Flooding and Inundation in Nepal Terai: Issues and Concerns
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Flooding and Inundation in Nepal Terai: Issues and Concerns Basistha Raj Adhikari Basistha Raj Adhikari Abstract: During the monsoon months from June to September, all the rivers in Terai are in spate with bank-full dis- charges and cause flooding and inundation. The problems of flooding and inundation in the Terai are more critical due to change in climate in general and change in rainfall pattern/intensity in particular. This article tries to highlight the issues and concerns of flooding and inundation in the Terai and suggests measures to mitigate these issues in light of climate change adaptation. Key words: Flooding, inundation, River Training Works, Terai, Nepal Introduction The medium rivers that originate from the Mahabharat he Terai region of Nepal occupies about 17 hills experience monsoon discharge of 2,000 m3/s to Tpercent of the land area (25,000 km2) and forms 8,000 m3/s and create havoc of flooding and inundation the northern edge of the Indo-Gangetic plain. The Terai in the Terai. These rivers transport significant amounts area is popularly known as the granary of the nation. The of sediment while flowing through the Churia range and topography of the Terai is almost flat with gentle slope exhibit lateral shifting. These rivers are wide as they towards the south. The elevation of the Terai ranges from enter into the Terai plain and start meandering after 65m to 300m from mean sea level with varying width of the Bhabar zone. These rivers are the Kankai in Jhapa, 20 km to 45 km. The climate of the Terai is subtropical Kamala in Siraha and Dhanusha, Bagmati in Sarlahi and with average temperature of 25oC. The rainfall of the Rautahat, Tinau in Rupandehi, West Rapti in Dang and Terai varies from 1,200 mm to 3,000 mm per annum Banke, and Babai in Bardiya districts. with occasional showers and cloudbursts. All the rivers The small rivers originating from the southern slope of Nepal debouch into the Terai plain at the foot hills of the Churia hills are named as Churia rivers. The length of the Churia and Siwalik ranges and provide water for of these rivers up to the Nepal-India border ranges livelihood of the Terai. During the monsoon months between 25 km to 85 km. These rivers are numerous from June to September, all these rivers are in spate with and cause local erosion and deposition in the Terai belt. bank-full discharges and cause flooding and inundation These rivers have special morphological characteristics in several parts of the Terai. The problems of flooding and that aggravate the flooding and inundation in the Terai inundation in the Terai are more critical due to change of Nepal. The most vulnerable Churia Rivers are the in climate in general and change in the rainfall pattern/ Biring and Ratuwa in Jhapa, Bakraha and Lohendra in intensity in particular. This article tries to highlight the Morang, Sunsari in Sunsari, Khando in Saptari, Balan issues and concerns of flooding and inundation in the and Gagan in Siraha, Rato in Mahottari, Jhim and Terai and suggests measures to mitigate these issues in Lakhandehi in Sarlahi, Lal Bakaiya in Rautahat, Pashaha light of climate change adaptation. in Bara, Rohini in Rupandehi, Banganga in Kapilvastu, Khutiya in Kailali and Dhondha in Kanchanpur districts. River Systems of Nepal These Churia rivers have special characteristics that are According to the origin, size and nature of the flow, described in their proper context below. Nepal's rivers are grouped into three categories: • Large rivers originating from the Higher Himalayas, River Morphology in the Terai • Medium rivers originating from the Mahabharat The river morphology in the Terai depends upon Hills, and the origin, flow magnitude and sediment load of the • Small rivers originating from the Siwalik and Churia river. The rivers originating from the Churia hills are Hills. characterized as flashy with negligible or low flow during The largest four rivers of Nepal, i.e. the Koshi, the the dry season. In addition, the large rivers also change Gandaki, the Karnali and the Mahakali originate from their morphology as they debouch into the Terai plain the high Himalayas and carry large discharges. These with flatter slopes and wide widths. The morphological rivers are incised in the hills and mountains and are characteristics of the rivers flowing through the Terai are comparatively less vulnerable until they debouch into briefly described hereunder: the Terai plain. The average discharge of the Koshi, Gandaki and Karnali range between 1,350 m3/s to 1,600 Severe Erosion in the Hill Slopes: The Churia hills m3/s while the average discharge of the Mahakali River are the youngest hills in the Himalayas formed about 2 is 726 m3/s. In the Terai these rivers often cause flooding million years ago (Neocene period). Their composition and inundation over large stretches of land adjoining is made of sedimentary rocks such as mud-stones, their banks. sand-stones, and conglomerates. The Terai areas of HYDRO NEPAL ISSUE NO. 12 JANUARY, 2013 59 Nepal experience relatively higher rainfall intensity High Peak Floods: Runoff in Churia Rivers (Carson 1985) which contributes to higher soil erosion concentrates only in the monsoon months from June to in the slopes of the Churia hills. The rivers that September. Factors like short steep slope, overgrazing, originate from the Churia hills bring a lot of sediment deforestation, and short time of concentration produce thereby eroding the hill slopes as well as river banks high peak flood during the monsoon. In general, the (Photo 1). runoff duration is less than one day and the flood hydrograph is very sharp with high peak discharge. Photo 1. Degradation of Catchment in the Churia Hills. Photo 3. Flood in Mahakali River in 2009. Excessive Sediment Load: The Churia range Narrow Width at Nepal-India Border: Most of the contributes maximum sediment load to the rivers Churia Rivers have relatively narrow width at the Nepal- originating from the southern face (Sharma 1977). India border. During heavy rainfall these rivers spill over According to a JICA study, the annual sediment yield of the banks and flooding and inundation occur for some the Lakhandehi river is estimated as high as 178,000 m3 time depending on the topography, slope of the river and per year. In some stretches of the Churia rivers, almost the waterway. A typical example of a narrowing river two meters of sediment has been deposited in the last width is the Rato River in Mahottari District. The Rato 45 years (Dixit 1995). Due to the higher sediment load, and Jangha rivers in Mahottari District lie 6 km apart at the the bed level of several rivers are rising significantly. The East-West Highway. The span of the Highway Bridge at the case of the Lothar river in Chitwan District is typical of Rato River is 204m. After joining the Jangha and Ankushi this phenomenon (Photo 2). rivers, the span of the bridge over the Rato River at the Bardibas-Jaleshwor road narrows down to about 100m. Further downstream near the Nepal-India border at the Jaleshwor-Bhirttamod road, the span of the bridge is narrowed again to only 30m. Topographical Depressions: Some of the Churia Rivers form topographical depressions such as marshy lands, swampy lands and oxbow lakes in the lower reaches of the Terai. These depressions have formed due to excessive meandering, abandoning of the old course, and avulsion of the rivers or their part. The Sunsari River at Maria Dhar, old course of the Kankai River are some of such examples. The medium sized river Babai in Bardiya District had also changed its course and formed topographical depressions in Photo 2. Sediment Accumulation under the Lother High Bridge. the west of Gularia some 80 years ago. Wide and Braided Channels: Most rivers in the Terai Rainfall-Runoff Process in the Terai are braided at the Bhabar zone and prone to change their Past experience shows that flooding and inundation course frequently. In many cases, these rivers find a new occur following high intensity rainfall in the Churia hills path and enter into the cultivated lands leaving the old and Terai. A rainfall intensity of 350 mm for consecutive course. This is also evident in some medium sized rivers. 48 hours is considered as high intensity rainfall (Sharma For example, the bifurcation of the Tinau River into the 1988). In addition, rainfall exceeding 70 mm per hour Tinau and Dano rivers just downstream of the East-West is considered as cloudburst rainfall (Gyawali 2011) Highway Bridge after 1978 flood is a typical example of which disrupts both the slopes and channel equilibrium the Terai river characteristics. at the local as well as regional scales. Precipitation 60 HYDRO NEPAL ISSUE NO. 12 JANUARY, 2013 events exceeding 375 mm in 24 hours have been Floods of 1978 in Butwal: With the intense rainfall in recorded in different parts of the Terai between 1959- the Churia hills, a landslide occurred in the Butwal area 1993 and maximum rainfall data is shown in Table 1. in September 1978. The landslide had blocked the river Tinau, and upon burst of the landslide dam, the high Mean Max Mean S. Station Station Annual Monthly surge of water had washed away the newly constructed No. Code Name District Rainfall Rainfall (mm) (mm) bridge over the river along the East-West highway, 1 0215 Godavari Kailali 2,279 700 July diversion weir of the Tinau Irrigation Project and other 2 0416 Nepalgunj Banke 1,338 426 July public property. This flood had not only damaged the 3 0705 Bhairahawa Rupandehi 1,609 509 July infrastructure and property adjacent to the river bank but 4 0906 Hetauda Makwanpur 2,283 566 Aug also had changed the flow pattern of the river.