Annapolis: July 18, 2006 Outline of talk: Objective: Improve BBL in 3D model. Estimates of shear stress. Evaluate bottom boundary layer model. Bottom Boundary Layer Representation Within Chesapeake Bay Models Courtney K. Harris J. Paul Rinehimer Bottom Boundary Layer Representation Within Chesapeake Bay Models Courtney K. Harris J. Paul Rinehimer Department of Physical Sciences Virginia Institute of Marine Sciences Chesapeake Bay Bathymetry

Annapolis: July 18, 2006 Draft grid of Chesapeake Bay Model from Carl Cerco; December Three-d models can estimate near-bed current. Can be linked to sediment models. Three-d models rarely have sufficient vertical resolution to resolve near-bed gradients. Objective: Improve representation of bottom boundary layer within Chesapeake Bay Model Bottom grid cell

Annapolis: July 18, 2006 Bottom Boundary Layer Model needed to provide shear stresses to Sediment Transport Model, and improve those in CH3D CH3D Sediment Transport Model ICM Transport: fluxes SS Wave Model Boundary layer model Wave : H,T Current : U r  b,  b ’ ? Turbulence:,  ? ? Size classes: Sand Silt-clay Clay-colloid Settling velocities Erosion Active bed Flocculation (?) C, N, P Figure by S.C. Kim, USACE

Annapolis: July 18, 2006 CH3D Sediment Transport Model ICM Transport: fluxes SS Wave Model Boundary layer model Wave : H,T Current : U r  b,  b ’ ? Turbulence:,  ? ? Size classes: Sand Silt-clay Clay-colloid Settling velocities Erosion Active bed Flocculation (?) C, N, P Figure by S.C. Kim, USACE Wave / current interaction model, coupled to movable bed roughness. Provide shear stress to sediment transport model. Bottom Boundary Layer Model needed to provide shear stresses to Sediment Transport Model, and improve those in CH3D

Annapolis: July 18, 2006 Characterize Roughness, Waves, and Currents Estimates of currents (CH3D) and waves (Young and Verhagen 1996); both provided by S.-C. Kim (USACE). Bed roughness estimated from mean grain size and hydrodynamic conditions. Use 1999 as a case study and to generate lookup table. Hurricane Floyd cm/s

Annapolis: July 18, 2006 Used Calendar Year 1999 as Case Study (c) (d) Shear stresses estimated for three sites. Spatial variability in shear velocities hard to predict. Time of storm, moderate and hurricane Floyd conditions used later.

Annapolis: July 18, 2006 Calculate Shear Stresses Ran Wiberg model for a range of conditions. Generated lookup tables:  (sf) = function(u 0, T, u 75, θ, d 50 )  b = function(u 0, T, u 75, θ, d 50 ) Used two roughness (z 0 ) parameterizations. Lookup tables in FORTRAN and matlab format. Provided FORTRAN lookup table routines to S.-C. Kim.

Annapolis: July 18, 2006 [Dynes/cm 2 ] Tidally Dominated Conditions: June, Arrows are winds.

Annapolis: July 18, 2006 Shear Stress has a lot of spatial variability Non-storm Conditions: June, Left: Currents near bed from CH3D Middle: Wave orbital velocities using waves from S.-C. Kim. Right: Bed shear stress (skin friction) from Wiberg model.. [Dynes/cm 2 ]

Annapolis: July 18, 2006 Evauluate Bottom Boundary Layer Model Using: –Full wave and current models for 1999 for Wolftrap and Cherrystone Flats site as reported by Wright, et al –Implementation of lookup table for BITMAX site (data provided by Suttles and Sanford, UMCES). Comparison between modeled and observed bed shear stresses water depth Lat

Annapolis: July 18, 2006 Available Data: Wright, Schaffner, and Maa, Cherrystone Flats Site Wolftrap Site

Annapolis: July 18, 2006  Wright, et al. (1997) say Cherrystone Flats is more energetic than Wolftrap, in terms of waves and tidal currents.  Modeled waves, however, are more energetic at Wolftrap than at Cherrystone Flats.  Estimates of shear stress sensitive to roughness height. Hurricane Floyd Day, 1999 Hurricane Floyd

Annapolis: July 18, 2006 Neither roughness formulation does well at both sites. Shear stress calculated with a high roughness do better at the Cherrystone Flats site. Shear stresses calculated with a (very) low roughness do better at the Wolftrap site. High roughness is similar to values used at other locations.

Annapolis: July 18, 2006 BITMAX Data ETM: Six Deployments: May, July, October 2001 – m deep BBL SWATT tripod Data and figures courtesy of Sanford and Suttles, UMCES Shear velocity estimated from velocity covariance ( ).

Annapolis: July 18, 2006 Shear stress better estimated with –high z 0 in May, 2001 (and October, 2001; May and July, 2002.) –……….. Estimates of z 0 show much less variance than observed. High z 0 estimates are more in line with values used for other studies. BITMAX Site: Higher Roughness Parameterization Accurate

Annapolis: July 18, 2006 BITMAX Site: Higher Roughness Parameterization Accurate Most of the Time Shear stress better estimated with –high z 0 in May, 2001 (and October, 2001; May and July, 2002.) –lower z 0 in July, 2001 (and October, 2002). Estimates of z 0 show much less variance than observed. High z 0 estimates are more in line with values used for other studies.

Annapolis: July 18, 2006 Considerable Scatter for Modeled vs. Measured shear stress at BITMAX sites. Recommend using the higher roughness parameterization: 1.Does better for peak conditions. 2.Consistent with formulation used at other sites.

Annapolis: July 18, 2006 Products and Deliverables Shear stress calculations: –Skin friction shear stress for sediment transport model. –Total shear stress for hydrodynamic model. Provided 1999 values of skin friction shear stress to S.-C. Kim in December, Provided lookup table in July, Dynes/cm 2

Annapolis: July 18, 2006 Conclusions Dynes/cm 2 Modeled shear stresses compare well to much of the available data when a standard roughness parameterization is used. A full model validation is difficult: Requires near-bed (<1m) measurements of suspended sediments, salinity. Bottom boundary layer model should include stratification from both sediments and salinity gradients. A wave model that better represents Chesapeake Bay mouth might be important.

Annapolis: July 18, 2006 Rejected Slides

Annapolis: July 18, 2006 [Dynes/cm 2 ] Storm Conditions: June, 1999.

Annapolis: July 18, 2006 BITMAX Site: Higher Roughness Parameterization Accurate Most of the Time Shear stress better estimated with –high z 0 in May, 2001 (and October, 2001; May and July, 2002.) –lower z 0 in July, 2001 (and October, 2002). Estimates of z 0 show much less variance than observed. High z 0 estimates are more in line with values used for other studies.

Annapolis: July 18, 2006 Shear stress better estimated with –high z 0 in May, 2001 (and October, 2001; May and July, 2002.) –lower z 0 in July, 2001 (and October, 2002). Estimates of z 0 show much less variance than observed. High z 0 estimates are more in line with values used for other studies. BITMAX Site: Higher Roughness Parameterization Accurate Most of the Time

Annapolis: July 18, 2006 Percent of Time that Currents Dominate Shear Stress