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Review File:Riverstone Sketch Plan Exhibits Engineering Review March 29, 2017 Page 3 of 5 If this process is pursued, the developer should consult with the LRRWMO Engineer before beginning the study to discuss acceptable methodologies. Stormwater Management Plan: 1. The proposed conditions section states 27.3 acres of new impervious will be created. The next paragraph states 35 acres of new impervious will be created. The locations of these areas must be identified in plan view. 2. The proposed infiltration areas do not meet the LRRWMO criteria for infiltration as outlined below: a. The volume to be infiltrated must be retained in the basin prior to any discharge leaving the basin. The large infiltration basin does not retain any water prior to discharge leaving the pond. b. The bottom of the basin must be at least 3 feet above the ground water level, The infiltration benches do not meet this separation requirement. 3. If water will be discharged directly to the infiltration area then allowed to overflow to the pond,then a sump manhole or water quality treatment structure must be installed up stream of the infiltration basin to capture sediment and debris. 4. Soil borings are required in the area of the infiltration practices to verify the soils are suitable for infiltration. 5. Water cannot be directly discharged to the wetland. It must be treated as noted above. 6. Detailed procedures for constructing the infiltration basins and bringing them online will be required in the final plans. 7. The narrative indicates 0.04 acres of wetlands will be lost. Wetland sequencing must be provided to document this loss. If this area is larger than the de-minimus then mitigation must be provided. 8. The table on page 4 lists Highway 10 as a discharge point. Drainage from this area does not reach Highway 10. The railroad tracks block flow to the south. 9. The title of the HydroCAD models must be revised to include the project name and designate Existing or Proposed conditions. 10. The rainfall used in the model must indicate Ramsey, not Andover. 11. The normal water level of the ponds is 866,the soils report indicates the groundwater level was 858—860. This is 6—8 feet below the normal water level and could result in dry dusty ponds. This possibility must be evaluated during development of the final plans. 12. The ponds must be less than 10 feet deep to comply with the recommended guidelines in the Minnesota Stormwater Manual. 13. A Minimal Impact Design (MIDS) model or P8 model must be submitted showing the water quality improvements provided by the stormwater system. 14. Staff downloaded the De Minimus calculation form and worked through it. This part of Ramsey is in the 2,500 sq. ft. exemption area. The proposed impact is 1,742 sq. ft. This is under the threshold; however, this wetland was described as seasonally flooded type. Staff looked at the several aerial photos from different years in GIS and measured the apparent area of the north east wetland for 1997. There were over 1.2 acres that appear to be wet. The delineation lists the area of this wetland as 0.68 acres.