Comparison of modeled and observed bed erodibility in the York River estuary, Virginia, over varying time scales Danielle Tarpley, Courtney K. Harris, Carl T. Friedrichs Virginia Institute of Marine Science

ETM STM Rinehimer, 2008

Motivation: Sediment availability impacts sediment distribution ETM STM Motivation: Sediment availability impacts sediment distribution Rinehimer, 2008 Scully and Friedrichs, 2007

Motivation: Impacts of bed erodibility on the near bed suspended sediment concentration Bed Stress Near-Bed C Red = 0.1 Pa 1 kg/m2 at 0.2 Pa 150 mg/L 7 Jul 2007 10 km Dominate flux during on spring Dominate flux on the shoal Indicates the bed erodibility may be variable and needed to better understand the sediment fluxes in the estuary. Fall et al., 2014 In an estuary, bed stresses are tidally driven and therefore relatively uniform spatially Patterns in SSC align with patterns in bed erodibility

Observations indicate cycles in sediment availability, impact sediment fluxes. While modeling efforts indicate bed erodibility impacts sediment availability. Objective: Examine the change in bed erodibility in the York River estuary over varying time scales: Observational and modeled seasonal variation Observational and modeled spring/neap variation

Observational measurements of erodibility GUST erosion microcosm Applies a known stress to the sediment surface Measures suspended sediment mass Turbidity meters Filtered water sample U-GEMS Manual Version 1.0, Green Eyes LLC

Numerical Methods: Hydrodynamic and sediment transport model Longitude Latitude Regional Ocean Modeling System (ROMS) Resolution (~150m) Forcings River discharge, wind, salinity & tidal height from observations Sediment-induced stratification Bed Consolidation and Swelling Resolution 170 m along-channel Open boundary is set to match observed salinities and open boundary at the river is fresh (salinity) USCG Every grid cell shown represents 5 grid cells

Sediment Bed Model Bed consolidation model adjustment to equilibrium Recent Erosion Deposition More Erodible Less Erodibility defined as amount of sediment mobilized at a shear stress threshold Erosion formula: Critical shear stress varies with depth in the bed, and time, following Sanford (2008): Define critical shear stress: the shear force applied to the bed in which erosion occurs

Model comparison to tidally averaged ADV observations near Clay Bank, York River estuary Fall et al., 2014 Comparable accelerating and decelerating current speeds Matching peak SSC but over estimating over the rest of the tidal phase Slight over estimate of the applied bed stress

Seasonal Erodibility Winter: December – February Spring: March – May Summer: June – August Fall: September – November Seasonal Erodibility Dickhudt et al., 2009

GUST erosion measurements at Clay Bank, York River Years 2011 - 2014 * Shoal GUST erosion measurements at Clay Bank, York River

Contrast Spring Season Summer Season High river discharge (top) More erodible. Summer Season Low river discharge (bottom) More consolidated bed.

Seasonal patterns Higher erodibility corresponding with elevated river discharge Locally at Clay Bank Throughout the York Potential drivers More recent deposition of fine sediment Shift in salinity stratification as observed in the Hudson River (Ralston et al., 2011).

Spring/neap cycle in bed erodibility Days starting from 1 April 2010 Gust eroded mass (kg/m2) at 0.2 Pa Tidal range (m) r = + 0.76 r = - 0.57 (a) (b) Kraatz et al, 2013 cores from spring of 2007 exhibited significantly more eroded mass (p < 0.0001) than those from 2010 Erodibility depended on the where the system was in the spring/neap cycle with the erodibility being highest during spring tides. Correlation with time alone is poor Time and tidal stage explain 94% of the variance Observations correlate best with a 5 day lag

On average higher erodibility during spring tide MODEL summer average for the entire timeseries during spring and neap tides On average higher erodibility during spring tide Channel no difference Values lower than 2010 observations Shoal Channel

MODEL combined daily average bed erodibility for spring and neap tides Shoal Erodibility (kg m-2) Channel Erodibility (kg m-2) Days past the onset of the tidal cycle

MODEL spatial variation in spring/neap daily averaged bed erodibility West Point Gloucester Point Clay Bank Longitude Log10 kg m-2 Latitude Fall et al., 2014 Neap Spring Higher erodibility during spring Higher erodibility in the upper and middle river

Conclusions Bed erodibility patterns from a three-dimensional model of sediment processes in the York River that includes sediment-induced stratification, and bed consolidation and swelling compare well with observational patterns. Both the observations and model results indicate that bed erodibility and therefore sediment availability vary over a range of timescales. At the STM erodibility was higher: In the spring season During spring tides The model also indicates that spatial differences in bed erodibility evolve between the channel and shoal. The sediment bed at the STM develops to be more erodible in the shoal.

Future Work Examine spatial variation in bed erodibility over longer time scales Consider variation of bed erodibility on additional time scales Flood/ebb tidal cycle Interannual Storms Since storms can increase sediment transport flux by a factor of 100 (Gong and Shen, 2009) Consider the addition of other cohesive processes Flocculation

Thank You! Courtesy of Pat Dickhudt

Additional slides

1. Introduction – What is bed erodibility. How do we measure it 1. Introduction – What is bed erodibility? How do we measure it? Why do we care? 2/16 -- Along most “drowned river” coastal plain estuaries, tidal velocity (and thus tidal bed stress) is relatively uniform with distance along the estuary. -- So changes in suspended sediment concentration (C) along estuaries must be due at lowest order mainly to changes in bed erodibility. -- So if you care about sediment concentration (C) in estuaries, you need to care about bed erodibility, which in turn is characterized by M = eroded mass in kg/m2 at a given stress. Bed Stress Bed Erodibility Near-Bed C 7 Jul 2007 7 Jul 2007 7 Jul 2007 10 km 10 km 10 km 00:40 – 01:30 Red = 1 kg/m2 at 0.2 Pa Red = 150 mg/L Red = 0.1 Pa Daily averaged magnitudes from 3-D ROMS model of York River Estuary (Fall et al., 2014) utilizing depth-limited, temporally-evolving cohesive erodibility model of Sanford (2008).

Fall et al., 2014

Highest sediment not correlated with highest stress but high erodibility Fall et al., 2014

Motivation: Sediment availability impacts sediment distribution Indication of possible changes in bed erodibility

Seasonal changes in bed erodibility

Kraatz, 2013

Observations show seasonal and spatial variation in bed erodibility Seasonal difference During some seasons the erodibility was higher than the shoal - - Can cut if needed Dickhudt et al., 2009

Spin up Neap Spring This time period should be after the secondary turbidity maximum has vacated the area Change in water level over a 6 hour period (big=spring tide)

Average higher erodibility during spring tide Shoal Channel Average for the entire timeseries during spring and neap tides Average higher erodibility during spring tide

Erodibility during spring tide remains higher ~6x higher Combined daily average bed erodibility for spring and neap tides Shoal Erodibility (kg m-2) Days past the onset of the tidal cycle Erodibility during spring tide remains higher ~6x higher Observations correlate best with a 5 day lag after the onset of the tide Model shows ~8 day lag for peak erodibility

On average higher erodibility during spring tide MODEL summer average for the entire timeseries during spring and neap tides On average higher erodibility during spring tide Channel: less of a variation Values lower than observations Shoal Channel