5E, Drainage Report for Townhouse Villages North and Townhouse Villages at Arden Hills

Home Browse Search

and prevent flooding from a 100-year storm event, or an event similar to that experienced in July, 2011. The estimated cost for Option 6 is shown in Table 1. An alternative that was investigated briefly involved routing the northern low point toward the nor[h of this development, into the old City Hall site property. The object was to see what impact this would have on the southern low point, by completely removing the northem catchment from the existing system. The result was a negligible lowering of the southern low point HWI,. Since this alternative would require watershed approval to reroute drainage patterns and that it would not provide an emergency overflow for the southern low point, no further action was taken. Option 6 (Figure 7) This option looked at what improvements to the existing storm sewer system would be necessary to keep the pipes downstream of the southem low point from surcharging during a 100-year event. Approximately 865-feet of storm sewer pipe would need to be removed and replaced from the southern low point to the last manhole, prior to outletting into Pond 4. Extensive removal of existing driveways would be required, along with removal of recent improvements that occurred in 2011 Pavement Management Program. Most of the existing storm sewer pipe would need to be increased several pipe sizes in diameter. Two of the largest stretches of new pipe would need to be upsized from 27" to 33" diameter pipe. The estimated cost for Option 6 is shown in Table 1. Estimated Costs of Options Proposed construction costs for each option are summarized in the table below. These cost estimates are based upon public construction cost information. Estimated construction costs presented in the following table include a 20 percent contingency factor. Table 1- Cost Summar . O tion 1- Berm & New Outlet from Pond 2 $17,400 O tion 2— Overflow Pi e from SE Low Point $20,300 O tion 3— Diversion Pi e $75,100 O tion 4— New Outlet Pi e from Pond 2 $59,000 O tion 5— New Outlet Pi from SE Low Point $68,900 O tion 6— Increased Storm Sewer Pi e Size $100,800 Recommendations The most cost effective solution would be Option 5. This option would provide an alternative route for storm water to discharge from the southern isolated low point, without relying on the existing storm sewer pipe system to carry runoff away from this area of concern.