Post on 08-Mar-2016
description
transcript
RUNOFF GENERATION
Riddhi Singh Lecture 6
Email: riddhi@iith.ac.inImage: http://upload.wikimedia.org/wikipedia/commons/5/52/Saturated_ground_due_heavy_rainfall_in_Wagga_Wagga.jpg
CE 4502
Today we will learn about
Runoff generation: conceptualization
Estimating ______ __________
2
RUNOFF GENERATIONCONCEPTUALIZATION
3
There are many __________ through which water may reach the river
channel.
4Image: http://www.hydrosconsult.eu/s/cc_images/cache_5828534.jpg
Generally runoff generation is conceptualized using two models: Hortonian
Overland Flow model and Saturation Overland Flow model
___________ overland flow occurs when rainfall intensity (i) exceeds the
infiltration capacity (f) of the soil.
5
Horton considered surface runoff to take the form of a sheet flow whose depth might be
measured in fractions of an inch.Image: http://geography.unt.edu/~williams/geog_3350/examreviews/exam2images/hydrol1.gif
Rainfall excess i f
Applicable to:
1. _______ surfaces in urban areas
2. Natural surfaces with ____ soil
layers and low infiltration capacity
in semiarid and arid areas.
3. Entire catchment
i>f
i-f
f
overland flow is produced when subsurface flow saturates the
soil near the bottom of a slope and overland flow then occurs as rain falls
onto the saturated soil.
6
_________________ are the area of the watershed actually contributing to the stream at
any time. It expands during rainfall and contracts thereafter.
Image: http://ks.water.usgs.gov/pubs/reports/wrir.99-4242.fig02.gif (left)
http://soilandwater.bee.cornell.edu/research/VSA/processes/im_fig2.jpg (right)
Applicable to:
1. ________ surfaces in humid
regions
2. ________ of hill slopes and near
stream banks
A streamflow ______________ is a graph showing the flow
rate as a function of time at a given location on a stream
7
A _________ is an integral expression of the physiographic and climatic characteristics that
govern the relations between rainfall and runoff of a particular drainage basin (Chow, 1959)
Image: http://www.xmswiki.com/w/images/2/2a/GSDAImage077.png (left)
http://blackpoolsixthasgeography.pbworks.com/f/1265744759/Storm%20Hydrograph.png (right)
Hydrographs tell the story of the catchment
Image: http://www.xmswiki.com/w/images/2/2a/GSDAImage077.png (left)
http://water.usgs.gov/edu/graphics/wcsnowmeltchart.gif (right)
Baseflow separation: the separation of the _______ runoff from
baseflow
9Image: http://www.engineeringexcelspreadsheets.com/wp-content/uploads/2011/03/Hydrograph-Example.jpg
A. Straight line method
B. Fixed base length method
C. Variable slope method
N
Inflection point
RUNOFF GENERATION
_____________________
10
__________ or effective rainfall is that rainfall which is neither retained on the
land surface nor infiltrated into the soil. After flowing across the watershed,
excel rainfall becomes _________ runoff.
11Image: http://upload.wikimedia.org/wikipedia/commons/5/52/Saturated_ground_due_heavy_rainfall_in_Wagga_Wagga.jpg
The graph of excess rainfall vs. time is called the _____________________ hyetograph (ERH).
The difference between observed total rainfall and excess rainfall is termed __________________.
Losses = infiltration + interception + surface storage + evaporation
12Image: http://echo2.epfl.ch/VICAIRE/mod_1a/chapt_1/pictures/fig1.3.gif
Assessing rainfall excess is the first step towards estimating streamflow.
Depending on whether streamflow data is available or not,
rainfall excess can be estimated in two ways:
13Image: http://faculty.ksu.edu.sa/saleh-alhassoun/CE4251/chapter1.files/image004.gif
1. Streamflow is available: index method
2. Streamflow is not available: infiltration equations, or by SCS method
1. The index is that constant rate of abstractions that will yield an excess rainfall hyetograph (ERH) with a total depth equal to the depth
of the direct runoff over the watershed.
14
Rai
nfa
ll [in]
Initial
abstraction
Losses
Rainfall excess
index
1. Pick a time interval, t
2. Judge the number of intervals, M of rainfall that actually contribute to direct runoff
3. Subtract t from the observed rainfall in each interval
4. Adjust the values of and M as necessary so that the depths of direct runoff and excess rainfall are equal (equation above)
1
M
d m
m
r R t
2a. Abstraction using infiltration equations: is estimated by determining
the ponding time and infiltration under a variable intensity rainfall
15
Derivation are based on the following principles:
1. In absence of ponding, cumulative infiltration is calculated from cumulative rainfall
2. The potential infiltration rate at a given time is calculated from cumulative
infiltration at that time
3. Ponding has occurred when the potential infiltration rate is less than or equal to
rainfall intensity
Rai
nfa
ll [in]
Initial
abstraction
Continuing
abstraction
Rainfall excess
index
2b. SCS method for estimating abstractions: developed by the Soil
Conservation Service (1972) for determining rainfall excess using information
of soil type, vegetation, and antecedent moisture conditions
16
Fa is the water retained
S is the maximum retention potential
Pe is the excess precipitation
P is the total precipitation
Ia is the initial abstraction
Pre
cipitat
ion r
ate
Ia
Pe
Time
Fa
Through experiments, the data for P and Pe was plotted for many watersheds
and the SCS curves were found. To standardize the curves, a dimensionless
curve number CN is defined such as:
17
100010S
CN
Curve number lie between 0 and 100
Curve number of 100 for impervious surfaces
For each soil type (A,B,C, and D) and land use type, a different
curve number exists
Image: http://www.professorpatel.com/uploads/7/6/5/6/7656897/7766438.jpg?606
Corrections can be made for antecedent moisture conditions: for dry (AMC
I) or wet (AMC III) conditions, equivalent curve numbers are calculated by:
18
4.2 ( )( )
10 0.058 ( )
23 ( )(II )
10 0.13 ( )
CN IICN I
CN II
CN IICN I
CN II
Image: http://www.professorpatel.com/uploads/7/6/5/6/7656897/7766438.jpg?606
AMC group Total 5 day antecedent rainfall (in)
Dormant Season Growing Season
I Less than 0.5 Less than 1.4
II 0.5 to 1.1 1.4 to 2.1
III Over 1.1 Over 2.1
Table 5.5.1 in Chow et al. (2010)
Curve numbers depend on soil type
19
Group Soil type
A High infiltration rates. Deep sand, deep
loess, aggregated soils
B Moderate infiltration rates. Shallow loess,
sandy loam
C Slow infiltration rates. Clay loams, shallow
sandy loam, soils low in organic content,
and soils usually high in clay.
D Very slow infiltration rates. Soils that swell
significantly when wet, heavy plastic clays,
and certain saline soils
Incre
asing cu
rve n
um
bers
And land use
20
Table 5.5.2 in Chow et al. (2010)
WATERSHED CONCEPTSTRAVEL TIME, STREAM NETWORKS
21
Reading assignment:
Section 5.7 Travel time
Section 5.8 Stream networks
In Chow et al. 2010