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The Importance of Maintaining Constructed Basins in Colorado River Sourced Managed Aquifer
Recharge Projects
Margaret SnyderHydrologist
City of Tucson/Water Dept.Planning & Engineering Div.
Water Resource Management Sec.
"An examination of three methods of recharge basin rehabilitation for
maintaining favorable infiltration rates and basin integrity in managed aquifer
recharge systems."
• Desiccation• Surface disturbance
• Sediment removal
Why do we need maintenance?• Tucson recharges >165,000AF annually• TSS was ~3mg/L in 2014• 3mg/L = 670 TONS/year!!
The Colorado River!
Central Avra Valley Storage and Recovery Project (CAVSARP)
• CAVSARP has been operational since 2000• Fine sediment blankets basin floors, drastically
retarding infiltration rates by sealing pores
Native Soil(Clayey Sand)
Silt Layer
Dirt Taco
Dead Algae
Organic layer
Silt
1) Desiccation
• Maintenance• Allowing basins to dry creates deep mud
cracks that partially restore infiltration pathways
• Free• No labor involved• Good for thin or thick layers of sediment
1) Desiccation
Surface vs Deep Cracks
Deep Cracks
Desiccation Drawbacks
• Can take months to dry an inefficient basin to an effective level
• Drying time may not be made up by increased performance
Desiccated
Ripped
2) Surface Disturbance
2) Surface Disturbance
• Maintenance• Breaks up the clogged layer• Restores infiltration pathways immediately• Quickly completed• Impact lasts much longer than desiccation
Original Ripper • 36 inch tines• 8 feet wide• Tracked Challenger tractor• Requires very dry/hard
conditions
• 36 inch tines• 8 feet wide• Tracked Challenger tractor
Original Ripper
OVERKILL!
• 18 inch tines• 8 feet wide• Uses smaller, wheeled tractor• Very maneuverable• $4000
Small Chisel Plow
Plow in parallel furrows for surface disturbance
But there was a problem with scale
• 18 inch tines• 20 feet wide• Tracked Challenger tractor• $37,000
Large Chisel Plow
OriginalRipper
Large Chisel Plow
Chevy Tahoe
ChallengerTractor
Surface Disturbance Drawback
• Need to dry almost as long as desiccation
• Rehabilitation as emergency response• Restores original infiltration pathways• Used in conjunction with chisel plowing,
infiltration rates can exceed a basin’s initial performance
3) Sediment Removal
RB101 Inlet Structure Sediment Removal
Direction of Flow
Buried FlowControl Teeth
RB101 Inlet Structure Sediment Removal
RB101 started receiving flow in 2003 and had not been touched since
RB101 Inlet Structure Sediment Removal
Flow
Flow
Flow
RB101 Inlet Structure Sediment Removal
RB101 Inlet Structure Sediment Removal
Secondary Dissipation
Tertiary Dissipation
Hard Hat
RB111 Sediment Removal
RB111 started in 1997 as a test basin and had not been touched since
• Grader• Front Loader• Min. 2 Dump Trucks
Sediment Removal Drawbacks
• Very time and labor intensive• Needs much more equipment• What to do with the dirt?– 1.3M ft3 removed from RB111– We used it to rebuild berms
Basin2013 Ave
ft/day infil2014 Ave
ft/day infil%
Changenone 101 1.04 0.79 -25%none 102 0.86 0.79 -8%103 0.71 1.69 138%104 0.37 0.93 151%105 0.44 0.77 75%106 0.52 1.15 121%107 0.59 0.88 49%108 0.48 1.27 165%none 109 0.84 0.86 negligible110 0.97 1.76 81%111 rehab Not Enough Data For 2014
Changes in Infiltration Rates with Maintenance
How to Monitor
• Use falling head infiltration rates for each wet/dry cycle• Choose a standard • I use how many days it takes to lose 1ft of head
RB108 Maintenance TrendsRecharge cycles since winter of 2013 (Only dessicated)
But aren’t you driving fine sediment further into the native soil?
• In short, YES• The rate that sediment accumulates is very
rapid• We anticipate necessity to remove sediment
at 5yr intervals, anyway
• Desiccation and plowing should be an ongoing maintenance plan incorporated into a facilities operation
• If basins are ignored a vicious cycle of compounding detriment occurs of extended flooding and slowing infiltration
• Maintenance reduces long term costs by maximizing time between rehabilitations
What Have We Learned?
Recap
• Desiccation– Cheap and easy– Takes a long time
• Mechanical Disturbance– Fast and easy– Some planning and up front costs
• Sediment Removal– Takes a really long time– More planning and up front costs
Conclusions
• Know your dirt!
• Monitor!
• Thank you to the other TW hydros and interns that helped me make this presentation happen and listened to it 8x
• Thank you to the technicians and interns who collected the data used in the graphs and tables