The USGS has published flood-inundation maps for the Yellow River from River Drive to Centerville Highway, in its Scientific Investigations Report 2019-5009. The Yellow River crosses Georgia 124 just north-east of town. This guide explains what those maps show, what they do not show, and how a homeowner can use them.
Key takeaways
- The USGS flood-inundation maps cover the Yellow River from River Drive to Centerville Highway.
- The maps are reported in Scientific Investigations Report 2019-5009.
- A flood-inundation map shows predicted extent for a river stage, not the depth in any one house.
- The map is a planning tool, and the high-water line and the readings are the record of a real flood.
- The Yellow River crosses Georgia 124 just north-east of Snellville.
- A homeowner near the mapped reach can see whether the house sits inside the mapped area.
What does the USGS mapping cover?
The mapping covers the Yellow River from River Drive to Centerville Highway, and it is reported in the USGS Scientific Investigations Report 2019-5009.
A flood-inundation map is built from a model of the river and its banks, run at a series of river stages. The map shows the area that would be under water at each stage along the mapped reach, so a homeowner can see which lots lie inside the predicted extent.
The Yellow River crosses Georgia 124 just north-east of Snellville, and the mapped reach is the part of the river that runs near the town. The report is the source to read for the detail of the mapping.
What does a flood-inundation map not show?
The map does not show how deep the water would be at a particular house, how long it would stay, or how the water would move through the drains and the lot.
The map is a prediction for the river stage, and the real flood depends on the rain, the drains and the ground. A house inside the mapped area can take more or less water than the map suggests, and a house outside it can still take water from the rain on its own lot.
The map does not replace the high-water line on the house or the readings in the walls and floors. Those are the records of an actual flood.
How can a homeowner use the map?
The map shows whether a house sits inside the mapped area for the river. It is a starting point for planning, and it helps decide where to look first in a storm.
A homeowner near the mapped reach can see whether the lot is inside the predicted extent, and can plan the first steps accordingly. The low room and the garage are the places to check first if the lot is inside the area.
The map also helps the owner see the path the water would take across the lot, and where the low corner is. Those are useful facts to note before a storm.
How does the map compare with the 2024 and 2009 events?
The Lenora Road high water in 2024 and the 2009 county flood are the two local records of real events. The map is a prediction, and the records are what actually happened.
Big Haynes Creek reached its flood stage at Lenora Road during Hurricane Helene in 2024, and the 2009 flood set off spillways and culvert failures across the county. Those events are the local reference for how water actually behaved.
A homeowner should use the map for planning and the records for the history, and should not treat either as a guarantee for a particular house.
What should the owner keep for the house?
The owner should keep a photograph of the lot, the grade and the drainage path, and a note of whether the house sits inside the mapped area.
Those records help the household see the risk for the lot, and they help the insurer and a buyer understand the property. The map's reference and the date it was consulted belong in the note.
A copy of the high-water line photographs and the restoration record should be kept with the house file, since they show what happened on the lot in a real storm.
Does the map change what a house needs?
The map does not change the steps for a flooded house. It changes where the owner looks first and what the owner plans for in advance.
A house inside the mapped area may benefit from moving valuables up in advance, keeping the lower room clear and knowing where the power and the main are. Those are planning steps, and they cost little.
The drying and the clean-up follow the readings in the house, whatever the map said, and the record shows the readings for each room.
Who should be asked about a particular lot?
The city and the county are the offices to ask about the mapping, the drainage and any permit that a change to the lot would need.
A homeowner who wants to know how a particular lot would be affected should ask the city or the county, and the USGS report can be read alongside that answer. A drainage change or a retaining wall may need a permit, and that question goes to the building department.
The restoration work covers the water in the house. The mapping and the lot's drainage are questions for the offices that hold those records.
How is a flood-inundation map made?
A map of this kind is built from a model of the river channel and its banks, run for a range of river stages. The model predicts the area that would be under water at each stage, and the result is drawn over the land as a map.
The map depends on the survey of the channel and the land, and on the assumptions in the model. It is a planning tool built from those inputs, and it is not a measurement of any particular flood.
The report that describes the mapping explains the inputs and the limits, and it is worth reading before the map is used for a decision about a house.
What do the stages on the map mean?
Each stage is a river height at a gauge, and the map shows the area flooded at that height. A higher stage floods more land, and the homeowner can see how much more.
A stage is a planning reference rather than a forecast. A storm can produce a stage that is higher or lower than the mapped value, and the map does not say when a stage will be reached.
A homeowner can use the stages to see how close the lot is to the edge of the mapped area, and where the water would reach first at a lower stage.
What should a lot-level check include?
A lot-level check looks at the grade, the low corner, the garage, the crawl space vents and the downspouts. Each one is noted with its height relative to the river reach, where that can be estimated.
The check is most useful before a storm, when the owner can still move valuables and clear the drains. A note of the lot's features and a photograph of each helps the owner and the insurer.
A lot that is inside the mapped area should be checked twice a year, and the owner should note any change to the grade or the drainage.
How does a map help with insurance questions?
A map shows where flooding was predicted, which can help an owner understand the risk of the lot. It does not decide whether a policy covers a loss, which the policy and the insurer decide.
An owner who knows the lot is inside a mapped area should ask the insurer in writing whether flood coverage is part of the policy, and what it would cover. The map and the answer belong in the house file.
The insurer's answer should be read against the policy wording, not against the map, since the two answer different questions.
How should the report be read?
The report describes the reach, the method, the stages and the limits of the mapping. It is a technical document, and the owner should read the summary and the limits before the maps.
A homeowner who does not want to read the full report can ask the county or the city for a summary of what it means for the local lots. The summary should be kept with the house file.
The report reference and the date it was read belong in the file, so the owner can show what was consulted and when.
What does a map change for a buyer?
A buyer who sees that a lot is inside a mapped flood area will ask about flood history, insurance and drainage. The house file is the answer to those questions, and it should be complete.
The map, the photographs, the readings, the restoration record and the insurer's answer on flood coverage make up that answer. A file that is missing one of them invites a question that the owner cannot answer.
Keeping the file current is the owner's best protection in a sale.
Frequently asked questions
Which part of the Yellow River is mapped?
The reach from River Drive to Centerville Highway, as reported in USGS Scientific Investigations Report 2019-5009.
Does the map show the depth in my house?
No. The map shows the predicted extent of flooding at each river stage. The depth in a house depends on the lot and the drains.
Is a house outside the mapped area safe?
Not necessarily. Rain on the lot and ground water can reach a house outside the mapped area, so the drainage still matters.
Where is the Yellow River relative to town?
It crosses Georgia 124 just north-east of Snellville.
Is flood insurance covered by a standard policy?
Standard homeowner policies generally exclude flood from rising water, and separate flood coverage is typically needed. The policy and the insurer decide.
Should I keep the map with the house file?
Yes. Keep the report reference and the date you consulted it with the photographs and the restoration record.
Do the maps change every year?
A mapping report is a snapshot of the model at its date. The owner should check the report's date and whether the county has a newer map before relying on it.
Can a homeowner request a lot-level map?
Ask the county or the city first. Any lot-level flood information they hold is the best place to start, and the report sets out the method used.
Will a river rise always come with a storm?
Not always. A river can rise after a storm far upstream, and the map's stages show the level, not the timing. Watch the river and the forecast together.
Sources
This guide is general information for Gwinnett County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.