ST0301-CS040130F Ii'E E SE -, NICHOLS
MEMORANDUM
TO:
FROM:
SUBJECT:
DATE:
CC:
Ken Griffith, Kevin Marvin, Suzan Taylor
Tammy ltoelscher
IIEC models oftributmy lo Grapevine Creek
January 30, 2004
Trish Harley, Rob Bergeron
Attached are various liE(? models of the hydraulics at thc Bethel and Coppell Road culverls as
described beloxv. Scenarios A through E are for Coppell Road and Scenario 1: is for Bethel Road.
Note that there is not a FEMA effective model at Bethel or Coppell Road. The effective model
ends several hundred feet east (downstream) of the Coppcll Road crossing.
Coppell Road
· Scenario A: HEC-2 model as received by Transystcms, of existing conditions with
effective Q (EXIST.OH2)
· Scenario B: ItEC-2 model, as received by Transystcms of proposed condilions with
existing and ultimate Q's (32FTALL.OII2)
· Sceuario C: HEC-RAS, FNI model of existing conditions. Based on Scenario A, with
FNi revisions including converting to IIEC-RAS from HEC-2, converling culvert
modeling approach in RAS using upstream flowline from Transyslems' plan and profile
sheets, FNI topo for downstream flowline and Transystems roadway elevations. (.P09)
· Scenario D: HEC-RAS, FNI model of proposed Transystems conditions, concrete lined
channel, n value of .025, and with FNI adjustments to the crossing ora 42'x10' Con/Span
arch section, rather than 4-10'x7' reinforced concrete boxes per the Transystems stndy
(.P07)
· Scenario E: HEC-RAS, FNi model of possible improvement including a natural channel
with an n value of.04, assuming the concrete lined channel will not meet COE 404
regulations (incomplete model - see note below)
Bethel Road
· Scenario F: HEC-2, Transystems models, of existing culvert geometry with existing
flows, and existing culvert geonretry with ultimate flows (EXi, NI.OI t2)
HE( models
JanumT 30, 2004
Page 2 of 2
· -:1
Note: The Scenario E fiowlmes at Bethel Road el 50..0 and 502.7'do not match the most
recent 1 ransystems Plan and Profile sheets' flowlines of 500.56' and 500.01' respectively. This
discrepancy is as yet unresolved and needs to be addressed.
According to the model Scenario F the following headwater clevatiuns result:
Headwater
Elevation
Existing Flow
512.18'
Ultimate Flow
512.45'
Required Top of 514.18' 513.45'
Curb Elevation
The City of Coppell design criteria requires the greater ora fi-eeboard of 2' above the existing
conditions and 1' above the fully developed conditions for new designs. The cmTent design of
the vertical alignment for Bethel Road has a top of pavement elevation at approximately 506 and
the existing roadway elevation is approximately 508. The current FNI vertical alignment ties into
the Transystems design at Freeport Parkway, lhlls to the east at a 1% slope, and has a low point at
the approximate location of the culvert. This design will not meet the freeboard design criteria.
A pavement design considering the ultimate flow headwater elevation and City of Coppell design
standards would require a top of pavement elevation of at least 514.18', which is approximately
6' above the existing pavement elevation at the culveil. Raising the roadway this amount would
cause other drainage issues, inclnding pending of water on private properties rather than flowing
to the streets and storm drain system and the inability to tie existing driveways rote the proposed
alignment.
Before further analysis is complete on thc culvert and befbre additional redesign of the vertical
alignment ~s started, several issues need to be resolved, including verifying the accuracy of
iufornmtiou in the models and development of a concept plan 1bt thc clmnucl improvements. At
this point v,e feel fairly confident that it is going to bc extremely difficult to design a roadway
section that meets thc criteria for freeboard at this location.
Please let us know if you need additional inIbrmatioi~ from FNI. XVe would be happy to meet
with you to discuss this issue.
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