Submerged Storm Pipes

[mes11]

Working on a project where we have limited depth to the point where storm sewers lack typical cover and are even not fitting into catch basin structures. The client wants to have a retention basin as an aesthetic feature on site. One way to remedy the storm sewer elevations would be to lower the pipes that discharge into the catch basin and therefore that lowers my storm sewers but submerges them below the normal water surface. If we did that, what are the pros and cons of doing so. I know I've seen it done from place to place but have not been comfortable enough to do it on one of my projects. I guess my biggest concerns would be freezing and siltation within the storm lines. It seems like this could have an adverse effect on the pavement over time also because I would think the trenching would stay wet also (maybe clay bulkheads could remedy wet trenches). Thanks for any insight in advance.

 

[gbam]

Will the basin always be wet or dry mostly?

 

[sam74]

Why don't you also lower the retention basin in addition to the storm sewer so that your storm sewer maintains minimum cover and stays above the normal water surface elevation? Maybe the elevation of your discharge point to the offsite area will not allow that?

If your storm sewer pipes are below the normal water surface elevation I would think siltation would definitely become a big issue. If you were to create inverts in your catch basins that were a foot or more below the outlets you could provide an area for siltation to potential settle out but you are looking at reoccurring maintenance issues to keep those cleaned out to keep your storm pipes from silting up. But that is a better alternative and easier to clean out than the pipes themselves.

Freezing issues would depend on where your located I suppose.

If the elevation of the offsite discharge point is the issue I'd almost insist on bringing in fill to get adequate cover to eliminate potential issues in the future which the EOR could be held accountable for.

 

[mes11]

gbam, yes this would a wet basin.

sam74, unfortunately the point that this pond discharges to is only about 4 feet lower than the catch basin grates. That's after raising the building finish floor about 2-2.5 feet above existing grade. There is talk now of making this a dry basin instead, which would solve this issue.

 

[Twinkie]

You need to eliminate the wet pond. You won't even be getting full capacity of your storm drains under these conditions.

What return periods are you modeling for the basin? Does the WSE ever top the grates in your storm drains?

 

[mes11]

Twinkie, I'm modeling 1 through 100 year storms. Fortunately, since they wanted this as a water feature I was able to oversize the pond and keep the peak elevation for all the storm events below the catch basin grates.

 

[Twinkie]

Maybe you do a weir at the last manhole before the outfall into the basin, then do sumped, open bottom inlets upstream to drain standing water. This way you eliminate the constant backwater into the pipe network, and maintain full capacity of the pipes.

You could probably even drop this network to the depth of cover you need, and use as supplemental storage, then WSE crests the weir and goes into the basin.