Application of satellite altimetry in understanding river–wetland flow interactions of Kosi river

Abstract

Flood-plain wetlands are the seasonal water bodies formed along a river. These wetlands become active during the monsoon season, which frequently grow in size with seasonal floods and eventually dry up during the non-monsoon season. The flow interaction between flood-plain wetlands and the river sometimes vary over a very short period in response to rapid rise in the river water level due to high precipitation in its upstream catchment. Understanding the complex flow interactions between the river and its associated flood-plain wetlands with field-based measurements of wetland hydrologic characteristics is always a challenging task. To overcome these challenges, an attempt has been made to utilise Topex/Poseidon satellite altimetry-derived water levels into a hydrodynamic model (HEC-RAS) to study river and wetland flow interactions in the lower reach of the Kosi river in India. The satellite altimetry-derived water levels and Landsat satellite images on the Kosi wetlands are used to develop volume-elevation relation. HEC-RAS is setup over the study area and calibrated for different values of manning’s roughness coefficient (n) for the river bank and the main channel of the river for the period of 1993–1996. Unsteady flow simulations are carried out for different monsoon seasons to simulate daily river flow interaction (inflow/outflow) between river and wetlands. Statistical analysis is performed between the altimetry-derived and the model-simulated water levels. It is found that simulated water levels are in good agreement ($R^{2}=0.87$, root mean square error of 0.84 m and Nash–Sutcliffe efficiency coefficient of 0.85) with altimetry-derived water levels. The analysis of simulations indicates that interactions between the wetland and the river are bidirectional with most of the flow coming out from the river during the month of August and leaving out from the wetlands during the month of September. The wetlands respond in three different ways, i.e., (i) gaining stage, (ii) wetland and river in equilibrium and (iii) loosing stage, which is reflected on water levels of the river and wetland. This study demonstrates complex interaction processes happening between the Kosi river and its surrounding wetlands.

Appendix: Field campaign (Kosi river)

The field campaign was done to understand in detail about the river–wetlands behaviour — a field cum interaction survey was conducted in the month of December 2014. The downstream wetlands in the study area were chosen to conduct the field survey. The river was completely within the main channel and flood plain was dry with partial vegetation on it. It was found that the wetland bed is composed of silty clay. During the survey, we approached the local people living nearby the wetlands to inquire about the response of the wetland for varying flood pulses in the nearby Kosi river. It was clear from the discussion that wetlands were formed in monsoon season and gradually dry up in the non-monsoon season. The gradual discharge of flow from wetland to river and wetland with no connectivity was also observed in the field. Table A1 represents the minimum and maximum depth of water in the wetland during different seasons collected from various local people in Nagrah village. Table A1 clearly indicates that the water depth variation in wetland during monsoon season was between 10 and 17 ft, i.e., 3–5 m, and in post-monsoon season, it was about 4 ft (1.2 m). Water depth in the wetland was also measured at different points along the longitudinal direction and the average depth was found to be 0.85 m.

The experience of extreme floods was shared by the local people, which occurred during the period of 1980s, 1990s and in recent years 2002 and 2003. They described that during the flood, these wetlands play an important role by gaining water from the river and act as a buffer for excess water. Wetlands were found to be integral part of the society living nearby. People were using water for multiple purposes such as irrigation (corn fields), domestic needs, livestock consumption (cattle and goat) and other household-related works. It is also a major source of income through fishing activity for the nearby people. The field cum interaction-based survey provided vital information in understanding the behaviour of wetland in terms of depth during monsoon and beginning of the post-monsoon season. The livelihood dependence of the people from the wetland was also observed in various ways.