Hydrology

- Frequency (how frequently a flood of a given magnitude occurs)

- How much (how high is the water level in the river)

- Damage (e.g. does the river overflow the leeves and what is the extent of the affected areas)

- Prevention and Mitigation (what are the best prevention and mitigation measures)

\(Q_p\) = peak discharge

Hazard = probability of occurence, within a specific period of time in a given area, of a potentially damaging flow
                --> 1 - \(F_{Q_p}\)  with \(F_{Q_p}\)= the non-exceedance probability

Vulnerability = degree of damage in probability terms inflicted on a structure by a natural phenomenon of a given                                        magnitude

Risk = Hazard and Vulnerability and Potential Loss

L = time horizon for hazard/risk assessment

Deterministic methods
based on:

- local analysis of empirical historical evidence

- concept of probable maximum precipitation (PMP)

Probabilistic methods
direct: --> statistical analysis at site

indirect:  -->  derived distribution techniques,  --> statistical regionalisation,  --> rainfall-runoff simulations

- direct at-site statistical methods --> length of data record

- statistical regionalisation --> broad range of scales

indirect R-R methods --> from small to very large scales depending on model complexity and rainfall input

PMF is a Deterministic method

Deterministic Method

Deterministic method

- based on a broad sample of large observed floods in a region (concept similar to that of the envelope curve)

- they typically provide the value of the "largest flood" without a frequency characterisation

- \(Q_p\) is expressed typically as a function of basin area A

Probabilistic Method used if you don't have data

Based on the concept that basins with similar characteristics have similar flood response, i.e. Homogeneous Region

2 procedures to compute the peak flow of an ungauged basin of area A for a return period R:

- Homogeneous region

- BFS analysis and computation of \(Q_{Px}(R^\ast)\)

- splash erosion

- surface erosion

- rill erosion

- gully erosion

The more arid the climate --> the less vegetation coverage --> bare ground erodes faster

The more humid the climate --> increasing rainfall --> erosion happens faster

"Optimal" condition is limited vegetation coverage and a significant amount of rainfall

- Suspended loads

- Bed load (slot trap, basket sampler, pan/tray sampler)

Required Data:

• channel hydraulic geometry
  • river cross section
  • hydraulic radius
  • river bed slope
  • discharge
  • bed roughness
• sediment characteristics
  • specific weight of particles
  • concentration of suspended material
  • granulometric curve and main diameters

--> semi-empirical formula: Meyer-Peter's Equation

Measured by:

• lysimeter
• percolation gauge (mass conservation)

Estimated by:

• empirical equations
• water budget equations
• energy balance equations
• mass transfer equations

Which al provide an estimate of PET!

measured by:

• infiltrometers
• lysimeters
• experimental plot

estimated by:

• soil water content measurement
• physically/empirical/conceptual models

gravimetric sampling:

Extraction of soil carrot in the field --> lab analysis

tensiometer:

Capillary flows through a porous ceramic tip from tensiometer to soil
(which is proportional to the water content gradient)

neutron probe:

attenuation of flux (or velocity) of radioactive neutrons
--> collision with hydrogen nuclei contained in soil water molecules

TDR - Time Domain Reflectometry:

velocity of propagation of a high-frequency signal
--> computation of the dielectric constant of the soil, which is related to its water content

Because the sediments (usually transported to sea via the rivers) are held back by manmade structures such as dams and reservoirs.

\(1.34*10^9 \text{ km}^3\)

Defines the conservation of mass across the different compartments of the hydrological cycle
(atmosphere, water bodies, soil and ground, vegetation, snowpack and ice)

- Control of excess water - (flood storm mitigation, storm drainage, bridges, sewerage)

- Conservation (quantity) - (water supply, irrigation, hydropower, navigation)

- Conservation (quality) - (pollution control)

Der Niederschlagsinput kommt zeitlich verzögert am Einzugsgebietsauslass (Pegel) an. Diese zeitliche Verzögerung (Retention) ist abhängig von den Speichereigenschaften und der Grösse des Einzugsgebietes.

The sun (solar energy) drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water.

Evaporation from soil and Transpiration from plants.

Precipitation that does not reach the soil, but is instead intercepted by the leaves, branches of the plants and the forest floor.

Infiltration is the process by which the water on the ground surface enters the soil.

Interflow is the lateral movement of water in the unsaturated zone, that first returns to the surface or enters a stream prior to becoming groundwater.

\(P(\Delta t)=Q(\Delta t)+E(\Delta t)+(S(t)-S(t-1))\)

\(P\) : precipitation

\(Q\) : runoff

\(E\) : evapotranspiration

\(S\) : reservoir

2.5 % of the global water reserves is sweet water.

- tilting siphon

- tipping bucket

- weighing

used for estimation of areal rainfall (basin precipitation)

1. Connect the raingauges (red)

2. Draw lines perpendicular to the connections (green)

For estimation of areal rainfall (basin precipitation)

Isoyets = Lines of equal precipitation

Method for estimation of areal rainfall (geostatistical methods)

Method for estimation of areal rainfall (geostatistical methods)