# Chapter 6 Study Guide, Version 2: Rivers and Flooding This version follows the textbook section by section. Review the summary, key ideas, examples, and questions for each section before moving to the next one. ## Chapter Learning Objectives - **LO:1:** Explain basic river processes. - **LO:2:** Compare and contrast flash floods, downstream floods, and megafloods and know why the differences are important. - **LO:3:** Summarize regions at risk from flooding and the global impact. - **LO:4:** Differentiate between the effects of flooding and the linkages with other natural hazards. - **LO:5:** Describe the benefits of periodic flooding. - **LO:6:** Discuss how human activity can significantly affect river processes and flooding. - **LO:7:** Summarize why being proactive with flood hazards is, in the long term, more significant in reducing the flood hazard than being reactive after a flood. - **LO:8:** Evaluate the adjustments we can make to minimize flood deaths and damage. - **LO:9:** Summarize how people perceive flooding. ## 6.1 An Introduction to Rivers (LO:1) ### Section summary Streams and rivers are part of the hydrologic cycle and are major agents of erosion, transportation, and deposition. Water evaporates, precipitates, infiltrates, becomes runoff, and eventually returns to the oceans. Runoff flows into small channels, which join as tributaries and form larger rivers. The land drained by a stream or river is its **drainage basin**, also called a watershed, river basin, or catchment. Large river basins contain many smaller watersheds. Virtually everyone lives in a watershed. ### Gradient, base level, and river profiles - **Gradient** is the slope of a river channel, calculated as vertical drop divided by horizontal distance. It is generally steep near the headwaters and gentler downstream. - **Base level** is the lowest elevation to which a river can erode, usually near sea level. A lake can act as a temporary base level. - A **longitudinal profile** shows the river's elevation from its headwaters to its mouth. - Steep gradients produce faster flow and stronger down-cutting, commonly creating V-shaped valleys, waterfalls, and rapids. - Near base level, the valley is wider and may contain a broad floodplain. ### Sediment transported by rivers The **total load** of a river includes three types: | Load | Material and movement | |--------------------|-----------------------------------------------------------------------------------------------------------------------| | **Bed load** | Sand and gravel that slide, roll, or bounce along the channel bottom; commonly less than 10 percent of total load | | **Suspended load** | Fine silt and clay held above the streambed; commonly nearly 90 percent of total load and responsible for muddy water | | **Dissolved load** | Ions and other materials carried in chemical solution, often produced by chemical weathering | ### Discharge **Discharge** is the volume of water moving through a river cross-section per unit time. It is calculated as: `Q = A x V` where `Q` is discharge, `A` is the cross-sectional area occupied by water, and `V` is mean water velocity. If the cross-sectional area becomes smaller while discharge remains constant, velocity must increase. This is why water moves faster in a narrow canyon than where a river spreads into a broad valley or enters a lake or ocean. ### Erosion, deposition, and channel patterns Rivers erode, transport, and deposit sediment. When a stream leaves a steep canyon and spreads onto flatter land, it may form an **alluvial fan**. When a river enters a larger body of water and slows, it may form a **delta**. A delta or fan may contain several **distributary channels**. The two main channel patterns are: - **Meandering:** A single channel with sweeping bends. Faster water erodes the outside of a bend, forming a **cutbank**. Slower water deposits sand or gravel on the inside, forming a **point bar**. An abandoned meander may become an **oxbow lake**. - **Braided:** Several wide, shallow channels divide and rejoin around bars and islands. Braided rivers are favored by steep gradients and abundant coarse bed load, often supplied by glaciers or rapidly uplifting terrain. Meandering channels often contain **pools**, which are deep areas formed by scour, and **riffles**, which are shallow areas where sediment accumulates. Pools and riffles create different aquatic habitats. ### Floodplain formation A **floodplain** is relatively flat land beside a river, formed by lateral migration of meanders and by overbank flow. When water spills over the channel banks, it slows and deposits fine sand, silt, clay, and organic material on the floodplain. Channels in fans and deltas can shift abruptly through **avulsion**, in which a river abandons part of its channel and forms a new course. A location that is dry during one flood may carry the main flow during another. ### The three zones of a river system 1. **Zone 1, water and sediment production:** Steep upper terrain, high flow velocity, down-cutting, erosion, and production of sediment. 2. **Zone 2, transport:** Broad valleys and floodplains where water and sediment move downstream. Channels may meander, braid, or alternate between the two. 3. **Zone 3, deposition:** Lower-gradient areas near base level where velocity slows and sediment accumulates on alluvial fans, deltas, lakes, and floodplains. ### Key terms - **Stream or river:** A body of water flowing in a channel. - **Tributary:** A smaller stream that joins a larger stream or river. - **Drainage basin or watershed:** Land whose precipitation drains to a stream or river system. - **Gradient:** The slope of a river channel. - **Base level:** The lowest elevation to which a river can erode. - **Total load:** All material transported by a river. - **Discharge:** The volume of water passing a cross-section per unit time. - **Floodplain:** Flat land beside a river built by river processes. - **Alluvial fan:** Fan-shaped sediment deposited where a stream leaves a steep canyon and spreads onto flatter land. - **Delta:** Sediment deposited where a river enters a larger body of water. - **Distributary:** A channel branching away from the main river. - **Avulsion:** An abrupt shift to a new river channel. ### Section 6.1 review questions 1. What is a drainage basin? 2. How do dissolved load, suspended load, and bed load differ? 3. Define an alluvial fan and a delta. 4. What are the two main channel patterns, and how and why do they differ? 5. Describe the three zones of a river system. ## 6.2 Flooding (LO:2) ### Section summary Flooding is the natural process of **overbank flow**, when water rises above a river's banks and spreads across the floodplain. Flooding is controlled by the amount and distribution of precipitation, infiltration, runoff, soil moisture, snowmelt, frozen ground, ice jams, drainage-basin size, and land use. Saturated soil acts like a wet sponge that cannot absorb much additional water. The same rainfall can cause flooding in a saturated basin but not in a dry basin. In mountainous and high-latitude regions, rain on frozen ground, snowmelt, and spring or midwinter thaws can produce flooding. ### Measuring floods - **Flood discharge:** The amount of water flowing when water overtops the banks. - **Stage:** The height of the water surface in a river. - **Flood stage:** A stage likely to damage property. Its meaning depends partly on how people use the floodplain. - **Hydrograph:** A graph of discharge, water depth, or stage through time. - **Flood crest:** The highest part of the flood as it travels downstream. Stream gauges record stage, and field measurements of velocity and cross-sectional area establish the relationship between stage and discharge. These data are combined to produce a flood hydrograph. ### Flood magnitude and recurrence interval Annual peak flows can be ranked and used to estimate a **recurrence interval**: `R = (N + 1) / M` where `R` is the recurrence interval in years, `N` is the number of years of record, and `M` is the rank of the flow. A 10-year flood has an approximate 10 percent chance of occurring in any given year. A 100-year flood has an approximate 1 percent chance in any given year. Recurrence interval is not a schedule. A 100-year flood can occur in consecutive years. The probability in the next year does not become zero because a similar flood recently occurred. Extrapolation beyond the length of the stream-gauge record is especially uncertain. **Bankfull discharge** is a flow that just fills the channel. It commonly has a recurrence interval of about 1.5 to 2 years, so a stream may overflow its banks every year or two even though much larger floods are less frequent. ### Types of floods | Flood type | Setting and cause | Important characteristics | |-------------------------------|---------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------| | **Flash flood** | Usually zone 1 or a small tributary basin; intense, brief rainfall, rapid snowmelt, dam or levee failure, or an ice-jam release | Peak discharge can occur in less than 10 minutes; little warning; fast water can sweep away vehicles and people | | **Downstream zone 2 flood** | Major rivers receiving runoff from a large drainage basin and many tributaries after long-duration storms | Flood crest moves downstream; broad areas may flood for weeks or months; large water volume threatens communities and infrastructure | | **Zone 3 fan or delta flood** | Alluvial fans and deltas with distributary channels | Flow paths can shift through avulsion, so a former or abandoned channel may reactivate | | **Megaflood** | Exceptional, prolonged storms over a large region, often involving atmospheric rivers | Rare and regional, with the potential to overwhelm ordinary flood-control systems | ### Flash floods Flash floods are common in arid and semiarid regions, steep terrain, sparsely vegetated areas, and small drainage basins. Intense rainfall can create a wall of water with little warning. Bridges, culverts, and floating debris can form temporary dams; when these dams fail, a sudden flood wave may move downstream. Many flash-flood deaths occur in automobiles. About 0.6 m (2 ft.) of fast-moving water can carry away most vehicles because buoyancy and lateral water force lift and push the vehicle off the road. ### Downstream floods Long-duration storms can saturate a large drainage basin. Runoff from thousands of tributaries combines, producing a large zone 2 flood. The flood crest travels downstream, and the flood may cover a wide area for an extended period. ### Zone 3 floods Alluvial fans and deltas are especially hazardous because channels may fill, scour, migrate, or suddenly shift. A current channel map may not show every historically active flow path. Historical maps and aerial photographs are important for identifying the full hazard. ### Megafloods and atmospheric rivers An **atmospheric river** is a narrow, moisture-rich band that transports enormous amounts of water through the atmosphere. When multiple atmospheric rivers reach mountains and produce prolonged precipitation, a **megaflood** may occur. The 1861-62 California flood turned much of the Great Valley into a lake and temporarily displaced the state government from Sacramento to San Francisco. Geological evidence indicates that similar extreme floods have occurred repeatedly. A rare event is not an impossible event, and ordinary flood programs may not be designed for its scale. ### Section 6.2 case studies - **Flash Flood survivor story, Santa Elena Canyon:** A group of canoeists encountered a sudden wall of water during the normally dry season. The flood originated far upstream, rapidly transformed a shallow river into dangerous rapids, overturned canoes, and separated the group until rescuers found them. - **Mississippi River floods, 1973-2008:** Long-duration regional rainfall produced major zone 2 floods in 1973, 1993, and 2008. The 1993 flood inundated about 55,000 km2, killed about 50 people, and caused more than $15 billion in damage. Levees and flood walls protected some areas but failed or constricted flow in others. - **Ventura River Delta, 1992:** An RV park was built on an active distributary channel. A flood with an approximately 22-year recurrence interval caused severe damage despite earlier studies that had estimated a 100-year recurrence interval. Historical maps would have shown that the channel had been active before development. - **California megaflood, 1861-62:** Prolonged atmospheric-river storms flooded the Great Valley and southern California. The event demonstrates why long-term geologic and historical records matter in hazard planning. ### Section 6.2 review questions 1. Define flooding. 2. What are flash floods, and where are they likely to occur? 3. Why are downstream zone 2 floods potentially so dangerous? 4. How do alluvial-fan and delta flows differ from zone 2 river flows? 5. What are atmospheric rivers and megafloods? ## 6.3 Geographic Regions at Risk for River Flooding (LO:3) ### Section summary Flooding can occur almost anywhere that water flows or accumulates. Important risk settings include river valleys, floodplains, alluvial fans, deltas, low-lying coastal areas, urban drainage basins, and areas downstream from dams. ### Global flood risk From 1980 to 2012, worldwide flooding caused approximately 6,000 deaths and $23 billion in economic losses per year, although losses vary greatly from region to region. Asia experiences the greatest losses of life, property, and affected people because large populations live along rivers in areas with intense precipitation. Exposure and vulnerability are both high. Historically, developing countries had higher vulnerability because of limited monitoring, warning systems, infrastructure, transportation, and disaster relief. Vulnerability has declined in many areas as countries have adopted structural and nonstructural flood-control measures, although death rates remain higher in lower-income countries. ### United States flood risk Virtually all areas of the United States are vulnerable to some type of flooding. Major settings include: - small, steep drainage basins subject to flash floods; - the Mississippi, Missouri, Ohio, and other major river systems; - broad river valleys in the central and eastern United States; - alluvial fans and deltas, including the Mississippi and Ventura River deltas; - low-lying coastal and estuarine areas; and - urban areas with extensive impervious cover and storm sewers. The chapter reports that river flooding averaged about 80 deaths per year in the United States. Flood risk depends on more than river size. Population, land use, floodplain development, local geology, warning systems, infrastructure, and evacuation capacity all affect losses. ### Reading the flood table The textbook's selected-flood table shows that different flood types produce different consequences. The 1972 Rapid City, South Dakota, flash flood produced the greatest listed loss of life, with about 242 deaths. The 2017 Houston flood produced the greatest listed economic loss, about $100 billion. Zone 1 flash floods can produce many deaths with little warning, while zone 2 floods can create very large economic losses across broad areas. ### Section 6.3 review questions 1. Why are flood losses so great in Asia? 2. Which flood in the textbook's table caused the most deaths, and which caused the greatest economic loss? What generalizations can you make about zone 1 and zone 2 floods? 3. Identify the major flood-risk settings in the United States and explain why they are vulnerable. ## 6.4 Effects of Flooding and Linkages between Floods and Other Hazards (LO:4) ### Section summary Flood effects may be **primary**, caused directly by moving or standing water, or **secondary**, caused by disruption and malfunction of services and systems. ### Primary effects - Drowning, injury, and loss of life. - Damage from swift currents, debris, and sediment. - Destruction of homes, businesses, farms, railroads, bridges, roads, and communication systems. - Erosion of soil, stream banks, and vegetation. - Deposition of sediment that buries land and structures. Flood damage depends on land use, water depth and velocity, rate of rise, flood duration, season, sediment load, and the effectiveness of forecasting, warning, and evacuation. Commercial and residential areas may suffer more structural damage than farms or recreation areas, but a long flood during the growing season can destroy crops. ### Secondary effects - Pollution from sewage, wastewater plants, septic systems, chemicals, oil, and animal waste. - Hunger, disease, and contaminated drinking water. - Displacement of people who lose their homes. - Fires caused by electrical shorts or broken natural-gas mains. - Loss of transportation, communication, water, and other services. - Economic losses caused by business interruption and damaged infrastructure. ### Linkages with other hazards - Hurricanes and tropical storms can produce flooding as a primary effect. - Earthquakes and landslides can produce flooding by damming or redirecting streams. - Dams, levees, bridges, and ice jams can fail and release water suddenly. - River erosion can destabilize banks and trigger landslides. - Flooding in estuaries and deltas can worsen coastal erosion and interact with storm waves. - Floodwaters can cause fire, pollution, disease, infrastructure failure, and long-term displacement. - Prolonged flooding can occur alongside severe weather and climate trends that increase intense precipitation. ### Section 6.4 review questions 1. What factors most strongly influence the damage caused by a flood? 2. Differentiate primary and secondary flood effects and give examples of each. 3. Explain how floods can be linked to hurricanes, earthquakes, landslides, fires, disease, and coastal erosion. ## 6.5 Natural Service Functions of Floods (LO:5) ### Section summary Flooding is a natural process, not automatically a disaster. It becomes a hazard when people build on floodplains or attempt to cross dangerous water. Periodic floods provide water, sediment, fertile soil, and habitat. ### Water resources and the Water-Energy-Food Nexus Runoff from rain and snow fills reservoirs and replenishes groundwater. Reservoirs can provide: - drinking water; - irrigation water; - hydroelectric power; - water for fisheries and ecosystems; and - flood storage. The **Water-Energy-Food Nexus** describes the connections among water supply, energy production, and agriculture. These uses can conflict. A reservoir kept low for flood control has more space for unexpected runoff but less water available for irrigation. A reservoir kept high for late-season agriculture has less flood-storage capacity. ### Fertile land Overbank flow slows across a floodplain and deposits fine sand, silt, clay, and organic matter. These deposits make floodplains highly productive agricultural land. Ancient Egyptians called the annual flooding of the Nile the “Gift of the Nile.” The Aswan Dam stopped much of this natural sedimentation, so farmers now rely more heavily on fertilizer. ### Riparian and aquatic ecosystems Floods form and maintain stream banks and riparian zones. They flush channels, scour pools, move boulders and woody debris, and transport nutrients downstream. These processes create varied aquatic habitats and support diverse plant and animal communities. ### Sediment supply and deltas Flood sediment can build deltas and help keep low-lying land above sea level. Levees that stop overbank sedimentation can cause deltas to subside. The Mississippi Delta illustrates how flood sediment is part of the natural balance between delta building and sinking. ### Case study: Experimental floods on the Colorado River Glen Canyon Dam reduced the Colorado River's natural high flows and trapped sediment. The mean annual flood was reduced by about 66 percent and the 10-year flood by about 75 percent. Reduced high flows contributed to disappearing sandbars, altered vegetation, and changed aquatic habitat. Experimental releases in 1996, 2004, and 2008 tested whether high flows could restore sandbars and habitats. The first test flood created 55 new beaches, added sand to 75 percent of existing beaches, and rejuvenated marshes and backwaters. Some sand later eroded away, and the limited sediment supply meant that the experiments could not fully restore the pre-dam river. ### Section 6.5 review questions 1. What are the main natural service functions of flooding? 2. How can river flow contribute to water resources while also causing damaging floods? 3. What is the Water-Energy-Food Nexus, and why is it important for understanding flood hazards? 4. What were the purposes of the three experimental floods through the Grand Canyon? ## 6.6 Human Interaction with Flooding (LO:6) ### Section summary Human activities can alter river processes, sediment supply, discharge, flood frequency, and flood magnitude. Rivers are open systems that tend toward **dynamic equilibrium**, an approximate balance between the sediment a river receives and the work it does to transport and deposit that sediment. ### Land-use change and sediment balance Changing forest to cropland generally increases erosion and sediment delivery. The stream may initially deposit the extra sediment, steepen its channel, and increase velocity until it can transport the new load. Converting farmland back to forest reduces sediment supply; the stream may then erode deposited sediment and adjust to a lower gradient. This sequence occurred in parts of the southeastern United States. Clearing forests for farming increased erosion and filled stream channels with sediment. Reforestation and conservation later reduced sediment delivery, allowing streams to clear and erode their channels. ### Dams and reservoirs Dams may provide water, irrigation, recreation, electricity, and flood control, but these purposes compete. A full reservoir provides little remaining flood-storage capacity. Dams also: - trap sediment; - change downstream discharge and channel shape; - alter flood frequency and magnitude; - affect deltas and downstream ecosystems; and - create a potential sudden-flood hazard if they fail. Because a dam traps much of the sediment that would normally move downstream, water released below the dam may have excess capacity to erode the channel and banks. This sediment-starved erosion can lower the channel and alter downstream habitat. The St. Francis Dam failed in 1928 because of adverse geologic conditions, killing about 500 people. The Teton Dam failed in 1976 because water moved through fractured volcanic rock beneath the dam; 11 people died and damage was about $2 billion. The examples show why geology, design, inspection, and maintenance matter. ### Urbanization Roofs, pavement, concrete, and other **impervious cover** prevent infiltration and increase runoff. Storm sewers move runoff to streams faster than natural drainage. Together, impervious cover and storm sewers shorten **lag time**, the delay between rainfall and peak runoff, producing **flashy discharge**, a rapid rise and fall in flow. In small urban basins, urbanization can increase both the magnitude and frequency of floods. An urban area with about 40 percent impervious cover and 40 percent storm-sewer service may experience about three times as many floods of a given magnitude as before urbanization. Urbanization also reduces groundwater recharge and dry-season streamflow, which can concentrate pollutants. Bridges and culverts can trap floating debris and form temporary dams. When the debris dam fails, a sudden flood wave may move downstream. ### Floodplain development and engineering Building homes, businesses, and infrastructure on a floodplain increases exposure. Levees and flood walls can protect existing development, but they may: - encourage additional development by creating a false sense of security; - constrict the river and produce a bottleneck; - raise water levels upstream; - fail through overtopping or breaching; and - increase the severity of flooding when they fail. Channel modifications intended for navigation or normal flow may behave as roughness elements during high flow and increase flood stages. ### Case study: Manila flood, 2009 A tropical storm produced about 44 cm of rain in 12 hours over Manila. A large population, crowded housing on a floodplain, limited infrastructure, and poor land-use decisions turned the flood into a catastrophe. The case shows how human exposure and vulnerability can dominate the consequences of a flood. ### Case study: Oroville spillway emergency, 2017 After a multiyear drought, intense storms filled Lake Oroville. Erosion damaged the main spillway, and concern grew that the emergency spillway could also fail. About 200,000 people downstream were evacuated. The event demonstrates the importance of geologic investigation, inspection, maintenance, and emergency planning for dams and spillways. ### Section 6.6 review questions 1. How does changing land use from forest to cropland affect stream-channel sediment balance? 2. Why might a stream channel erode below a dam? 3. Why do impervious cover and storm sewers help define the degree of urbanization? 4. What are the main effects of urbanization on flooding in small drainage basins? ## 6.7 Minimizing the Flood Hazard: Structural Control to Floodplain Management (LO:7 and LO:8) ### Section summary Historically, people have attempted to control floods by constructing dams, levees, flood walls, and channels. These measures can protect existing development but can also encourage more people to build in hazardous areas. No dam, levee, or channel can control the largest possible runoff. The most effective long-term approach combines structural measures with floodplain regulation, hazard mapping, flood-proofing, insurance, relocation, natural storage, public education, and emergency planning. ### Structural approaches - **Levees and flood walls:** Contain water and protect existing development, but may fail, constrict flow, raise upstream water, and encourage development. - **Dams and reservoirs:** Store and regulate water, but have competing uses and can fail or release large amounts of water. - **Channelization:** Straightens, deepens, widens, clears, or lines a channel to control flooding, improve drainage, or aid navigation. It may reduce flooding locally but can damage fish and wetland habitat and alter downstream erosion and deposition. - **Stormwater controls:** Retention ponds, special parking areas, planters, pervious concrete, vegetated roofs, and other features temporarily store or infiltrate runoff and lower peak discharge. ### Flood insurance The U.S. National Flood Insurance Program, administered by FEMA, provides financial protection and encourages flood-hazard mapping and land-use standards. A 100-year flood has a 1 percent chance of occurring in any given year. It also has about a 25 percent chance of occurring at least once during a 30-year mortgage. ### Flood-proofing Flood-proofing uses structural changes to reduce damage. Options include: - raising buildings on piles, columns, or fill; - building flood walls or earthen mounds around structures; - waterproofing doors, windows, and basement walls; - installing improved drains and pumps; and - designing lower floors to withstand and be cleaned after floodwater enters. ### Floodplain regulation and mapping Floodplain regulation seeks to obtain beneficial uses of floodplains while minimizing loss and protection costs. The floodway is the channel and adjacent floodplain needed to carry floodwater; development should not be allowed there. Flood-hazard maps can show historical floods and areas inundated by floods of a particular frequency. They support zoning, building codes, open-space preservation, park planning, and decisions to buy out or relocate repeatedly flooded properties. Historical maps, aerial photographs, high-water marks, flood deposits, scour marks, and trapped debris can all help identify flood-prone areas. A FEMA Flood Map Service Center search can be used to enter a property address and inspect its mapped flood zones, but maps may be outdated or incomplete and should not replace site-specific evaluation. ### Relocation and buyouts Governments may purchase and remove repeatedly flooded homes, demolish them, and preserve the land as open space. Relocation reduces future exposure, but leaving a home can be emotionally difficult even when residents understand that flooding will continue. ### Forest and flood management Forests can act like a sponge, storing precipitation and releasing it slowly. Protecting or restoring forests can reduce flood frequency and duration in small and moderate basins. Forests may not prevent large floods caused by extreme storms over large drainage basins. ### Social media Social media can provide real-time information about water depth, road conditions, flood timing, evacuation, shelters, and rescue needs. During the 2011 Bangkok flood, communication through social media helped people adapt to the slowly developing flood. ### Case study: Nicholas Pinter Nicholas Pinter's work demonstrates how geologists can use hydrologic, statistical, geospatial, and modeling analyses to identify human effects on flood stages. River structures built for navigation can increase roughness and raise flood stages during large floods. His work also illustrates the conflict between navigation at low flow and safe conveyance of a 100-year flood. ### Section 6.7 review questions 1. Why might levees, flood walls, and dams produce a false sense of security? 2. What is channelization, and how has it been used in flood control? 3. Describe the main flood-hazard adjustments that do not involve engineered structures. 4. How would you use the FEMA Flood Map Service Center to examine the flood hazard at a home address, and what limitations would you consider? 5. What role can social media play in reducing flood losses? ## 6.8 Perception of the Flood Hazard (LO:9) ### Section summary Institutional understanding of flooding is generally adequate. Government agencies, planners, and policy makers produce maps, forecasts, warning systems, and floodplain-management plans. Individual understanding is much more variable. People may underestimate flood risk because: - they have not experienced a flood recently; - they trust a levee, flood wall, dam, or flood map too much; - they value a home or business located on a floodplain; - they believe the next flood will not be as large as past floods; - they resist relocation or land-use restrictions; or - they do not understand how quickly shallow-looking water can move a vehicle. Public awareness is especially important for flash floods. The National Weather Service slogan **“Turn Around, Don't Drown”** summarizes the rule that people should never drive onto flooded roads. In a narrow valley, people may need to abandon a vehicle and climb to higher ground if water is rising or a roaring sound is heard upstream. ### Personal adjustments #### Before a flood - Check with a local flood-control agency or flood-map service to determine whether the property is at risk. - Purchase flood insurance if possible and learn how to file a claim. - Prepare sandbags or flood boards, a flashlight, blankets, rain gear, food, water, a battery-powered radio, a first-aid kit, gloves, and important documents. - Keep emergency supplies above likely flood levels. - Learn how to shut off gas and electricity. - Make a household flood plan and discuss it with family or housemates. #### When a flood warning is issued - Be prepared to evacuate and monitor official radio, television, weather, and emergency information. - Move people, pets, vehicles, valuables, and supplies to higher ground or upper floors when safe. - Check on neighbors who may need assistance. - Complete preparations during daylight when possible. - Keep warm and dry because a flood may last longer than expected. #### What not to do - Do not underestimate floodwater. - Do not drive through flooded roads. - Do not walk in water above knee level; holes, debris, and contaminated water may be hidden. - Do not enter a building or touch items exposed to floodwater until they have been checked, cleaned, and disinfected. #### After a flood - Photograph damage and file insurance claims if applicable. - Obtain professional help to remove, dry, and clean damaged materials. - Have gas, electrical, and water systems checked before turning them back on. - Ventilate buildings. - Treat water as potentially contaminated until the water supplier or officials declare it safe. - Continue following official instructions because flooding and contamination may persist. ### Flood awareness and community action Maps, public safety campaigns, warning systems, evacuation routes, and clear communication can improve individual decisions. Education is particularly important because at least half of flood deaths may involve people in automobiles. Awareness programs should explain flood depth, velocity, rate of rise, hidden hazards, evacuation routes, and the limits of engineered protection. ### Section 6.8 review questions 1. What is being done to raise public awareness of the flood hazard? 2. Summarize the personal adjustments that can reduce flood risk. 3. Why is individual perception of flooding often different from institutional perception? ## Applying the Fundamental Concepts: Houston Flood of 2017 The Houston flood illustrates the chapter's five fundamental concepts: 1. **Science helps predict hazards:** Weather forecasts, rainfall measurements, stream monitoring, and flood modeling provided information about the approaching storm. 2. **Risk information can support decisions:** Scientists had documented Houston's flood risk, but rapid population growth and outdated maps did not prevent development in vulnerable areas. 3. **Hazards are linked:** Hurricane Harvey produced prolonged intense rainfall, urban runoff, drainage failure, flooding, transportation disruption, pollution, and displacement. 4. **Humans can turn disasters into catastrophes:** Low-lying terrain, impervious cover, extensive development, poor land-use planning, and outdated flood maps increased exposure and vulnerability. 5. **Consequences can be minimized:** Boats and helicopters rescued people, shelters provided assistance, flood-proofing protected the Texas Medical Center, and future losses could be reduced through better mapping, planning, drainage, flood-proofing, insurance, and land-use decisions. ### Houston facts to remember - Hurricane Harvey dropped as much as 125 cm (50 in.) of rain over Houston. - About 1 million people were displaced, about 200,000 homes were damaged, and hundreds of thousands of automobiles were affected. - Evacuating the entire city was not practical because traffic could have trapped people on flooded roads. - About 40 percent of the metropolitan area had become impervious to water. - Nearly half of those flooded were outside the mapped flood-hazard areas. - Flood-proofing the Texas Medical Center with flood doors and barriers prevented major damage during the 2017 flood. ## Connections among the Sections - **6.1 to 6.2:** River gradient, drainage basins, runoff, discharge, and channel shape control how flooding develops. - **6.2 to 6.3:** Flood type and drainage-basin setting help identify where losses are likely to be greatest. - **6.3 to 6.4:** Exposure, vulnerability, water depth, velocity, duration, and warning determine flood consequences. - **6.4 to 6.5:** The same overbank flow that damages a community can supply water, sediment, fertile soil, and habitat. - **6.5 to 6.6:** Dams and levees can reduce some flood effects while disrupting sediment transport and natural services. - **6.6 to 6.7:** Human changes to land, channels, and drainage create a need for integrated flood-hazard reduction. - **6.7 to 6.8:** Maps, structures, and regulations reduce risk only when people understand them and act on the information. ## Chapter-Wide Comparison Questions - How do drainage basins, watersheds, tributaries, and river systems relate? - How do gradient, velocity, discharge, erosion, and deposition interact? - How do bed load, suspended load, and dissolved load differ? - How do meandering and braided channels differ in shape, sediment, and setting? - How do alluvial fans and deltas differ from zone 2 river valleys? - What are the three zones of a river system? - How do flash floods, downstream floods, zone 3 floods, and megafloods differ? - Why can a 100-year flood occur more than once in a short period? - Why is flood stage partly determined by human use of the floodplain? - Why does Asia experience large flood losses? - How do primary and secondary flood effects differ? - How can floods be linked to hurricanes, earthquakes, landslides, fires, disease, and coastal erosion? - What natural services do periodic floods provide? - How do land-use changes, dams, levees, channelization, and urbanization alter rivers and floods? - Why can flood-control structures create a false sense of security? - Why are proactive adjustments more effective over the long term than reactive response alone? - How do flood insurance, flood-proofing, floodplain regulation, buyouts, forests, and social media reduce risk? - How does institutional perception of flooding differ from individual perception? - What should and should not be done before, during, and after a flood? ## Practice Questions 1. Define a drainage basin, watershed, tributary, gradient, base level, and longitudinal profile. 2. Explain how gradient changes from a river's headwaters to its mouth. 3. Compare bed load, suspended load, and dissolved load. 4. Calculate or explain discharge using `Q = A x V`. 5. Describe how meanders, cutbanks, point bars, oxbow lakes, pools, and riffles form. 6. Compare meandering and braided channels and identify conditions favoring each. 7. Describe the three zones of a river system. 8. Explain how alluvial fans, deltas, distributaries, and avulsions affect flood risk. 9. Define flooding and identify the factors that control it. 10. Compare flash floods, downstream floods, zone 3 floods, and megafloods. 11. Explain why flash floods are especially dangerous to people in automobiles. 12. Calculate a recurrence interval for a ranked flood and explain its annual probability. 13. Why does a 100-year flood not occur only once every 100 years? 14. Explain why Asia experiences greater flood losses than many other regions. 15. Describe the primary and secondary effects of flooding. 16. Trace links between flooding and hurricanes, earthquakes, landslides, fires, disease, and coastal erosion. 17. List the natural service functions of periodic floods. 18. Explain the Water-Energy-Food Nexus and its relationship to reservoirs and flood control. 19. Explain how changing forest to cropland affects sediment and river channels. 20. Explain how dams, levees, flood walls, channelization, and urbanization change flood risk. 21. Why can impervious cover and storm sewers increase flood frequency and shorten lag time? 22. Contrast proactive flood-hazard reduction with reactive response. 23. Evaluate the benefits and limits of levees, dams, channelization, flood-proofing, flood insurance, and floodplain regulation. 24. List personal actions to take before, during, and after a flood. 25. Explain why institutional and individual perceptions of flooding may differ. 26. Use Houston, the Mississippi River, Ventura River, the California megaflood, or the Colorado River to explain how science and human decisions affect flood losses. ## Quick Review Checklist Before the quiz or test, make sure you can: - define drainage basin, watershed, tributary, gradient, base level, discharge, floodplain, alluvial fan, delta, distributary, and avulsion; - explain the hydrologic cycle and the three zones of a river system; - distinguish bed load, suspended load, and dissolved load; - calculate discharge using `Q = A x V`; - explain gradient, velocity, erosion, deposition, and sediment transport; - compare meandering and braided channels; - explain floodplain formation, cutbanks, point bars, oxbow lakes, pools, and riffles; - define flooding, stage, flood stage, hydrograph, flood crest, bankfull discharge, and recurrence interval; - calculate recurrence intervals and annual probabilities; - explain why recurrence intervals are not schedules; - distinguish flash floods, downstream floods, zone 3 floods, and megafloods; - explain atmospheric rivers and the 1861-62 California megaflood; - identify global and U.S. flood-risk regions; - explain why Asia experiences large flood losses; - describe primary and secondary flood effects; - trace flood linkages to hurricanes, earthquakes, landslides, fires, disease, and coastal erosion; - list the water, agricultural, ecosystem, sediment, and restoration services of floods; - explain the Water-Energy-Food Nexus; - describe how land use, dams, levees, channelization, and urbanization alter flooding; - explain impervious cover, lag time, flashy discharge, and temporary debris dams; - explain why flood-control structures can create false security; - compare proactive and reactive flood responses; - explain flood insurance, flood-proofing, floodplain regulation, buyouts, forest management, and social media; - explain the importance and limitations of FEMA flood maps; - describe what to do and what not to do before, during, and after a flood; - explain the difference between institutional and individual flood-risk perception; and - use the Houston, Mississippi River, Ventura River, California megaflood, and Colorado River examples to explain flood hazards and adjustments.