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

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disruptive to the existing landscape. To maximize pipe flow potential and limit head build up, a 24" arch pipe was assumed for cost estimating purposes. This was an option originally considered in the preliminary analysis. Once the survey data was obtained it was recognized that this may not be a viable alternative. Because the invert of an emergency overflow pipe at the east end of the driveway would be higher than the low garage floor elevations, this option would not serve a practical purpose. Modeling Option 2 showed that this action would have no impact on either the north or south low points. The estimated cost for Option 2 is shown in Table 1. Option 3 (Figure 4) Option 3 would serve to isolate the low point catch basins on Arden View Court from the main line storm sewer. A 24" storm sewer bypass pipe would be constructed between units 4401 and 4402 of Arden View Court. Currently, the entire flow upstream of the low point catch basins in Arden View Court must make a 90-degree turn at the catch basin to continue downstream. Given the momentum and velocity of a 24" pipe flowing full, it is likely that the storm sewer pipe and grate in the driveway to the east of this low point is surcharged and backs up during heavy rainfall events, as occurred in July of 2011. It was thought that a proposed bypass pipe would help to relieve flow pressure from the low point in Arden View Court and likely reduce the flooding potential in this area. To minimize disruption to the existing landscape in this area, a new pipe was considered to be jacked. Modeling Option 3 showed insigniiicant improvement to either the north or south low points. The HWL at the north low point was 973.33 (-0.02') and the HWL at the south low point was 964.98 (-0.17' ). The estimated cost for this improvement is shown in Table 1. Option 4 (Figure 5) This option involves constructing a new outlet from Pond 2 towards the west, which would outlet near Pond 5. A new outlet pipe from Pond 2 would reduce the outflow from the downstream storm sewer conveyance system. This would provide extra carrying capacity of the existing system, thus allowing for greater storm events to be handled before surcharging occurred. A directionally drilled pipe from Pond 2 towards Pond 5 would limit the overland disruption. Modeling Option 4 showed no significant improvements at either low point. The HWL at the north point was 973.29 (-0.06') and the HWL at the south low point was 965.09 (-0.06'). The estimated cost for this improvement is shown in Table 1. Option 5 (Fieure � This option focused on reducing the flooding in the southern low point and is similar to Option 2. The difference is that a new storm sewer outlet was simulated being constructed near the existing rectangular driveway grate, which is east of the south low point on Arden View Court. This pipe system was routed toward the southeast pond (Pond 3), where it would outlet. This scenario placed a new grate near the e�cisUng low point, which could start taking water immediately once runoff from the catchment occurred and divert some of this flow to the southeast. This scenario was modeled simulating the July, 2011 storm, with the existing low point catch basins in this area essentially plugged. Constructing a new catch basin structure and a 24" outlet pipe resulted in a HWL of 966.4 at the southern low point. The existing low garage elevation is 966.63. This option would protect the low townhomes adjacent to the low point