Ask yourself what would happen if the main control device at the pond becomes clogged. In these parts we often size for undetained flow so that if the facility is not working as intended and goes into overflow it will not blow out downstream conveyances.
depends on your design criteria. you may design an outlet to handle the 100-year flow. If the outlet gets clogged or the storm is larger than what you used for design, overflow would occur at your emergency spillway. This is often just a low spot in the embankment or channel which outlets to a relatively safe area, ie. not directly at somebody's front door but into a public right of way such as a street.
I say case specific.
If development is humming in the area post developed for the drainage plan is normally good. But if you may be waiting for upstream improvements make sure you check on what happens with the emergency spillway. In the country spillways tend to have unimpeded outlets, but your spillway may have a major road or neighborhood there now or in the future. In this case your 100 or less frequent spillway could harm people and your only defense is that you hoped upstream improvements would have been built quicker.(especially if your basin and improvements coveredup a natural drainageway)
You see this in some detention basins or flood control dams where orifices are added after upstream improvements go in. It is also pretty common for large upstream tracts to get opened up making for an interim condition that is pretty bad for runnoff events.
