2012-01-17_REVISION - M2005050 (2)

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Cazier, Tim From: Cazier, Tim Sent: Tuesday, January 24, 2012 4:08 PM To: 'jamespirc @aol.com' Cc: Means, Russ Subject: RE: Latest Update Attachments: PENN_RIPRAP.pdf Mr. Pierce, Apparently I'm also afflicted with a solid inability to get my point across. I've enlarged the Pennsylvania Detail #8 and added some notes in an effort to improve my ability to express my understanding of the spillway design. Unfortunately, Detail 8 also shows portions beyond the plane of the section drawn parallel to the direction of flow in the section —thereby "muddying" the picture. I've colored the portion of the section that is in the aforementioned plane: blue is the earthen plug and magenta is the riprap layer. As best as I can tell, the geotextile is protected by the 2xD50 thickness of riprap (except where the riprap pinches out near the earthen plug). If the geotextile is continued under the earthen plug (represented by the red dashed line), there is a potential seepage path along the geotextile as well as a potential break in the cohesive continuity between the "plug" and the embankment itself. If I understand your proposed approach correctly, this would be a relatively thin layer of protection and potentially problematic. In regards to your first point regarding geotextile use, please note the recommendation by the Pennsylvania authors in the middle of p. 24 regarding riprap and channel gradients. I've included p. 24 in the attached file and underlined their note addressing channels with gradients steeper than 10 %. My take away from this note is that the authors are not altogether comfortable with loose riprap on steep slopes. Following their recommendation, I would not be opposed to placing geotextile under a reno mattress or gabion revetment. With respect to you second point, please note the green line I sketched in to represent the water surface profile. Above the earthen plug, the flow velocity can be expected to be near zero, but as the profile goes to supercritical flow, the velocity will increase and would likely exceed the permissible shear stress / velocity in Table 6 (p. 21) and Table 9 (p. 24), respectively in the subject design manual. If you can demonstrate that the permissible shear stress and velocity is not exceeded in the spillway throat in the sections not to be lined with riprap then the Division will consider the design. However, I'm not altogether convinced a computer model can really demonstrate that, given the boundary conditions. If it is possible, the Division would require an extension of the riprap revetment to a point upstream of the that point of permissible velocity /shear stress by at least a few feet as a "factor of safety ". I hope this helps. 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.cazierkstate.co.us From: jamespirc@aol.com [mailto:jamespirc @aol.comj Sent: Friday, January 20, 2012 12:24 PM