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Steep Slope Stormwater Conveyance Ideas

Steep Slope Stormwater Conveyance Ideas

Steep Slope Stormwater Conveyance Ideas

(OP)
This may be impossible, but I would like some ideas on how to route this stormwater.

Project is a bridge replacement project where the bridge spans a river with very steep slopes.  The distance from the bridge deck to the 100-year W.S. elevation is approximately 115-feet.  I need to route the water from the bridge deck down near the river so we can treat it with a bioinfiltration swale.  The preferred swale site is located 70-feet downstream of the bridge deck.

What type of techniques can be used to convey this stormwater down this steep slope to this location?

Any ideas?

Rob

RE: Steep Slope Stormwater Conveyance Ideas

YOU SAY SPAN SO I ASSUME THERE ARE NO PIERS.  COULD YOU PLACE A METAL FRAME STRUCTURE FROM THE BRIDGE DECK TO THE GROUND BELOW (A MAKESHIFT PIER)AND ROUTE A VERTICAL STORM PIPE DOWN IT AS IF IT WERE A NORMAL BRIDGE/ROOF DRAIN?  

RE: Steep Slope Stormwater Conveyance Ideas

Hi Rob

Often the street cross-slope and gutter are sized to convey bridge runoff to the approach as gutter flow without using drains on the span (the preferred WSDOT method), but this is not always possible.

When drains are unavoidable, ductile iron pipe would withstand the elements and is typically hung beneath the span.  This pipe would lead to the bridge approach, where you could tie it to an HDPE tightline to the river below.  Some large rock for energy dissipation or a stilling well may be needed at the outfall.  You should provide a slip connection where the HDPE transitions from above/below ground, and likely would need some anchors into the face of the bluff

You might also consider alternate treatment BMPs.  For a project I know of in Portland, small single-canister Stormfilters were mounted beneath the drains on the span so that runoff was filtered prior to discharge.  Note that Stormfilter is an approved basic BMP in Washington now.  Then the treated water could freefall to the river below.  This might sound expensive, but any work to install a pipe down the steep could quickly exceed the cost of several single-canister systems.

Regards,
BLT

RE: Steep Slope Stormwater Conveyance Ideas

(OP)
BLT,

Thanks for the ideas.  We are fortunate that we can convey bridge runoff to the approach as gutter flow without using drains on the span.  Also we don't have to detain the stormwater because we can directly discharge to the Elwha River.  But I still have to convey the stormwater down to the river even though we only have to treat the 6-month, 24-hour storm.

I also understand tying into an HDPE tightline to the river below.  My main concern is the slope of the pipe.  If I conveyed the stormwater as you mention above, the slope of the HDPE pipe would need to be 65%.  This seems way too steep and that’s why I'm looking for other alternatives.

I have used Stormfilters before on other projects, but I'm not sure I want to treat the stormwater and discharge it on top of the bluff.  I'm concerned about adding additional runoff to the bluff which could cause slope stability problems.

Rob

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