Actually, if I understand the question, weir flow wouldn't apply to this situation. For situations where a broad-creasted weir exists, if the flow depth divided by the length of the broad-crest (measured in the direction of flow) is less than 0.08, then flow over the weir crest is subcritical...
CarlB is right in that there are a lot of statistics involved in determining the X-year rainfall event. Although the largest rainfall event in the past may have a bearing on the 100-yr event, there is no real correlation, partly because there are few, if any, events that are exactly 24 hours in...
