Flood Frequency Analysis
Introduction
Hydrology of rivers is essential to
water engineers for studying
watersheds, design of dams,
diversions, and flood control through
reservoirs or dykes, etc. Information
and data are collected through a
network of gauges along rivers and
tributaries. The hydrologic cycle
explains the movement of water on,
above and below the surface of the
ground as shown in Figures (1) and
(2).
Hydrologic Parameters
Fig. (1)
Intensity of precipitation and duration needed for design any water construction
Rate of infiltration (type of soil and moisture condition)
Type of cover of the watershed and roughness of land
geometry of watershed drainage
network
Slope of watershed
Time of concentration
Stream flow measurement
The rate of water flow is measured in
units of cubic meter per second or
cubic feet per second. The
measurement of flow rates requires
knowledge of the cross sectional
velocity profile, which varies with
flow rate. While it is hard to measure
flow rate directly, it is easy to measure river stage, for example by a gauge Fig. (2)
marbles. Therefore, rates of flow are measured only a few times, enough to
establish a rating curve that correlates the flow rate and water level readings. Regular
measurement of water level is combined with the rating curve to produce time series of
stream flow of a certain river.
1
, Precipitation and Hydrograph
Precipitation is defined as the falling water from the atmosphere in either liquid or solid
phase. This process is known as the condensation of moisture in the atmosphere due to
cooling of air. Dynamic or adiabatic lifting of the air is the cause of air cooling. Adiabatic
lifting means that a given parcel of air is rise with resultant cooling and possible
condensation into small cloud droplets. If these droplets coalesce and have of sufficient
size to overcome the air resistance, precipitation in some form produced.
There are three major types of storms as follows:
1. Convective Storms
2. Orographic Storms
3. Cyclonic Storms
Hydrographs and River Discharge
The amount of water flowing in a river at
any given point and time is known as the
discharge which is measured in cubic
meters per second. It can be calculated by
multiplying the velocity of the river by
channel volume at a given point and time.
The hydrographs as shown in Figure (3) is
a graph that shows river discharge over a
given period.
A storm hydrograph shows how the river's
discharge changes following a period of Fig. (3)
heavy rainfall. The hydrograph indicates that flood is shown as a peak above
the base (normal) flow of the river. Analysis of hydrographs can help engineers and
hydrologists to forecast the likelihood of flooding in a river basin. The response of a river
to the rainfall event is measured in terms of the lag time - the time between the peak of the
rainfall and the peak discharge. Rivers with a short lag time respond quickly to rainfall
events and are therefore more likely to flooding than rivers with a longer lag time.
2
Introduction
Hydrology of rivers is essential to
water engineers for studying
watersheds, design of dams,
diversions, and flood control through
reservoirs or dykes, etc. Information
and data are collected through a
network of gauges along rivers and
tributaries. The hydrologic cycle
explains the movement of water on,
above and below the surface of the
ground as shown in Figures (1) and
(2).
Hydrologic Parameters
Fig. (1)
Intensity of precipitation and duration needed for design any water construction
Rate of infiltration (type of soil and moisture condition)
Type of cover of the watershed and roughness of land
geometry of watershed drainage
network
Slope of watershed
Time of concentration
Stream flow measurement
The rate of water flow is measured in
units of cubic meter per second or
cubic feet per second. The
measurement of flow rates requires
knowledge of the cross sectional
velocity profile, which varies with
flow rate. While it is hard to measure
flow rate directly, it is easy to measure river stage, for example by a gauge Fig. (2)
marbles. Therefore, rates of flow are measured only a few times, enough to
establish a rating curve that correlates the flow rate and water level readings. Regular
measurement of water level is combined with the rating curve to produce time series of
stream flow of a certain river.
1
, Precipitation and Hydrograph
Precipitation is defined as the falling water from the atmosphere in either liquid or solid
phase. This process is known as the condensation of moisture in the atmosphere due to
cooling of air. Dynamic or adiabatic lifting of the air is the cause of air cooling. Adiabatic
lifting means that a given parcel of air is rise with resultant cooling and possible
condensation into small cloud droplets. If these droplets coalesce and have of sufficient
size to overcome the air resistance, precipitation in some form produced.
There are three major types of storms as follows:
1. Convective Storms
2. Orographic Storms
3. Cyclonic Storms
Hydrographs and River Discharge
The amount of water flowing in a river at
any given point and time is known as the
discharge which is measured in cubic
meters per second. It can be calculated by
multiplying the velocity of the river by
channel volume at a given point and time.
The hydrographs as shown in Figure (3) is
a graph that shows river discharge over a
given period.
A storm hydrograph shows how the river's
discharge changes following a period of Fig. (3)
heavy rainfall. The hydrograph indicates that flood is shown as a peak above
the base (normal) flow of the river. Analysis of hydrographs can help engineers and
hydrologists to forecast the likelihood of flooding in a river basin. The response of a river
to the rainfall event is measured in terms of the lag time - the time between the peak of the
rainfall and the peak discharge. Rivers with a short lag time respond quickly to rainfall
events and are therefore more likely to flooding than rivers with a longer lag time.
2
