The first assumption is that the channel is much wider than it is deep, and the equations can be solved as if the channel were infinitely wide. This means that side-wall effects can be ignored, and that the hydraulic radius, , can be assumed to be equal to the channel depth, .
where  is the cross sectional area of flow and  is wetted perimeter. For a semicircular channel, the hydraulic radius would simply be the true radius.
For an approximately rectangular channel (for simplicity in the mathematics of the explanation of the assumption),
where  is the width (breadth) of the channel, and
For b>>h,
and therefore
Formally, this assumption can generally be held to hold when the width is greater than about 20 times the height; the exact amount of error accrued can be found by comparing the height to the hydraulic radius. For channels with a lower width-to-depth ratio, a better solution can be found by using the hydraulic radius instead of the above simplification.