City of Morgan Hill Sewer System Master Plan 2017

August 2017

ES-4

City of Morgan Hill

Sewer System Master Plan

ES.4 SYSTEM PERFORMANCE AND DESIGN CRITERIA

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. Design criteria include

capacity requirements for the sanitary sewer collection facilities, flow calculation methodologies

for future users, flow peaking factors, and accounting for infiltration and inflows.

Partial Flow Criteria (d/D)

Partial flow in gravity sewers is expressed as a depth of flow to pipe diameter ratio (d/D). For

circular gravity conduits, the highest capacity is generally reached at 92 percent of the full height

of the pipe (d/D ratio of 0.92). This is due to the additional wetted perimeter and increased friction

of a gravity pipe.

During max day dry weather flows (MDDWF), the maximum allowable d/D ratio for all proposed

pipes (all diameters) is 0.75. The maximum allowable d/D ratio for all existing pipes (all diameters)

is 0.90. The criterion for existing pipes is relaxed in order to maximize the use of the existing pipes

before costly pipes improvements are required.

During max day wet weather flows (MDWWF), to avoid premature or unnecessary trunk line

replacements, the capacity analysis allowed the d/D ratio to exceed the dry weather flow criteria

and surcharge. This condition is evaluated using the dynamic hydraulic model criteria that

stipulates that the hydraulic grade line (HGL), even during a surcharged condition, should be at

least three feet below the manhole rim elevation

The

City’s design standar

ds pertaining to the d/D criteria are summarized in

Table ES.1

ES.5 EXISTING SEWER COLLECTION SYSTEM OERVIEW

The City provides sewer collection services to approximately 12,400 residential, commercial,

industrial, and institutional accounts.

The City’s collect

ion system consists of approximately 158

miles of up to 30-inch gravity sewer pipes, which includes part of the Morgan Hill-Gilroy Joint

Sewer Trunk, that convey flows towards the South County Regional Wastewater Authority

(SCRWA) Wastewater Treatment Plant (WWTP), located southeast of the City of Gilroy, as shown

Figure ES.2

A system-wide modeled pipe inventory, listing the total length by pipe diameter, is shown on

Table ES.2

. This table is based on information extracted from the City’s GIS and upd

ated to

reflect review of construction drawings provided by City staff. The 8-inch, 10-inch and 12-inch

diameter pipes account for approximately 50 percent of the total sewer pipeline length.

ES.6 SANITARY SEWER FLOWS

The wastewater flows collected and treated at the SCRWA WWTP vary monthly, daily, and

hourly. While the dry weather flows are influenced by customer uses, the wet weather flows are