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City of Morgan Hill
Sanitary Sewer Flow Monitoring and Inflow/Infiltration Study
12-0248 AEG CofMorganHill FM Rpt.docx
TOC - v
Term
Definition
backfilled with a granular material. In this case, the pipe trench serves as a
conduit similar to a French drain, conveying storm drainage to defective joints
and other openings in the system.
Inflow
Inflow
is defined as water discharged into the sewer system, including private
sewer laterals, from
direct
connections such as downspouts, yard and area
drains, holes in manhole covers, cross-connections from storm drains, or catch
basins. Inflow creates a peak flow problem in the sewer system and often
dictates the required capacity of downstream pipes and transport facilities to
carry these peak instantaneous flows. Overflows are often attributable to high
inflow rates.
Normalization
To run an “apples-to-apples” comparison amongst different basins, calculated
metrics must be
normalized
. Individual basins will have different runoff areas,
pipe lengths and sanitary flows. There are three common methods of
normalization. Depending on the information available, one or all methods can
be applied to a given project:
Pipe Length: The metric is divided by the length of pipe in the upstream
basin expressed in units of inch-diameter-mile (IDM).
Basin Area: The metric is divided by the estimated drainage area of the
basin in acres.
ADWF: The metric is divided by the average dry weather sanitary flow
(ADWF).
Normalization,
inflow
The peak I/I flow rate is used to quantify inflow. Although the instantaneous flow
monitoring data will typically show an inflow peak, the inflow response is
measured from the I/I flow rate (in excess of baseline flow). This removes the
effect of sanitary flow variations and measures only the I/I response:
Pipe Length: The peak I/I flow rate is divided by the length of pipe (IDM) in
the upstream basin. The result is expressed in gallons per day (gpd) per
IDM (gpd/IDM).
Basin Area: The peak I/I flow rate is divided by the geographic area of the
upstream basin. The result is expressed in gpd per acre.
ADWF: The peak I/I flow rate is divided by the average dry weather flow
(ADWF). This is a ratio and is expressed without units.
Normalization,
GWI
The estimated GWI rates are compared to acceptable GWI rates, as defined by
the Water Environment Federation, and used to identify basins with high GWI:
Pipe Length: The GWI flow rate is divided by the length of pipe (IDM) in the
upstream basin. The result is expressed in gallons per day (gpd) per IDM
(gpd/IDM).
Basin Area: The GWI flow rate is divided by the geographic area of the
upstream basin. The result is expressed in gpd per acre.
ADWF: The GWI flow rate is divided by the average dry weather flow
(ADWF). This is a ratio and is expressed without units.
Normalization,
RDI
The estimated RDI rates at a period 24 hours or more after the conclusion of a
storm event are used to identify basins with high RDI: