Stratigraphic Data Calibrates Predictive Modeling of Holocene-Present Floodplain Sediment Storage In the Mid-Atlantic U.S.

Storage of sediment on floodplains delays downstream sediment delivery, increasing the timescale of catchment responses to forcing by tectonics, climate changes, and watershed sediment management practices.  Including floodplain storage in catchment sediment routing models, however, is challenging because the long timescales involved exceed the duration of stream gaging station and other observational data sources.  As a result, floodplain storage is typically ignored in catchment sediment modeling.  To quantify timescales of sediment storage for mid-Atlantic U.S. floodplains since the early Holocene, floodplain sediment thickness distributions are defined for three time periods by analyzing stratigraphic data: presettlement (deposited before 1750), legacy (deposited 1750-1950), and modern (deposited after 1950).  These data are used to calibrate a model that predicts the thickness, age, and storage time distributions of floodplain deposits through time.  The model uses empirical equations to estimate changes in flood magnitude and duration caused by changes in forest cover and urban development.  Simple hydraulic models predict the occurrence of overbank flow based on channel geometry (which changes through time as floodplains accrete) and the potential for backwater induced by nearby milldams during the 19^th Century. Overbank deposition during overbank flows is predicted based on sediment concentration, sediment settling velocity, and overbank flow duration.  Sediment erosion is predicted based on the age distribution of stored sediment and a power law function that specifies the exposure of sediment to erosion by age category, an approach that is similar to the StorAge Selection Functions often used in catchment hydrologic modeling.   The calibrated model, “tuned” to reproduce observed stratigraphic data, predicts monotonically increasing fluvial sediment concentrations from presettlement to modern time periods, and sediment budget components (input and output fluxes and rates of sedimentation and erosion) that also increase through time.  Predicted sediment residence times (mean age of stored sediment) vary from ~450 years in 1750 to ~300 years in 2017, and the model accurately reproduces the full age distribution (0 to > 5000 yr) of stored sediment documented by contemporary stratigraphic data.  This calibrated model can accurately represent floodplain storage for improved watershed scale sediment routing computations in the mid-Atlantic region, improving our ability to manage Chesapeake Bay restoration and other important watershed sediment management issues.