HomeMy WebLinkAboutLetter to StaffCarol Cotter, P.E.
City of College Station
P.O. Box 9960
College Station, Texas 77842
iv1iTCHELL
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MORC3.A.N
Re: Creekview Elementary Eagle Avenue Culvert
Dear Carol,
April 23, 2009
During the construction of the culvert for the Eagle Avenue exten.sion, it had come to
our attention that the concrete cap covering the sanitary sewer line downstream of the
culvert was higher than shown on the as-built plans. Once we looked at the field
cond itions, we came to the conclusion that there would be about six inches of ponding
behind the concrete cap and inside of the box culverts. In the future this ponding area
will accumulate silt and reduce the capacity of the cu lvert due to this silting .
Fortunately, silting was anticipated in the design of this culvert and the culvert was
designed assuming 25% clogging. The model shows that the bottom 1.5' of the culvert
is silted and the ponding created with this issue is 0.5'. I have included the following
ture for our use. ~~~~t:r-:1!~~~~~.~~
In reality, this design technique accommodates silt in the culvert but the actual concrete
cap on the sewer line is downstream of the culvert, so I modified the model to show
exactly the conditions that I see in the field: 6" of standing water in the culvert and a little
concrete dam 20' downstream of the culvert. I've included a Table of Results
comparing the approved design to the as-built conditions. As you can see when the
25% blockage is changed to the as-built conditions, the water surfaces are lower than
the design conditions.
s 11 UNIVERSITY ORIVf EAST. SUITf lv~ • co LEGE STATION. TX 77840. I 9i9 260 69c~. f. 379 260.356d
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rf~rH(.hc:lanomtr;;m c.1;;•:n • 'I'/\V":y mnth~ioodff". 190~.(or,
Table of Results
River Q W.S. Vel Flow Top Froude
Sta Profile Plan Total Elev Chnl Area Width #Chi
(cfs) (ft) (ft/s) (sq ft) (ft)
8553.03 2% SewCap 1065 289.36 3.15 609.74 217.94 0.23
8553.03 2% ApprovedDsn 1065 289.53 3.03 647.88 237.69 0.22
8553.03 1% SewCap 1183 289.62 3.28 670.64 252.95 0.24
8553 03 1% ApprovedDsn 1183 289.77 3.16 709.25 275.01 0.22
8496.27 2% SewCap 1071 289.37 2.56 517.14 215.43 0.18
8496.27 2% ApprovedDsn 1071 289.54 2.46 557.11 253.6 0.17
8496.27 1% SewCap 1190 289.64 2.67 581 .86 269.04 0.19
8496.27 1% ApprovedDsn 1190 289.78 2.58 622.5 291 .63 0.18
8481 .13 2% SewCap 1073 289.36 2.68 403.61 179.09 0.18
8481 .13 2% ApprovedDsn 1073 289.52 2.61 429.15 259.76 0.18
8481 .13 1% SewCap 1192 289.61 2.85 447.86 301 .51 0.19
8481.13 1% ApprovedDsn 1192 289.79 2.43 620.59 443.77 0.16
8449.57 Proposed Eagle A Culvert
8411 .52 2% SewCap 1073 288.91 3.08 394.79 188.64 0.21
8411 .52 2% ApprovedDsn 1073 288.91 3.08 394.64 188.56 0.21
8411 .52 1% SewCap 1192 289.09 3.07 575.78 197.17 0.21
8411 .52 1% ApprovedDsn 1192 289.09 3.07 575.6 197.13 0.21
8406 2% SewCap 1073 288.91 3.08 394.73 188.61 0.21
8406 1% SewCap 1192 289.09 3.07 575.63 197.14 0.21
8405.5 2% SewCap 1073 288.9 3.19 382.07 188.07 0.23
8405.5 1% SewCap 1192 289.09 3.16 562 .26 196.63 0.22
8405 2% SewCap 1073 288.91 3.08 394.41 188.41 0.22
8405 1% SewCap 1192 289.09 3.07 575.02 196.82 0.21
8351 .7 2% SewCap 1088 288.78 5.04 382.33 196.94 0.4
8351 .7 2% ApprovedDsn 1088 288.78 5.04 382.33 196.94 0.4
8351 .7 1% SewCap 1209 288.94 5.23 410.52 206.25 0.41
8351 .7 1% ApprovedDsn 1209 288.94 5.23 410.52 206.25 0.41
8160.12 2% SewCap 1110 288.66 2.6 738.93 227.07 0.19
8160.12 2% ApprovedDsn 1110 288.66 2.6 738.93 227.07 0.19
8160.12 1% SewCap 1233 288.8 2.79 771 .81 240.43 0.2
8160.12 1% ApprovedDsn 1233 288.8 2.79 771 .81 240.43 0.2
As you can see the modeling was sufficiently conservative to accommodate this as
found condition.
I understand that there is interest in why the concrete cap was not field verified. The as-
built sewer drawings do no show the sewer line to be a conflict. The elevation of the
cap was verified during construction but after the culvert was poured, and even if it had
been identified before the culvert was poured, I would not have recommended adjusting
the culvert because the roadway is designed at minimal grades and I knew that the
modeling was sufficiently conservative to accommodate the issue with no adverse
impact.
Questions have been ra ised about how this sewerline cap impacts maintenance of the
culvert and sewer line: The concrete cap was left in place to continue to provide
protection to the sewer line, and rock rubble was placed around the cap such that scour
would not undermine the sewer line. Field observations reveal that the rubble is doing
exactly what it is supposed to do and is not undermining. The concrete cap is located
exactly where culverts routinely deposit debris; downstream of the culvert where the
flows are expanding and slowing down and the flow loses the energy to carry the debris.
I would expect no additional maintenance on this culvert based on th is as-found
condition .
In conclusion, my review of this condition reveals that the water surface is not adversely
impacted by this sewerline, the maintenance of the sewerline or culvert is not adversely
impacted , and that the original modeling was sufficiently conservative . Thank you for
your time.
Cc: File