A channel has a rectangular cross section, a width of \(40 \mathrm{~m}\), and a flow rate of \(4000 \mathrm{~m}^{3} / \mathrm{s}\). The normal water depth is \(20 \mathrm{~m}\). The flow then encounters a 4.0-m-high dam. Find the water depth directly above the dam if the flow is critical. Assume frictionless flow.