When constructing a synthetic design storm using the alternating block method, what is the primary goal in placing the largest intensity block at the center of the storm?

A synthetic design storm is a hypothetical rainfall event constructed for hydrological design purposes, rather than being an observed historical storm. It is typically derived from statistical rainfall data, such as Intensity-Duration-Frequency (IDF) curves, which relate rainfall intensity to duration and return period. The alternating block method is a common technique used to construct the hyetograph, which is a graphical representation of rainfall intensity over time, for such a synthetic storm. This method involves disaggregating rainfall depths obtained from IDF curves into a series of constant-intensity rainfall blocks over the total storm duration. Each block represents a discrete time interval with a specific rainfall intensity. The largest intensity block refers to the time interval within the storm that experiences the highest rainfall rate. The primary goal of placing this largest intensity block at the center of the storm duration is to produce the maximum plausible peak runoff from a watershed for the given storm frequency and total duration. This central placement is critical because it simulates the most hydraulically efficient conditions for peak flow generation. By having the highest intensity rainfall occur in the middle of the storm, the ground surface and soil are already wetted by the preceding, lower intensity rainfall blocks. This pre-wetting significantly reduces initial abstractions, which are the initial losses of rainfall due to processes like infiltration into dry soil, evaporation, and filling of surface depressions. With reduced initial losses, a greater proportion of the peak intensity rainfall becomes effective runoff. Furthermore, centering the peak intensity block allows the watershed's hydrological response, represented by the hydrograph (a graph of discharge over time), to fully develop. The initial rainfall primes the watershed, establishing flow paths and increasing soil moisture. By the time the peak intensity block occurs, the watershed is in a state of heightened responsiveness, with water already contributing to flow from various parts of the catchment. This timing ensures that the maximum rainfall intensity coincides with a watershed that is already generating significant runoff, thus maximizing the peak discharge rate. If the peak intensity were at the beginning, more rainfall would be absorbed by initially dry ground. If it were at the end, the full watershed response and rising limb of the hydrograph might not develop before the storm concludes. Therefore, central placement ensures the maximum hydraulic loading occurs when the watershed is most efficient at converting rainfall into runoff, resulting in the highest design peak flow.