[email protected] Annapolis: July 18, 2006 Outline of talk: Objective: Improve BBL in 3D model. Estimates of shear stress. Evaluate bottom boundary layer.

[email protected]: July 18, 2006

Outline of talk:• Objective: Improve BBL in 3D

model.• Estimates of shear stress.• Evaluate bottom boundary layer

model.

Bottom Boundary Layer Bottom Boundary Layer Representation Within Representation Within

Chesapeake Bay ModelsChesapeake Bay Models

Courtney K. HarrisCourtney K. HarrisJ. Paul RinehimerJ. Paul Rinehimer

Department of Physical SciencesVirginia Institute of Marine Sciences

Chesapeake Bay Bathymetry

[email protected]: July 18, 2006

Draft grid of Chesapeake Bay Model from Carl Cerco; December 2003.

• 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

Bott

om

gri

d

cell

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Bottom Boundary Layer Model needed to provide shear stresses to Sediment

Transport Model, and improve those in CH3D

CH3D

Sediment Transport Model

ICM

Trans

port:

flux

es

Transport: fluxes

SSWave Model

Boundary layer model

Wav

e : H

,TC

urre

nt :

Ur

b, b

’

?

Turb

ulen

ce:

,

?

?

•Size classes:•Sand•Silt-clay•Clay-colloid

•Settling velocities•Erosion•Active bed•Flocculation (?)

C, N

, P

Figure by S.C. Kim, USACE

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CH3D

Sediment Transport Model

ICM

Trans

port:

flux

es

Transport: fluxes

SSWave Model

Boundary layer model

Wav

e : H

,TC

urre

nt :

Ur

b, b

’

?

Turb

ulen

ce:

,

?

?

•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

[email protected]: 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

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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.

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Calculate Shear Stresses

•Ran Wiberg model for a range of conditions.

•Generated lookup tables:

(sf) = function(u0, T, u75, θ, d50)

b = function(u0, T, u75, θ, d50)

•Used two roughness (z0) parameterizations.

•Lookup tables in FORTRAN and matlab format.

•Provided FORTRAN lookup table routines to S.-C. Kim.

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[Dynes/cm2]

Tidally Dominated Conditions: June, 1999.

Arrows are winds.

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Shear Stress has a lot of spatial variability

Non-storm Conditions: June, 1999.

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/cm2]

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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. 1997.

– Implementation of lookup table for BITMAX site (data provided by Suttles and Sanford, UMCES).

Comparison between modeled and observed

bed shear stresses

wa

ter

de

pthL

at

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Available Data: Wright, Schaffner, and

Maa, 1997.

CherrystoneFlats Site

Wolftrap Site

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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 FloydDay, 1999 Day, 1999 Hurricane Floyd

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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.

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BITMAX Data

ETM:Six Deployments: May, July, October 2001 – 2002.12 m deepBBL SWATT tripod

Data and figures courtesy of Sanford and Suttles, UMCES

Shear velocity estimated from velocity covariance (<u’w’>).

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• Shear stress better estimated with – high z0 in May, 2001 (and October, 2001; May and July, 2002.)– ………..

• Estimates of z0 show much less variance than observed.• High z0 estimates are more in line with values used for other studies.

BITMAX Site: Higher Roughness Parameterization Accurate

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BITMAX Site: Higher Roughness Parameterization Accurate Most of

the Time

• Shear stress better estimated with – high z0 in May, 2001 (and October, 2001; May and July, 2002.)– lower z0 in July, 2001 (and October, 2002).

• Estimates of z0 show much less variance than observed.• High z0 estimates are more in line with values used for other studies.

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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.

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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, 2005.

•Provided lookup table in July, 2006.

Dynes/cm2 Dynes/cm2

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Conclusions

Dynes/cm2 Dynes/cm2

• 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.

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Rejected Slides

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[Dynes/cm2]

Storm Conditions: June, 1999.

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BITMAX Site: Higher Roughness Parameterization Accurate Most of

the Time

• Shear stress better estimated with – high z0 in May, 2001 (and October, 2001; May and July, 2002.)– lower z0 in July, 2001 (and October, 2002).

• Estimates of z0 show much less variance than observed.• High z0 estimates are more in line with values used for other studies.

[email protected]: July 18, 2006

• Shear stress better estimated with – high z0 in May, 2001 (and October, 2001; May and July, 2002.)– lower z0 in July, 2001 (and October, 2002).

• Estimates of z0 show much less variance than observed.• High z0 estimates are more in line with values used for other studies.

BITMAX Site: Higher Roughness Parameterization Accurate Most of

the Time

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Percent of Time thatCurrents Dominate Shear Stress