NATURE OF RIVERS B-1. Channel Function... ALLUVIAL FEATURES. ... to successfully carry sediment and water from the watershed. ...dissipate energy.

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Shana Randall
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of a stream is to carry water and sediment TERRACE

stream flows, sediment supply, and bank stability.

1 1 2 Function... Sevier River... to successfully carry sediment and water from the watershed....dissipate energy. 3 ALLUVIAL FEATURES 4 CHANNEL DIMENSION The purpose of a stream is to carry water and sediment TERRACE Abandoned floodplain width and depth is determined by stream flows, sediment supply, and bank stability. Wide, shallow channels are less effective at transporting sediment. FLOODPLAIN Carries high flows Narrow, deep channels are more effective at transporting sediment. BANKFULL CHANNEL Carries sediment Sevier River B-1

5 CHANNEL MEANDER MEANDERING IS COMMON TO ALL STREAM CHANNELS 6 CHANNEL BEDFORMS

through turbulance Walla Walla River STRAIGHT CHANNELS WORK TO CREATE MEANDER 7

Large floods less common than small floods Width of flooding is related to

Greatest velocities/stress Point bars are depositional and form on inside of

2 5 CHANNEL MEANDER MEANDERING IS COMMON TO ALL STREAM CHANNELS 6 CHANNEL BEDFORMS Riffle bedforms provide natural grade control Runs and pools disappate energy through turbulance Walla Walla River STRAIGHT CHANNELS WORK TO CREATE MEANDER 7 CHANNEL BEDFORMS 8 FLOODING Riffles are located in transitions between meanders Large floods less common than small floods Width of flooding is related to elevation Elevation increases away from central channel Lower velocities/stress Greatest velocities/stress Point bars are depositional and form on inside of meanders Floods rarely 1% - 4% Probability Floods infrequently 10% - 25% Floods often 50% Floods 100% B-2

9 GEOMORPHOLOGY 10 VELOCITIES Alluvial features rise in elevation away from central channel Highest velocities occur in central channel bed has greatest inherent strength Velocities decrease away

3 9 GEOMORPHOLOGY 10 VELOCITIES Alluvial features rise in elevation away from central channel Highest velocities occur in central channel bed has greatest inherent strength Velocities decrease away from central channel Bank strength decreases away from channel Lower velocities/stress Greatest velocities/stress Floods rarely 1% - 4% Probability Floods infrequently 10% - 25% Floods often 50% Floods 100% 11 VELOCITIES 12 DISTURBANCE Highest CHANNEL velocities occur ALTERATIONS: in central channel Narrowing the floodplain bed has greatest increases inherent velocities strength drastically!! Disturbance is function of velocity, slope, & water depth Velocities Increased decrease velocities away from means central increased channel erosion potential Bank strength Narrow decreases floodplain away from increases channel flood risk Disturbance- velocities/stress decreases Disturbance greatest around central channel Lower velocities/stress Greatest velocities/stress Soil moisture decreases Floods rarely 1% - 4% Probability Floods infrequently 10% - 25% Floods often 50% Floods 100% Rarely disturbed Low soil moisture Infrequently disturbed Low soil moisture Often disturbed Mod. soil moisture disturbed Highest soil moisture B-3

13 STREAM MEANDER 14 STREAM MEANDER Meander creates zones of deposition & erosion

velocity Low shear stress Deposition on point bars (low shear stress) MEANDER SECTION

in channel RIFFLE/TRANSITION SECTION 15 VEGETATION 16 VEGETATION Vegetation creates

4 13 STREAM MEANDER 14 STREAM MEANDER Meander creates zones of deposition & erosion Velocity & stress are non-uniform in meanders High velocity High shear stress Low velocity Low shear stress Deposition on point bars (low shear stress) MEANDER SECTION Erosion along outer meander banks (high shear stress) High velocity High shear stress in channel RIFFLE/TRANSITION SECTION 15 VEGETATION 16 VEGETATION Vegetation creates roughness; slows velocities Roughness should increase Strengthens stream banks to reduce erosion. Perennial Streams Least roughness In central channel Roughness dissipates energy; reduces velocities. Keep high velocities in central channel Ephemeral Streams B-4

17 STREAM STABILITY 18 LATERAL INSTABILITY Stream stability can be defined as the ability to carry

aggrading or degrading over time. Rosgen, 1996 Lateral instability is characterized by eroding banks.

Understand the causes Lateral Instability Meander cutbanks Vertical Instability gullies Alternating

5 17 STREAM STABILITY 18 LATERAL INSTABILITY Stream stability can be defined as the ability to carry the water and sediment of a watershed while maintaining dimension, pattern, and profile without aggrading or degrading over time. Rosgen, 1996 Lateral instability is characterized by eroding banks. Generally eroding banks are in meanders. Understand the causes Lateral Instability Meander cutbanks Vertical Instability gullies Alternating banks erode. 19 VERTICAL INSTABILITY After channel incision Commonly both banks are actively eroding. narrow channels widen. 20 REVIEW STREAM CHANNEL FUNCTIONS FUNCTION Transports sediment Carries water Dissipates energy FORM Bankfull channel carries sediment /terraces spread high flows Meander and bedforms dissipate energy Stream channels are dynamic Colorado B-5

The central channel is strongest part of stream For stability, highest

6 21 REVIEW VELOCITY/VEGETATION 22 QUESTIONS? Erosion is a function of velocity, slope, & depth of water. The central channel is strongest part of stream For stability, highest velocities should remain in central channel. Riparian vegetation roughness to slow velocities and strength to the banks.. B-6

Why Stabilizing the Stream As-Is is Not Enough

Why Stabilizing the Stream As-Is is Not Enough Several examples of alternatives to the County s design approach have been suggested. A common theme of these proposals is a less comprehensive effort focusing