Chapter 9
Why drains back up when the streets flood
The questionWhy does sewage come up my drains when the streets flood?
The short answerDrains back up when the streets flood because the public sewer fills with rain and river water, and the overflow finds the lowest open drain, often a shower or floor drain on a slab house.
By the editors of High Water at Home · Updated · about 7 minutes
Sections in this chapter
9.1 How does floodwater get into a sewer line?
Floodwater gets into a sewer line through gaps that were never meant to hold back a flood. FEMA's guide to protecting utility systems names three routes: water running in through unsealed manholes, pipe joints and line breaks; sewers that carry storm runoff becoming overloaded; and a sewage pump station that fails. When the pipe is full and under pressure, engineers call it surcharged.
A full sewer has nowhere to send new water except backward. FEMA's homeowner retrofitting guide puts it plainly: water can travel the wrong way through sewer lines and come out of toilets or drains.
9.2 Why does it come up in my shower and not the street?
It comes up in the lowest opening on your line because gravity is what moves most sewage, and gravity works both ways. On a slab house in Wharton or El Campo, the lowest openings are usually a shower pan, a tub, a floor drain in a utility room or a toilet. The line from your house joins the public main, so when the main fills higher than your lowest drain, that drain becomes the overflow.
FEMA notes that because most wastewater systems rely on gravity flow, sewage backup can happen even when the building itself does not flood. The same guide warns that backflow may start from a backup some distance from the house and arrive with little or no notice. That is why a dry yard is no promise of a dry bathroom.
9.3 Which Wharton County floods do this?
Both kinds of flood can fill sewers here: the slow river rise and the fast local downpour. A USACE presentation hosted by the city on 19 April 2022 names four sources of flooding in Wharton, with Baughman Slough, Caney Creek and Peach Creek alongside the Colorado River. Rain alone can do it too. A U.S. Army article dated 21 March 2023 reported water marks four feet above ground in neighbourhoods that flooded in 2020 rain events.
On the river, the National Weather Service gauge at Wharton (WHAT2) puts major flood stage at 43 ft, the level at which its impact table has many streets on Wharton's west and southwest sides under water. That number is feet above the gauge's zero, not depth on your lot; the chapter on river stages and house depth shows how to translate it.
9.4 Is a septic house safe from backup?
A septic house is not safe from backup; the route is just different. FEMA explains that floodwater can enter a septic tank through the perforated drainfield pipe and then flow back into the home through the drain lines. On rice-country lots around Louise, Boling and Glen Flora, a soaked field can push water the wrong way just as a full city main can.
The EPA's rule for septic owners is direct: do not use the sewage system until water in the soil absorption field is lower than the water around the house. Septic systems when the ground is soaked covers the field itself.
| Route | What happens | Who it affects |
|---|---|---|
| Infiltration | Floodwater runs into the sewer through unsealed manholes, joints and breaks | City sewer customers |
| Overloaded sewer | Storm runoff fills a line that also carries sewage | City sewer customers |
| Pump station failure | A lift station stops and sewage backs up toward houses | City sewer customers |
| Drainfield route | Floodwater enters the tank through perforated field pipe | Septic houses |
9.5 What should you do when drains start to back up?
When drains start to back up, treat what comes out as sewage and keep people away from it. CDC says a house that has flooded may also be contaminated with sewage, and the protective gear and cleanup steps in Floodwater in the house apply to a bathroom floor as much as a flooded street.
- Keep children and pets out of the room and close the door.
- Stop using water on a septic house, as EPA advises, and hold off on laundry and dishwashers on a city line too, since every gallon you send down has to go somewhere.
- Stay off any wet floor near outlets, cords or appliances.
- On the City of Wharton system, report a sewer problem to the Water/Sewer Department; after hours, the city's page (checked 11 Nov 2026) routes utility emergencies through Wharton Police Department dispatch.
- Take photos before cleanup for any insurance claim.
We found no federal page that gives city-sewer customers a specific "stop using water" rule during a surcharge, so ask the city what it wants on the day.
9.6 Can anything stop it next time?
A valve on the house sewer line can lower the chance of a repeat, if your city allows one. FEMA's 2023 property brochure tells homeowners to consult a plumber about a sewer backflow valve where flooding pushes sewage up through drains. The types, their upkeep and who may install them are in Backwater valves and cleanouts.
For new building in the city floodplain, the City of Wharton's floodplain permit form already calls for automatic backflow devices on water and sewer hookups. Older houses may have nothing of the kind.
Questions readers ask
Does flood insurance pay for sewage that came up the drains?
As FEMA's policy text read on 11 Nov 2026, an NFIP policy leaves out sewer and drain backup unless there is a flood in the area and that flood is the direct cause. A backup during a dry-weather clog is not a flood claim. See Flood insurance and plumbing damage.
Is the water coming out of a floor drain clean rainwater?
Assume it is not. Sewer lines carry household waste, and a surcharged line mixes it with whatever the floodwater picked up. Clean it as sewage, with gloves, boots and eye protection.
Why does the backup stop once the river falls?
When the public main drains below your lowest fixture, gravity runs the right way again and the water in your pipes can leave. The mess left behind still needs cleaning and disinfecting before anyone uses that bathroom.
Who owns the sewer pipe between my house and the main?
This book found no published City of Wharton or El Campo rule on where its part ends. Ask the city's Water/Sewer Department about your lateral before hiring work on it.
Can a stopped-up drain look like flood backup?
Yes. A blockage inside your own line can push water up the lowest drain on a dry day. If the streets are dry and only your house is backing up, the problem may well be in your own line, and a licensed plumber can find it.
What this chapter rests on
The backup routes come from FEMA's P-348 utility guide (2017 edition), backed by its retrofitting guide, its 2023 property brochure, EPA and CDC flood pages, and City of Wharton and USACE material, all checked 11 Nov 2026. This book could not confirm how El Campo's sewer system handles backups; ask the City of El Campo. It gives no rule for water use during a city sewer surcharge because none was published.
Notes to chapter 9
- FEMA — P-348, Protecting Building Utility Systems From Flood Damage (2017) · three backup routes, gravity backup without house flooding, little-notice backflow, septic drainfield route · checked 11 Nov 2026
- FEMA — P-312, Homeowner's Guide to Retrofitting, 3rd ed. (2014) · water flowing backward out of toilets and drains · checked 11 Nov 2026
- FEMA — Protect Your Property From Flooding (2023) · consult a plumber about a sewer backflow valve · checked 11 Nov 2026
- EPA — Frequent Questions about Flooded Homes · septic use only after field water drops · checked 11 Nov 2026
- CDC — Reentering Your Flooded Home · flooded homes may be contaminated with sewage · checked 11 Nov 2026
- USACE / City of Wharton — Levee Lunch and Learn presentation, 19 April 2022 · four flooding sources for Wharton · checked 11 Nov 2026
- U.S. Army — Corps of Engineers work to mitigate flood risk for Wharton, Texas (21 March 2023) · 2020 rain water marks, Santa Fe ditch drainage share · checked 11 Nov 2026
- NOAA/NWS — National Water Prediction Service, gauge WHAT2 · 43 ft major stage and impact statement · checked 11 Nov 2026
- City of Wharton — Public Works · Water/Sewer Department duties · checked 11 Nov 2026
- City of Wharton — After Hours Utility Emergency · after-hours routing through police dispatch · checked 11 Nov 2026
- City of Wharton — Floodplain Development Permit Application · automatic backflow devices on new floodplain connections · checked 11 Nov 2026
- FEMA — NFIP Standard Flood Insurance Policy, Dwelling Form F-122 (Oct 2021) · sewer backup exclusion unless a flood in the area is the cause · checked 11 Nov 2026