In broad shallow areas with depths less than about 2 m, wind-waves most often are

in the transition range where wave length Lw is greater than twice the local depth D

and wave bed friction affects wave growth and depth-limited wave conditions.

Atmospheric drag coefficients are also affected since they depend on wave

conditions, and since:

τ a = ρ a Cd U 2

where

=

air density

ρa

Cd

=

atmospheric drag coefficient

Ua

=

wind speed corrected to 10 m reference height

atmospheric shear stress is affected as well. It was found in Laguna Madre Texas that

atmospheric drag coefficients can be approximated by:

2

⎛

⎞

0.4

⎟

⎜ 16.11 - 0.5 ln( *D*' ) - 2.48 ln(*U*

⎟

⎝

⎠

in water depths less than 2 m, where D' is an effective depth. Wave height and period

were found to be related to Cd and D' at this site, and D' was found to be the water

depth or depth to the vegetation canopy top.

A momentum balance approach is used to estimate wind-wave shear stress, since the

uncertainties in specifying widely varying wave friction factors makes it possible to

over estimate momentum transfer. Wind-waves are assumed to be fully-developed

with dissipation equal to momentum input from the atmosphere. Atmospheric shear

stress is partitioned between currents and waves. In deep water, dissipation by white

capping and wave-wave interaction leads to most atmospheric momentum eventually

going into the currents. In ultra-shallow water the shear stress going into bed by

wave action is a fraction of the input such that:

τ wb = 5.38 -0.5 , for U > 5 m/sec

a

τa

When RMA2 has used the marsh porosity option (DM cards) shear stresses should

be adjusted in the SED2D WES simulation for more accurate estimates of the bed

exchange. Therefore, the marsh porosity information must be provided (in

appropriate units) and the program will compute the needed adjustments. The

adjustment is made by computing a conveyance distribution within the marsh

porosity depth distribution based on Manning's equation. This is then extended to a

shear stress distribution that is averaged and a correction factor developed for the

conventionally derived shear stress from one of the options above

10

Title

Integrated Publishing, Inc. |