Storm water management assessment memorandum

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Page 13 of 20 Storm Water Management System Assessment Orono Truck Station IV. Recommendations As previously stated, a follow-up site visit will be completed in spring 2014 to observe the inlet structures, outlet control structures, vegetation, and sheet flow perimeters. A completed Stormwater Basin Questionnaire has not yet been received from the facilities manager. Completion of this survey would allow for greater understanding as to whether the ponds are functioning properly or if problems are arising with inlet/outlet structures, vegetation, or erosion. Based on the first inspection, the following actions are recommended: �► Any vegetation encroaching on the inlet, outlet, or emergency overflow structures should be removed. This maintains proper flow through the pond and ensures that all structures are accessible for maintenance. y Further analysis of the east pond outlet control structure design should be completetl to verify if the permanent water level in the pond is at the design elevation. This analysis should include review of adtlitional design documents (subject to availability) as well as visual inspection of the outlet control structure, which will be completed as part of the follow-up inspection in spring 2014. If the permanent water level is lower than the design elevation, further evaluation should be completed to determine the cause of the low water level. y Accumulated sediment should be removed from the east pond. A scaled drawing of the pontl was not available, and no estimate of sediment to be removed has been calculated. y Accumulated setliment should be removed from the west pond. Since the west pond is a wetland area and there is no documentetl record of a stormwater maintenance plan, approval from the Minnehaha Creek Watershed District will be required prior to performing maintenance activities in the pond. A scaletl drawing of the pond was not available, and no estimate of sediment to be removed has been calculated. � The buck thorn near the west pond is an invasive species. It could be removed and the area routinely mowed to prevent reestablishment, but its removal does not affect performance of the pontl. As noted above, approval from the Minnehaha Creek Watershed District will be needetl prior to performing maintenance activities in the west pond. � We recommend that Hennepin County consider a 15-year cycle of sediment survey antl removal for both ponds. �► Please refer to the attached maintenance schedule. When removing vegetation, care should be taken not to allow any removed vegetation to clog the outlet structure, to re-stabilize bare areas to prevent erosion, and to avoid exposing the pond to hazardous chemicals. A permit is not required to perform sediment removal in retention ponds. However, in June of 2012 the MPCA published Managing Stormwater Sediment Best Management Practice Guidance for Municipalifies, which outlines how to efficiently test and remove sediment from retention ponds. Since the purpose of the stormwater pond is to collect and retain pollutants, sediment should be tested according to this guidance document and the sediment disposed of properly.