Chapter Fourteen Streams & Floods

Earth’s Water Water in, on, and above Earth is ~ 1.36 billion km 3 (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 water  97.2 % in ocean; 2.15% ice/glacier; 0.65% Lakes, Streams, Groundwater, Atmosphere Types of water  Juvenile- initial Earth’s water  Meteoric- nearly all surface water originates in the atmosphere

Distribution of the Earth’s water: by volume

Hydrologic Cycle

A stream system network

Anatomy of a Stream A 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 & 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. 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 - Regrading  Degradation- occurs when there is less sediments – steeper gradient - Regrading  Abrasion: Scouring of Particles  Hydraulic lifting – Erosion by water pressure  Dissolution: Dissolution of soluble bedrock

Stream 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 streams

Four types of drainage pattern

Four types of drainage patterns – contd.

Four types of drainage pattern – contd.

Four types of drainage patterns – contd.

Channel Patterns Straight Channels: In areas of active uplife Braided streams: Networks of converging and diverging stream channels separated by sand & gravel bars Meandering Streams: Oxbow Lakes

Sediment 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 format  Suspended load  Bed load  Dissolved load

Superposed streams

Superposed streams – contd.

Antecedent streams

Antecedent streams – contd.

Evolution of meandering streams

Pronounced stream meander bends

Waterfalls & Rapid Waterfall Formation Waterfall migration upstream Waterfall reduction to rapids Niagara Falls Stream 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 rapids

Evolution of waterfalls & rapids – contd.

The creation of stream terraces

Creation of stream terraces – contd.

Sediment Distribution

Floodplain features

Anatomy of a delta

Stream 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 loads

The evolution of the Mississippi River delta plain

Hydrographs - Illustrations

Hydrographs - Charts

A flood frequency curve for a hypothetical stream

Controlling Floods Floods occur every 2 to 3 years Flooding caused when runoff is greater than stream channel can carry Flood Prediction: – Seasonal flooding – Predicting Extreme Floods Statistical probability Stream hydrographs 10-yr and 100-yr floods Difficulty of Predicting Floods Inadequate Records underestimate 100-yr floods Human development increases maximum discharge Inadequacy of statistical probability

Flood Prevention Artificial 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 wells

Artificial levees and flood walls – contd.

Artificial levees & flood walls – contd.

Flood-hazard map