The analysis undertaken through interactive process to mark the neighbourhood area which has elevation lower or equal to a particular gauge level calculated by Gumbel’s method shows the probable extent of flood water in case of failure of embankment on both sides at Sisapathar. The analysis for various return period flood leads us to the following conclusions.
- For a 10 year return period flood which gauge level is 88.806 M (&) the total area inundated by flood will be of 155.95 sq.km. The details of area affected in each landuse class is given in the table No 4. The population that will be affected by a 10 year return period flood is assessed to be 48033 nos. on basis of population density figure.
- For a 25 year return period flood with a gauge level of 89.58 metre, the total area affected will be 160.49 sq.km. The area affected in each landuse class is shown in the table No. 4. Population affected will be 49431 nos.
- For a 50 year return period flood, the area inundated will be of 161.40 sq.km. Area affected in each category of landuse class is shown in the table No. 4. The population that will be affected is in the order of 49712 nos.
- For a 100 year return period flood with a gauge level of 90.74 metres, the area flooded will be of 162.94 sq.km. Area affected in each category of landuse class is shown in the table No. 4. The nos. of population affected will be 50186 nos.
A rating curve showing gauge levels against discharge at Sisapathar is also drawn and shown in fig. 1. This will enable us to find discharge against a particular gauge level. The data considered to draw the curve is shown in
table No. 5. Another curve showing relation between area of inundation against gauge levels is also shown in figure 2.
The river Dikrong is embanked along the left bank from Harmoti to Dahgharia and in the right bank, two portions are embanked near Bihpuria and Madhupur village. These embankments have rendered reasonable protection to the flood affected areas. However, the effectiveness is decreasing due to the rising of the river bed gradually. This is due to the fact that the river carries heavy silt annually. Cuts and breaches in the embankments occur due to overtopping, seepage and erosion. The materials used for construction of the embankments are mostly sand and silt which are susceptible to erosion. Due to lack of proper maintenance of existing inadequate section of embankment and meandering nature of the river, the erosion problem leading to breach of embankments is increasing year after year.
Although at Sisapathar both banks of Dikrong is embanked, considering the frequent phenomenon of breach in the embankments, this study has been undertaken at Sisapathar gauge discharge sites where hydrologic data for a long period is available. Moreover, river Dikrong is not embanked throughout its entire length on its right bank, for which flood water will definitely inundate a huge portion of land. The extend of flood can be calculated in any outlet point if gauge discharge data are available at that point.
Overtopping of embankment are also frequent in this region, but details could not be studied for lack of elevation data of embankments. But in general, it can be said that the embankments should be raised and strengthened for a 25 year return period flood level.
The basic idea of flood risk mapping as undertaken in this study is to regulate land use by flood plain zoning in order to restrict the damages. The Rashtriya Brah Ayog in their report of 1980 has recommended that flood plain management measures should be under-taken and suitable legislation enacted whenever necessary. In the light of above discussion, it can be said that flood risk mapping, being an important non-structural flood management technique, will go long way in reducing flood damages in areas frequented by flood.
References:
- Introduction to the use of Geographic Information System for Practical Hydrology I.T.C. Publication No. 23, Netherland / Flood study in the Mahgna-Dhonagoda Polder, Bangladesh, Allard M.J. Meijerink, Hans A.K. de Brouwer, Chris M. Mannaerts, Carles R. Valenzuela.
- K. Subramanya, Engineering Hydrology
- ILWIS user’s Manual Version 1.41, International Institute of Aerospace Survey and Earth Sciences (ITC), The Netherlands.
- ILWIS user’s Manual Version 2.00, ITC, Netherlands.
- Master Plan of Dikrong sub basin, 1996, Brahmaputra Board.