Short answer: Onion Creek floods when heavy rain falls across its 321-square-mile basin, including Hill Country land upstream in Hays County, so gutters can’t protect a house from creek floodwater. What a homeowner can do is check whether the lot is in a mapped floodplain (City of Austin’s ATXFloodPro), understand that a “100-year” flood has about a 26% chance of happening at least once in 30 years, carry flood insurance if the risk is real, and route roof water so it doesn’t add to ponding near the house. Creek flooding, street flooding and roof runoff are three different problems with three different fixes.
What to do
- Look up your address in ATXFloodPro and note whether it touches the 25-year or 100-year floodplain.
- Sign up for alerts at Warn Central Texas and know how to check road closures at ATXFloods.
- Ask an insurance agent about flood coverage. Texas regulators say most home policies don’t cover flood, and most flood policies take 30 days to start.
- Keep roof water from adding to the problem: downspouts discharging away from the house, toward the street or a low point on your own lot, never at the slab or across the property line.
- Plan an overflow route for when gutters, barrels or drain lines are overwhelmed.
- Ask the City before regrading, adding fill or building walls on a lot in a mapped floodplain.
October 30–31, 2013
Thirteen years ago this week, on the evening of October 30, heavy rain began falling across Central Texas. The National Weather Service’s summary describes 12 to 14 inches in a narrow swath from Wimberley to Driftwood in Hays County, and 6 to 10 inches across parts of Hays, western Comal and central Travis counties. Camp Mabry, in central Austin, recorded 4.52 inches in the 24-hour reports.
Onion Creek at U.S. Highway 183 rose above 40 feet. USGS’s alert that day put the preliminary crest at about 41 feet, above the gaged record of 36.5 feet set in 2001 and the historic peaks of about 38 feet in 1869 and 1921. The Austin Fire Department’s after-action report gives a crest of 40.17 feet and says neighborhoods east of I-35 along the Onion Creek floodplain were hit hardest. AFD reported roughly 370 people rescued and more than 400 evacuated.
People died. The AFD report records three deaths in its response; a later FEMA article on the Lower Onion Creek buyouts says the flood killed five people and damaged more than 500 homes. The two sources give different counts. Either way, the loss was real, and it is why this post exists.
What the City did afterward
According to FEMA, the hardest-hit lower Onion Creek neighborhoods originally had 855 properties in the 100-year floodplain. Earlier acquisition projects had already removed 323. After the 2013 flood, the City offered recovery buyouts to the remaining properties. FEMA’s article says the Austin City Council authorized $60 million for buyouts in September 2014 and approved buyouts for 232 houses in March 2015. The City describes its buyouts as converting floodplains back to open space and helping families find safe housing away from flood-prone areas.
The lesson for any lot near a creek: no gutter, drain or yard project changes how high a creek can rise.
Three different water problems
| Source | What it looks like | What helps | Who to contact |
|---|---|---|---|
| Creek flooding | Water rising from the creek channel onto the floodplain, sometimes fast | Floodplain awareness, flood insurance, evacuation plans, buyouts and City projects | City of Austin Floodplain Office |
| Local flooding | Streets and yards flooding before water reaches a creek, often where storm drains are too small | City storm-drain projects; reporting drainage problems | City of Austin Watershed Protection |
| Roof runoff and yard ponding | Water pooling by the slab, at downspouts, or in low spots | Gutters, downspout routing, grading, overflow paths | You and your contractor |
The City’s local-flooding page notes that flooding can happen before stormwater reaches a creek, and that storm drains may be undersized in neighborhoods built before drainage rules took effect in the early 1980s. Texas regulators add that, on average, 40% of National Flood Insurance Program claims come from outside high-risk flood areas.
So a lot can pond badly without being in a mapped floodplain, and a lot in a floodplain can have well-routed gutters. Treat each problem separately. A 2026 r/Austin forum post described exactly this: flooding from the street, not the creek.
Scale: your roof versus the creek
USGS lists Onion Creek’s drainage area at the U.S. 183 gauge as 321 square miles. Texas Parks and Wildlife’s stream-segment listing describes the creek running from Blanco County to its confluence with the Colorado River in Travis County. In between, it crosses Hays County.
Here’s the math:
- 321 square miles is about 8.9 billion square feet.
- A 2,000 sq ft roof is about one 4.5-millionth of that.
- In 2013, a gauge just north of San Leanna in south Travis County reported 6.73 inches. On a 2,000 sq ft roof, that’s 2,000 × 6.73 × 0.623 = about 8,400 gallons.
Eight thousand gallons is plenty to pond against a slab if it all leaves one corner. It’s also nothing next to the water that comes down a 321-square-mile basin. Gutters manage the first. They don’t touch the second.
Step 1: Look up the floodplain
The City’s floodplain page walks through the lookup:
- Go to ATXFloodPro.com.
- Click “I want to,” then “Find Floodplain Information.”
- Enter your address.
Things to know when you read the result:
- Austin reviews development in the 25-year and 100-year floodplains. Pending its updated floodplain studies, the City says its regulatory floodplain is based on the current FEMA 500-year floodplain, which better reflects Atlas 14 rainfall.
- The City is remapping floodplains using NOAA’s Atlas 14 rainfall study. In 2018, NOAA reported that Austin’s 100-year, 24-hour rainfall estimate rose by as much as 3 inches, to 13 inches. Maps can change.
- The City says every creek and lake has a floodplain, whether or not it is mapped, and areas in the 100-year floodplain may flood in much smaller storms too.
- If your address is outside City of Austin jurisdiction, check with your city or Travis County as well.
What “100-year” means over a 30-year mortgage
A “100-year flood” is a flood with a 1% chance of being equaled or exceeded in any given year. It’s not a once-a-century schedule. Over many years, the chance of at least one adds up:
Chance of at least one in 30 years = 1 − (1 − annual chance)^30
| Floodplain label | Annual chance | Chance of at least one in 30 years |
|---|---|---|
| 25-year | 4% | about 70.6% |
| 100-year | 1% | about 26.0% |
| 500-year | 0.2% | about 5.8% |
This assumes the chance stays the same every year and that years are independent. Real risk can change as maps, rainfall estimates and development change, and a flood-level event is not the same as damage to a particular house. But the table explains why a “100-year” lot deserves a real plan.
Plan a safe overflow route for roof water
Every gutter system overflows in a big enough storm. The question is where that water goes when it does.
- Downspouts discharge away from the house. PNNL’s guidance is to direct roof drainage away from foundations, at least 5 feet out.
- Pick the destination before you dig. The street, a swale or a low part of your own lot that drains away. Not a low spot against the slab.
- Keep it on your side. Texas Water Code §11.086 addresses diverting surface water in a way that damages another person’s property. Don’t aim downspouts or a new swale at a neighbor.
- Rain barrels need an overflow, too. PNNL says harvesting systems need a properly designed overflow. A full barrel in a storm is just another downspout; route its overflow like one. Barrels don’t reduce creek flooding.
- Be careful with “soak it in” ideas. On the clay soils common east of I-35, water soaks in slowly once the soil is wet. A gravel pit or drywell near the house can simply hold water next to the foundation. Any infiltration design needs storage, infiltration and overflow checked. Look up your soil first with our Web Soil Survey walkthrough.
- Don’t block drainage paths. Fences, raised beds and fill that dam a swale send water somewhere else, often into a neighbor’s yard or back toward your house.
- In a mapped floodplain, ask first. The City reviews development in floodplains during plan review. Regrading, walls and fill can need review.
Our downspouts and drainage page covers buried lines and outlets. If you collect rainwater, see rainwater collection gutters for overflow setups.
What gutters can and can’t do on a flood-prone lot
Gutters can: – Collect roof water and move it to a planned outlet. – Keep roof runoff from soaking one side of the slab. – Keep water off entries, walks and patios. – Feed a rain tank with a planned overflow.
Gutters can’t: – Keep creek water out of a house. – Change your floodplain status or your flood insurance needs. – Fix a storm drain that’s too small for the street. – Make up for grading that slopes toward the house.
A checklist for a lot near Onion Creek (or any creek)
- [ ] FloodPro result saved, with the date you checked
- [ ] Flood insurance discussed with an agent, with the 30-day wait in mind
- [ ] Warn Central Texas alerts set up for your household
- [ ] Every downspout’s endpoint marked on a simple sketch
- [ ] An overflow path from the house to the street or a low point that drains away
- [ ] No discharge crossing the side lot line
- [ ] Floodplain review asked about before regrading, fill or walls
- [ ] Street or storm-drain problems reported to the City
FAQ
How do I know if my house is in a floodplain in Austin?
Use the City’s ATXFloodPro map: click “I want to,” then “Find Floodplain Information,” and enter your address. Note whether it shows the 25-year or 100-year floodplain. Maps are being updated with Atlas 14 rainfall data, so check again before buying or building.
Will gutters stop my yard from flooding?
Only the part that comes from your roof. If water comes from the creek or the street, gutters won’t stop it. Well-routed gutters keep roof water from adding to ponding next to the house.
What does a 100-year flood mean?
A flood with a 1% chance of happening in any year. Over 30 years, that’s about a 26% chance of at least one, assuming the odds stay constant. It can happen in back-to-back years.
Does homeowners insurance cover flooding in Texas?
The Texas Department of Insurance says most home policies don’t cover flood damage and that most flood policies have a 30-day waiting period. Ask your agent, and see FEMA’s FloodSmart site.
Do rain barrels help with flooding?
They hold a small amount of roof water and can reduce what lands near the house in a light rain. In a big storm they fill quickly, so their overflow needs a planned route. They don’t change creek flooding.
Sources
- NWS Austin/San Antonio: October 30–31, 2013 Halloween flash flood event (PDF)
- USGS project alert, October 31, 2013
- Austin Fire Department: Halloween Flood after-action report (PDF)
- FEMA: Lower Onion Creek flood mitigation buyout project
- USGS: Onion Creek at U.S. Highway 183, station 08159000
- Texas Parks and Wildlife: ecologically significant stream segments, Region K
- City of Austin: floodplain management
- City of Austin: local flooding
- City of Austin: creek flooding
- City of Austin: ATX Flood Safety
- Texas Department of Insurance: flood insurance
- NOAA: Texas rainfall frequency update (2018)
- TWDB: rainwater harvesting FAQ
- PNNL: gutters and downspouts
- Texas State Law Library: neighbor law, water damage
For more on gutters that spill in heavy rain, see why gutters overflow; for flooding history farther south on Onion Creek, our Buda page notes the city’s flood-warning sensors; and general questions are on the FAQ. If you’re replacing gutters anyway, seamless gutters explains the options.
Request a gutter estimate. On a lot near a creek, we’d map where each downspout can discharge and where overflow would run, and tell you plainly which problems are roof water and which aren’t.