detention volume Calculation

You'll need to ask this question of others. Specifically, you need to ask the agency which is reviewing your calculations. Every agency does things differently and most rely upon local, arbitrary regulations. Most will allow you to include pipe and CD volumes but ther is NO general, scientific answer to your question.

Good luck

The storage volume of pipes and other appurtenances are physically used by storm water as it moves in the system. This is usually modelled in a sewer routing computer routine, which takes into account the slope of the hydraulic gradient. This is a similar to flood routing in a river reach. For the calculation of runoff to obtain the hydrograph for routing, you can assume free flow in the pipes.

If you're modelling the system storage based on simplified methods that assume that the velocity is very small and the hydraulic gradient is "flat" (stage-storage-discharge curves), such as those used to model ponds, you can include the volume of the pipes and appurtenances as you compute the stage-storage-discharge curves. When you do the routing calculation, the storage in the pipes et al will be already taken into accounted.

If the pipes are set low enough to be used for detention storage, you must calculate their hydraulic capacity using backwater calculations to determine the hydraulic and energy grade lines in the pipe system; most likely some of the pipes will flow full under pressure. The high water levels in the pond will give you the starting water levels for your calculations. You cannot assume free-surface flow for the entire system.

After all this, the short answer is yes, you must carry out the calculations assuming pressure flow.

Thank you guys for these responses, give me enough leads....