ABE 57 2nd Lec

• Characteristics of a rainstorm

  • Intensity
  • Duration
  • Areal Extent
  • Frequency
• N represents what?

  Time period of Interest
• P represents what?

  Probability of occurence
• T represents what?

  Return Period
• Formula for Pexceedance?

  -

  
• Formula for Pnon-exceedance?


  -

  
• Hydrologic Frequency Analysis is the probabilistic analysis of hydrologic processes
• Hydrologic process - is any hydrologic phenomenon which undergoes continuous changes with time.
• Deterministic - govern by physical law
• Probabilistic & Stochastic - govern by chance
• Stochastic - time dependent
• Probabilistic - time independent
• Objective of FA - relate magnitude of events to their frequency of occurence through probability distribution, that is, a frequency curve
• RIDF - Rainfall Intensity Duration Frequency
• DDF - Depth Duration Frequency
• Steps in FA

  1. Selection of Data
  2. Determination of Statistical Parameters
  3. Fitting of Data
  4. Testing the Significance of Results
• Annual Series - only one data point per year, usually the maximum
• Partial Duration Series - all data above a given theshold value
• Statistical Distributions

  • Normal Distribution
  • Log-Normal Distribution
  • Log Pearson Type III Distribution
• Log Pearson Type III Distribution - also known as Gamma Distribution
• Plot of Mokus - testing the adequacy of length of record
• independence - an event is not influenced by another event
• homogeneity - events under the same type of conditions or statistical properties are not changing
• Gumbel Distribution Formula?

  -

  
• Runoff - normally applies to flow over a surface
• Streamflow - describes the drainage after it reaches a defined channel
• Surface Runoff - that part of runoff which travels over the ground surface and through the channels to reach the basin outlet.
• Types of Surface Runoff

  • Infiltration Excess
  • Saturation Excess
• Interflow - runoff due to the part of precipitation which infiltrates into the soil surface and moves laterally through the upper soil horizons towards streams as perched groundwater above the major groundwater level.
• Groundwater Runoff - groundwater discharge into a stream due to deep percolation of the infiltrated water into groundwater aquifers.
• Factors Affecting Runoff

  1. Climate
  2. Physiographic Factors
• Large Watersheds - Channel Characteristics
• Small Watershed - basin characteristics
• Small Watershed - very sensitive to high intensity rainfall of short duration with land area of less than 250 sq. km
• River discharge increases with basin area.
• Rivers rise and fall more slowly in large basins than in small basins
• After Urbanization

  • high runoff
  • baseflow is reduced|
  • flood peaks sooner and higher
• Runoff Measurement and Estimation

  • Direct Method
  • Indirect Measurements
  • Use of Empirical Formula
  • Frequency Analysis
• Canal <1 m depth - 0.6 depth measurement
• Canal >1 m depth - 0.2 and 0.8 depth measurement