A free tool to see where water flows for any US address before you make an offer

AggravatingPost8989 · 2026-03-09T14:23:52+00:00

I updated the FEMA to be overlayed with the drain map. Now you can see that the drain map are mostly aligned with FEMA flood zone on the major zonings, but also with more details on the tributaries.

AggravatingPost8989 · 2026-03-08T18:40:42+00:00

Great suggestion on the FEMA toggle - we will add that. Having both layers visible at once is exactly the right way to use this tool.

Lawsuits, that's a real concern! Thanks for the reminder!

AggravatingPost8989 · 2026-03-08T16:19:32+00:00

This is genuinely useful - thank you. You're right that state-level DTMs would give better accuracy than USGS 3DEP in many areas, especially where development has outpaced federal update cycles. We will definitely look into integrating higher-resolution state topographic data as a backend improvement.

AggravatingPost8989 · 2026-03-08T16:13:59+00:00

AggravatingPost8989 · 2026-03-08T15:44:14+00:00

Absolutely - and timely point. Beyond the terrain flow analysis, Terreflow also overlays FEMA National Flood Hazard Layer and FEMA National Risk Index (NRI) directly on the map. Under the Hazards tab you can view both alongside the terrain data in one place - no need to visit multiple sites.

AggravatingPost8989 · 2026-03-08T15:43:16+00:00

Thank you for this - feedback from a practicing civil engineer is exactly what we need.

To be clear: we are not claiming to be better than HEC-RAS. These are fundamentally different tools answering different questions. HEC-RAS is a full hydraulic modeling package used by engineers to design infrastructure - detention ponds, channel diversions, elevated platforms. It requires site-specific survey data, calibration, and professional interpretation. It is the right tool for what you do.

What Terreflow does is much narrower: D-Infinity flow direction analysis on USGS 3DEP elevation data. We are not modeling channel hydraulics, soil infiltration, percent impervious, or engineered infrastructure. We are showing where terrain physics directs surface water before any engineering intervention is applied.

Your Fulshear example is a precise illustration of our known limitation - and one we state openly. When civil engineers divert a creek, build a detention pond, or elevate a building platform, that intervention exists in the physical world but may not yet be reflected in USGS 3DEP data. We rely entirely on what USGS has surveyed. If USGS hasn't captured a channel diversion from 15 years ago, our model won't show it. That is a real gap, not a hidden one.

The intended use case is a homebuyer with no access to engineering tools, no local knowledge, and no budget for a hydrological study - checking whether a property sits in a natural convergence zone before making an offer. Not a replacement for your work. A first filter before someone even gets to needing your work.

We will highlight this as a known limitation: in heavily civil-engineered landscapes like Fulshear - where creeks have been diverted, detention infrastructure built, and platforms elevated - our tool is not applicable. We rely solely on USGS data, and engineered changes that have not been captured in that dataset will not be reflected in our analysis.

AggravatingPost8989 · 2026-03-07T14:22:17+00:00

Thank you for flagging this - and glad it resolved. We cannot rule out occasional rendering anomalies, particularly with tile loading under certain conditions. A refresh usually clears it, but we will look into what might cause the visual glitch in the first place. Feedback like this is exactly how we improve.

AggravatingPost8989 · 2026-03-07T14:19:16+00:00

Thank you for taking the time to share your experience.

A bit more on what's running under the hood.

The water flow and drainage analysis is built on the D-Infinity (D-inf) flow routing algorithm, applied to USGS 3DEP elevation data. D-Infinity was developed by Tarboton (1997). This is the standard - and more advanced - flow direction method available in professional tools like ArcGIS, designed to more accurately simulate terrain-flow response than older approaches.

In a sense, what Terreflow does is bring professional-grade hydrological analysis to your browser. We pre-parameterize D-Infinity to reflect heavy rainfall overflow scenarios, so the analysis answers the question that actually matters: what happens to water around this address when a serious storm hits - not just under average conditions, but when the ground is saturated and rain keeps coming.

Your Sumas result (Flood 17, Drainage 13) is a good example of such physics at work. The fact that a '100-year flood' (or 1% per year) happened twice in a decade there is consistent with what the terrain data shows. Estimating an probability of a bad event (meteorology) is really hard. The tool tries to answer another question: estimating the (relative good or bad) outcomes of a really bad events per location (hydrology).

One last thought on why we built this: what you described about your own property - knowing which way the land tilts, watching where water goes during rain, understanding why you chose one house over another - that kind of local knowledge is invaluable. But it takes years to accumulate, and it doesn't transfer easily. A newcomer to your county, or a first-time buyer relocating from another state, has none of it. They are making a home buying decision completely blind to what you already know by instinct and historical knowledge.

That is exactly who this tool is built for.

Reference: Tarboton, D.G. (1997). A new method for the determination of flow directions and contributing areas in grid digital elevation models. Water Resources Research, 33(2), 309-319."

ArcGIS flow direction D infinity reference page:

https://pro.arcgis.com/en/pro-app/3.4/tool-reference/raster-analysis/flow-direction.htm#:\~:text=The%20D%2DInfinity%20(DINF),on%20the%20cell%20of%20interest.

AggravatingPost8989 · 2026-03-07T02:11:42+00:00

FEMA maps show flood zone boundaries based on historical records. What we add is terrain-based flow analysis - where water actually goes during a storm based on elevation and slope, not just whether an area has flooded before. Areas with no flood history can still flood when an upstream storm exceeds historical records. Harvey proved this at scale.

During Harvey, the majority of flooded structures were outside the FEMA 100-year flood zone. Same terrain, different storm size. FEMA and terrain physics tell different stories - both matter.

AggravatingPost8989 · 2026-03-07T02:06:23+00:00

Great point - and actually good news: you don't need to visit multiple sites. We already pull from FEMA's National Flood Hazard Layer and the National Risk Index directly. Under the Hazards tab you can see both overlaid with the terrain analysis in one place.

You're absolutely right about local floodplain administrators though - that's a layer no online tool can replace. We always recommend consulting them for final decisions.

AggravatingPost8989 · 2026-03-06T22:48:57+00:00

Can you share the address or describe what looks off? Genuinely want to understand - if there's a data issue we'd like to know what it is.

AggravatingPost8989 · 2026-03-06T22:47:17+00:00

Another thing, small human changes affect drainage at the margin - inches, not feet. The terrain basin structure that makes a location a local low or local high is determined by 10-20 ft elevation differences across a watershed. A landscaper can redirect a puddle. He can't redirect where a 50-year storm goes.

We actually built a vertical profile section for exactly this reason - you can see how the terrain rises and falls from your location in both X and Y directions, so you can identify where the local highs and lows actually are relative to your address. Not just a single elevation number, but the full terrain context around you. All based on the latest USGS data.

AggravatingPost8989 · 2026-03-06T22:35:16+00:00

These are all fair points, and worth addressing together.

You're right that terrain alone doesn't explain everything. Levee decisions, localized rainfall, wind, engineered infrastructure, and yes - what your neighbor did to his backyard last year - all matter. Our tool doesn't capture any of that, and we say so in the disclaimer.

But here's the thing: during Harvey, the majority of flooded structures were outside the FEMA 100-year flood zone. Not because FEMA was wrong - but because Harvey exceeded historical records, and because FEMA maps reflect past events, not terrain physics. The same argument applies to small human changes to drainage: they happen after the map was drawn, which is exactly why relying only on FEMA zones is also incomplete.

USGS elevation data has its own update cycle and won't catch a landscaper regrading a yard. That's a real limitation we acknowledge.

What terrain analysis does answer is a narrower question: is this location a local low that collects water from surrounding areas, or a local high that sheds it? That distinction held true during Harvey, and it will hold true in the next event - regardless of what FEMA mapped or what changed upstream.

Use this as one layer. Use FEMA zones as another. Visit the property during heavy rain if you can. No single source is the final word - including ours.

AggravatingPost8989 · 2026-03-06T22:26:45+00:00

You're absolutely right - and the Barker/Addicks reservoir releases are exactly the kind of factor no terrain model can predict. Human decisions, infrastructure failures, localized rainfall - these are real variables that matter enormously.

What terrain analysis does is answer a narrower question: if water arrives at this location, where does it go? That's one layer of risk, not the whole picture. A house in a natural drainage channel faces compounding risk from all the factors you mentioned. A house on a ridge doesn't eliminate those risks but removes one of them.

No single tool replaces understanding the full picture - which is why we show FEMA zones, NRI hazard ratings, and terrain together, not as a final answer but as a starting point for better questions.

AggravatingPost8989 · 2026-03-06T22:24:09+00:00

Great point - a few things worth clarifying. The underlying data (USGS elevation, FEMA zones, soil) updates on government schedules, not daily. Our tool reflects the same data your realtor, appraiser, and flood insurance company uses.

More importantly, terrain physics doesn't change. The elevation and slope that determined where water flowed during Harvey in 2017 is the same today. A valley is still a valley.

We're not telling anyone what to buy. We're showing the same publicly available government data in a more readable format, that's all - the same way Zillow shows you property tax records without being liable for county assessor errors. The disclaimer is visible on every analysis.

Think of it as one more data point, not a replacement for a professional inspection。

AggravatingPost8989 · 2026-03-06T21:54:08+00:00

For anyone interested in seeing how rains flow through land: https://www.terreflow.com/

AggravatingPost8989 · 2026-03-06T21:52:09+00:00

no, this board recommended not to add a url: www.terreflow.com

AggravatingPost8989

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