HomeMy WebLinkAboutFlow Line DiscrepancyCarol Cotter
City of College Station
Planning & Development
PO Box 9960
College Station, TX 77840
MITCHELL
MM
MORGAN
April 15, 2008
Re: College Hills Elementary Sanitary Sewer line Relocation -Flow line Discrepancy
Dear Carol :
John Logan brought to our attention that the flow line of the existing sanitary sewer line at station
0+00 is 0.6 feet lower than what is shown on our plans. The plans have been updated, reducing
the slope in the unconstructed portion of 8" sanitary sewer to 0.758%. The 8" sewer line between
station 6+00 and 7+27 ha s already been constructed, and will remain at 0.926%.
The reduction in slope will stil l allow capacity for the ultimate flow (0.6 cfs) within the system. The
portion of the 8" sewerline with 0.758% slope has a capacity of 1.03 cfs . The portion of the 8"
sewerline with 0.926% slope has a capacity of 1.13 cfs . Please refer to the College Hills Elementary
Sewer Study for sewer flow calculations. A:-
a E' l\\ ·(cj .::-¢' ~--······~&°:..\"'. ''1, S1 c€rely, ,? <::?~ ...... * .... ~IS' '11 /.~ "'* .· -.... l <1lUa~· · ./ (.~~.;c;iiii'D~ ~ii~~:.N.~ LJ r---'I. -o: 1/689 .. er~ . ~ -~··. .:{# ~ Veronica J.B . rga , \ .. E., C.F.M. 11 c··. 'r~ ,,x,.'>.-·..:s: .::-. 1 ~·· . .'-G1sT\~'"'"·~-Manag1ng Partner li,~\-,;:;·-....... ·;;;_~05
Attachment: Revised Sheet 01
cc: File
-~~·~·-1 ,0NAL ~ ...... ~ .... -
j l5'fof~
S:\Proj\0739-CollegeHillsRenovations-VLK\docs\0739-sewer_slope_change.doc
511 UNIVERSITY DRIVE EAST, SUITE 204 • COLLEGE STATION, TX 77840 • T 979.260.6963 • F 979.260.3564
CIVIL ENGINEERING • HYDRAULICS • HYDROLOGY • UTILITIES • STREETS • SITE PLANS • SUBDIVISIONS
1nfo@mitchellandmorgan.com • www.m1tchellandmorgan.com
INTRODUCTION
COLLEGE HILLS ELEMENTARY
SEWER STUDY
College Station Independent School District (CSISD) has chosen to replace the existing
College Hills Elementary School, located on Williams Street between Francis Drive and
Gilchrist Avenue, with a new elementary school at the same location . The proposed
building footprint will overlay an existing sanitary sewer line, requiring the realignment of
the sewer line with the construction of the school. This report will provide flow analysis and
sizi ng for the sanitary sewer lines to be relocated with the school, using parameters as set
forth by the Texas Commission on Environmental Quality (TCEQ) and the Bryan/College
Station Unified Design Gu idelines.
GENERAL LOCATION AND DESCRIPTION
The future College Hills Elementary will be located on Williams Street between Francis Drive
and Gilchrist Avenue, replacing the existing elementary school at this location. An eight
inch (8") sewer line currently transects the 16.1 acre site, flowing east toward Gilchrist
Avenue along Wolf Pen Creek Tributary A. Approximately 65.8 acres sewer to this line, with
half of the acreage in residential use . Please see Exhibit A for the Landuse and Sewershed
Map. Undeveloped lots and parks were assumed to contribute no flow to the system.
FLOW CALCULATIONS
The approximately 65.8 acre sewershed is composed of many land uses, as shown in Exhibit
A, with single family residential and institutional the predominant uses. Because this
sewershed is near full development, single family residential land use flow rates were based
on flow per lot (267 gpd/lot) instead of an acreage basis . To best estimate wastewater
generation for the proposed College Hills Elementary school, the average daily sewer
generation rate of a similar school (Creekview Elementary) was determined on a
square footage basis . Flows for College Hills were determined by multiplication of this
figure by the proposed square footage of the new school. B/CS Unified Design Guidelines
(Appendix A) Normal Flow Method 2 was used to generate a flow values for all remaining
land uses, assuming 16 units per acre for apartment land use.
Peaking Factor and Inflow & Infiltration
To account for peak demands on the sanitary sewer system, the B/CS Unified Design
Guidelines specify a peak hour factor of four times the average daily demand (dry peak).
This factor was thus applied to all flow values to simulate a maximum demand on the
system . Additionally, Inflow and Infiltration (l&I), at a rate of 25%, is added to the dry peak
flow rate in pipe design to account for flow that may enter the system via cracks or loose
joints (wet peak). The sewershed generates 0.56 cfs dry peak flow, and 0.60 cfs wet peak
flow. Please see Exhibit B for flow calculations.
PROPOSED FACILITIES
The proposed sanitary sewer facilities for College Hills Elementary and the surrounding area
can be seen in Exhibit B with the site plan detailed plan and profile drawings in Exhibit C.
Also shown in Exhibit B are the existing sewer lines that will be abandoned. The proposed
sewer line will fully carry the capacity of the abandoned lines and new development.
College Hills Elementary: Sewer Report
Existing Facilities
As seen in Exhibit C, the proposed building footprint of the new College Hills Elementary
lies directly on top of an eight inch (8") sewer line that carries flow to an existing twelve
inch (12 ") sewer line on Stallings Drive. The sewer line to be abandoned is approximately
675 linear feet in length running through the CSISD property.
Proposed Facilities
The eight inch (8") sewer line is to be relocated northeast of the current alignment as
shown in Exhibit C, outside of the proposed building footprint. Rerouting the sewer line will
require approximately 707 linear feet of eight inch (8") PVC pipe. The proposed facilities
can be viewed in detail in the attached site plan and sewer line plan and profile sheet as
presented in Exhibit C.
Once existing wastewater flows were tabulated the capacity of the proposed sewer line was
determined using Manning's Equation . It was calculated that the proposed eight inch (8")
sewer line with a slope of 0.988% can carry 1.17 cfs flowing full, which is greater than the
calculated peak wet generation rate of the sewershed of 0.60 cfs.
CONCLUSION
The sanitary sewer system proposed in this report is expected to provide sewer service to
the new College Hills Elementary School as well as the surrounding area served by this
sewer line. Assumptions and standards were based upon TCEQ regulations and B/CS
Unified Design Guidelines. Calculations show that the proposed sewer line will provide
adequate capacity for ~xisting land use conditions.
College Hills Elementary: Sewer Report 2
Land Use Area
(Acres)
Residential 32.69
Commercial 3.26
Apartments 6.24
Church 1.10
School 15.18
Park 1.50
Undeveloped 5.86
Total 65.83
Exhibit B
College Hills Elementary
Sanitary Sewer Study
#of Flow Avg Daily
Lots (gpm) Flow (cfs)
130 24.10 0.054
2 3.39 0.008
2 18.51 0.041
2 0.40 0.001
1 16.80 0.037
1 0.00 0.000
19 0.00 0.000
157 63.20 0.141
Dry Peak l&I
(cfs) (cfs)
0.215 0.0134
0.030 0.0019
0.165 0.0103
0.004 0.0002
0.150 0.0094
0.000 0.0000
0.000 0.0000
0.563 0.035
Total New Pipe Capacity (cfs)
Wet Peak
(cfs)
0.228
0.032
0.175
0.004
0.159
0.000
0.000
0.598
1.17
Exhibit B