maintaining basins in colorado river sourced managed aquifer recharge projects — margaret snyder,...

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 started receiving flow in 2003 and had not been touched since


Flow

Flow

Flow


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

[email protected]

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