Chapter FourteenChapter FourteenStreams & FloodsStreams & Floods

Earth’s WaterEarth’s Water

Water in, on, and above Earth is ~ 1.36 billion km3 (326 million mile 3) and this amount is fairly constant

The continuous circulation of water through the ocean, land and the atmosphere is the process called hydrologic cycle

Distribution of Earth’s water97.2 % in ocean; 2.15% ice/glacier; 0.65% Lakes,

Streams, Groundwater, Atmosphere Types of water

Juvenile- initial Earth’s waterMeteoric- nearly all surface water originates in the

atmosphere

Distribution of the Earth’s water: by volumeDistribution of the Earth’s water: by volume

Hydrologic CycleHydrologic Cycle

A stream system networkA stream system network

Anatomy of a StreamAnatomy of a StreamA stream is a surface water flow confined to a channel Floodplain- flat land immediately surrounding a stream which

may be submerged if a river overflows its bank Drainage Basin- areas of land that supply their water Drainage Divide- topographic highland that separates two

adjacent drainage basins Tributaries- streams that contribute water to main (trunk) stream Distributaries- main river splits into small channels that empties

into a lake/sea Graded Stream- state of temporary equilibrium Base level- lowest point to which a river can erode Discharge- the amount of water that flows through a given area

(Q=V*A)

Streamflow & DischargeStreamflow & Discharge Gradient = Change in elevation per distance

– Ranges from 66 m/km (in mountains) to 0.1 m/km (on lowland plains)

– Turbulent Flos: Non streamline flow– Stream Velociy:

Velocity = Distance traveled in a given time Ranges from >10 m/s to 0.27 m/s Local velocity depends on: continental gradient, location of water within

channel (slowest in straight segment at sides & bottom; velocity greatest at the outside of the curve; fastest in straight segment in top center, in curved segment: top on inside of curve, narrow places than in wide ones)

Velocity greater downstream than upstream because greater volume of water and stream bed is smoother

Stream discharge: Volume of water passing a given point for a given time

Discharge = (Width X Depth X Velocity) / 2 (for a hypothetical square or rectilinear stream, factor 2 is not there)

Streamflow and Discharge – contd.Streamflow and Discharge – contd. Discharge depends on:

– Size of drainage basin– Amount of precipitation in basin– Ranges from 200,000 to 5 m3/s (one day in Amazon for

more than 5 yrs of New York need)

– GEOLOGICAL WORK OF STREAMS Stream Erosion Graded Streams- No net erosion, no net deposition

(dynamic equilibrium)Aggregation- too much sediments-increased sediment load

– steeper gradient – increases stream’s velocity - RegradingDegradation- occurs when there is less sediments – steeper

gradient - RegradingAbrasion: Scouring of ParticlesHydraulic lifting – Erosion by water pressureDissolution: Dissolution of soluble bedrock

Stream TransportStream Transport Streams erode their networks of tributary valleys and distinctive

drainage patterns. A drainage pattern is a reflection of the underlying rock material or structure.

Drainage Types (Patterns) Dendritic: Branching drainage pattern – develop on relatively flat

sedimentary rocks Radial: Streams typically drain from a Central high area like spoks of a

wheel Rectangular: Looks like a grid of city streets Trellis: Develops where narrow valleys are separated by parallel ridges

Stream Piracy Superposed/Antecedent streams Channel patterns- straight, braided or meandering, oxbow lake

Aggradation & Degradation of graded streamsAggradation & Degradation of graded streams

Four types of drainage patternFour types of drainage pattern

Four types of drainage patterns – contd.Four types of drainage patterns – contd.

Four types of drainage pattern – contd.Four types of drainage pattern – contd.

Four types of drainage patterns – contd.Four types of drainage patterns – contd.

Channel PatternsChannel Patterns

Straight Channels: In areas of active uplifeBraided streams: Networks of converging and

diverging stream channels separated by sand & gravel bars

Meandering Streams: Oxbow Lakes

Sediment DepositionSediment Deposition

Velocity of sediment transport is controlled by its discharge

Stream Capacity- the total volume of sediments it carries is controlled by its discharge

Competence- the maximum size of sediments a stream carries is controlled by its velocity

Sediments are transported in the following formatSuspended loadBed loadDissolved load

Superposed streamsSuperposed streams

Superposed streams – contd.Superposed streams – contd.

Antecedent streamsAntecedent streams

Antecedent streams – contd.Antecedent streams – contd.

Antecedent streams – contd.Antecedent streams – contd.

Evolution of meandering streamsEvolution of meandering streams

Pronounced stream meander bendsPronounced stream meander bends

Waterfalls & RapidWaterfalls & RapidWaterfall FormationWaterfall migration upstreamWaterfall reduction to rapidsNiagara FallsStream Transport:

– Capacity = Maximum possible sediment load that a stream can transport; proportional to stream discharge

– Competence = largest possible sediment that a stream can transport; proportional to the square of a stream’s velocity.

Evolution of waterfalls and rapidsEvolution of waterfalls and rapids

Evolution of waterfalls & rapids – contd.Evolution of waterfalls & rapids – contd.

The creation of stream terracesThe creation of stream terraces

Creation of stream terraces – contd.Creation of stream terraces – contd.

Creation of stream terraces – contd.Creation of stream terraces – contd.

Sediment DistributionSediment Distribution

Floodplain featuresFloodplain features

Anatomy of a deltaAnatomy of a delta

Stream DepositionStream Deposition

Alluvium- sediment materials that settle from water Point bar- sediments deposited within the channel Flood plain Deposits

Levees Backswamp- deposits that make a flood plain’s wetland

Alluvial fans- formed where stream valley widens Delta- formed by the deposition of sediments in

standing water where the main stream breaks into smaller channels

Large sediment loadsLarge sediment loads

The evolution of the Mississippi River delta plainThe evolution of the Mississippi River delta plain

Hydrographs - IllustrationsHydrographs - Illustrations

Hydrographs - ChartsHydrographs - Charts

A flood frequency curve for a hypothetical streamA flood frequency curve for a hypothetical stream

Controlling FloodsControlling Floods

Floods occur every 2 to 3 yearsFlooding caused when runoff is greater than

stream channel can carryFlood Prediction:

– Seasonal flooding– Predicting Extreme Floods

Statistical probability Stream hydrographs 10-yr and 100-yr floodsDifficulty of Predicting Floods

Inadequate Records underestimate 100-yr floodsHuman development increases maximum dischargeInadequacy of statistical probability

Flood PreventionFlood PreventionArtificial Strategies:

– Artificial Levees & Flood walls– Flood-control dams– Channelization: Benefits and problems– Drawbacks to structural solutions: cost and false

sense of security– Nonstructural Strategies:

Identifying high-risk areas Zoning against floodplain development Managing resources to minimize floodwaters

Artificial levees & flood wellsArtificial levees & flood wells

Artificial levees and flood walls – contd.Artificial levees and flood walls – contd.

Artificial levees & flood walls – contd.Artificial levees & flood walls – contd.

Flood-hazard mapFlood-hazard map