How many gallons of new stormwater runoff will result from the Sembler MEGA mall? The short answer is that at this point, from a technical perspective, that there's insufficient information to know.
We DO know from information that Sembler initially submitted, and then withdrew, that they expect that more than 70% of their site will be impervious. That's HUGE! They're saying that 70% of the rainfall that falls on their site will run off.
We know that the average rainfall in this area is approximately 52 inches per year. So why can't the volume of stormwater runoff that will reach the Okatie be calculated? 52 inches times the area of the site times 70%? It's a relatively simple calculation.
The volume of runoff that will reach the Okatie can't be calculated at this point for several reasons. One of the biggest reasons is that we don't know at this point how stormwater runoff from the site will be handled. Will it all go to the Okatie? Or will some of it go to the New River (where SCHH's stormwater runoff goes)? Also, how will on-site storage be handled? Will it be handled in wet ponds (lagoons) like SCHH's stormwater is handled?
The average 52 inches rainfall per year comes from MANY rainfall events. If wet ponds are used for storage, then runoff from some of those events will be totally contained in the storage ponds. None will be released to the Okatie or the New River.
Predicting the volume of stormwater runoff that results from a developed site is a relatively complex process that involves mathematical modeling. It should be left to experts.