Equilibrium detention storage is an important parameter as it has a proportional effect on flood attenuation. In this paper, based on the kinematic wave theory, a working formula for the equilibrium detention storage of an overland plane with upstream inflow has been derived. Since the flow regime over a concrete plane can vary throughout the entire range laminar to turbulent, this case has been selected to examine the effect of flow regime on the equilibrium detention storage. In the examination, the derived formula has been applied to four flow regimes: (a) laminar, (b) transitional, (c) near turbulent, and (d) turbulent. The examination shows that for planes with a small discharge, laminar flow gives the maximum detention storage. For planes with a medium discharge, transitional flow gives the maximum detention storage, and for planes with a large discharge, near turbulent flow gives the maximum detention storage. The flow regime can cause more than two-fold increase in detention storage. All these results can be attributed to the respective flow resistance, and have been endorsed with analyses of the water surface profile and the rising limb of the hydrograph. Finally, relating the results to real-life situations, it shows that the flow regime that gives the maximum detention storage is also the dominating flow regime in nature. Hence, extraordinarily, the flow regimes that exist in nature in fact provide maximum flood attenuation. Copyright © 2008 John Wiley & Sons, Ltd.