Snow Melt Design

[EngWade]

I've been assigned the task of designing a stormwater management system for a proposed residential development to be built on the side of a mountainside (with a contributing drainage area of ~500acres). The project is located in a snowy region of the northeast. Typically, designs require analysis of the 1-, 10-, and 100-year rainstorm events.

My concern is related to the effectiveness of my conveyance system, relative to snowmelt. It may be, that by designing the site to convey the 100-year storm, I already will have sufficient conveyance capacity for snowmelt - I just want to be sure, and do not know how to quantify the snowmelt runoff.

Is anyone familiar with a situation like this, or can direct me to a published resource?

Thanks in advance.

 

[bpattengale]

In my area of the country (Minnesota)a 10-day melt event is usually used when analyzing stormwater facilities. Basically it is assuming that the entire snowpack will melt over a 10 day period, a worst case scenario.

Generally with snowmelt events the peak flows from the melt are relatively low while the total volumes involved are large.

If your conveyance system is designed to convey the 100-yr storm event you shouldn't exceed your capacity, but if you have detention/retention ponds or other volume control structures it may be possible to exceed the capacity of those.

USACE has a manual on predicting snow melt, but its fairly indepth and more directed at predicting runoff for flood stage predictions. EM 1110-2-1406 is the manual if you're interested in using it.

 

[sknight701]

The worst case scenario for you would be a rain on snow event, which includes the rainfall and the snowmelt caused by the rainfall. Assuming you used the the SCS method to calculate your runoff, you could change the antecedent runoff condition to type III (fully saturated antecedent conditions), which will give you a higher peak Q to use in your system design. You might also want to look up papers on the subject authored by Dr. Henry Froehlich of Oregon State University, who did quite a bit of research in this subject (I was his grad student).

 

[emmgjld]

I have understood the melting of a snow pack to be one, long term event, with fluctuations due to the heat of the day and cool nights, and a rainfall on the snow to be treated as a short term event, with no soil infiltration, as given by sknight701. The problem of a relatively thin snowpack, melting rapidly, with a rainfall, may be a little more difficult to define.