Hydrology

cumulative height frequency curve of for example a basin.

Image: Hypsometric curve of the earth

Method of determening the basin characteristic: average slope

Method to determine the basin characteristic: slope

\(D = \frac{\text{total length of streams}}{\text{area}}=\frac{\sum_i L_i}{A}\)

- measures the efficiency of the basin drainage

- for equal climatic charactersistics --> can be used as proxy information for permeability

Quantitative study of the surface landform.

measures of geometric similarities among watersheds and stream networks

--> parametrization of R-R models

1st Horton's law of stream numbers

2nd Horton's law of stream length

3rd Horton's law of stream areas

4th Horton's law of stream slopes

Deutsch: Abflussganglinie

Used for IUH identification

Used to measure the response of a watershed to a rain event. (precipitation --> runoff)

- many empirical approaches for calculating

The GIUH is the probability density function of a drop's travel time in a basin.

- 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