Causes of Flooding Summarized
9/14/2026 – “Causes of flooding summarized” is part of my continuing effort to boil down more than 2 million words in 3100 posts since Hurricane Harvey into short summaries of key topics. I’ve seen the 83 causes of flooding listed below happen over and over again in different places in different combinations at different times. I’ve organized them into three main groupings: Nature, What Humans Do, and What We Fail to Do.
The Three Large Groupings
1. NATURE — Conditions We Largely Cannot Control
These establish the underlying hydrologic and hydraulic setting.
| Factor | Principal effect |
|---|---|
| Extreme rainfall intensity | Overwhelms infiltration and drainage capacity |
| Extreme rainfall duration | Produces large runoff volumes and sustained high flows |
| Tropical storms and hurricanes | Can produce both extreme rainfall and surge |
| Training/stalled thunderstorms | Repeatedly dumps rain over the same watershed |
| Antecedent rainfall | Saturated soils produce much greater runoff |
| Naturally low infiltration soils | Converts a larger percentage of rainfall to runoff |
| Flat terrain/low gradients | Slows drainage and increases backwater sensitivity |
| Watershed size and shape | Controls concentration and timing of runoff |
| Tributary geometry | Can cause flood peaks to arrive simultaneously |
| Coincident tributary peaks | Produces unusually high flows at confluences |
| High downstream water levels | Creates backwater and reduces drainage efficiency |
| Tides and storm surge | Can impede discharge from coastal watersheds |
| Natural sediment transport | Gradually changes channel and reservoir capacity |
| Natural channel migration/erosion | Alters conveyance and can threaten development |
| Naturally high groundwater | Reduces infiltration and drainage capacity |
For Houston, flat terrain deserves special emphasis. A relatively small change in downstream water level, sediment accumulation, subsidence, obstruction, or discharge can propagate upstream as backwater because there isn’t much gravitational head available to overcome it.
2. WHAT WE DO — Human Actions that Increase Flood Risk
These can be divided into five mechanisms: more runoff, less storage, less conveyance, altered timing, and greater exposure.
| Human action | Primary mechanism |
|---|---|
| Add impervious cover | ↑ Runoff volume and peak |
| Clear vegetation | ↑/accelerates runoff |
| Compact soils during development | ↓ Infiltration |
| Install efficient storm sewers | Accelerates concentration |
| Drain natural depressions | ↓ Storage |
| Destroy/fill wetlands | ↓ Storage/infiltration |
| Fill floodplains | ↓ Flood storage |
| Build in floodways | ↓ Conveyance |
| Encroach on channels | ↓ Conveyance |
| Undersize culverts | Creates bottlenecks/backwater |
| Undersize bridges/openings | Creates bottlenecks/backwater |
| Elevate roads with inadequate cross-drainage | Creates barriers to overland flow |
| Channelize/straighten streams | Accelerates downstream flow |
| Construct levees | Redistributes floodwater/removes storage |
| Build inadequate detention | ↑ Downstream runoff |
| Release detention at unfavorable times | Causes peak stacking |
| Make reservoir releases during downstream peaks | Adds to downstream peak |
| Operate reservoirs independently | Can synchronize peaks |
| Withdraw excessive groundwater | Causes subsidence |
| Mine sand in active floodplains/floodways | Can alter channels, erosion and sediment movement |
| Allow excessive construction erosion | ↑ Sediment load |
| Discharge sediment into waterways | ↓ Conveyance/storage |
| Dump debris into drainage systems | ↓ Conveyance |
| Build in flood-prone areas | ↑ Exposure |
| Put critical infrastructure in flood-prone areas | ↑ Consequences |
| Continue development upstream of constrained channels | ↑ Runoff delivered to existing bottlenecks |
This category contains an important concept. A project can make flooding worse by changing where the water goes, how fast it gets there, when it arrives, or how much room is available for it when it gets there.
3. WHAT WE FAIL TO DO — Human Omissions that Allow Flood Risk to Increase
This category captures institutional and cumulative failure, not simply engineering mistakes.
Failure to understand changing risk
| Failure | Consequence |
|---|---|
| Fail to update rainfall criteria | Infrastructure designed for obsolete rainfall assumptions |
| Fail to update flood maps | Risk is systematically understated |
| Fail to model future development | Infrastructure becomes undersized |
| Fail to account for upstream development | Downstream systems inherit unexpected runoff |
| Fail to account for subsidence | Elevations/gradients become obsolete |
| Fail to account for changing rainfall | Design assumptions become outdated |
| Fail to analyze cumulative impacts | Individually small impacts become significant |
| Fail to analyze runoff timing | Peak synchronization goes undetected |
| Fail to consider downstream tailwater | Drainage performance is overstated |
Failure to maintain
| Failure | Consequence |
|---|---|
| Don’t maintain channels | ↓ Conveyance |
| Don’t remove sediment | ↓ Channel/reservoir capacity |
| Don’t maintain storm sewers | ↓ Drainage capacity |
| Don’t clean inlets | Local street flooding |
| Don’t maintain detention basins | ↓ Effective storage |
| Don’t maintain detention outlets | Alters discharge performance |
| Don’t maintain culverts | ↑ Backwater |
| Don’t repair erosion | Channel instability/sedimentation |
| Don’t remove debris/obstructions | ↓ Conveyance |
| Don’t maintain pumps/gates | Reduced system reliability |
Failure to regulate and enforce
| Failure | Consequence |
|---|---|
| Don’t enforce floodplain regulations | Encroachments accumulate |
| Don’t enforce floodway regulations | Conveyance deteriorates |
| Don’t enforce detention requirements | Runoff increases |
| Don’t enforce no-net-fill requirements | Floodplain storage disappears |
| Don’t enforce construction Best Management Practices | Sediment enters waterways |
| Don’t inspect permitted facilities | Problems remain undetected |
| Don’t correct known violations | Violations become effectively permanent |
| Grant repeated variances/exceptions | Standards gradually lose effectiveness |
Failure to mitigate known problems
| Failure | Consequence |
|---|---|
| Don’t fix known bottlenecks | Predictable flooding continues |
| Don’t enlarge undersized crossings | Backwater persists |
| Don’t preserve floodplain | Natural storage disappears |
| Don’t preserve wetlands | Natural attenuation disappears |
| Don’t acquire repeatedly flooded properties | Known exposure continues |
| Don’t complete mitigation projects | Residents remain exposed |
| Don’t fund maintenance | Infrastructure deteriorates |
| Don’t fund mitigation proactively | Recovery substitutes for prevention |
| Don’t adapt infrastructure after major floods | Previous vulnerabilities remain |
Failure to coordinate
| Failure | Consequence |
|---|---|
| Jurisdictions plan independently | Upstream actions impose downstream consequences |
| Reservoirs operate independently | Releases can stack |
| Counties use different standards | Watershed protection becomes inconsistent |
| Cities/counties regulate only within boundaries | Hydrology ignores political boundaries |
| Agencies optimize individual projects | Basin-wide impacts can be overlooked |
| No watershed-wide authority | Nobody owns the cumulative problem |
Conclusion
That last point deserves special attention in the San Jacinto Basin. Some political leaders have actually fought against creating a river-basin-wide flood authority.
But water follows topography, not jurisdictional boundaries. A watershed fragmented among cities, counties, drainage districts, flood-control districts, groundwater districts, river authorities, state agencies and federal agencies creates opportunities for individually defensible decisions to produce collectively undesirable outcomes.
Flood disasters rarely have a single cause. They are usually the product of natural extremes interacting with decades of accumulated human actions and omissions.
And that distinction matters enormously when discussing responsibility. We cannot prevent extreme rainfall. But rainfall alone does not determine how high the water gets, where it goes, how many people are exposed, or how much damage results.
When floods happen, the sheer length of this list encourages finger pointing by those wishing to evade their own responsibility.
Avoiding floods requires everyone to do their part and avoid creating an adverse impact for others.
Posted by Bob Rehak on 9/14/2026
3303 Days since Hurricane Harvey
