Runoff
Runoff, also called overland flow or direct surface runoff (DSRO), is
generated after precipitation when interception and infiltration are
less than precipitation. Some of the infiltrated water reappears on
the surface (interflow) and meet the overland flow. The runoff
flowing down the slope creates/meets rivulets (small streams)
which then meets large rivers and then to sea. The flow in a
river/stream is known as stream flow. When GWT is high, GW
flows into stream which is largely responsible for Base flow in a
stream flow. Thus stream flow consists off DSRO and base flow.
Natural flow/Virgin flow is stream flow unimpeded by any
obstruction like reservoir, lake etc.
Some of the generated surface runoff is stored in depression/puddles. It is called
depression storage. The generated water which does not reach streams is called
surface detention or detention storage. The water stored in the banks of river during
high flood is called Bank Storage. Bank storage produce base flow during lean period.
CATCHMENT CHARACTERISTICS
The area surrounding a river whose surface runoff drains into the river is
called drainage basin, watershed or catchment area of the river. The boundary
line, along a topographic ridge, separating two adjacent drainage basins is called
drainage divide. The river location at which all surface drainage from
a basin comes together or drains as outflow from the basin in the stream
channel is called concentration point or measuring point.The time required for
DSRO from the outerrmost point in a drainage area to reach the concentration
point is called the concentration time. This is a very significant variable since only
such storms of duration greater than the time of concentration will be able to produce
runoff from the entire catchment area and cause high intensity floods.
The characteristics of the drainage net may be physically described by:
(i) the number of streams (ii) the length of streams (iii) stream density (iv) drainage
density
The stream density of a drainage basin is expressed as the number
of streams per square kilometre.
stream density, Ds = N/A
where N = number of streams; A = area of the basin
• The Drainage density is expressed as the total length of all stream
channels (perennial and intermittent) per unit area of the basin and
serves as an index of the areal channel development of the basin
Drainage density, Dd =L/A
• Average stream slope = total fall of the longest water course/length
of the longest water course
• The shape of a drainage basin can generally be expressed by:
(i) form factor (ii) compactness coefficient
Form factor, Ff =W/L= A/L2
where W = axial width of basin, L = axial length of basin, i.e., the
distance from the measuring point (MP) to the most remote point
on the basin.
Compactness coefficient, Cc = perimeter of the basin/
circumference of circular area, which equals the area of the basin.
Runoff characteristics of streams
• Streams can be classified as:
(i) Perrenial: The streams with flows throughout the year. The
continuous flow may be because of snowmelt (ex. Ganga),
high baseflow due to ground water.
(ii) Intermittent: Baseflow is not significant. Dries up during
lean period except occasional flow due to rainfall.
(iii) Ephemeral: Such streams have no groundwater/baseflow
contribution. During storms, flow is high and sudden and then
dries up.
Ephemeral
Runoff Volume
Yield: The total quantity of surface water that can be expected
during a given time period at the outlet of its catchment is called
yield for that period. Annual yield and Seasonal yield
.
Rainfall-runoff relationship
May be of the form:
R = aP+b, a and b being constants
R= cPd , c and d being constants
(i) Empirical Relationships:
Binnie’s Percentage
Barlow’s Table
Strange’s table
In addition, we have several other formulae:
(iv) Overland flow hydrograph
(v) Unit hydrograph
SCS-CN method of runoff estimation
• Soil Conservation Services, USA, developed a method for estimating
runoff from rainfall. This method is also referred as the CN (curve
number) method. It is based on the water balance equation & two
fundamental hypotheses:
(1) ratio of the actual direct runoff to the potential runoff is equal to
the ratio of the actual infiltration to the potential infiltration, and
Q/(P-Ia) = F/S
(2) the amount of initial abstraction is some fraction of the potential
infiltration:
Ia = λ S
where, F=the cumulative infiltration excluding Ia, Q = actual runoff
(mm), P = rainfall (mm), Ia = initial abstraction, which represents all
the losses before the runoff begins and is given by the empirical
equation. S = the potential maximum retention.
Combining (1) and (2):
For conveniece, S (mm) is represented by curve number (CN)as:
CN = 25400/(S+254)
value of CN lies between 0 and 100. CN=0 indicate S being very high, i.e.,
retention is very high where as CN=100 represents zero potential
retention (impervious basin).
CN depends upon Soil type [Group A (low runoff potential),B,C and D (high
runoff potential]; Antecedent moisture condition (AMC, AMC-I (dry),
AMC-II (avg cond.) AMC-III (saturated soil condition), and Land Use.
SCS determined λ=0.2, thus, Q= (P-0.2 S)2/(P+0.8 S) for P>0.2 S
(Standard SCS-CN equation)
SCS-CN equations for Indian Conditions
SCS-CN estimates runoff volume while Rational formula estimates peak
discharge
Advantages:
• SCS-CN method is simple and relies on only one parameter, CN
Flow Duration Curve: Stream flows vary with time, highest during
monsoon and lowest in summer. Flow Duration Curve is the plot of the
percent of time specified discharges were equaled or exceeded during
a
given
period.
• Step 1: Sort (rank) average daily discharges for period of record
from the largest value to the smallest value, involving a total of n
values.
• Step 2: Assign each discharge value a rank (M), starting with 1 for
the largest daily discharge value.
• Step 3: Calculate exceedence probability (P) as follows:
P = 100 * [ m / (N + 1) ]
P = the probability that a given flow will be equaled or exceeded (%
of time)
m = the ranked position on the listing (dimensionless)
N = the number of events for period of record (dimensionless)