Guwahati: Development activities in the hill states surrounding Assam, if undertaken without detailed hydrological studies and ecological safeguards, can significantly worsen flood conditions in downstream areas, particularly during periods of high-intensity rainfall, according to IIT Guwahati hydrologist and Professor of Civil Engineering Arup Kumar Sarma.
Responding to questions on whether development activities in Arunachal Pradesh, Nagaland and Meghalaya influence flood patterns in downstream areas of Assam, Sarma said roads, infrastructure projects, mining and other developmental activities in hilly areas can significantly alter downstream flood conditions if they are carried out without detailed hydrological studies or adequate measures to restore vegetative cover.
“Developmental activities like construction of road or other infrastructures, extensive deforestation for mining or other developmental work in the hilly area, if done without detail hydrological study and without taking proper care for recreating vegetative cover, can alter the flood situation significantly at downstream, particularly when high intensity rainfall occurs in the hilly terrain causing high water and sediment yield,” he said.
He also pointed to poorly designed road crossings as another factor contributing to floods and landslides.
“Inadequate openings kept for the hilly streams in road crossings can lead to flooding and landslides, causing disruption in communication. Inadequate openings kept in roads of plains can block such flash floods, causing flooding at the upstream of such roads due to water stagnation,” he added.
Explaining the recent flood pattern in upper Assam and the Khanapara-Jorabat area of Guwahati, which differed from the state’s usual floods, Sarma identified two major reasons.
“There are two distinct reasons. The first is extensive deforestation in the immediate upper catchments. Extensive deforestation is visible in the upstream hills of Nagaland (affecting Sibsagar and Jorhat), Arunachal Pradesh (affecting Dhemaji and Dibrugarh), and Meghalaya (affecting Khanapara, Jorabat and Guwahati), as evident from satellite imagery.”
Sarma said such deforestation, driven by mining, road construction and other developmental activities, has reduced the ability of rivers to carry water and sediment.
“Such deforestation maybe due to boulder mining, road construction and other developmental activities. Such deforestation can generate high water and sediment yield. Sediment deposition makes the river shallow and thus they loss their water carrying capacity.”
He added that changing rainfall patterns linked to climate change have further intensified the situation.
“We are experiencing more high intensity rainfall due to climate change. Extreme event in Eastern Himalaya due to impact of El-Nino this year cannot be nullified, generally in some part El-Nino causes low precipitation though. Cyclonic moisture movement from Bay of Bengal towards Assam moving over Nagaland and Manipur is seen, which can cause high precipitation due to orographic lift. Continuous precipitation causes land saturation and a high intensity precipitation over saturated deforested land generates very high water yield,” Sarma said.
On the possibility of similar flood-related challenges in districts such as Karbi Anglong and Dima Hasao, he said the plains in these districts could also experience flooding if similar rainfall events occur.
“If such precipitation happen, plain area of these districts can also experience flood. Kapili has reservoir at upstream. If high precipitation happens dam authority will be bound to release the excess rainwater once it crosses the reservoir capacity. So, reservoir situation should be monitored, so that early warning can be issued to mitigate flood disaster. Also reservoir operation can be augmented to reduce flood marginally.”
Sarma also stressed the need for coordinated disaster preparedness and an early warning mechanism involving both hill and plains states.
“There should be a coordination committee for disaster preparedness and to develop an early warning mechanism. Such committee should involve Academician, Reservoir Authority, relevant District Authority of both hilly state and state in plains. Hydrodynamic and hydrological modelling with sensor based input can help in forecasting flood disaster and issuing early warning.”
He said better monitoring and data collection would strengthen flood forecasting and disaster mitigation efforts.
“Installation of sufficient rain gauge in the hilly terrain will give better information about flood generation and will help disaster mitigation measures. Awareness generation can help utilizing data through crowd sourcing. Drone based regular monitoring can also help in identifying critical situations.”
Sarma also called for immediate ecological restoration of degraded catchments.
“Immediate steps should be taken collectively for reforestation of the degraded catchment through Ecological Management Practices (EMPs) with economically beneficial plants and selecting proper varieties of plant that are suitable for controlling surface erosion. EMPs, in addition to vegetation, include structural measures like retaining wall etc. to support vegetation growth, water harvesting structures and suitable plantation and land-use augmentation considering due right of land ownership,” he added.