Because the flowrate over an ungated ogee spillway depends on the net head of the water conveyed in the approach channel and because the energy losses depend on the flowrate, computation of the spillway discharge for a given gross head of water in the reservoir entering the approach channel necessitates a trial-and-error scheme. For given information of (a) the elevations of the lake water surface and of the spillway apex, (b) the energy loss coefficient at the entrance, the side wall inclination, the length, and the roughness coefficient of the trapezoidal approach channel, an iterative method for computing the discharge over an ungated ogee spillway is presented. Next, for given information of (a) the (volume) ↔ (water surface elevation) relationship of the reservoir, (b) the spillway apex elevation, (c) the maximum allowed lake water surface elevation, and (d) the design flood hydrograph, an iterative method for computing both the length of the ungated ogee spillway and the outflow hydrograph simultaneously such that the maximum water surface elevation reached during routing of the design flood hydrograph becomes equal to the maximum allowed elevation is presented. Matching of the maximum water surface elevation reached in the reservoir while routing of the design flood hydrograph to the pre-specified maximum lake elevation requires a trial-and-error scheme of reservoir routing computations over many different-length spillways. The iterative method presented in this study which is executed in a single run of the coded computer program is a short-cut alternative to the long approach. The developed method is applied to Catalan Dam, which is one of the large dams in Türkiye from reservoir capacity, flood attenuation, and hydroelectricity production aspects, as a case study. The length of an ungated spillway is computed by the method presented here as an alternative to the existing radial-gated spillway, and the reservoir routing computations are done with the design flood hydrograph given in its final project by both the ungated and the gated spillways, and the maximum lake water surface elevations and the outflow hydrographs are found to be fairly close to each other.