CE 3205 Water and Environmental Engineering

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CE 3205 Water and Environmental Engineering

• Watershed and Introduction to Precipitation

Mdm. Norhidayah Rasin
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Hydrologic Cycle
Water moves throughout the Earth by different
pathways and at different rates
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Hydrologic Cycle
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The Watershed or Basin

• Area of land that drains water, sediments and
  dissolved materials along a stream channel to a
  single outlet and is separated from other
  watersheds by a drainage divide.
• Rainfall that falls in a watershed will generate
  runoff to that watershed outlet.
• Topographic elevation is used to define a
  watershed boundary

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Watershed

• Watershed Area of land draining into a stream
  at a given location
• Streamflow Gravity movement of water in
  channels
• Surface and subsurface flow
• Affected by climate, land cover, soil type, etc.

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Watershed Hydrologic System
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Watershed/Drainage Basin Terms

• Catchment
• Catchment area
• Catchment basin
• Drainage area
• River basin
• Water basin
• Watershed

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Drainage Patterns/Networks
Parallel
Trellis
Deranged
Dendritic
Centripetal
Rectangular
Radial
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Watershed Characteristics
Divide

• Size
• Slope
• Shape
• Soil type
• Storage capacity
• Land use / cover

Reservoir
Natural stream
Urban
Concrete channel
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Basin size

• Delineate watershed according to the height of
  land that separates water draining to the point
  of interest from water that drains to adjacent
  basins
• Watershed area (km2, ha)
• smaller watersheds tend to have a more peaked
  hydrograph, more intermittent water supply
• larger watersheds have flatter hydrographs
  because larger channel network can store more
  water

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Watershed Land Slope

• The slope of the sides of a watershed govern how
  fast water will drain to the channel
• steep slopes - peaked hydrograph
• gentle slopes - flat hydrograph
• slope is vertical over horizontal distance,
  derived from topographic maps
• An objective repeatable formula for land slope

where L is the total length of contours, CI is
the contour interval and A is the watershed area.
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Strahlers Order of Streams

• A headwater stream with no tributaries is a first
  order stream
• When two first order streams join they form a
  second order stream
• Two second order streams form a third order
  stream etc.

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2
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3
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Bifurcation Ratio (RB)
The ratio of the number of stream segments of a
given order, Nn, to the number of segments of the
next highest order, Nn1, is called the
bifurcation ratio, RB
Bifurcation-splitting of a main body into two
parts
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Bifurcation Ratio - Example
Stream order No.of stream order Bifurcation ratio
1 17 17/6
2 6 6/2
3 2 2/1
4 1 -
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Watershed Delineation
Catchment Surface
DEM Data
GIS
SKC Bridge
Upper Bernam Basin
Outlet
Upper Bernam River Basin
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Typical Drainage Basin Langat River Basin
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SubWatershed Bagan and Sat Rivers
Sg. Bagan
Sg. Sat
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Watershed Delineation
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Draw Sub-watersheds
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Stream Networks
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Precipitation
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Introduction

• All forms of water that reach the earth from the
  atmosphere is called Precipitation.
• The usual forms are rainfall, snowfall, frost,
  hail, dew. Of all these, the first two contribute
  significant amounts of water.
• Rainfall being the predominant form of
  precipitation causing stream flow, especially the
  flood flow in majority of rivers. Thus, in this
  context, rainfall is used synonymously with
  precipitation.

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Introduction.

• In nature water is present in three aggregation
  states
• solid snow and ice
• liquid pure water and solutions
• gaseous vapors under different grades of
  pressure and saturation
• The water exists in the atmosphere in these three
  aggregation states.

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Introduction.

• Types of precipitation
• Rain, snow, hail, drizzle, glaze, sleet
• Rain
• Is precipitation in the form of water drops of
  size larger than 0.5 mm to 6mm
• The rainfall is classified in to
• Light rain if intensity is trace to 2.5 mm/h
• Moderate if intensity is 2.5 mm/hr to 7.5 mm/hr
• Heavy rain above 7.5 mm/hr

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Introduction.

• Snow
• Snow is formed from ice crystal masses, which
  usually combine to form flakes
• Hail (violent thunderstorm)
• precipitation in the form of small balls or lumps
  usually consisting of concentric layers of clear
  ice and compact snow.
• Hail varies from 0.5 to 5 cm in diameter and can
  be damaging crops and small buildings.

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Temporal and Spatial Variation of Rainfall

• Rainfall varies greatly both in time and space
• With respect to time temporal variation
• With space Spatial variation
• The temporal variation may be defined as hourly,
  daily, monthly, seasonal variations and annual
  variation (long-term variation of precipitation)

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Measurement of Rainfall

• Rainfall and other forms of precipitation are
  measured in terms of depth, the values being
  expressed in millimeters.
• One millimeter of precipitation represents the
  quantity of water needed to cover the land with a
  1mm layer of water, taking into account that
  nothing is lost through drainage, evaporation or
  absorption.
• Instrument used to collect and measure the
  precipitation is called rain gauge.

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Rainfall measurement
1. Non recording gauge
Precipitation gauge 1 - pole 2 - collector 3 -
support- galvanized metal sheet 4
funnel 5 - steel ring
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2. Recording gauge / graphic rain gauge

• The instrument records the graphical variation of
  the fallen precipitation, the total fallen
  quantity in a certain time interval and the
  intensity of the rainfall (mm/hour).
• It allows continuous measurement of the rainfall.

The graphic rain gauge 1-receiver 2-floater
3-siphon 4-recording needle5-drum with
diagram 6-clock mechanism
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3. Tele-rain gauge with tilting baskets

• The tele-rain gauge is used to transmit
  measurements of precipitation through electric or
  radio signals.
• The sensor device consists of a system with two
  tilting baskets, which fill alternatively with
  water from the collecting funnel, establishing
  the electric contact.
• The number of tilting is proportional to the
  quantity of precipitation, hp

The tele-rain-gauge 1 - collecting funnel 2 -
tilting baskets 3 - electric signal 4 - evacuation
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4. Radar measurement of rainfall

• The meteorological radar is the powerful
  instrument for measuring the area extent,
  location and movement of rainstorm.
• The amount of rainfall overlarge area can be
  determined through the radar with a good degree
  of accuracy
• The radar emits a regular succession of pulse of
  electromagnetic radiation in a narrow beam so
  that when the raindrops intercept a radar beam,
  its intensity can easily be known.

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Rain gauge Network

• Since the catching area of the rain gauge is very
  small as compared to the areal extent of the
  storm, to get representative picture of a storm
  over a catchment the number of rain gauges should
  be as large as possible, i.e. the catchment area
  per gauge should be small.
• There are several factors to be considered to
  restrict the number of gauge
• Like economic considerations to a large extent
• Topographic accessibility to some extent.

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Raingauge Network..

• World Meteorological Organization (WMO)
  recommendation
• In flat regions of temperate, Mediterranean and
  tropical zones
• Ideal ? 1 station for 600 900 km2
• Acceptable ?1 station for 900 3000 km2
• In mountainous regions of temperate ,
  Mediterranean and tropical zones
• Ideal ? 1 station for 100 250 km2
• Acceptable ? 1 station for 250 1000 km2
• In arid and polar zone
• 1 station for 1500 10,000 km2
• 10 of the rain gauges should be self recording
  to know the intensity of the rainfall

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END THANK YOU

• Next topics to be continued..
• -Preparation data
• -Estimation of missing data
• -Test for consistency record
• -Mean Precipitation over an area
• arithmetic mean
• the method of the Thiessen polygons
• the isohyets method
• -Runoff estimation