Agenda - Environmental Policy Board - 03/02/2015

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Updated Storm Water Management Plan (SWMP) City of Ramsey, Minnesota across the watershed, etc.). Based on local conditions, the SCS hydrograph was used for development of the Ramsey storm water models in this plan. A SCS 24-hour Type II storm distribution with 100 -year intensity was used for the design of ponds and drainage systems. The Soil Conservation Service has determined from National Weather Bureau data that a Type II distribution is the storm event recommended for the upper -Midwestern United States. The SCS hydrograph method is based on sound hydrologic theory and is commonly used to analyze runoff for the design and analysis of flows and water levels. The detailed modeling computations for this plan have been performed using the SSA Modeling Software as developed by Boss International, Inc. 3. Rainfall Probability Rainfall amounts for hydrologic analyses should be based on: Technical Publication 10 (TP 10) rainfall data for the United States shows that a 5.9 inch rainfall has a statistical probability of occurring once every 100 years in the Ramey area. This is not to say that a 5.9 inch rainfall cannot occur more often, in subsequent years, or even on multiple occasions within the same year; it having a 1 percent chance of occurring in any given year. NOAA Atlas 14: Sanja Perica, Deborah Martin, Sandra Pavlovic, Ishani Roy, Michael St. Laurent, Carl Trypaluk, Dale Unruh, Michael Yekta, Geoffrey Bonnin (2013). NOAA Atlas 14, Volume 8, Version 2, Precipitation -Frequency Atlas of the United States, Midwestern States. NOAA, National Weather Service, Silver Spring, MD. More recent updates of these documents shall be used, if available. The SCS National Engineering Handbook snowmelt data shows the 100 -year, 10 -day snowmelt event is 7.3 inches over 10 days. 4. Pond and Pipe Design Criteria To provide reasonable protection of downstream facilities, analysis of flood levels, storage volumes and flow rates for water bodies and detention basins shall be based on the range of rainfall and snow melt durations producing the critical flood levels and discharges. This report recommends a 10 -year frequency design for storm sewer pipe using the Rational Method'. It is further recommended that pond design be based on the greater of the 100 -year, 24-hour frequency SCS rainfall event, or the 100 -year, 10 -day snowmelt event for overland drainage and pond storage design. In comparing the peak pond elevations for each of these events, the 100 -year SCS rainfall event, with the assumption that the infiltration rate was negligible, created the highest peak pond elevations. Hence, throughout the remainder of this report, the peak 100 -year pond rates are discussed for typical pond High Water Levels (HWL). These design criteria were selected for the analysis and design of the drainage system for this SWMP. In addition, a 10 - inch, 24-hour rain event has also been modeled over the entire city to analyze all ' The Rational Method is markedly different than the SCS methodology in that it does not deal with runoff volumes, only flow rates. An explanation of the Rational Method is made later in this report. Section IV February 20, 2015March 6, 2015 Page 35