anybody have the formula to calculate the basin capacity to store the "first flush" of stormwater?
regrettably, i have not heard of a first flush calc
It must be something in Pennsylvania. New York State has several rain falls we have to deal with. In my area the smallest rain fall is 1.2" and in the Town and Villages I work in it is a 2" rain fall.
stormwater "first flush" calc. (North Carolina)
its actually related to North Carolina. Had a question about it on the exam.
It totally threw me off, the first time I had heard the term myself.
Typically, you have to size the basin such that the post development outflow is less than or equal to the predevelopment outflow. Trying to contain the entire storm event would result in a basin much larger than required. Usually, orifices and weirs in the outflow are used to regulate the outflow so that it maintains a rate less than pre-development.
See if this helps:
Usually that means the first inch of run-off from a storm.
"First Flush" Should Be Defined In The Regs
Most likely in the Best Management Practices Manual. The first flush usually refers to the amount of rainfall defined by the regs that is assumed to pick up and transport all the crap on paved surfaces to the stormwater system, such as oils, antifreeze, brake shoe fines, fecal matter, in essence all the bad stuff. It is a mythically quantitative number, such as the one or two year storm, at a two hour time. Defined as such it can vary across the state or may simply be defined as so many inches of rainfall anywhere in the state. It is mythical in that it can never be ruly detained in a basin due to the requirement to have minimum drainage opening sizes to prevent clogging. Typically to effectively treat that dirty runoff structural, organic or vegetative filters are required.
It may or may not be the same as the stormwater quality design storm which is the lowest level in a retention basin design. In NJ that would be a 1.25"/2 Hr. variable rate rainfall event.
That the number did not pop into your head means you failed to read something that the test takers asked you to study.
Paul in PA
Exactly correct. Some jurisdictions call it the "water quality event". Somewhere between the 1-year and 2-year storm typically.
New York State calls it Water Quality Volume (WQv). In Section 4.2 of the Stormwater Management Manual it does not come close to mentioning "first flush".