Comprehensive Study Notes on Flooding and Flood Management

Introduction to Flooding and the Hydrologic Cycle

• Definition of Flooding: A flood is defined as an event that occurs when water inundates land areas that are normally dry. This typically happens when the volume of water in a stream exceeds the capacity of the stream channel.

• Contextual Example: The aftermath of Hurricane Katrina in New Orleans serves as a primary visual example of the devastating impact of major flooding.

• The Hydrologic Cycle Components: Understanding the behavior of the hydrologic cycle is essential to explaining why floods occur. The cycle involves the following continuous processes:

  • Water Storage in Ice and Snow: Long-term storage of water in solid form.

  • Precipitation: Water falling from the atmosphere as rain or snow.

  • Water Storage in the Atmosphere: Water vapor and clouds.

  • Transpiration: Release of water vapor from plants.

  • Snowmelt Runoff to Streams: Water from melting snow flowing into surface water bodies.

  • Infiltration: Water soaking into the ground surface.

  • Ground Water Storage and Discharge: Water stored beneath the surface that eventually flows back into larger bodies of water.

  • Surface Runoff: Water flowing over the land surface into streams.

  • Freshwater Storage: Lakes, ponds, and reservoirs.

  • Condensation: The process of water vapor turning into liquid droplets.

  • Evaporation: Conversion of liquid water from oceans and other bodies into vapor.

  • Water Storage in Oceans: The largest reservoir of water on Earth.

Watershed Dynamics and Characteristics

• Definition of a Watershed: A land area from which all water drains to a single point, known as the watershed outlet. It is also referred to as a drainage basin and is bounded by a ridge.

• Upstream vs. Downstream Characteristics:

  • Headwaters (Upstream): Located at higher elevations where small stream channels begin. Channels tend to be inside narrow, steep valleys with a steep gradient (slope in the direction of flow).

  • Lower Reaches (Downstream): Near the watershed outlet, the valleys are broader and flatter. The stream gradient is significantly flatter compared to the headwaters.

Mechanics of Flood Processes and Triggering Factors

• Frequency of Flooding: Floods are infrequent; most of the time, water remains contained within the channel. Floods occur during exceptionally large precipitation events or when storm intensity exceeds the soil's capacity to absorb water.

• Watershed Characteristics Influencing Floods:

  • Soil Infiltration Capacity: Coarse-grained (sandy) soils absorb water faster than fine-grained (clayey) soils.

  • Soil Thickness: Shallow soils have less storage capacity and become saturated quickly, leading to runoff.

  • Slope Steepness: Steeper slopes cause surface runoff to move more rapidly into streams, increasing the likelihood of exceeding bankfull capacity.

  • Antecedent Moisture: If the soil is already wet from recent rainfall, it has limited storage for new water.

  • Vegetation Cover: Dense vegetation intercepts and stores water, preventing immediate surface runoff.

Types of Floods: Flash Floods vs. Downstream Floods

• Flash Floods:

  • Location: Typically occur in headwater portions of a watershed with steep slopes.

  • Cause: Rapid surface runoff from intense rainstorms over small sub-watersheds. Other causes include dam failures, sudden breaking of debris jams, or landslides into water bodies.

  • Behavior: Characterized by a very sudden and rapid rise in water levels.

• Downstream Floods:

  • Cause: Produced by rainstorms of wide geographic extent and long duration.

  • Behavior: Runoff from across the large watershed takes a long time to arrive. Because slopes are flatter, water moves slowly, leading to a slow development of the flood.

  • Impact: While they provide ample warning time, they affect large, densely populated areas and are slow to recede, causing long-term disruption.

  • Historical Example: The flooded villages and fields in Bangladesh after the $1991$ Bangladesh cyclone.

Measuring and Predicting Floods

• Flow Rate (Discharge): The volume of water flowing past a specific point per unit of time.

  • Units: Measured in cubic meters per second ($m^{3}/sec$) or cubic feet per second ($ft^{3}/sec$, abbreviated as cfs).

• Hydrographs: A graphical plot of flow rate versus time, used to track the progress of a flood event. For example, a major flood event might reach a flow rate of $30,000\,ft^{3}/sec$.

• Flood Frequency and Recurrence:

  • Recurrence Interval ($T$): The average time between floods of a certain size (e.g., a "$100$-year flood").

  • Frequency ($P$): The probability of a flood of a particular size occurring in any given year ($P = 1/T$). A $100$-year flood has a $1\%$ annual probability.

  • $100$-Year Floodplain: Land areas expected to be underwater once every $100\,\text{years}$. These are considered risky areas, and residents are typically required to have flood insurance.

Hazards and Global Impacts of Flooding

• Injury and Loss of Life: Flooding causes an average of $100\,\text{fatalities}$ annually in the U.S. Globally, numbers are much higher.

  • Case Study: The $1931\,\text{Yangtze–Huai River}$ floods in China affected tens of millions. Death toll estimates, including subsequent famine and disease, range from $400,000$ to $4\,\text{million}$.

• Infrastructure Damage: Currents, debris, and sediment can destroy farms, homes, buildings, railroads, bridges, and roads, leading to mass homelessness.

• Erosion and Deposition: Severe erosion can degrade productive farmland, while sediment deposition elsewhere can cause further damage.

• Pollution: Floods can damage wastewater treatment plants and petroleum storage tanks, releasing pollutants into the river system.

• Secondary Hazards: Disease outbreaks from pathogens in the water and food shortages/famine due to destroyed farmland.

Urbanization and Increased Flood Risk

• Effects of Land Conversion: Moving from wildland or farmland to urban use changes the hydrologic cycle.

• Impervious Surfaces: Pavement and buildings cannot absorb water, increasing runoff.

• Reduced Smoothness: Urban surfaces are smoother than vegetated land, accelerating runoff speed.

• Storm Drains: These systems further accelerate the transport of water into streams.

• Physical Obstructions: Bridges and structures in streams can create debris jams, leading to localized flooding.

• Comparative Data: Hydrographs for Newaukum Creek ($12108500$) and Mercer Creek ($12120000$) in January and February $2000$ demonstrate the differences in hourly unit area discharge between different land uses.

Flood Control Strategies: Dams, Levees, and Channelization

• Dams:

  • Mechanism: Create reservoirs to store flood water and release it slowly over time. This reduces the peak flow downstream.

  • Drawbacks: Expensive to build and maintain; risk of sudden failure due to landslides, erosion, overtopping, or earthquakes.

• Levees:

  • Natural Levees: Formed by sediment deposited outside the channel during periodic flooding, increasing channel capacity.

  • Artificial Levees: Human-built networks to contain water within the channel.

  • Problems: Increased downstream velocity (worsening problems for neighbors), erosion of the stream channel, ecological damage, and catastrophic consequences if a breach occurs.

• Channelization:

  • Process: Clearing, straightening, or paving a channel (e.g., the Los Angeles River).

  • Mechanism: Lowers channel roughness to increase flow capacity.

  • Problems: Increased flow velocity leads to downstream erosion, destruction of habitats/ecology, and danger to people using concrete channels who may be caught in flash floods.

Site Improvements and Land-Use Planning

• Structural Site Improvements:

  • Raising building foundations above the hazard level.

  • Constructing flood walls or protective mounds.

  • Using waterproofing materials in construction.

  • Installing improved drainage systems and pumps.

  • Relocation: Government buyouts of homes in high-risk zones.

• Land-Use Planning: Cited as the best strategy for flood damage control.

  • Method: Avoiding the placement of vulnerable structures in floodplains using FEMA (Federal Emergency Management Agency) maps.

  • Challenges: Pressure for urban expansion into vacant floodplain land, public resistance to regulation, and a general lack of hazard perception.

  • Solutions: Education and monetary incentives to encourage flood-safe planning.