Characterizing Flash Floods in Nepal

Day by day, natural disasters are increasing because of global population growth and climate change (Dixit, 2009). Recent mudflows, landslides, floods, and hurricanes have shown the differences between developing and developed nations. In South Asia, recent flood disasters have killed more than one-thousand lives and led to the destruction of properties. Nepal is viewed as one of the most disaster-prone nations in the world in this context. Including a 2017 flood event in Terai, 2008 Koshi embankment breach, and 1993 central Nepal flood occurrences, Nepal has encountered many flood occurrences (Shukla & Mall, 2016). In the southern area of Nepal, flash floods are widespread, especially in Churia-originating rivers. The significant factors for the destruction of property and loss of life are rising bed levels and intense rainfalls. Urbanization, changes in land use, and other natural causes lead to rivers changing in Nepal’s southern areas, provoking significant consequences because of extreme floods (Gautam & Phaiju, 2013). Along with other socio-economic drivers, the increase of settlements and agricultural land at the expense of forest covers also predisposes Nepal to more disaster vulnerability in both intensity and frequency. The work addressed here further elaborates the data taken from satellites over a period providing evidence of the occurrence of flash flooding conditions, specifically precipitation and soil moisture.

The main purpose of this project is to provide the evidence of occurrence of flash flooding using the precipitation and soil moisture.

Background:

Early warning system applications can minimize damage and loss (Dixit, 2009). Excellent early warning systems comprise identifications and estimations of dangers, communication, dissemination, monitoring, and responses of the related stakeholders and communities. For a wide variety of natural hazards such as snow avalanches, landslides, volcanic eruptions, and floods, several early warning systems have been designed. There are very current communication technology, computer modeling, and precipitation-sensing advancements that make early warning systems of flash floods progressively sustainable, affordable, and useful. However, it is worth noting that flash floods’ current prediction requires distributed hydrologic models, high-resolution computer models of atmospheric processes, satellite algorithms, radar coverage, and dense rain gauge networks. Most flash-flood prone nations such as Nepal with susceptible populations have various alternatives for establishing regional or local flash flood early warning systems. Some of the early warning systems of flash floods are heavy-rain-event detections via streamflow/rainfall satellite sensors, gauge networks, radar networks, or combinations of the three (Shukla & Mall, 2016). Thus, attempts have been made to design community-based early warning systems for flash floods in Nepal.

Flood warning and forecasting affect non-structural flood management approaches (Gautam & Phaiju, 2013). The public and the planners have accepted this method of managing floodplains and mitigating flood disasters. Appreciating these facts, the meteorology and hydrology department in Nepal should work to develop flood forecasting and warning systems in major Nepal rivers. Every year, floods of differing magnitudes take place in Nepal during monsoons. Some of the flash floods become destructive. Catastrophic floods can be described as “floods that have very large impact in terms of life and property losses and major disruptions to infrastructures (Torrents can be categorized into the three severity classes based on the recurrence intervals: extreme floods with recurrence intervals surpassing 100 years, massive floods with recurrence intervals between 20–100 years, and huge floods with recurrence intervals less than 20 years.

Teleconnections has become a new word in the world of meteorology (Dixit, 2009).This is particularly true with El Niño and La Niña because there is no consistency to the impacts.  There are two flooding causes. These are anthropogenic and natural. The natural flooding cause in Nepalese rivers consists of extremities in seismic activities, climates, topographical extremities, and fragile geological conditions (Shukla & Mall, 2016). The anthropogenic flooding causes include developmental activities (such as urbanization, hydro-powers, irrigations, and roads), land-use changes, unscientific agricultural practices, deforestations, and socio-economic changes, such as illiteracy, poverty, and population growth.

The worst flooding disasters in Nepal are widely considered the flash flood events of1993. In the history of Nepal, these events are considered to be the costliest flash floods that destroyed almost 18,000 acres and destroyed virtually 2,000 structures (Shukla & Mall, 2016). The meteorology and hydrology department in Nepal has meteorological and hydrological stations to collect climatic, river stage, and river discharge data throughout Nepal.  The meteorology and hydrology department in Nepal publishes these data records yearly.

Figure 1: Showing the discharges rate from 1979 to 2007.

From 1979 to 2007, The Bagmati River instantaneous maximum discharges along with the floods of 20-year and 100-year return periods, according to meteorology and hydrology department publications, are indicated in Figure 1 (Dixit, 2009). The flood discharges differ from 2,000m³/s to 16,000m³/s. With a return period of about 100 years, it is noted that huge floods that were catastrophic took place in 1993.

Low-Cost early warning system development

After analyzing the community interactions and field conditions, the low-cost early warning systems are developed. These systems include a set of water and rainfall lever measuring points installed over major stations in its sub-basins or watersheds (Gautam & Phaiju, 2013). All stations transmit their data in real time to repeater stations connected to the master station in which information from all the water bodies such as rivers and basins are obtained and processed. The water level changes at various interest points are closely monitored and controlled. The central units issue flood warnings when flash floods are likely to take place, and relevant actions are taken to save lives and reduce damages.

Working Sensor mechanisms:

The water levels are sensed by the ultrasonic sensor, and the sirens will start automatically. The thresholds may be set up according to time and space. These systems have diverse alternatives. The sensors first detect the initial water levels, and sirens will begin automatically and repeat after stopping for five seconds (Shukla & Mall, 2016). The sensors detect the second water levels, and sirens will begin automatically and repeat after stopping for five seconds. The gauge readers adjacent to the stations shall transfer the data to the downstream office or community when the sirens are on.