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   The City of Longville is seeking assistance from the Minnesota Department of Natural Resources (DNR) as concerns continue over falling water levels on Girl Lake following construction of the Rock Arch Rapids project at the lake’s outlet.

   The city met Friday, October 2 with Barr Engineering to discuss the situation and has asked the firm to provide technical assistance in determining whether the newly constructed rock arch rapids has changed the outlet conditions and contributed to lower-than-normal lake levels.

   In a Sept. 24 proposal to the city, Barr Engineering said there is “strong evidence” that the Rock Arch Rapids, as constructed, has reduced the normal upstream lake level. The proposal noted that the DNR has pointed to local drought conditions as the primary cause of the reduced lake levels. Barr proposed reviewing project plans, surveys, construction records, lake-level data and hydrologic information, as well as inspecting the outlet and evaluating the relationship between lake elevation and discharge.

   The proposed engineering review would also examine whether the Rock Arch Rapids was constructed substantially in accordance with its design and whether any differences could affect lake levels or hydraulic performance. The estimated cost of the initial work is $37,100.

   Barr’s Joel Toso, PhD, PE, a senior water resources engineer, has more than 40 years of experience involving hydrologic and hydraulic modeling, flood forecasting and dam engineering. He also taught applied hydrology and hydraulics at the University of Minnesota for nearly 20 years.

   Following a meeting with Longville officials Oct. 2, Toso prepared a letter dated Oct. 5 for the city to provide to the DNR. The letter states that the city wants to address the falling lake levels “apparently due to the configuration of the newly replaced lake outlet.” It also notes that the DNR had requested a letter signed by an engineer supporting the city’s concerns.

   In the letter, Toso explains that lake levels are controlled by the configuration of an outlet. While engineered dams with concrete spillways and gates provide a more defined and controlled reservoir level, the use of rock at the Girl Lake outlet creates a natural control that can be more difficult to precisely establish.

   Barr is asking the DNR to allow the city to temporarily place sandbags at the Girl Lake outlet to reduce outflow while additional information is gathered. According to the letter, the temporary measure would provide time to investigate possible leakage through the outlet’s subgrade, use changes in lake levels to estimate lake inflows and test other configurations if necessary to better match pre-project conditions.

   The proposed sandbag placement would raise the low-flow notch approximately one foot above its current elevation. Barr said the city would need approval relatively quickly, given the approaching cold weather, and anticipated the sandbags could be removed before the lake freezes. 

   The city’s concerns center on whether the current outlet configuration is contributing to water levels that are below the normal levels experienced before the Rock Arch Rapids project.

   Barr’s proposed investigation is intended to provide additional technical information so the city, DNR and other parties involved can better understand the cause of the lower lake levels and determine whether corrective action is necessary.

   The city is now awaiting the DNR’s consideration of the request for temporary sandbag placement and continued discussions regarding the Rock Arch Rapids outlet.

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