One of the technical aspects of analysing flooding is 'time of concentration'. Basically, if a drop of rain falls at the furthest point of the catchment, how long does it take to flow to the point you are interested in? Repeat that process at every location within the catchment and work out when the greatest volume of water occurs at the point you are interested in, but it isn't that simple.
You need to consider how rain actually falls. Storms do not drop all their rain everywhere at the same time and at the same intensity throughout the storm. Intensity varies depending on duration. Typically, storms start out with heavy rain which tails off in some way. Short storms can deliver rain faster than long storms; think cloudburst vs. drizzle. You usually do this analysis from records of actual storms.
If the storm moves from the far end of the catchment towards the point you are interested in, the accumulated water arrives in a shorter period than if the storm moved from the point you are interested in towards the far end of the catchment. A shorter period will usually worsen flooding.
You calibrate your catchment model by comparing historical storm events with recorded flood levels, adjusted for changes in runoff and water detention (more concrete/ asphalt, deforestation, etc.) over time.