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RE: Clarification of comments on JBird drainage http: / /mail.aol.comi35478- 111 /aol- 6 /en -us /mail /PrintMessage.aspx
<br />From: Caner, Tim <Tim.Caner @state.co.us>
<br />To: jamespirc <jarnespirc@aol.com>
<br />Cc: Means, Russ <Russ.Means @state.co.us>
<br />Subject: RE: Clarification of comments on JBud drainage
<br />Date: Wed, Jan 4, 2012 12:10 pm
<br />Mr. Pierce,
<br />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
<br />typically greater than 10 acres.
<br />I will attempt to provide satisfactory responses to your questions/comments below:
<br />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
<br />channels and spillways in the mining and solid waste industry. 1 have seen properly sized riprap in channels as flat as 14 % simply wash off a
<br />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
<br />suddenly becoming wet as a result of a short, but very intense thunderstonn.
<br />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
<br />(usually 10 to 30 minutes) while measurements and data are collected, then the flows are increased in a stepwise fashion for fixed time periods
<br />again, until the riprap in the flume fails or the system flow capacity is achieved. I have done extensive intemet searches for studies on using
<br />geotextiles vs. granular filters under riprap and have not found anything useful. I've also consulted with geotextile experts on how the "friction
<br />angle" changes on wet vs. dry fabnc and again have not seen any useful data.
<br />Based on your comment related to Ten Cate's usage suggestions, I looked at their "GEOTEXTILE FILTER DESIGN, APPLICATION, AND
<br />PRODUCT SELECTION GUIDE - Drainage and Erosion Control Applications ". On page 8, it does mention the dynamic flow conditions, but it
<br />does not talk about slope limitations (that I could find).
<br />I have also researched several Federal, State & local government entities for guidance. Typically (as for DOT sites, for example) they do allow
<br />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
<br />metro area's Urban Drainage and Flood Control District (UDFCD) also allows fabric filters for riprap, but their riprap design method is only
<br />applicable to subcritical flows. UDFCD requires supercritical channels to be designed with concrete.
<br />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
<br />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
<br />described in the "Riprap Characteristics" section). Furthennore, they state "The bedding layer thickness and size were designed according to
<br />standard Reclamation criteria." I have a pdf version of the Bureau's design standard if you're interested.
<br />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
<br />annually. If you have a strong desire to use a geotextile filter, the Division will consider a stipulation/conditional approval of such. However, if
<br />subsequent inspections reveal any sign of failure, it will have to be replaced with suitably approved materials. This will require a Technical
<br />Revision (TR) to show the replacement materials are suitable for permanent placement and no release of the site can be considered until
<br />corrected.
<br />Item 2: The proposed spillway design is unconventional. I understand the flow velocities in the throat will be low. However, as the flow
<br />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
<br />here: A) Unless you can accurately demonstrate where this transition occurs (under several design flows), it would be best to line the entire
<br />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
<br />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
<br />the spillway throat to hold the sand/gravel in place, then we may consider approval).
<br />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
<br />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
<br />must be replaced with suitably approved materials. If inappropriate sandstone is used, a TR will be required to show replacement materials are
<br />suitable for permanent placement and no release of the site can be considered until corrected.
<br />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
<br />separate. The 10 -year design volume is fairly straight forward — provide sufficient storage capacity to retain runoff from the 10 -year, 24 -hour
<br />design storm.
<br />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
<br />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
<br />case, the lowest invert is the spillway crest. The reason for this is that weather patterns that are capable of generating significant storm events
<br />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
<br />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
<br />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
<br />lowest outlet structure. If you prefer, you may route the 100 -year design stone through the full pond to account for attenuation, and size the
<br />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.
<br />I hope this clarifies your questions and comments. If you have any further questions or comments, please feel free to contact me.
<br />1 of 3 2/9/2012 12:40 PM
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