Investigating River Channel Changes as a consequence of Continuous Flood Hazard in Terai Region of Napal using Remotely Sensed Data
Lal Samarakoon*, Akichika Ishibashi*, Yasushi Mabuchi*, Kioshi Honda**, Shigechika Miyajima***
*Nippon Koei Co., Ltd., Research & Development Center, 2304 Takasaki, Kukizaki, Japan
**STAR/SERD, Asian Institute of Technology, Bangkok, Thailand
***Water Induced Diaster Prevention Technical Center, Katmandu, Nepal
Abstract
Wide spread damage due to floods is one of the very common water induced natural disasters in Nepal that account for heavy losses during the monsoon period every year. Perennial rivers that flow through Siwalik area transport heavy sediments loads than rivers located in other areas during rainy season due to very fragile nature of the soils in Siwalik Hills. As a consequence, high sediment depositions are observed in the border of the Siwalik and Terai regions where the rugged terrain of Siwalik Hils change to relatively flat floodplain. This causes heavy losses during flood events in this areas due to overflow of river banks and planform changes of rivers. In this study, satellite data covering twenty year period was used in identifying the changes of river channel in the floodplain of Ratu river that originates from Siwalik hills in Central Nepal. Reason for changes were examined with field observation and the present trend in the planform change was established in predicting flood prone areas in future flood events.
Introduction
In July 1993 high intensity rainfall occurred in the Central Nepal in two occasions, July and August causing severe flood damage throughout Nepal. It is reported that July flood was more severe than the August flooding causing extensive damage as a consequence of more than 50mm of rainfall within 24 hours.This rainfall was concentrated in the central hills of Nepal extending from mahabarth mountain region, Siwalik hills and extending to Terai region. This natural phenomena caused heavy toll of damage. The death toll rose to 1460, and about 39495 houses were damaged. The transportation between to and from Katmandu was cutoff for few days hindering timely rescue work of affected people.
These types of storm and natural calamities have been occurred even in the past in Nepal. The causes for some of these unprecedented floods are storms, glacier lake burst and landslide. The physiographic nature of these Himalayan region, where climate is varying form tropical to alpine, and topography from alluvium plains to the mountain peak of the worl are some of the natural features that cause the flood hazards, and the ever increasing burden on the land could turn these hazards into disasters.
Proper countermeasures for flood prevention or mitigation are hardly carried out even though the disasters occur in time to time. This is partly due to the lack of understanding of flood hazards, hazard prone areas, causes of events, and consequences, as hardly any studies are carried out focusing on these factors.
This research project was aimed at studying the sedimentation in the riverbed of the watershed and in the floodplain using remotely sensed data. The analysis was focused on the process of accumulation, river channel aggradation, degradation, and formation of new channels as a consequence of heavy sediment unloading in the monsoon period of the Siwalik originate rivers. Flood prevention and controlling work that has been carried out in the past were investigated for identify their effectiveness.
Study Area and Data
Outline of the study area
Present study was carried out in the Siwalik area where the sediment transport is very high depositing enormous amount in the Terai Plain. Sub-streams comparatively short in length with higher gradient, and very flat riverbed is typical to the rivers originate in the Siwalik hills. With the increase of the gradient, the watershed become narrow. Further, the dendritic drainage pattern develops towards the upper stream of the River with the increase of riverbed gradient.
Ratu watershed selected for the study belongs to the Siwalik area located in the South of Sun Koshi and west of Kamala river. This river originates at an altitude of 708 meters, south of Sindhulimadi, and flow inn the direction of north-south from Siwalik Hills to the Terai region. Ratu river could be consider as wadji, as water flow in the upper reach is visible only during the during the monsoon season. The East-West Highway (hereinafter EWH) could be considered as the turning point of this river system dividing the
watershed and floodplain, Figure 1.The length of the river in the upper stream up to EWH is about 35 km, and average gradient of the riverbed is about 1/100. Width of the river changes dramatically below the EW, where to the north of highway the width is about 600 meters, and below the highway the width has been extended to 2 to 3 km.
Figure 1 Spectral response patterns of selected sample for the Landsat MSS 1973 and TM 1993
Acquisition of Remote Sensing and Reference Information
Landsat MSS, TM and IRS-LISS-II data were considered for the present study. Long term investigation of the river planform changes could carry out with MSS and TM data acquired in the early and newest Landsat missions. IRS data was obtained for comparison with TM for their suitability in using land cover information extraction and the usability in lieu of Landsat TM data. Considering the objectives of the study, the seasonal variations that could present in the satellite data it was decided to selected the multi-sensor temporal data listed in Table 1.
Table 1 Information on the satellite data obtained for the study
| Sensor |
Date |
Sun Angle |
Sun Azimuth |
Processing Level |
| MSS |
1973.03.14 |
Not Known |
Not Known |
System Corrected |
| MSS |
1977.03.20 |
Not Known |
Not Known |
System Corrected |
| TM |
1993.03.16 |
47° |
127° |
System Corrected |
| TM |
1995.03.22 |
122° |
46° |
System Corrected |
| LISS-II |
1995.03.15 |
47° |
127° |
System Corrected |
Referring to the selected data and major flood events, aerial photographs acquired in 1979 and 1992 were obtain for comparison and establishing ground reference information for calibration and validation satellite data interpretation. Also, field investigation were carried out to collect information on extents on floodplain, change, and distribution of the sediments. A detailed discussion of the method of the field survey, obtained information, and the detail discussion of the soil erosion phenomenon, and its transportation is found in JICA, 1996.
Database Creation for the Analysis
In order to make use of all the information, satellite data, maps, aerial photographs and field visit information it was decided to create a geographic database referred to as GIS for easy reference of source of information.
The acquired satellite data had been received by sensors having different field of views resulting different ground resolution. Further, their spatial orientation also dissimilar as the ground processing are carried out independently. Therefore, the procured satellite data were brought into a common map projection (UTM) by constructing mapping function through identifying control points on 1:25,000 topographical maps. Also, the conventional topographical and vegetation map were digitized and incorporated into the GIS database. Further, the aerial photographs and conventional photographs obtained in the field were scanned, rectified and registered into UTM projection. Thus the completed GIS database for the present study included multi-sensor temporal satellite data, aerial photographs, elevation and land cover information.
Results and Discussion
Spectral Characteristics of Riverbed Material
Dry season riverbed appears very bright on aerial photographs, and on the conventional photographs indicating very high reflectance in the visible spectral region of the electromagnetic spectrum. Further, some form of different gradation was observed in the old deposits and newly deposited areas. It was attempted to observe these characteristics properties on the radiance values observed by the sensors to establish the best suitable spectral band or the combination of bands for delineating of deposited material, and assessing the change in the planform of river channel.
With reference to the aerial and other conventional photographs, reference area were established and corresponding digital counts for dominated over classes, forest, crop lands and river sediments covering most of the study area were extracted. Statistical parameters of representative sample were calculated and analyzed for differentiation of these cover classes, and the establish the spectral characteristics of deposited materials. The
Spectral response pattern of the selected sample are depicted graphically in Figure 1 for 1973-MSS, 1993-TM datasets. The spectral response patterns of the other datasets were similar to one shown here. Comparatively very high reflectance was observed for riverbed, followed by crop lands and forest. It is said that the bareland are highly reflective as there is not energy absorption capacity unless the surface is wet. Further, when the materials are very fine and sandy where the water retention power is very low, the reflectance tend to be very high compared to other earth feathers. This is clearly visible in this area as field survey reported that the very fine sandy particles are found in the lower part of the river, and there is no visible water flow during the dry period. The riverbed material in the upper stream showed less reflectance than the materials in the floodplain particle size or the composition of the materials, presence of moisture or the traceof the river flow. Further, the old deposits identified in the aerial photographs showed relative lower spectral response when compared to new deposits. The may be due to change of color of the deposited material with time, or sparsely grown grass ver the deposited materials. Spectral pattern of crop lands resembled the patterns of old-riverbeds. Most of the crop lands are partially cultivated or abandoned during the dry season accounts for this similarity. It could be said that any of the TM spectral bands, specially, band 1, band 1, band 3, band 3, band 5 and band 7 are very much suitable for classification of river bed degradation and aggradation or planform changes, and there is no necessary to establish any other derived index. For MSS, the best band would be band 4, band 5 or band 6, band 7 may not be very much suitable due to its narrow dynamic range.
