🌊Hydrology Unit 9 – Flood Hydrology and Flood Frequency Analysis

Key Concepts and Definitions

• Flood hydrology studies the occurrence, distribution, and movement of water during flood events
• Flood frequency analysis estimates the probability of a flood event of a given magnitude occurring in a specific time period
• Return period represents the average number of years between flood events of a certain magnitude or greater (100-year flood)
• Hydrograph depicts the rate of flow versus time past a specific point in a river, or other channel or conduit carrying flow
  • Peak discharge is the maximum flow rate during a flood event
  • Time to peak is the time from the beginning of the flood event to the peak discharge
• Catchment refers to the area of land that drains water to a particular point in a river system
• Runoff coefficient expresses the fraction of total rainfall that appears as runoff
• Antecedent moisture conditions describe the relative wetness or dryness of a watershed before a precipitation event

Causes and Types of Floods

• Fluvial floods occur when water levels in rivers, lakes, or streams rise and overflow onto surrounding land
• Pluvial floods result from heavy rainfall events that cause surface water runoff to exceed the capacity of drainage systems
• Coastal floods happen when storm surges, high tides, or tsunamis cause sea water to inundate coastal areas
• Flash floods are characterized by rapid onset and high velocity water flow, often due to intense rainfall or sudden release of water from a dam or ice jam
  • Flash floods can be particularly dangerous due to their sudden nature and powerful flow
• Urban floods occur in cities and towns when the built environment alters natural drainage patterns and increases impervious surfaces
• Groundwater floods result from a rise in the water table above the land surface, often due to prolonged rainfall or changes in land use
• Ice jam floods happen when floating ice accumulates and blocks river flow, causing water to back up and overflow

Hydrologic Cycle and Flood Generation

• The hydrologic cycle describes the continuous movement of water on, above, and below the Earth's surface
• Precipitation, such as rainfall or snowmelt, is the primary input of water into a catchment
  • Intensity, duration, and spatial distribution of precipitation influence flood generation
• Infiltration is the process by which water enters the soil surface and moves downward
  • Soil type, land use, and antecedent moisture conditions affect infiltration rates
• Surface runoff occurs when the rate of precipitation exceeds the rate of infiltration and other losses
• Evapotranspiration is the combined process of evaporation from the Earth's surface and transpiration from vegetation, which returns water to the atmosphere
• Groundwater flow contributes to baseflow in rivers and can influence flood magnitude and duration
• Catchment characteristics, such as size, shape, slope, and land use, affect the timing and magnitude of flood events

Flood Hydrology Basics

• Flood hydrographs are used to analyze the magnitude, timing, and duration of flood events
  • Rising limb represents the increase in discharge from the start of the flood to the peak
  • Falling limb represents the decrease in discharge from the peak to the end of the flood
• Baseflow is the portion of streamflow that comes from groundwater or other delayed sources, rather than direct surface runoff
• Direct runoff is the portion of streamflow that comes from precipitation or snowmelt that reaches the stream channel quickly
• Unit hydrograph theory assumes that the direct runoff hydrograph resulting from one unit of excess precipitation is constant for a given catchment
  • Unit hydrographs can be used to predict the flood hydrograph for any amount of excess precipitation
• Routing is the process of determining the timing and magnitude of flow at a downstream point based on the flow at an upstream point
  • Hydraulic routing considers the physical characteristics of the channel or conduit
  • Hydrologic routing uses mathematical models to simulate the storage and movement of water through a catchment

Data Collection and Analysis

• Streamflow data is collected using stream gauges that measure water level and convert it to discharge using a rating curve
  • Rating curves are developed by measuring discharge at various water levels and fitting a curve to the data points
• Precipitation data is collected using rain gauges, weather radar, or satellite imagery
  • Thiessen polygons or isohyetal maps are used to estimate areal precipitation from point measurements
• Historical flood data, such as high water marks or paleoflood evidence, can provide information on past flood events
• Flood frequency analysis requires a sufficiently long and reliable record of streamflow or precipitation data
  • Data quality control and gap filling techniques may be necessary to ensure a complete and consistent record
• Statistical analysis of flood data involves fitting probability distributions to the observed data and estimating parameters such as the mean, variance, and skewness
• Regional flood frequency analysis pools data from multiple sites to improve estimates of flood quantiles, particularly for ungauged or data-scarce locations

Flood Frequency Analysis Methods

• Annual maximum series (AMS) considers only the largest flood event in each year of the record
• Partial duration series (PDS) includes all flood events above a certain threshold, regardless of the year in which they occurred
• Probability distributions, such as the Gumbel, log-Pearson Type III, or generalized extreme value (GEV) distributions, are fitted to the flood data
  • Parameter estimation methods include moments, maximum likelihood, or Bayesian inference
• Plotting positions, such as the Weibull or Gringorten formulas, are used to estimate the empirical exceedance probabilities of the observed flood events
• Goodness-of-fit tests, such as the Chi-square or Kolmogorov-Smirnov tests, assess the adequacy of the fitted probability distribution
• Confidence intervals quantify the uncertainty in the estimated flood quantiles due to sampling variability and model uncertainty
• Regional flood frequency analysis methods, such as the index flood method or regional regression equations, are used to estimate flood quantiles at ungauged or data-scarce locations

Flood Risk Assessment and Management

• Flood hazard maps delineate the areas that would be inundated by floods of different magnitudes or return periods
  • Hydraulic models, such as HEC-RAS or MIKE FLOOD, simulate the flow and inundation extent of floods
• Flood vulnerability assessment identifies the people, property, and infrastructure that are exposed to flood hazards
  • Exposure analysis considers the spatial intersection of flood hazard zones and vulnerable elements
  • Vulnerability indicators, such as building materials or socioeconomic status, influence the degree of potential damage or loss
• Flood risk is the product of the probability of a flood event and its potential consequences
  • Risk matrices or risk curves are used to visualize and communicate flood risk
• Flood risk management involves a combination of structural and non-structural measures to reduce the likelihood or impact of floods
  • Structural measures include dams, levees, flood walls, and channel modifications
  • Non-structural measures include land use planning, building codes, flood forecasting and warning systems, and insurance
• Benefit-cost analysis compares the expected benefits of flood risk reduction measures to their costs over the lifetime of the project
• Participatory flood risk management engages stakeholders in the decision-making process to ensure that local knowledge and values are considered

Real-World Applications and Case Studies

• The 2005 Hurricane Katrina caused catastrophic flooding in New Orleans due to the failure of the levee system, highlighting the importance of flood defense infrastructure and emergency preparedness
• The 2011 Bangkok floods in Thailand resulted from a combination of heavy monsoon rainfall, high tides, and urbanization, causing extensive damage to industry and transportation networks
• The 2013 Colorado Front Range floods were characterized by extreme rainfall and flash flooding, leading to erosion, landslides, and damage to roads and bridges
• The 2015-2016 UK winter floods were caused by a series of storms that brought heavy rainfall and storm surges, testing the country's flood defenses and insurance system
• The 2017 Houston floods from Hurricane Harvey demonstrated the vulnerability of urban areas to pluvial and fluvial flooding, particularly in low-lying or poorly drained neighborhoods
• The 2019 Midwest US floods were triggered by rapid snowmelt and heavy rainfall, causing prolonged inundation of agricultural land and small towns along the Mississippi and Missouri Rivers
• The 2021 European floods, particularly in Germany and Belgium, resulted from intense rainfall that overwhelmed river and drainage systems, causing flash floods and landslides in hilly regions
• Ongoing research and development of flood forecasting and warning systems, such as the European Flood Awareness System (EFAS) or the Global Flood Monitoring System (GFMS), aim to provide early warning and decision support for flood risk management