[GBlist] erosion under bridge

[Julie Rehmeyer <julie@math.mit.edu>]

I have a bridge across a stream, and recent floods have started to
erode the streambed.  I need to do some kind of erosion control
project to keep the erosion from going further and endangering the
footings for the abutments.  

My image of this at this point is to put in gabions from abutment to
abutment at the downstream end of the bridge.  I'm wondering first,
whether this sounds like the right approach to you folks who are more
knowledgeable than I am (or whether it is sufficiently dependent on
the details of the situation that I really need to hire someone
knowledgeable to come look at it), and second, whether there are
details about how the gabions should be installed that I should know
about.

A few details that may or may not be relevant: 

-- The bridge has a 25' span, and the construction overall is quite
   beefy.

-- The stream experiences significant flooding with summer rains.
   This summer there have been about four floods so far, which is
   somewhat higher than normal.  The concern, obviously, is both for
   normal summer flooding and larger, 50-100 year floods.

-- The stream bends just after the bridge.  There is a log placed in
   the stream at an angle as a checkdam at the upstream end of the
   bridge, which seems to be helping the stream to make that turn.  It
   seems to be doing a pretty good job of this generally, though
   during the biggest flood this year, the stream managed to find its
   away underneath that log.  That was when the worst of the erosion
   happened.  Since then, it has filled in again and is going over the
   top.  I have wondered whether this should be replaced with gabions.

Thanks in advance for your thoughts.

Julie Rehmeyer
julie@math.mit.edu

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