Carter Road Detent Basin DesignMEM
ORANDUM
July 7, 2003
TO:
FROM:
SUBJECT:
The Honorable Mayor and City Council Members
Michael C. Van Milligen, City Manager
Carter Road Detention Basin Update
I had received an inquiry about the outlet structures on the Carter Road Detention
Basin. Assistant City Engineer Gus Psihoyos has provided additional information
related to the outlet structure.
As you are aware, the building of the Carter Road Detention basin is on a fast track so it
can be completed prior to next year's spring storm events. If the City Council does not
feel comfortable approving this project at the July 7 City Council meeting, I would ask
that you establish a date for a special council meeting to give further consideration to
the project to help minimize any project delays.
MCVM/jh
Attachment
cc: Barry Lindahl, Corporation Counsel
Cindy Steinhauser, Assistant City Manager
Gus Psihoyos, Assistant City Engineer
CITY OF DUBUQUE, IOWA
MEMORANDUM
July 7, 2003
TO:
FROM:
Michael C. Van Milligen, City Manager
_ ...----~..
Gus Psihoyos Assistant City Engineer ~'Q'j'(.~
SUBJECT: Carter Road Detention Basin Update
INTRODUCTION
It was brought to my attention that Option 3 as discussed in the July 2, 2003
memorandum on the Carter Road Detention Basin needed further clarification.
Option 3 would be a 15" pipe that would be open at all times and a normally-
gated closed 24" pipe. The 15" and 24" pipe would be routed through a manhole.
These two pipes would then go into a 30" pipe, which would outlet into an energy
dissipater at the end of the Carter Road embankment. This dissipater would
reduce the outlet velocity and assist in the prevention of downstream erosion.
The 15" pipe would discharge during all storm events; the second 24" pipe would
be manually opened when the downstream drainage condition warrants. The 15"
pipe would reduce the peak outflow allowing the drainage to be discharged at
slow rate. Any pipe smaller than 15" would be prone to maintenance problems
(i.e., clogging).
DISCUSSION
Storm sewer pipes that are under embankments are typically categorized as
culverts. There are basically two types of controls that govern the manner in
which the culvert will function as the embankment is impounding water.
Inlet Control. The control section is located at the culvert entrance; and the
discharge is dependent only on the inlet geometry and the headwater depth.
Inlet control only occurs when the rate of water flowing through the culvert is
greater than the rate water that can enter the inlet.
Since the control of the section is at the inlet, the capacity is not affected by any
hydraulic factors beyond the culvert entrance such as the pipe slope, the culvert
length, and the surface roughness of the pipe.
Outlet Control. The pipe under the proposed Carter Road Detention Basin will
function with outlet control. The control section for this pipe is located near the
culvert outlet; and the discharge is dependent upon all upstream hydraulic factors
such as the pipe shape, the slope, length, and surface roughness as well as the
tailwater depth, the headwater depth, and inlet geometry.
Outlet control occurs when water enters the culvert at a greater rate than water
can flow through it. The calculations showed that this 15" pipe under a 100-year
storm with a headwater depth of 39 feet will discharge approximately 30 cubic
feet per second under outlet control.
The question was raised if the 15" pipe could be gated and closed, if the City
would want to operate under this operational mode. According to our consultant,
Veestra & Klm (V&K), the DNR could view this mode as an unfavorable condition
and would treat it much like Option 1 stated in the V&K of July 2. If the gate was
closed by accident, the DNR would view this outlet structure not viable and would
consider it closed at all times, and thus dam height would be raised accordingly.
Property acquisitions would be gt'eater because of the increased depth of
impoundment upstream of the dam. V&K could review the gated 15" gated
structure option and determine the impact of this scenario. Our V&K
representative is on vacation today; however, it is estimated that this review
could take perhaps two weeks. With this additional two-week delay, the
construction of this basin may be impacted for this calendar year.
GP:vjd
cc: Michael A. Koch, Public Works Director
Deron Muehring, Civil Engineer
MEMORANDUM
July 2, 2003
TO:
FROM:
SUBJECT:
The Honorable Mayor and City Council Members
Michael C. Van Milligen, City Manager
Carter Road Detention Basin Design Parameters
Preliminary recommendations regarding the outlet structure design were provided by
Veenstra & Kimm, the consulting firm designing the Carter Road Detention Basin. The
outlet structure will dictate the height of the dam that will establish the flood easement
areas and/or property acquisitions that the City must secure to meet IDNR requirements
for the project.
Attached you will find a copy of the preliminary recommendations regarding the outlet
structure design from Veenstra & Kimm, the consulting firm designing the Carter Road
Detention Basin. This outlet structure will dictate the height of the dam that will
establish the flood easement areas and/or property acquisitions that the City must
secure to meet IDNR requirements for the project.
OPTION I calls for a single 24-inch diameter concrete pipe that would be
gated under normal conditions (always closed until after a rainstorm). This
outlet structure would require constant attention because of base flow from
groundwater discharging into the basin. If the structure is closed on a daily
basis stagnant water would result causing health and nuisance concerns.
OPTION 2 called for two 24-inch diameter concrete pipes with one being
gated or closed under normal conditions. The second pipe could be opened
after a storm event to drain the basin in a timelier manner if downstream
conditions warrant.
OPTION 3 would be a 15-inch diameter pipe that would be open all the time
and a gated 24-inch diameter pipe. This option would be similar to Option 2.
The smaller 15-inch pipe would reduce the peak flows allowing drainage to be
discharged at slower rates. A pipe smaller than the 15-inch pipe would be
prone to clogging and would be difficult to maintain.
Engineering recommends Option 3 for two reasons:
The basin should have a small diameter opening to pass ground water to
prevent the maintenance and nuisance concerns due to standing water.
The 15-inch diameter pipe is preferable to the 24-inch diameter pipe because
the 100-year peak flow with a 15-inch diameter pipe is only one-third that of
the 24-inch pipe.
Option 3 would result in a peak outflow from a 2-year storm from 428 cubic feet per
second (cfs) to 24 cfs, a 94% reduction. Runoff from the 10-year storm would be cut
from 839 cfs to 27 cfs, a 97% reduction, and finally the 100-year storm would go
from 1480 cfs to 30 cfs, which is a 98% reduction in peak flows. We feel that these
reductions meet the parameters set forth in the Drainage Basin Master Plan
prepared by HDR.
Engineering staff held a public open house on Tuesday, June 17, 2003 to discuss the
proposed Carter Road Detention Basin. Notice was sent to over 30 abutting property
owners about the proposed basin and asked them to attend the meeting to express their
views and ask questions concerning the proposed improvement. About one-third of the
residents attended.
The main concerns voiced by residents were safety, issues with trees, and the length of
time that stormwater would be impounded.
I concur with the recommendation and respectfully request Mayor and City Council
approval.
MiChael C. Van Milligen ~
MCVM/jh
Attachment
cc: Barry Lindahl, Corporation Counsel
Cindy Steinhauser, Assistant City Manager
Gus Psihoyos, Assistant City Engineer
MEMORANDUM
July 2, 2003
TO:
FROM:
SUBJECT:
Michael C. Van Milligen, City Manager
Gus Psihoyos, Assistant City Engineer
Carter Road Detention Basin Project Update
INTRODUCTION
The purpose of this memo is to give a brief update on the status of the Carter Road
Detention Basin and seek the City Council's approval for the outlet structure design for
the detention basin.
BACKGROUND
As I shared with you on June 16, 2003, City Engineering held a public open house on
Tuesday June 17, 2003 to discuss the proposed Carter Road Detention Basin.
We sent a notice to over thirty abutting property owners about the proposed basin and
asked them to attend the meeting to express their views and ask questions concerning
the proposed improvement, about one-third of the residents attended.
The main concerns voiced by residents were safety, issues with trees, and the length of
time that stormwater would be impounded (standing water). Overall the meeting went
very well and we relayed to them that we would get back to them as soon as the dam
height and outlet structure are designed to the satisfaction of the DNR.
DISCUSSION
Attached you will find a copy of the preliminary recommendations regarding the outlet
structure design from Veenstra & Kimm, the consulting firm designing the Carter Road
Detention Basin. This outlet structure will dictate the height of the dam that will
establish the flood easement areas and/or property acquisitions that the City must
secure to meet IDNR requirements for the project.
OPTION I calls for a single 24-inch diameter concrete pipe that would be
gated under normal conditions (always closed until after a rainstorm). This
outlet structure would require constant attention because of base flow from
groundwater discharging into the basin. If the structure is closed on a daily
basis stagnant water would result causing health and nuisance concerns.
OPTION 2 called for two 24-inch diameter concrete pipes with one being
gated or closed under normal conditions. The second pipe could be opened
after a storm event to drain the basin in a timelier manner if downstream
conditions warrant.
OPTION 3 would be a 15-inch diameter pipe that would be open all the time
and a gated 24-inch diameter pipe. This option would be similar to Option 2.
The smaller 15-inch pipe would reduce the peak flows allowing drainage to be
discharged at slower rates. A pipe smaller than the 15-inch pipe would be
prone to clogging and would be difficult to maintain.
RECOMMENDATION
Engineering recommends Option 3 for two reasons:
The basin should have a small diameter opening to pass ground water to
prevent the maintenance and nuisance concerns due to standing water.
The 15-inch diameter pipe is preferable to the 24-inch diameter pipe because
the 100-year peak flow with a 15-inch diameter pipe is only one-third that of
the 24-inch pipe.
Option 3 would result in a peak outflow from a 2-year storm from 428 cubic feet per
second (cfs) to 24 cfs, a 94% reduction. Runoff from the 10-year storm would be cut
from 839 cfs to 27 cfs, a 97% reduction, and finally the 100-year storm would go
from 1480 cfs to 30 cfs, which is a 98% reduction in peak flows. We feel that these
reductions meet the parameters set forth in the Drainage Basin Master Plan
prepared by HDR.
ACTION REQUESTED
Engineering is requesting Council approval to proceed with the design of the Carter
Road Detention Basin under Option 3.
attachments
cc: Michael A. Koch, Public Works Director
Pauline Joyce, Administrative Svs. Mgr.
Deron Muehdng, Civil Engineer
Leo Foley, V&K
07/82/2~03 12:39 131946G1008 VEENSTRA & KI~,I P~GE 02
VEENSTRA & KIMM, INC.
860 22rid Avenue- SuRe 4 * Coratvltlet tov~a 5224 I-!565
319~66-1000 * 379466-1008(FAX') * 888.24t~OOH!~EAT$)
July 2, 2003
City of Dubuque (Engineering)
C/O Gus Psihoyos
50 West 13* Street
Dubuque, Iowa 52001-4864
DUBUQUE, IOWA
· CARTER ROAD DETENTION BASIN
PRINCIPAL SPILLWAY ALTERNATIVE RECOMMENDATION
The preliminary hydrology and hydraulic modeling for the Carter Road Detention Basin is
complete. A decision on principal spillway must be made in order to submit preliminary
plans to the Iowa Department of Natural Resources (IDNR) for review, Based on
guidelines from the iDNR, a prindpal spillway must pass the design flood without need
for operation of the emergency spillway, The Carter Road Dam will be classified as high
hazard and the design flood will be the 100-year flood.
Since it is desired to keep the Carter Road Basin as a dry beltom detention some type of
Iow flow or principal spillway will be required. A 36-inch conduit is the minimum size
recommended in the IDNR guidelines. Anything less than a 36-inch conduit will be
subject to clogging and must be modeled as non-useful. The IDNR has informally agreed
to allow a minimum 24-inch conduit if proper design was given to trash racks and inlet
structures. However the IDNR will require any gated slmctures to be modeled assuming
worst-case drcumstances. Therefore any gated structures will have to be modeled with
the assumption that the gates are closed during the design flood.
Based on our review of the hydraulic model and discussions with City engineering staff
Veenstra & Kimm, Inc. proposes the following options for a principal spillway:
Option 1 - A gated 24-inch conduit at the base of the dam. If the gate were opened
after the flood peak then 100% of the 100-yr storm would be detained and released
in about a 36-hour duration after the flood peak. The IDNR would require the top
of dam height be set above the 100-yr elevation as modeled with the gate closed.
The top of dam elevation will be at least 792 feet. The dam would be designed to
withstand an over topping event. However the IDNR may want the dam to be
modeled with a pool since there is no guarantee that the gate would be open and
the basin would be dry when the design flood occurred. Option 1 does not appear
to be the most acceptable to IDNR.
West De~ Moilles ,, CoralWtle · Omah~ * Mo£me e Mason Ci~ · Sioux Cie/
07/O2t2003 12:39
Gu~
~uly 2, 2003
Pn~e2
131945G1008
VEENSTRA & KT~I
PA~E
03
Option 2 - Two 2~-inch conduits at the base of the dam. One gated 24-inch and
one open pass through 24-inch conduit. The operation of the this dam would
ensure the dam is classified as a dry bottom basin and provide flexibility to drain
the basin quickly if it appears there are no downstream problems. If the gated
conduit were kept dosed for modeling purposes, 94% of the lO0-yr storm would.
be detained and released in a 22-hour period after the flood peak. The duration of
time to release the flow could be theoretically cut in half if the gated structure is
opened after the flood peak. The IDNR would require the top of dam height be set
above the lOO-yr elevation as modeled with the gate closed. The top of dam
elevation will be at least 785.5 feet. The dam would be designed to withstand an
over topping event.
Option 3 - A 15-inch open conduit and a g~ted 24-inch conduit at the base of the
dam. This option would operate similar identically to option 2. However the 15-
inch pass through conduit would allow detainage of 98% of the lO0-yr storm
assuming the 24" gate is modeled as closed. The detained flow would be releas~
in about 75 hours. The 24" gated structure could be opened and would allow the
re~ease rate to be more than cut in half if there are no downstream problems. Since
the 15-inch structure is less than 24-1rich the IDNR will require some type of high
flow inlet structure to release an equivalent flow prior to overtopping. Therefore
the top of dam will need to be slightly higher than the 100-yr elevation. ^ high
flow inlet could be designed to be located at the 100-yr level or just below. The
top of dam elevatien would be about 791 feet for this Option. The dam would be
designed to withstand an overtopping event.
Veenslra & Kimm, Inc. recommends the city consider the flexibility and operational
convenience of Options 2 & 3. If there is only a gated structure as the primary spillway
. the City must ensure the gate is opened after each rainfall or the basin will not be a dry
bottom basin.
We have not finalized co~ estimates for these options at this time, however due to the
height of dam and potentially no need for a high flow outlet requirement (If IDNR
approves a 24-inch conduit) it is apparent that Option 2 will be the least cost alternative.
IDNR has indicated they would prefer Option 2 or 3. Since Options 2 and 3 beth detain a
very significant portion of the 100-yr flow (94°£ and 98% respectively) Veenstra and
Kimm, Inc_ recommends the City select the preferred Option based on downstream
considerations.
As soon as the principal spillway is selected the computer models will be finalized and
sent to IDNR for review. We plan to submit a preliminary plan for Corps of Engineers
clearance this week. The soil borings are completed and the final geotechnical report
should be available very soon. The final construction plans will rake about 1-month to
07/82/2003 12:39 13194B61~08
VEk-NSTRA& KIMM
~'u .ly 2, 2003
complete after the spillway has been selected. The IDNR has indicated the,/need at least
60-days to review final construction plans.
We have previously provided hydraulic figures on the respective inflows and outflows. If
you have any questions or need additional information concerning our model, please
contact the undersigned at 1~888~241-8001.
04
VEENSTRA & KIMM, INC.