Flood Risk, Alluvial Fans
Most of the West Valley is nowhere near a river that runs. The Agua Fria, the New River, the Gila and the Hassayampa are dry beds for most of the year, and much of the flooding here happens far from any of them: shallow sheet flow on the alluvial fans below the White Tank Mountains, across ground the map may call a moderate risk.
This page is the zone letters read as behavior, what a flood policy does that a homeowners policy does not, and how to find out about your own address before the season.
Water across the floor right now?
Water that came in over the ground is treated as contaminated. Keep children and pets out of the wet rooms, photograph it, and call {{PHONE}}. A person answers at any hour.
The Flood Control District of Maricopa County says it in one line: "You do not need to live near water to be flooded." In a large storm the Agua Fria, which the district gauges at Buckeye Road, the New River, the Gila and the Hassayampa all run, but the water that reaches a garage in Buckeye or Goodyear usually never saw a channel. It came down off the mountains as sheet flow across a fan.
West of the valley the ground rises into the White Tank Mountains. Rain on those slopes lands on rock and thin soil, so most of it runs, and where a drainage reaches the flat the water spreads, slows and drops the sand and gravel it was carrying. Over a very long time that debris builds a broad, gently sloping apron out from the mountain front. The county’s floodplain regulations define it: an alluvial fan is "a sedimentary deposit located at a topographic break such as the base of a mountain front", with "the shape of a fan". The Flood Control District has mapped the two fans on the Buckeye side of the White Tanks in a study it calls White Tank Fans 1 and 2.
FEMA’s description of alluvial fan flooding is the whole problem in a sentence: it originates at the apex, it is characterized by high velocity flow with active erosion and deposition, and it has "unpredictable flowpaths". What crosses the fan is sheet flow, shallow unconfined water moving across a wide apron in shifting braided threads. A channel that carried water down one side of a property this year can fill with its own sediment, and the next storm finds a new low line some distance away. On a fan, last time tells you very little about next time.
Flood zone letters look like paperwork. They are short descriptions of how water is expected to behave on a piece of ground, in FEMA’s own definitions.
The high risk boundary is drawn around a one percent annual chance. A property just outside it has a chance somebody judged to be under the threshold that triggers a federal insurance requirement, on a fan where the channels move and in a valley that has been paved since the panel was drawn.
That requirement is the mandatory purchase rule of the Flood Disaster Protection Act of 1973. The Flood Control District states it for the county: owners whose property is in a regulated 100 year floodplain are required to purchase flood insurance if they have a federally backed mortgage. Where a lender does not require the policy, a regulatory trigger was not met; nobody walked the drainage.
Nearly a third of flood claims come from outside the high risk zones.
FEMA’s FloodSmart program puts it at 29 percent of NFIP claims from 2014 to 2024, and the county’s own page says "One in four flood insurance claims nationally" are for properties outside the mapped floodplains. The map describes what a lender is required to do about a mortgage. It does not describe what the water is going to do.
The Arizona Department of Insurance and Financial Institutions: "Damage caused by flooding is not covered by standard homeowners insurance. To cover your home for flood damage, you must purchase a separate flood insurance policy through the National Flood Insurance Program (NFIP)." The Flood Control District says the same: "Flood damage is not covered by homeowners’ insurance policies."
FEMA says there is typically a 30 day waiting period before an NFIP policy goes into effect, with two exceptions: coverage bought because a government backed lender requires it, and coverage tied to a community flood map change.
The Flood Control District gives floodplain determinations for an address at no fee: 602-506-2419, ask for a floodplain representative. The FEMA Flood Map Service Center is "the official online location to find all flood hazard mapping products created under the National Flood Insurance Program, including your community’s flood map". Read the letter, then the depth if there is one.
Drive from Verrado or Estrella Mountain Ranch out to the farm ground near Arlington after a storm and it looks like two climates. It is one climate and two pieces of engineering.
The master planned communities were designed around the water. Maricopa County requires every subdivision, regardless of lot size, to retain on site all of the runoff it generates in the 100 year, 2 hour storm, so the lots are graded to a drainage plan, streets carry a depth on purpose, and retention basins hold the design storm and release it slowly. A storm above the design limit does not degrade gently.
The older rural ground was laid out around agriculture. On much of it the road is the lowest line in the landscape, so the road becomes the channel, and a house whose driveway meets it stands at the edge of a watercourse nobody mapped. Irrigation ditches and field berms were built to move water in one direction, and a berm built to hold irrigation water is a dam when the water comes the other way. Keep the drainage you were given, too: the county’s regulations count a fence among the obstructions that can divert or retard a watercourse and create an "adverse impact", and filling the swale across a side yard moves that water into somebody’s wall.
Water that crossed open ground is not clean, whatever it looks like on the tile. It has run over roads, yards, fields and, in the rural areas, septic ground. The IICRC S500 standard lists rising water from rivers or streams among its examples of Category 3, grossly contaminated water, and ground water off fields and septic ground is treated as Category 3 once the crew has inspected it. Porous material the water touched is removed rather than dried: carpet and pad, particle board, the lower band of drywall and its insulation. The structure is dried to a logged standard, and the rebuild follows with the same team.
Photograph the yard, the street and the point of entry before anything is moved. Where a claim turns on whether water came in over the ground or through the roof, that is the evidence that answers it. The first hour after water damage is the order to work in, and Why a dry yard floods in half an hour covers the openings the water uses.
No. The lender’s requirement attaches to a mortgage on a building inside the Special Flood Hazard Area. A lender that does not ask is telling you the panel put the parcel outside the one percent line on the day the panel was drawn, and nearly a third of NFIP claims come from outside that line.
It means FEMA expects shallow flooding there in the one percent annual chance event, usually as sheet flow, with an average depth the panel states, from one to three feet. On the fans below the White Tank Mountains that is a description of water crossing the yard in shifting threads, not of a river rising. Ask the Flood Control District at 602-506-2419 for the determination, then walk the lot after the next real storm, while the sediment lines and the flattened weeds show where the water actually went.
You can buy it, but it will not respond to that storm. FEMA says an NFIP policy typically takes 30 days to go into effect, unless a government backed lender is requiring it or it is tied to a community map change, and a homeowners policy will not pay for flood. The spring is when to ask.
The response is time sensitive and the water is not clean. A person answers 24 hours a day, and one team takes it from the removal to the rebuild.