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Climatology and Hydrology

Hydrological cycle

Hydrometeorology (wind and storm, flood, and drought)

Surface hydrology/river hydrology (flood, drought, and pollution)

Ground water hydrology (flood, drought, pollution, subsidence)

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References

Nagle G, and K.Spencer. 1997. Advanced Geography. Oxford University Press,New York.

Horst L. 1974. Hydrometry. International Course in Hydraulics and Environment Engineering, Delft The Netherlands.

Seyhan E. 1977. Fundamental Hydrology. Institut der Rijkuniversiteit Utrecht, Netherland.

Seyhan E. 1977. Watershed as a Hydrological Unit, Geografisch Institut der Rijkuniversiteit Utrecht, Netherland.

Wilson E.M. 1975. Engineering Hydrology. The Macmillan Press, New York.

Van Dam J.C., Raaf W.R. and Volker A. 1972. Veldboek Volume D: Climatology. ILRI: Wageningen, The Netherlands.

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Definition Hydrology is that branch of Physical

Geography dealing with the waters of earth with special reference to properties, phenomena, and distribution. It treats specially of the occurrence of water on earth, the description with respect to water, the physical effects of water on the earth, and the relation of water to life on earth (Linsley, 1949)

Hydrology ia an earth science. It encompasses the occurrence, distribution, movement, and properties of the waters of the earth and their environmental relations (Knapp, 1989)

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Hydrology: the distribution and movement of water.

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WatershedAn area contributing runoff and

sediment.

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Drainage Basin Concept

River Basin or Drainage Basin is the entire area drained by a stream or system of connecting streams such that all stream-flow originating in the area is discharged through a single outlet (Linsley,1949, Applied Hydrology)

Watershed area supplies surface runoff to a river or stream, whereas drainage basin for a given stream is the tract of land drained of both surface runoff and groundwater discharge (Knapp, 1989, Introduction to Hydrology)

Catchment area (related to precipitation)

CONCEPT OF SYSTEM

INPUTSTRUCTURE SYSTEM

OUTPUT

Precipitation

Discharge

Sediment

Pollutan

River Basin

Reservoir

River Segment

River Discharge

Water Quality

Sediment

Pollutan

Black Box / Grey Box / White Box Approaches

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INPUT OUTPUTBASIN SYSTEM

Precipitation

Morphometry

Geology

Soil

Vegetation

Human

Discharge

Sediment

Pollutant

Sub-Surface Flow

Rainfall-Runoff

RelationshipErosion & Sedimentation

Disolving

Chemical MaterialsDischarge Sediment Load

Surface Flow

Precipitation

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Hydrograph

McCuen, 1989

Measuring Streamflow

Stream

Flow

Groundwater

Runoff

Streamflow = Surface Runoff + Baseflow

Discharge is a measure of the volume of water passing a

given point over a period of time.

Units?

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Losing vs Gaining Streams

Humid AreasArid Areas

BASIN MORPHOMETRY

Dealing with the measurement of River Basin or Watershed geometry;

Basin Morphometry is useful in development of the empirical methods for the rainfall-runoff relationship.

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Spatial/Areal Aspects :

Area (A) and Shape Forms (Rf, Rc, Re)

Topographical/Relief Aspects:

Basin Slope (Sb), Main Stream Slope (Ss), Median Elevation

Stream length Aspects:

Length of longest water course (Li), Length of main stream to Center of Gravity (Lca), Length of main channel (Lb), Length of Overland Flow” (Lg)

Stream drainage Aspects:

Stream Order, Bifurcation Ratio of Stream (Rb), Drainage Density (Dd), Center of Gravity of Basin (Cg), Stream junction system

Spatial Aspect

Basin Area (km2)

Shape of Watershed:

(1) Form Factor (Rf),

(2) Circularity Ratio (Rc),

(3) Elongation Ratio (Re)

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Shape of Watershed

Form Factor (Rf) = A / Lb2

A = Basin Area ( km2 )

Lb = Main Stream Length ( km )

Notes:

1. If Rf moreless 1, the basin is in circle

shape

2. If Rf far from 1, long shape basin

Shape of Watershed

Circularity Ratio (Rc) = A / Ac

A = Area (km2)

Ac = π r2 = Area of a circle having

the perimeter as the watershed

Notes:

If Rc > 0,5 the basin is toward circle (dendritic)

If Rc < 0,5 the basin is toward length shape (trellis)

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Shape of Watershed

Elongation Ratio (Re) = D / Lb

D = Circle diameter is same as Basin

area (km)

Lb = Main stream length (km)

Notes:

1. If Re moreless 1, the basin is in circle

shape

2. If Re far from 1, long shape basin

Topography / Relief Aspects

Mean Slope of Watershed(Sb )

Mean Slope of Main Channel( Ss )

Median Elevation

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O Outlet

A Φ1=Φ2

B

Φ3<Φ4

Φ1

Φ2

Φ3

Φ4

NS

ON = Longest Stream

OS = Main Stream

Watershed BoundaryZ

STREAM LENGTH ASPECTS

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Stream Drainage Aspects

1. Stream Orders

2. Bifurcation Ratio (Rb)

3. Drainage Density ( D / Dd )

4. Center of Gravity (Lca)

5. Stream junction system

STREAM ORDER

Strahler’s scheme is most

commonly used

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WATERSHED

BIFURCATION RATIO (WRb)

u=k

Σ Rb u/u+1 (Nu + Nu+1)

u=1

WRb = ------------------------------------------------

u=k

Σ Nu u=1

Nu = Number of stream order u

Nu+1 = Number of stream

order u+1

Rb = Bifurcation Ratio

Rb between 3 – 5 is normal condition due to geology

Rb <3 and >5 the stream pattern are influence by geology

Rb >5 usely trellis and Rb <3 usely dendritic

Drainage density

Drainage density depends on climate and geology (these are the independent variables that control many aspects of fluvial geomorphology).

If infiltration dominates over runoff, tend to have lower drainage density.

D or Dd = Σ L / A , ΣL: sigma stream length and A: Basin Area

D = 1 – 5 is normal condition , (unit in mile/square miles)

D = < 1 abnormal, more flooded area

D = > 5 abnormal, large areas will be drained

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Discharge Measurement

Volumetric and Hydraulic Structures

Velocity Area Method (Currentmeter and Floating Method),

Slope Area Method (Manning’s “n”),

Dilution Method (Continous and Sudden Injection)

Discharge is very easy to calculate:

cross-sectional area of the channel multiplied

by the velocity of the water

So how do we measure discharge?

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Measurement of Stream Discharge

From Ritter et al., 1995

Q = A x VQ : Stream Discharge

A: AreaV: Velocity

Floating Method :

Q = A x KUK = V/U = 1 – 0.116 {(1-λ)1/2 – 0.1}

K normal 0.85

K < 0.5 m 0.60

K > 4.0 m 0.90 – 0.95

Q = W x d x a x L/T

Manning’s Formula Q = A x 1/n x R 2/3 x S1/2

A = Arean = Manning’s Coefficient

R = Hydraulic Radius S = Slope of energy line

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Velocity

USGS

The rate which the flow travels along the channel reach.

Measured in feet per second or meters per second

How do we measure velocity?

Most Simplistic

Float Method

Current Meter

Average at .6 of the total depth

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Dilution Method

Continous Injection: Q=q(C1-C2)/(C2-C0)

Sudden Injection: Q=(V/T) x (C1/C2)

I II

C0

(EC-meter)

C2

C1

T

High consentrationof salt water whichused to measure

C1

C2

Measure discharge

at different flows

How can we relate stage to discharge?

Rating Curve – relates stage to discharge

Empirical relationship

from observations

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Straightline Method (1)

Fixed Base Length Method (2)

Variable Slope Method (3)

(1) A-E

A

B

C D

E

(2) ABDE

(3) ABCE

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Flood Measurement

Frequency Analysis

Unit Hydrograph

Rational Method Q=FCIA

Synthetic Unit Hydrograph.

1. Snyder (USA)

2. Clarke (Australia)

3. Nakayasu (Japan)

4. GAMA I (Indonesia)

Frequency Analysis

Frequency Analysis is to test the calculation using “empherical” and “theoritical” formulas

Use Probability papers

Data should have historical long and good quality data.

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Flood-frequency curve (with error bars) for the

Skykomish River, at Gold Bar, WA

(from US Geological Survey gauging records)

Exceedence probability

Unit Hydrograph

A unit hydrograph is defined as the hydrograph of surface runoff which would be generated from a unit depth of rainfall excess uniformly distributed over the watershed and occuring within a specified duration of time.

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Unit Hydrograph

There are two prinsiples/assumptions:

(1) proportional principle: with uniform-intensity nett rain on particular catchment, different intensities of rain of the same duration produce runoff for the same period of tme, although of different quantities.

(2) superposition principle: applies to hydrographs resulting from contiguous and/or isolated periods of uniform-intensity nett rain, where it may be seen that the total hydrograph of runoff due to the sum of the separate hydrographs

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Superimposition of Hydrographs

Unit hydrograph averaged from four recorded

hydrographs, normalized to one inch of runoff(27.4 sq mi. watershed, Coshocton Ohio)

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Advantage of the Unit Hydrograph Method The method takes logical account of all factors

which influence the flood hydrograph resulting from rainfall excess.

The concept is easy to understand Application of unit hydrograph to a hyetograph of

rainfall excess to estimate the resulting flood hydro-graph is simple process

Use with care, under appropriate circumstances (spatial uniformty of rainfall excess and linearity of catchment behaviour conditions) the unit hydrograph approach can give at least as accurate flood estima-tes as any other method of estimating a flood from rainfall data.

The method can be used with confidence on catch-ments where no streamflow observations have been made provided Synthetic UH relationships have been developed, or can be developed from observed data in the region of interest

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Procedure of the application of the Rational method:

1. Determine the area of the catchments from map

or aerial photograph

2. Determine the length of main stream and its slope

3. Determine the time concentration (tc)

4. Determine the rainfall intensity, in which its

duration equal to the time of concentration = tc

5. Find the coefficient of runoff C from table or

diagram

Time of concentration could also be estimated by:

Tc = time of concentration (minute)F = correction factor, 58,5 when the catchments

area in km2

L = length of main stream (km)A = area of the catchments (km2 )S = slope of main stream (m/km)

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Estimation of runoff coefficient ( C )

No Type of Area Values of C *

1

Topography

Flat land, with average slopes of 1 ft. to 3 ft. per mi0,3

Rolling land, with average slopes of 15 ft. to 20 ft. per mi 0,2

Hilly land, with average slopes of 150 ft. to 250 ft. per mi 0,1

2

Soil

Tight impervious clay 0,2

Medium combinations of clay and loam 0,4

Open sandy loam0,1

3 Cover

Cultivated lands

Woodlands

0,1

0,2

Deductions from unity to obtain the Runoff Coefficient ( C ) for Agricultural Areas. (From Bernard, 1935).

HydrographsFrom Chernicoff and others, 1997

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Synthetic Unit Hydrograph

(Wilson,1974)

The best known approach is due to Snyder who selected the tree parameters of hydrograph base width, peak discharge and basin-lag as being sufficient to define the unit hydrograph.

i/tr

Rainfall

intensity

Qp (Peak

Discharge)

ft3/sec

tp = basin lag in h

T = Hydrograph Baselength in days

Snyder Synthetic Unit Hydrograph

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tp = Ct (Lca L) 0.3 ; qp = Cp ( 640/tp )

Qp = Cp { (640 A)/tp } ; T = 3 + 3 (tp/24)

tp = basin lag in h

Lca = distance from gauging station to centroid of catchment area,

measured along the main stream channel to the nearest point ,

in miles.

L = distance from station to catchment boundary measured along

the main stream channel, in miles

Ct = a coefficient depending on units and drainage basin characte-

ristic and varying between 1.8 – 2.2 for the Appalachian High-

land catchments studied.

Cp = a coefficient depending units and basin characteristics and va-

rying between 0.56 – 0.69 for the Appalanchian catchments and

generally approaching its largest value as Ct approaches its lo-

west and vice versa.

T = hydrograph baselength in days

Macam-macam HSS

1. Snyder (1938): asal dari U.S.A (dataran benua)

2. US-SCS (dapat ditambah routing), lebih fleksibel, tetapi untuk di luar U.S.A harus lebih hati-hati, dan perlu dilakukan kalibrasi pada stasiun duga (SPAS, river gauging station)

3. Nakayasu: asal Jepang (kepulauan subtropis)

4. Clarke : asal Australia (dataran benua, ada routing)

5. Gama I : asal Jawa (kepulauan tropika, Prof. Sri Harto Br 1985)