Help understanding a theory

[wesman07]

Hello,

I am working on the stormwater management design for my home. My county requirements are not real clear and the people I talk to give me conflicting information. I'm hoping someone here can explain things a little differently. The requirements I can find are:

Retain the first 1/2" of water from all impervious surfaces
Detain the delta of pre and post construction for a 25 year storm event based on time of concentration
Detain the delta of pre an post construction for a 25 year/ 24 hour storm event.

What is hanging me up is the 24year/25 hour storm event. The volume needed is coming out to 17,000 cubic feet where the other two calculation are roughly 700 cubic feet. Does "detention" mean temporarily hold or collect and pass through?

 

[MintJulep]

Do the requirements you're working with offer a contextual definition of "detain"?

Does "detention" mean temporarily hold or collect and pass through

Those mean the same thing to me.

You need a defined time.

Probably you also need an allowed discharge rate.

 

[wesman07]

Do the requirements you're working with offer a contextual definition of "detain"?



Those mean the same thing to me.

You need a defined time.

Probably you also need an allowed discharge rate.

No, not of specifically of detain. This is the best definition I could find

12-804: DEFINITIONS - D:

DESIGN STORM:

A rainfall event of specific return frequency and duration which is used to calculate stormwater runoff volume and peak discharge rates. For the purposes of this title, the “design storms” are:

A. 25-year design storm shall mean a rainfall event of one-tenth inch (0.1") per hour for a twenty four (24) hour duration;

B. 100-year design storm shall mean a rainfall event of one hundred twenty five thousandths inch (0.125") per hour for a twenty four (24) hour duration.

 

[MintJulep]

based on time of concentration

What's that mean?

 

[wesman07]

The time it takes for water to flow from the furthest point to your catchment area. Distance, slope of grade and type of vegetation all are factors in that equation.

“Tc” determines the peak flow of using the rational method.

 

[wesman07]

What's that mean?

Forgot to reply.

 

[zelgar]

Based on the rules, it appears that this is for Bonner County, ID: link
SUBCHAPTER 7.2 - GRADING, STORMWATER MANAGEMENT AND EROSION CONTROL: link
I'd make certain that this rule is applicable to your case looking at 12-720.2: APPLICABILITY: and 12-720.3: ACTIVITIES TO WHICH THIS SUBCHAPTER IS NOT APPLICABLE:

If it is, from what limited amount of reading is that you need to make certain that your future activities are not contributing more run-off from prior to the construction from what I'm reading in 12-726: PERFORMANCE STANDARDS:
D. For new subdivisions, planned unit developments, commercial and industrial development and public projects, as set forth at section 12-720.2 of this subchapter, there shall be no increase in the peak rate of runoff from the site when compared with the dissipation of stormwater from the site prior to the start of any construction, for the first one-half inch (1/2") of runoff. Within project boundaries, sufficient retention capacity shall be constructed to retain stormwater flow from the first one-half inch (1/2").