500 years Hydrograph at downstream of highly regulated catchment

[Allen1362]

What is the best approach for producing a hydrograph with 100 and 500 year return period for a highly regulated catchment? I have a gauging station with 50 years data at the downstream of the catchment . The max flood in this period has just 40 year return period. I don’t want to use a rainfall run off model because I do not have enough information about the reservoirs within the catchment. Is it possible to build 100 and 500 years hydrographs from available time series? . The only method which I have found in text books is unit hydrograph method which is very simple and I am sure it is not enough accurate for this study. Any idea?
cheers