Stormwater Management for Residential Pool 1

[MrMatthew]

Hi all,

I'm with a small company and we've recently started taking on jobs for residentail, below-grade pool installations in Florida. We're getting a lot of comments back on our first few submissions related to Stormwater/Runoff Management and I'm struggling to understand what they want, find example calculations, etc.

For example, we received this comment:

"The adopted level of service for drainage is to retain the first inch of rainfall on-site; post-development runoff shall not exceed predevelopment runoff rate for a 25-year storm event, up to and including an event with a 24-hour duration."

I understand that a pool installation reduces the pervious area in the lot. We have a swale calculation showing that the swale designed in the project is adequate to handle 1" of runoff from the newly impervious installed area - what is the point of explicitly requiring that calculation if we're also required to show retention for 11" of rain (25 year event for the area)?

Is it as simple as sizing a swale for 11" of water for the newly impervious area? Is it appropriate to reduce that rainfall value by the ~4" of gap between pool water level and deck level? That's free water retention.

Any help or resources would be appreciated. Thank you

 

[CarlB]

Better chance of responses if posted in Storm/Flood forum.

This runoff/treatment analysis is commonly performed by Civil Engineers.
Typically a project would not have to include any stormwater improvements (say swales, infiltration systems) if it can be shown the improvements do not increase runoff, or in some jurisdictions beyond some threshold value.
A swimming pool, along with deck, walkways, etc, will cause an increase in runoff for areas converted from grass/soil to hardscape, but decrease runoff for the portions including the pool, and areas that drain into the pool, since rainfall on those portions will be captured. If captured runoff is later pumped out onto the surface, it would not be considered reducing volume of runoff, but would decrease rate of runoff.

The requirement to capture/treat 1" runoff means just that-capture runoff in swales, depression, infiltration gallery, etc, such that it does not run off the property.

The requirement for larger storms (e.g. 25-yr event) is to somehow retain/detain runoff such that the peak rate of runoff post-development does not exceed the pre-development rate. The runoff rate would increase if the site's overall runoff coefficient (C-value) has been increased by the project. Detention methods involve capturing runoff, and slowly releasing it after peak of storm has passed.

 

[MrMatthew]

The C-Factor was exactly what I was looking for. I was looking for a factor that accounted for some amount of retention on site, but couldn't and so assumed that the 25-year values were the runoff to design for -

Nope! Found a state design guide with recommended C-Factors and additional scalars for increasingly severe rain events (25- & 100-year).

I love standardized design factors.