53 / 431
August 2017
3-1
City of Morgan Hill
Sewer System Master Plan
2017
City of Morgan Hill
3.0
CHAPTER 3 - SYSTEM PERFORMANCE AND DESIGN CRITERIA
This chapter presents the City’s performance and design criteria, which were used in this master
plan for evaluating the adequacy of capacity for the existing sanitary sewer system and for sizing
improvements required to mitigate deficiencies and to accommodate future growth. The design
criteria include: capacity requirements for the sanitary sewer facilities, flow calculation
methodologies for future users, flow peaking factors, and accounting for infiltration and inflows.
3.1 HYDRAULIC CAPACITY CRITERIA
In addition to applying the City design standards for evaluating hydraulic capacities; this master
plan included dynamic hydraulic modeling. The dynamic modeling was a critical and essential
element in identifying surcharge conditions resulting from downstream bottlenecks in the gravity
sewers.
3.1.1
Gravity Sewers
Gravity sewer capacities depend on several factors including: material and roughness of the pipe,
the limiting velocity and slope, and the maximum allowable depth of flow. The hydraulic modeling
software used for evaluating the capacity adequacy of the
City’s
sewer system, InfoSWMM by
Innovyze Inc., utilizes the fully
dynamic St. Venant’s equation which
has a more accurate engine
for simulating backwater and surcharge, in addition to manifolded force mains. The software also
incorporates the use of the Manning Equation in other calculations including upstream pipe flow
conditions.
Manning’s Equation for Pipe Capacity
The Continuity equation and the Manning equation for steady-state flow are used for calculating
pipe capacities in open channel flow. Open channel flow can consist of either open conduits or, in
the case of gravity sewers, partially full closed conduits. Gravity full flow occurs when the conduit
is flowing full but has not reached a pressure condition.
x
Continuity Equation:
Q = V A
Where:
Q = peak flow, in cubic feet per second (cfs)
V = velocity, in feet per second (fps)
A = cross-sectional area of pipe, in square feet (sq. ft.)
x
Manning Equation:
V = (1.486 R
2/3
S
1/2
)/n
Where:
V = velocity, fps
n =
Manning’s
roughness coefficient
R = hydraulic radius (area divided by wetted perimeter), ft
S = slope of pipe, in feet per foot