Hydrology and Water Resources
RG 744
Institute of Space Technology
December 18 2013
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Response of river to precipitation
An unusually high stage of a river
May fill up the stream up to its banks and often spills
over to the adjoining flood plain
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Flood may be considered as a wave that propagates
downwards
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In an ideal channel with frictionless fluid, flood wave may be
considered traveling with no change from its point of origin
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In natural channel energy is lost due to frictional forces
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As a result magnitude of flood wave reduced or attenuated as
it travels downstream
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But discharge may also increase in downstream reaches due to
increase in watershed area
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Estimate of extreme flood flow is required for the
design of hydraulic structures
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Proper selection of design flood value is of great
importance
 A higher value results in an increase in the cost of hydraulic
structures,
 An under-estimated value is likely to place the structure and
population involved at some risk
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Magnitude of flood may be estimated in accordance
with the importance of the structure
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Design flood may be defined as
 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
Routing is the process of predicting temporal
and spatial variation of a flood wave as it
travels through a river (or channel) reach or
reservoir
There are Hydrologic and Hydraulic Routings – here we will
study Hydrologic Routing
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Once overland flow arrives at a stream it becomes
channel flow
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Routing
 To know how outflow from a reservoir/stream is related to its
inflow?
 What the downstream hydrograph (outflow) will be if
upstream hydrograph (inflow) is known?
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Known parameters:
 you have a hydrograph at one location (I)
 you have river characteristics
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Need:
 a hydrograph at different location (O)
1
2
3
4
Hydrograph computed at outlet of
each subarea
Hydrographs
routed to the outlet
of the watershed
Hydrographs routed thru one reach of
the watershed
I – Q = dS/dt
Assume no seepage, leakage, evaporation or inflow other than main inflow
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In hydrologic routing techniques, equation of
continuity and an analytical/empirical relationship
between storage and outflow in a river or reservoir are
used
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Continuity Equation:
I – O = ΔS/Δt
Where:
I = Inflow rate
O = outflow rate
ΔS/Δt = rate of change of storage
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Flood prediction and flood warning
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Design of hydraulic structures (dams, spillways,
etc.)
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Evaluation of flood control measures
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Etc.
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Reservoir Routing (storage routing):
 Study of effect of a flood wave entering a reservoir
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River Routing (channel or stream flow routing):
 change in shape of hydrograph as it travels down a
channel is studied
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Level pool reservoir
During the advance of a flood
wave, inflow exceeds outflow
producing a wedge of storage
During the recession, outflow
exceeds inflow, resulting in a
negative wedge
Also, there is a prism of storage
which is formed by a volume of
constant cross-section along the
length of prismatic channel
Assumption: X-sectional area of
the flood flow is directly
proportional to the discharge at
that section S = KQ
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Predicting temporal and spatial variation of a flood
wave as it travel through a river reach
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Flood waves passing down a river have their peaks
attenuated due to storage characteristics of the
stream reach if no lateral inflow is added
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Estimates the transformation of flood wave as it
moves through a river channel
S = KO + KX(I-O)
S = storage
O = Outflow
I = inflow
X = weighting factor that varies between 0 and 0.5
K = storage time constant (T)
Homework: show derivation of the above simplified equations
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tp = 4 hr
L = 2 miles
Vavg = 2.5 ft/s
K=?
X=?
Δt = ?
Co =?
C1 =?
C2 =?
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Reservoirs are important in flood control because
of large storage capacity
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Downstream hydrograph peaks are smaller in
magnitude and delayed in time
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Peak flow at the upstream side of the reservoir is controlled
in such a way that the flow at the downstream side is
reduced to safe discharge
In figure below upstream hydrograph has higher peak with
shorter base
Flood waves passing through a reservoir have their peaks
attenuated and time base enlarge due to storage
Downstream hydrograph with lower
peak and broader base achieved by
detaining flood water for some time
by closing the spillways and then
gradually releasing it
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Fixation of maximum reservoir level up to which
the structure is completely safe
Implementation of outflow pattern from the
reservoir so that it may not create any danger in
the downstream side
Inflow = outflow + change in storage
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Two relationships specific for reservoir
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Data required
 Inflow hydrograph
 Starting elevation above spillway
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Consider a reservoir having an ungated spillway
(weir, outlet discharge pipe) or gated spillway
with fixed position
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Use relationship between outflow (Q) and
elevation head (H) for a sharp crested
rectangular weir
Q = CLH3/2
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Q = Discharge at the outlet (cfs)
C = Discharge coefficient of weir (cfs)
L = Length of crest (ft)
H = Depth above spillway
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Estimation of Δt
 Δt < tp/5
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Rainfall-runoff modeling
Flood Frequency Analysis (Statistical
methods)
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There may be floods exceeding the design
specification of a structure
What is a probability of those floods occurring in
any given year
The probability of a flood exceeding or equaling
a given magnitude?
Highest or peak discharges (floods) in each year
used for calculation
Procedure for frequency calculation: Refer
lecture 3 under “ Recurrence Interval of a Storm”
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Floods occurring along rivers, streams and in
coastal area are natural events that have
always been occurring throughout the history
Due to excessive rain runoff increases and
streams or rivers overflow
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Floodplain: Normally dry land area adjoining rivers, streams, lakes, bays
or ocean that is undated during flood events
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Lots of damages and death due to river flooding
each year throughout the world
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Damages caused by floods are sometimes
aggravated by manmade factors e.g.
 Increasing urbanization
 Deforestation
 Uncontrolled development restricting waterways
 Floodplains used for agriculture or settlement purposes
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Flood control is an important issue throughout
the world
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Measures to reduce or alleviate the negative
consequences of flooding
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Range of options to be considered in flood
protection schemes
 Both structural and non-structural approaches
 These approaches must be weighted in terms of costs
and benefits
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Levees (dikes or flood embankments)
Detention/retention Ponds, Dams/reservoirs
Diversions
Channel widening/modification
Due consideration to be given to the design of hydraulic
structures to prevent from collapsing
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Collapsing may also cause further damage by the force of
water released from behind the structures
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Involves huge sums of capital investment
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Earthen banks along the river course
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To confine river into a limited cross-sectional width
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Heights of levees are higher than the design flood
level with sufficient free board
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Floodplain zoning and Management
Preservation of natural wetlands
Flood forecasting and warning system
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Corrective and preventive measures for reducing
flood damages.
Some of the measures are
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Floodplain management program
Emergency preparedness plan
Flood control works
Floodplain management regulations
Defines flood hazard area
Investigates problems arisen in developed areas
and potential problems due to future
development
Zoning features of a regulated floodplain
 The flood hazard area is generally defined at the
100-year floodplain.
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Increasing urbanization leads to increased
overbank flows (floods)
More runoff and more stream runoff
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Flood season 15th June- 15th October
1.
Most modern dams are designed so that they can afford to lose some
storage capacity without their performance being impaired – the part of a
reservoir known as "dead storage" which lies beneath the elevation of the
dam’s lowest outlet. However sediments do not build up evenly along a
horizontal plane, so that some "live storage" is usually lost long before the
dead storage is filled. At Tarbela Reservoir in Pakistan, for example, 12 per
cent of the live storage had been lost by 1992 (after 18 years of operation)
while 55 per cent of the dead storage was still empty of sediment.
2.
Absolute control over flood is rarely feasible either practically or
economically. What we seek to do is to reduce flood damage to a
minimum consistent with the cost involved.
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Barriers: levees, floodwalls, storage basins,
riprap
Adjustment: floodplain regulation (i.e.
zoning)
Redesign: channelization (e.g. Trinity River)
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To add erosion-resistant material to a stream
bank
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Provide flood control
Treat urban runoff
Recreational spots