06-09-2004 Council Agenda - Laserfiche WebLink

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Gervais Lake Association, March 31, 2004 Page 2 The current plans for the West Vadnais Lake Outlet are for a roughly 2,000 foot -long pipe to terminate at a 1.5 acre pond behind the Levitz Furniture building. This pond would be considerably larger than the current pond and would be 3 to 4 feet deep. Hydrologic modeling has indicated that the outlet velocities from this pond would be considerably less than currently exists. One of the other conditions of the Ramsey- Washington Metro Watershed District in considering this proposal is that we would have control of the pipeline and would be allowed to block flows into the pipeline, should downstream flooding conditions warrant. We would work out an operations procedure for this closure. This procedure would be available for review and comment. It is our intention that the pipeline would be blocked during periods of high water, so additional flows from West Vadnais do not contribute to, or exacerbate, an existing problem. It should be clearly understood that high water in West Vadnais is not a result of one sto ni, or a short-term series of storms. The level of West Vadnais currently is high due to an almost decade - long, higher- than - normal precipitation period. It is theorized that the level of West Vadnais is more influenced by regional groundwater levels than precipitation levels. Flooding conditions in Gervais Lake and the Phalen chain of lakes is influenced largely by an extremely high spring snowmelt event and big summer storm events. West Vadnais would not respond as quickly as does the Phalen chain of lakes. We will consider the elevation noted in your letter as a trigger for sandbagging the outlet; however, your elevation of 860.0 can be achieved with one summer rainfall event. I think the elevation of 860.0 would be a good trigger for inspection of the outlet to determine if discharge volumes are significant enough to warrant shutting off flow. We will draft this operations procedure and provide it to the lake association for review. 2. Possible new waters entering from I -35E / I -694 Unweave the Weave Project I am not sure if your figure of more - than - double hard surface area is correct, but there will indeed be a considerably larger amount of hard surface with the new project than the existing freeway configuration. However, MnDOT is providing numerous ponds within the Unweave the Weave Project to maintain flows at or below current discharge rates. There will also be considerable improvement in water quality exiting from the freeway system compared to existing conditions. Due to the increase in impervious surface, the total annual volume of water discharging from the freeway system will be larger; however, the rate of runoff which contributes to flood levels will remain at, or be lower than, current levels. Therefore, this system will not increase flood levels in the chain of lakes. The Unweave the Weave Project design has been incorporated into the District's hydrologic modeling for the chain of lakes watershed. This information was recently incorporated, double- checked, and used to verify that flood levels will not be increasing. Your concern for waters from Twin Lake and other areas of Vadnais Heights is a good question; however, there are no new drainage areas contributing to the watershed. No new areas will be connected that are not currently connected to the drainage system. The Twin Lake drainage area is currently landlocked and does not outlet, and it has not outletted for a considerable period of time. The drainage system will be improved in the Unweave the Weave Project but we still do not anticipate any outlet volume coming from Twin Lake. 3. Possible new waters from the next phase of the I -694 reconstruction I have not seen any plans for the reconstruction of I -694 between Rice Street and I-35W but this reach of I -694 is outside of our drainage area and would not contribute additional flows to Ramsey - Washington Metro Watershed District. -19-