Characterization of the Spatial and Temporal Dynamics of Floodplain Surface-Water Circulation

Floodplains are spatially complex landscapes with a variety of surface features and landforms. These low-relief, low-gradient landscapes may contain extensive networks of well-defined channels and topographically connected depressions, and in some cases, the channels originate as cuts across levees or banks of the main river, referred to as through-bank channels. Corresponding complex patterns of inundation and flow have been detected through remote sensing and in field observations. The complex floodplain flows in response to the passing of flood waves is, however, poorly quantified. This work aims to increase fundamental understanding of floodplain flows and to highlight controls on floodplain wetting, circulation and draining. Surface water flows dynamics over a topographically complex floodplain are quantified using a novel and robust observation approach, the Triangular Facet method. The approach requires a minimum of three stage measurements and can be applied to any inundated surface, across a range of temporal and spatial scales. Water level and flow directions are synchronously measured. The Triangular Facet revealed complex and highly variable flow patterns, including abrupt flow reversal. Flows during sub-bankfull inundation (via through-bank channels, river stage is below bankfull stage) are particularly variable, including multiple abrupt flow reversals, while overbank inundation produces a more subdued response with slow rotations. The “triangular facet” approach provides a new type of mesoscale insight of flow processes over inundated landscapes, highlighting flow complexity. Moreover, the approach provides information on floods that represent a compromise between in-situ point velocimeter measurements and satellite remote sensing. Systemwide understanding of floodplain flow dynamics is obtained by process-based numerical modeling under realistic and unsteady forcing conditions. Calibration and validation of the hydrodynamic model was conducted using in-situ field observations obtained with Triangular Facet approach. Simulations highlight floodplain inundation dynamics for two conditions, the passage of sub-bankfull and overbank flood waves. Sub-bankfull inundation commences with the passage of the flood wave crest beyond the lower elevation levee breaches, and floodplain wetting is guided by the channel network. Hence, the upstream sub-bankfull inundation area expands while much of the downstream floodplain remains dry. The onset of overbank flow is spatially variable but becomes continuous, and the through-bank channels persist as preferential pathways that produce higher velocity flows several kilometers inland. Meanwhile, near-stagnant zones develop between through-bank channel mouths, where water is temporarily stored and routed to the channel network. Also, the majority of the inundation water enters the study area from the upstream floodplain (intra-floodplain flow). To glean deeper insight into processes governing floodplain circulation, reduced complexity simulations were conducted using synthetic hydrographs, constrained by observations. Findings indicate that shallow rising gradients produce enhanced floodplain inundation and result in lower river water levels. Steeper rising gradients deter inundation processes and cause higher water levels in the river. On the contrary, shallow falling stages enhance overall floodplain drainage processes. Steep falling stages, however, result in greater floodplain water retention. The substantial variation in inundation and drainage processes result from highly variable and complex flow patterns resulting from various rates of change of stage that appear to be dependent on flow thresholds associated with submergence and emergence of topography. Collectively, these findings highlight complex flows in response to floodplain wetting and draining processes, mutually depending on stage, rate of change of stage, inundation process (sub-bankfull or overbank), intra-floodplain flows, initial floodplain wetting conditions, submergence and emergence of topography. Floodplain wetting and draining processes significantly influence flow direction, and characteristics of the flood wave over the floodplain surface such that a single stage in the main channel does not uniquely define floodplain flow hydraulics. Given these findings we propose that assessments of floodplain hydraulic connectivity account for the effects of heterogeneous levee structure, intra-floodplain exchanges, as well as the typical flow thresholds associated with submergence and emergence of topography. Understanding floodplain flows has important implications as increasingly susceptible to flooding, and this view is especially relevant given the role of modern climate change in compound flooding. Detailed insight on water circulation can aid in, for example, flood hazard and mitigation analyses and in maintaining floodplain ecosystems and water quality, and for dam management