huangyhg

bridge foundation scouring please help!!!
i have a small 32 foot bridge with some minor bridge scouring. before the bridge gets scoured anymore i am looking at placing concrete around and underneath the bridge footings. i will lower the water level to try first but if unsuccessful i then will try underwater concrete bags and place them in the scour spots.
does this sound reasonable to everyone here, i am a young engineer and this seems the most practical to me. if you have any other ideas please throw them out there.

how you will place concrete under the footings. to reduce scour you should place riprap stone above the soil surface, thickness of which you have to calculate based on reduced scour depth. it's more durable and a permanent solution.
extending concrete down can sometimes just lead to extending the scour as well...
i think ali has a good recommendation with the riprap (i would call it gabion cages); essentially these are galvanized steel cages filled with rocks.
alternatively you can dig a trench down stream from the piers (normally about two times the river depth- this assumes a fairly shallow river), and optionally fill this with large boulders. this causes the rocks to build up and effectively re-buries your piers...
there are much more experienced bridge engineers around than i; hopefully someone chimes in with an easier solution!
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
as a rule of thumb, take a survey of the channel and observe the largest material you see deposited in the riverbed, usually on the inside of a bend in the river/stream. mesure this material and supply rip-rap for your site that is at least 1.5 times this size. in this way, you supply rip-rap that won't be sent downstream next spring.
determine the proper channel size through your bridge area. you may have to go with your best estimate of the historical profile. place the rip rap several feet above and below the crossing on each bank to a depth approximately twice the size identified earlier.
you will almost certainly need a permit from someone before you can do this work. you will have to show that your proposal will not change the flow nor back up the water at your site. because of this, you will probably have to get soemone involved that really knows how to run the hec software. good luck.
you need to get an engineer on the project with bridge scour experience as this is a specialized area that most engineers do not have experience with. this is a high risk structure and expensive to repair and you will want to do it right the first time. you could gain a little knowledge by reading the following references and others available at the link below:
usdot, 1995b. evaluating scour at bridges, hydraulic engineering circular no. 18, federal highway
administration, fhwa-ip-90-017, 3rd ed.

richardson, e.v., d.b. simons, and p.f. lagasse, 2001, "highways in the river, environment," fhwa nhi 01-004, federal highway administration, hydraulic series no. 6, washington, d.c.
scour is a function of velocity. lower the velocity and lessen the scour possibilities. increase the cross-section area available for flow under the bridge at each water elevation by shaping the slopes or adding a parallel culvert.
yes, velocity is a major component, however there are many more. it is also related to:
depth of flow
diameter of piers
geometry and configuration of piers and abutments
channel geometry such as bends and constrictions
grade controls downstream or upstream
soil types
natural armoring or lack of armoring
vegetation
flow duration

of the list of factors affecting scour, which may be modified after the bridge is built to lessen scour? only two, armoring or velocity, are amenable to change in the post constructed bridge. the others need to be considered in initial design.
all should be considered during your scour mitigation project as they may have a large affect on the design of your scour protection or mitigation.

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