This bridge consists of a double 20-foot span with ramps, for a total length of 55
feet. It was designed to fail in a safe manner in case of a large flood. The bridge is
only lightly attached to the footings so it can be washed away without structural failure
of the parts. To prevent the two sections of the bridge from going down stream they are
chained to the end abutments. The intent was that with only minor repairs to the handrails
the bridge could be reset after a flood.
The bridge withstood the flows of Hurricane Floyd without any damage or needing to be
reset. The debris line resulting from that storm indicated that the peak elevation was
about even with the bridge deck.
According to the Somerset County Engineering Department the flow in the tributary (WBR2 in
their computer model) that the bridge spans peaked at about 960 cubic feet per second
(CFS). This tributary drains an area of 1.43 square miles. This is far less than the 2,300
CFS of a 100 year storm and is about equal to the flow expected of a five or ten year
storm. The peak flow, during Floyd, of the West Branch of the Middle Brook at the junction
of the tributary was 2,400 CFS, which was almost half the 5,200 CFS of a 100-year storm.
The Middle Brook up to this point drains an area of 2.32 square miles extending westerly
to its' headwaters located near Argonne Farm.
The reason that the Hurricane Floyd flows are less than a 100-year storm while other areas
such as Bound Brook were hit hard is a result of the "time of concentration" of
the streams. The "time of concentration" ("Tc") is the time that a
raindrop furthest away from the point of the study takes to reach that point. At the
"Tc" the rain falling at the point of the study and the rain from the furthest
point are all there at the same time trying to flow through that point. Streams and pipes
with short times of concentration rise and fall quickly. During a hurricane they can rise
and fall several times as bands of rain sweep over the area as if they were a series of
small storms. Streams and rivers with large times of concentration continue to rise for
the full duration of the storm and sometimes even after the rain stops. This is why small
streams and street gutters may not flood as much during a long storm, especially one with
breaks in the rain, than a river like the Raritan. On the other hand, a typically short,
pounding, summer rain may flood streets and small streams but does not last long enough or
have enough volume to cause the Raritan to flood.
Michael Kammerer and his volunteers completed the bridge in May of 1996.
View from the Bridge
This bridge was designed to fail, but in a safe manner in case of a large flood. The
bridge is only lightly attached to its footings so it can wash away without damage to the
structure. The two sections of the bridge are chained to the abutments to prevent them
from sailing away downstream after a large flood.
After hurricane Floyd, debris proved that floodwaters rose to the bridge deck, but did no
damage to the bridge itself. The estimated amount of water flowing in the brook was 960
cubic feet per second during Floyd. This figure may sound high when in summer you look
down upon a trickle of water!}
Small headwater streams such as this tributary to the Middle brook, which drain small land
areas, are often better able to deal with the volume of water that falls as rain, because
they rise and fall many times over the course of a storm. However, much larger rivers,
such as the Raritan river, caused dramatic flooding during Floyd for several reasons. This
larger body of water continued to accept water from all parts of central and western New
Jersey gradually over the entire course of the storm, reaching its peak flow only long
after the rains had ceased. Older towns and cities at the downstream ends of large rivers
tend to be built within the floodplain of the river, whereas laws limiting how close to
floodplains structures may be built govern new developments.
Long Road Tributary. This tributary is the same stream as seen crossing
Crim and Washington Valley Roads near the Presbyterian Church. It then extends into
Bernards Township by means of the same gap in the second Watchung Mountain as Long Road.
It has a drainage area of 1.43 square miles and a time of concentration (see above) of
about forty-five minutes from its headwater to the West Branch of the Middle Brook.
In the Somerset County computer model it is known as "WBR-2" and has its
drainage area broken down into six smaller drainage areas for greater accuracy. The
greater the number of pieces the drainage area is divided into the better the model will
predict how the stream will behave. Actual storm data is compared to the models
predictions in a process known as calibration. The model is run through the computer with
small changes made to its data until an acceptable result is found. This is made more
difficult because no storm produces rain in the ideal distribution the model expects.
The nature of the drainage area (soil type, land use, etc.) and the time of concentration
largely determine the storm runoff from that area for a given storm.
