Floodplain storage and its influence on sediment transport: new insight from field and modeling studies
Date
2025
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Publisher
University of Delaware
Abstract
As sediment moves through a watershed, it cycles between periods of channelized transport and immobile storage in adjacent reservoirs. Floodplains serve as the largest and longest-lived reservoir in many river systems, storing sediment for years to millennia before remobilization. This dissertation investigates how floodplain storage influences sediment transport across space and time through a combination of field and numerical modeling studies. ☐ A field-based study of the Powder River in Montana quantifies the duration of storage and remobilization using a new methodology to reconstruct the age distribution of eroded material. Results indicate an exponential storage time distribution with a mean of ~800 years, suggesting sediment delivery is governed by meander belt reworking in laterally confined valleys. ☐ A numerical sediment routing model is then developed to simulate channel floodplain sediment exchange while tracking the age of stored sediment. The model tests both age-independent and age-dependent erosion functions. Results show that age-dependent erosion, where younger sediment is preferentially eroded, increases sediment delivery timescales. Model results are consistent with analytical solutions and reveal the importance of age structure in floodplain storage. ☐ The previous model is restricted to a single river valley with constant width and channel morphology. In the final chapter, these restrictions are addressed, and a model is developed to route sediment through a network of river valleys with spatially variable floodplain width, water discharge, and tributary sediment supply. This expanded framework is used to simulate a branched watershed network based on the White Clay Creek in southeastern Pennsylvania. This network model successfully reproduces sediment fluxes compared to sediment budget data and is used to test idealized restoration scenarios within a watershed network. Reducing upstream sediment input or limiting floodplain erosion produced only modest decreases in suspended sediment flux reaching the watershed outlet, indicating that common restoration practices may have limited effects. However, further validation of this is needed before it should be used to evaluate specific restoration scenarios. ☐ Together, these studies highlight the impact that floodplain sediment storage and remobilization have on sediment delivery timescales, as well as offering new tools for evaluating restoration strategies in sediment-impaired watersheds.
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Keywords
Sediment transport, Floodplain storage, Sediment delivery, Erosion function, Sediment exchange