Spatial and Temporal Dynamics of the Amazon Flood Wave

Water flow in rivers is typically measured at channel cross sections with simple geometry producing essentially a one-dimensional measurement. Floodplain and wetland flows are much more spatially complex with both large-scale diffusive and locally confined hydraulics. This complexity leads to a wide range of residence times and of carbon, nutrient, and sediment dynamics within the wetland ecosystem. However, our ability to model and hence predict the hydrologic, ecologic, and societal consequences of floods is greatly limited by the nearly complete lack of water height (h) measurements across floodplains virtually anywhere in the world. Using spaceborne interferometric synthetic aperture radar (SAR) measurements, we show the first ever spatially dense hydraulic mapping of the passage of a flood wave through a large, topographically complex floodplain. We find that temporal changes in flood water heights (dh/dt) are more complex than typically assumed and the water flows are not easily prescribed by flows down the main river and bathymetry alone. When lacking floodplain measurements of h, floodplain water surfaces are assumed to be horizontal extensions of those measured in the adjacent main river. Such assumptions do not match our measurements. Instead, during the passage of a flood wave, some floodplain channels are spatially coincident with sharp changes in dh/dt. Some floodplain channels separate adjacent locations with different rates of infilling. However, near the peak of the flood wave, some of these floodplain channels are no longer evident as controls on dh/dt. It is not clear from SRTM topography alone which channels serve as flow controls on dh/dt, rather dh/dt mapping of changing water levels reveals these patterns. During the passage of the flood wave, flow paths change from bathymetrically influenced to hydraulically controlled, thus it is difficult to a-priori know the flow path from bathymetry alone. The implications are that two-dimensional measurements of floodwater elevations and changes (dh/dt and slope, dh/dx) are needed to constrain two-dimensional models for predictions of floodwater movement in large floodplains.

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