Set of flashcards River Engineering (Page 3 of 4)

Flashcards	147
Language	English
Category	Technology
Level	University
Created / Updated	05.02.2025 / 05.02.2025
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Explain the governing parameters to determine a bedform

Geometry Antidunes

Antidunes

Fmax: maximum Froude number for bedforms:
Fmin: minimumFroude numberforantidunes: Antidunes start to form if 0.844 ≤ F ≤ 1
For 0.844 < F < 1.5
- 6h< Λ< 12h
- (Δ/Λ)max≈ 0.2
- Δ ≈ h

Differnet bedforms

Microforms: Ripples scale with d
Mesoforms: Dunes and antidunes scale with h
Macroforms: Bars

Bedforms of lower regime

Lower Regime
– Plane bed: No motion θ < θc
– Ripples (Riffel): Steep microform width d ≤ 0.6 mm, Ripple Δ and length Λ ∼ d, water surface
is not affected, high flow resistance
– Dunes and ripples superposed: Transition stage
– Dunes (D¨unen): Mesoform, Dune height Δ and length λ ∼ h, water surface is affected
– Washed out dunes: Transition stage

bedforms of upper regime

Upper regime

– Plane bed: Intensive transport, transition from the lower to the upper regime

– Antidune standing wave (Antid¨unen): F ≥ 0.8, apparent upstream movement, high standing

waves

– Antidune breaking wave: Breaking of unstable waves

– Chute and pool (Stau und Ablauf): Riffle-pool sequence, flow transition (supercritical ↔ subcritical),

steep river sections (mountain streams)

Describe the concept of the roughness composite model to determine the flow resistance due to bedforms

Effect of bedforms on flow resistance can be considered using the “roughness composite model”

Apply composite roughness model: Total resistance of bed equals sum of individual resistances:

bed shear stress = grain roughness + shape roughness

Concept of excess shear stress

Concept of excess shear stress as basic structure of all modern bedload transport formulas

(Stress –Resistance)

Stress: Determined by flow conditions
Resistance: Determined by particle properties

Applied shear stress (τ) = The force exerted by flowing water on the bed or banks.
Critical shear stress (τ_c) = The minimum force needed to initiate sediment movement (resistance).
Excess Shear Stress (τ-τ_c) = The net force available for erosion or transport.

Concept of excess shear stress

Implications

If τ<τ_c: No sediment movement.
If τ>τ_c\ Erosion begins, increasing with excess stress.
Higher excess stress = faster erosion and sediment transport.

Identify the concepts of excess shear stress within different bedload transport formulas

view the FS

Bedload TRansport Formulas

MPM: suitable for flat gravel bed rivers (e.g. Rhein)

WP: suitable for plane bed equilibrium in flat gravel bed rivers

SJ: 2 Formulas; gravel bed, fixed bed

Bedload TRansport Formulas

Uncertainties

Uncertainties: boundary conditions more significant than equations

Quality of input data
Bedforms in gravel bed rivers
Channel geometry
Sediment mixtures

Signifcant Discharge Values

Q₀: start of bedload transport

Q_D: breakup of armor layer

Special case: Wide channels

Depends on discharge and not is not a state of the river

Resistance of bed is same across whole channel because breakup of armor layer is the central part

Calculate the minimum and maximum total bedload (Fracht) over a certain time from a given bedload transport function and a flow duration curve

By combining the bedload function G_b with flow hydrographs or duration curves, the total bedload F over a certain time can be calculated

Describe how the principle of the minimum and maximum total bedload influences the actual bedload transport rate (Geschiebeführung) and therefore the evolution of the bedload transport diagram along a river course

Bedload diagram allows for a rough estimate of the actual bedload transport & bedload budget along the river course

Interpretation: Q0 und Qd are significant

if F_min,i is < F_min,i+1 - Erosion
if F_i is between F_{min,i+1 and} F_max,i+1 - Equilibrium, bedload transport stays the same
if F_min,i is > F_max,i+1 - Deposition

Relation between

bedload process and bed slope

Deposition leads to increase in bed slope → transport capacity increases
Erosion leads to decrease in bed slope → transport capacity decreases

this simplified model can only be applied for very small changes in the transport capacity within a certain time of interest

Interpretation of bed level changes over time (graph)

massive bed level changes (negative):

Box 1:
- Reason for Strong erosion 1: sediment trap been built
- Reason for Strong erosion 2: massive gravel withdrawals
Box 2:
- Blockramps: Obstacle in the river: fillin the upstream, little erosion downstream

bedload budget:

interpretation of bedload transport diagram

Diagram:

x: Distance [km]
y: Volume/a [m³/a]

Interpretation

upslope: eroded
level slope: equilibrium
Downslope: deposited

bedload budget:

interpretation of grain size diagram

diagram

x: diameter [m]
y: Distance [km]

Interpretation

sudden increase in diameter: tributary rivers
decrease of diameter over river course
- Abraiding
- Breaking mechanisms, crushing
- Sorting, desorting

Abrasion

Abrasion is one of the reasons for the decline of total bedload over the rover course. Due to abrasion, bedload converts into suspended sediment