Stormwater Management and Flood Mitigation Study – Implementation Plan is Now Underway

Posted by PLSLWD Staff - May 16, 2018 - News, Flooding Updates, Prior Lake, Spring Lake

Photo of erosion at Gonyea Culvert

As if starting her new job as the District Administrator for the Prior Lake-Spring Lake Watershed District just one month before a nearly 60-year flooding event wasn’t difficult enough, little did Diane Lynch know that she would still be working on flooding damages nearly four years later. Over 13 Inches of rain fell over a short period in June, causing local flooding of homes and streets and overwhelming the Prior Lake Outlet Channel.

Damages From 2014 Flooding

The Prior Lake Outlet Structure and Channel was built in 1983 primarily for stormwater conveyance for land-locked Prior Lake in Scott County. The part constructed/part natural channel meanders its way over 7 miles to the Minnesota River.

During the June 2014 rain events, it is estimated that the lake level of Prior Lake could have reached approximately 912.26 feet without the outlet structure; whereas it peaked at…

View Full Article →

View Full Article Flood Study, flooding

Keeping an Eye on Springtime Lake Levels – UPDATE 5/2

Posted by PLSLWD Staff - May 3, 2018 - News, Flooding Updates, Prior Lake, Spring Lake

The City of Prior Lake and Prior Lake-Spring Lake Watershed District (PLSLWD) staff continually monitor local conditions including winter snowpack, precipitation, and lake levels to assess flooding risk. Early spring is one time of year when flooding potential receives some extra attention. According to the National Weather Service office in Chanhassen, the weather outlook and the potential for snow or rain are the biggest wild card every year. At this point, additional rainfall would be the main spring flood driver.

Based on an analysis done by City staff, there isn’t a strong correlation between the lake level at ice-out and the eventual high-water elevation for Prior Lake in any given year. For example, in 2014 the early spring level for Prior Lake was very close to the long-term average of about 901.6 on April 20; the record flooding level of 906.17 recorded on June 30 was caused by above-average rainfall events over a period of a few weeks in May and June. Other years, such as 2001, start out with high lake levels (902.75 on 4/15/2001) but the lake’s high-water level for the year is recorded shortly thereafter (904.33 on 4/30/2001); in that case melting snow likely contributed to the high-water levels.

CURRENT CONDITIONS AS OF MAY 2, 2018: Ice-out occurred this week and there is no longer any snowpack in the watershed above Spring Lake and Prior Lake. Spring Lake levels have been declining for about a week, although it continues to discharge to Prior Lake. Prior Lake appears to have View Full Article →

View Full Article Prior Lake, Spring Lake, flooding, lake levels, spring

Keeping an Eye on Springtime Lake Levels

Posted by PLSLWD Staff - April 23, 2018 - News, Flooding Updates, Prior Lake, Spring Lake

The City of Prior Lake and Prior Lake-Spring Lake Watershed District (PLSLWD) staff continually monitor local conditions including winter snowpack, precipitation, and lake levels to assess flooding risk. Early spring is one time of year when flooding potential receives some extra attention. According to the National Weather Service office in Chanhassen, the weather outlook and the potential for snow or rain are the biggest wild card every year. At this point, additional rainfall would be the main spring flood driver.

Based on an analysis done by City staff, there isn’t a strong correlation between the lake level at ice-out and the eventual high-water elevation for Prior Lake in any given year. For example, in 2014 the early spring level for Prior Lake was very close to the long-term average of about 901.6 on April 20; the record flooding level of 906.17 recorded on June 30 was caused by above-average rainfall events over a period of a few weeks in May and June. Other years, such as 2001, start out with high lake levels (902.75 on 4/15/2001) but the lake’s high-water level for the year is recorded shortly thereafter (904.33 on 4/30/2001); in that case melting snow likely contributed to the high-water levels.

CURRENT CONDITIONS AS OF APRIL 23, 2018: There is currently very little snowpack in the watershed above Spring Lake and Prior Lake after the warm weather this past weekend. Lake levels are rising and will continue to rise as the last of the snow melts. The water level of Prior Lake was 902.89 on April 23 which is slightly above the long-term early spring average of about 902.6. The lake continues to discharge through View Full Article →

View Full Article Prior Lake, Spring Lake, flooding, lake levels, spring

Lake Level Update – May 25, 2017

Posted by Jaime Rockney - May 25, 2017 - News, Flooding Updates, Prior Lake

The City of Prior Lake and Watershed District staff continue to monitor the weather and lake levels. Due to an abundance of rain, Prior Lake levels increased by almost 9 inches over the last week and a half to an elevation of about 903.44’ (as of May 25) and Spring Lake rose by about a foot. The ordinary high water (OHW) and no-wake level on Prior Lake is 903.9’. Spring Lake level and flow has already started to drop and is currently 912.19’ (OHW and no-wake zone is 912.8’).

Prior Lake continues to rise because more water is coming into Prior Lake than can leave. Currently, approximately 68 cubic feet per second (cfs) is entering Prior Lake (from Spring Lake) and 50 cfs is leaving Prior Lake (through the Outlet Structure). Spring Lake cfs does not need to drop all the way to the Prior Lake cfs before Prior Lake level starts to drop, because some infiltration and evaporation will occur on Prior Lake. For example, Prior Lake may level off when Spring Lake is discharging 60 cfs and Prior Lake is discharging 50 cfs, but it is largely dependent on variables like sunshine and wind.

Large rains could increase the lake levels quickly due to the saturated conditions in the watershed and little upstream holding capacity. However, according to National Oceanic and Atmospheric Administration (NOAA) on May 25, there is no more than a 30% chance of rain through the Memorial Day weekend. Even though no immediate threat of a large rainfall is in the forecast, the City and Watershed are preparing to dam Spring Lake, if needed. The Watershed is currently in communication with DNR to get approval. City staff would be installing the dam and they have all the materials ready, if needed. If a dam is installed on the Spring Lake outlet, that would likely not occur until next week. Unless the forecast changes significantly, it does not appear Prior Lake would surpass the No-Wake Zone for the Memorial Day weekend.

If the dam does get installed, an emergency procedure is in place and the dam would be removed, or lowered, if the level of Spring Lake nears the OWH level of 912.8’ to prevent damage to structures on Spring Lake.

The Watershed has realized some relief from even higher lake levels by opening the low-flow gate on April 25. The Watershed continues to operate the structure with the gate open (visit https://www.plslwd.org/2017/04/low-flow-gate-opened for more information) and will close the low-flow gate when the lake level drops to 902.5’. At lake levels above 903.4’, the low-flow gate has no impact on discharge because the pipe is full and cannot pass more water through the pipe. However, as the lake rises, pressure will slowly push more water through the pipe.

Staff will continue to provide lake level updates through various media sources and the Public Works Department has initiated a few early flood response protocols in the event water levels reach concerning levels that could start to impact city infrastructure and residents. Stay up to date by watching the City of Prior Lake website (www.cityofpriorlake.com) or the Watershed’s website (www.plslwd.org). The easiest way to find the current lake level for Spring Lake or Prior Lake is to visit the Prior Lake-Spring Lake Watershed District website and click on the links near the top, listed after “Current Lake Levels”.

View Full Article

Keeping an Eye on Springtime Lake Levels: spring flooding potential update

Posted by PLSLWD Staff - March 31, 2017 - News, Flooding Updates, Prior Lake

Update supplied by the City of Prior Lake.

Although the City of Prior Lake and Prior Lake-Spring Lake Watershed District (PLSLWD) staff continually monitor local conditions such as winter snowpack, precipitation, and lake levels to assess flooding risk, early spring is one time of year when flooding potential receives a lot of attention. City staff performed an analysis last year and showed there isn’t a strong correlation between the lake level at the ice-out date and the eventual high water elevation for Prior Lake for any given year.

In 2014, the early spring level for Prior Lake was very close to the long-term average of about 901.6 on April 20, and the record flooding level of 906.17 recorded on June 30 was caused by above average rainfall events over a period of a few weeks in May and June. Other years, such as 2001, start out high (902.75 on 4/15/2001) and the high level for the year is recorded shortly thereafter (904.33 on 4/30/2001); in this case melting snow could have contributed to the high-water levels.

There is currently no snowpack in the watershed above Spring Lake and Prior Lake. Water levels for Prior Lake are above the long-term early spring average of 902.58 as of March 27 and the lake has been discharging through the Prior Lake Outlet Channel since last fall. Our region has been experiencing some very wet weather lately, leading to full storage areas upstream (ponds, wetlands, depressions) and therefore lots of runoff every time we get more precipitation.

Our average annual precipitation is about 31.1 inches and the 365-day precipitation sum (amount of water we’ve received since March 22, 2016) is 40.8 inches. Even with the lack of snow, we could still experience spring flooding, but lake levels will depend only on the precipitation we receive. The National Weather Service in Chanhassen, in their initial Spring 2017 Flood Outlook, agrees with this assessment, stating: “The main flood threat will be determined by the occurrence (or lack of) heavy snow events in March and/or heavy rain events in March and April.” We will continue to keep an eye on the situation.

City staff is currently working on finalizing a plan that includes flood trigger points (lake elevations) for flood response actions and the role the city will play in those actions. The plan will be presented to the City Council soon in the form of a City Policy.

The PLSLWD is also working on short-term flood solutions by looking at how to optimize the operation of the Prior Lake Outlet Structure, which also will involve action at certain lake elevations. Per the recently-completed joint Flood Study, these short-term flood solutions are needed, combined with a long-term strategy of upstream water storage, to help reduce flood impacts on our residents and businesses.

View Full Article Prior Lake, Spring Lake, flooding, flooding potential, lake levels