References 25/05/2012

Transcription

1 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) 1

2 References Nagle G, and K.Spencer Advanced Geography. Oxford University Press,New York. Horst L Hydrometry. International Course in Hydraulics and Environment Engineering, Delft The Netherlands. Seyhan E Fundamental Hydrology. Institut der Rijkuniversiteit Utrecht, Netherland. Seyhan E Watershed as a Hydrological Unit, Geografisch Institut der Rijkuniversiteit Utrecht, Netherland. Wilson E.M Engineering Hydrology. The Macmillan Press, New York. Van Dam J.C., Raaf W.R. and Volker A Veldboek Volume D: Climatology. ILRI: Wageningen, The Netherlands. 2

3 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) 3

4 Hydrology: the distribution and movement of water. 4

5 5

6 Watershed An area contributing runoff and sediment. 6

7 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 INPUT STRUCTURE SYSTEM OUTPUT Precipitation Discharge Sediment Pollutan River Basin Reservoir River Segment River Discharge Water Quality Sediment Pollutan Black Box / Grey Box / White Box Approaches 7

8 INPUT BASIN SYSTEM OUTPUT Precipitation Morphometry Geology Soil Vegetation Human Discharge Sediment Pollutant Precipitation Rainfall-Runoff Relationship Erosion & Sedimentation Discharge Disolving Chemical Materials Sediment Load Sub-Surface Flow Surface Flow 8

9 Hydrograph McCuen, 1989 Measuring Streamflow Runoff Stream Flow Groundwater Streamflow = Surface Runoff + Baseflow Discharge is a measure of the volume of water passing a given point over a period of time. Units? 9

10 Losing vs Gaining Streams Arid Areas Humid 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. 10

11 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 (km 2 ) Shape of Watershed: (1) Form Factor (Rf), (2) Circularity Ratio (Rc), (3) Elongation Ratio (Re) 11

12 Shape of Watershed Form Factor (Rf) = A / Lb 2 Notes: A = Basin Area ( km 2 ) Lb = Main Stream Length ( km ) 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 Notes: A = Area (km 2 ) Ac = π r 2 = Area of a circle having the perimeter as the watershed If Rc > 0,5 the basin is toward circle (dendritic) If Rc < 0,5 the basin is toward length shape (trellis) 12

13 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 13

14 Watershed Boundary Z STREAM LENGTH ASPECTS S N Φ 3 Φ 1 Φ 2 Φ 4 B Φ3<Φ4 A Φ1=Φ2 ON = Longest Stream OS = Main Stream O Outlet 14

15 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 15

16 WATERSHED BIFURCATION RATIO (WR b ) u=k Σ Rb u/u+1 (N u + N u+1 ) u=1 WR b = u=k Σ N u 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 16

17 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) So how do we measure discharge? Discharge is very easy to calculate: cross-sectional area of the channel multiplied by the velocity of the water 17

18 Measurement of Stream Discharge From Ritter et al., 1995 Q = A x V Q : Stream Discharge A: Area V: Velocity Floating Method : Q = A x KU K = V/U = {(1-λ) 1/2 0.1} K normal 0.85 K < 0.5 m 0.60 K > 4.0 m Q = W x d x a x L/T Manning s Formula Q = A x 1/n x R 2/3 x S 1/2 A = Area n = Manning s Coefficient R = Hydraulic Radius S = Slope of energy line 18

19 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 19

20 Dilution Method Continous Injection: Q=q(C 1 -C 2 )/(C 2 -C 0 ) C 1 I II (EC-meter) C 0 C 2 Sudden Injection: Q=(V/T) x (C 1 /C 2 ) C1 High consentration of salt water which used to measure C 2 T How can we relate stage to discharge? Rating Curve relates stage to discharge Empirical relationship from observations Measure discharge at different flows 20

21 Straightline Method (1) Fixed Base Length Method (2) Variable Slope Method (3) A (3) ABCE B C D (2) ABDE E (1) A-E 21

22 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. 22

23 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. 23

24 Unit Hydrograph There are two prinsiples/assumptions: (1) proportional principle: with uniformintensity 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 24

25 Superimposition of Hydrographs Unit hydrograph averaged from four recorded hydrographs, normalized to one inch of runoff (27.4 sq mi. watershed, Coshocton Ohio) 25

26 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 hydrograph 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 catchments 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 26

27 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: T c = time of concentration (minute) F = correction factor, 58,5 when the catchments area in km 2 L = length of main stream (km) A = area of the catchments (km 2 ) S = slope of main stream (m/km) 27

28 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 mi 0,3 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 loam 3 Cover Cultivated lands Woodlands 0,1 0,1 0,2 eductions from unity to obtain the Runoff Coefficient ( C ) for Agricultural Areas. (From Bernard, 1935). From Chernicoff and others, 1997 Hydrographs 28

29 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 basinlag as being sufficient to define the unit hydrograph. Snyder Synthetic Unit Hydrograph i/tr Rainfall intensity tp = basin lag in h Qp (Peak Discharge) ft 3 /sec T = Hydrograph Baselength in days 29

30 tp = Ct (Lca L) 0.3 ; qp = Cp ( 640/tp ) Qp = Cp { (640 A)/tp } ; T = (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 characteristic and varying between for the Appalachian Highland catchments studied. Cp = a coefficient depending units and basin characteristics and varying between for the Appalanchian catchments and generally approaching its largest value as Ct approaches its lowest 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) 30