Frank Burdick’s Log Number: N/A
Submitted by: Resident, via email
Category (1,2,3): 1
Status: Closed
Estimated date to
address: N/A
Q. The only dispute I have ever had with the volume and rate of
footing water is in using either a dry weather number or a wet weather number
and extrapolating them beyond the storm event. If you look at the
"spike" chart, the peak might be shown as producing high volume for,
say, three minutes. During the event the volume rises from a "normal"
flow to that peak then drops back toward that normal level. Yet in almost every
instance, the elevated level is multiplied for a duration of an hour or
longer. It is the peak/storm duration volumes we have to deal with, and that is
what points to short term retention. This can be clarified. It does not make
sense to me, to burden over 18,000 households (plus an unknown post disconnect
number) with a sump pump penalty instead of retention, if a projection of new
water/sanitary/surface customers is considered. We have serious surface water
problems. With all the technology available to us, we need to develop a means of
determining which business or resident has a high volume footing drain problem,
and resolve them and not impose sump pumps in low volume instances. If you
recall the initial SSWWE meeting, there were accusations that the CAC's were
biased and not necessarily representative of the public. Possibly their
responsibility and duties need to be more clearly explained. For this reason I
will remain independent. Thank you though for your inclusive mail, especially
for your Retention work.
A. Each sanitary
collection system that we have studied is unique and has unique characteristics
that drive the cost-effective engineering solution. The engineering options
depend on the magnitude of the wet weather flows, the location that wet weather
flows are generated, the location of hydraulic bottlenecks in the system, and
many other factors. Examining these characteristics is the next step in
the process - hydraulic capacity and alternatives evaluation. The first
step was to quantify the impacts of FDD on the sanitary sewer flow. We
had to understand that impact first, before we can evaluate further
alternatives.
Storage is a very
common method of addressing peak wet weather flow, and we will be including
many examples in our presentation to the Best Practices sub-group of the CAC
next week. A few things to keep in mind about storage:
· As you pointed out, storage can be very effective for systems
with “spiky” peak flows and the timing of the peak flows has a tremendous
impact on the storage size. That is why it is very important to
understand the flow characteristics of the system (the step we are in now, and
will be reporting on Dec 12).
· The viability of storage depends on the location of the
bottlenecks, relative to the location of the wet weather flow generation.
For example, if high wet weather flows and bottlenecks are upstream in
the system, building storage downstream won’t help. The storage has to be
located upstream of the bottlenecks to work. That is why it is critical
that we understand the hydraulic capacity of the system - that’s the next
step.
· For a traditional storage tank, a large amount of land has to be
available near the location the storage is needed (very common for 2-5 acres or
more needed). That can be a significant challenge in a built-out area. We
understand that it was a concern with the SSO task force in 2001 - we have
heard stories of concerns by the task force about putting tanks in parks and
wooded areas and negatively impacting these natural resources.
· There are other alternatives to traditional storage tanks to
store flow - linear storage and storage shafts. Linear storage can be
accomplished by constructing an oversized pipe - perhaps on the order of 6-12
feet in diameter for a length of several thousand feet to store the flow.
Often, these are constructed by tunneling, which can reduce the surface
impacts. One disadvantage of tunneling is that it can be more expensive than a
traditional storage tank. We will be including several examples of
tunnels in our material for the Best Practices group. Some communities have
also built deep shafts for storage. Tunnels and shafts have the added
risk of complex, deep underground construction. There are many examples of
failed tunnels and storage shafts.
· Depending on the depth of pipes, and conflicts with other
utilities, it may be possible to build linear storage with “open-cut”
construction techniques. This has a short-term disturbance to the surface
during construction, but can be less costly and have less risks than tunneling.
The viability of this option depends on the location, the depth of the
sewer, the presence of conflicting utilities, the extent of surface restoration
impacts and other items.
As you can see,
evaluating alternatives is complex and entails many competing decisions and
values. The City and the OHM team can present engineering costs and impacts,
but the optimal engineering solution is not necessarily the best solution for
the community. Hence the need for a Citizens Advisory Committee to weigh
these options and recommend what is best for the community to balance the ever
competing challenges. These are the types of discussions we plan on having as
part of the next step with the CAC.
A few technical
items to address a few of your other comments:
· It is fairly straightforward to show that stormwater flows are
much, much larger than FDD flows, and that FDD flows are much larger than
normal sanitary flow. The conclusion is the same whether the computation
is done based on stormwater volume or peak flows. This is not unique to
Ann Arbor - it is a very common observation from systems around the Country.
I’d be happy to review the basis of this conclusion with you.
· In making computations of stormwater volume, the duration should
be matched with the duration of the rainfall being applied. Often, for
stormwater computations, a peak-hour rainfall is used (something like 1.8-inchs
in an hour), and so the volumetric computations are made on an hourly basis.
This is often done to simplify the computations to illustrate basic concepts.
You are correct that the impact depends on the pattern of the rise from
the base condition to the peak condition and back to the base again. This
is called a hydrograph. We examine the impact of the full hydrograph on the
sewer system, and will often summarize the results in terms of the “peak flow”
or hydrograph "volume” to provide some simple metrics for comparison.
Underlying these metrics are the detailed hydrographs. I’d be happy to
review the underlying hydrographs with you, or the CAC. We will have
hydrographs available for viewing at the Dec 12 CAC meeting if there
is interest, and time depending.
· The City did indeed target the “wettest" area of the system
with the five priority districts for FDD. The metering is showing that these
five priority areas had peak flows that were 20-30 times average during wet
weather events in 2000 before the FDD, when more common rates are 3-8 times
average. As we presented in the October CAC, the post-FDD flows in these
districts from 2013 appear to have been reduced to the more common range of
peak flows during large rain events. On December 12, we will be
reviewing the results of the full-evaluation of these flows, including the
application of three scientific methods to quantify the impact of the FDD on sanitary
flows.
· We are not aware of a technology that can determine which
specific houses or businesses will have high FDD flow. It is very
challenging to meter the flows from an individual footing drain before it is
disconnected. While there are some indicators, the flow variations from
house to house are very sporadic. There are instances of a very high flow
FDD right next to a house that is very low. The best technique that we are
aware of is to perform flow metering at the neighborhood level, and target
those areas with the highest flow. Areas with a high propensity for basement
backups are also an obvious target area. Pilot FDD with sump monitoring is then
an effective method of verifying the appropriate areas. We understand that
these techniques were the basis of the five priority areas identified in 2001.
Please note that I have intentionally kept this
message at a summary level. I’d be happy to get into more details, but it is
not effective to do so via email. If you would like to get into more details, I
would be happy to meet with you. I did this a few weeks ago with Frank Burdick
with a pad of paper, pencil and calculator, and I think it was very helpful.
There are no comments.