ST0301C-CS120926
Rhonda Adloo
From:Caleb Thornhill <bct@freese.com>
Sent:Wednesday, September 26, 2012 1:53 PM
To:Keith Marvin
Cc:Tricia Hatley; CPL03243; Mike Wayts; Thomas Caffarel; Jon Bunner
Subject:FW: Coppell Response
Keith,
From the meeting that Mike Wayts, Trish and I attended on September 14, there were three questions/concerns
brought up that we need to respond to. Take a look and then call Trish for us to discuss.
B. Caleb Thornhill, P.E.
Freese & Nichols, Inc.
214-217-2282
1)The exhibit dated May 20, 2008 shows a proposed floodplain to be very narrow between Bethel Road and
Coppell Road (possibly contained in the channel). Coppell wants to know why this was possible in 2008 and not
now.
a.After looking into this, I have come to the conclusion that this floodplain was mapped based on a model
that was built on the best available data at the time, however it didn’t reflect modifications to the
channel flowline or top of bank.
2)How do the Qs compare between the TranSystems model and the FNI model?
a.The below table best summarizes the information requested. The FNI model accounts for all of the
ponds in the upstream watershed at the time of the study (2008). This was in an effort to reduce the
peaks through the channel. The results show that the peak discharges are close to one another.
TranSystems Ex. Q TranSystems Ult. Q FNI Ex. Q FNI Ult. Q
100100100100
Bethel Road 1,924 2,515 1,867 2,215
Coppell Road 2,174 2,755 2,621 3,000
3)Have you considered the upstream pipe system designed for the 25-YR event along Bethel Road near the
intersection with Royal Lane? This was designed for the 25-YR event on purpose to help reduce the amount of
flow from the DFW airport property.
a.The hydrologic model was built using the Lag routing option which does not account for storage. It
simply lags the hydrograph. The lag time used in the model reflects the travel time in the pipe system
and does not reflect the travel time through Bethel Road or overland flow.
Typical hydrologic methods wouldn’t account for storage in pipe systems, but would rather be lag routed or
considered in the time of concentration calculations. Since the lack of capacity of the pipe system wasn’t
considered, we crudely evaluated what impact this lack of capacity might have on the hydrology near Bethel and
Coppell Road. To do this we assumed that whatever runoff couldn’t be conveyed in the pipe system would be
conveyed in the roadway above the pipe system. There are four sumps in the roadway that we calculated the
storage volume to compare against the total volume of the hydrograph at this location. These values are
summarized below in the table. As you can tell the lack of capacity of the pipe system would result in an
extremely small percentage of the total hydrograph volume and therefore wouldn’t significantly reduce the
peak discharges.
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Volume in sags along Bethel Road Total volume of hydrograph at Freeport Difference (%)
(ft3) (ft3)
46,468 14,845,248 0.31
In summary, we think that the hydrologic model developed in 2008 by FNI is still a valid model without the above
mentioned storage accounted for. This is in line with standard industry practice and while the concern is valid, I don’t
think the results will be significantly altered. There will remain the overbearing problem of too much flow for the
channel capacity. As mentioned previously, upstream detention should be investigated to help reduce the peak
discharges in the channel near the Bethel Road and Coppell Road crossings. Development in the floodplain is not
encouraged and would aggravate the problem.
Thomas N. Caffarel, PE, CFM
Project Manager
Stormwater Management
Freese and Nichols, Inc.
1701 N. Market St., Ste. 500 LB51
Dallas, Texas 75202
214-217-2200 ext. 2234
214-217-2234 direct
214-217-2201 fax
www.freese.com
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