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POLYTECHNIC UNIVERSITY OF THE PHILIPPINES
College of Engineering
Department of Civil Engineering NDC Compound, Anonas St., Sta. Mesa, Manila
DAM ENGINEERING
GROUP 1
Written Report
HYDROLOGIC STUDIES
GROUP MEMBERS:
Alcaraz, Aprilito
Allones, Harold John
Bacho, Edison
Benliro, Jezreel
Bernal, Divina
Estrabo, Jonas
Nadala, Jermaine Benjch
Sevial, Nico
Rosales, Angelica May
Yuzon, Jaycee
BSCE 5-4
HYDROLOGY STUDIES
Hydrology studies are performed by environmental consultants or hydrologic engineers.
The studies are based on the review of existing maps and records, as well as the
collection of site- specific hydrologic measurements.
Hydrologic Studies involves learning about the properties of water and its maintenance.
Hydrologic technicians are vital to the health and sustainability of water resources,
providing maintenance and management skills necessary for environmental protection.
Characteristics of a hydrologic technician include having a love of nature and passion for
the environment. Many technicians spend a majority of their time working outdoors.
EVALUATION AND REVISION OF EXISTING RECORDS
Before beginning any hydrologic analysis it is important to be sure that the data are
homogeneous. This requires checking history and / or double mass analysis. Stream flow data
can be further evaluated by checking the changes which have occurred in the basin during the
period of record. In each case, the data should be appropriately adjusted to either current
conditions or natural conditions. The errors resulting from lack of homogeneity in data are
especially serious because they lead to bias in the final answers.
Double Mass Analysis
Changes in gage location, exposure, instrumentation, or observational procedure may
cause a relative change in the precipitation catch. Frequently, these changes are not disclosed in
the public records. To test the consistency of the record double-mass analysis can be used.
Double-mass analysis-tests record consistency at a station by comparing its accumulated
annual or seasonal precipitation with the concurrent accumulated values of mean
precipitation for a group of surrounding stations.
For example, a change in slope about 1961 indicates a change in the precipitation regime
of Dillon, Colorado. A change due to meteorological causes would not cause a change in slope,
as all base stations would be similarly affected. The station history of Dillon discloses a change
in gage location in June 1961. To make the record prior to 1961 comparable with that for the
more recent location, it should be adjusted by the ratio of the slopes of the two segments of the
double-mass curve (0.74/1.19). The consistency of the record for each of the base stations should
be tested, and those showing inconsistent records should be dropped before other stations are
tested or adjusted.
ESTIMATING DISCHARGE OF PAST FLOODS
Flood stage record in conjunction with a stage-discharge relation is the preferred method
for estimating the magnitudes of past floods.
Factors affecting the above procedure:
1. The greatest known flood may have occurred prior to the establishment of a
gaging station or after its discontinuance.
2. The greatest flood may have destroyed the gaging station.
3. Records of past floods are only available at other points in the river basin.
In populated river basins the heights reached by great floods of the past have often been
noted on buildings and bridges, and estimates have been made of the corresponding
discharges.
In sparsely developed areas it is necessary to make a search for floodmarks noted by local
inhabitants.
The inflow-outflow storage relations at existing dams and reservoirs can also be used to
estimate flood flows.
If past floods have been determined elsewhere on a river, the corresponding discharge at
the dam site may be estimated by assuming that flood magnitudes are proportional to
some exponential power of the drainage area, usually 0.53
EXTENDING THE PERIOD OF RECORDS
Correlation of Runoff Records
There is often a need to estimate the additional periods in runoff records to increase
the length of a short record or to fill the periods, where the record was interrupted for
various reasons.
THE PROCEDURE IS TO CORRELATE FLOWS with those at another gaging
station in the same or in an adjacent basin which has a longer record.
The relation established between the stations can then be used to extend the stream
flow record at the dam site.
For example, if X and Y are to variables, one with data for the period of N time units, and
the other for K time units respectively with N>K.
The area to be compared should be:
AREA- about the same size
Having similar hydrologic
Topographic
Geologic characteristics
Average flows for a month are generally adequate for reservoir capacity studies and reservoir
operation studies.Estimates of daily flows are not generally reliable unless based on records
collected at points on the same stream and where differences in drainage area are small.
The correlation was used to extend the dam site record, 1964-71, to the additional period
available at Montague, 1936-63.
If the number of values to be estimated is not to great, a graphical comparison of the hydrographs
at two stations can be made. The discharge values are plotted on a logarithmic scale as this
permits equivalent accuracy for a wide range of magnitudes.
Correlation Of Runoff And Precipitation Records
If a drainage basin has a sufficient number of precipitation records to provide an index of
the average basin precipitation, a RUNOFF- PRECIPITATION RELATION may be
developed and used to extend monthly runoff records.
Multiple correlation may be required in which runoff is related to various percentages of
precipitation in previous.
Use of an electronic computer for this type of analysis can save considerable time.
Generation of synthetic records by statistical methods
In recent years, several statistical procedures have been proposed for the sequential
generation of synthetic records.
The generated record exhibits the same characteristics and statistical parameters (mean
and standard deviation) as the basic record, but may indicate possible sequences of high
and low flows which are more critical than those in the record.
The time unit in such studies is usually a month.
ANALYSIS OF RUNOFF RECORDS
(a) HOMOGENITY OF RECORDS
Before a record of river runoff can be analyzed for the use in the design of water resources development
project it must be determined that the hydrologic character of the river basin particularly in regard to man-
made developments, has not change significantly through out the record. In other words, the record must
be statistically homogeneous.
Changes that may take place in a river basin include the following:
*diversions of flow into or out of the basin
*changes in artificial storage by construction of reservoirs
*changes in natural storage by drainage of swamps and lakes
*progressive changes in cover and land use caused by deforestation, afforestation, agricultural practices,
or construction of impervious areas.
(b) HYDROGRAPH
The chronological record of flow is termed the hydrograph. It is basic data for all statistical analyses of
the runoff of a drainage basin. The hydrograph reveals many aspects of the runoff characteristics of the
basin including:
*the seasonal distribution of high and low flows.
*the character of flood flows, whether occurring isolated flood rises or in a succession of floods rising
above a high seasonal base and lasting several months.
*the influence of snowmelt.
*the effect of valley storage.
*the contribution of ground-water flow.
Example of hydrograph
The time unit used in compiling and analyzing the hydrograph will vary with its intended use. Annual
discharges are usually of interest only in comparing sequences of high and low years. Monthly flows are
the most useful unit in reservoir design for municipal water supply, irrigation development, and
hydroelectric power. Hydrographs in units of a day or less are needed to develop design floods.
(c) MASS CURVE
A useful device in the analysis of runoff records is the mass curve which is a plot of the cumulative runoff
from the hydrograph against time. The time scale is the same for the hydrograph and may be in days or
months. The volume ordinate may be in cfs-days, cfs-months, acrefeet, million gals per day, etc. The
slope of the mass curve is the derivative of the volume with respect to time or the rate of discharge.
Example of Mass Curve
(d) STORAGE –DRAFT CURVE
The results of a mass curve analysis can be plotted as a storage-draft curve. This curve gives the storage
needed to sustain various draft rates.
If the storage is unlimited, the storage-draft curve will approach the available mean flow as an asymptote.
It is rarely possible to develop the mean annual flow of a river basin. For most projects, some spillage will
occur in years of high runoff. To impound all flood flows will require an excessively large reservoir.
Such a reservoir may not fill in many years, and probably could not be justified economically. The
selected rate of regulated flow to be developed will depend on:
*the demands of the water users,
*the available runoff,
*the physical limits of the storage capacity,
*and the overall economic project
The storage draft curve indicates only the specific storage needed to achieve different rates of regulated
flow.
Example of Storage-Draft Curve
(e) Selection of Design Flow
The hydrologic analyses, combined with economic analyses of costs and benefits for different heights of
dam and reservoir capacities will lead to the selection of the reservoir capacity and the corresponding
dependable flow that can be justified. The selected design flow may not necessarily be available 100
percent of the time. The proposed water use may permit deficiencies at intervals, for example, a 15
percent storage once in 10 years. Irrigation water supplies permit greater deficiencies than those for urban
and industrial use. Hydroelectric power plants, connected to a large systems, may tolerate substantial
water supply deficiencies.
OPERATION AND ROUTING STUDIES
A plan devised to achieve the greatest value of benefit from the storage capacity.
Based on:
1. Knowledge of the flow characteristics of the stream ( how it perform in the past years)
2. Purposes of the reservoir ( must be analyzed to determine how the hydrograph of flow should
be altered to produce the greatest benefits).
a. Single-Purpose Operation
b. Multi-Purpose Operation
3. Effect of sudden releases on stream banks and long sustained flows from the reservoir on
agricultural development in the valley below the reservoir.
ROUTING TECHNIQUES
The Calculus Method
(Applicable only for preliminary studies of reservoir behavior to save time and labor involved in making
the exact analysis)
Assumptions:
1. The area –depth curve for a reservoir is a semi- cubical parabola with area denoted by A and depth by
H, while B and C are constants
A= CH 3/2
2. The outflow depth curve for the outlet is an ordinary parabola
A = BH 1/2
3. The inflow hydrograph of the flood maybe replace d by a rectangle representing a uniform inflow rate
I, lasting for a length of time T.
The above assumptions lead to simple mathematically derived conclusions as below:
1. The average rate of outflow during a uniform flood is approximately 5/6 of the maximum
rate of outflow during this period.
2. During the emptying period, the average outflow is 4/5 the maximum rate of outflow
during this period.
The two conclusions are known as the 5/6 and 4/5 rules.
Example 1.
The average intensity of the flood to be controlled in 20hrs is 3.0 Mm3/hr, and the
maximum possible outflow is 0.84 Mm3 /hr. Find the storage capacity needed for the proposed
reservoir.
Sol’n:
Ave. outflow during filling period = 5/6 x 0.84 Mm3/hr = 0.70 Mm
3/hr
Total outflow during this period = ave. outflow x duration
= 0.70 x 20 =14.0 Mm3
Storage Capacity required = total inflow – total outflow
= (20 x 3.0) – 14.0
= 46.0 Mm3
DESIGN FLOODS
Flood
A flood is commonly considered to be an unusually high stage of a river. For a river in its natural state,
occurrence of a flood usually fills up the stream up to its banks and often spills over to the adjoining
flood plains.
Design Flood
• The maximum flood that any structure can safely pass.
• The flood considered for the design of a structure corresponding to a maximum tolerable risk.
• The flood which a project (involving a hydraulic structure) can sustain without any substantial
damage, either to the objects which it protects or to its own structures.
• The largest flood that may be selected for design as safety evaluation of a structure.
Design Flood is also known as the Inflow Design Flood (IDF). It is the flood adopted for design purpose,
and could be:
o The entire flood hydrograph, that is, the possible values of discharge as a function of time.
o The peak discharge of the flood hydrograph.
Factors for Selecting the Method of Peak Flow and Frequency Determination
1. Desired Objective- A distinction can be made between the method to determine the magnitude
of the peak flow and the method to determine the maximum volume of flow during a flood period. The
peak may be important in one design problem, while volume in another.
2. Available Data- Long term records of hydrologic data permit the rational application of
statistical procedures, but success in the use of these techniques is inhibited by short term records.
3. The area and characteristics of the watershed – these factors affect the runoff process and
consequently govern the way in which the runoff and hence the peak flow occurs.
4. The importance of the project and time available for analysis- The time available for analysis
governs mainly the sophistication attempted in the analysis.
PEAK FLOW DETERMINATION
1. Unit Hydrograph Method
-Is the most popular and widely used method for predicting flood hydrograph resulting from a known
storm
-First suggested by Sherman in 1932
-A unit hydrograph is defined as the hydrograph of direct runoff resulting from one unit depth (1cm) of
rainfall excess occurring uniformly over the basin and at a uniform rate for a specified duration (D
hours)
Standard Project Flood (SPF)
Standard project flood (SPF) is the flood that would result from a severe combination of meteorological
and hydrological factors that are reasonably applicable to the region. Extreme rare combinations of
factors are excluded. SPF is often used where the failure of a structure would cause less severe damages.
Typically the SPF is about 40 to 60% of the PMF for the same catchment.
Probable Maximum Flood (PMF)
Probable maximum flood (PMF) is the extreme flood that is physically possible in a region as a result of
severe most combinations, including rare combinations of meteorological and hydrological factors. The
PMF is used in situations where the failure of the structure would result in loss of life and catastrophic
damage and as such complete security from potential floods is sought.
To estimate the design flood for a project by the use of a unit hydrograph, one needs the design
storm. This can be the storm producing PMF or SPF as per the design case.
2. Flood Routing
-is the technique of determining the flood hydrograph at a section of a river by utilizing the data of
flood flow at one or more upstream sections. Flood routing is used in
flood forecasting
flood protection
reservoir design
design of spillway and outlet structures
Flood routing types
reservoir routing
channel routing
Routing methods
hydrologic routing (I − O = dS/dt)
hydraulic routing
3. Flood Frequency Analysis
This is the calculation of the statistical probability that a flood of a certain magnitude for a given river
will occur in a certain period of time. Each flood of the river is recorded and ranked in order of
magnitude with the highest rank being assigned to the largest flood. The return period here is the likely
time interval between floods of a given magnitude and can be calculated as:
number of years of river records + 1rank of a given flood
FLOOD ESTIMATION by Rational Method
Rational Method is based on the principle that if a rainfall of uniform intensity and unlimited duration
falls on a basin, the runoff rate will be maximum at the time of concentration tc , as it is only at this
time when the first drop of rainfall, which fell at the remotest part of the basin, reaches the outlet point.
The method is suitable for peak flow prediction in small size (< 50 km2) catchments.
Qp = CIA
Qp = peak runoff rate (m3/s)
C = runoff coefficient / ratio of total volume of runoff to rainfall
-integrated effect of the catchment losses and hence depends upon the nature of the surface, surface
slope and rainfall intensity
I = rainfall intensity (cm/h) of a storm whose duration is equal to the time of concentration of the basin
A = area of watershed (km2)
For a rainfall of uniform intensity and very long duration over a catchment the runoff increases
as more and more flow from remote areas of the catchment reach the outlet. If the rainfall
continues beyond the time of concentration (t > tc), the runoff will be constant and at the peak
value (Qp) equal to Qp = CiA.
FLOOD ESTIMATION by Empirical Formula
The formula is based on statistical correlation of the observed peak and important catchment and storm
properties.
Most of the formulae use the catchment area as a single parameter affecting the flood peak and other
factors are clubbed in a region specific constant parameter.
Dickens Formula
Qp = CD A3/4
in which Qp is in m3/s and A is in km2
CD = Dicken’s constant with value between 6 to 30 depending upon the region (catchment type and
average rainfall)
Ryves Formula
Qp = CR A3/2
in which CR = Ryves’s constant with value between 6.8 to 12 depending upon the region (catchment
type and average rainfall)
Inglis Formula
Q p= 124A/ √(A +10.4)
SAMPLE PROBLEM
Determine the 10-year peak flow over a downtown drainage area of 3 ha with the length of water course
as 1.0 km and slope 0.5%. The IDF curve may be used.
A= 3 ha = 0.03 sq km
L = 1.0 km =1000 m
S = 0.5% = 0.5/100 = 0.005
tc= 0.50 h or 3o min
I = 3.3 cm/h for a tc = 3o min
C = 0.80
10- year peak flow rate, Qp = CIA
Qp = 0.80 x 3.3 x (3.0/100) m3/s
Qp = 0.0792 m3/s
ANALYSIS OF STORMS AND FLOOD OF RECORDS
Analysis of Storms of Records
The major storms over a project basin and its surrounding area are analyzed with regard
to its orientation, isohyetal patterns, areal coverage, total depth, duration and short-term
intensities. First step in the analysis is to plot the isohyetal map of total storm rainfall. Such maps
are used to determine depth-area relations for the total storm or for selected time periods. In
mountainous topography, it may be advisable to draw isohyetal lines considering the topographic
and orographic effects where affects. Another method of determining the depth-area relations is
called the Thiessen method. Second step in the analysis of storm rainfall over a particular basin
is the determination of the depth-time distribution. The time distribution is determined by
analysis of observations registered by continuous recording gages. For a given storm, the
cumulative rainfalls from various recording gages may be plotted as mass curves for a visual
appraisal of consistency and a check for obvious errors. And by comparing cumulative rainfalls,
it may be concluded that the entire basin rainfall could be represented by the direct or weighted
average time distribution shown by the recording gages. Once the representative time
distribution pattern and the desirable time increment have been selected, the difference between
rainfall depths at successive time intervals can be plotted to form a hyetograph , which is a bar
graph pictorially representing basin average rainfall intensities.
Analysis of Flood of Records
Flood frequency method is a method dealing with the runoff directly. This method does
not provide a hydrograph shape but gives only a peak discharge known frequency.
Frequency studies interpret a past record of events to predict the future possibilities of
occurrence. In analyzing by statistical method, it is assumed that the occurrences are:
Individual events
Factors influencing character of each event remain unaltered.
Measurement technique and size of observation are identical.
It may be difficult to find data conforming to all requirements. Thus a preliminary step
the basic data should first be screened and adjusted to remove.
The following are the most important conditions:
Effect of man-made changes in the regime of flow should be investigated and
adjustment made as required.
For small watersheds, a distinction should be made between daily maxima and
instantaneous or momentary flood peaks.
Changes in the stage-discharge relationship make stage records nonhomogeneous
and unsuitable for frequency analysis. It is therefore preferable to work with the
discharges. In case stage frequencies are required, refer the results to the moist
recent rating.
Any useful information contained in data publications and manuscripts should be
made use of after proper scrutiny.
Determination of Frequency
The first problem that a hydrologist faces in practice is to decide on the frequency
of the flood to be adopted in the design of hydraulic structure. This obviously depends
upon the degree of risk he is prepared to take.. The degree of risk that we run in designing
a structure during an anticipated service of life of n years for a flood of a particular
frequency can be theoretically evaluated from the binomial distribution is reproduced as:
P(X = x) = [n!/x!(n-x)!](1-ρ) n-x
Where:
x= number of times a given flood magnitude is to be equalled or exceeded
ρ = the probability of occurrence of a flood equal to or greater than the given
flood magnitude.
n = number of trials, or in other words, the life of the project.
Method of Frequency Analysis
Frequencies can be evaluated graphically by plotting magnitudes of a hydrologic
variable against the frequencies with which they have been equalled or exceeded and
fitting a smooth curve through the plotted points and assuming the same as representative
of future possibilities. To standardize the basis for fitting a curve , the concept of
theoretical distribution is employed. Once a distribution is employed, it is a simple and
straightforward process to calculate the required probabilities.
Two Distributions Commonly Used
Logarithmic normal
Curve fitting methods, graphical or mathematical
Extreme value
Gumbel Method
Log-normal Method
Foster Method
Hazen Method
The methods based on frequency factors use the equation:
x = X + Ks eq. (13.25)
Where:
x = flood magnitude of given return period T
X = mean of recorded flood
s = standard deviation of recorded floods
K = frequency factor
Gumbel’s method
- This form of distribution law with a bearing on the nature of the data is accepted
as the best suited for the frequency analysis.
The magnitude of the flood x for the desired return period T years can be
estimated from eq. (13.25) provided the value of K is known as X and s can be estimated
from the observed flood data. The value K for Gumbel’s distribution has been has been
derived and its values based on the sample size and given return period.
Log- Normal Method
- This is based on the log-normal probability law and assumes that the flood is
such that their natural logarithms are normally distributed.
Foster’s Method
- Foster suggested the use of Pearson’s skew functions for fitting observed flood
data. Pearson adopted the general differential equation,
Where:
x = deviation of the variable X from its mean
J = frequency corresponding to x
a = a constant
f(x) = function of x
Hazen’s Method
- adopted the same values of ô x and C s as calculated in foster’s method.
However, instead of the table of skew factors based on the Pearson’s curve, he
developed a series of factors on the assumption that the logarithms of the variable are
normally distributed. He recognized that the resulting curve might not fit the actual data
in all cases and suggested that several values of C s may be tried and one giving the best
fit selected. Reducing the coefficient of skew tends to reduce extreme values and increase
those near the centre of the distribution.
Design Storms
Reported by: Aprilito D. Alcaraz
A design storm is a precipitation pattern defined for use in the design of a hydrologic system.
Usually the design storm serves as the system input, and the resulting rates of flow through the system
are calculated using rainfall-runoff and flow routing procedures. A design storm can be defined by a
value for precipitation depth at a point, by a design hyetograph specifying the time distribution of
precipitation during a storm, or by an isohyetal map specifying the spatial pattern of the precipitation.
Design storm can be based upon historical precipitation data at a site or can be constructed
using the general characteristics of the precipitation in the surrounding region. Their application ranges
from the use of point precipitation values in the rational method for determining peak flow rates in
storm sewers and highway culverts, to the use of storm hyetographs as inputs for rainfall-runoff analysis
of urban detention basins or for spillway design in large reservoir projects. This chapter covers the
development of point precipitation data, intensity-duration-frequency relationships, design
hyetographs, and estimated limiting storm based on probable maximum precipitation.
DESIGN PRECIPITATION DEPTH
POINT PRECIPITATION
A point precipitation is precipitation occurring at a single point in space as opposed to areal
precipitation which is precipitation over a region. For point precipitation frequency analysis, the annual
maximum precipitation over a region for a given duration is selected by applying the method outlined in
Sec. 3.4. to all storms in a year, for each year of historical record. This process is repeated for each series
of duration. For each duration, frequency analysis is performed on the data, to derived the design
precipitation depths for various return periods; then the design depths are converted to intensities by
dividing by the precipitation duration.
By analyzing data in this way, Hershfield (1961) develop isohyetal maps of design rainfall depth
for the entire United States; these were published in U.S. Weather Bureau technical paper no. 40,
commonly called TP 40. The maps presented in TP 40 are for durations from 30 mins. To 24 hours and
return periods from 1 to 100 years. Hershfield also furnished interpolation diagrams for making
precipitation estimates for duration and return periods not shown on the maps. Fig. 14.1.1 shows the TP
40 map for 100-year 24-hour rainfall. The U.S. Weather Bureau (1964) later published maps for
durations of 2 to 10 days.
In many design situations, such as storm sewer design, duration of 30 minutes or less must be
considered. In publication commonly known as HYDRO 35 (Frederick, Meyers, and Auciello, 1977), the
US National Weather Service presented isohyetal maps for events having durations from 5 to 60
minutes, partially superseding TP 40. The maps of precipitation depths for 5-, to 15-, and 60-minute
durations and return periods of 2 and 100 years for the 37 eastern states are shown in fig 14.1.2. Depths
for 10- and 30-minute durations for a given return period are obtained by interpolation from the 5-, 15-,
and 60-minute data for the same return period.
P10min =0.41P5min + 0.59P15min (14.1.1a)
P30min =0.51P15min + 0.49P60min (14.1.1b)
For return periods other than 2 or 100 years, the following interpolation equation is used, with the
appropriate coefficients a and b from table 14.1.1.
PT yr. =aP2 yr. + bP100 yr. (14.1.2)
Miller, Frederick, and Tracey (1973) present isohyetal maps for 6- and 24-hour durations for the 11
mountainous states in the Western United States, these supersede the corresponding maps in TP 40.
Table 14.1.1. Coefficients for interpolating design precipitation depths using Eq. (14.1.2)
Return period T years a B
5 0.674 0.278
10 0.496 0.449
25 0.293 0.669
50 0.146 0.835
EXAMPLE: Determine the design rainfall depth for a 25-year 30-minute storm in Oklahoma City.
Solution: Oklahoma City is located near the center of the state of Oklahoma and the values of 15- and
60-minute precipitation for 2- and 100-year return periods are read from Fig. 14.1.2 as P2,15= 1.02in,
P100,15= 1.86in, P2,60= 1.85in and P100,60= 3.80in, respectively. Using (14.1.1b) the values for 30-minute
precipitation depth are calculated:
P30min =0.51P15min + 0.49P60min
For T= 2 years, P2,30= 0.51 x 1.02 + 0.49 x 1.85= 1.43 in.
For T= 100 years, P100,30= 0.51 x 1.86 + 0.49 x 3.80= 2.81 in.
Then (14.1.2) is used with coefficients a=0.293 and b=0.669 from table 14.1.1 to give the 25-year
30minute precipitation depth:
P25,30 = aP2,30 + bP100,30
P25,30 = 0.293 x 1.43 + 0.669 x 2.81
P25,30 = 2.30 in.
INTENSITY-DURATION-FREQUENCY RELATIONSHIPS
One of the first steps in many hydrologic design projects, such as in urban drainage design, is the
determination of the rainfall event or events to be used. The most common approach is to use a design
storm or event that involves a relationship between rainfall intensity (or depth), duration, and the
frequency or return period appropriate for the facility and site location. In many cases, the hydrologist
has standard intensity-duration-frequency (IDF) curves available for the site and does not have to
perform this analysis. However, it is worthwhile to understand the procedure used to develop the
relationships. Usually the information is presented as a graph, with duration plotted on the horizontal
axis, intensity on the vertical axis, and a series of curves, one for each design return period, as illustrated
for Chicago in Fig. 14.2.1.
The intensity is the time rate of precipitation, that is, depth per unit time (mm/h or in/h). it can
be either the instantaneous intensity or the average intensity over the duration of the rainfall. The
average intensity is commonly used and can be expressed as
𝑖 =𝑃
𝑇𝑑
Where P is the rainfall depth (mm or in) and 𝑇𝑑 is the duration, usually in hours. The frequency is usually
expressed in terms of return period, T, which is the average length of time between precipitation events
that equal or exceed the design magnitude.
Table 14.2.1. Design precipitation depths (in) at Oklahoma City for various durations and return periods
Return period T (yr)
Duration Td (min)
5 10 15 30 60
2 0.48 0.80 1.02 1.43 1.85
5 0.57 0.94 1.20 1.74 2.30
10 0.63 1.05 1.34 1.97 2.62
25 0.72 1.21 1.54 2.30 3.08
50 0.80 1.33 1.70 2.56 3.44
100 0.87 1.45 1.86 2.81 3.80
EXAMPLE:
Determine the design precipitation intensity and depth for a 20-minute duration storm with a 5-year
return period in Chicago.
Solution: From the IDF curves for Chicago (Fig. 14.2.1), the design intensity for a 5-year, 20-minute storm
is 𝑖 = 3.50 in/h. the corresponding precipitation depth is given by Eq. (14.2.1) with 𝑇𝑑= 20 min= 0.333h.
𝑃 = 𝑖𝑇𝑑
𝑃 = 3.50 𝑥 0.33
𝑃 = 1.17 𝑖𝑛
DESIGN PRECIPITATION HYETOGRAPHS
Most often hydrologists are interested in precipitation hyetographs and not just the peak estimates.
Techniques for developing design precipitation hyetographs
1. SCS method
2. Triangular hyetograph method
3. Using IDF relationships (Alternating block method)
SCS METHOD
SCS (1973) adopted method similar to DDF to develop dimensionless rainfall temporal patterns
called type curves for four different regions in the US. SCS type curves are in the form of percentage
mass (cumulative) curves based on 24-hr rainfall of the desired frequency. If a single precipitation depth
of desired frequency is known, the SCS type curve is rescaled (multiplied by the known number) to get
the time distribution. For durations less than 24 hr, the steepest part of the type curve for required
duration is used
SCS Method Steps
Given Td and frequency/T, find the design hyetograph
1. Compute P/i (from DDF/IDF curves or equations)
2. Pick a SCS type curve based on the location
3. If Td = 24 hour, multiply (rescale) the type curve with P to get the design mass curve. If Td
is less than 24 hr, pick the steepest part of the type curve for rescaling
4. Get the incremental precipitation from the rescaled mass curve to develop the design
hyetograph
Example – SCS Method
Find - rainfall hyetograph for a 25-year, 24-hour duration SCS Type-III storm in Harris County using a
one-hour time increment a = 81, b = 7.7, c = 0.724 (from Tx-DOT hydraulic manual).Find
◦ Cumulative fraction - interpolate SCS table
◦ Cumulative rainfall = product of cumulative fraction * total 24-hour rainfall (10.01 in)
◦ Incremental rainfall = difference between current and preceding cumulative rainfall
hrin
bt
ai
c/417.0
7.760*24
81724.0
inhrhrinTiP d 01.1024*/417.0*
TRIANGULAR HYETOGRAPH METHOD
Td: hyetograph base length = precipitation duration
ta: time before the peak
r: storm advancement coefficient = ta/Td
tb: recession time = Td – ta = (1-r)Td
Given Td and frequency/T, find the design hyetograph
1. Compute P/i (from DDF/IDF curves or equations)
2. Use above equations to get ta, tb, Td and h (r is available for various locations)
ALTERNATING BLOCK METHOD
Given Td and T/frequency, develop a hyetograph in Dt increments
1. Using T, find i for Dt, 2Dt, 3Dt,…nDt using the IDF curve for the specified location
2. Using i compute P for Dt, 2Dt, 3Dt,…nDt. This gives cumulative P.
3. Compute incremental precipitation from cumulative P.
4. Pick the highest incremental precipitation (maximum block) and place it in the middle of
the hyetograph. Pick the second highest block and place it to the right of the maximum
block, pick the third highest block and place it to the left of the maximum block, pick the
fourth highest block and place it to the right of the maximum block (after second block),
and so on until the last block.
DESIGN AERIAL PRECIPITATION
Point precipitation estimates are extended to develop an average precipitation depth over an
area.Depth-area-duration analysis
d
d
T
Ph
hTP
2
2
1
Time
Rainfall intensity, i
h
t t
d
a
T
tr
Td
◦ Prepare isohyetal maps from point precipitation for different durations
◦ Determine area contained within each isohyet
◦ Plot average precipitation depth vs. area for each duration
ROUTING OF SPILLWAY DESIGN FLOOD
A spillway for a dam may serve one or more of three principal functions, defined by Cochran as follows:
1. Provides protection against overtopping of non-overflow sections of the dam, acting in
conjunction with other outflow facilities, such as regulating outlets or turbines.
2. Limits water surface elevations in the reservoir above the normal full pool elevation to avoid
damages upstream from the dam.
3. Supplements regulating outlet for flood control operation when reservoir levels are above the
spillway crest.
The spillway design flood is the most important flood to be provided for in the design of a dam. The
selected magnitude and probable recurrence interval of this flood is related to the importance of a dam,
its functional use, the economic value of the investment , and the potential damages to property and
even loss of life that would result total or partial failure of the structure.
Cochran has proposed the following general standards for the hydrologic design of spillways:
Standard 1: Design the dam and spillway large enough to assure that the dam will not be
overtopped by floods up to the probable maximum categories.
Standard 2: Design the dam and appurtenances so that the structure can be overtopped without
failing and, insofar as practicable, without suffering serious damage.
Standard 3: Design the dam and appurtenances in such a manner as to assure that breaching of the
structure from overtopping would occur at a relatively gradual rate, such that the rate and magnitude of
increases in flood stages downstream would be within acceptable limits.
Standard 4: Keep the dam low enough and storage impoundments small enough that no serious
hazard would exist downstream in the event of breaching.
ROUTING OF SPILLWAY DESIGN FLOOD
(a) General Principles
This section is concerned with the routing of the spillway design flood through the reservoir, including a
discussion of possible modifications to the total inflow hydrograph resulting from the creation of the
reservoir, and the assumptions to be made regarding reservoir elevation and outlet conditions. The
routing is carried out for a number of assumed spillway designs and corresponding hydraulic capacities
to determine, along with structural and cost analyses, the most economic design. The important factor
in the routing procedure is the evaluation of the effect of storage in the upper levels of the reservoir,
termed “surcharge storage,” on the required outflow capacity. In computing the available storage, the
water surface is generally considered to be level. There will be sloping water surface at the head of the
reservoir due to backwater effect and this condition will create an additional “wedge storage.” However,
in most large and deep reservoirs this incremental storage can be neglected.
(b) Total Inflow Hydrograph
The hydrograph of the spillway design flood, developed as outlined in Section 11, is derived initially for
the drainage area at the head of the reservoir. Additional adjustments are necessary to make it
applicable to the total drainage area at the dam site. Such adjustments are necessary to make it
applicable to the total drainage area at the dam site. Such adjustments are necessary, because large
reservoirs, many miles in length and covering many square miles, may significantly change the runoff
characteristics of the lower reaches of the river, resulting in modifications to the shape and timing of the
natural flood hydrograph.
The spillway design flood hydrograph may be made up of three or more components as follows:
1. The hydrograph of the main river at the point where the channel intersects the reservoir
surface. There may be two or more principal tributaries for which hydrographs must be
developed.
2. The hydrograph of the tributary area surrounding the reservoir. This may be composed of
several small streams. Usually a hydrograph is developed for one typical stream and then its
characteristics are considered applicable to the total contributing area. The resulting
hydrograph will peak sooner than the hydrograph from the main channel.
3. The hydrograph resulting from the design storm rainfall falling directly on the reservoir area.
This hydrograph will also peak sooner than the contributions from the main channels, and is
generally significant only as a contribution to the total volume of runoff.
The total inflow hydrograph is the sum of the ordinates of the component hydrographs. In
combining the hydrographs, no allowance is usually necessary for the time of travel through the
reservoir, as the effect of the inflow will be transmitted to the spillway as a pressure wave. With very
large reservoirs, the contribution from the peripheral areas and/or the rainfall on the water area may
cause an initial peak which is higher than that contributed by the main channel at the head of the
reservoir.
(c) Initial Reservoir Elevation
If a reservoir is drawn down at the time of occurrence of the spillway design flood, the initial increments
of inflow will be stored, with the corresponding reduction in ultimate peak outflow. Therefore, for
maximum safety in design it is generally assumed that a reservoir will be full to the spillway crest in the
case of an uncontrolled spillway and to the normal operating pool elevation when a gated spillway is
used.
There may be exceptions to the above criteria in the case of reservoirs with large reservoirs with
large reservations for flood control storage. However, even in such cases a substantial part (50 percent
or more) of the flood control storage should be considered as filled by runoff from antecedent floods.
The effect on the economics and safety of the project should be analyzed before adopting such
assumptions.
When the storage is to be used for power, irrigation, or water supply, the reservoir should be
assumed to be full to the normal operating pool at the beginning of the spillway design flood.
Any assumption that a reservoir can be significantly drawn down in advance of the spillway
design flood (other than one with definite flood control storage reservation) as the result of a short-
term flood warning system, is generally not acceptable for several reasons. The volume that can be
withdrawn is the product of the total rate of discharge at the dam times the warning time. Since the
warning time is usually short, except on great rivers, the released rate must be the greatest possible
without flood damage downstream. Even under the most favorable conditions, it will be found that the
volume that can be released will not be significant relative to the volume of the spillway design flood.
(d) Outflow Conditions
The facilities available for discharge of the inflow from the spillway design flood will depend on the type
and design of the dam and its proposed use. A single dam installation may have two or more of the
following discharge facilities:
Uncontrolled overflow spillway
Gated overflow spillway
Regulating outlet
Power plant
Uncontrolled and gated spillways may take several forms such as conventional gravity (ogee)
section, side-channel overflow, chutes, or shaft (morning glory) spillways.
Uncontrolled Overflow Spillway. With a reservoir full to the spillway crest at the beginning of the design
flood, discharge will begin at once and continue at a rate proportional to the three-halves power of the
head on the spillway. Surcharge storage is created as the head increases and thus storage will also be
proportional to some function of the head, depending on the characteristics of the elevation-capacity
curve. The peak outflow will always be less, to some degree, than the inflow.
Gated Overflow Spillway. With a gated spillway, the normal operating level is usually near the top of
the gates, although at times it may be drawn below this level by other outlets. The usual purpose in
selecting a gated spillway is to make maximum use of available storage and head and at the same time
to limit backwater damages by providing a high initial discharge capacity. In routing the spillway design
flood, an initial reservoir elevation at the normal full pool operating level is assumed. Operating rules for
spillway gates must be based on careful study to avoid releasing discharges that would be greater than
would occur under natural conditions before construction of the reservoir. The effect of a large reservoir
on the inflow hydrograph has been discussed in Section 12(b). In general, releases should be less than
the inflow during the progress of a flood, thereby causing limited surcharge to build up. Furthermore, it
is generally the practice in the design of gated spillways for the peak discharge from the spillway design
flood to exceed the capacity of the gates at full opening with the results that the maximum water
surface will rise above the normal pool operation.
Regulating Outlet. Most dams are designed with low-level outlets, either through the dam or the
abutments for normal downstream releases in carrying out their single or multipurpose functions. The
discharge capacity of these outlets is usually small relative to the potential flood flows. These outlets
may be assumed to be operating, at least in the initial stages of the design flood, and with due
consideration for needs for flood control downstream. As spillways flow increases, it is conservative to
assume that these outlets are closed.
Power plants. When a hydroelectric plant is located at a dam and reservoir, it may be assumed that the
turbines are discharging initially, thereby delaying use of the spillway. As in the case of the regulating
outlets, the discharge should be limited to non-damaging channel capacity downstream, considering
inflow from contributing areas below the dam. As the spillway discharges increase, the elevations of the
tailwater below the dam may limit turbine discharge capacity. It is common practice to assume that the
turbines are discharging at 75 percent of capacity, unless special conditions may prevent it. An
assumption that the plant discharge is completely stopped, as in the case of the regulating outlets, may
be unrealistic, as the power production may be required in the region regardless of flood conditions.
(e) Selection of Maximum Water Surface Elevation
Determination of the maximum reservoir level by routing of the spillway design flood hydrograph under
various assumed lengths, heads, and possible types of spillway is a basic step in the selection of the
elevation of the crest of the dam. The spillway length and corresponding capacity may have an
important effect on the overall cost of the project, and the selection of the ultimate spillway
characteristics is based on an economic analysis. Among the many economic factors that may be
considered are damage due to backwater in the reservoir, cost-height relations for gates, and utilization
in the dam of material excavated from the spillway channel.
After the economic water surface elevation is selected, an additional vertical distance to the
crest of the dam must be provided for wave action as described in Section 13. Actually this computation
may be done concurrently with the reservoir routing in order to have a correct estimate of the total
volume in the dam, for use in the economic studies.
Freeboard Against Wave Action
FREEBOARD – vertical distance between the crest of a dam or embankment and some specified pool
level.
It is provided to protect dams and embankments from overflow caused by wind induced tides and wave.
Riprap or other types of slope protection are provided within the freeboard to control erosion that may
occur even without overtopping.
The reference elevation for setting freeboard is generally the normal operating level, or the maximum
flood level.
There are generally three basic considerations in establishing freeboard allowance:
1. Wind tide
2. Wave characteristics
3. Wave runup
Wind Tide
Wind Tide is the tilting of the reservoir surface above the horizontal water surface level on the
leeward side and below it on the windward side.
Wind blowing over an enclosed body of water exerts a horizontal force that causes a buildup in
level along the leeward shore. There is a similar reduction in the opposite direction. This phenomenon is
called wind tide or setup.
𝑆 = 𝑉𝐴2𝐹
1400𝐷
Where:
S = setup in feet above the reference level
V = wind velocity in miles per hour
F = fetch or water distance in miles over which the wind blows
D = mean reservoir depth in feet along the fetch
Wave Characteristics
The characteristics of wind generated waves measured in several large inland reservoirs have
been analyzed using the dimensionless parameters, gT/V, gHs/V2 and gF/V
2 where:
g = gravity constant of 32.2
T = time between wave crests in seconds
F = effective fetch in miles
Hs = average of the highest one third of the waves occurring in a particular series
The wind velocity used in the equations was that measured over water and is greater than velocities
measured at land stations, which are the data usually available in reservoir studies. Wind speeds recorded
over land should be adjusted as follows:
Fetch
in Miles
Percentage
Increase
>4 30
2 20
<1 10
To evaluate the effect of differences in reservoir shapes and to obtain a better correlation between
fetch and wave height, a weighted or effective fetch has been devised. The steps in a typical computation
for estimating the effective fetch in a reservoir are summarized as follows.
1. The maximum fetch line in the reservoir in the direction of the wind is located.
2. Seven secondary fetch lines radiating from the dam and on each side of the maximum fetch
are drawn at 6° intervals.
3. The length of each fetch line is multiplied by the cosine of the angle between the line and the
maximum fetch line.
4. The sum of the products in step 3 divided by the sum of the cosines to obtain the effective
fetch distance.
Wave Runup
- is the maximum vertical extent of wave uprush on a beach or structure above the still water
level.
The vertical height that a deep-water wave will run up a slope are functions of the wave
characteristics as measured by the ratio between wave height and wave length, the slope of the
embankment, the permeability and the relative surface rougness.
Freeboard Allowance
Design freeboard for wave action or the vertical distance between the selected still-water level
and the crest of the dam is the computed runup from a selected wave height plus the wind tide. For major
embankments a minimum freeboard of 5.0 ft is customary. For other embankments a lesser minimum
may be used.
Sedimentation of Reservoir
Sediments
Sediment, a naturally occurring material that is broken down by processes of weathering and
erosion, and is subsequently transported by the action of wind, water, or ice, and/or by the force of
gravity acting on the particle itself.
The matter that settles to the bottom of a liquid.
Geology: mineral or organic matter deposited by water, air, or ice.
Importance and reasons of reservoir sedimentation
• Morphological and hydrological characteristics of reservoirs
• Reasons of reservoirs sedimentation
– Transport of bed load and suspended sediments into the reservoir – Creation of sediments by biological processes – Vegetation growth (creation of swamps and marshy land)
• Mean volume loss of reservoirs due to sedimentation in worldwide
Measures against reservoir sedimentation
Measures in the catchment area
1. Erosion protection (soil conservation)
– Plantations (forestation) – Stabilization of slopes – Erosion protection at rivers (against bank and bed erosion)
2. Bed load retention basins:
– Built on torrents and mountain rivers – Regular emptying is required – Insignificant retention of suspended load – Small effect on reservoir sedimentation
3. Measures in the catchment area
4. Sediment retention dams upstream of reservoir:
– Bed load and suspended load is captured – Degree of retention for fine sediments is 90% for – example if the capacity of the retention basin is in – minimum 10 % of the yearly inflow – Mechanical cleaning or flushing is required
Measures in the reservoir
1. Mechanical removal
1. Dredging – at full or empty reservoir
2. Hydrosuction (Airlift) – at full lake (limited depth)
3. SPSS - Slotted Pipe Sediment Sluicer – at full lake (limited depth)
2. Hydraulic removal = flushing
a. Negative impacts downstream of the dam:
– damaging or destruction of aquatic organisms, especially fishes – covering of benthos by sediments – local depositions in the river bed and increase of bed elevation – blockage of diversion and intake structures – reduction of flood safety – increased abrasion at turbines and gates – decrease of water quality especially at diversion structures – hindering of leisure activities – sedimentation of downstream reservoirs – damaging or destruction of aquatic organisms, especially fishes
– covering of benthos by sediments
Measures at the dam
1. Heightening of the dam
2. Heightening of water release structures (bottom outlet, power intake)
3. Cone flushing in front of water release structures under pressure
4. Venting of turbidity currents (high capacity release structures required)
a. Drawdown of reservoir during yearly floods
b. Release of highly sediment charged water trough
c. turbines under controlled concentration
REFERENCES:
Mutreja, K.N. Applied Hydrology, Tata McGraw- Hill Publishing Company limited, 1986.
Alfred R. Golze. Handbook of Dam Engineering. Manila : National Book Store, 1980.
Ven TE Chow, “Applied Hydrology”, Mc Graw-Hill Book,Co.,1998.
Web resources for TP 40 and rainfall frequency maps
http://www.tucson.ars.ag.gov/agwa/rainfall_frequency.html
http://www.erh.noaa.gov/er/hq/Tp40s.htm
http://hdsc.nws.noaa.gov/hdsc/pfds/