Conti… 18 2, 10, 50, 100 and 200 years Return Periods Flood Frequency Analysis based on Maximum Instantaneous flow recorded at (Station 470) from year 1964 – 2004

Slide 19: 

19 Return Periods and area inundation relationship generated from steady flow analysis. c

Slide 20: 

Discharge versus flow area relationship c

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Slide 22: 

22 c

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Conclusion : 

Conclusion 27 Flood Frequency Analysis values by Log Pearson III type method is higher for different return period in comparison to other method. Area inundated by 2, 10, 50, 100 and 200 years return period flood is 230, 239, 246, 249 and 252 Hectares respectively. The classification of flood depth area shows that most of the flooding area has water depth greater than 3 meter. The assessment of the flood area shows that a large percentage (more than 40 %) of vulnerable area lies on flood plain area i.e. sand area and followed by forest, cultivation area, etc. 23% and 18% respectively.

Recommendation : 

Recommendation 28 Use high resolution topographical data to get more accurate results. Use Remote Sensing technology to generate high resolution TIN. Measure river cross section data from topographic survey.

Limitation : 

Limitation 29 The study was carried out under major constraints of limited data. TIN was generated from Digital contour data provided by Department of Survey, as the topography is irregular, TIN thus prepared only represents approximate terrain. The cross section data, which is one of the major layer for the model is extracted by the software on the basis of generated TIN which may not represent the river geometry properly. Due to these limitations, the results are therefore subject to some degree of uncertainties.

Acknowledgement : 

Acknowledgement Respected Supervisors Prof. M.K. Balla Dr. Ripendra Awal Associate Prof. B. M. Pradhan My Parents THANK YOU FOR YOUR PATIENCE

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