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Erosion at Stormwater Pipe Outfall in Marine Waters

Erosion at Stormwater Pipe Outfall in Marine Waters

Erosion at Stormwater Pipe Outfall in Marine Waters

I work for a local government in Puget Sound, WA. We have numerous stormwater outfall pipes that, at one time, discharged to the edge of a marine bluff (the marine bluff could be anywhere from 2 ft to 50 feet high). Over time, the bluff has eroded into Puget Sound, but the pipe now remains suspended over the "new" beach. In some cases, the pipe is now discharges to the top of a slope - in either case it's not good.

Our challenge is to replace the existing outfall with one friendly to fish and other wildlife and resistant to further erosion.  Having said that, all the marine bluffs will continue to erode no matter what we do.  But, we need to do something.

Splash pads on beach and gabion baskets on beach as an outfall may help disspate energy, but have proven to give the state and federal regulators heartache.

Any suggestions?

RE: Erosion at Stormwater Pipe Outfall in Marine Waters

Might be possible to use fabriform concrete mattress to prevent erosion down  the slope and as a splash pad.They can be colour matched to the background reducing the  visual impact.Plants will also grow on the mat in time as the
unevenness produces anchorages for soil and plants.

Intrusion Prepakt

RE: Erosion at Stormwater Pipe Outfall in Marine Waters

Could you bury a splash basin or pad?  During small normal events you get just normally outfall erosion (small gully to waters edge), but during large events you have that scour protection.  You could use a high cement content soil cement for better appearance on the slope.  

You might also be able to build a splash pool with just sheet piles (colored plastic ones may be more acceptable) and then a colored slope protection.  As a beach, you have to assume erosion and depositon will occur.

For slope protection you might consider cribs in your climate since you could get good vegetation.  Small will percolate through the crib with large events cascading down the side.  Otherwise, an option to gabions is just a filter under large riprap.  You can also grout a channel where the concrete isn't very obvious.

RE: Erosion at Stormwater Pipe Outfall in Marine Waters

What is the regulatory concern?  I would think a splash pad of large rock would be a preferred option...

Consider HDPE tightline to get flow to the bottom without aggravating the slope; attach it to slope using Manta Rays, helical anchors, or other method recommended by geotech that takes into account the degree of slope movement.

For energy dissipation devices consider gabion structure, stilling well (precast Type2 CB with deep sump), in-pipe flow expansion, concrete baffle structure, etc per FHWA HEC-14.  Which one to pick depends on regulatory issues you mentioned, flow rate, and site access/constraints.  I would start with the stilling well as you consider the options as it would likely be least expensive and easiest to construct, and last longer with less maintenance than gabions.

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