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Cazier, Tim From: Cazier Tim Sent: Wednesday, January 04, 2012 12:10 PM To: 'jamespirc @aol.com' dam z s !', cr� 1 Cc: Means, Russ Subject: RE: Clarification of comments on JBird drainage Categories: Hard Rock Mr. Pierce, N1-2Oa5- - 2_ You are most welcome to the formula. I'm curious as to which state does not allow WINTR -55 for sites < 1 acre. I'm used to dealing with areas typically greater than 10 acres. I will attempt to provide satisfactory responses to your questions /comments below: Item 1: My comment on using a granular filter vs. a geotextile is based more on my 14 years of consulting experience in designing steep channels and spillways in the mining and solid waste industry. I have seen properly sized riprap in channels as flat as 14 % simply wash off a geotextile filter during the design event. Our working theory was that it was the dynamic environment of going from dry rock on dry fabric to suddenly becoming wet as a result of a short, but very intense thunderstorm. Typical hydraulic testing is done under a more or less steady state condition. The test flume is subjected to low flows for a fixed period of time (usually 10 to 30 minutes) while measurements and data are collected, then the flows are increased in a stepwise fashion for fixed time periods again, until the riprap in the flume fails or the system flow capacity is achieved. I have done extensive Internet searches for studies on using geotextiles vs. granular filters under riprap and have not found anything useful. I've also consulted with geotextile experts on how the "friction angle" changes on wet vs. dry fabric and again have not seen any useful data. Based on your comment related to Ten Cate's usage suggestions, I looked at their "GEOTEXTILE FILTER DESIGN, APPLICATION, AND PRODUCT SELECTION GUIDE - Drainage and Erosion Control Applications ". On page 8, it does mention the dynamic flow conditions, but it does not talk about slope limitations (that I could find). I have also researched several Federal, State & local government entities for guidance. Typically (as for DOT sites, for example) they do allow geotextile filters. However, if you dig a bit deeper, their riprap channels are typically no steeper than 8 or 10 %. I've also noted that the Denver metro area's Urban Drainage and Flood Control District (UDFCD) also allows fabric filters for riprap, but their riprap design method is only applicable to subcritical flows. UDFCD requires supercritical channels to be designed with concrete. I acknowledge the "Conclusions" section of the PAP 790 paper says "A filter cloth (geotextile) or filter layer should be placed under the riprap if there is no bedding layer." However, you should also note that the Bureau and CSU used a granular bedding layer in their test setup (as described in the "Riprap Characteristics" section). Furthermore, they state "The bedding layer thickness and size were designed according to standard Reclamation criteria." I have a pdf version of the Bureau's design standard if you're interested. All this being said, the J Bird spillways are expected to be subjected fairly low design flows and being a DMO, the site will be inspected annually. If you have a strong desire to use a geotextile filter, the Division will consider a stipulation /conditional approval of such. However, if subsequent inspections reveal any sign of failure, it will have to be replaced with suitably approved materials. This will require a Technical Revision (TR) to show the replacement materials are suitable for permanent placement and no release of the site can be considered until corrected.