6.7. SR 04-19-2004 - Laserfiche WebLink

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Mr. Scott Harlicker February 16, 2004 Page Two B. Drainage Calculations The general concept of the drainage calculations is to collect storm runoff, provide quality and quantity control in a series of detention ponds, and then outlet at predevelopment conditions in one of three directions: Either to the south, through the existing wetland complex on the School District property; to the south and west, through the series of ponds in the Nordic Woods development, which ultimately outlet to Ditch 31; and finally, directly west into Ditch 31. There was discussion at the TEP, and an answer will be Coming forth whether or not a two-tiered pond system will be required for the drainage area that outlets directly into Ditch 31 because of its protected water status. The ponding area located in the rear lots of Block 7 needs to have an identified means of access, which appears to be through the rear of Lot 10. This area should be called out on the plan and an additional easement document provided after the plat is filed, showing access across that lot so that the eventual owner of Lot 10 will have knowledge of the use of their property for pond access and will understand that no structures and/or landscaping may be placed in this area. The specific comments we have on the drainage calculations are as follows: Submit all drainage calculations whether they show any runoff or not. The curb numbers (CN) for the existing subcatchments seem Iow. The CN of 25 should be raised to a CN of 35. The Time and Concentration (Tc) value shown for the existing subcatchments seems high. Sheet flow was used for the entire runoff calculation on some of the subcatchments. The 300 lineal feet maximum sheet flow length is not always applicable for the Tc calculation due to steep slopes and/or swales formed in the drainage area. Shallow concentrated flow or channel flow may be more appropriate. Reevaluate the Tc calculations to be sure that the Tc is accurately modeled. The hydrologic soil group was shown as an "A" after development. We believe that this will change when lots are developed and topsoil along with irrigation systems in many cases are used in the development of the lots. Submit storm sewer tabulation and calculations when complete. Block 6 shows a highpoint in the back of several lots with approximately 4:1 slopes toward the houses. Block 7, Lots 8, 9, 10, 11, and 12 are all showing a lower level house elevation that is lower than the required elevation in relationship to the HWL of the pond. These houses should have an elevation one foot higher than the 100-year HWL of the pond. The grading from Lot 1, Block 1 down to Pond B shows the disturbance of a delineated wetland. This item will need to be taken up with the TEP. > Land locked ponds with no outlet must be modeled for back-to-back 100-year storm events or the ten-day snow melt, whichever is greater. The area intake located in Block 2 behind Lots 10 and 11 should be modeled as a pond to determine how much ponding will take place during the 2, 10, and 100-year storm events. The same issue applies for the intake located in Block 7 between Lots 41, 42, and 43. Ltr-O21604-Harticker.doc Howard R. Green Company