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Module 4 (L12 - L18): Watershed Modeling

for watershed modeling, overall description of different hydrologicprocesses, modeling of rainfall, runoff process, subsurface flowsand roundwater flow

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1


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Watershed characteristics, GeometricWatershed characteristics, Geometric

representation of watersheds, Linearrepresentation of watersheds, Linear

, , ,, , ,

Drainage & dischargeDrainage & discharge

Keywords:Keywords: Watershed characteristics, geometricWatershed characteristics, geometricre resentation draina e linear areal relief: as ectsre resentation draina e linear areal relief: as ects

22Prof. T I Eldho, Department of Civi l Engineering, IIT Bombay


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Watershed characteristics

Important Characteristics

Size

Shape

Physiography

Drainage

Land use

Vegetation

Geology and Soils

Hydrogeology

Socioeconomics

33Photos: Singh, 07. 2001Photos: Singh, 07. 2001Prof. T I Eldho, Department of Civi l Engineering, IIT Bombay


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Description of Characteristics

CLIMATE

Evaporation

Wind

Relative humidity etc. PHYSIOGRAPHY

Size and sha e of watershed

Elevation

Slope and aspect

Drainage features (pattern, density, etc.)

Parent rock t es i neous sedimentar metamor hic



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Description of Characteristics.. SOILS

Soil depth

Soil infiltration capacity

Soil erosiveness etc.Photo A.K. Sin h 2002

LAND USE AND COVER CONDITIONS Land use types (forest, grassland, agriculture, urban, etc.)

, ,

Forest land conditions Major forest types

Rangeland condition and types Agricu tura practices

Road networks and condition

Recreational use (resort, wildlife, fish resource, etc.)



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Description of Characteristics..

WATERSHED HYDROLOGY

Floods

Stream flow (quantity and quality)

SOCIO-ECONOMIC FEATURES/WATERSHED USE Water use and needs (sources of water, domestic use,

irrigation, industrial, power generation, etc.)

Water use problems (erosion, flooding, siltation, watersupply, water quality, etc.)

management


Photo, A.K. Singh, 2002


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Important Watershed characteristics

Drainage Area (A): - most important for hydrologic

design; reflects volume of water - generated from rainfall.

as product of rainfall depth & drainage area.

drainage area input to all models.

Watershed length (L): increases as the drainageincreases; L is important in hydrologic computations; L-defined as distance measured along the main channel from thewaters e out et to t e as n v e; s measure a ong t eprincipal flow path

A & L- both measures of watershed size; they may reflectdifferent aspects of size. A-indicate potential for rainfall toprovide a volume of water; L- used in computing timeparameter -measure of travel time of water through a

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watershed.

Prof. T I Eldho, Department of Civi l Engineering, IIT Bombay


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Important Watershed characteristics

Watershed Slope (S): - Flood magnitudes reflect the

momentum of the runoff. Slope is an important factor in the.

Watershed slope reflects the rate of change of elevationwith respect to distance along the principal flow path.

=

end points of the principal flow path); L- hydrologic lengthof the flow path .

a ers e s ape: a ers e s ave an n n e var e y oshapes, and the shape supposedly reflects the way that runoff

will bunch u at the outlet.

Eg. A circular shaped watershed would result in runoff from

various parts of watershed reaching outlet at the same time




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Basin Shape Watershed Parameters

Length to the center of area (Lca): distance in miles measuredalong main channel from basin outlet to the point on the mainchannel opposite the center of area.

Shape Factor (L1) = (L Lca)0.3; Where L is the length of the

Circulatory ratio: Ratio of basin area (Au) to the area of circle(Ac) having equal perimeter as the perimeter of drainage basin

ange o c- . o .

c

uc

AR

c area as the basin to the maximum basin length (Lbm)

cDR


bmL


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Channel GeomorphologyChannel Geomorphology

C anne engt : use requent y in y ro ogiccomputation.

The distance measured along the main channel from thewatershed outlet to the end of the channel.

Channel slope: Sc = Ec / Lc

defining the upper & lower ends of the channel & Lc is the

length of the channel between the same two points.

If the channel slope is not uniform- a weighted slope mayprovide an index -reflects effect of slope on the hydrologic

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response of the watershed



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Geometric Representation of Watersheds

Grid Method oncep ua me o

Grid Method:

r angu ar or rec angu ar gr s

Stream channel system:- based on slope, channeldimension conditions

Flow in elemental areas:- travel to channel and finallyto watershed outlet

ver an ow an c anne ow

1313Prof. T I Eldho, Department ofCivil Engineering, IIT Bombay


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Geometric Representation

eps:

A rectangular grid system is

watershed

Grid size:-watershed boundaries ofc anne s approx ma e y gr segmen s

Overland units are grid units insidewatersheds boundaries and channel unitsmay create grid units.

Principal flow direction of each overland

Water is assumed to flow in direction ofland slope to next overland flow unit or

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adjacent channel: cascade and channelProf. T I Eldho, Department of Civi l Engineering, IIT Bombay


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Grid Method

Discretization as

rectangular strips

Channel

1

1 Overland flow

element node

network in the

watershed

Overland flow strip

Channel meeting

point

Single discretized channel

4

3

3

4

w over an ow a ng

at channel nodesW L

1515Channel flow element

Overland flow

element



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Conceptual Method

aters e geometry us ng a networ o e ementa sector

Plane; Triangular section ; Converging section; Diverging section

Channel

Join together:-complete watershed

Plane:-

Horizontal or inclined

Defined by slope, length and area

Converging section

Diverging section

Channel element:- by hydraulic geometry (cross sectional area,wetted perimeter, hydraulic radius, width etc.) and bed profile

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- , , , - .



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Assemblage of Geometric Elements

Assemblage of geometric elements by topographiccharacteristics like grade, direction of flow, land use,ve etation rou hness and channel network

Two methods

Based on topographic characteristics :

different portions of watersheds are represented bygeometric elements

watershed and element representing it.

Geomorphologic characteristics of watershed are used toeve op a networ representation

Model flow paths are analogous to watershed flow paths



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Geomorphologic Characteristics

Quantitative land form analysis: Flowing

water & associated mass gravity movementsac ng over ong per o s o me responsible for development of surface

Geomorphologic characteristics ofwatershed:- systematic description of watershed

geometry and its stream channel system

o measure e near aspec s o ra nage ne wor ,

aerial aspects of drainage basin and

relief aspects of channel network



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Linear aspects of drainage networks-

First order:-unbranched tributary

Second order:-two or more first order Third order:-two or more second order streams

n or er s ream s orme y or more s ream o or er n-& stream of lower order

Bifurcation ratio:-Ratio of number of stream of an order to thenumber of stream of the next lower order

- 1

Nu

NuRb

, .

(Nu+1)- No. of stream of u+1 order; Rb- range from 2 to 4 (generally)

High in steep areas; represents effect on maximum flood discharge of the

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wa ers e .



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If bifurcation ratio Rb and rinci al order k of stream of

Linear aspects of drainage networks

watershed are known, then total number of streams of allorders of a drainage network

1K kR

Law of stream numbers: relate number of stream of

1-u

1i bR

order u (Nu )to bifurcation ratio and principal order (k)uk

bu RN

Stream lengths: reveals characteristics of various

components of drainage network and its contributingsurface area

Where mean length of channel of order u

N

i

uL

L

1uL

2020

- .u

N



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Linear aspects of drainage networks L

any order to average length of stream of next lower order

1

u

uL

LR

aw o s ream eng s:- re a es average eng ostream of order u (Lu) to stream length ratio (RL) andaverage length of first order streams(L1)

1

1

u

Lu RLL

aw o s ream num er an s ream eng s can ecombined to yield on equation for finding total channellength of watershed N

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1

1

.

Lbi

u RRLL


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Areal aspects of watershed-

and inter basin (total)

Stream basin:-area of steam basin Interbasin area:-contributing surface flow directly to

stream of higher order

Total basin area (Au of order u):-Total area projectedon a horizontal plane, contributing overland flow to thestream of given order plus all tributaries of lower order

NNNNNNN

---Stream basin area--- ---Inter basin area---

.............1

04

1

03

1

02

1

1

111

iiii

u

iii

u



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Areal aspects of watershed

Law of Stream Areas: relates uneven area of basinof order u (A

u

) to the mean drainage area of first orderA and the s stem area rate R

Ra:-average basin area of streams of one order toaverage area of basin of next lower order

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uau RAA

Relationship between basin area and stream length

where, L=stream length; A=basin area; m, n=constants

m



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Drainage & Discharge

Relationship between drainage area anddischar e

mJAQ

where, J, m=constants; m varies from 0.5 to 1.0

the horizontal plane of basin map

It has significant effect on stream dischargecharacteristics

Basin can be characterized by: Form factor;

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Drainage & Discharge..

Form factor: Ratio of basin area to square of basinlen th

Circulatory ratio: Ratio of basin area (Au) to the area

2

b

uf

LR

of circle (Ac) having equal perimeter as the perimeterof drainage basin

uAR

Range of Rc- 0.6 to 0.7

Elongation Ratio: Ratio of diameter of a circle (Dc)

c

aving same area as asin to maximum asin engt(Lbm)

cl

DR

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bm



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Drainage & Discharge

Drainage Density (Dd): Ratio of total length of allstream L of all order within a watershed to the totalarea of watershed (Au) k N

uL

u

r

d AD

relatively high density of stream & thus a Rapidstream response

Constant o c anne maintenance inverse o rainagedensity: C = 1/Dd



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Drainage & Discharge.

Stream Frequency: Number of stream segment perunit area of watershed

k

i

u

F 1

k

where, NuNo. of stream segments of u order and Ak

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asin area o princip e or er



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Relief Aspect of Drainage Basin and

Relief: elevation difference between reference pointslocated in the drainage basin.

Maximum Relief: Elevation difference between highest

Maximum basin Relief: Elevation difference beforebasin outlet and highest point located in the perimeter ofas n

Relief Ratio:H

Rn

ratio of relief H to horizontal distance on which relief wasmeasured (L)




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Relief Aspect H

where, H- Max. basin relief; p-basin perimeter

.p

np

Channel slope: slope of a channel segment increaseswith increase in orders

streams of order u( Su ) to average slope of first orderstream( S1 ) and stream slope ratio (Rs)1SuS

1

1.

u

su RSS

drainage density (Dd)

du HDR



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Relief Aspect

Geometric number: ratio of ruggedness

g

Geometric number= d

S

HD

ti

onmodel

igitaleleva

Photo A.K. Sin h 2002



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Hypsometric Analysis of watershed

To develop relationship between horizontal cross-sectional

area of watershed and elevation. urve p o e w re a ve e g an re a ve areas(a/A); h=height of given contour; H=relief; a=cross-sectional area of contour; A=total watershed area

Curve is called Hypsometric curve Useful for comparing area elevation characteristics of

Slope of hypsometric could be changed with

stages of watershed developments Watershed development in three stages:

Inequlibrium stage

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Monadnock stage


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Hypsometric Analysis of watershed

Photo A.K. Sin h 2002

Inequlibrium stage: young stage : watershed is underdevelo ment rocesses

Equilibrium: Mature stage of watershed steady stateconditions reached

prominent hills are found above subdued surface

Hypsometric curves for different stages of watershed



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Hypsometric curves for differentstages of watershed

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ReferencesReferences Raj Vir Singh (2000), Watershed Planning and

Management, Yash Publishing House

J.V.S Murth 1991 Watershed Mana ement New A einternational Publications

Ghanshyam Das (2000), Hydrology and soil conservation,

Horton R.E., (1945), Erosional development of streams &their drainage basins: Hydro-physical approach toquantitative morphology. Bull. Geol. Soc. Am. 56:275-370.



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Tutorials - Question!.?.

Criticall anal ze the im ortantcharacteristics of a typical agriculturewatershed.

Illustrate various parameters and try toIllustrate various parameters and try toquantify them.quantify them.

Discuss t e or er o importance.Discuss t e or er o importance.

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Self Evaluation - uestions!.

Classify the various watershed characteristics & its

im ortance in watershed mana ement.

Describe different methods of geometricrepresentation of watersheds.

Describe linear aspects of watershed & its importance

in geomorphological study of watershed.

Discuss relief as ects of watershed & its im ortance ingeomorphological study of watershed.

W at is Hypsometric ana ysis o waters e ?.

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ro . o, epar men o v ng neer ng, om ay


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Assignment- Questions?.

What are the important watershed factors to be

considered in watershed mana ement?

In watershed analysis, what are the important channelgeomorphology parameters to be considered?

Illustrate the geometric representation of watershed

step by step.

Describe the areal as ects of watershed & itsimportance in geomorphological study of watershed.

What are the different stages of watershed

.

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Unsolved Problem!.Unsolved Problem!.

For your watershed, identify variousFor your watershed, identify various

characteristics & list them in the order ofcharacteristics & list them in the order ofmpor ance.mpor ance.

Analyze the linear aspects of the watershed.Analyze the linear aspects of the watershed.

..

Analyze the relief aspects of the watershed.Analyze the relief aspects of the watershed. Collect dataCollect data area, channel length, slope etc.area, channel length, slope etc.

Illustrate the importance of each characteristics in theIllustrate the importance of each characteristics in thewatershed management plans.watershed management plans.

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Dr. T. I. EldhoDr. T. I. Eldho

Professor,Professor,

Department of Civil Engineering,Department of Civil Engineering,Indian Institute of Technology Bombay,Indian Institute of Technology Bombay,

Mumbai, India, 400 076.Mumbai, India, 400 076.

Email:Email: [email protected]@iitb.ac.in

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Phone: (022)Phone: (022) 25767339; Fax: 2576730225767339; Fax: 25767302http://www.http://www.civil.iitb.ac.incivil.iitb.ac.in

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