GSC 1620 Chapter 6

Streams and Flooding

Running Water Running water performs

much of the erosional activity of the Earth’s surface and therefore significantly influences landform development

The streams formed by running water provide drinking and irrigation waters, create a habitat for freshwater aquatic species, and are commonly used as transportation routes and sources of power (e.g., hydroelectricity)

However, stream floods can claim many lives and destroy significant amounts of property development

Background Water runoff and

streamflow are part of the surface reservoir component of the hydrologic cycle (see figure)

Hydrologic cycle – powered by solar energy, the ongoing circulation of Earth’s waterthrough the atmospheric, surface, and subsurface reservoirs.

Background Water runoff along the surface may

generate streamflow or sheetflow (also called overland flow)

Difference? Sheetflow refers to the unconstrained flow of water across the surface while a stream is a body of water that transports sediment and flows downslope within a clearly defined channel (see slides)

Effects of sheet erosion caused by sheetflow

Note the erosional rills (small gullies)

Background Rills (tiny channels) develop during

sheet flow – one or more stream channels might develop depending on the erosive energy of the runoff and the susceptibility of the underlying material to erosion

Note Stream Channel

Background The terms brook, creek, river are

colloquial and vary in meaning – we’ll learn later how to better describe the magnitude of streamflow

Terminology Drainage basin

(watershed) – the total land area that drains water into the stream channel; every stream possesses a drainage basin and drainage basins may overlap to form larger drainage basins (see slides)

section ofhigher groundthat separatesadjacentdrainagebasins

Terminology A stream’s origin point is called the

head (headwaters), its point of termination is called the mouth

The term “longitudinal profile” describes the change in stream bed (bottom) elevation from the head to the mouth; all streams have a similar concave upward longitudinal profile (see slide)

Stream gradient – measureof steepness of the stream channel

Terminology Measure of stream

magnitude: discharge Stream discharge – the

volume of water that passes by a fixed point in the stream per unit of time; in the U.S. discharge is typically expressed in cubic feet per second (ft3/s)

Discharge is calculated by stream gage stations

Will a stream’s discharge at a particular point fluctuate or remain constant? How will the discharge vary from the stream’s head to mouth? (see slides)

Terminology Floodplain – relatively

flat ground adjacent to the stream channel that can be covered by water during a flood

The width of stream floodplains varies; since flood severity also varies the entire floodplain may not be inundated by a flood (see figure)

Stream Channel Formation Streams create their

channels by eroding (downcutting) through the underlying sediment and rock – what primary factors influence the depth of downcutting? (see figure)

Stream Sediment Transport Streams can transport tremendous

volumes of sediment – especially during high discharge periods like floods

Most of the sediment is derived by weathering of rocks in the stream’s drainage basin and its transport by runoff or tributary into the channel; some is also derived from erosion of the stream bed and/or embankments

Stream Sediment Transport

Stream load – the sediment transported by a stream; subdivided into bed, suspended, and dissolved loads (see figure)

Stream Sediment Deposition Sediment deposition obviously occurs

where the transport energy (velocity) of the stream diminishes – the material deposited by streams is called alluvium

Sediments deposited within the stream channel are called bars

Alluvial fans and deltas are the primary sediment deposits associated with streamflow (see slide)

Deltas oftenform atthe mouth ofmajor streams

Alluvial fansoften form near thehead of streams thatoriginate in mountains

Alluvial Fan

Mississippi Delta

Typical Stream Channel Shapes Although stream channel

characteristics (shape, width, depth) (see slide) vary widely, most streams can be classified as having a straight, braided, or meandering channel shape

Each of these shapes develops due to specific geologic or topographic conditions

Stream Evolution As most streams mature their channels

develop a meandering (sinuous) shape Erosion on the cut bank (where water

velocity is highest) and deposition of sediment point bars causes these channels to migrate (shift position)

This channel migration can have significant consequences (see slides)

Figure 16.13

oxbow lake

meander scar

Stream Channels Other stream channel shapes do

exist, such as braided streams (see slide)

Braided Stream Channel

Stream Floods Humans have

been living in floodplains for millennia – what are some of the benefits? (see slide)

Stream Flooding A stream channel has a finite (fixed)

volume at each point along the channel; if the volume of water and sediment input exceeds that finite volume the stream overflows its banks (floods)

We could view a flood as a stream “out of equilibrium” with its input of water and sediment (see slides)

Stream Flood Factors Six major natural factors determine the

likelihood and severity of stream flooding: Stream Discharge (most floods result from rapid

discharge increases due to intense/prolonged rainfall or rapid snow melting)

Stream Velocity Sediment Load Topography (steepness and water infiltration

capacity) Vegetation Type and Distribution Channel Characteristics (width, depth, shape,

erodability of bed and embankment materials)

Steep Valley

Shallow Valley

Flood Categories Upstream (flash) – occur in small,

steep drainage basins (e.g., mountain canyons)

Downstream – occur in larger, more shallowly-sloped drainage basins (see slides)

Aftermath of Upstream (Flash) Flood

Stream/Flood Terminology Stream stage – height of water surface

above stream bed Flood stage – stream stage at which the

stream overflows the channel Flood crest – maximum stream stage

associated with a flood event Peak lag time – maximum time between

the flood-initiating event (e.g., onset of rainfall) and the flood

Which drainage basin type should have the shorter peaklag time?

Upstream Downstream

Arkansas flood toll now 19, 1 missingBy the CNN Wire Staff June 13, 2010 6:18 p.m. EDT

                                                                                          

Services for Kylee Sullivan, 7, will be Tuesday morning. Sullivan was one of 19 killed by flash flooding on Friday.STORY HIGHLIGHTS•NEW: 19 dead, 1 missing after Arkansas flood •More than 100 people still searching rugged woodlands •Debris piles hide bodies •Funerals for two victims set for Tuesday RELATED TOPICS •Arkansas •Accidents and Disasters •Floods

The latest victim was discovered about half to three-quarters of a mile downstream from the campground, Arkansas State Police Capt. Mike Fletcher told reporters. Eighteen of the 19 bodies recovered so far have been identified.

A wall of water swept through the Albert Pike Recreation Area before dawn Friday, when most campers were sleeping. The park is located along the Caddo and Little Missouri rivers, and the flood struck so quickly there was little chance for campers to escape.

Short-term Flood Prediction Stream hydrograph – figure

illustrating how the discharge (or stage) of a stream varies with time as a function of accumulated water input (see slide)

Stream hydrographs are compiled from past observations of a stream’s behavior

What advance in weather forecasting could enable a stream hydrograph to be used as a short-term (days, hours) flood prediction tool?

Short-term Flood Prediction Describe to me how stream gage

monitoring can be used to make short-term (hours, minutes) prediction of flood events

River Flow Path

= stream gage station

0 10mi

The United States Geological Survey communicates thisinformation to the National Weather Service

Long-term Flood Prediction Flood frequency curve – figure

illustrating how often, on the average, a stream segment hosts a flood of a particular magnitude (see figure)

Flood Frequency Curve – charts how oftena flood of a certain magnitudeis likely to occur

Average time between floodsof a particular magnitude

Long-term Flood Prediction The flood frequency curve is

typically constructed from historical records of a stream’s flooding

The inverse of the flood recurrence interval is the mathematical probability that a stream will host a flood of that magnitude in any given year

Long-term Flood Prediction

Can the flood frequency curve be used to make accurate prediction of flood events? If not, what is the usefulness of this information?

Source: Geotimes,August, 2008

Consequences of Floodplain Development Note how floodplain development

(e.g., urbanization) decreases the peak lag time and increases the typical flood discharge in the following slides

Be able to tell me why

Flood Prevention/Control Measures Water retention/diversion basins Levees Stream channelization Dams Restrictive zoning (see slides)

Least damaging to stream ecosystem

Effect of subsurface drainage increases flood potential

Flood Prevention/Control Measures Levees – raised, artificial

embankments; often constructed of compacted earth, rock, poured concrete or sandbags

Problems arise (see slides) if the levees become breached (e.g., sustain a hole) or get overtopped by floodwaters (see slides)

What happens if waters overflow the levee or the levee gets breeched?

Levee Problems

Original stream channel

City A City B

Channelization – modification ofthe stream (e.g., deepening, widening, straightening)

channelized segment of stream

Possible consequences ofcity A’s actions for city B?

Flood Prevention/Control Measures An obvious benefit of a dam is its

potential to impound water and reduce the chance of flooding downstream

What are some of the other advantages and disadvantages of dams? (see figures)

Geotimes: May, 2008

Benefits and disadvantages of dam construction?Global climate change consequences for western U.S. dams?

Approximately80,000 large dams(> 7.6 meters high)exist in the U.S.

Flood Prevention/Control Measures Restrictive zoning – land

development ordinances which regulate the type and degree of development