01 Introduction

FUNDAMENTALS OF ENGINEERING

HYDROLOGYWR 321

Instructor: Dr. Nobert, J

OBJECTIVE

To provide basic knowledge to facilitate understanding of hydrological processes

Estimation and technique for measurement of water balance components

Technique for data processing

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MODE OF DELIVERY+ASSESSMENT

2 hour lecture per week (30 hrs)

Assessment: 2-Tests (40%), UE (60%)

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RECOMMENDED REFERENCES/TEXTBOOKS

K. Subramanya, Engineering Hydrology, Tata McGraw-Hill Pub. Co, New Delhi

Linsley, R.K.Jr., et.al; Hydrology for Engineers

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COURSE CONTENT Role of Hydrology in hydraulic and Water Resources

Engineering Projects: Components of the hydrological cycle, Drainage basin as a hydrologic unit

Precipitation: Forms of precipitation, measurement of precipitation, preparation of data, presentation of rainfall data, Mean precipitation over an area, Depth-Area-Duration relationships, Double curve, IDF curves, Probable maximum precipitation

Abstractions from precipitation: Evaporation process, evaporimeters, Empirical evaporation equations, analytical methods of evaporation estimation, infiltration, etc

Runoff:

Hydrometric measurements

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INTRODUCTION

Water is vital for all living organisms on Earth. For centuries, people have been investigating where water comes from and where it goes, why some of it is salty and some is fresh, why sometimes there is not enough and sometimes too much.

All questions and answers related to water have been grouped together into a discipline.

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INTRODUCTION

The name of the discipline is hydrology and is formed by two Greek words: "hydro" and "logos" meaning "water" and "science".

Hydrology is the science concerned with the occurrence, distribution, movement and properties of all the waters of the Earth.

A good understanding of the hydrologic processes is important for the assessment of the water resources, their management and conservation on global and regional scales.

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GLOBAL WATER RESOURCES

Humankind cannot survive without water. Water is vital to everyday life. It is used: Drinking Household use (e.g. washing, cooking, bathing,

cleaning); Sanitation; Agriculture Industrial processes.

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GLOBAL WATER RESOURCES

We are forever striving to tap into additional sources of water.

As population increase (globally, regionally, nationally, locally): There is increased water demand; and There is increased pollution due to human

activities reducing the volumes of ‘clean’ water available.

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APPLICATIONS IN ENGINEERING Hydrology finds its greatest applications in the

design and operation of water resources engineering projects Irrigation Water supply Flood control Hydropower Navigation

In all these projects hydrological investigations for the proper assessment of the following factors are necessary: (i) Capacity of the storage (ii) the magnitude of flood flows to enable safe disposal of the excess flow (iii) the minimum flows available at various seasons (iv) the interaction of the flood wave and hydraulic structures, such as levees, reservoirs, barrages and bridges.

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SOURCES OF FRESH WATER

Water can be obtained from a variety of sources at different stages in the water cycle.

(1). Rainwater- Rainwater can be collected directly by channelling water falling on roofs, or other impermeable surfaces, into storage vessels or tanks.

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RAINWATER

The quantity of water available from this source will depend on the prevailing climate, the area of the collecting surface, and the available storage capacity.

The quality of rainwater is generally good; any impurities present are generally due to debris and dust washed from the roofs or collection channels.

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(2) GROUNDWATER

Water may be extracted from the ground in a number of ways, including springs, wells, and boreholes.

Groundwater is likely to contain natural chemical impurities derived from the composition of the soil and rock with which it has been in contact.

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(3) SURFACE WATER

Surface water from streams, rivers, lakes and reservoirs may be plentiful, but it is likely to be of the poor quality unless abstracted from the upper reaches of the catchment.

It is exposed to bacterial and chemical pollution from many sources.

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WATER CYCLE

In order to satisfy man’s ever increasing need for new water supplies it is first necessary to assess the quantity and quality of water available. We therefore need to understand the water cycle.

The water cycle is the term used to describe the continual movement of water between the sea, air and land.

The water cycle is sometimes referred to as the hydrological cycle.

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HYDROLOGICAL CYCLE

Components:- Precipitation- Surface runoff- Evaporation- Transpiration- Groundwater flow

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HYDROLOGICAL CYCLE Precipitation falls on to the surface of the

earth and either reaches streams and rivers as surface runoff, or percolates through the ground, most of it eventually arriving at the sea.

Water from both land and sea evaporates and water from plants transpires into the atmosphere.

The subsequent water vapour condenses into clouds and eventually falls to earth again as rain, thus completing the cycle.

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HYDROLOGICAL CYCLE The water cycle can also be expressed in

terms of the water balance equation:

Precipitation – (Evaporation + Transpiration + Runoff + Groundwater outflow) = Change in storage

Methods based on this general equation can be used to assess the water resources of individual catchments areas so that we know how much water can be extracted without depleting the resource over a number of years.

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HYDROLOGIC CYCLE

Movement of water through various phases in the environment erratic in time and space magnitude and frequency of extremes important

to engineer extremely complex

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MASS BALANCE IN HYDROLOGIC SYSTEMS

General form: Rate of accumulation of mass in system = Input rate - output rate ± reaction

Hydrologists: Change in storage = Inflow - Outflow

Assumptions: no reaction volumes, pressure, temperature do not change

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MASS BALANCE EQUATION FOR A LAKE

sec

h

mm

m

onaccumulati of rate Mass

3600

1

1000

1

)(

)(

watersoutTin

wateroutin

AIEEIP

QRQ

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MASS BALANCE TERMS

Qin = streamflow into lake (m3/h)

Qout = streamflow out of lake (m3/h) R = runoff (m3/h) E = evaporation (mm/h) P = precipitation (mm/h) ET = evapotranspiration (mm/h)

Iin = seepage into lake (mm/hr)

Iout = seepage out of lake (mm/hr)

As = area of lake (m2)

ρwater = density of water (kg/m3)

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EXAMPLE PROBLEM 1

A Lake has a surface area of 708,000 m2. In May, the river A flows into the lake at an average rate of 1.5 m3/s. The Meandering River flows out of the Lake at an average rate of 1.25 m3/s. The evaporation rate was measured as 14.0 cm/mo. A total of 22.5 cm of precipitation fell in May. Seepage losses are negligible. The average depth in the lake on May 1 was 19 m. What was the average depth on May 30th?

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WHAT DO WE KNOW?

Inputs to the lakeAverage inflow =

1.5 cm3/sP = 22.5 cm/mo

Outputs to the lakeAverage outflow =

1.25 cm3/sE = 14.0 cm/moSeepage = 0

• Surface area of lake = 708,000 m2• Average depth on May 1 = 19 m.

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PICTURE OF SYSTEM Ap

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WHAT ASSUMPTIONS HAVE WE MADE?

Flow into the lake is only from the river, no overland flow

Seepage is negligible Can use average values

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SOLVING THE PROBLEM

Need to write equation:Inflow - outflow + Precipitation - Evaporation = Change in volume of water in the lake during this month = S = change in storage

Need to worry about units, since some values are given in units of volume/sec, others in depth/mo.

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SOLVING THE PROBLEM The equation: S = I – O + P – E =

(1.5 m3/s)(86,400 s/day)(30 d/mo) – (1.25 m3/s)(86,400 s/day)(30 d/mo) + (22.5 cm/mo)(m/100 cm)(708,000 m2) - (14.0 cm/mo)(m/100 cm)(708,000 m2) = 3,888,000 m3/mo – 3,240,000 m3/mo + 159,300 m3/mo – 99,300 m3/mo

Solving the above equation, yields S = 708,000 m3/mo

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SOLVING THE PROBLEM Since S = 708,000 m3/mo and the average

surface area is 708,000 m2 , the change in depth during the month = (708,000 m3/mo)/708,000 m2 = 1 m or about 3.25 ft.

Note S is positive, this means that the volume increases and therefore the depth increases. The new average depth on May 30th would be 20 m.

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WATERSHEDS

(From: Introduction to Environmental Engineering, Davis and Cornwell, 3rd. Ed., Mc Graw Hill Pub., ©1998)

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Original (natural)

Partiallydeveloped

Fullydevelope

d

(a) (b) (c)

Q

time

(From: Hydrology and Floodplain Analysis, 2nd ed. P.B. Bedient and W.C. Huber, Addison-Wesley Pub. © 1992)

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UNIT HYDROGRAPH

Rain Stops

Q is flow atoutlet (drain)

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HYDROLOGIC CONTINUITY EQUATION

Same concept can be applied to a watershed At any given time:

Accumulated inflow - Outflow = Storage

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HYDROLOGIC CONTINUITY EQUATION

Rainfall accumulates on surface (surface detention) storage increases

Rain stops storage decreases as water flows out of the system

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EXAMPLE PROBLEM 2

In a given year, the X watershed, with an area of 2500 km2, received 150 cm of precipitation. The average rate of flow measured in the River, which drained the watershed, was 40 m3/s. Seepage is estimated to occur at a rate of 9.2 x 10-7 cm/s. Evapotranspiration was estimated to be 45 cm/yr. What is the change in storage in the watershed?

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SOLUTION

Draw picture List information Write question in symbolic form

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WHAT DO WE KNOW?

Area = 2500 km2

P = 150 cm/yr Seepage = Infiltration=

Groundwater flow = 9.2 x 10-7 cm/s

ET = 45 cm/yr Assume all flow in river is

due to runoff R = Qout

Qout = 40 m3/s

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HYDROLOGIC CONTINUITY EQUATION - SOLUTION

S = P - ET - G - R

22

23

kmm1000km2500

mcm

100yr

days365

daysec

400,86seccm

40

yr

cm29

yr

cm45

yr

cm150

Ryr

days

day

hr

hr

cmx

yr

cm

yr

cm

365

24min60

min

sec60

sec102.945150 7

= 150 - 45 -29 - 50.5 = 25.5 cm/yr = 150 - 45 -29 - 50.5 = 25.5 cm/yr = S S

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RUNOFF COEFFICIENTS

Runoff Coefficient = R/P = 50.5/150 = 0.37

Typical values: Lawns: 0.1 - 0.2 Roofs: 0.75 - 0.95 Streets: 0.70 - 0.95 Playgrounds: 0.20 - 0.35 Suburban areas: 0.25 - 0.40 Commercial areas: 0.70 - 0.95

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