Riprap alternative - under a Bridge 3

[vicki932]

Any suggestions to protect the banks under a bridge besides a hard-armor treatment measure? with little to no sunlight, soil bioengineering techniques won't really be possible. any other suggestions?

 

[Ryb01]

What is the design crieteria of the erosion protection? (i.e. what storm event, shear stress,longevity, etc)

 

[vicki932]

For high storm events - 100yr event. Urban area. Shear stress on ave 2 lb/ft^2. flood control. to prevent scouring/erosion from happening at the bridge. any other examples besides rock that could be used...something a bit more pleasing aesthetically, or more friendly to the environment?

 

[cvg]

it's all in the eye of the beholder, rock is both pleasing to the eye and very environmentally friendly. However, this is not a place to skimp on erosion protection just to go "green" Your best bets are either dumped rock, grouted rock, concrete, gabion mats, articulated concrete revetment mat and possibly a turn reinforcement mat (however without sunlight, that is unlikely to work). There are other methods such as jetty jacks and groins, but they aren't recommended under a bridge. Soil cement or RCC would work, maybe that looks more environmentally friendly. The final option would be to lengthen your bridge to completely span the river, bank to bank without any constriction. Keep the abutments out of the river. If you do that, then the river banks may not need protection.

 

[gbam]

Are your channel banks/bottom cohesive or non-cohesive? cvg mentioned many of the alternatives I am aware of. You may want to consider soil cement. We use soil cement mainly here in the southwest.

 

[vicki932]

well, thanks to all. yes, rock can be somewhat aesthetically pleasing, but not as much if a soil bioengineering or combination of one is available. Also, debatable on the "very environmentally friendly" if the rock provides little benefit to the aquatic life when compared to something else that may work. but from the responses, i agree that a hard-armor method is about the best choice possible given the criteria. THANK YOU!

 

[Ryb01]

cvg hits the nail on the head, this is not the place to skimp on erosion protection.

I've not seen many bridges without stone or other hard types of reinforcement to protect the river banks within and around the shadow zone of the bridge.

You will have to assess the acceptable risk if choosing an alternative type of erosion protection.

You could introduce plantings and other types of bioengineering outside the shadow zone to produce a greener look. Live stakes, shrubs and small caliber trees planted upstream and downstream of the bridge, in conjunction with a fescue seed mix native to your area would do a great job of visually hiding the hard armouring of the channel banks in the shadow zone. Not to mention provide additional stability to the channel banks upstream and downstream of the bridge.

 

[vicki932]

regarding soil cement....i'm looking into it. the advantages of uses it over riprap, and does it look like concrete or soil? questions i'm looking into.

 

[vicki932]

Ryb01 - excellent response/advice. thanks.

 

[cvg]

If you keep the cement content low, soil cement looks a lot like soil. With higher cement contents, it looks more like concrete and is usually stronger. It usually is not formed, but placed with earthmoving equipment and compacted in a stair step method using vibrating rollers. The rough edges make it look more natural. The portland cement association has some good information on soil cement, see below

http://www.cement.org/water/dams_sc.asp

http://www.cement.org/water/dams_sc_embankment.asp#levees

 

[Drumchaser]

Have seen various proprietary modular block-mat systems used
with good results. Some were labor intensive. I do not recall the cost but it did look better than concrete Rip Rap.

 

[gbam]

vicki932 - do a google images search for Tucson, AZ floods on the Rillito Creek/River or Santa Cruz River. These water courses have soil cement bank protection. Typically the color of the soil cement matches the surrounding soils.

 

[cvg]

the Rillito River is featured on the PCA website I linked previously. Tucson specifies 600 psi at 7 days and after a year or so you get 2,000 psi compressive strength or better. They use a relatively low cement content to get this strength and the soil is relatively clean sand and gravel from the river bed. 700 psi is specified in Phoenix and even higher for grade control structures.

 

[Prepakt1]

You can use a fabriform concrete mat and colour as necessary to blend into the background. We have done this for a large 14th century heritage bridge where the foundations were exposed at low tide and we did not want to clash with the old structure.

Intrusion Prepakt /marineconcrete.com

 

[GabionGuy]

gabions and reno mattresses have been used for decades in this type of application. The correct design may allow you to minimize the amount of stone required to provide adequate erosion protection.

They can also be filled with organic soil and vegetated. While the vegetation may not take hold, it might be worthwhile to attempt as a gesture to the local agencies controlling the permitting.

 

[cfj104]

We used vegetated reno mattresses and articulated concrete block (at a NPDES discharge point) along a stream bank recently.

We used the vegetated reno mattresses along the bank. The mattress were lined with erosion control blankets and then filled with a stone and topsoil combination (with seed). We had great success with this method and a year later you can not see the mattresses. At the toe, we used coir logs and live stakes. In addition to this, we pulled some large rocks from the creek and placed them along the toe of the slope to keep the main channel flow in the center of the stream. The large rocks matched the natural surroundings b/c they were natural from the creek and kept the flow in the center.

For your situation, if you will not have sunlight, you might want to consider ACB. I have had a lot of success with this material, plus, certian types have holes that will alow vegetation to grow through. Using large boulders at the toe might help give you a natural effect and minimize erosion by keeping the flow in the center of the channel.

Good luck!

 

[cvg]

For bridges, gabions need to be designed and installed correctly and properly maintained to function well. It can be done successfully, but must be done right. We nearly had a bridge failure a few years ago here where the abutments and channel slopes near the bridge were protected by 18" thick gabion mattress with large size rock. This was an approximately 1,000 foot long bridge with gabions which failed along the both abutments. Erosion caused a headcut which took out the approach road on one side, bridge was closed for months and the gabions were replaced with soil cement to repair it. Flow was no more than about 30,000 cfs which was only 25% of the 100-year flood.