water depth in calculating wave orbital velocity

[mapeifeng]

In the BBL models, the still water depth h is used to compute wave bottom orbital velocity if *_CALC_UB is defined such as that of ssw_bbl below. I think total water depth h+zeta should be a more appropriate one to be used here as in shallow waters the surface elevation can change water depth substantially and therefore cause quite big difference to wave bottom stress which is proportional to Ub². For example, in a typical condition of h=10m, Pwave=8s, Hwave=1m, the value of Ub² can be increased by about 17% if water depth is dropped by 1m and by 39% if it is dropped by 2m.

______________________________________________________________________________
#ifdef SSW_CALC_UB
Kdh=h(i,j)*Fwave_bot(i,j)**2/g
Kbh2=Kdh*Kdh+ &
& Kdh/(1.0_r8+Kdh*(K1+Kdh*(K2+Kdh*(K3+Kdh*(K4+ &
& Kdh*(K5+K6*Kdh))))))
Kbh=SQRT(Kbh2)
Ab(i,j)=0.5_r8*Hwave(i,j)/SINH(Kbh)+eps
Ub(i,j)=Fwave_bot(i,j)*Ab(i,j)+eps
#else
_______________________________________________________________________________

[jcwarner]

when you start to get into the shallow water areas, these approximations may not be that appropriate. If you want a better solution, use the roms+swan coupling, and allow the wave model to get a more accurate representation of the bottom orbital motions.

if you want to add zeta to h, it is not that big of a deal. but you would have to bring zeta into the bbl. i dont think they use it right now.
-john

[nganju]

I ran into this problem a few years ago and added in zeta...but to be consistent you should really be updating the wave field (height, period, dir) as water level (zeta+h) changes. Which is why a better option would be ROMS-SWAN coupling. If you don't have the computational power to do that, you could alternatively run SWAN in stationary mode under different water level cases, and see where the water level changes do spatially. This all depends on what you're actually trying to do (simulate actual morphological change, or just sediment concentrations).

[mapeifeng]

Thanks a lot for the comments. I agree that SWAN should be coupled in order to predict more accurate wave characteristics and the neglect of "zeta" in the simple formulation usually makes minor differences although adding "zeta" should make the calculation more physically consistent as far as water depth is concerned.