Frank Burdick’s Log Number: 68, 10
Submitted by: Burdick’s Log
Category (1,2,3): 1
Status: Closed
Estimated date to
address: Closed
Q. Craig Hupy was quoted in the Ann
Arbor.com article on 4/20/2012 as follows: "When we started the FDD
program, we were concerned about the additional load on the stormwater
system," he said. "And even in the largest events, the modeling we
did showed that it was a fraction of an inch — between an eighth and a quarter
inch more water in the street — so it's diminutive compared to the water
falling in the big events."
"If we didn't
have the FDDs in that rain event, we would have had basement backups downstream
of them. We didn't have any," Hupy added. "Given the intensity of
that storm, she would have had surface flooding whether we had done or not done
FDD, so I feel very comfortable saying FDD did not have a material effect on
the surface flooding. "
The
above Craig Hupy quote implies, on one hand, that the flow from FDDs is
relatively small or "diminutive compared to water falling in the big
events." The quote also implies
that FDDs prevented sanitary back ups from some basements. How can the claim of "70% to 90% of
total sanitary flow caused by footing drains" (as previously presented to
the CAC) be reconciled with the "diminutive" statement above?
(12/1/13)
Q.10. Can the City
provide a “lay man's" diagram that illustrates OHM's analysis of storm and
sanitary flows? (11/12/13)
A.68., A.10. Craig Hupy was
speaking about the impacts of FDD flow on the stormwater system.
Stormwater flows are much, much larger than the flows from the FDD, and
the “diminutive” statement was in reference to that comparison. The
statement that 70-90% of the total sanitary flows is caused by footing drains
is a comparison of the flows in the separate sanitary sewer pipe. These are
comparisons and percentages of flow in different systems. Because stormwater
flows are much larger than FDD flows, and FDD flows are much larger than normal
sanitary flow, the FDD flows are a relatively large component of the flow in
the sanitary system, but a relatively small component of the flow in the
stormwater system.
To further illustrate this point, we have outlined below a
comparison between the flow generated from a single house from typical sanitary
sewer flow, footing drain flow and stormwater flow. We have also prepared a
comparison between a typical sanitary sewer pipe and a typical storm sewer pipes
in the figure below.
Comparison of stormwater runoff volume to FDD volume - a
simple example:
Ann Arbor characteristics from 2010 census (from
semcog.org):
- 6,345 acres of single family housing
- 19,725 single family units
- 0.32 acres per avg single family parcel
- 43,560 square feet (sf) per acre
- results in 13,940 sf per avg single family parcel
Runoff from 1.8-inches of rain in an hour (common storm
sewer design rainfall):
- 1.8 inches / 12 = 0.15 feet
- rain volume from average parcel = 0.15 ft x 13,940 sf =
2,091 cubic feet (cf)
- fairly common for residential area for 35% - 50% of rain
to runoff as stormwater.
- stormwater runoff
range of 732 - 1046 cf
Compare to sump pump volume:
- 3-5 gpm from a sump pump
- 180 gallons - 300 gallons in an hour
- 7.48 gallons per cubic foot
- sump runoff range
is 24 - 40 cf
Compare to typical sanitary volume:
- 100 gallons per person per day typical (see reference
below on typical household use)
- 2.2 people per household in Ann Arbor (from semcog.org)
- 2.2 people per house x 100 gallons results in 220 gallons
per day per house
- That is 9 gallon per hour
- 7.48 gallons per cubic foot
- Typical sanitary
flow in an hour is about 1.2 cf
Note: the sump pump volume is probably a little over
estimated, as 3-5 gpm is more typical for a larger storm in the 2-4 inch range
over more than an hour. The rule of thumb for FDD flow is about 1 gpm per inch
of rain on average. So for this example, 1.8 inches in an hour might produce
something like 2 gpm. I used 1.8-inches of rain in an hour because that is a
common storm sewer design rainfall, and keeping everything to the volume in an
hour made the computations simple. Actual hydrograph computations of runoff
would be more complex and more precise, but would show the same
conclusions.
Comparison between a typical sanitary pipe and a typical
stormwater pipe:
Because storm water flows are much larger than sanitary
flows (as demonstrated above), the stormwater pipe tends to be much larger than
the sanitary pipe in a typical street. Storm and sanitary pipes can have a
tremendous range in sizes depending on the size of the area served and the
slope or grade of the pipes. To
illustrate the different carrying capacities, we have provided the illustration
below that compares an 8-inch sanitary pipe to a 24-inch storm pipe, which are
very common pipes sizes for city streets.
We have also compared the capacity of the pipe to the flow from 50
footing drains, which would be a very common number of houses serves by pipes
of this size.
Notes:
· These illustrations are to scale (the 24-inch
pipe is three times the size of the 8-inch pipe in the illustration).
The percent of pipe capacity used by 50 FDDs was
based on 4 gpm per FDD, which results in a flow of 0.44 cubic feet per second
(cfs) from 50 FDDs.
1. 1. Sump
pump volume is a small fraction of stormwater runoff volume. Footing drain
volume is a large portion of sanitary sewer wet weather volume.
2. 2. Typical
storm pipes have a much larger carrying capacity than typical sanitary pipes
(15 times more in this examples).
3. 3. Footing
drain flow rates are a large fraction of the capacity of a typical sanitary
sewer pipe. Footing drain flow rates are a small fraction of the capacity of a
typical storm sewer. (12/5/13)
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