Chapter 6 - Flooding

Chapter 6 - Flooding

Learning Objectives

• Understand:

  • Basic stream processes.

  • The process of flooding, and know the difference between upstream and downstream floods.

• Know:

  • What geographic regions are at risk from flooding.

  • The effects of flooding and the linkages with other natural hazards.

• Recognize:

  • The benefits of periodic flooding.

• Understand:

  • How people interact with and affect the flood hazard.

• Be Familiar:

  • Adjustments that can be made to minimize flood deaths and damage.

Water Cycle – Important Definitions

• Reservoirs:

  • Include oceans, ice, atmosphere, groundwater, streams, and lakes.

• Water return methods to ocean:

  • Underground flow of groundwater.

  • Surface drainage or runoff, eventually making its way into streams.

• Streams:

  • Any channeled flow of water, regardless of size.

• Rivers:

  • Large streams composed of tributaries.

• Each River is Unique:

  • Variability based on:

  • Drainage basin

  • Slope/gradient

  • Sediment size and volume

  • Water characteristics

  • Climate

  • Human uses

Drainage Basin

• Also Known As: Watershed or catchment.

• Definition:

  • Precipitation that flows to a single stream.

• For Large Rivers:

  • Composed of smaller basins, which feed tributaries.

River Terminology

• Headwaters:

  • The starting point of the stream.

• Mouth:

  • The point where the stream empties into a larger water body.

• Floodplain:

  • Flat surface adjacent to a channel that is filled with water during high water stages.

Floodplain Gradient

• Definition:

  • Slope of the stream channel represented as:
    G = \frac{\Delta h}{\Delta l}

• Characteristics:

  • Steepest at high elevations, levels off as it approaches base level (the lowest point a stream can erode).

• Base Level:

  • Generally at sea level or temporary levels (like lakes).

Steepness and Stream Characteristics

• Headwaters:

  • Typically V-shaped, steeper-sided, and deeper valleys.

• Mouth:

  • Less steep, broader, potentially with a wide floodplain.

Materials Transported by Streams

• Movement of Water:

  • Water transports sediment, which is categorized as follows:

  • Load: Total amount of sediment a stream carries.

  • Capacity: Maximum load that a stream can carry at any given moment.

• Types of Load:

  • Bed Load:

  • Larger particles rolling, sliding, and bouncing along the streambed.

  • Suspended Load:

  • Comprises 90% of total load; consists of silt and clay particles carried above the streambed.

  • Dissolved Load:

  • Materials carried in solution as ions.

Erosion & Discharge

• Role of Streams:

  • Primary agent for erosion, sediment movement, and deposition.

• Factors Affecting Erosion and Deposition:

  • Discharge (Q):

  • Volume of water flowing through a cross-sectional area of the stream channel per unit time.

  • Typically measured in cubic meters per second.

  • Increases with added water:

    • From rainfall events

    • Downstream as tributaries feed into the river.

Velocity

• Definition:

  • Speed of water, measured in meters per second.

• Factors Determining Velocity:

  • Based on channel area; narrower channels = higher velocity.

  • Widens as gradient decreases.

• Velocity Examples:

  • Low Velocity:

  • Occurs when entering oceans, lakes, and ponds.

  • Moderate Velocity:

  • Found in wider channels of plains.

  • High Velocity:

  • Characteristic of steep channels in canyons/mountains.

Velocity and Sediment Load

• Settling of Particles Based on Velocity:

  • High Velocity:

  • Large pebbles and rocky material settle.

  • Moderate Velocity:

  • Sandy materials.

  • Low Velocity:

  • Mud and fine particulates.

• Sudden Velocity Changes:

  • When streams transition from high to low velocity suddenly, sediment falls out of suspension, forming:

  • Alluvial Fans:

    • Landforms created through sediment deposit.

  • Deltas:

    • Formed when sediment extends into a larger body of water.

Channel Types

• Meandering Streams:

  • Characterized by a single sinuous channel that moves back and forth in a floodplain; often occurs in flat areas.

  • Floodplains are formed from repeated flooding depositing finer grain sediments.

• Braided Streams:

  • Composed of two or more intertwining channels, separated by sand and gravel bars; often found in steep gradient rivers with coarse sediment.

Features of a Meandering Stream

• Cut Banks:

  • Areas of greater velocity on outside curves causing erosion.

• Point Bars:

  • Sediment deposits on inside curves where water slows.

• Pools:

  • Deep areas formed by erosion during low flow.

• Riffles:

  • Shallow areas formed by sediment deposition at high flows.

• Meander Scrolls:

  • Indicate historical migration of the channel.

• Oxbow Lake:

  • Crescent-shaped lake formed when a channel is cut off.

River System Zones

• Zone 1 – Zone of Erosion:

  • Headwaters formed from melting snowmelt, rainfall, or groundwater in steep areas, characterized by high-velocity water causing severe erosion.

• Zone 2 – Zone of Transport:

  • Moderate velocity water that moves sediment through the channel; may have meandering or braided channels.

• Zone 3 – Zone of Deposition:

  • Low velocity, allowing sediment to settle; deposits can form alluvial fans, lakes, or deltas.

Flooding

• Definition:

  • Natural process where water flows over the banks of a stream or channel.

• Flood Influencers:

  • Amount and distribution of precipitation in the drainage basin.

  • Rate of soil absorption of precipitation.

  • Rate of surface runoff entering streams.

  • Soil moisture levels; saturated soil cannot hold additional moisture, similar to a wet sponge.

Flood Definitions

• Stream Stage:

  • Water level (depth) in a channel at any time.

• Flood Stage:

  • Indicates water has reached a height likely to cause damage, varying by location.

• Flood Graphs:

  • Stream gauge records produce stage-time graphs and stage-discharge graphs showing stream flow at different stages.

Magnitude and Frequency of Flooding

• Peak Annual Discharge:

  • Highest discharge event each year as shown on a hydrograph.

• Recurrence Interval (RI):

  • Average time between flood events of a specific size, enabling size and frequency estimation.

• Discharge-Frequency Curve:

  • Graph comparing peak annual discharge to RI.

• Recurrence Interval and Flood Odds:

  • 10 Year Flood:

  • 10% chance of occurring annually.

  • 20 Year Flood:

  • 5% chance of occurring annually.

  • 100 Year Flood:

  • 1% chance of occurring annually.

Regions at Risk for Flooding

• Factors Increasing Risk:

  • High population density.

  • Prolonged intense rainfall.

  • Poor land use planning.

• Summary of Geographic Risk:

  • All areas receiving precipitation can potentially flood.

  • Highest risks in areas with poor infrastructure and no monitoring or disaster plans (especially urban areas).

• Flood Statistics (U.S.):

  • Floods ranked as the number-one disaster in the 20th century.

  • ~80 deaths due to flooding annually in 1900s; ~125 deaths in 2000s.

  • Average property damage exceeds $4 billion annually.

Types of Flooding

• Flood from Meltwater:

  • Common in early spring and midwinter thaws in mountainous/high latitude areas.

  • Ice jams can occur, causing back-up floods downstream.

• Flash Floods:

  • Result from intense short-duration rainfall over small areas.

  • Can also be caused by dam breaks, levees, and ice jams.

  • Particularly severe in urban areas, steep terrains, and arid environments.

  • Example:

  • 1976 Rocky Mountain floods in Colorado resulting in 139 fatalities and $35 million in damages.

Downstream Floods (Zone 2)

• Characteristics of Downstream Floods:

  • Large scale, covering extensive areas, and usually from long-duration storms saturating the soil.

• Example: 2004 Floods:

  • Caused by hurricanes resulting in extreme saturations and increased runoff.

Zone 3 - Alluvial Fans and Delta Floods

• Flood Hazards:

  • Defined paths can change in subsequent floods; unpredictable flow channels.

• New Orleans Example:

  • The Mississippi delta system shifts locations frequently due to geological and hydrological changes.

Atmospheric River Flooding

• Definition:

  • Moisture channels in the atmosphere can carry immense water volumes, potentially causing substantial flooding.

• Megaflood Example (California 1861):

  • Caused substantial rainfall leading to a massive flood that submerged the California Great Valley.

Effects of Floods

• Primary Effects:

  • Injury and loss of life.

  • Damage from currents, debris, and sediment.

  • Erosion and sediment deposition.

• Secondary Effects:

  • Short-term water pollution.

  • Risk of disease and hunger.

  • Displacement and homelessness.

• Example:

  • 1998 New England Flood led to sewage contamination in Boston Harbor.

Factors Influencing Flood Damage

• Determinants of Flood Damage:

  • Land use on floodplain.

  • Floodwater depth and velocity.

  • Rate of rise and flooding duration.

  • Seasonal timing of floods.

  • Quality of transported sediment.

  • Effectiveness of forecasting, warnings, and evacuations.

• Detail on Influencing Factors:

  • Land Use: Residential and commercial properties face more damage versus agricultural land.

  • Seasonal Timing: Crop damage is worse during growing seasons.

  • Warning Systems: Can enhance evacuation and preemptive measures.

Case Studies

• 2017 Houston Flood:

  • Largest urban flood in U.S. history due to Hurricane Harvey, causing $200 billion in damages and displacing over 1 million people with 30 deaths.

• Mississippi River Floods:

  • Significant floods in 1973, 1993, and 2008 resulted in substantial property and human losses despite flood control measures.

Fundamental Concepts on Floods

• Meteorological Link:

  • Floods are often linked to weather phenomena capable of prediction.

• Frequency of Flood Events:

  • Houston has experienced 26 floods since 1989, mainly correlated with hurricanes.

Predicting Floods

• Methods of Prediction:

  • Rely on historical records, geological history, and past flood evidence (e.g. high water lines, debris distribution).

• Limitations:

  • Flooding is less periodic than tectonic events; historical records may not demonstrate recent flooding events.

Risk Perception and Management

• Perception Gaps:

  • Policymakers tend to understand floods better than individuals.

  • The federal government promotes floodplain management despite individual ignorance.

• Risk Calculation:

  • Risk is estimated as: Probability x Consequences.

  • Misestimating recurrence intervals can lead to complacency in flood preparedness.

• Recent Example:

  • 2025 Texas floods leading to large casualties in known high-risk areas.

Linkages with Other Natural Hazards

• Flooding Relationships:

  • Primary effects from hurricanes and tsunamis; secondary effects from earthquakes and landslides.

• Additional Consequences:

  • Floods can cause fires, coastal erosion, and landslides due to altered water flow dynamics.

• Climate Change Impact:

  • Increased intensity of storms and flooding occurrences due to climate variations.

Recent Flooding Incidents

• Example: Southern China (Summer 2025):

  • Approximately 34 million people affected by extreme rainfall attributed to climate change.

Human Impact on Flooding

• Human Contributions to Flood Risk:

  • Urban development and land use decisions can exacerbate flooding hazards.

• Example: Manila 2009 Flood:

  • Tropical storm resulted in 18 inches of rain in 12 hours; 1/5th of the population lived in vulnerable areas.

• Changes in Land Use:

  • Affect stream equilibrium; can lead to flooding increases.

System Modifications and Urbanization

• Effects of Urbanization:

  • Increased flood frequency and magnitude due to impervious surfaces (e.g. roads, buildings).

• Example: 2017 Texas Flood:

  • Urbanization contributed to rapid runoff and floods beyond typical historic patterns.

Minimizing Flood Hazards

• Prevention Strategies:

  • Education, land use planning, and structural controls increase resilience against flooding impacts.

• Public Awareness:

  • Education programs can positively influence community behavior around flood risk, exemplified by campaigns like “Turn around, don’t drown.”

Structural Controls

• Types of Barriers:

  • Earthen levees, concrete walls, inflatable fences, and flood doors to mitigate flooding.

• Downsides of Barriers:

  • Can generate a false sense of security; require regular maintenance, with some causing upstream water backups.

• Drawbacks of Levees:

  • Reduce natural floodplain areas, impacting ecosystems and increasing risk of failure.

Flood Proofing Techniques

• Methods:

  • Raising building foundations, constructing barriers, using impermeable materials, and installing pumps/drains.

• Flood Insurance:

  • Requires understanding risk levels and purchasing flood insurance where necessary.

Summary and Conclusion

• Floods represent a significant natural hazard due to geographical, meteorological, and human factors.

• Management strategies combining education, physical barriers, and land use planning can effectively reduce the risks and impacts of flooding.