Congo Floodplain Hydraulics using PALSAR InSAR and Envisat Altimetry Data

Abstract

Wetlands of lowland rivers and lakes are massive in size and in volumetric fluxes, which greatly limits a thorough understanding of their flow dynamics. The complexity of floodwater flows has not well been captured because floodwater moves laterally across wetlands and its movement is not bounded like that of typical channel flow. Water flow across wetlands is more complex than implied by one-dimensional, point-based measurements. Interferometric Synthetic Aperture Radar (InSAR) has been proven to be an effective tool to map wetland’s relative water level changes \( \left(\partial h/\partial t\right) \) with high spatial resolution and accuracy. In this study, we integrated relative \( \partial h/\partial t \) from ALOS PALSAR interferograms and Envisat altimetry data to estimate absolute \( \partial h/\partial t \) in the Congo floodplain near Lisala in Democratic Republic of the Congo. Three fringe patterns from PALSAR interferograms were identified due to spatiotemporal water level changes in the floodplain. We observed (1) dense fringes parallel to the Congo mainstem in high water season, (2) broad fringes across the floodplain, (3) and fringes around the floodplain boundary in low water season. The absolute water level change maps generated by integrating InSAR and Envisat altimetry data suggest that \( \partial h/\partial t \) can reach up to 1.2 m in the proximal floodplain in high water level season. During low water season, \( \partial h/\partial t \) can reach up to several decimeters. Based on the maps of absolute \( \partial h/\partial t \) and the principle of mass continuity, analysis of temporal hydraulic variations is also presented. Our hydraulic analysis suggests that \( \partial h/\partial t \) is subtle and the water flow in the floodplain is not well confined during low water season. On the other hand, the proximal floodplain has greater \( \partial h/\partial t \) than the distal floodplain during high water season, which suggests that water mostly flows from floodplain to river.