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on the drawing, and proofing did not catch the drafting error. That value was too high, and the use of <br />this same large of number, as suggested by the DRMS reviewers, is also not correct. We must <br />remember that this is a quite a small site; not even as large as the average strip mall parking lot. How <br />many times do we see 1.55 cfs flowing off of that asphalt at the strip mall out in Lakewood? Not very <br />often. Another way to make a second comparison let's assume this amount of runoff rate on Grassy <br />Valley located just to the east of the permit area. The value of roughly 1.55 cfs per acre would equate to <br />a flow rate of nearly 1,200 cfs at the point that the Victor road crosses the lower portion of the valley. <br />This number is of the same magnitude as the current flow in the upper Colorado River above its <br />confluence with the Eagle River. That volume of water surely has not flowed down that valley since the <br />end of the last ice age. <br />A numerical review of the area of the calculation shows that it is covered with a good stand of grass over <br />a well- drained coarse - grained soil. The initial abstraction value was calculated to be 2.8 inches, so <br />runoff, using the SCS model, will not occur within the grassed areas at less than the 6 -hour duration <br />storm (per NOAA Atlas 14). If we assume the 2 -hour duration storm (from the NOAA) of 2.36 inches for <br />the 100 -year event, the total average flow from a hypothetical asphalt surface with 100% runoff for an <br />area of 48,000 square feet would only be 1.3 cubic feet per second. The total average flow off of asphalt <br />for the one -hour event would still only maximize at 2.2 cubic feet per second. Thus, it obvious that the <br />1.55 cfs number is way too large and it makes no sense to use it. <br />As the reviewer correctly noted, the SCS program (TR -55) is capable of producing runoff estimates for <br />larger areas, but is not quite so useful for basins as small as we are dealing with in this permit <br />application. Thus, a different method must be used to calculate estimated runoff. Since the size of the <br />basin is so small, the flows can be approximated using a model that assumes instantaneous runoff. <br />Using the basic SCS parameters to calculate the necessary variables, we get the following values for the <br />100 -year event for the grassed and disturbed areas: <br />For the grassy area: <br />Area <br />1.1 acres <br />Runoff Curve Number (CN) 42 <br />Surface Storage (S) 13.8 inches <br />Initial Abstraction (la) 2.8 inches <br />RunOfflooyear storm (CO 0.13 inches <br />Volume100yearstorm (V) 537 cubic ft (24 hrs) <br />Average Flow 24h, 0.006 cfs <br />4 <br />