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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.