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The Division's engineer has reviewed the submitted • lans and has the following issues that need <br />to be addressed regarding the stormwater controls a d inlet designs proposed: <br />1) There are no conceptual drawings of the spill ays. However, the "Revised Drainage" <br />drawing suggests they are to be riprap -lined •tructures. The weir coefficient used in the <br />calculations implies a smooth concrete spillway section, A riprap -lined spillway will be <br />irregular and have more head Toss, therefore •e less efficient. As such they would be <br />undersized. The Division is requesting drawi gs to clarify the intended spillway design <br />concept and revised calculations if the spillway is not a concrete section. <br />2) If the spillways are in fact to be riprap - lined, t <br />discharge) is at the elevation of the bedding I <br />(i.e., flows will seep /leak through the riprap). <br />storage capacity of the pond to the bedding I. <br />installed in the spillway. The Division request <br />design concept) and if no cutoff wall is to be ii <br />should be adjusted accordingly. <br />3) There are no riprap size calculations. Given tl <br />almost certainly adequate, but we should req <br />(to include gradation, shape, and density, lay <br />Specifications for Road and Bridge Constructi <br />suffice). <br />4) The calculations do not address weir depth, o <br />information to assess freeboard. I think 6 incl <br />case if there is no cutoff wall, a foot should b <br />6) Comment 2: The introduction also states the <br />inconsistent with the Soil Survey Map (Exhibit <br />in 2009. The soil map unit on Figure 3 is prim <br />according to TR -55 is HSG "C ". In conjunction <br />e effective spillway crest (for storage, not <br />yer below the riprap, not the top of riprap <br />This effectively reduces the available <br />yer elevation unless a cutoff wall is <br />a clarification (again of the spillway <br />cluded the design, the available storage <br />e low expected flows, a six -inch D50 is <br />west some calculations and design specs <br />r thickness, etc.). CDOT's Standard <br />n, Division 506 (or equivalent would <br />ly flow depth. We need enough <br />es (above flow depth) is adequate in this <br />required if there is a cutoff wall. <br />Attachment 4 — Surface Water Management : nd Controls Analysis <br />5) Comment 1: The introduction on the first page of Attachment 4 states the calculations <br />for the pond /weir sizing is performed using T• -55. However, the calculations on this <br />page clearly use the rational method, not TR- 5. The rational method is used for peak <br />flow estimates and it appears, detention basi sizing. The Division requirement for <br />stormwater control is based on a 24 -hour des gn storm event (10- and 100 -year <br />recurrence intervals). The rational method d.es not utilize 24 -hour precipitation; it is <br />solely based on the estimated storm intensity for the related sub basin time of <br />concentration. Furthermore, runoff volume ca lculations using the rational method are <br />unreliable and again do not account for a 24- our design storm. Please resubmit the <br />peak flow and volume calculations using the 2 -hour design events and TR -55 <br />methodology as alluded to in the introduction. <br />ydrologic: soil group (HSG) is "D ". This is <br />T, Figure 3) submitted in the original EPP <br />rily 108 /Wrayha stony clay loam, which <br />with the response to Comment 1, please <br />