Why Tucson Washes Turn Dangerous So Fast: Q&A With UA Engineer Dr. Jennifer Duan

By the Tucson Restoration Pros editorial team · September 2026

A dry wash behind a Tucson house can become fast, sediment-heavy floodwater in minutes. We asked Dr. Jennifer G. Duan, a University of Arizona professor who studies hydraulics and sediment transport in arid and semi-arid regions, what homeowners get wrong about monsoon flooding, why the first hours after water gets inside matter, and how to judge your own property’s risk. Her answers are below, reproduced as she wrote them, followed by our practical notes on putting them to use.

Key points from Dr. Duan’s answers

• Tucson monsoon floods usually come from short, high-intensity rainfall, so washes can rise suddenly and reach nearby homes “with little warning.”
• Sediment is a big part of the damage. Floodwater that picks up loose channel sediment can become sediment-laden or hyper-concentrated flow, and sometimes debris flow, with forces strong enough to damage walls, culverts, roads and homes.
• In some washes, flow can reach 15–25 feet per second (roughly 10–17 mph).
• Once water is inside, it moves into drywall, insulation, framing, flooring and cabinets quickly. She calls removing standing water and starting the drying process as soon as it is safe “extremely important.”
• Check your parcel on the Pima County Regional Flood Control District flood hazard map, and don’t rely only on older floodplain maps, because channels change over time.

The Q&A

Dr. Duan answered four questions by email in late August 2026. Her answers appear in full. Two misspellings are corrected in [brackets]; nothing else has been changed.

1. For Tucson-area homeowners, what makes monsoon-season flooding and washes so damaging to homes hydrologically? What is happening with the water and soil that people tend to underestimate?

Tucson’s monsoon-season flooding is typically caused by short-duration, high-intensity rainfall. During these storms, runoff can develop very quickly, causing water levels in washes to rise suddenly and inundate nearby residential areas with little warning.

Another important factor is sediment. Because Tucson’s washes are dry for much of the year, loose sediment can accumulate in channels and banks are [vulnerable] to erosion. During intense storms, rapidly moving floodwater can erode and transport large amounts of this sediment, producing sediment-laden or hyper-concentrated flows and, under some conditions, debris flows. These flows can exert substantial forces and cause severe damage to bridges, culverts, roads, walls, and homes along their paths.

Because significant rainfall and flooding are relatively infrequent in Tucson, homeowners may underestimate both how quickly a wash can change from dry to dangerously rapidly moving, sediment-laden floodwater. Flooding impact is a combination of water depth, high velocity, sediment, and debris that can create powerful erosive and impact forces. In severe cases, fast-moving floodwater can undermine foundations, erode the soil supporting a structure, and cause major structural damage within minutes.

2. Once flooding reaches a home, what should a homeowner understand about how quickly water moves into building materials, and why prompt drying matters?

Floodwater can move very quickly. In some washes, flow velocities can reach 15–25 feet per second, or approximately 10–17 miles per hour. At these speeds, floodwater can travel a considerable distance in just a few seconds and can enter a home very rapidly once it reaches the property. Homeowners in flood-prone areas should consider appropriate barriers and other flood-protection measures to help prevent water from entering their homes.

Once water enters a house, it can quickly penetrate porous building materials such as drywall, insulation, wood framing, flooring, and cabinetry. Because many homes contain wood structural components, prolonged exposure to moisture can cause swelling, deterioration, and, over time, weakening of structural materials. Trapped moisture can also create conditions favorable for mold growth.

For these reasons, removing standing water and beginning the drying process as soon as it is safe to do so is extremely important. Prompt drying can significantly reduce damage to building materials, limit mold growth, and lower the extent and cost of repairs.

3. Are there wash-proximity or watershed factors that make certain Tucson neighborhoods meaningfully higher risk, and how might a homeowner gauge their own exposure?

Tucson [residents] can check their property flood risk at Pima County Regional Flood Control District (https://gis.pima.gov/apps/floodhazard/). For flooding concerns, [residents] can reach out to the District for questions.

4. Is there a research-based step you wish more homeowners took before monsoon season to reduce flood damage?

FEMA provides nationwide flood hazard maps, including maps of the 100-year floodplain. However, flood risk can change over time as watersheds and channels evolve. For example, sediment accumulation in Tucson-area washes can raise channel-bed elevations and potentially increase flood levels and alter inundation areas. Therefore, homeowners should not rely solely on existing floodplain maps to understand their current flood risk.

Advances in computational hydraulics, weather forecasting, and cloud computing now make it increasingly feasible to develop county-wide, real-time flood forecasting and warning systems. Such systems could combine rainfall forecasts with hydrologic and hydraulic models to provide several days of advance information about potential flood risk across the county. More localized warnings could then be sent to homeowners in areas expected to experience flooding.

Before monsoon season, I would encourage homeowners—particularly those living near washes or in low-lying areas—to understand their property’s flood risk and have a flood-preparedness plan in place. With sufficient advance warning of a major storm, homeowners could take appropriate protective measures, such as deploying temporary flood barriers, protecting vulnerable entrances, and moving valuable belongings to safer locations.

Putting it to use: our notes for Tucson homeowners

The rest of this article is written by the Tucson Restoration Pros editorial team. It is our practical guidance, not Dr. Duan’s.

What 15–25 feet per second looks like on your lot

Run the numbers on Dr. Duan’s range and a 100-foot stretch of wash is covered in roughly four to seven seconds. That is why watching the wash and waiting to see whether it rises is not a plan. By the time water is visibly climbing the bank, the time for moving cars, sandbagging a door or carrying boxes out of a garage has mostly passed. Anything you want to do has to happen when the forecast turns, not when the wash starts running.

The same speed is why a running wash is never safe to cross on foot or by car. The water is often opaque with sediment, so you cannot see how deep it is or whether the crossing underneath has washed out. In Arizona, drivers who go around barricades into a flooded wash and need rescue can be billed for the cost of that rescue under the state’s “Stupid Motorist Law.”

Check your parcel, not just the FEMA map

Dr. Duan’s two map-related answers point the same direction. Start with the Pima County flood hazard map she names, and read the District’s Flood Hazard Map & Information page. Pima County notes that local floodplains, erosion hazards and riparian areas cover more ground than the FEMA-mapped floodplains, because FEMA mapped only the major watercourses. A house that sits outside a FEMA zone can still sit inside a county-mapped floodplain or erosion hazard setback.

When you look at your parcel, check:

• How close the nearest wash is, including small unnamed washes and drainage channels that run behind houses or along a street.
• Whether your lot sits lower than the street or the wash bank. Water follows grade. If the driveway slopes toward the garage, so will runoff.
• Erosion hazard or setback lines near the wash. Dr. Duan’s point about banks eroding and soil being undermined is exactly what these setbacks are meant to address.
• Whether the channel near you has filled in with sand and gravel since the map was drawn. She notes that sediment buildup can raise the channel bed and change where water goes.

If anything is unclear, the Regional Flood Control District is the right place to ask. Dr. Duan suggests contacting them directly with flooding questions.

A simple flood-preparedness plan for wash-adjacent homes

Dr. Duan recommends having a plan before monsoon season. For most Tucson households, that plan fits on one page:

1. Know your trigger. Decide in advance what forecast or warning makes you act. Have weather alerts turned on for your phone and know which wash near you floods first.
2. Name your weak entrances. The usual ones are a garage door that faces a slope, a sliding door at patio level, a casita or detached studio closer to the wash, and any door that sits lower than the yard.
3. Stage your barriers. If you use sandbags or temporary barriers, keep them where you can reach them in the dark and rain, not behind stored boxes.
4. Get valuables up and out. Garages and casitas flood first. Keep documents, photos, electronics and anything you can’t replace off the floor and away from the side closest to the wash.
5. Write down your shutoffs. Know where the main water shutoff and electrical panel are, and how to reach them from a dry spot.
6. Photograph the house now. A quick walk-through video of each room, taken on a dry day, is a useful record if you ever need to show what was damaged.

If water already got in

This is where Dr. Duan’s second answer matters most. Water that reaches drywall, insulation, framing, flooring and cabinets doesn’t stay on the surface, and the time it spends in those materials drives both mold risk and how much has to be removed.

• Safety first. Don’t walk through water that may be in contact with outlets, cords or the panel. If you can’t reach the breaker from a dry spot, leave it and call the utility or an electrician.
• Treat floodwater from outside as contaminated. Water that came across the yard or out of a wash carries soil and whatever it picked up on the way. Restoration contractors generally classify it as Category 3, which changes what can be dried in place and what has to come out. Our guide to Category 1, 2 and 3 water explains the difference.
• Document before you clean. Photograph the waterline on walls, the sediment left behind, and damaged items before anything is moved.
• Start water removal and drying as soon as it is safe. Surfaces in Tucson can feel dry within a day, while the bottom of the wall, the carpet pad and the framing stay wet. Our article on why “it looks dry” isn’t the same as dry explains how moisture readings are used to decide when drying is done.
• Check your coverage. Standard homeowners policies usually exclude flood from surface water, so it’s worth reading our overview of flood insurance in Tucson before you file.

For a step-by-step timeline after a storm, see Monsoon Flooding in Tucson: Your First 48 Hours. If a wash backed water into a garage or casita, our guide to wash and arroyo flooding in garages and casitas covers that situation.

Sources

• Dr. Jennifer G. Duan, University of Arizona, email answers to Tucson Restoration Pros, August 2026. Faculty profile.
• Pima County Regional Flood Control District, Flood Hazard Map and Flood Hazard Map & Information.
• National Weather Service Tucson, Monsoon information (the official monsoon season runs June 15 to September 30).

Dr. Duan’s answers are her own and are published with her participation. They are general educational information, not an engineering assessment of any specific property. Her participation is not an endorsement of Tucson Restoration Pros or of any contractor. For questions about a specific parcel, contact the Pima County Regional Flood Control District.

Tucson Restoration Pros is a dispatch and referral service for Tucson and Southern Arizona. We connect homeowners with independent, licensed local restoration contractors. We are not a restoration company and we do not perform the work ourselves.