Search for: All records

Abstract The elevation of natural river levees can vary considerably along the length of a river, and low‐lying features such as secondary floodplain channels allow for hydrologic exchange between a river and its floodplain over a range of discharges. This hydrologic, “river‐floodplain connectivity” plays a role in attenuating flood waves and transporting fluvial material to floodplain ecosystems. However, flood wave attenuation and transport are also limited by the available storage provided by floodplains. In this study, we explore the combined controls of river‐floodplain connectivity and floodplain width on flood wave attenuation and transport, and how those controls change as flood magnitude increases. We develop idealized river‐floodplain models based on the geometry of the lower Trinity River in Texas, USA, varying floodplain width, peak discharge, and degree of river‐floodplain connectivity, which we prescribe by varying the width of a secondary channel connecting the river to the floodplain. We show that attenuation transitions from connectivity‐limited to storage‐limited as discharge increases. Secondary channel conveyance allows for floodplain inundation at lower discharges, but also fills the floodplain faster and, for larger floods, can cause higher flood peaks downstream. Greater secondary channel conveyance and wider floodplains increase fluxes to the floodplain, but secondary conveyance allows the floodplain to drain faster while wider floodplains have longer average residence times. This study presents a framework for understanding how secondary channel conveyance and floodplain width combine to modulate lateral flow exchange, residence times, and flood wave attenuation, and can guide successful management of river systems and future restoration efforts. 

Abstract Hydrologic connectivity controls the lateral exchange of water, solids, and solutes between rivers and floodplains, and is critical to ecosystem function, water treatment, flood attenuation, and geomorphic processes. This connectivity has been well‐studied, typically through the lens of fluvial flooding. In regions prone to heavy rainfall, the timing and magnitude of lateral exchange may be altered by pluvial flooding on the floodplain. We collected measurements of flow depth and velocity in the Trinity River floodplain in coastal Texas (USA) during Tropical Storm Imelda (2019), which produced up to 75 cm of rainfall locally. We developed a two‐dimensional hydrodynamic model at high resolution for a section of the Trinity River floodplain inspired by the compound flooding of Imelda. We then employed Lagrangian particle routing to quantify how residence times and particle velocities changed as flooding shifted from rainfall‐driven to river‐driven. Results show that heavy rainfall initiated lateral exchange before river discharge reached flood levels. The presence of rainwater also reduced floodplain storage, causing river water to be confined to a narrow corridor on the floodplain, while rainwater residence times were increased from the effect of high river flow. Finally, we analyzed the role of floodplain channels in facilitating surface‐water connectivity by varying model resolution in the floodplain. While the resolution of floodplain channels was important locally, it did not affect as much the overall floodplain behavior. This study demonstrates the complexity of floodplain hydrodynamics under conditions of heavy rainfall, with implications for sediment deposition and nutrient removal during floods. 

The morphology of river levees and floodplains is an important control on river-floodplain connectivity within a river system under sub-bankfull conditions, and this morphology changes as a river approaches the coast due to backwater influence. Floodplain width can also vary along a river, and floodplain constrictions in the form of bluffs adjacent to the river can influence inundation extent. However, the relative controls of backwater-influenced floodplain topography and bluff topography on river-floodplain connectivity have not been studied. We measure discharge along the lower Trinity River (Texas, USA) during high flow to determine which floodplain features are associated with major river-floodplain flow exchanges. We develop a numerical model representing the transition to backwater-dominated river hydraulics, and quantify downstream changes in levee channelization, inundation, and fluxes along the river-floodplain boundary. We model passive particle transport through the floodplain, and compute residence times as a function of location where particles enter the floodplain. We find that bluff topography controls flow from the floodplain back to the river, whereas levee topography facilitates flow to the floodplain through floodplain channels. Return flow to the river is limited to locations just upstream of bluffs, even under receding flood conditions, whereas outflow locations are numerous and occur all along the river. Residence times for particles entering the floodplain far upstream of bluffs are as much as two orders of magnitude longer than those for particles entering short distances upstream of bluffs. This study can benefit floodplain ecosystem management and restoration plans by informing on the key locations of lateral exchange and variable residence time distributions in river-floodplain systems.