20130319 Ver 1_Application_20130404 (3)

We are working on a more detailed cost comparison for #130 and #189 individually. We are having to
wait for others to get those numbers since aluminum box culverts are relatively new to us and we don't
have a great feel for their costs at this point. Cost comparison to come in the next day or two. Culverts,
beside being much cheaper to construct, are also generally cheaper to maintain over their entire life
span.
From a hydraulics standpoint, there is no "magic number" where we will change from a bridge to a pipe.
It is really a case by case scenario as I'm sure you've seen.
#130:
The drainage area for this bridge is 4.8 square miles with a 100 -year discharge of 1,975 cfs (FIS Report,
November 2009). The existing bridge is a single span, approximately 20' in length with a superstructure
thickness of 1' -11 ". If this was replaced with a bridge, it would need to be approximately 35' long with
21" cored slab units and 30' wide. This width is due to spread encroaching into the travel lane which is
not allowed on bridges. This longer structure increases the impacts to the adjacent property owners,
particularly in the northeast quadrant with the structure so close to the stream and road. Hydraulically,
replacing the bridge with a bridge is not desirable as the proposed superstructure would need to be
approximately 2" thicker than the existing which would lower the existing low chord of the bridge and
reduce the bridge opening area.
Replacing a bridge with a bridge also increases the amount of blockage in the 100 -year floodplain for the
HEC -RAS Model. The existing bridge has rail on the bridge only with none on the adjacent roadway
approaches. When the bridge length is increased, the bridge rail length is also increased which adds
additional blockage in the model. When replacing a bridge, the current standards for guardrail also
need to be implemented, meaning, an additional 12' of guardrail (the portion of the guardrail with the
posts very close together) is to be included in the HEC -RAS model at each end of the bridge. Therefore,
in increasing the bridge length from 20' to 35', we increase the bridge rail length by 15' plus add an
additional 12' to each end for an additional blockage length of 39'. In HEC -RAS models that operate
where the 100 -year storm passes entirely through the bridge, this additional blockage is not seen by the
100 -year storm and does not cause an issue. However, in this particular model, the bridge is
significantly overtopped in the 100 -year storm event, so the additional blockage can cause a rise in the
water surface elevations between the existing conditions and the proposed conditions. Since there are
structures immediately adjacent to the bridge, we cannot have a water surface rise without adversely
affecting their property. Using a culvert benefit us, despite a small 6 sf decrease in hydraulic opening,
because guardrail is not typically included in the HEC -RAS model for culverts. This is because the
guardrail across the culvert has the wider post spacing and it is assumed that water can flow through
this. With using the culvert, we were able to eliminate the 20' length of blockage that the existing
bridge had which resulted in a decrease in water surface elevations between the existing and proposed
conditions.

#189:
The drainage area for this bridge is 1.6 square miles with a 100 -year discharge of 1,196 cfs (FIS Report,
November 2009). The existing bridge is a single span, approximately 19' in length with a superstructure
thickness of 1' -8 ". If this was replaced with a bridge, it would need to be approximately 50' long with
21" cored slab units and 33' wide. The proposed bridge width is based on maintaining the minimum
clearance to the guardrail at the horizontal curve on the west side just before the structure. The
proposed bridge length is significantly longer than the existing since the existing structure is skewed at
approximately 140 degrees and the most the cored slab units can be skewed is 120 degrees. This longer
structure increases the impacts to the adjacent property owners, particularly in the northern quadrant
with the structure so close to the road. Hydraulically, replacing the bridge with a bridge is not desirable
as the proposed superstructure would need to be approximately 5" thicker than the existing which
would lower the existing low chord of the bridge and reduce the bridge opening area.
Replacing a bridge with a bridge also increases the amount of blockage in the 100 -year floodplain for the
HEC -RAS Model. The existing bridge has rail on the bridge only with none on the adjacent roadway
approaches. When the bridge length is increased, the bridge rail length is also increased which adds
additional blockage in the model. When replacing a bridge, the current standards for guardrail also
need to be implemented, meaning, an additional 12' of guardrail (the portion of the guardrail with the
posts very close together) is to be included in the HEC -RAS model at each end of the bridge. Therefore,
in increasing the bridge length from 19' to 50', we increase the bridge rail length by 31' plus add an
additional 12' to each end for an additional blockage length of 55'. In HEC -RAS models that operate
where the 100 -year storm passes entirely through the bridge, this additional blockage is not seen by the
100 -year storm and does not cause an issue. However, in this particular model, the bridge is
significantly overtopped in the 100 -year storm event, so the additional blockage can cause a rise in the
water surface elevations between the existing conditions and the proposed conditions. Since there are
structures immediately adjacent to the bridge, we cannot have a water surface rise without adversely
affecting their property. Using a culvert benefit us because guardrail is not typically included in the HEC -
RAS model for culverts and by providing a 5 sf increase in the hydraulic opening. This is because the
guardrail across the culvert has the wider post spacing and it is assumed that water can flow through
this. With using the culvert, we were able to eliminate the 19' length of blockage that the existing
bridge had, which resulted in a decrease in water surface elevations between the existing and proposed
conditions.