Ce 3205 water and environmental engineering
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CE 3205 Water and Environmental Engineering. Watershed and Introduction to Precipitation. Mdm. Norhidayah Rasin. Cloud Formation. Rain Clouds. Evaporation. Precipitation. Soil. Storage. Runoff. Transpiration. Stream. Vegetation. Infiltration. Ocean. Percolation. Groundwater Flow.

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CE 3205 Water and Environmental Engineering

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Ce 3205 water and environmental engineering

CE 3205 Water and Environmental Engineering

Watershed and Introduction to Precipitation

Mdm. Norhidayah Rasin


Ce 3205 water and environmental engineering

Cloud Formation

Rain Clouds

Evaporation

Precipitation

Soil

Storage

Runoff

Transpiration

Stream

Vegetation

Infiltration

Ocean

Percolation

Groundwater Flow

Hydrologic Cycle

Water moves throughout the Earth by different pathways and at different rates


Ce 3205 water and environmental engineering

Hydrologic Cycle


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

Outlet

The Watershed or Basin


Watershed

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.


Watershed hydrologic system

Watershed – Hydrologic System


Watershed drainage basin terms

Catchment

Catchment area

Catchment basin

Drainage area

River basin

Water basin

Watershed

Watershed/Drainage Basin Terms


Drainage patterns networks

Drainage Patterns/Networks

Parallel

Trellis

Deranged

Dendritic

Centripetal

Rectangular

Radial


Ce 3205 water and environmental engineering

Watershed Characteristics

Divide

  • Size

  • Slope

  • Shape

  • Soil type

  • Storage capacity

  • Land use / cover

Reservoir

Natural stream

Urban

Concrete channel


Basin size

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


Watershed land slope

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.


Strahler s order of streams

Strahler’s 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.

1

1

1

1

2

1

2

1

1

2

2

3

1

3


Bifurcation ratio r b

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, Nn+1, is called the bifurcation ratio, RB:

**Bifurcation-splitting of a main body into two parts


Bifurcation ratio example

Bifurcation Ratio - Example


Watershed delineation

Watershed Delineation

Catchment Surface

DEM Data

GIS

SKC Bridge

Upper Bernam Basin

Outlet

Upper Bernam River Basin


Typical drainage basin langat river basin

Typical Drainage Basin – Langat River Basin


Ce 3205 water and environmental engineering

SubWatershed – Bagan and Sat Rivers

Sg. Bagan

Sg. Sat


Watershed delineation1

Watershed Delineation


Draw sub watersheds

Draw Sub-watersheds

19


Stream networks

Stream Networks


Precipitation

Precipitation


Introduction

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.


Introduction1

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.


Introduction2

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


Introduction3

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.


Temporal and spatial variation of rainfall

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)


Measurement of rainfall

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.


Rainfall measurement

Rainfall measurement…

1. Non recording gauge

Precipitation gauge

1 - pole

2 - collector

3 - support- galvanized metal

sheet

4 – funnel

5 - steel ring


2 recording gauge graphic rain gauge

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


3 tele rain gauge with tilting baskets

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


4 radar measurement of rainfall

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.


Rain gauge network

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.


Raingauge network

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


Ce 3205 water and environmental engineering

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


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