Hydrology Prefinals

• Run-off


  means the draining or flooding off of precipitation from a catchment area through a surface channel.
• Overland Flow


  is the excess precipitation moving over the land surfaces to reach smaller channels.
• Surface runoff


  a flow travelling all the time over the surface as overland flow and through the channels as open channel flow and reaching the catchment area.
• Interflow


  is a part of the precipitation that infiltrates and moves laterally through upper crusts of the soil and returns to the surface at some location away from the point of entry into the soil.
• Interflow


  This is sometimes called as through flow, storm seepage, subsurface flow, or quick return flow.
• TYPES OF INTERFLOW

  1. Prompt interflow – an interflow with the least lag
  2. Delayed flow
• Groundwater flow or Groundwater runoff


  is a part of run-off that percolates deeper into the soil and reaching groundwater storage.
• CLASSIFICATION OF RUNOFF

  1. Direct Runoff
  2. Baseflow
• Direct Runoff


  The part of runoff that enters the stream immediately after the precipitation. It includes surface runoff, prompt interflow, and precipitation on the channel surface. This is sometimes called as direct storm runoff or storm runoff.
• Baseflow


  The delayed flow that reaches the stream essentially as groundwater flow including delayed interflow.
• Watershed


  the entire area of a river basin whose surface runoff (due to a storm) drains into the river in the basin. Also known as drainage basin, or catchment area.
• Parts of Watershed

  • Drainage Divide
  • Concentration point or Measuring point
  • Concentration time
• Drainage Divide


  the boundary line, along a topographic ridge, separating the two adjacent watersheds.
• Concentration point or Measuring point


  the single point or location at which all surface drainage from a watershed comes together or concentrates as outflow of the basin in the stream channel.
• Concentration time


  the time required for the rain falling at the most distant point in a watershed to reach the concentration point.
• Physical Characteristics of a Watershed

  • The number of streams
  • the length of streams
  • stream density
  • drainage density
• Stream Density


  which is expressed as the number of streams per square kilometer area of the watershed.
• Drainage Density


  which is expressed as the total length of all stream channels per unit area of the watershed.
• Other Characteristics of a Watershed

  1. Slope of the watershed
  2. Shape of the watershed
• Slope of the watershed

  Average Stream Slope

  A) Counter Interval

  B) number of contours crossed by all subdividing lines

  C) = total length of subdividing lines

  
• Shape of the Watershed

  FORM FACTOR

  Where

  Wb = axial width of basin

  Lb = axial length of basin, i.e., the distance from the measuring point to the most remote point on the basin

  
• Shape of the watershed

  COMPACTNESS FACTOR

  Where,

  Pb = perimeter of the basin

  2√𝜋𝐴 = circumference of circular area, which equals the area of the basin

  
• Classification of Streams

  1. Influent Streams and Effluent Streams
  2. Intermittent and Perennial Streams
• Influent Streams 


  when the seepage from the stream feeds the groundwater resulting in the build up of water mound. This happens when the ground water table is below the bed of the stream. Such streams will dry up completely in rainless period and are called ephemeral streams.
• Effluent Streams


  the ground water feed the stream. This happens when the ground water table is above the bed of the stream.
• Intermittent Streams


  when the ground water table lies above the stream during the wet season but drops below the bed during the dry season. The stream flows during wet season due to surface runoff and groundwater contribution but becomes dry during dry seasons.
• Perennial Streams 


  when the ground water table never drops below the bed of the stream even in the most severe droughts and therefore they flow throughout the year.
• Run-off 


  is that balance of water, which flows or runs over the natural ground surface after losses by evaporation, interception and infiltration.
• Yield of Catchment 


  the net quantity of water available for storage, after all loses, for the purposes of water resource utilization and planning, like irrigation, water supply, etc.
• Maximum Flood Discharge 


  it is the discharge in times of flooding of the catchment area, i.e., when the intensity of rainfall is greatest and the condition of catchment regarding humidity is also favorable for an appreciable run-off.
• Run-off Estimation Methods

  1. Empirical Formulae, curves, and tables
  2. Infiltration Method
  3. Rational Method
  4. Overland Flow Hydrograph
  5. Unit Hydrograph Method
• Infiltration Method 


  By deducting the infiltration loss, i.e., the area under the infiltration curve, from the total precipitation or by the use of infiltration indices, which are already discussed. These methods are largely empirical and the derived values are applicable only when the rainfall characteristics and the initial soil moisture conditions are identical to those for which these are derived.
• Rational Method 


  a rational approach is to obtain the yield of a catchment by assuming a suitable run-off coefficient.

  𝑦𝑖𝑒𝑙𝑑 = 𝐶𝐴𝑃

  Where ,

  A = area of catchment

  P = precipitation

  C = run-off coefficient
• Runoff Coefficients for various types of catchments

  Rocky and impermeable: 0.8-1.0

  Slightly permeable, bare: 0.6-0.8

  Cultivated or covered with vegetation: 0.4-0.6

  Cultivated absorbent soil: 0.3-0.4

  Sandy soil: 0.2-0.3

  Heavy forest: 0.1-0.2
• Overland Flow Hydrograph 


  Overland flow occurs as a thin sheet of water over the ground surface (soon after a storm starts), joins a stream channel, and then flows in the channel to the concentration point.
• The Reynolds Number


  Where, d uniform depth of flowv = kinematic viscosity of waterq discharge per unit width

  V = velocity of flow

  
• Unit Hydrograph Method


  The hydrograph of direct surface discharge measured at the outlet of drainage area, which produces a unit depth of direct runoff (i.e., a Pnet of 1 cm over the entire area of the catchment) resulting from a unit storm of specified duration (called unit period) is called a unit hydrograph of that duration.
• Water Demand


  the quantity of water that is needed by the population to be covered. The water to be supplied should be sufficient to cover both existing and future consumers. In addition to the projected consumptions, an allowance for non-revenue water (NRW) that may be caused by leakages and other losses should be included.
• Water demand are influenced by the following factors:

  • Service of water to be implemented
  • Size of the community
  • Standard of living of the populace
  • Quantity and quality of water available in the area
  • Water tariffs that need to be shouldered by the consumers
  • Climatological conditions
  • Habits and manners of water usage by the people
• Service Level

  1. Level I (Point Source)
  2. Level II (Communal Faucet System or Stand Post)
  3. Level III (Waterworks System or Individual House Connection)