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Item 2: The proposed spillway design is unconventional. I understand the flow velocities in the throat will be low. However, as the flow transitions to the riprap, I would expect it to go supercritical (as a result of the proposed steep out slope). The Division has a couple of concerns here: A) Unless you can accurately demonstrate where this transition occurs (under several design flows), it would be best to line the entire spillway section as is standard practice; B) in the dry, windy environment at the site, the Division expects the wind alone will remove most of the sand in the proposed 1- 2 inch layer of sand /gravel protecting the geotextile (if you can demonstrate that sufficient vegetation can be established in the spillway throat to hold the sand /gravel in place, then we may consider approval). Item 3: I was unaware of sandstone riprap investigation by Steve Abt, et al. Thank you for bringing it to my attention. The Division is willing to approve the use of the sandstone as long as it passes the hardness field test with the stipulation that if the stone used shows signs of degradation it must be replaced with suitably approved materials. If inappropriate sandstone is used, a TR will be required to show replacement materials are suitable for permanent placement and no release of the site can be considered until corrected. Item 4: To clarify the Division's position on pond design, the 10 -year volume design capacity and the 100 -year peak flow capacity are really separate. The 10 -year design volume is fairly straight forward — provide sufficient storage capacity to retain runoff from the 10 -year, 24 -hour design storm. The spillway design flow capacity is based on the scenario that the water in the pond is at the level of the invert of the lowest outlet structure at the onset of the 100 -year design event (i.e., to the level at which it can be lowered via gravity flow only — no pumps, valves, gates, etc.). In this case, the lowest invert is the spillway crest. The reason for this is that weather patterns that are capable of generating significant storm events may linger or be repeated over relatively short time periods ( a few days to perhaps a week). As such, the spillway should be designed to pass the 100 -year design peak flow regardless of the level of water in the retention /detention structure. Even if the pond is designed to store the 100 - year event runoff, the spillway needs to be designed to pass the 100 -year design peak flow with a beginning water level at the invert of the lowest outlet structure. If you prefer, you may route the 100 -year design storm through the full pond to account for attenuation, and size the spillway accordingly. However, in this case, the ponds and peak flows are so small, I doubt it would be worth the effort — but it's your call. I hope this clarifies your questions and comments. If you have any further questions or comments, please feel free to contact me. Tim Cazier, P.E. Environmental Protection Specialist Colorado Division of Reclamation, Mining and Safety 1313 Sherman Street, Room 215 Denver, CO 80203 ph: 303 - 866 -3567 x8169 fax: 303-832-8106 tim.cazier state.co.us From: jamespirc @aol.com [mailto:jamespirc @aol.com] Sent: Tuesday, January 03, 2012 12:58 PM To: Cazier, Tim Subject: Clarification of comments on JBird drainage Mr. Cazier - First I wish to thank you for providing the riprap sizing formula to be used in conjunction with the PAP 790 riprap design 2