Investigating River Channel Changes as a consequence of Continuous Flood Hazard in Terai Region of Napal using Remotely Sensed Data
Channel changes and sedimentation in the Ratu Watershed
Using the statistical parameters of the samples selected over interested cover classes explained in the previous section, satellite datasets were classified into sediment deposited and non-deposited areas. In the present study, radiometric correction or rectification was not considered. The spectral response patterns of sediments and riverbeds are significantly different from those of surrounding permitting easy classification. Further, the objectives of the work was to identify the spread of the deposition, and the riverbed changes, hence the compassion of digital counts of multi-temporal data was not required. The comparison was carried out with the classified images to identify changes. The riverbed extents obtained from these classification are shown in Table 1.
Table 1 Extent of the riverbed of the Ratu river in the upper watershed
| MSS-1973 |
MSS-1977 |
TM-1993 |
TM-1995 |
LISS-1995 |
| Sq.km.5.18 |
sq.km.5.23 |
sq.km.5.48 |
sq.km.5.45 |
sq.km.5.53 |
There was no appreciable change was found in the extents of the riverbed area of the upper stream except for some minute difference in the extents. Given the geometric resolution of the sensors. 79m, 30m, and 34m for Landsat MSS, TM and LISS-II it is questionalble to interpret these changes as actual riverbed expansions. These observations were further analyzed for any spatial variation by considering five river segments, Figure 2. These segments were defined considered confluence of major tributaries of Ratu river. The main aim of these segmentation was to find may riverbed change with the contribution of sediments from tributaries. It also could
facilitate to recognize the contribution of human activities on the sedimentation as the settlements are concentrated only in some part of the watershed.
Figure 2 River segments in the Ratu watershed
Extents of the Ratu river sub-divisions shown in Table 2. As for the entire river wihin the watershed up to EWH, no trend was found for the sub-divisions also. The highest increase was observed for the L1 segment for 1993, but the riverbed area has return back to 1973 extent in the 1995 complicating the explanation. The increase could be only due to presence of some mix-pixels in the 193 observation.
The maximum difference observed for a time span was 0.2 sq. km., and this represents 220 TM pixels or 110 MSS pixels (MSS pixels were resampled into 60m during the geometric rectification). The length of the river in the unper watershed is about 35 km, and the average width is more than 60 meters. Simple arithmetic show the ratio of the riverbed change with pixels that required to represent the Ratu riverbed within the watershed is less than 10% of TM pixels, and less than 5% of MSS pixels. Therefore, it is questionable to consider these differences in temporal estimations of the riverbed areas as a actual riverbed deviations that have been occurred within the time span under consideration. These marginal differences might have occurred because of the limitations of the spatial resolution of the sensors, and the orientation of the sensor field of view at the time of satellite pass. In concluding, it could be said that there was no appreciable change in the Ratu riverbed in the upper watershed during the 1973 to 95 time span.
Table 2 Cumulative area of the riverbed of the main stream
| |
1973 |
1977 |
1993 |
1995 |
| L1 |
0.1692 |
0.1548 |
0.1917 |
0.1557 |
| L2 |
0.4824 |
0.5292 |
0.5913 |
0.5868 |
| L3 |
1.3932 |
1.4976 |
1.6344 |
1.6812 |
| L4 |
5.4468 |
4.5976 |
4.8744 |
4.8195 |
| L5 |
5.1768 |
5.2308 |
5.4855 |
5.4410 |
Channel Changes and Sedimentation in the Ratu Floodplain
As for the upper reach of the river, the sediment deposition and its change was estimated for the Ratu floodplain using spectral patterns described earlier. The extents of the sediment deposition in the floodplain for the four datasets and for the LISS dataset is shown in Table 3.
Table 3 Floodplain deposition of sediment and its change
| 73-MSS 77-MSS |
77-MSS |
93-TM |
95-TM |
95-LISS |
| 9.17sq.km |
11.58sq.km |
9.12 sq.km |
9.61sq.km |
10.07 sq.km |
The figure given in the table represents the whole sediment deposited are below the EWH along the downstream until the satellite sensor can discern the sediment from its surrounding. Further, the given extents exclude the old riverbed deposits that was differentiable by respective sensors of the four dates data. The estimated extents for the four dates shows the sediment deposited area is increasing gradually, except for drastic increase in the time span between 1973 to 77. This increase could be a consequence of the comparatively high precipitation prevailed in the period of 1974 to 75, where the total annual precipitation was over 6000 mm, (Karmacharya, 1995). Also, the political situation in the country during 1970 to 80 period leading to noticeable deforestation in eastern and central Nepal, (Sharma, 95) might have increased soil erosion leading to noticeable change in the Ratu floodplain. The classification accuracy of LISS shows that LISS sensor data can also be used in lieu of Landsat TM data in sediment deposition estimations.
Multi-temporal image integration was carried out to further enhance the pattern of depositin and spatial variability of river channels in the floodplain. A set of three images were produced by ingrating two dataset at a time defining three different pattern of changes. Figure 3, 4 and 5 show the newly deposited, unchanged or re-deposited, and unaffected areas during the time intervals 1973-77, 1977-93, and 1993-95, respectively. Newly deposited, and unaffected areas are shown in different shades, and the unchangedor redeposit areas are shown in hard lines. Unchanged represents sediments in both dates.
Figure 3 Change of sedimentation from 1973-77
Figure 4 Change of sedimentation from 1977-93
Figure 5 Change of sedimentation from 1993-95
A consider increase during the period 73 to 79 was clearly visible in the Figure 3, specially in the most westward channel. In this channel sediment deposition has been active even active even before 1973, but the sediment deposition during the period 1973-77 is comparatively larger than that has been occurring prior to 1973. This change could be due to continuous aggradation of the riverbed just below the Highway in the eastward channel
resulting change of flow to west. Decrease in the sediment transportation in the lower portion of the eastern channel would further support this assumption.
Figure 4 shows the difference of the 16 year period 1977 to 1993. The most westward channel, which was very active before 1977 had become inactive in sediment transportation. 1979 aerial photographs shows presence of a dike at point X across this channel that was built to control the flooding along the down stream of the channel. The width of the Ratu river just below the EWH, and eastern portion of the river above it has been decreased. This is a as a result of a dike constructed in the eastern bank of the river, just above the bridge after 1977. A critical riverbed deposition could have been occurred near point Y with the introduction of the dike above the bridge changing the watercourse westward after passing the bridge. This sedimentation might have split the channel into two very distinctive river courses. Aggradation around the point Y could have occurred due to an increase in material to be carried by the river, a loss of discharge or velocity flow, or a rise in occurred due to an increase in material to be carried by the river, a loss of discharge or velocity flow, or a rise in occurred due to an increase in material to be carried by the river, a loss of discharge or velocity flow, or a rise in base level. Extension of the floodplain just below the EWH into few kilometers could support these reason, and high sediment activities just below the EWH. Similar explanation could be made to the division of the most eastern part of the river identified in Figure 4. Comparing the observations of these two time spans, it could be said that the planform deviation of the river was much active during the 1973 to 79 period than the next 16 year. No much difference were observed between 1993-95 within the floodplain. Few newly deposited areas are visible in the lower par to the most eastern sub-stream, and a decrease was observed in the westward channels. This could be an indication of the increasing activity in the eastern side of the floodplain due to continuos deposition and aggradation in the west side of the Ratu riverbed just below the EWH. These findings shows the potential of satellite data in identifying, and monitoring sedimentation process of a watershed, and tracing river channel planform changes.
Remote Seining for Flood Protection Planning
A natural hazard turns into a disaster when an event causes heavy loss of life and property damage. This is a mostly happens when risk inherent human activities take place in hazardous areas. With the increasing population in this mountainous country, the burden on the land is in the rise resulting further land degradation and risking to settle in flood prone areas for basis human needs.
The observation carried out in this study shows that heavy sedimentation is occurring in this Ratu floodplain for a long time, and increasing gradually requiring counter measures. It is required to identify the status of floods to consider appropriate prevention or mitigation strategies. Lessen the impacts of flood could be status of floods to consider appropriate prevention or mitigation strategies. Lessen the impacts of flood could be considered through structural measures or non-structural adjustments. Structural implementation consider out to change, modify or prevent the phenomenon, whereas the non-structural implementation consider considerably high within few kilometers below the EWH, hence the people living in the vicinity of the river in this area are very much vulnerable to flood hazards. The two dikes, one above the bridge, and another in the western channel (Figure 4) showed their influence in deviating and splitting the river to flow in different directions. Impact of these structural approaches is questionable as the total loss may have not reduced. Further, the twenty year period data showed that heavy sedimentation has taken place only in a limited area below the EWH. The most active sedimentation take place in the riverbed just below the EWH. Aggradation of the riverbed in this area of the river, and its spatial distribution would decide the direction of sediment transportation in a future flood event as there is not alarming change in the volume of sediment produced in the watershed for the time period considered here. These observations may be very critical in deciding appropriate counter measures, and would have not obtained without remote sensing data. Limiting the settlements in this area leaving natural overflow during monsoon may have better chance in mitigating hazards than considering expensive structural measures. Also, riverine beyond 10km form EWH may not experience severe damage as a consequence of sedimentation except for inundation due to excessive runoff during the monsoon period.
Conclusion
Potential of multi-temporal remote sensing data acquired from different sensing can satisfactorily be used in investigating and assessing the form of riverbed sedimentation, and the planform deviation of river channels. Historical satellite information are invaluable in evaluating the efficiency of flood protection measures carried out in the past, and their consequences that could useful in future flood management strategies. Also the present trend of the channel formation, spatial distribution of sedimentation observed by satellite data may infer the current sedimentation activities and future channel developments.
