Infiltration chamber design 1

[murdock]

I am designing a subsurface infiltration system for a small (0.8 ac) drainage area. Due to state requirements, I am forced to do this, as I have to infiltrate the entire volume of a 1-year storm into the native soil. The site is nice and sandy, so getting all that volume (0.08 ac-ft) in isn't a problem (in theory). I have been looking at the Stormtech (stormtech.com) arch-type chambers in a stone bed.

For those who have designed these structures, what kind of inlet pipe layout have you used to maximize the surface area the runoff will use in the bed? Is a single pipe inlet sufficient to distribute the flow, or are inlets to multiple chamber rows necessary?

As for the fouling concerns, I am spec'ing out a swirl concentrator to remove 80% of the sediment before it hits the infiltration device. Has anyone else done this and started to evaluate its performance? Does the SC device keep your beds free of silt? Real-world input would be a great help.

 

[daddymem]

Hi, I just designed one of these systems for Nantucket MA. I used a sediment filter that discharges to a manhole with a weir wall in it to direct low flows to a single isolator row. Larger flows jump the weir and exit the manhole via another pipe connected to a single row. An additional row is next to that and there is no pipe connected to it. Since all units sit in a bed of stone the units will fill fairly evenly. Since the low flows are directed to the isolator row, this should be the only area with sediment accumulation, and a jet-vac can take care of that easily. The pipes connecting to the units are 12" hdpe(the ends of the units have punchouts for various size pipes depending on the model). All of this information is available from their website.

 

[lindbls]

Be sure you spec out exactly what you are looking for in a "swirl concentrator." Reading manufacturer brochures, you'd think they can all remove 80% of sediment. But, what does the 80% represent? Is it 80% by weight? Volume? On an annual basis or a specific design storm? Whose design? What flow?, etc. The risk of designing based on a storm event is that two manufacturers could design and end with a different design flows than you designed and hence undersize the device.

Many of the swirl devices are really debris racks or screens. While removal of garbage (trash and debris) is important, so is removal of the fine suspended sediment (total suspended solids - TSS). Read the fine print of the literature. For example, I've spec'd that a "swirl device" needs to remove 80% of TSS based on a mean particle size of 50 microns at a flow rate of 2.5 cfs. It seemed to work pretty well but was pretty restrictive. We saw several suppliers having to upsize their original proposal to meet the spec. Several suppliers scoffed at the idea of the spec saying it was too restrictive, but in the end, at least two or three met the spec.

Will the device really stop the silt from accumulating? No, but it will help. Depending on the site, it may be most cost effective over the life of the system to upsize the treatment device now for a few thousand dollars and prolong the effective life of the infiltration system. Once the system is clogged, it's time for a new one. You may even consider a filter system (Stormwater Management has a good one). It's heavy on maintenance and expensive, but more effective than the swirl systems.

I haven't worked with the Stormtech systems before (I've used 60" diam perf pipe), but keep in mind how the maintenance will be performed. If Stormtech has an open bottom, a vac-con truck will easily suck out the floor (and foundation) of the Stormtech units and potentially undermine them. Make sure the access ports can accomodate whatever size vac-con hose and nozzle will be used.

Hope it helps.