Ch 7 - Rivers and Flooding

100-year flood

a 100-year flood is an estimate of the size of a flood having a recurrence interval of 100 years, based on extrapolations of historical annual flood and recurrence interval data. it is the likelihood that a flood of a given magnitude will happen every 100 years, but there is no guarantee.

structural approach- flood control dams

a dam captures river discharge in the reservoir behind it to provide irrigation and drinking water, hydroelectric power, control water levels for shipping, and control flooding.

watershed

area of land where all of the water that is under it or drains off of it goes into the same place. all the land that contributes water to a trunk river. individual streams in the watershed carry water and sediment. also called drainage basins and catchments.

flood recurrence interval

average time between past flood events of similar magnitude, calculated using flood hydrographs. the recurrence interval (RI) for a river's annual flood rank = [(T2 - T1) + 1]/r, where T1 = first year of record, T2 = last year of record, and r = annual flood rank.

structural approach- channel alteration

channel alterations are designed to make channel cross sectional area greater or to increase flow velocity. may be done by straightening, deepening, widening, or clearing a channel.

sediment size classes

clay<silt<sand<gravel<pebbles/cobbles<boulders

braided channels

composed of a series of shallow, wide, low-flow channel strands that branch, diverge, and converge again that form the characteristic braided pattern. these rivers frequently change their course and their floodplains are completely occupied by the channel.

crevasse splays

crevasse splays form when a stream breaks its natural or artificial levees and deposits sediment on a floodplain.

criteria for braided channels

criteria that promote the development of a braided river system: high slope, erodible banks, high sediment load, frequent variations in discharge, lack of bank-stabilizing vegetation.

cutbank erosion

cutbank erosion on the outside of the channel bends and deposition on the inside of bends drive lateral migration of the channel across the floodplain.

meandering channels

have sinuosity values > 1.5. they have a single, deep channel with a few islands. the thalweg will migrate back and forth to a greater extent than it does in straight channels.

ice jams

ice blocks that can partially obstruct river flow caused by frozen rivers experiencing spring thaw that break apart.

channel response to increased water

if the amount of water entering a channel increases, it can respond in 2 ways to maintain its equilibrium: 1) the water can move more quickly or 2) the river can scour the channel bed and increase its cross-sectional area.

failure of ice damns and flooding

lakes can be formed upstream of ice dams- if the ice dam should fail (which often occurs in just a few hours) the additional water causes flooding downstream.

alluvium

large amounts of sand or gravel carried by a stream that is rushing down a steep mountain canyon to a nearby valley. where the stream enters the valley there is an abrupt decrease in gradient (and therefore abrupt decrease in velocity) and so deposition takes place.

levees

levees form because 1) water slows slightly near the channel banks, depositing sediment over time, and 2) when a river overtops its banks, depositing sediment in the process. levees may be natural or artificial.

avulsion

may occur from meander cutoff, logjams, and excess discharge that may cause the river to erode through and jump its bank.

straight channels

often form where they are confined by erosion-resistant topography. they follow geologic structures such as faults. straight channels have been confined by engineered levees to prevent lateral migration. upland streams are often straight.

sediment deposition outside river

outside the river, deposition occurs into a lake or ocean (delta) or onto a desert landscape (terminal floodout).

flood frequency curve

plot of annual flood discharges and their recurrence intervals. rivers with >30 yrs of annual flood records show a systematic relationship between peak discharge and recurrence interval- allows the peak discharge of a flood with a recurrence interval of 100 yrs to be estimated.

flood hydrograph

plots of river discharge over time. the river stage is recorded using river gauges- measures above a standard reference point the height of the river bed. >7000 stream gages in the U.S. have over 30 yrs of hydrograph recordings- some date back to late 1800s.

problems with channel response

response 1) there are limits to how much the velocity can increase & response 2) before a river is able to scour the channel bed it rises and overtops its banks.

structural approach- retention ponds

retention ponds or basins are locally constructed reservoirs that collect runoff water and then slowly release it back into a stream or let it seep into the ground. they decrease the rate that surface runoff would otherwise run off into rivers.

stream carrying capacity

the max amount of load a stream can carry at a specific velocity. material transported in large rivers consists mainly of suspended load like clay and silt.

floodplain civilizations

throughout history many civilizations have been attracted to rivers and adjacent floodplains for their life-giving water and fertility, but there are also perils living so close to dynamic systems of the hydrosphere.

limits of NFIP

1) not all communities participate in NFIP b/c they can't all afford insurance premiums, 2) taxpayers share cost of uninsured properties, and 3) 100-yr flood protection won't help participants during 500-yr flood event. ultimate solution to mitigating flood hazards is to get out of the way of floods.

channel morphology

4 forms a river channel can take: straight, meandering, braided, and anastomosing.

Damages of Pineapple Express

All the way from Oregon to Southern California, residents battled with power outages, flooded roads & mudslides. avalanches of mud and debris blocked part of the Pacific Coast Highway. heavy rain was falling in Ventura County and LA County where possible flash flooding was a concern.

Ex: Voluntary Buyouts in Vestal

Buyouts began in 2013, 2 years after the 100-yr flood. 75% of the money is coming from FEMA and the other 25% from the state. In total, the town of Vestal purchased 58 homes.

Ex: Glacial Lake Missoula

Largest ice-dammed lake known to have occurred. Glacial Lake Missoula formed by an ice dam that was caused by southern encroachment of ice sheet at west end of a lake. Glacial Lake Missoula was extremely long and held large amounts of water. It formed repeatedly upstream from high ice dams that blocked the valley in Western Montana.

rating curve

a rating curve is a graph that shows what stage the river reached during various discharges over a period of historical time. allows us to make predictions as to how much discharge we can expect given a particular river stage. rating curves allow for design of 100-year flood maps, which guide community planning and construction away from areas that a 100-yr flood would inundate.

capacity increase

capacity increases when velocity and discharge increase, and therefore more erosion and sediment transport occurs during flooding.

suspended load

carried in suspension

removal of vegetation

removal of vegetation from an area, such as clear cutting forests or crop harvesting, can increase surface runoff.

ice dam

the obstruction that occurs if the river is completely obstructed by ice blocks.

flood probability

the probability (P) that a specific flood will occur is the inverse of its recurrence interval: P = 1/RI. 100-year flood has a P = .01, or 1% chance of occurring in any year. 1-year flood has a P = 1, or 100% chance of occurring in any year.

sea level

the ultimate base level.

overland flow/runoff

the water that flows over the land surface to feed rivers.

high discharge

when a river's discharge is particularly high, the water may overtop the channel banks and flooding may occur

load

all of the material that a river carries.

braided river sediment depositon

in rivers with abundant sediment, lower velocity zones within the river's channel can be places of sediment deposition (braided rivers).

Jokulhlaups

A glacial outburst flood- abrupt release of water from a subglacial or proglacial lake or reservoir. the floods are episodic and can be large in magnitude, and are often associated with volcanic eruptions. Occur mainly in Iceland.

Ex: Three Gorges Dam in Yangtze River

The Three Gorges Dam in China is designed to stop the seasonal flooding that has plagued the area, but the enormous size of the reservoir meant displacement of 1.3 million people. Trapped sediment has meant a loss of nutrients downstream and industrial runoff from the reservoir causes a health hazard. In 2003, nine years after construction began, reservoir was filled and resulted in a lake 2x width of the natural river channel. 2 months later, 700 million cubic feet of rock slid into Qinggan River (2 miles from where it flows into Yangtze) spawning enormous waves that killed 14 people. Despite this, the reservoir was filled again and its water levels raised 3 years later, and the same thing happened.

nonstructural approach- Flood Control Act of 1928

aimed to reduce or eliminate flooding on the nation's waterways. came about in wake of catastrophic Mississippi River flood of 1927- government built world's longest system of levees floodways, dams, and diversion projects to prevent future floods. though some of these measures were successful, later 1993 Mississippi flood (billions in damages, worst U.S. disaster since first flood of 1927) forced policymakers to change their approach.

Ex: Pineapple Express (timing of precipitation and effect on flooding)

Warm, moist air from the Pacific Ocean is carried by high-altitude winds to the U.S. Pacific Coast. 1) When it reaches the snow-covered western slopes of the Cascade and Sierra Nevada mountains, it releases its moisture as rain. 2) Combination of rain and melting snow can cause severe flooding.

precipitation and flooding

flooding occurs if 1) a lot of rain falls during a short period of time (ex: rapid creek watershed in South Dakota- 38 inches of rain in 6 hours) or if 2) some rain falls for a long period of time (ex: missouri floods from the Mississippi River- weather system stalled and rained for two days straight).

distributaries

in a delta, decrease in channel velocity is so rapid that the channel may become clogged with sediment and new channels (distributaries) form that branch off the sides. distributaries disperse flow, unlike tributaries, which brings it together.

flood hazards

include property damage, fatalities, impacts on human health (waters have sewage, carcasses of drowned livestock, and pollutants), and reduction of ability to obtain potable (safe to drink) water.

dissolved load

ions carried in solution

alluvial fan

alluvial fans are cones of debris where a mountain stream opens into a valley. river discharge is spread out over several channels and sediment is deposited in an alluvial fan. common in arid environments.

floodplain

an area of land adjacent to a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge.

Huang Ho River

Huang Ho (Yellow River) in China is the world's muddiest river. It carries a tremendous suspended load of silt, up to 70% of the river's volume, and its watershed has much loess (silt-rich windblown sedimentary deposit).

Ex: Binghamton 100-Year Flood

In September 2011, Binghamton had already received nearly double its normal total of rain for the month and the Susquehanna River Valley received additional 3-5 inches of rain, saturating the soils. Rain from a stalled front continued for 4 days, and amplified by the arrival of Tropical Storm Lee + stream of moisture emanating from far-away Hurricane Katia. By noon on September 8th, the Susquehanna River crested in Binghamton at its highest level since records began in 1846.

nonstructural approach- National Flood Insurance Program (NFIP)

NFIP offers government-subsidized flood insurance to communities if they adopt floodplain management policies that discourage development in most hazardous areas. covers structures that lie within a river's 100-yr flood zone. effects: regulatory floodway must be set aside to accommodate 100-yr floods, and buildings must raise the lowest floor to the height floodwaters from a 100-yr flood would reach.

failure of landslide dams and flooding

some landslides may be large enough to extend completely across valleys, damming rivers upstream in the process and creating lakes. these dams are also temporary can fail within a few months or years, and once again cause flooding.

groundwater

some of the precipitation that falls will infiltrate the ground and supply water to groundwater. some of the runoff stored in ponds or dammed lakes will also infiltrate the ground.

mitigating floods- structural approach

structural approach to flood control uses engineering- channel alteration, flood control dams, diversion channels, and retention ponds. structural approaches attempt to decrease discharge, increase channel's cross-sectional area, and increase a river's flow velocity.

surface runoff

surface runoff from precipitation onto the land flows together toward lower areas such as lakes or oceans.

discharge

the amount of water flowing through a river at any given time. discharge is a rate- the volume of water flowing past a specific point in the river each second. discharge = area x velocity.

channel

the confined area through which the water in a river flows.

cross-sectional area

the cross-sectional area of a river is the key factor in how much water a river channel can carry

factors of discharge

the discharge of a river depends mainly on two factors: the size of its watershed, and the amount of precipitation.

flood crest

the highest level above flood stage that a river achieves during a flood.

braid bars

the sediment bars that divide the flow into multiple strands

pros and cons of structural approach

while structural approach does help to mitigate flooding around rivers, it does so at a cost: 1) structural approach is expensive, 2) levees/floodwalls raise a river's flood stage so if they overtop the flooding is even worse, 3) restricting the lower course of a river creates bottlenecks and increases risk of flooding upstream, 4) dams create local base levels and are sediment traps, 5) flood control may encourage development of floodplain land that shouldn't be developed, and 6) environmental consequences- implications for wildlife and fish.

flood hydrograph data interpretation

hydrographs can tell us 1) what time of year we can expect a river to reach its peak flow, 2) steep slopes suggest that the river built quickly to flood stage (ex: flash flood), 3) gentler slopes suggest that the river built to peak discharge over a long period of time (ex: riverine floods- more time to prepare but also more time before water recedes).

living with floods- flood controlled engineering

in engineered levees and floodwalls, preventing water from spilling onto river's floodplain means increase in overall level of water in channel. Rather than over-engineering higher levees and floodwalls, many floodplains should purposely not be urbanized and in this way the floodplains can contribute to sustaining valuable soil resources and biodiversity.

land use and flooding- effects of cultivation

land cultivation makes the land surface more prone to erosion --> increasing surface runoff transfers this material into the rivers --> increased deposits will decrease cross-sectional area of the river --> river is more prone to flooding.

mitigating floods - nonstructural approach

nonstructural approach to flood control encourages changes in how people occupy/use floodplains in order to reduce the number of people at risk in floods. also tries to decrease the damages associated with floods.

terminal floodout

occurs when channel capacities decline to the point where flooding is diverted overbank. in this floodout zone, channels generally disappear and large floods spread across low-gradient, alluvial surfaces known as floodouts. in terminal floodouts, unchanneled floodwaters spread out across a desert landscape and ultimately dissipate.

base level

places of lower elevation (lakes, enclosed basins, the ocean) that serve as the endpoint of a river's flow. base level may be thousands of kms from a river's source

1994 Amendment to NFIP

provided funds for at risk communities to relocate buildings, or purchase and demolish them. this reclaimed land can be developed as parks, open space, or agricultural land. >17000 homeowners in 36 states have opted for voluntary buyout.

Ex: 1993 Rain Machine

regional weather patterns created a rain-machine over the upper midwest that led to great 1993 Mississippi and Missouri River floods.

sustainable floodplain management- restoring/preserving a floodplain

restoring or preserving a floodplain involves the following- 1) giving the river more room to spread out when it floods, 2) restoring wetlands that restore water and gradually return it to river, 3) protecting forests and other vegetation, which maintain soil permeability and prevent erosion or excess surface runoff, and 4) recognizing the extra storage capacity of meandering rivers, which are longer than straight rivers.

water supply

river water supply is determined by area and precipitation

bedrock flow

rivers flowing over bedrock erode by plucking out loose rock fragments and carrying them downstream.

longitudinal profile

shows the change in elevation of a river's channel along its length, from its headwaters to its mouth.

land use and flooding- effects of urbanization

increased development/increased concrete and cement causes a decrease in the ground's/plant's ability to soak in water --> increased surface runoff off of pavement and roofs pour into rivers in short amount of time --> river stage will rise higher and faster than it would otherwise --> river is more prone to flooding. if storm sewers and surface runoff channels can't keep up, localized flooding will occur.

drainage divide

individual watersheds are separated from one another by a drainage divide- the highest topographic divide in the area.

sediment deposition inside river

inside a river, deposition may occur in the inside of a meander (point-bar deposition), or in the center.

sustainable floodplain management- Challenge 21

Congress has allocated funds for restorations on a variety of rivers and their floodplains around the U.S. The Flood Mitigation and Riverine Restoration Program, part of the Water Resources Development Act of 1999, has the approach that when floodplains are preserved or returned to a natural state, they help to mitigate floods. Projects include relocation of threatened structures, conservation & restoration of wetlands, and planning for future floods.

point bar

divergent flow on the inside of the channel results in localized deposition that builds up a crescent-shaped point bar.

low gradients

low gradients cause the river to flow more slowly, usually in a broad, meandering channel.

failure of constructed dams

~75,000 constructed dams in U.S. and some of them are over 100 yrs old- age may contribute to dam failure, as can natural disasters like earthquakes. Typically, dam failure is caused by unusually heavy precipitation --> river discharge that stress dams beyond their capacity.

Ex: Missoula Floods

Due to periodic rupturing of the ice dam, cataclysmic floods swept across Eastern Washington and down the Columbia River Gorge about 40x in a 2000 year period. Resulted in massive excavation of loess, sediment, and basalt from the Channeled Scablands of Eastern Washington and the transport of all this material downstream. The Channeled Scablands were scoured into their present state during the Pleistocene when the Lake Missoula floods swept throughout the area.

Ex: Gros Ventre Landslide

Example of failing landslide dams: In 1925, following melt from a heavy snow pack and heavy rain, a massive landslide of rock debris flowed down Sheep Mountain, crossed the Gros Ventre River, and flowed up the other side of the valley before settling in the valley floor. The Gros Ventre landslide created an enormous rock dam which formed Lower Slide Lake. A portion of the rock dam failed 2 years later, causing a flash flood to destroy most of the nearby town of Kelly. 6 deaths. This geological instability continues to the present, where a wide crack in the soil and new evidence of movement in 2014 caused an evacuation of some of the town's residents and businesses in case the hillside let loose.

Ex: Ru River dam failure

In 1975, Typhoon Nina brought extremely heavy rainfall to China's Ru River, causing the Banqiao Dam to fail. Resulted in 26,000 deaths from flooding and 150,000 deaths from subsequent disease and famine.

Ex: Flash Flood in Big Thompson Canyon

In July 1976, a thunderstorm dropped torrential rains over the headwaters of the Big Thompson River. river discharge rose extremely rapidly, and the rocky canyon walls and steep gradient funneled the water toward people and towns downstream. a wall of water up to 6m high rushed down the canyon and destroyed everything in its path. 150 deaths.

Ex: Grand Canyon -manmade flood

Man-made floods reestablished habitat in Grand Canyon: Sandbar downstream of Glen Canyon Dam gained area/volume after an experimental flood in 1996. sediment-starved floodplains and channels downstream from dams result due to sediment trapped behind dams unless water and sediment are released from dams periodically. such controlled floods replenish some fertility to floodplains downstream.

sediment deposition

a river deposits sediment when there is a decrease in velocity (less sediment transport).

anastomosing channels

anastomosing channels bifurcate and rejoin to flow around relatively stable vegetated islands. they are high sinuosity channels with cohesive banks which prevent lateral migration, typically narrower and deeper and have lower gradients than braided channels. they do not migrate, but jump their banks and move by avulsion.

structural approach- diversion channels

diversion channels locally decrease a river's discharge. upstream from a town, a dam is constructed with floodgates that can be opened (during time of high water to release some water, which is diverted around the flood prone area and released back into river farther downstream or into lake/reservoir) or closed.

bedload

dragged, skipped, or bounced over the river bottom- this process is called saltation.

steep gradients

erosion through downcutting occurs where gradients are steep and flow is rapid, producing a straight, narrow, steep-banked channel.

measuring and forecasting floods

estimating the magnitude and recurrence intervals of large floods allows for planning/protection of those who live around rivers. tools and concepts for understanding floods include flood hydrographs, flood recurrence interval, 100-year flood, flood probability, historical data vs. present or future probability of flooding.

delta

fan-shaped mouth of the stream where sediment is deposited, if the base level of the stream is a lake or ocean.

natural floods- flash floods

flash floods occur when a storm dumps large amount of rain over a watershed in short period of time. flash floods crest rapidly and recede rapidly. arid climates often experience flash flooding because of the rocky, dry impermeable ground surface.

flooding

flooding occurs when the discharge of a river is so great that the water rises and overtops the river's natural/artificial levees.

natural floods- riverine floods

riverine floods (river floods) commonly involve larger volumes of water than flash floods b/c they develop in larger watersheds. the floods are more widespread, and the flood stage takes more time to build and recede than flash floods. riverine floods are result of multiple rainstorms and spring snowmelt. may develop over days/weeks and take months to completely recede. These floods cause more damage but fewer deaths, as people have plenty of water to evacuate area.

channel erosion

rivers erode down gradient and laterally into their banks. when rivers meander, they erode into the cutbank and deposit material into the point bar.

sediment flow

rivers flowing over unconsolidated sediment pick up varying sized material and transport them as bedload, suspended load, or dissolved load.

rivers sort sediment

rivers sort sediment so that as a river deposits its sediment load, it will deposit coarser boulders, cobbles, and gravel first-closest to a river source. finer material, such as sand, silt, and mud, are deposited last- farthest from the river source.

tributaries

streams that flow into other streams. they contribute to another river. tributaries merge into the main trunk river.

recognizing gifts from the Nile

the annual spring flooding of the Nile was welcomed. in their basin irrigation, farmers parceled the fields into areas covering 20,000 acres surrounded by earthen levees. when the Nile flooded, sediment-laden water became trapped by the levees, depositing a layer of mineral-rich silt. this was great soil to grow crops such as wheat, barley, and fruit. the 1970 construction of the Aswan changed all of this.

bank-full stage

the bank-full stage of a river is the level at which the channel is completely full, and marks the water level at which people need to be concerned about flooding. the highest point above flood stage on a flood hydrograph represents the flood crest- the peak discharge of that particular flood.

flood stage

the level at which the surface of a river, stream, ocean, or other body of water has risen to a sufficient level to cause sufficient inundation (flooding) of areas not normally covered by water.

gradient

the loss of elevation along a river channel. the steeper the gradient, the faster the elevation decrease.

urbanization

urbanization that covers the ground with buildings and pavement decreases infiltration and increases surface runoff.

land use and flooding- loss of wetlands

wetlands are a natural flood-control mechanism along rivers that soak up water like a sponge and allow some transfer to the groundwater. when we convert wetlands for agricultural or urban use, draining or filling them means that more water will run off into the rivers, making them more prone to flooding.