HomeMy WebLinkAbout20 - Design Report - Flanders Way - Sewer Main Diversion to Norton East Ranch Interceptor Design Report - Page 2 of 4
INTRODUCTION
The proposed project involves construction of an 8” sanitary sewer main between the existing
sanitary sewer main in Flanders Way and the Norton East Ranch Interceptor in Cottonwood Road.
The construction of this sewer main is intended to divert flow from Flanders Creek Subdivision
away from the Laurel Glen Lift Station in order to provide additional capacity in the lift station for
construction of several projects in the lift station’s drainage basin. A capacity analysis of the
Laurel Glen Lift Station titled “Laurel Glen Lift Station Capacity Analysis” has been prepared to
assess the capacity provided to the lift station as a result of this proposed diversion, and is included
with this submittal.
The proposed sewer main diversion will be made on the 8” sanitary sewer main in Flanders Way, 1
block south of Oak Street, and will divert 33 acres of Flanders Creek Subdivision to the 21” Norton
East Ranch Interceptor in Cottonwood Road through Lot 3, Block 5 Flanders Creek Subdivision.
C&H Engineering has submitted a site plan application for this lot on behalf of Vision
Development, LLC. for construction of 12 dwelling units around a central drive access. Since this
project already proposes a sanitary main extension off of Flanders Way, and proposes an east/west
drive access from Flanders Way with public utility easement, installing the sanitary sewer main
diversion through this drive access does not negatively impact the proposed project. This project is
also currently on hold at the city until additional sewer capacity is provided for the lot.
SANITARY SEWER SYSTEM
The diversion will be made by installing a cut-in manhole on the existing 8” sanitary sewer main in
Flanders Way and installing 351 LF of 8” sanitary sewer main east through Lot 3 to another cut-in
manhole to be installed on the existing 21” sanitary sewer main located in the median of
Cottonwood Road. Four 4” sanitary sewer service stubs will be installed on this proposed sewer
main as well to serve the future buildings on the proposed Lot 3 site plan.
As explained in the Laurel Glen Lift Station Capacity Analysis, the proposed sewer main will
divert flow from the majority of Flanders Creek Subdivision. The blocks in Flanders Creek which
will be diverted are shown on Figure 1 in Appendix A. It should be noted that the block numbers in
Design Report - Page 3 of 4
this figure were taken from the preliminary plat of Flanders Creek Subdivision so that they would
match the original flow allocations to each block that are outlined in the 2006 Laurel Glen Lift
Station Design Report. Because of this, the block numbers shown on Figure 1, and used in this
report do not all match the final block numbering recorded on the final plat of Flanders Creek
Subdivision.
With the exception of Block 1, and Block 5, all of the proposed blocks that will be diverted have
been fully built-out. Lot 1, Block 1 has been approved for a 41-unit site plan, and the majority of
the units are constructed. Lot 3, Block 5 is the lot that the diversion will take place through and is
under review at the city for a 12-unit site plan. The proposed dwelling units for these two lots have
been included in this analysis to estimate the total flow to be diverted to the Norton East Ranch
Interceptor. The following calculations tabulate the total number of dwelling units to be diverted
and calculate the peak flow rate that will be diverted:
Dwelling Units Diverted
Block 1: 41 units (proposed)
Block 2: 14 units
Block 3: 14 units
Block 4: 32 units
Block 5: 28 units (16 constructed, 12 proposed)
Block 6: 12 units
Total: 141 units
141 dwelling units x 2.17 ppl / du = 306 people
Average Daily Flow = 306 ppl x 65 gpcd = 19,890 gpd
Qmax 18+P1/2
Peaking Factor = ------- = --------- (P =.306 = Population in thousands)
Qave 4+P1/2
Peaking Factor = 4.07
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The assumed infiltration rate for the area that contributes to the Laurel Glen Lift Station was
determined to be 271 gallons per day per acre as outlined in the Laurel Glen Lift Station Capacity
Analysis. This infiltration rate is used for this diversion as well to estimate to total peak flow for
the diversion:
Assumed Infiltration Rate
(271 gal / day / acre) x (33-acre diversion area) = 8,943 gal / day
Peak Flow Rate
(19,890 gpd x 4.07) = 80,952.3 gpd + 8,943 gpd = 89,895.3 gpd = 62.43 gpm (0.139 cfs)
The capacity of an 8-inch main is checked using Manning’s Equation:
Q = (1.486/n)AR2/3S1/2
Q0.75 = 0.573 cfs > Qpeak = 0.139 cfs ∴ adequate
CONCLUSION
Based on the above calculations, the proposed 8” sanitary sewer main diversion from Flanders way
to Cottonwood Road will divert 62.43 gpm of peak sanitary sewer flow to the Norton East Ranch
Interceptor.
8-inch PVC Pipe @ 75% Capacity
Pipe Diameter = 8 in
Manning's (n)= 0.013 (for PVC)
Min. Slope (S) = 0.004 ft/ft
A = p(r2)0.35 ft2
P = 2pr 2.09 ft
R = A/P 0.17 ft
Qfull = 0.764 cfs
Q0.75 = 0.573 cfs
APPENDIX A
Figure 1 – Dwelling Units Served
By Laurel Glen Lift Station