HomeMy WebLinkAboutLetter Report on Floodplain*CIVIL DEVELOPMENT, Ltd.*
CIVIL ENGINEERING & DESIGN-BUILD SERVICES
P.O. Box 11929 · College Station, Texas 77842 · Phone: 979-764-7743 · Fax: 979-764-7759
June 27, 2006
Mr. Alan Gibbs
City of College Station
P.O. Box 1000
College Station, TX 77842
RE: Spring Creek Townhomes, Phase 4
Letter Report on the Floodplain Analysis
Dear Alan:
Following conversations with you and Josh Norton regarding the floodplain surrounding Springcreek
Townhomes, Phase 4, I began an analysis of existing computer models to detem1ine how the channel
modifications on the adjacent property have affected the floodplain shown on the cmTent FEMA maps.
With your help, I was able to piece together the available data and make the following comparison
between the effective and current models for your review.
The channel adjacent to Springcreek Townhomes, Phase 4 is referred to as Spring Creek, Tributary C.
Using the digital file of the effective model you prov~ded, I summarized the R EC-RAS output for this
sh·eam segment. Next, I obtained a digital copy of thelrevised model for Spring Creek from Mitchell &
Morgan, who are currently analyzing the entire basin as part of a project fo r the City of College Station.
Since I was not interested in the other parts of the Spring Creek drainage basin, I modified their model so
that only Tributary C was analyzed. The Mitchell & Morgan mode l includes recently surveyed
topographic data and cross-sections along Tributary C, so it provides the most accurate geometric
representation of existing conditions that is available. The flowrate through Tributary C in the Mitchell
& Morgan model is significantly different from the effective model, so I changed the flowrate in their
model to match the effective model.
The results are summarized on Exhibit Al. Exhibits A2 and A3 are full printouts from the two models of
Tributary C that were used to prepare Exhibit Al. A~ you know, the channel modifications that were
undertaken about 5 years ago changed the ali gnment of the stream, so the River Stations where the cross-
sections are taken do not match on the two models. In order to compare floodplain elevations, I have
shown the River Stations from each model adj acent to the corresponding River Stations on the other.
While they are not exact correlations, they provide a comparison at approximately the same location.
(These are slightly adj usted from the e-mail I sent you previously, but the results do not change much.)
Exhibit B shows a map of the area with the stationing from the effective model and the cross-sections
from the new model so you can see where I made the c9mparisons.
The two models indicate that the floodplain in th e vicinity of the Springcreek Tovvnhomes, Phase 4 is
significantly lower as a result of the channel modifications. This is due in large part to the flowline
elevations (Min. Ch Elev.) that are much lower in the current conditions model than in the effective
model. The trapezoidal channel in the new model is also much more efficient at ca rryin g the same
flowrate than was the shallow. natural channel that existed previously.
Mr. Alan Gibbs
Page 2
My conclusion from analyzing these computer models and the associated maps is that there is no increase
in the 100-year floodplain as a result of the channel modifications that were constructed in 2000, or as a
result of the Springcreek Townhomes, Phase 4 development. In fact, the floodplain has been lowered
significantly in this area as a result of the channel modification, just as it was designed to do. LJA
Engineering is in the process of preparing a LOMR for this area that will reflect these changes to the
floodplain. Until that work is completed, the data in this report provides the best available infonnation on
floodplain boundaries in the vicinity of this project. If you have any questions, please feel free to contact
me at your convenience.
Kent Laza, P.E.
Project Manager
Attachments: Exhibit Al
Exhibit A2
Exhibit A3
Exhibit B
CD containing digital copies of Effective and Cunent HEC RAS Models
Exhibit A1
Comparison of HEC-RAS Models
Spring Creek, Tributary C
Current Effective Model Civil Development, Ltd.
City of College Station 6/26/06 6/23/2006
River Sta Q Total Min Ch El W.S. Elev River Sta Q Total Min Ch El W.S. Elev
(cfs) (ft) (ft) (cfs) (ft) (ft)
1600 665 271 .00 273.93
I
I 1542 665 270.00 271 .82
1342 665 268.00 270.20
1436 665 269.03 271 .94
1408 665 269.00 271 .08
1372 665 268.10 269.84
1359 665 265.00 266.24
1309 665 265.00 268.35
1221 665 264.00 267.94
1083 665 263.00 266.55
1043 665 261 .90 266.44
1190 -+ 665 268.00 269.26
982 665 T-266.00 266.63
993 Culvert
941 1 665 1 261 .00 + 264.95
923 665 262.00 265.93
828 665 260.00 264.51
560 665 ~· 260.00 264.60
304 665 258.00 264.39
31 665 258.00 264.42
0.1 665 255.27 264.41
840 665
I
261 .00 264.07
583 665 259.00 262.26
267 665 ~258.00 260.33
0.1 665 259.88 256.00
Springcreek
Townhomes.
Ph 4
Note: The River Stations on the two models are not directly comparable. Stream modifications that occurred subsequent to
adoption of the Current Effective Model have changed the length of the channel, partucularly in areas upstream of the Arrington Road
culvert. The River Stations have been moved to in the spreadsheet above to approximate their location relative to each other. The
attached sketch shows the stationing for the two models overlaid on a map of the area.
Exhibit A2
EFFECTIVE MODEL FOR SPRING CREEK TRIB C
From City of College Stati on (6-26-06)
HEC-RAS Plan: existing River: Trib C Reach: upper Profile: 100 year
Reach River Sta Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude #Chi
(cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft)
upper 1542 665 270 271.82 271 .82 272.41 0.02357 6.17 107.85 95.69 1.02
upper 1342 665 268 270.20 269.33 270.33 0.006352 2.86 232.39 244.39 0.52
upper 1190 665 268 269.26 268.87 269.36 0.006306 2.6 256.16 310.24 0.5
upper 982 665 266 266.63 266.63 266.91 0.028974 4.22 157.49 288.55 1.01
upper 923 665 262 265.93 265.67 266.25 0.006935 6.22 225.73 198.82 0.64
upper 828 665 260 264.51 264.37 265.32 0.013194 7.25 98.35 70.72 0.85
upper 560 665 260 264 .60 261.93 264.63 0.000288 1.31 507.98 169.06 0.13
upper 304 665 258 264.39 261.46 264.50 0.000903 2.66 256.19 100.59 0.24
upper 31 665 258 264 .42 259.15 264.42 0.000052 0.77 858.16 172.01 0.06
upper 0.1 665 255.27 264.41 257.49 264.41 0.000039 0.75 890.64 158.92 0.05
Exhibit A3 ,,,
~
Civil Development Ltd's HEC-RAS Model for Springcreek Town homes, Ph 4 (with effective flowrate)
6/23/2006
HEC-RAS Plan: old flo River: Trib C Reach: upper Profile: 1 .0 %
Reach River Sta Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude #Chi
(cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft)
upper 1600 665 271 .00 273.93 273.63 27 4.11 0.004696 4.21 277 .53 283.57 0.51
upper 1436 665 269.03 271 .94 271 .94 272.73 0.016966 7.24 101 .01 74.47 0.94
upper 1408 665 269.00 271 .08 271.38 272.33 0.009176 8.99 74.03 54.08 1.33
upper 1372 665 268.10 269.84 270.45 J 271.83 0.016534 11.31 58 .82 45.38 1.75
upper 1359 665 265.00 266.24 267.46 271 .18 0.05048 17.83 37 .31 33.29 2.97
upper 1309 665 265.00 268.35 267.43 268.70 0.003051 4.76 139.75 60.27 0.55
upper 1221 665 264.00 267.94 266.99 268.34 0.005713 5.03 132.15 54.37 0.57
upper 1083 665 263.00 266.55 266.18 267.21 0.011 888 6.49 102.41 49.8 0.8
upper 1043 665 261 .90 266.44 264.62 266.94 0.003049 5.63 118.16 52 .22 0.47
upper 993 Culvert
upper 941 665 261 .00 264.95 263.73 265.61 0.005094 6.48 102.61 57.84 0.57
upper 840 665 261.00 264.07 264.07 264.96 0.007797 7.56 87 .97 50.47 1.01
upper 583 665 259.00 262.26 262.05 262.97 0.005676 6.77 98 .17 52 .3 0.87
upper 267 665 258.00 260.33 260.21 261 .06 0.006399 6.86 96.99 55.64 0.92
upper 0.1 665 256.00 259.88 258.33 260.01 0.002003 2.9 229.7 99.58 0.34