River Engineering

• Bed and Bank Protection:
  • Riprap: Placing large stones or boulders along banks and beds to dissipate flow energy.
  • Gabions: Wire mesh baskets filled with rocks to stabilize banks and reduce erosion.
• Hydraulic Structures:
  • Weirs & Check Dams: Reduce flow velocity and control sediment transport.
  • Sills & Spurs: Deflect flow away from vulnerable areas, preventing local scouring.
• Channel Modifications:
  • River Training Works: Guide flow to prevent excessive turbulence near structures.
  • Widening or Deepening Channels: Reduces velocity and shear stress on the bed.
• Vegetation & Bioengineering:
  • Planting Grass & Shrubs: Stabilizes soil and reduces flow impact.
  • Root-Based Reinforcement: Tree roots help bind soil and prevent erosion.
• Scour Protection Around Structures:
  • Scour Aprons & Concrete Mattresses: Protect bridge piers and abutments from localized erosion.
  • Scour Holes Filling: Use of filter layers and graded materials to fill and stabilize scour holes.
• Flow Control Measures:
  • Energy Dissipation Structures: Reduce velocity before it reaches vulnerable sections.

• Nothing, if not necessary; can be ecologically beneficial
• Working at the root of the problem
  • Improve bedload continuity
  • Supply bedload
  • Reduce the hydraulic load
• Increase resistance to erosion

Block Ramps

Different failure mechanisms have to be taken into account

• direct erosion, block entrainment -> shear stress> crit. shear stress
• direct erosion, ramp toe scour scour depth not met 
• indirect erosion -> filter criteria not met 

• Block carpet - Riprap 
  • Interlocked blocks (Blocksatz)
  • Dumped blocks (Blockwurf) 
• Block cluster 
  • structured blocks 
  • unstructured blocks 
  • self-structured blocks 

River groynes are mainly used to:

• prevent bank erosion (inhibit meander development)
• constrict a channel into a concentrated single channel (land use, navigation…)
• increase heterogeneity (improved ecological conditions)

• perpendicular to the main velocity:
  • are primarily used to decrease the width and to deepen the river bed (for example for navigation)
• pointing downstream (deklinante Buhnen)
  • create scours on the embankment behind the groyne

• pointing upstream (inklinante Buhnen)
  • are used to protect the embankment

Goals (engineering view):

• Increased water level during low flow conditions impact on ground water table
• Increased river cross section withincreased discharge capacity decreased water surface during flood event (depending on sediment management)
• River bed stabilization and reduced erosion upstream
• Sediment retention (depending on sediment management)

Goals (ecological view):

• Increased heterogeneity (spatial variation) and dynamics (temporal variation) improved habitat quality and quantity (aquatic and terrestric)
• Improved habitat connectivity (transverse, longitudinal and vertical direction)
• Improved water quality due to increased bioactive surfaces
• Increased availability of refugia during harsh conditions (flood, drought, heat…) increased resilience

Goals (social/economic view):

• Site-development / attractivity
• Local recreation

• Improved acceptance of engineering / restoration activities

Change in cross section:

• local flow acceleration
• local scouring

Reduced transport capacity in widened section

• Bedload deposition
• Bedload deposition leads to vertical offset s in widened section
• Bed slope in widened section >channelized section

•                      Δz = Bed aggradation upstream

• Machine based implementation
  • Final design reached immediately
  • High investment costs
  • Heavy impact on flow and water quality during implementation (turbidity)
  • Immediate result and immediate effect and hydraulics, bedload budget and ecology
  • Effects on hydraulics and bedload transport are predictable
• Self-dynamic implementation by removing bank protection
  • No bedload discontinuity
  • Less influence on water quality during implementation
  • Eventually initial measures to provoke bank erosion (to reduce the time for self-dynamic widening)
  • Definition of observation and intervention line (max. bank erosion) needed
  • Influence on bedload budget, hydraulics ... are less predictable, therefore good monitoring is needed
  • Final design and the related effects reached only after decades which is often less accepted by society.
• Partly self-dynamic & partly machine based
  • River partly widened (e.g. not full length or width, or just one-sided)
  • Design width is protected from the beginning

Prediction of and s Prediction ofΔzand s Δzs

• Assumption: Longitudinal profile in equilibrium
• For channelized situation: Meyer-Peter&Mülle rHunziker….
• For braided situation: Zarn Marti….

• Influences on the flood protection
  • Locally increased discharge capacity leads to locally smaller water depths. Normal river widenings however are too short to have a real retention effect.
  • Local river widening has almost no influence on the flood peak discharge downstream, because of the negligible retention effect
  • River widenings in river reaches with bedload deficit can lead to increased bed erosion upstream
  • Bedload deposition during the initial phase (before equilibrium conditions) can lead to bedload
  • deficit and increased bed erosion downstream. Monitoring needed!

• Influences on Ecology
  • Heterogeneity and dynamics, river morphology and flow conditions lead to improved habitat quantity and habitat quality
  • Availability of refugia during floods (low flow zones)
  • Note: Distance to another ecological hotspot (river widening) is important in terms of habitat connectivity on reach scale, the impact of hydropeaking in widened sections with low bank slopes can be increased and monitoring of neophytes is needed.

WBG - Wasserbaugesetz + WBV - Wasserbauverordnung

Gewässerschutzgesetz (Gschg) + Gewässerschutzverordnugn (GschV)

Bedload and large wood transport shape morphology of gravel bed rivers

Rivers tend towards a dynamic equilibrium between sediment transport, bed slope and discharge!

Relation: Discharge -> Sediment/Wood Transport ->Morphology ->

Bedload:

• Gravel lands and transported close to the bed.
• Changes particle shape from angular to round due to abrasion.
• In alpine rivers, bedload shapes the morphology.
• Discharge determines a maximum transport capacity of bedload.

Suspended load :

• Clay, silt, and fine sand transported in suspension, distributed in the water column.
• Bedload turns into suspended sediment due to abrasion.
• Deposition happens in reservoirs, backwater zones, or floodplains.

River is aiming at: Adjust bed slope to a minimum value so that a given amount of water and sediment can be transported. ->Minimum stream power (discharge × bed slope).

Upper course: erosion, with cobble-gravel 

middle course: equilibrium, WIth gravel-sand

lower course: aggradation, with sand-silt

• Detailed hydrological data are available online at the hydrological webpage of the Federal Office for the Environment (FOEN) and on cantonal websites

Discharge cannot be measured directly

Discharge distinguished by a Discharge calculation 

via waterlevel-discharge relation: rating curve

• x: discharge Q [m³/s]
• y. water level [m.a.s.l]

Return period n

probability of occurence (single year) q = 1/n

during period of m years 1 -(1-q)^m

e.g. 10 a -> 0.096

Integrated flood control

1) hazard assessment (intensity x probability)

2) assign differentiated protection levels (damage potential)

3) action planning

• prevention
• constructional flood protection measures
• emergency planning

Basic river cross section: described by a dam on both sides

parameters:

• height of dam crest
• height of flood plain
• bed level + bed width
• Definiton of embankment base points

Longitudinal slope of a river: linear fit of average bed level

Survey data of the bed level allow to elaborate a sediment transport diagram, calculate the annual total bedload, and give ideas about the longitudinal bed slope

Interpretations:

• Negative changes in transported bedload volume: Deposition
• Positive changes in transported bedload volume: Erosion
• Discontinuities: hint for tributary (Nebenfluss), Gravel extraction, changes in river infrastructure (e.g. weirs)

There are diverse techniques for hydrological measurements and surveying of the river topography -> they need to be adjusted to the actual question

• Tachymeter
• Multibeam Echosounder
• Airborne River Monitoring
  • Green Laser – LiDAR
  • Structure from Motion – Multiview Stereo Imaging
  • Surface PIV

The vertical velocity profile in turbulent open channel flows follows a logarithmic distribution

Velocity Distribution:

• The velocity profile in open channel and pipe flow exhibits a logarithmic distribution near the boundary due to turbulence and shear stress effects
• The Manning-Strickler equation is an empirical approach that simplifies flow resistance based on roughness, while Keulegan’s equation incorporates a logarithmic law of velocity distribution.
• The log-law approach (used in Keulegan's formulation) better represents velocity profiles in channels with rough beds because it accounts for the balance between turbulence production and dissipation.

Flow Resistance:

• Flow resistance depends on roughness elements that influence velocity distribution, especially in turbulent flows.
• Manning-Strickler provides a simplified empirical equation for estimating flow resistance but does not explicitly describe how velocity changes with height.
• Keulegan’s logarithmic approach describes flow resistance based on the interaction between roughness height, flow depth, and Reynolds effects.

Key Difference:

• Manning-Strickler: Simpler, empirical, and widely used in engineering for practical flow estimations.

Keulegan: More physically based, logarithmic, and accounts for detailed turbulence effects in rough-bed flows.