River Engineering

• Sediment availability is a key driver for river morphodynamic character
• Sediment availability is a key driver for ecological conditions
• Assumption: river can transport all the bedload material that is available. Often not the case: transport limit or supply limited system
• supply limited system:        transport capacity > available bedload
  • e.g. mountain torrents; excess energy is dissipated by development of roughness structures
  • River bed incision (erosion9
  • Bed armoring - bed structuring
  • Width reduction
• Transport limited system: transport capacity =< available bedload
  • E.g. swiss rivers before channelization, braided rivers are always transport limited
  • River bed aggradation (deposition)
  • Braiding
  • Width increase
• Sediment deficit
  • Enhanced bed erosion and reduced width change in river planform (e.g. braiding changes to alternate bars)
  • Reduced morphological variability (bank height, scour depth…)
  • Reduced variability in the flow field
  • Reduced morphological dynamics (less changes in time)
  • Changes in substrate quality: armoring, coarsening, colmation (clogging with fine material)…
  • Reduced habitat availability and quality
  • Reduced resistance (e.g. reduced availability of refugia during floods reduced abundance of certain species)

• Leads to destabilization of river engineering measures
• Leads to pools which increase waterdepth, substrate and flow velocity variation -> important for ecology

Reason: local erosion

• Increased flow velocity
• Increased turbulence
• Secondary currents

3 Types of scours:

• Contraction Scour; Flow constriction:
  • Bridges
  • Construction sites
  • Natural lateral constrictions
• Near-Structure Scour; Flow disturbance at infrastructure
  • Weirs
  • Drop structures
  • Bridge Abutments (Widerlager)
  • Bridge pier (Brückenpfeiler)
  • Groynes(Buhnen)
  • Boulders, trees, root wads…
• Scour due to morphodynamic activity
  • River bend scour
  • Bedform scour (confluence, braided rivers, alternate bars)

Key parameters for scour prediction are: U_m, b, h, d_m, D, shape factor

S = K_yd*K_l*K_d*K_S*K_δ

S     Final scour depth
K_yd Basic scour depth f(D,h) (max. scour depth= 2.5D)
K_l    Flow velocity in relation to critical velocity (incipient motion) and critical velocity of armour layer
K_d   Grain size (D/d50)
K_S   Bridge pier shape
K_δ   Flow direction towards bridge pier

• 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.