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Mapping gully erosion patterns in foothills of lower Shiwaliks
Results and discussion
The physical damage to permanent structures, which constituted about 7.5 per cent of total structures, was comparatively less compared to other structures. About one-third structures in Brahmanawala and Basu Khad catchments were partially damaged and an equal number were completely damaged (Table 1). On the other hand in all the catchments where such structures were installed, these were silted on the upstream side to different levels. The siltation up to 50-75 per cent occurred in about 25-75 per cent of the structures in different catchments whereas complete siltation occurred in 75-100 per cent of the structures in various catchments. Permanent structures were least damaged physically as these could resist the energy of runoff water more efficiently due their strength. However, the siltation on the upstream side within 2-3 years resulted in a situation where the runoff water fell from a greater height thereby becoming more erosive and carried the silted particles along with it. The presence of gully control structures in the highest order gully means that all the lower order gullies were contributing freely to this gully resulting in build up of runoff of sufficient amount and velocity. Before planning the installation of gully control structures in a catchment, no information on gully networks including distribution and extent of different ordered gullies, gully density, gully texture etc. was collected. These gullies collect runoff from each nook and corner of the catchment and poured it into the highest order gullies via other intermediate order gullies. In many instances, the catchment boundaries were not delineated while
treating for gully control in the area. This results in non-availability of the expected runoff volume from that area which probably leads to under-design of the structures.
Table 1. Extent and status of permanent structures in different catchments in foothills of
lower Shiwaliks
| Catchment |
Total number of structures installed |
Per cent damaged structures |
Per cent structures silted upstream with different levels of
siltation |
| Partial |
Complete |
25-50 |
50-75 |
100 |
| Brahmanawala |
3 |
30 |
30 |
-- |
30 |
70 |
| Bathalaur |
6 |
-- |
-- |
-- |
70 |
-- |
| Sahora |
5 |
-- |
-- |
80 |
20 |
-- |
| Sadhu Chak |
9 |
-- |
-- |
-- |
25 |
75 |
| Basu Khad |
15 |
33 |
33 |
-- |
-- |
100 |
The mapping of gully networks in four catchments of Saleran showed that gully texture--number of first-order gullies per unit area, ranged from 253.8 to 767.9 km-2 (Table 2) The gully density--total length of gullies per unit area ranged from 8.73 to 31.9
km km-2. The gully density and gully texture were both a function of average relief of the catchments studied. The first order gullies constituted 57-85 per cent of the total gullies whereas second order gullies ranged from 15-33 per cent in the four catchments (Table 3). About 60-89 per cent of the total length of gullies was represented by first order gullies and 12 -27 per cent length was represented by the second order gullies (Table 4). Thus both length and number of gullies were dominated by first order gullies which are a neglected lot while deciding the installation of gully control structures.
Table 2. Extent of gully erosion in study catchments
| Catchment |
Gully texture(km-2) |
Gully Density (km/km-2) |
Average Relief |
| I |
767.91 |
31.93 |
1.36 |
| II |
436.89 |
15.50 |
0.09 |
| III |
742.86 |
16.31 |
0.13 |
| IV |
253.8 |
8.73 |
0.07 |
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