River Engineering

• 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.

u/U* = i/k*ln(y)+C

Trapezoidal channel can be described by the Log-law of Keulegan

Bed shear velocity: U_*=sqrt(τ/rho) = sqrt(ghJ)

Flow Resistance (Chezy9 = cU_* = c*sqrt(ghJ)

Keulegan (Log-Law of resistance) c = 2.5 ln(aR/k_s) ,     a = 12

sand roughness in gravel bed rivers: k_s= 2 d₉₀

unifrom grains on plain bed: ks = d

uniform grains loosely packed: ks = (1.5 - 2)d

natural grain size distribution with armouring layer: ks = 2 d90

natural grain size distribution loosely packed with rough elements on top: ks = (2.5 - 3)d90

Q = v*A is not homogenous in reality

Discharge calculations, by splitting the cross section: 

• method of equivalent roughness 
• Method of partial areas
• Stripe method
• methods for compound cross sections

• stream tubes (regions of uniform velocity) separated perpendicular to the isotachs; roughness values differ -> different P and A
• no exchange of mass and momentum along the separating lines (no shear stresses)
• Assumptions: Approximately the same energy slope and velocities in all stream tubes
• Calculate discharge Q for a given cross section and a specific water depth h

lines of equal velocity in a flow field 

The most suitable discharge calculation method can be determined by having a look at the type of cross section , the wall influence and the given water depths

Influence of Vegetation:

• Separating line “with roughness” new wetted perimeter PV
• Assumption: no flow in vegetated area; removal of crossectional area in calculation
• Strickler coefficient for the separating line: kst,V= 18 to 25 m1/3/s
• kst,V depends on vegetation density along the embankments

1. Discharge (Q=A×VQ = A \times V)
   • Higher discharge increases water depth, potentially exceeding channel capacity and causing floods.
   • Extreme events (heavy rainfall, snowmelt) lead to flash floods and rapid depth changes.
2. Channel Roughness (Manning’s nn)
   • Higher roughness (vegetation, bedforms, debris) slows velocity, increasing depth.
   • Smooth channels allow faster flow but may intensify flooding downstream.
3. Sediment Transport & Deposition
   • Sediment deposition raises the riverbed, reducing capacity and increasing flood risk.
   • Erosion and sediment transport can change river morphology, deepening some areas while filling others.
4. Wood Transport & Blockages
   • Large wood (fallen trees, branches) can block culverts, bridges, and narrow sections, causing localized flooding.
   • Floating wood increases roughness, reducing velocity but raising water depth upstream.
5. Slope & Bed Morphology
   • Steep slopes accelerate flow, reducing depth but increasing erosion and downstream flood hazards.
   • Flat areas slow flow, increasing sediment deposition and raising flood levels.
6. Freeboard & Infrastructure
   • Low freeboard (levee height margin) reduces flood protection.
   • Poorly maintained levees, dams, and culverts lead to overtopping and infrastructure failure.

• Wood: Floating, rolling 
• Sediment :   
  • Bedload:  rolling /saltating
  • Suspended sediment:   transported suspension
• Dissolved solids:  transported in suspension

freq =sqrt( f²_w + f²_v + f²_t )

f_w Uncertainties in the determination of the water table
f_v Wave formation and backwater effects
f_t Clogging of large wood below bridges; only in case of bridges

The freeboard is an important design parameter to improve flood safety. It needs to be applied due to several uncertainties. Differ between the required and the actual freeboard

Definition Freeboard: as the vertical distance between water table and dam crest / bridge deck

The total required freeboard f_req refers to the design flood. It is calculated from 2-3 partial freeboards:

for bridge with with rough deck 

wood with small dimensions (branches):   0.5m

Singlelogs:    1m

single rootstocks:     1m

congested wood transport (wood carpet):     1m

Methods for LW Estimation:

• GIS Analysis (by software of wsl)
• Field examination (test area; counting)
• Remote sensing tools
• Empirical formulas

E: Catchment size [km²]

F: Sediment load [m³]

Q_max: Peak discharge Q_max [m³/s

• Hazard: Blockage, unintentional dam build, destructive forces
• Benefit: improved Habitat, increased flow heterogeneity; backwater upstream & increased flow velocity at side, affects local sediment transport; deep scour & fine sediment deposition

• Large wood (LW): Individual logs with a length ≥ 1.0 m and diameter ≥ 0.1 m
• Driftwood: Commonly used for LW that is floating (drifting) [Schwemmholz]
• Organic fine material: Branches or leaves
• Selected wood types from different sources (ZF)

• Potential wood volume: stock of wood in the catchment area that can theoretically be entrained into the river during a flood
• Effective wood volume: actual wood volume entrained during a flood:

Effective wood volume = reduction factor * estimated pot. w.v.

• Log orientation
• Log diameter
• interaction with river bed