Flash Flood — Maricopa, Arizona

2021-08-14 · near (gbn)gila Bend Muni, Maricopa, Arizona

Event narrative

Following 2 to 4 inches of rain in 1 to 2 hours over the Sand Tank Wash watershed, catastrophic flooding occurred in the small town of Gila Bend with feet of water, mud, and debris moving through town. Storm surveys were completed by NWS Phoenix, Maricopa County Department of Emergency Management (MCDEM), American Red Cross, and Flood Control District of Maricopa County (FCDMC). FCDMC gages on Sand Tank Wash, Bender Wash, and in the Maricopa Mountains measured rainfall amounts around 3 to 4 inches in 1 to 2 hours for an annual exceedance probability of 0.1%. FCDMC found the most significant floodwaters were within Sand Tank Wash, with lesser flows in Bender and Scott Avenue Washes. The NWS Phoenix survey found evidence of out of bank flooding in Sand Tank Wash 1 to 3 miles south of Interstate-8 (I-8) with 2 to 6 feet of water and sizable debris. The streamflow gage on Sand Tank Wash at I-8 reached alarm stage at 0157MST, bank full at 0341MST, and flood stage at 0352MST. The gage crested at 7.69 feet with 5000 cfs of flow at 0408MST. The floodwaters and debris that moved through town caused flood damage to at least 140 homes and 25 businesses. Most structures impacted were between the Gila Bend Canal and Main Street and then along Pima Street. During the flooding, Maricopa County Sheriff's Office rescued at least 30 individuals from their roofs along with others pulled from stranded vehicles. Photos after the water receded showed the canal was eroded and breached where Sand Tank Wash flows over it. Photos also showed 2 to 5 foot water lines both on the outside and inside of many of the structures, piles of debris, damaged property and vehicles, and erosion. Of the homes MCDEM assessed, 21 were deemed uninhabitable after the flooding. Unfortunately there were 2 direct fatalities with this event. One made a heroic effort to save a stranded motorist before they themselves were swept away in their own vehicle. The other was swept away in one of the flooded washes. Additionally, many farm animals were killed. Besides the fatalities, no other injuries were reported. A state of emergency was declared for this event.

Wider weather episode

A classic bow echo developed along the Mogollon Rim in central Arizona around 1900MST on the 13th and merged with thunderstorm clusters that developed in south-central Arizona around 2030MST to form one southwestward-moving MCS that progressed all the way to the town of Yuma in far southwest Arizona by 0230MST on the 14th. The MCS produced widespread strong winds with localized damaging winds from individual microbursts across the region. The strong winds also generated dense blowing dust in southwest Arizona. In addition to the wind, storms were producing average rain rates of 1 to 2 inches per hour with briefly higher rates in the strongest cells. This led to flash flooding in multiple areas. The town of Gila Bend suffered catastrophic flash flooding after 2 to 4 inches of rain fell over a large portion of the basin containing Sand Tank and Bender Washes, which flow into Gila Bend. A breach of the canal in town due to erosion added to the flash flooding. Significant flow through the washes impacted numerous homes and businesses and there were 2 fatalities. A state of emergency was declared for this event.

View location on OpenStreetMap → (32.8914, -112.6763)

Source: NOAA Storm Events Database, event_id 983830. Narrative written by the NWS forecast office that issued the report.