HomeMy WebLinkAboutWPO200800044 Review Comments 2008-06-10A
9
County of Albemarle
Department of Community Development
County Engineer's Commentary
Number 5: Erosion and Sediment Control for Roads Across Streams
11 Mar 2008
Question: Do I have to put in a basin? Can I put in traps instead? Can I just use silt fence?
Background: While these may sound like the same questions asked on every project for every
erosion control plan, I am referring specifically to roadway stream crossings, usually with
culverts and fill. The county review generally follows a form like this;
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Submittal 1: The culvert crossing with silt fence protection. This is usually seen as the least
destructive solution. It has the smallest footprint, assuming the fill comes from another part of
the roadway. Unfortunately, it does not meet minimum standards. There is no protection for the
stream when the pipe is being placed. Most of the silt fence is useless, because it is running with
the grade, and blowouts occur on either side of the pipe inverts. County comments ask for
revision, requesting a basin.
Albemarle Cou mf Community Development
County Engineer's Commen Qry #
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Submittal 2 After county comme ts,asecond su bmial might look so m&y!ng £k this. Th
designer usually replies that the culvert area G too big for a basin mb practical. #A also easier
m design traps, and cheaper and quicker m build them. Unfortunately, the culvert drainage area is
too big for the CIIP also, and this solution, like the one before, only operates after placement o
the pipe. County comme ts are repeated, rq etingabasin,bmrecognizing 36min may never
be provided, ands also offering a silt trap solution, wit four #a s around the pipe inverts.
Submittal 3 Atthis point, the designer haStake the silt trap r c mme d bon but still faithful
to te 6£R submittal, is k e in/ only silt fe n of It esm&1 Idrainage are s. The county w!#
usually insist that the silt fence is not adequate, and point out that the drainage area mST2
exceeds the maximum. Th drainage are to ST- I also might be exceeded d 6ngtein!al land
disturbance (this is discussed later).
Albemarle County Community Development
County Engineer's Commentary #5
Page 3 of 6
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Submittal 4: The four -corner compromise: At this point, the plan is usually approved. The
county has given up on obtaining a sediment basin, and a compromise has been reached to
provide traps at the four points of drainage concentration. Where areas are too big for traps
(bottom left, ST -4), diversions have been provided to direct "clean" water around the trap.
Commentary:
There are a number of problems with the four -corner compromise. For designers and reviewers,
it takes about four revisions to get here, and that wastes everybody's time. For inspectors and
contractors, it does not work well in the field. It is difficult to get these traps dug into the
hillsides, and it is even more difficult to maintain. A stream crossing and diversion are still
needed to install two of the traps, and the pipe. From the Program Authority perspective, there is
still a period of time when the two traps downstream have drainage areas above the maximum,
before the road fill cuts off the upper areas, and so the project does not meet minimum standards.
During the life of the project, one or more of these traps typically fails, and the "clean" diversion
is usually dirty, at least temporarily. Lastly, owners often do not see the sense in providing traps
uphill of the project, and complaints and confusion result.
In order to avoid these problems, it appears necessary to set some rules on what the county will
accept. The basin is still the preferred solution, both for ease of maintenance and inspection, and
for performance. However, some point must be recognized when it is a greater disturbance and
headache to build a large basin than to fool around with the four -corner compromise. Likewise,
at some point the disturbance or drainage area is small enough that a basin or trap is unnecessary.
Lastly, there are often objections from DEQ or the Army Corps to basins on some jurisdictional
streams.
It seems logical to base rules on expected performance, drainage areas, and consideration for the
potential for damage, based on the amount of proposed disturbance and fill. The following tiered
approach is proposed;
Albemarle Cou mf Community Development
County Engineer's Commen Qry #
Page 4o6
£ For drainage areas less than 50ars, antra fills grea tert an 4'inhih,abasinmutb
used. (5Q acres Ga arbitrary number chosen k approximately 4 times what the four-comer
#a arrangement cou J protect, 12 acre / Live S£teamsmust b pumped around, or diverted
with temporary piping and an off-line culvert placement, o some other strategy. Basins
cannot be placed in the live flow. _
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2 # the culvert drainage area G above 50 acres, o the road fill G less than 4in height, the four-
comer ea arrangement may be used. C e n water diversions must be built with a fabric
liner. Down-stream traps must b designed m the largest drainage encountered d 6ngte£f
of the project. For example, looking at #a ST-1, most designers use th is drainage are;
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Albemarle Cou mf Community Development
County Engineer's Commen Qry #
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However, du 6ngteintal stages of construction, bfr fill is in place, the draina ema be
something like this;
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The use of basins in the con a may need to be considered ins some cases.
Uphill, where drainage aresma be less th an 0.25acres, wire reinforced silt fence may be
used alone.
In the example, th is may be tees with #a ST-3, which could be removed;
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Notice the silt fence Gplaced along the contour as much m possible, m catch sediment
instead of diverting it
Every project is a unique finge rp6m msome extent, adt esepoo&Sma not always
work, but this should catch the majority ostlualton. Please let meknow / you have
Albemarle County Community Development
County Engineer's Commentary #5
Page 6 of 6
concerns with these proposals, or suggestions for improvement.
file: CountyEngineersCommentary _5_E &SforRoadsAcrossStreams.doc