07-28-1999 Council Agenda - Laserfiche WebLink

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with riprap. The riprap would be placed on top of a geotextile fabric to prevent any erosion under the riprap. The riprap channel will be designated for velocities below a maximum allowable flow velocity of 11.5 fps. In the memo dated September 30, 1998, we estimated a cost of $18,000 for installation of a riprap channel. This cost included only installing riprap along the existing steeply cut swale eroded by stormwater flowing through this back yard area. At our meeting on September 30, 1998, several in attendance expressed concern over placing riprap on the steep banks of the existing eroded channel. Our new option includes clearing and grubbing and re- grading of the ditch area and also a riprap scour basin area to transition from the Enkamat to the riprap channel. Estimated costs for this improved Enkamat/Riprap channel are $32,000.00. Storm Sewer along Maple Leaf Court This option would include constructing a manhole to connect the culverts under County Road D and Maple Leaf Court and conveying the storm water in a pipe along the east boulevard of Maple Leaf Court to the existing catchbasin at the south end. This catch basin outlets into the existing pond at the south end of the cul -de -sac. The pipe from the catch basin to the pond would also be replaced. The two culverts mentioned above would need to be extended into a manhole constructed at their junction at the north end of Maple Leaf Court. A 24 -inch RCP pipe would be extended down the east edge of Maple Leaf Court, just behind the curb. This 24 -inch pipe would connect into the existing catchbasin at the south end of the cul -de -sac. The flow capacity of this 24 -inch RCP storm sewer would be 32.0 cfs at a slope of 2 %. This would provide enough capacity for the existing 22.1 cfs flow for a 5 -year event from the two culverts as mentioned in the memo dated September 30, 1998. As part of this option, the 12 -inch RCP storm sewer outlet, with a flow capacity of 5.0 cfs, from the existing catchbasin would also need to be replaced. This 12 -inch pipe would be replaced with a 24 -inch RCP outlet from the catchbasin, also at a slope of 2% with a flow capacity of 32.0 cfs. This 24 -inch RCP outlet with a flow capacity of 32.0 cfs would handle the flow from both the proposed 24 -inch storm sewer, 22.1cfs, from the north and the flow into the catchbasin in the cul -de -sac, 4.5 cfs, with a total flow of 26.6 cfs (storm discharges for 5 -year events). During larger storm events, the catchbasin at the south end of the cul-de -sac would overtop into an overflow swale over the outlet pipe from the catchbasin. This swale would be restored with Enkamat to minimize erosion susceptibility. There are several problems that exist with the construction of this option. As indicated by a site visit, there are four driveways that would need to be partially removed along with the removal of mailboxes, and some landscaping. Also, there is indication from the above ground pedestals for the power, cable television, and telephone along this side of Maple Leaf Court that there is an abundance of underground utilities along the proposed storm sewer route. Additional erosion problems would continue to exist along the gully during larger storm events, even if the inlet on the north side of County Road D was replaced, from water continuing to overtop County Road D to the south where it would flow into the existing gully. This gully would still need to be stabilized with this option for erosion control during larger storm events. The estimated cost for this option is $42,000.00. This cost represents all storm sewer, driveway removal and replacement, and property restoration. OAPROM04490A0325 \ 490-1304 cWarC F PAGE 129 1y CONE