Fluvial Geomorphology · 2017-08-23 · fluvial geomorphology • critically important to...
Transcript of Fluvial Geomorphology · 2017-08-23 · fluvial geomorphology • critically important to...
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FLUVIAL GEOMORPHOLOGY
CHAPTER 6 (CHANNELS) AND 7 (DRAINAGE BASINS)
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FLUVIAL GEOMORPHOLOGY• CRITICALLY IMPORTANT TO UNDERSTANDING LANDSCAPE EVOLUTION
• REGIONAL (TECTONIC AND CLIMATIC) VS. LOCAL CONTROLS (DISCHARGE, VEGETATION, SEDIMENT TYPE/LOAD)
• DYNAMIC CHANNEL MORPHOLOGY (SHAPE)
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GLOBAL SEDIMENTARY TRANSPORT
• RIVERS 85 TO 90%
• GLACIERS 7%
• GROUNDWATER AND WAVES 1 TO 2 %
• WIND AND VOLCANO LESS THAN 1%
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ANTHROPOGENIC VS. NATURAL CHANGE
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2016 FLOOD
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ENERGY TO THE FLUVIAL SYSTEM• THE CONVERSION OF POTENTIAL ENERGY (SOLAR DISTILLATION
AND GRAVITY) TO KINETIC ENERGY AND HEAT PROVIDES POWER TO THE FLUVIAL SYSTEM.
• MOST OF THIS ENERGY IS LOST TO INTERNAL TURBULENT FLOW.
• ONLY 2 TO 4 % OF THE POTENTIAL ENERGY OF WATER MOVING DOWNHILL IS CONVERTED TO MECHANICAL (EROSION) WORK AND TRANSPORTATION
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ENERGY CONSIDERATIONS
• FLUVIAL INTENSITY VARIES AMONG CLIMATIC REGIONS, ALONG TEMPERATURE GRADIENTS, PRECIPITATION, ALTITUDE, VEGETATED ZONES, AND SEASONAL CHANGE.
• 1/3 OF THE EARTH’S LAND SURFACE DOES NOT HAVE RUN-OFF TO THE OCEANS.
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OVERLAND FLOW
• SUBSURFACE AND SURFACE WATER AVAILABLE TO A RIVER IS DEPENDENT UPON CLIMATE, SOIL TYPE, BEDROCK LITHOLOGY, SLOPE, VEGETATION, AND MANY MORE.
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PRIMARY OVERLAND FLOW TYPES
• INFILTRATION
•THROUGH (INTERFLOW)
•SHEET FLOW
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INFILTRATION
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INFILTRATION CONT.
• INTENSE RAINFALL MAY EXCEED THE CAPACITY OF SOIL TO INTAKE WATER, CREATING HORTONIAN OVERLAND FLOW.
• AN IMPORTANT FACTOR THAT OCCURS IN SATURATED OVERLAND FLOW AND NOT IN HORTONIAN FLOW IS: SATURATION ADDS SEEPAGE PRESSURE INCREASING THE PROBABILITY OF LOOSE PARTICLES BEING TRANSPORTED DOWN SLOPE.
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THROUGH FLOW (INTERFLOW)
• RAPIDLY FLOWING GROUNDWATER MOVING DOWN SLOPE WITHIN A SOIL CATENA’S INTERCONNECTED CRACKS, BURROWS, ROOT CHANNELS, AND SOILVOIDS WITH OR WITHOUT C OR R-HORIZON SATURATION.
• THIS WATER MOVEMENT CREATES SEEPAGE PRESSURE THAT IS CAPABLE OF EROSION BY INCREASING SOIL VOID SPACES THROUGH TURBULENT FLOW.
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SHEET FLOW
• OCCURS ON BARE LOW GRADIENTS, THAT HAVE A SCATTERING EFFECT THAT SUPPRESSES IMPACT ENERGY AND CHANNEL FORMATION.
• MUDDY WATER FLOWS AS A THIN, SLOW MOVING SURFACE LAYER.
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CHANNEL DEVELOPMENT
• PIPING AND SAPPING
• RILLS
• GULLY
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RILL AND GULLY FORMATION
• A CRITICAL POINT IS REACHED ONCE THE LANDSCAPE THRESHOLD IS REACHED AND EROSION IS INITIATED.
• PIPING IS A FUNCTION OF SEEPAGE PRESSURE IN SOIL THAT AGGRESSIVELY UNDERMINES SMALL PITS OR RILL HEADS WHERE SEEPAGE PRESSURE IS THE GREATEST. START OF HEADWARDPROPAGATION.
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RILLS
THREADS OF HIGHER VELOCITY AND MORE TURBULENT CURRENT ERODED SMALL CHANNELS.
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GULLY
• A STREAM CHANNEL WITH DISTINCT CUTBANKS AND COMMONLY A STEEP HEAD.
• AGRICULTURAL PRACTICES.
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STREAM VOCABULARY
• PERMANENT (YEAR ROUND DISCHARGE)
• INTERMITTENT (SEASONAL DISCHARGE)
• EPHEMERAL (ONLY DURING AND AFTER RAINFALL)
• INFLUENT – (ARID/DRY – LOSS OF SURFICIAL WATER – LOW WATER TABLE)
• EFFLUENT (HUMID/MOIST – GAIN IN SURFICIAL WATER – HIGH WATER TABLE)
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SEDIMENT SUPPLY / BANK STABILITY
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CHANNEL TYPES
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VEGETATION/RIPARIAN SYSTEM
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FLUVIAL EROSION AND TRANSPORT
SEDIMENT LOAD
1. DISSOLVED LOAD
2. SUSPENDED LOAD
3. BED LOADS- ROLLING
- SLIDING
- SALTATION
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FLUVIAL COMPETENCE
THE MEASURE OF A RIVER’S ABILITY TO TRANSPORT A PARTICULAR MAXIMUM PARTICLE SIZE.
COMPETENCE IS A FUNCTION OF:
1. FLOW VELOCITY
2. CHANNEL SHAPE
3. AMOUNT OF SUSPENDED LOAD
4. PARTICLE SHAPE
5. DEGREE OF SORTING
6. WATER TEMPERATURE
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RIVER CAPACITY
THEORETICAL MAXIMUM AMOUNT OR MASS OF SEDIMENT THAT A RIVER CAN TRANSPORT.
• 1. DISSOLVED LOAD HAS NO EFFECT ON THE HYDROLOGIC FLOW OF RIVERS
• 2. BED LOAD IS VIRTUALLY IMPOSSIBLE TO MEASURE
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SUSPENDED LOAD
• IS MEASURED AND DETERMINES CAPACITY.
Capacity
Discharge
Suspended load source
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FLOW VELOCITY / MANNING’S EQUATION
U = (R2/3 S1/2 )/N• U = STREAM FLOW VELOCITY
• R = HYDRAULIC RADIUS ((CROSS SECTIONAL AREA ( ACS ) / WETTED PERIMETER ( PW ))
• S = WATER SURFACE SLOPE
• N = MANNING ROUGHNESS COEFFICIENT
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FROUDE NUMBER / FLOW TURBULENCE
FR = U/(DG)0.5
• U = FLOW VELOCITY
• D = FLOW DEPTH
• G = GRAVITATIONAL ACCELERATION
• TRANQUIL / SUBCRITICAL FLOW
• FR < 1
• CRITICAL FLOW• FR = 1
• SUPERCRITICAL FLOW• FR > 1
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