Chattanooga Rainfall by the Numbers: What a Drain Has to Carry

Last reviewed 2026-07-25. Journalists, researchers, and community groups are welcome to cite any figure on this page with attribution and a link.

Drainage arguments in this city tend to run on impressions. It rains a lot here, the clay is bad, the yard floods. All true, and none of it is a number you can design against. The figures below are pulled from published federal and municipal sources and nothing on this page is estimated or rounded from memory. They cover how much rain Chattanooga actually receives, how hard it falls, what the regulations require a site to hold back, and what one recent storm looked like against those design depths.

Every figure carries its source, and every source URL points at the page or dataset that holds the number rather than at a homepage. Where a figure could not be confirmed from a primary source it was left off, which is why there is no city stormwater fee quoted here. Precipitation frequency values are point estimates for latitude 35.0456, longitude -85.3097, near the center of Chattanooga, and they shift as you move across the county.

How much rain does Chattanooga actually get?

The 1991-2020 U.S. Climate Normals put annual precipitation at the Chattanooga Lovell Field station at 55.00 inches. That is the reference figure for the metro and the one worth designing against. Rain totals are only half the story, though. The same normals record 83.4 days a year with 0.10 inch of precipitation or more, which works out to roughly one wet day in four, spread across every month on the calendar. Drainage here is not a seasonal problem that arrives in spring and clears up; it is a year-round load on whatever system a house has.

The monthly distribution is flatter than most people expect. March is the wettest month at 5.34 inches and October the driest at 3.59 inches, a spread of under two inches between the extremes. There is no dry season to schedule work into and no month where a wet crawl space can be relied on to dry out by itself. The full set of monthly normals for the Lovell Field station is below.

What size storm does a drain have to survive?

NOAA Atlas 14 Volume 2 Version 3 gives point precipitation frequency estimates for any coordinate in the Ohio River Basin project area. For 35.0456, -85.3097, the partial duration depths run from a 1-year, 24-hour total of 3.07 inches up to a 100-year, 24-hour total of 7.38 inches. For yard drainage the 60-minute column usually matters more than the daily one, because a French drain fails on intensity rather than on volume. An inch and a half in an hour is roughly the 2-year event here; three and a half inches in an hour is the 100-year. A trench sized for a slow soaking rain will surcharge in either one.

These are the same numbers regulators work from, and they sit behind the 2-, 5-, 10- and 25-year 24-hour storms named in Chattanooga's peak rate control requirement. Because they are point estimates, a table pulled for downtown is not the table for Soddy-Daisy or for Fort Oglethorpe. The values below are the estimate for latitude 35.0456, longitude -85.3097, which sits inside the city limits.

What do the rules require a site to keep on the lot?

Chattanooga's stormwater performance standards start at 1,000 square feet of land disturbance. Above that line a site has to manage a stay-on volume, which is the rainfall retained on site rather than discharged off it. The baseline is the first inch of rainfall, evaluated against a storm preceded by 72 hours of dry weather. Inside the South Chickamauga Creek watershed the requirement climbs: new development must manage the first 1.6 inches and redevelopment the first 1.0 inch. Individual home gardening, repairs and maintenance are named among the exemptions.

The thresholds are different in the surrounding jurisdictions, and the gaps are wide. The seven cities in the Hamilton County Water Quality Program require a land disturbance permit at one acre or more, or below an acre when the work is part of a larger common plan of development or sale, a rule in force from the start of 2008. Fort Oglethorpe, under Georgia EPD's Phase II permit, triggers at more than an acre or more than 5,000 square feet of impervious surface. Three jurisdictions, three different numbers, inside a twenty-mile radius.

What does moving this water cost the region?

The Hamilton County Water Quality Program is funded by an annual stormwater fee of $9.00 per residential unit and $108 per impervious acre on commercial property, collected with the property tax bill. That money pays for pollution investigations, construction plan review and inspection across the participating cities and the urbanized part of unincorporated county. It is a small number per household, and a useful reminder that the public drainage network is neither free nor unlimited in capacity.

The sanitary side is where the large figures live. A federal Clean Water Act consent decree entered in 2013 required Chattanooga to eliminate untreated sewage overflows and to pay a civil penalty of $476,400, split equally between the United States and the State of Tennessee. At the settlement announcement the city estimated the required sewer improvements at $250 million and agreed to an $800,000 stream restoration project. On December 4, 2023 the decree was modified to add further work, including equalization basins for wet weather flows, at an estimated $185 million. Every gallon of clean stormwater kept out of a sanitary line is a gallon that spending does not have to handle.

How did August 12, 2025 compare to the design depths?

On August 12, 2025 the Chattanooga Airport gauge recorded 6.42 inches of precipitation in its daily summary. Set against the Atlas 14 point estimates above, that calendar-day total falls between the 25-year 24-hour depth of 5.98 inches and the 50-year depth of 6.67 inches. The comparison is not exact, because a calendar-day total and a sliding 24-hour duration are not the same measurement, and rainfall varied sharply across the metro that day. It does put the event in a bracket, though, rather than in the vague category of very heavy rain.

The aftermath is the more useful data point. In East Ridge, water entered houses on South Moore Road, and the city manager told a council meeting that a hydrology study was needed for the entire city, citing density, wetland development and impervious surfaces, and describing water shooting up out of manholes. Council members asked residents to clear brush, branches and grass clippings out of the drainage ditches on their own property. A storm in that bracket is a design event rather than an anomaly, and how the local system responded is a fair measure of the margin available.

Sources & Methodology

Every figure above was taken from the primary sources below (accessed 2026-07-25). Where a number is derived (arithmetic on published figures) or covers a broader period or area, the text says so.

1. NOAA National Centers for Environmental Information - 1991-2020 U.S. Climate Normals — Annual precipitation normal of 55.00 inches and the 83.4 days per year with 0.10 inch or more, station USW00013882, Chattanooga Lovell Field.
2. NOAA NCEI - 1991-2020 Monthly Precipitation Normals, Chattanooga — Every monthly value in the normals table, including the March high of 5.34 inches and the October low of 3.59 inches.
3. NOAA Atlas 14 Precipitation Frequency Data Server — All 60-minute and 24-hour depth-duration-frequency values for latitude 35.0456, longitude -85.3097, Volume 2 Version 3, partial duration series.
4. NOAA NCEI - Daily Summaries, Chattanooga Airport — The 6.42 inches of precipitation recorded at station USW00013882 on August 12, 2025.
5. City of Chattanooga - Stormwater Performance Standards, Chapter 3 — The 1,000 square foot land disturbance trigger, the stay-on volume definition, the first inch and 1.6 inch South Chickamauga requirements, the peak rate control storms and the listed exemptions.
6. Hamilton County Water Quality Program - Stormwater FAQ — The $9.00 per residential unit and $108 per impervious acre fees, the list of participating cities and what the fee funds.
7. U.S. Environmental Protection Agency - Chattanooga Clean Water Act settlement modification — The 2013 consent decree, the $476,400 civil penalty split between the United States and Tennessee, and the $185 million December 4, 2023 modification.