Lessons - Reduce Flooding

In the years since Hurricane Harvey, I’ve published more than 3,000 posts about flooding and flood mitigation. They contain more than 2 million words – the equivalent of more than 25 average-length novels.

That’s daunting for even dedicated readers. So, this page will summarize the lessons learned since then. It will expand over time with subpages that focus on:

Generic factors that contribute to flooding
Partly generic factors amplified in the Lake Houston Area
Relatively distinctive factors in the Lake Houston Area, rarely found in combination elsewhere.

I hope it will ultimately become a sort of “quick guide.” I wish to acknowledge the help of ChatGPT in summarizing those 2 million words. I also wish to acknowledge help from dozens of engineers, and elected and appointed officials leading the fight to reduce future flood risk. My hope is that this section can help educate and frame policy decisions.

I will add hyperlinks to subpages over time as I develop them.

I. UNIVERSAL (GENERIC) DRIVERS OF FLOODING

These show up in virtually every developed watershed in the U.S. and globally.

Urbanization & Impervious Cover
- Where it applies: Everywhere growth is occurring
  - Increased runoff volume
  - Faster hydrograph response (reduced time of concentration)
  - Higher peak discharges
- Examples:
  - Atlanta (Chattahoochee Basin)
  - Dallas–Fort Worth (Trinity River)
  - Phoenix (flash flooding in washes)
- Conclusion: This is the single most consistent, anthropogenic driver of flooding worldwide.
Outdated Rainfall Assumptions
- Where it applies: Nationwide (especially where Atlas 14 updates lag implementation)
  - Infrastructure designed to obsolete rainfall curves, maps
  - Underestimation of short-duration intensity
- Examples:
  - Midwest levee overtopping events
  - Northeast urban flooding (e.g., NYC cloudbursts)
- Conclusion: A systemic, engineering/design lag problem. Not region-specific.
Floodplain Encroachment
- Where it applies: Nearly all growing metro areas
  - Loss of natural storage
  - Increased downstream stage elevations
- Examples:
  - Mississippi River floodplain development
  - California Central Valley
- Conclusion: A classic externality problem seen globally.
Fragmented Governance
- Where it applies: Most U.S. metro regions
  - Upstream vs. downstream conflicts
  - Jurisdictional silos
- Examples:
  - Denver (South Platte Basin)
  - Chicago region (multiple drainage districts)
- Conclusion: A structural governance issue, not hydrologically unique.
Infrastructure Bottlenecks
- Where it applies: Universal
  - Bridges, culverts, and channel constrictions control flood elevations
  - Local choke points drive regional outcomes
- Examples:
  - Undersized crossings in Appalachia
  - Urban culvert failures nationwide
- Conclusion: A fundamental hydraulic constraint issue.
Prevention vs. Recovery Imbalance
- Where it applies: Nationwide (FEMA/HUD model)
  - Chronic underinvestment in mitigation
  - Overreliance on post-disaster funding
- Conclusion: An economic and policy pattern across the U.S.

II. PARTLY GENERIC, BUT AMPLIFIED IN LAKE HOUSTON

These exist elsewhere but are more intense, more frequent, or more consequential in the Lake Houston Area.

Detention Limitations at Scale
- Generic aspect:
  - Site-by-site detention doesn’t account for cumulative watershed effects
- Lake Houston amplification:
  - Rapid upstream growth (Montgomery County)
  - Large contributing drainage area relative to downstream capacity
- Why it matters here:
  - The mismatch between distributed detention and centralized bottlenecks (Lake Houston) is unusually stark
Reservoir Operations & Coordination
- Generic aspect:
  - Multi-reservoir systems require coordination
- Examples elsewhere:
  - Tennessee Valley Authority system
  - California reservoir networks
- Lake Houston amplification:
Two key reservoirs with different missions:
- Lake Conroe (water supply + some flood control)
- Lake Houston (primarily water supply, limited flood pool)
- Why it matters here:
  - Timing of releases can materially affect downstream flooding in a densely developed floodplain
  - Dam Gates and Redesign Projects
Low Gradient / Backwater Sensitivity
- Generic aspect:
  - Flat coastal plains are prone to slow drainage
- Examples:
  - Louisiana bayous
  - Florida coastal systems
- Lake Houston amplification:
  - Extremely low slopes near Lake Houston
  - Strong backwater effects extending far upstream
- Why it matters here:
  - Small obstructions → large upstream water surface increases
Subsidence
- Generic aspect:
  - Happens throughout Southeast Texas
  - Driven by population growth and water extraction
- Examples:
  - San Joaquin Valley
  - Atlantic Coast (NYC, Baltimore, Norfolk)
  - Las Vegas
  - Chicago
- Lake Houston Amplification
  - Differential subsidence (Areas in Spring subsiding by 4 feet, much faster than at Lake Houston Dam)
  - Forms bowls in landscape that increase erosion upstream and lead to deposition/backwater effects downstream
  - Erases freeboard factor, makes homes/businesses more susceptible to flooding
- Why it matters here:
  - Fast growth of region
  - Aquifer compaction irreversible
  - Amplifies every other factor

III. RELATIVELY DISTINCTIVE TO LAKE HOUSTON (OR RARE IN COMBINATION)

Industrial-Scale Sand Mining in Active Floodways
- Uniqueness:
  - Few urban watersheds have dense clusters of sand mines directly adjacent to major river channels
- Mechanisms:
  - Pit capture
  - Dike failures
  - Pumping over dikes
  - Chronic sediment mobilization during floods
- Why this stands out:
  - Converts floods into sediment delivery events, not just water events
- Comparable but less intense examples:
  - Wisconsin frac sand regions
  - Parts of the Lower Mississippi
- Conclusion:
  - This is one of the most distinctive features of the San Jacinto watershed.
Sediment-Induced Loss of Conveyance in a Major Urban River
- Generic aspect:
  - Sedimentation occurs everywhere
- Lake Houston distinction:
  - Scale + location:
    - Mouth bars
    - Channel infilling near a critical reservoir
- Why it matters:
  - Directly reduces discharge capacity into Lake Houston
  - Creates persistent hydraulic choke points
- Conclusion:
  - Not unique in existence, but unusual in magnitude and consequence.
Terminal Reservoir with Limited Gate Capacity
- Uniqueness:
  - Lake Houston historically had minimal discharge capacity relative to inflows
- Implication:
  - Water “backs up” into tributaries during major events
- Comparison:
  - Many reservoirs are designed with larger flood-control outlets
- Conclusion:
  - A design mismatch that is particularly consequential in this basin.
Extreme Upstream Growth Feeding a Constrained Outlet
- Uniqueness (in combination):
  - Rapid development in Montgomery County
  - Drainage funnels into a relatively constrained downstream system
- Why it stands out:
  - Classic “funnel into a bottleneck” configuration at metropolitan scale
The “Stacking Effect” (Most Important Insight)
- Individually, none of these are entirely unique. But together, Lake Houston has:
  - Rapid upstream urbanization
  - Extensive floodplain development
  - Industrial sediment sources
  - Low-gradient hydraulics
  - Constrained reservoir outflow
  - Fragmented governance
- Result:
  - A system where runoff is increasing, conveyance is decreasing, and control is fragmented—all at the same time.
  - This combination is rare and particularly unstable.