effective mass coefficient in liquid-containing concrete structures in ACI 350.3

[Rainbowtrout]

I am designing an underground pump station in OK which means seismic forces are small but existent. I am following ACI 350.3-06. In section 9.6, effective mass coefficient is defined and is a factor used to calculate concrete wall's inertia forces. I am having trouble understanding what it is, how that equation is derived, and why it changes with L and H as it is.

I tried to track down the two references but couldn't locate either even in the local academic library. I am hoping someone in the forum will shed a light.

The equation is
epsilon = 0.0151*(L/H)^2-0.1908*(L/H)+1.021

Thanks!