Thoma area

D,Thoma

Thoma condition represented by the Thoma surge shaft area

Fth       = L * Q² / (2*g*Hb*A*dH)
 T         = 2 π*((F*Hb*L)/ (A*g*(Hb-2*dH))) ^0.5

 dH =  head loss at flow Q in tunnel with area A. Hb is gross head.  Surge shaft area is F and T is peak to peak time.
T is also influenced of pressure wave propagation speed. An increase must be considered.