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  • Flood Control, Oro Valley are stormwater superstars

    Aug 09, 2018 | Read More News
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    Charcoal gray clouds billowed up above a sun-drenched Tucson valley, looming over the area like something out of a 1950s science fiction film, eliciting both fascination and anticipation in anyone in sight of the Santa Catalina Mountains. Fed by warm, wet monsoon air drawn up the slopes of Pusch Ridge and the rest of the range, the clouds gained height and power for several hours, building until the front reached an inevitable tipping point when it made its move and unleashed its fury.

    What followed that July 10 afternoon was a massive downpour that soaked first Oro Valley then the rest of metro Tucson’s northwest side. It packed quite a punch in that area. The Pima County Regional Flood Control District’s ALERT rain gage at Arthur Pack Park near the intersection of Thornydale and Overton Roads recorded 2.4 inches of rainfall in just over an hour with nearly half of that total coming down in the first 10 minutes. The downpour spawned a flood so powerful it contributed to a freight train derailment in Marana.

    Still, for all its bluster, in most places the storm provided little more than a few hours of entertainment for weather watchers and caused only minor inconveniences due to localized street flooding. Even saturated from a long, soaking rainfall just two days earlier, the flood control infrastructure in the area held up.

    “We received no reports of significant flooding problems in any location on the northwest side that was designed with drainage channels and detention basins,” Flood Control Deputy Director Eric Shepp said. “Our ongoing maintenance and sediment removal efforts of this infrastructure ahead of the monsoon really paid off. Some significant problems did develop in the parts of the Tortolita Alluvial Fan that had developed through lot-splitting without the benefit of such infrastructure. The natural channels there aren’t capable of handling the volume of rain generated that day.”

    As the rain came down, teams from the Flood Control District and the Town of Oro Valley Stormwater Utility got to work, inspecting neighborhoods, drainage channels, culverts, detention basins and flood-prone sections of road. Oro Valley Assistant Director of Public Works Aimee Ramsey saw the storm roll in from her back porch, following its path on her phone’s weather app and monitoring email reports from the field.

    Ramsey credits Flood Control’s cooperative, proactive approach to waterway maintenance with helping ensure the Town’s infrastructure is always up to the task.

    “It is the work that the Town and the Flood Control District do before the rains come down that makes all the difference,” Ramsey said of the relationship with RFCD.

    North Ranch Wash on July 10/Photo Credit: Town of Oro ValleyThe two governments divide flood control maintenance responsibilities within the Town limits by size of the watercourse, with the District responsible for publicly-owned watercourses and improvements capable of conveying 3,000 cfs (cubic feet of water per second, equivalent to 22,500 gallons of water) during a 100-year flood and Oro Valley handling everything below that. Older developments built before modern planning techniques often provide the Town’s most challenging flood control issues.

    “New development is less of a problem because you have your stormwater engineers reviewing plans, looking at storm drainage, building infrastructure that meets requirements,” Ramsey said. “Older neighborhoods can be tough. How do you retrofit something into a 40-year-old area without significantly changing the neighborhood, making it acceptable to the community?   And of course many design options are just not within our budget.”

    Staff from both Flood Control and the stormwater utility also collaborate with private landowners, providing advice and expertise to help minimize potential problems. Oro Valley’s geography presents unique engineering challenges. Sitting on the northwestern edge of the Santa Catalinas, most wash systems start as deep, defined mountainside ravines capable of carrying huge volumes of water and debris at great speed but as they approach the Cañada del Oro the slopes flatten, the water slows and the channels become braided, separated by small, often temporary, sandbars.

    “The Flood Control District has been very, very helpful to us in undertaking projects that would be bigger than our operations staff can handle,” Stormwater Utility Project Manager John Lynch said. “They’ve taken out thousands of cubic yards of sand and other material that’s built up in these washes. We work closely with them to dovetail our projects and utilize their services when they are working on washes they are responsible for up here.”

    Oro Valley stormwater engineers compared the event last month to a similar front in 2014 that overtopped the banks of Rooney Wash near Oracle Road and First Avenue, flooding nearby subdivisions and leaving many roads impassable. A joint Town-District effort to remove sediment in that same stretch of wash since then proved highly effective and kept the flow in the channel this time around.

    Monsoon sky“It’s a location we need to carefully monitor because sediment keeps building up there,” Oro Valley Stormwater Engineer Justin Turner said. “Rooney Wash used to go straight into the CDO and now there’s a state highway and development that required redirecting thechannel and flattening the slope causing more sediment to drop out. The County Flood Control District has been very generous in helping us maintain that segment of wash.”

    Engineers as a group are famously dry and detailed-oriented, even staid, but the sight and sound of roaring water careening through a wash or arroyo makes the entire Oro Valley Stormwater Utility team gawk just like everyone else who lives in the Sonoran Desert. Lynch likened their field work during rainy weather to storm chasers in the Midwest who seek out tornadoes. But instead of twisters, they follow the flooding. Going into the field also gives them practical feedback on their designs and models.

    “When the rain started falling, we ran up to North Ranch Wash off of Shannon near Lambert. We’d just done some minor improvements to try and make the wash flow better through a dip section. It was our first opportunity to see how they functioned,” Lynch said. “It’s always amazing to me on smaller washes when you see standing waves, waves that appear stationary within the current. That’s a sign of a powerful flood.”  

    As in Oro Valley, the storm provided a good test for the northwest side’s entire flood control system. A pair of existing RFCD stormwater detention/retention basins off Camino de Oeste passed with flying colors, holding strong as water crested less than two feet below the lip of their spillways. The infrastructure retained almost 100 acre-feet of stormwater, significantly reducing the potential for downstream flooding. Even a neighborhood-scale water harvesting project still under construction at Overton Road and La Cholla Boulevard met its mission.

    File: rentention basin project under construction at Overton/La Cholla“We were happy to see that the flood control infrastructure we had planned and constructed worked as designed,” Shepp said.

    Working together also helps both governments stretch taxpayer dollars and accomplish more while keeping down costs. Lynch estimates cooperating with Flood Control enabled Oro Valley to nearly double the division’s $100,000 maintenance budget and allowed Oro Valley to address more priorities.

    “The Flood Control District has been a very good resource for us, helping us take our limited budget and helped us do more with the limited dollars we have,” Lynch said. “The Flood Control District has been very receptive and accommodating and willing to assist us in removing sediment and vegetation in washes that typically are a little larger and making sure water and debris doesn’t escape those channels.”

    “Oro Valley is a great partner, with a keen understanding of how to optimize our inter-governmental relationship, and is a very active member on the Flood Control District Advisory Committee,” Shepp said.

    Moving forward, Oro Valley and Regional Flood Control plan to continue their collaboration with studies of numerous washes underway, including Carmack Wash, Big Wash, and Mutterers Wash with the intent to help develop sensible, cost-effective, long-range plans to remove more property from federal and local floodplains and save lives.

    Photos (in order from top to bottom):

    1. Storm water running strong in North Ranch Wash on July 10 (credit: Town of Oro Valley Stormwater Utility)

    2. The massive July 10 Monsoon storm as seen from Downtown Tucson. The dark blue band below the clouds obscuring the Tortolita Mountains is the deluge that dropped more than 2 inches of rain in an hour (Pima County)

    3. Oro Valley’s and Pima County Flood Control District’s retention basin project under construction at Overton Road and La Cholla Boulevard. Though not yet complete, the incomplete basins retained hundreds of thousands of gallons of water that otherwise would have flowed into the CDO wash. (credit: Regional Flood Control District).