STUDY ON WATERSHED ANALYSIS AND SEDIMENT REGIME IN THE SWALIK
REGION OF NEPAL
M. B. Shrestha* *, A. Yuasa? , D. Z. Seker^, T. Sadao ©, K. Kensuke*
? River Basin Research Center, Gifu University, 1-1 Yanagido, Gifu-city 501-1193, Japan - madhu@ina.janis.or.jp
? ITU, Civil Engineering Faculty, 80626 Maslak Istanbul, Turkey - seker@itu.edu.tr
* United Graduate School of Agriculture, Gifu University, 1-1 Yanagido, Gifu city 501-1193, Japan
KEY WORDS: GIS, Landsat, Geomorphology, Floods, Analysis
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 geology of the Siwaliks. Investigating the sediment production and
transportation regime in the Siwalik regions of Nepal, drainage analysis was conducted. Satellite data was used in identifying the
changes of land coverage, floodplain and river channels. Reasons for these changes were examined with field observation and aerial
photograph interpretation. These series of studies evidently showed the crucial causes of the massive sediment production were due
to intense erosion that led by the fragile geology, intense rainfall, lack of vegetative coverage, exploitation of lands due to human
activities and root morphologies of the indigenous vegetation.
1. INTRODUCTION
Nepal is a mountainous country; approximately 86 percent of
the area is characterized by steep hills and mountains. Elevation
ranges from 70 m to 8849 m within a radius of less than 200 km.
The country comprises five distinct physiographic regions
distributing E-W belt form north to south: the High Himalayas
(23.7% of the total area), the High Mountains (19.7 %), the
Middle Mountains (29.5 %), the Siwalik (12.7 %) and the Terai
(14.4 %). All the regions are stretched in a E-W direction and
arranged collaterally form north to south.
The southern belt of the Terai region, the Siwalik region, is
composed of north dipping interbedded sandstone, silt stone,
shale and weakly/unconsolidated conglomerates. It has a
subtropical climate and dominated by Sal (Shorea robusta)
forests. The soils (red laterite soil) are highly vulnerable to
water erosion, and flash floods occur frequently in the low-
lying areas during monsoon, causing massive sediment
production.
Due to the clearance of a total of 103,968 ha of forest in Siwalik
hills and Terai plains under the government's resetticiment
programmes from the 1950s to the mid. 1980s, and an
introduction of modern healthcare and a malaria suppression
programmes, unpredictable growth of population, which led to
increasing demand for fuelwood, timber, fodder and agricultural
land (Awasthi ef al., 2002). These factors caused the decline of
the forest cover, accelerating erosion process and producing
huge mass of sediments from the Siwalik Hills resulting severe
flood-induced deterioration on the nation's exclusive granular
plains, the Terai. Controlling high sediment production in the
Siwaliks and protecting cultivable lands in Terai region from
floods are the most prominent concerns to Nepal.
While implementing sediment control measures, understanding
the characteristics of a watershed is inevitable. The main
objective of this study is to illustrate the characteristics of
sediment regime of Thadokhola watershed, one of the typical
* Corresponding author.
564
watersheds of the Siwalik region by analysing its
geomorphometric parameters incorporated with land-use/land-
cover changes within the watershed.
2. MATERIALS AND METHOD
2.1 Study Area
The study area, Thadokhola watershed, derives from its river
Thadokhola, one of the tributary of Rapti River, lies in the
central part of the Siwalik regions. Thadokhola river could be
considered as wadji, as water flow in the upper reach is visible
only during the intense rainfall period of monsoon (wet season).
Water is only visible at downstream (near the confluence with
Rapti River) during the dry season. The channel bed is covered
by highly porous materials: sand, silts, gravels/cobbles and
boulders. Occurrences of frequent floods at downstream are
very common during the high rainfall period. The southern
(upstream) border of the Thadokhola watershed adjoins with the
Parsa Wildlife Reserve. The slender-shaped (form factor is
0.14) watershed has total area of 14.10 km and elevation varies
from 400 to 740 m (Figure 1). The width of Thadokhola river
bed varies from 50 to 210 m and average channel gradient is
less then 3 degrees.
Thadokhola watershed is a remote hilly area having neither
access to roads nor infrastructures. People residing in the
watershed rely fully or partially on traditional agriculture. Large
areas of cultivated lands (mostly rice fields) and vegetable
farms are located near and around the settlements (mud and
brick houses with thatched roof). Settlements are mostly
concentrated at middle to lower part of the watershed.
Surface erosions on cultivated lands, bank cuttings,
aggradations of channel bed, formations of small fans and talus
near/around river trunks, and expansion of floodplains are
prevalent over the watershed. Few gabions (0.5m width, 1.0m
in height), set up along the river banks are the one and only
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