Rain on snow event 1

[dpa]

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

 

[LHA]

Does the ordinance specify ice-pack as the ground cover for your calcs? I've never seen that. If it doesn't, just use normal ground conditions.

 

[dpa]

The county is requiring me to use the assumption that the ground is frozen and no infiltration occurs. They say they have had some cases of serious soil erosion and property damage in the past in urban areas downhill from this project caused by rain on snow events. They don't really know what numbers to use however.

Thanks for the quick response
DPA

 

[LHA]

Your first sentence settles it: "...no infiltration occurs." means impervious. If they are "requiring" that, then you must be allowed to use impervious for both pre and post. That will minimize the storage volume, not increase it. It will probably even eliminate it, as opposed to pre as grass and post as grass and impervious. Pre=Post=100% impervious, therefore you are not changing CN at all, and probably not increasing Tc by much, if at all.

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.

 

[bltseattle]

It sounds like the County really doesn't have a good handle on this, as LHA points out in several ways. I'm not sure I agree with the comment about Tc though, if it is not a redevelopment or a steep site then there could be a substantial change in Tc (think about how long it takes snowmelt to concentrate from a pasture or flat woodland compared to a parking lot with developed drainage).

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.

 

[LHA]

I agree with BLT, they probably want to solve an existing problem. My last post reads like I am quick to call in the lawyers; I am not. This guarantees everyone loses something, including time and money. Usually helping a muni. or county solve an existing problem makes good will, and actually saves time and $ over the entire project.

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.

 

[dpa]

Thanks for the comments

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

 

[dpa]

I forgot to mention, I have not seen this requirement in writing just a verbal from one of the county's engineers. Compared to most building departments they have a huge number of PE's.s Surprising for such a rural area (central Washington)

DPA

 

[dpa]

If you are interested there is a draft with a lot of public comments for the Storm Water Management Manual for Eastern Washington available in pdf at this site.

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

 

[gbam]

I must take issue with the no change concept. This may be true in the snow melt senario but what about in non-winter conditions. There will be increased discharge possibly requiring detainment. Should you evaluate for multiple senarios? Are you just looking at worst case? I just think it would be prudent to chk flows for summer condition to determine the flooding impact.

Just a Thought

 

[dpa]

Did that already and sized the pond for summer conditions. However for rain on snow there really is no change. Except now there will be some reduction in winter runoff vs pre development.

DPA

 

[LeoSC]

Usually the rain part on rain-on-snow events is less intense than for usual runoff calculations. This is because the large, high intensity storms occur during the non-frozen part of the year. So in effect, you should calculate the n-year storm using peak rainfall data applicable for the months of March, April and maybe May, for example, and not the annual maximum.

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.