This is a very basic question, but why is it that as time of concentration for a watershed goes up, peak flow goes down?
The simple answer is just that you have the same relative volume of water.
short tc – the rate at which the water leaves the basin is very high.
long tc – there is a longer period of time that the water is leaving the basin, hence lower peak.
As the tc is the time for the most hydraulically distant part of the watershed to reach the basin outlet point then with a longer tc the lower parts of the basin are all contributing water as that most hydraulically distant point winds its way down. Try four or five different tc’s, say 5, 10, 30 minutes, etc., and while your peak rate will vary substantially your volume will probably stay within 5%.
Detention can have high unintended consequences if the entire receiving water (stream) is not carefully considered. You really need to evaluate what’s downstream from you. What does this channel empty into? What are you actually needing to design for? You may have a regulatory requirement to detain to a pre-development peak (or some other parameter), and you can fairly easily do that, but by doing so you might be creating a whole other set of problems. And these would be problems that you’re not mandated to check by the reviewing authority you have to satisfy.
If you have a lake right below you it might not make much difference (detention-wise). But if there is another development right below you then you need to evaluate a larger range of events than the reviewer might require to see if your detention is having some other adverse effect.
There’s a school of thought that says staged detention with some form of water quality treatment included is the way to go. Another puts higher stock in the water quality. Then there’s the client (developer). They just want you to meet whatever the minimum requirements are that the regulating authority is looking for. And there are plenty of engineers that will do that and feel they are meeting their obligations just fine.
Above is a snippet.