Rain on snow event
Rain on snow event
(OP)
Does anyone know of any data or even SWAGs for the amount of runoff you get when it rains on snow? For example,
Does the rain melt the snow and double or triple the amount of runoff in the same time of concentration?
Or does the rain melt the snow but the time of concentration increase because of a delay caused by the rain being absorbed at first and then the melting occuring over a longer period?
Is there a SWAG factor for ambient temperature?
Or, does a Patty Melt melt quicker if...? Mind boggling the number of possibilities.
Unfortunately for the client the size of the retention facility may get to be unneccesarily large and therefore expensive if there really isn't an answer yet. I understand this area is still being researched.
Any help would be greatly appreciated.
DPA
Does the rain melt the snow and double or triple the amount of runoff in the same time of concentration?
Or does the rain melt the snow but the time of concentration increase because of a delay caused by the rain being absorbed at first and then the melting occuring over a longer period?
Is there a SWAG factor for ambient temperature?
Or, does a Patty Melt melt quicker if...? Mind boggling the number of possibilities.
Unfortunately for the client the size of the retention facility may get to be unneccesarily large and therefore expensive if there really isn't an answer yet. I understand this area is still being researched.
Any help would be greatly appreciated.
DPA





RE: Rain on snow event
RE: Rain on snow event
Thanks for the quick response
DPA
RE: Rain on snow event
As for the effect of thaw, it is not beign changed by your development; it either melts and increases pre AND post, or it does not.
If they are insisting on thawed grass pre, and frozen grass post, make them cite an ordinance (which they can't do, because it would never pass public comment). If they can't cite an ordinance, explain how unfair and illogical (indeed, impossible) this is to your client's lawyer.
RE: Rain on snow event
What the County appears to want is a flood control reservoir to prevent snow melt damage- which as LHA points out, could be caused by existing snow melt problems just as much before as after a proposed development, if you assume everything is frozen. It may make more sense to increase conveyance sizes than detention sizes if a simplified approach is used, to prevent flow related damaages, if it is not feasible for the County to use a more sophisticated approach to give you better guidance.
RE: Rain on snow event
I would explain this to your client, run a conservative basin based on reducing current flooding and conveyance based on current erosion potential. Then see if the system works for what the Ordinance actually requires. If you show good faith on offering them a solution to existing problems, they should relax (or waive altoghether) any Ord. requirements you can't meet. If they make you meet Ord. and give them this snow melt remediation, that is unfair and probably prohibitively costly, and then call the lawyer.
RE: Rain on snow event
It's definitely going to be costly. Even if you assume no snow melts the runoff volume from zero permeability is huge compared to the infiltration rate. lha is right the snowmelt runoff does exist before and after development. I guess what they really want is a flood control project at my client's expense. Of the two counties I have to deal with in this area this one is definetly harder to please. It also has a much slower growth rate which maybe is the real agenda.
Actually the logic of lha's second response (and it really is logical) means that no matter what development occurs on the site there is no increase in runoff post development and therefore no detention required at all.
The site is just over 0.6 acres but it is adjacent to a large valley of several thousand acres of desert. That is where the real flooding comes from. My design no matter how grandiose will not really have an effect.
DPA
RE: Rain on snow event
DPA
RE: Rain on snow event
http://www.ecy.wa.gov/biblio/0310050.html
There is a whole lot of comment about rain on snow events and a lot of disagreement about whether snowmelt factors should even be used. There are no conclusion however. It's 222 pages so a long download time. You have to really want it. Not much useful there anyway.
DPA
RE: Rain on snow event
Just a Thought
RE: Rain on snow event
DPA
RE: Rain on snow event
In addition, snow, no matter how much it has melted, will delay the runoff because it will tend to store part of the rain.
Normally you will find that the runoff volume produced by such an event is relatively large, but the peak flows are lower than those from the "full year" rainfall intensities.