1. Ingeniería

Civil Engineering
Hydrology
HYDROLOGY
TABLE OF CONTENTS
Chap No
DESCRIPTION
P NO
1
Introduction
2-4
2
Measurement of Precipitation
5 – 15
3
Abstractions form Precipitation
16 – 27
4
Surface Water Hydrology (Runoff)
28 – 41
5
Stream Flow Measurement
42 – 54
6
Surface Water hydrology
55 – 65
(Hydrographs)
7.
Floods
66 – 72
8
Flood Routing
73 – 80
9
Ground water
81 – 94
10
Level 1
95 – 106
11
Level 2
107 – 121
12
Level 3
122 - 130
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Civil Engineering
Hydrology
Chapter 1
Introduction
Hydrology is an earth science. It encompasses he occurrence, distribution, movement and
properties of water of earth.
Hydrological cycle
The hydrological cycle is a global sun driven process whereby water is transported form the
oceans to the atmosphere to the land and back to the sea.
The hydrology cycle is usually described in terms of six major components: precipitation (P)
infiltration (1). Evaporation (E). Transpiration (T) infiltration (1), G round Water flow (G). for
computational purposes, evaporation and transpiration are sometimes lumped together as
evapotratpiration (ET) figure I.I define these components and illustrates the paths they define
tin the hydrological cycle.
NOTE:
A convenient starting point to describe the cycle is in the oceans.
Evaporation is the transfer of water from a liquid state to a gaseous state, i.e., it is the
conversion of liquid to the vapour phase.
Precipitation is the deposition of water on the earth's surface in the form of rain, snow, hail,
frost and so on.
Interception is the short-term retention of rainfall by the foliage of vegetation.
Infiltration is the movement of water into the soil of the earth's surface.
Transpiration is the soil moisture taken up through the roots of a plant and discharged into the
atmosphere through the foliage by evaporation.
Percolation is the movement of water from one soil zone to a lower soil zone.
Storage is the volume of water which gets stored in natural depressions of a basin.
Runoff is the volume of water drained by a river at the outlet of a catchment.
Residence Time
Average duration of a particle of water to pass through a phase of the hydrological cycle is
known as the residence time of that phase.
Volume of water in a phase
Volumeof water in a phase
Residence time =
Average flow ratein that phase
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Civil Engineering
Hydrology
Average flow rate in that phase Average
residence time of ocean is larger than that of
global ground water.
Catchment Area A catchment area is an area
of land where surface water from rain and
melting snow or ice converges to a single
point, usually the exit of the basin, where the
waters join another water body. Such as a
river, lake, reservoir, sea. or ocean.
In closed catchment the water converges to a
single point inside the basin, known as a Sink,
which ma) be a permanent lake, dry lake, or a point where surface water is lost underground.
The catchment acts as a funnel by collecting all the water within the area covered by the
catchment and channeling it to a single point. Each catchment area is separated topographically
from adjacent catehment area by a geographical barrier such as a ridge, hill or mountain.
The line which divides the surface runoff between two adjacent river basins is called the
topographic waterdived, or the watershed divide, or simply the divide.
The Hydrological Budget
For a given catchment in a time interval t , Inflow - Outflow7 = Storage [continuity equation]
This continuity equation expressed in terms of various phase of hydrological cycle is called
water budget equation/hydrological budget equation.
For surface flow:
P + R1 + Rg – R2 – Es – Ts – I = Ss storage ………….. A
P = ppt.
P = ppt.
RI = Surface water inflow
Rg = ground water appearing as surface water
ES = Evaporation
Ts = Transpiration
I = Infiltration
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Civil Engineering
Hydrology
For underground flow;1 + G1 – G2 – Rg – Eg – Tg = Sg storage ……………… B
I = Infiltration
G1 = Ground water inflow
G2 = Ground water outflow
Rg = Ground water appearing as surface water
Eg = Evaporation
Tg = Transpiration
Combined hydrological budget (water budget eq.) is obtained by addition eq. (A) and (B)
P – (R2 – R1) – (Es – Eg) – Ts + Tg) – (G2 – G1)
= (Sg  Sg )
= P - R – E – T – G = S water budget equation
P = precipitation
R = Net runoff
E = Net transpiration
T = Net transpiration
G = Net ground water flow
S = Net storage increase
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Civil Engineering
Hydrology
Chapter 2
Measurement of Precipitation
The total amount of precipitation on a given area is expressed as the depth over the horizontal
projection of the area. Thus 1 cm of rainfall over a catchment area of 1 km 2 represents a
volume of water equal to 104 m3.
Terms such as pluviometer, ombrometer and hyetometer are also sometimes used to designate
a raingauge.
Types of Gauges
The various types of precipitation gauges used are broadly classified as
(a) non-recording gauges, and
(b) recording gauges.
Non-Recording Gauges
The non recording gauge extensively used in India is the Svmons' raingauge. It is installed in an
open area on a concrete foundation. The distance of the rain gauge from the nearest object
should be at least twice the height of the object. It should never be on a terrace or under a tree.
The gauge may be fenced with a gate to prevent animals and unauthorized persons from
entering the premises.
Measurements are to be made at a fixed time, normally at 08:30 hrs. In case of heavy rainfall
areas, measurements are made as often as possible.
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Civil Engineering
Hydrology
Recording Gauges
The recording gauges produce a continuous plot of rainfall against time and provide valuable
short duration data on intensity and duration of rainfall for hydrology analysis of stroms, the
commonly used recording gauges
Tipping bucket type
Weighing type, and
Natural syphon type
The weighing type is suitable for measuring all kinds of precipitation (rain,, sleet etc.).
Tipping-Bucket Type
The catch from the funnel falls onto one of a pair of small buckets. These buckets are so
balanced that when. 0, 25 mm of rainfall collects in one bucket, it tips and brings the other one
in position. The tipping actuate an electrically driven pen to trace a record on clockwork-driven
chart The record from tipping bucket gives dat on the intensity of rainfall. The main advantage
of this type of instrument is that it gives an electronic puis output that can be recorded at a
distance from the rain gauge.
Weighing-Bucket Type
The catch from the funnel empties into a bucket mounted on a weighing scale. The weight of
the bucket an its contents are recorded on a clock work-driven chart. This instrument gives a
plot of the accumulate* rainfall against the elapsed time, i.e. the mass curve of rainfall
(accumulated precipitation against time
Natural Syphon Type
This type of recording rain-gauge is also known as float type gauge. Here the rainfall collected
by a funnt shaped collector is led into a float chamber causing a float to rise. As the float rises, a
pen attached to the hot. through a lever system records the elevation of the float on a rotating
drum driven by a clockwork mechanism A syphon arrangement empties the float chamber
when the float has reached a pre-set maximum level which resets the pen to its zero level. This
type of raingauge is adopted as the standard recording type rain gaug in India.
This type of gauge gives a plot of the mass curve of rainfall.
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Civil Engineering
Hydrology
Other Precipitation Measurement Method Storage Gauges
in a desert or a mountainous terrain, storage gauges are use to measure total seasonal
precipitation. These gauges are read monthly, seasonally, or whenever it is possible to inspect
the stations.
Telemetering Rain Gauges
These rain gauges are of the recording type and contain electronic units to transmit the data on
rainfall to base station both at regular intervals and on interrogation. The tipping -bucket. Type
raingauge. Being ideal suited. Telemetering gauges are of utmost use in gathering rainfall data
from mountainous and general inaccessible places.
Radar Measurement of Rainfall
Radar permits the observation of the location and movement of areas of precipitation in the
atmosphere; and certain types of radar equipment can yield estimates of rainfall rates over
areas within the range of the radar.
Observations by Satellites
Satellite images can be used for estimating precipitation over areas ranging from the global to
the very local scale in real or near-real time. This complements the conventional precipitation
measurements in areas of sparse rain gauge networks and can improve the accuracy of
estimating precipitation for short time periods (several hours).
Rain Gauge Network
For proper assessment of water resources, a good network of rain gauges is a must. More is the
variability of rainfall; denser should be the rain gauge network.
As per the IS: 4987-1968, the recommended rain gauge network density is as follows:
In plains
On in every 520 km2
Moderately elevated area
(av. elevation up to 1000 m) One in 260 to 390 km2
Hilly areas
One in 130 km2
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Civil Engineering
Hydrology
As far as possible. 10 percent of the rain gauge stations should be equipped with automatic rain
gauges.
Adequacy of Raingauge Station
Number of raingauge stations for an area to give necessary average rainfall with certain
percentage of error can be obtained as follows:
(P1  P2  ...  Pn
n
Pl is precipitation magnitude of the 1st station and so on and n is the total-number of raingauge
stations .in. the catchment.
Step 1: Calculate mea rainfall, Pm Pm 
Step 2: Calculate the standard deviation, 
(P1  Pm )2  (P2  Pm )2  ...(Pn  Pm )2
n 1
Step 3: Calculate coefficient of variation, Cv

CV  / Pm
Step 4: Optimal number of stations, N
2
C 
N  v 

Where,  is the allowable degree of error in the mean rainfall (in fraction).
Step 5: Additional number of raingauge station required = N – n in routine hydrological
investigation, error of estimate should not exceed 10% i.e.  = 0.1
Normal Precipitation
The normal rainfall is the average value of rainfall of a particular date, month or year over a
specified 30-ye; period
Normal rainfall is used to find out the missing data a certain raingauges
Before using the rainfall records of a station it is necessary to 1st check the data for continuity
and consistency Continuity means availability of continuous record of previous rainfall and
consistency means that rainfall of previous years should be consistent with the present
environmental and land use conditions (like if there
A jungle in particular area which did not axis 15 years ago then previous records will not be
consistent with current record
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Civil Engineering
Hydrology
Estimation of Missing Data
Sometimes a station has a break in record due to absence of observer or failure of the
instrument. It is then necessary to estimate that missing data. To estimate the data, three or
more stations close to this station are selected. Following are the different ways of calculating
the missing data.
Arithmetic Mean Method
If the normal precipitation at each of these selected stations is within 10% of that for the
station with missing data, then simple arithmetical mean of the precipitation of those stations
will give the value of the missing station.
P  P  P  ...  Pm
Px  1 2 3
m
Normal ratio method
If normal precipitation at any of these selected stations is above 10% of that for station with
missing data then
Px 1  P1 P2
P 
    ...  m 
Nx m  N1 N2
Nm 
Where P1 = Precipitation of 1st station
N1 = Normal precipitation of the 1st station
m = No of additional station chosen
Px = Precipitation (missing data)
Nx = Normal precipitation of the station at which data is missing
Inconsistency of Records
Some of the common causes for inconsistency of the records are:
Shifting of raingauge station to a new location Neighborhood of the station undergoing a
marked change
Change in the ecosystem due to calamities such as forest fires, landslides etc. Occurrence of
Observational error from a certain date Inconsistency of record is corrected bv using double
mass curve
Technique. Thus on correction, the previous record becomes consistent with the present day
environmental and land use condition.
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