River Inundation and Hazard Mapping a Case Study of North Zone Surat City

Transcription

1 River Inundation and Hazard Mapping a Case Study of North Zone Surat City Patel Chandresh G, Assistant Professor, Ganpat University, Kherva Dr.P.J. Gundaliya, Associate Professor, LDCE, Ahmedabad Abstract: New technologies, such as Geographical Information System (GIS) and Remote Sensing (RS) are helping the researchers to create accurate floodplain maps with improved efficiency. This paper illustrates generation of Digital Elevation Model (DEM) and GIS applications for developing floodplain models of North Zone of Surat City, fastest growing city of Gujarat state, India. Flood event occurred in 2006 and discharge observed near Nehru Bridge is taken as reference to workout probable submergence depth of different area lying in North Zone of Surat city. Based on flood frequency analysis of the study area flood inundation map for flood return period of 30, 32 (Worst flood event-2006) and 35 years (future flood event) are generated in GIS environment. Keywords: DEM, Flood, North Zone, River I. INTRODUCTION Flooding is one of the serious natural hazards in the world [1],and it accounts for 40% of all deaths caused by natural disasters, with most flood events occurring in developing, tropical regions [2].Floods are of many types including flash flooding [3], flooding due to rising ground water [4], coastal flooding [5] and flooding due to the opening or breaking of a dam or reservoir [6].Floods resulting from excessive rainfall within a short period of time and consequent high river discharge damage crops and infrastructure. They also result in siltation of the reservoirs and hence limit the capacity of existing dams to control flood [7]. Flood is a natural disaster in India, however human activities in many circumstances change flood behavior. Activities in the flood plain and catchment such as land clearing for urbanization or agriculture, construction of infrastructures such as highway and road and bridges in the flood plain may increase the magnitude of flood which increases the damage to the properties and life. The behavior of flood due to flood mitigation project such as river widening and straightening must well be understood since this kind of work may transfer the flood problem form upstream to the downstream part of the river. The combination of GIS software and hydraulic software able to speed up the process of producing flood risk map which is suitable for a decision support system [8]. II. STUDY AREA Tapi basin is divided in three zones, Upper Tapi Basin, Middle Tapi Basin, and Lower Tapi Basin (LTB).The portion between Ukai Dam to Arabian Sea is known as LTB, includes Surat city and surrounded industrial and rural area. The Tapi river basin in Gujarat, encompassing an area of about lakh hectares [9] covers parts of Surat district. The topography in LTB comprises narrow valley and gently sloping ground [10]. The LTB receive an average annual rainfall of 1376 mm, and these heavy downpours are one of the main reasons for occurrence of flood and water loggings downstream. Surat city, situated at latitude 21 o 06 to N and longitude 72 o 45 to 72 o 54 E on the bank of river Tapi having coastline of Arabian Sea is on its West. Surat city is divided in seven zone visually North zone, South zone, South-West zone, South-East zone, Centre zone, East zone, and West zone by Surat municipal corporation as seen in Fig.1. Surat had experienced so many flood events before and after construction of Ukai dam. Flood occurred in August, 2006 is described as worst flood in history 70

2 of lower Tapi basin. Centre Water Commission(CWC) along with department of irrigation, local Municipal Corporation and other state government agencies have invested large amount of resources for protecting the fastest growing city of the India by making structural and non-structural measures. Fig. 1 Study Area- Surat City with Zones III. DATA COLLECTION For the research Geo-coded Indian Remote Sensing (IRS-1D) satellite image is used, Topo-Sheets at 1:50,000 scale collected from Survey of India, High resolution Google-earth image down loaded through internet. Physical measurements for river hydraulic parameters obtained after monsoon of 2006 and Contour maps of study area at 0.5 m interval were collected from Surat Municipal Corporation (SMC). In order to find out discharge carrying capacity of river Tapi, detailed drawings of 309 river channel sections which were measured at a mean distance of 150 to 200 m by irrigation department were collected. The water level and river discharge data from hourly to daily scales for different stations viz., Ukai dam, Kakarpar weir, Mandavi, Ghala, Singanpur weir, Hope (Nehru) bridge were collected from Central Water Commission, State Water Data Centre (SWDC), and Irrigation Department. Map showing name wise different area lying in North Zone was collected from Surat Municipal Corporation. IV. METHODOLOGY DEM is a computerized representation of the earth relief. Special data stored in the digital database of the GIS, such as a DEM, can be used to predict the effects of future events. The TIN model allows for a dense network of points where the land surface is complex and detailed, such as river channels, and for a lower point density in flat or gently sloping areas used TINs to model storm drainage in an urban setting. [11] [12]. Digital Elevation Model of North zone of Surat city is generated by digitizing contours at 0.5 m interval. Following procedure is adopted. ArcGIS software requires database in form of map, for that Toposheet at 1:50000 scale collected from Survey of India department, Ahmedabad Scanning is a process of converting any paper-based material into a digital format, which is usually integrated into the GIS database. The toposheets are scanned at 400 dpi resolution. 71

3 Georeferencing is a process of assigning map coordinates to an image data.data that are already georeferenced can be used as reference in georeferencing. Required topographical features such as contours, Gauge level and Discharge data are collected. Then contour map is geo referenced over downloaded Google earth image and a spatial adjustment is applied for better accuracy. Contours of North zone are digitized with help of ArcGIS software. There after spatial and temporal data are interlinked using GIS software and customized Data Base Management System. Tool 3D analyst is used to convert digitized Contours in to Triangulated Irregular Network (TIN) and using interpolation method Digital Elevation Model of North Zone generated. TIN and DEM of North Zone can be seen in Fig.2 [A] and Fig.2 [B] respectively. Fig.2 [A] TIN of North Zone [B] DEM of North Zone In order to identify submerged area of different zone at various levels surface analysis in 3D analyst tool is used. In order to identify submerged area of North Zone at various levels surface analysis in 3D analyst tool is used. For finding out submergence area of North Zone shape files of the zone is required. In order to find out probable submergence depth of different area falling in North Zone, following procedure is done in ArcGIS software. Arc Toolbox - Analysis tool - Extract-Clip-Area of interest/contour. Arc View- Toolbars-3DAnalyst Tools-Create / Modify TIN-Create TIN from feature. Arc Toolbox- 3DAnalyst Tools- Conversion- TIN to Raster. Arc Toolbox-Spatial Analysis Tool- Hydrology Flow Direction. Output of the operation is utilised to generate watershed. 72

4 Arc Toolbox Spatial Analysis Tool- Hydrology- Watershed. Output of the operation is shown in Fig.3 Fig.3 North Zone Watershed Arc Toolbox- Conversion Tools- From Raster choose Raster to Polygon. Arc Toolbox- Spatial Stastic Tool Utilities-Calculated Areas After following all the steps in the end choose shape file of calculated area and use Query function. Through the function Query find GRIDCODE and F_ AREA. In GRIDCODE put the figure of desired contour (e.g. 6, 7). Finally GRIDCODE indicate the area of contour. [V] RESULT AND DISCUSSION DEM of the Surat city is generated by collecting contour map of 0.5 m interval (Source SMC). Considering safe river gauge level with reference to lowest bank height near Nehru Bridge and applying this limit to prepared DEM of North Zone. The analysis is based with reference to two water surface level 7.55 m threshold value for the bank height near Nehru bridge and m water level measured at Nehru bridge in last flood event of Different area like Lal-Darwaja, Katar Gam, Tunki, Singanpor, Ved and Dabholi falling under this zone. The analysis is carried out with the help of DEM. Hence Steep slope indicates high flood potential area while gentle slope represents low flood potential area. Results obtained for probable submergence area versus reduced level of different area belong to North Zone is shown in Fig.4. It seems that Katar gam is low lying and Lal Darwaja is high rise area in this Zone. When water level reach up to 11 m there is no submergence in Lal- Darwaja area but then after due to increase in water level by each one meter the total submergence in this area would be 0.73 Km 2. Katar Gam is very t low lying area of this zone, when water level reach to 7 m the submergence area would be Km 2. Dabholi is also a low lying area in this Zone. When water level reaches from 8m to 19 m in rare condition most of the area, 1.66 Km 2 would go in submergence. Tunki is low lying and comparatively flat area of the Zone, as indicated in graph from 8m to 10m the whole area about 1.29 Km 2 would submerged. Ved is also high rise area fallowing Lal Darwaja and gets start to submerge when water reaches to 11m. 73

5 Fig.4 Reduced Level versus Probable Submergence Area of North Zone VI CONCLUSION This research work has shown clearly that Geographic Information System coupled with DEM model and remotely sensed data is vital in geospatial analysis of flood plain delineation and hazard mapping. North Zone area of the Surat city has been successfully modeled. Current satellite images of high resolution are needed so that an effective DEM can be created since the accuracy of any hydrological model depends largely on the accuracy of the DEM used.. The generated DEM of study area has an accuracy of 0.5 m in vertical and has been used for analysis of delineation of flood prone areas. Probability of occurrences of flood in this area is about once in a five year. Considering recurrence interval of 5 years if flood occurs up to 25 years return period few law lying area submerged for few inch depth. If flood return period of 30 years, 32 years ( equals to magnitude of discharge released in worst flood event of 2006 ) and 35 years ( considering extreme future flood event ) is taken as references then most areas lying in North Zone will be under water for considerable depth of flood water and can be seen in Fig.5 [A], [B] and [C] respectively. Fig.5 Flood Delineation Map [A] 30 Years Return Period [B] 32 Years Return Period ( Worst Flood Event of 2006 )[C] 32 Years Return Period ( Extreme Future Flood Event ) 74

6 In order to reduce effect of flood in North Zone river bank height should be improved by raising bank height because left hand side bank of river Tapi is just near to periphery of the North zone. It is found that bank heights near left hand side river sections namely L-6 to L-22 are very low comparatively and should be raised to improve water carrying capacity near this zone. With the possibility of increasing frequency and intensity of rainfall events as a result of climate change, increasing population which is resulting to land use changes in urban areas, it is more prudent to model floodplains and low lying areas for both emergency management and development planning. VII REFERENCES [1] Seyedeh, S. S.,et.al, Integrated Modelling for Flood Hazard Mapping Using Watershed Modelling System, American Journal of Engineering and Applied Sciences 1 (2): , [2] Ohl, C., and Tapsell, S., Flooding and Human Health -The Dangers Posed are not Always Obvious, British Medical Journal 321, and ISSN , [3] Gruntfest, E. and Handmer, J., Coping with Flash Floods, Kluwer Academic Publishers, Netherlands, ISSN , pp 11-13, [4] Burt, T., et.al, Water Table Fluctuations within the Floodplain of the River Severn, England, Journal of Hydrology 262(1-4), 1 20, [5] Nicholls, R., Analysis of Global Impacts of Sea-Level Rise: A Case Study of Flooding, Physics and Chemistry of the Earth 27, , [6] Forkuo, E. K., Digital Elevation Modelling using ASTER Stereo Imagery, Journal of Environmental Science and Engineering, Volume 52, Issue 2,pp , 2010, [7] Sanyal, J., and Lu, X. X., Application of Remote Sensing in Flood Management with Special Reference to Monsoon Asia: A Review, Natural Hazards, 33(2), pp ,2004. [8] Abd Jalil Hassan, Devlopment of Flood Risk Map Using GIS for SG. Selangor Basin [9] Annon, Watershed Atlas of India, All India soils & Land Use Survey, Ministry of Agriculture and Cooperation,Govt. of India,1990 [10] Central Water Commission (CWC), Water Year Book Tapi Basin, Hydrological Observation Circle, Gandhinagar, Gujarat, India, [11] Carter, J.R., Digital Representations of Topographic Surfaces,. Photogram metric Engineering and Remote Sensing [12] Djokic, D. and Maidment, D.R., Terrain Analysis for Urban Storm Water Modeling, hydrological processes