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3.4 Preliminary Planning Level Cost Estimation Preliminary planning -level cost estimates for returning Silver View Pend to its estimated design conditions were prepared since it is recommended that the city remove the significant accumulation of sediment. Sediment removal costs are based on the cubic yards of material to be removed and the three possible MPCA disposal options depending on the level of contamination that may be found in the sediment. The unit casts used in the estimate are based on recent past bidding and project work experience. The estimates are generalized and do not include specific considerations for Silver View Pond as the sediments have not been sampled or analyzed. The opinions of costs exclude wetland restoration/mitigation, maior storm sewer work, mobilization, pennitting, engineering and design, and easernent acquisition. The estimates include excavation, loading, off -site disposal, and short - distance hauling. For SRV I material (low contamination, residential re -use), the estimated unit cost is $ 2 per cubic yard. For SRV Il material (medium contamination, industrial re -use), the estimated unit cast is $30 per cubic yard. For SRV III material (highly contaminated, landfill disposal), the estimated unit cost is $45 per cubic yard. The costs for additional work not included in these estimates could easily equal the cost of sediment removal, effectively doubling the cost of the project. Table 6 shows the results of the cost estimate. A cost estimate to modify the pond's outlet to convert it to an extended detention basin was performed. This cost is based on adding a section of pipe and a special manhole that is equipped with an oil skimming and a flow attenuation device that detains flows up through a I -year or ?-year storm event for approximately 24 ]tours to the pond's outlet to enable the pond to act as an extended - detention basin with a floatable material skimmer. This option may be a viable way to enhance water quality treatment and enable the city to delay the time and expense for sediment removal. It will not eliminate the need to eventually remove sediment, but it may provide a way for the city to plan for and save over time so it can perform the needed removals with saved funds as opposed to borrowed funds at a later date. A detailed hydrologic and hydraulic study would be needed to address if such an extended detention option would be viable. A preliminary review of the existing outlet structure indicates that to add an extended detention structure, it would require a 4-foot-deep, 72-inch- diameter manhole with interior weir and appurtenances and an 8-foot section of 15-inch-diameter RCP pipe. A conceptual sketch of these types of structures is shown in Section 4.4 of this report. A more in-depth analysis would be needed to refine the cast estimate and extent of the project after additional hydrologic and hydraulic analysis is performed to understand the possible impacts on surrounding properties that could result from an extended detention outlet modification. The opinions of cost described in Section 4 for the extended detention structure are also included in Table I . As F Mpis 23 MN CC 21h110*2 4iircr Vic% Pund %ssyssikicw M orLhIcs kcporl SO%cr View Pond Nsscisinenl-FimLdoex 16