City of Morgan Hill Sewer System Master Plan 2017

August 2017

3-3

City of Morgan Hill

Sewer System Master Plan

assumptions are made for use of St. Venant equations in the modeling software. First, flow is one

dimensional. This means it is only necessary to consider velocities in the downstream direction

and not in the transverse or vertical directions. Second, the flow is gradually varied. This means

the vertical pressure distribution increases linearly with depth within the pipe.

Manning’s Roughness Coefficient (n)

The Manning roughness coefficient ‘n’ is a friction coefficient that is used in the Manning formula

for flow calculation in open channel flow. In sewer systems, the coefficient can vary between

0.009 and 0.017 depending on pipe material, size of pipe, depth of flow, root intrusion,

smoothness of joints, and other factors.

For the purpose of this evaluation, and in accordance with City standards, an “n” value of 0.0

was used for both existing and proposed gravity sewer pipes unless directed otherwise by City

staff based on pipe structural condition.

This “n” value is an acceptable practice in

planning

studies.

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.

When designing sewer pipelines, it is common practice to use variable flow depth criteria that

allow higher safety factors in larger sizes. Thus, design d/D ratios may range between 0.5 and

0.92, with the lower values used for smaller pipes. The smaller pipes may experience flow peaks

greater than planned or may experience blockages from debris. The

City’s design standards

pertaining to the d/D criteria are summarized on

Table 3.1

During peak dry weather flows (PDWF), the maximum allowable d/D ratio for 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 peak wet weather flows (PWWF), 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 and the criteria

listed on

Table 3.1

, which stipulates that the hydraulic grade line (HGL), even during a surcharged

condition, should be at least three feet below the manhole rim elevation.