Conference Call Summary Three solids classes –Clay settles at 5 m/d –Silt settles at 50 m/d –Sand settles at 1000 m/d Major recommendation was use formulae for erosion and critical shear stress from Sanford and Maa (2001) for clay and silt.

Conference Call Summary For erosion of sand, look at Courtney and Weber, or the Van Rijn formulation. Mud erosion rate kg / sq m / s / P Bed should have a number of thin layers (0.1 cm) on top of a thick layer (1 m)

Sanford Model of Stress-Limited Erosion Rate (same formulation applied to analyze data) Sanford Model of Stress-Limited Erosion Rate (same formulation applied to analyze data)

Near surface profiles, comparing sandy and muddy cores Heavy black line:

Full profiles of M(m), comparing sandy and muddy cores Blue lines in left panel are from Sea Carousel Measurements in Baltimore Harbor

The Big Question These relationships were derived for the Sea Carousel. Short-term experiments (4 hours) under constantly increasing shear stress. Parameter “m” is the total eroded mass (kg/sq m). As m increases, critical shear stress for erosion increases. The function simulates bed consolidation.

The Big Question The problem is to apply these relationships for long-term (years) under variable shear stress. The big question is when do we initialize m? For how long do we keep track of eroded mass? From the initial application of stress (beginning of model run)? At the beginning of each erosion event (tidal cycle, wind event)?

The Big Question A second, related question involves deposition. We are using constant deposition. Does the amount deposited count in the summation of m?

It Makes a Big Difference m initialized at each event m initialized once

An Experiment Work with the 30-box model Impose constant shear stress, continuous deposition For one case, keep a running total of m For the second case, declare an event terminated whenever instantaneous deposition exceeds erosion

The 30-Box Test Grid

Sensitivity to Erosion Rate, M Constant shear stress Continuous deposition m initialized once Three erosion rates: 1, 2, 5 g/sq m/s/P

1 g/m2/s/P 5 g/m2/s/P 2 g/m2/s/P

Sensitivity to Settling Rate Sinusoidal shear velocity Continuous deposition m initialized once Three settling rates: 1, 2, 3 m/d Three settling rates: 12, 23, 35 u/s

1 m/d 2 m/d 3 m/d

Sensitivity to Parameters in Critical Shear Stress Formula Sinusoidal shear velocity Continuous deposition m initialized once Exp Exp Exp Exp Exp

Tau_c = … Tau_c = … Tau_c = 1.37 …

Tau_c = … Tau_c = … Tau_c = …

Sensitivity to Shear Velocity Sinusoidal shear velocity Continuous deposition m initialized once Three maximum velocities: 1, 2, 3 cm/s

U* = 2 cm/s U* = 1 cm/s U* = 0.5 cm/s

Simulated Sea Carousel

Sensitivity with Transport Turn on transport in 30-box model. Two- year run duration. All parameters at recommended values Sinusoidal shear velocity Continuous deposition m initialized once Sensitivity to Ustar, Tau_crit, Erosion Rate, Settling Velocity

Double maximum U* from 1 to 2 cm/s

Halve Tau_crit parm from to 0.457

Increase erosion rate from 2 to 5 g/sq cm/ s/P

Order of magnitude reduction in settling rates

Conclusions Major sensitivity to: –Initiation of m. Once or at each event? –Bottom shear velocity –First parameter in critical shear stress formula Lesser sensitivity to: –Maximum erosion rate –Settling velocities