Subsurface drain hydrology 2

[bltseattle]

I would like recommendations for a method to develop a hydrograph for a subsurface drainage or filter system. The simpler, the better.

Specifically, I would like to be able to model the flow attentuation that occurs when rain falls onto an athletic field. The field would consist of say 12 inches of sand with 6-inch underdrains at 15-ft o.c. How can I detemine the discharge hydrograph from the underdrains for a given design storm?

It occurs to me that the hydraulics would be similar to a sand filter, without any surface ponding...

Thanks,
BLT

 

[dicksewerrat]

You will have to find the speed of the water through the sand, the amount of water the sand holds, the rainfall duration and rate. You also have to know if the water will stop at the bottom of the pipes or will it continue down? Lots of things to look at before you can make some determinations.

 

[LHA]

This method has been approved for me in the past:

Calculate the perc rate times the infiltration area.

Just set that as a constant rate discharge (not precise, but close enough, head water won't affect perc rate greatly.

Route that "outlet" to the pipe.

My guess is the sand will continue to govern with that many 6" pipes, but over a large field, it may switch to pipe control at some point.

 

[bltseattle]

Thanks for the tips. I've developed a routing method that models the downward percolation that is similar to lha's recommendation. Based on what I've heard from some drainage plan reviewers, and have observed, lateral percolation to the drains provides substantial additional flow attenuation that I would like to be able to model, as well...

 

[schurman]

bltseattle,

Email me at schurman@gmail.com. I work at the firm you use to work at in Bellevue.

Rob

 

[dicksewerrat]

You should get lateral flow if you have a layer of low permiability below the sand. Otherwise the water will continue down without any lateral movement.

 

[bltseattle]

Dicksewerrat,
That is indeed the scenario, permeable soils over impermeable layer. And I also know the design storms and soil characteristics.

The problem is that while the downward percolation is relatively easy to predict, the time for the flow to reach the underdrains via lateral flow is quite challenging to compute accurately, especially if analyzing a 24-hour design storm rather than a "steady state uniform loading" type of scenario. Part of the difficulty is that Q varies depending on how far from the underdrain you are looking, so any HGL would be somewhat parabolic rather than linear, and Darcy's law doesn't readily fit.

So where LHA said "route the flow to the pipe", I need to find a method to do that calculation that will enable me to compute the flow attenuation that is expected.

I am looking for either a steady-state formula to relate a uniform hydraulic loading (the downward percolation rate) to the HGL that would arise as the water moves laterally over distance L through soil with known saturated hydraulic conductivity, or a spreadsheet or other model that can generate the same information.

Using just the downward percolation yields flows higher than would occur, due to the added "travel time" introduced by the lateral flow.

The reason I care is that it doesnt' make sense to have to provide stormwater detention for an athletic field drained as described, some drainage reviewers know this, but others want it demonstrated numerically.

Thanks for all the responses so far!