THE EFFECTS OF SNOW AVALANCHES ON THE HYDROLOGIC REGIME OF THE KUNHAR RIVER, WESTERN HIMALAYAN, PAKISTAN:

Transcription

1 THE EFFECTS OF SNOW AVALANCHES ON THE HYDROLOGIC REGIME OF THE KUNHAR RIVER, WESTERN HIMALAYAN, PAKISTAN: ANALYSIS AND APPLICATION TO RIVER FLOW FORECASTING. By MOHAMMAD INAMULLAH KHAN B.E. (Civil Engineering), N.E.D. University f Engineering and Technlgy, Karachi, Pakistan, 1984 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES Department f Civil Engineering We accept this thesis as cnfrming t the requirement standard THE UNIVERSITY OF BRITISH COLUMBIA September 1995 Mhammad Inamullah Khan, 1995

2 In presenting this thesis in partial fulfillment f the requirements f an advanced degree at the University f British Clumbia, I agree that the Library shall make it freely available fr reference and study. I further agree that permissin fr extensive cpying f this thesis fr schlarly purpses may be granted by the head f my department r by his r her representatives. It is understd that cpying r publicatin f this thesis fr financial gain shall nt be allwed withut my written permissin. Department f Civil Engineering The University f British Clumbia Vancuver, Canada Date g»b. \.\ l45

3 ABSTRACT This study sets ut t investigate the significance f snw avalanches n the hydrlgy and runff generatin in the Kunhar basin in Nrthern Pakistan. The bjectives f this research are, t analyze the snwmelt and snw avalanche effects using the U.B.C. Watershed Mdel, and t prduce a flw frecasting system which takes accunt f the snw avalanche effects. The Kunhar River is a majr tributary f the Jhelum River in the western Himalayas f Pakistan. The basin area is abut 2,340 km 2 with an elevatin range frm 800 t 5,300 m abve sea level. The watershed has a seasnal snw cver which develps frm early Nvember nwards, reaching a maximum depth in March r April. Als, the snwpack increases greatly at upper elevatins. In the Kunhar basin the avalanching is a majr surce f snw redistributin frm higher t lwer elevatins. It is estimated that n average ver 200 x 10 6 m 3 water equivalent f snw is avalanched annually. The percentage f the ttal affected area (runut and starting znes) by avalanches in Kunhar basin is estimated t range frm 12% t 21%. The starting zne lies at a mean elevatin f abut 4,000 m and runut znes are at mean elevatins f 2,450 and 2,800 m abve sea level. This means that the avalanche activities in the lwer elevatins are dependent n the snw precipitatin at elevatin 4,000 m. This study shws that abut 20% f the snwpack at 4,000 m is, n average, subject t avalanching.

4 Avalanche cntributin is fund t be very significant in calibrating the watershed mdel. On average the verall Nash-Sutcliffe cefficient f efficiency f the mdel was imprved frm 77 t 84% after intrducing avalanches in the calibratin which shws imprved time distributin f runff. Snwmelt pattern in the avalanche areas is significantly mdified by avalanche activity. Firstly, the snwmelt in the runut znes starts abut seven days later and lasts abut 30 days mre than in areas nt affected by avalanches. The snwmelt vlume in runut areas is increased by abut 200 t 300% in affected areas. The maximum snwmelt frm the avalanche runut areas is abut 100% higher than the maximum snwmelt in the un-affected areas. The timing f the maximum snwmelt is delayed by abut 15 days in the runut znes f avalanche affected area, due t high accumulatin f snw. These results shw that the snw avalanches increase bth the vlume and the perid f the snwmelt in the runut znes and als change the time distributin f the snwmelt. Since the snwmelt increase in the runut znes is cmpensated by the decrease in snw in affected areas f the starting znes, the ttal snw melt frm the basin is unchanged. The abve results f flw simulatin by using redistributin f snw were used t prduce a frecasting system f avalanche activity. Linear regressin analyses were perfrmed and the linear relatinships fr each band were estimated. Regressin analyses shw very strng crrelatin between avalanche vlume and snwpack accumulatin at the upper elevatins, i.e., the cefficient f determinatin (R 2 ) is fund t be in a range f The extra snw depth acquired at elevatins 2,450 t 2,800 m in the frm f iii

5 avalanche is als strngly crrelated with the existing snw depth at 4,000 m, R 2 ranged between 0.93 and If the snwpack at 4,000 ra elevatin is measured then the maximum snw accumulatin, which ccurs in late March r early April, can be estimated. Frm the develped equatins the ttal avalanche vlume, the snw avalanche depth, and the affected areas fr runut and starting znes can be estimated. These estimates can then be used in the U.B.C. Watershed Mdel t frecast the flw fr the cming seasn. Applicatin f this prcedure shwed that the prpsed frecasting system gives an imprved and reliable estimatin f the seasnal flw vlume and the time distributin f runff fr the Kunhar river. IV

6 Table f Cntents Abstract Table f Cntents List f Tables List f Figures List f Symbls Acknwledgment Dedicatin ii v ix x xii xv xvi Chapter ONE INTRODUCTION PREAMBLE RESEARCH OBJECTIVES 4 Chapter TWO THEORETICAL BACKGROUND AND LITERATURE REVIEW INTRODUCTION GOVERNING FACTORS AND RELEASE MECHANISMS HYDROLOGICAL ROLE OF AVALANCHE RESEARCH ON HYDROLOGICAL ASPECT OF AVALANCHE Summary 22 v

7 Chapter THREE GEOGRAPHY AND CLIMATE OVERVIEW GEOGRAPHY OF THE NORTHERN MOUNTAINS Reginal Setting The Upper Indus Basin ' CLIMATE Brad Climatic Cntrls Lcal Climate STUDY AREA Avalanche Activity in Kunhar Basin 38 Chapter FOUR METHOD OF ANALYSIS HYDROLOGICAL MODEL CALIBRATION PROCEDURE CLIMATIC DATA PRECIPITATION ADJUSTMENTS AVALANCHING SUMMARY OF CALIBRATION REGRESSION ANALYSES 60 VI

8 4.7.1 Avalanche Vlume Avalanche Depth 61 Chapter FIVE RESULTS AND DISCUSSIONS INTRODUCTION MODEL CALIBRATION Stage-I Calibratin Stage-II Calibratin Avalanche Cntributin DISTRIBUTION PATTERN OF AVALANCHE Avalanche Vlume Avalanche Area EFFECTS OF AVALANCHES Snwpack Cnditins With and Withut Avalanche Avalanche Effects n Snwpack Accumulatin Avalanche Effects n Snwmelt AVALANCHE FORECASTING MODEL Avalanche Vlumes Avalanche Depth (POPADJ) and Areal Distributin 109 vii

9 Chapter SIX CONCLUSIONS AND RECOMMENDATIONS CONCLUSION Calibratin Prblems Significance f Avalanche Avalanche Frecasting Mdel RECOMMENDATIONS Knwledge f the Snwpack at Higher Elevatins Snw Curse Surveys Testing f the Strategy Mdel Mdificatin 127 REFERENCES 128 APPENDIX 137 viii

10 List f Tables Table 4.1. Descriptin f watershed by elevatin bands 55 Table 4.2. A typical wrking sheet shwing prcedure t calculate the magnitude f avalanches 59 Table 5.1. Statistics reprt f Stage-I calibratin fr year Table 5.2. Statistics reprt f Stage-II calibratin fr years Table 5.3. Statistics reprt f Stage-I, Stage-II, and Stage-Ill calibratins fr the Kunhar River Basin 67 Table 5.4. Distributin pattern f avalanche vlume in each band Table 5.5a. Areal distributin f avalanches in each band 78 Table 5.5b. Regressin analysis reprt (A vs. A) 80 Table 5.6. Illustratin f calculating the mean snwpack (mm) in band 3 fr the year , with and withut avalanches 83 Table 5.7. Snwpack depth in millimetres f water equivalent 85 Table 5.8. Snwmelt (Sm) pattern with and withut avalanches 95 Table 5.9. Results f regressin analyses (A vs. SP' fi ) 107 Table Extra snwpack (mm) required in bands 3 & 4 and snw t remve frm band 6 '. 110 Table Results f regressin analyses (Sp vs. SP' ) Ill Table Avalanche frecasting fr years and Table Avalanche frecasting fr years and Table Avalanche frecasting fr years and Table Avalanche frecasting fr year Table Cmparisn f efficiency and deviatin in discharge between baseline, avalanched, and frecasted calibratins 121 ix

11 List f Figures Figure 2.1. A typical sketch f lse-snw avalanche 9 Figure 2.2. Crss sectin f a typical snw slab 9 Figure 2.3. Stresses in a snwpack due t weight 11 Figure 3.1. Gegraphical map f the Karakram-Himalayan Ranges and the Upper Indus Basin 27 Figure 3.2. Kunhar River Basin 36 Figure 5.1. Cmparisn f hydrgraphs befre and after avalanches with the bserved flw (a) year (b) Figure 5.2. Cmparisn f hydrgraphs befre and after avalanches with the bserved flw (a) year (b) Figure 5.3. Cmparisn f hydrgraphs befre and after avalanches with the bserved flw (a) year (b) Figure 5.4. Cmparisn f hydrgraphs befre and after avalanches with the bserved flw fr year Figure 5.5a. Avalanche distributin pattern fr active bands as a percentage f snwpack in band 6 76 Figure 5.5b. Avalanche in bands 3 & 4 against avalanche in band 6 as a percentage f snwpack in band 6 76 Figure 5.6a. Avalanche area distributin pattern fr active bands as a percentage f ttal watershed area 79 Figure 5.6b. Avalanche area against vlume in bands 81 Figure 5.7. Snwpack depths in mm water equivalent at different elevatin band fr years, (a) ; (b) ; and (c) Figure 5.8. Snwpack depths in mm water equivalent at different elevatin band fr years, (a) ; (b) ; and (c) Figure 5.9. Snwpack depths in mm water equivalent at different elevatin band x

12 fr years, (a) ; and (b) Figure 5.10 Snwpack accumulatin in avalanched and nn-avalanched part f a band fr year Figure 5.11 Snwpack accumulatin in avalanched and nn-avalanched part f a band fr year Figure Snwmelt patterns in elevatin bands withut and with avalanches fr year Figure Snwmelt patterns in elevatin bands withut and with avalanches fr year Figure Snwmelt patterns in elevatin bands withut and with avalanches fr year Figure Snwmelt patterns in elevatin bands withut and with avalanches fr year Figure Snwmelt patterns in elevatin bands withut and with avalanches fr year Figure Snwmelt patterns in elevatin bands withut and with avalanches fr year Figure Snwmelt patterns in elevatin bands withut and with avalanches fr year Figure Avalanche vlume in bands with respect t snwpack in band Figure Extra snw w.e. required t increase in runut band and t decrease in starting zne with respect t snwpack w.e. in band 6 (a) band 3; (b) band 4; and (c) band Figure Cmparisn f bserved, avalanched, and frecasted hydrgraphs fr years (a) and (b) Figure Cmparisn f bserved, avalanched, and frecasted hydrgraphs fr years (a) and (b) Figure Cmparisn f bserved, avalanched, and frecasted hydrgraphs fr years (a) and (b) Figure Cmparisn f bserved, avalanched, and frecasted hydrgraphs fr year xi

13 List f Symbls A = Area f the Catchment in m 2 (Eq: [2.4]) A = Avalanche Area (km 2 ) ~ 6 3 A = Avalanche Vlume Water Equivalent (xlo m ) Av = Avalanche C = Cnstant (427.35) C = Degree Centigrade Dv = Deviatin in Vlume (%) E! = Nash-Sutfcliffe Cefficient f Efficiency Ept = Cefficient f Efficiency fr Optimizatin Prcess / = Yield Cefficient r Prprtin f Snw n the Avalanche Path which Avalanches (t m ) g = Gravitatinal Acceleratin h = Height H = Catchment Area f Avalanches in Hectares K = Cncentratin Factr (Starting Zne Area/Runut Area) M = Annual Avalanche Mass a m = Meter mm - Millimeter Q = Discharge (m 3 ) xii

14 R = Cefficient f Determinatin is a = Amunt f Precipitatin (water equivalence f snwfall and rain int snw) Sm = Snwmelt (mm water equivalent) Sp = Extra Snw Depth Water Equivalent (mm) Required t Increase r Decrease Frm Active bands (Frm Eqs. [5.5], [5.6], and [5.7]) SP = Snwpack Water Equivalent (x 10 6 m 3 ) Sp' = Extra Snwpack Depth water Equivalent in Part f 'b' f a Described Band frm Band CAL File SP' = Snwpack Depth Water Equivalent (mm) V = Mean Vlume f Avalanches in 1,000 m 3 w.e. = Water Equivalent ASP = Change in Snwpack depth (mm w.e.) x = Shear Stress a = Nrmal Stress p = Average Snw Density at Depth h a = Surface Slpe Angle Measured frm the Hrizntal (Degrees) xiii

15 Band Numbers Part 'a' f a Band (i.e., Un-affected Area f a Band) Part 'b' f a Band (i.e., Affected Area f a Band) Estimated Discharge Observed Discharge Ttal xiv

16 Acknwledgment I wish t express my mst sincere appreciatin and gratitude t Dr. M. C. Quick, wh has had a prfund and psitive impact n my academic and prfessinal attitude. I greatly appreciate his advice, cntinuus guidance, and invaluable encuragement thrughut the curse f this research. Many thanks t Dr. Sakis Lukas, fr valuable suggestins and cmments. I am particularly grateful t the Civil Engineering Department f UBC as a whle and Dr. Warren Bell f BC Hydr fr excellent wrking facilities and cperatin in all aspects. I wuld like t acknwledge Mr. Edmnd Yu fr his supprt and assistance in cmputer stuff. The supprt f Heiki Walk is als gratefully acknwledged. The research was supprted financially by IDRC (Canada) and WAPDA (Pakistan). xv

17 T my wife and children whse lve and encuragement were cnstant surce f strength xvi

18 Chapter 1 INTRODUCTION 1.1 PREAMBLE Pakistan is primarily dependent upn irrigated agriculture in the Indus Basin. This irrigatin system includes inter-river link canals and tw majr strage reservirs at Mangla and Tarbela that regulate as well as supplement the water supplies. The Indus River is Pakistan's main surce f water fr irrigatin, pwer generatin, and urban and industrial water supply. The waters f Indus derive largely frm high-altitude snwfalls and the glaciers f the nrthern muntain ranges, the Karakram, Khistan, and Himalayas. The hydrlgy f the Upper Indus Basin (U.I.B.) is mainly determined by the snw and ice cnditins in these muntains and supply the main stem f the Indus with mst f their water. Spring snwmelt is a significant runff cmpnent in the tributaries f the Indus River lcated n the suth side f the Himalayan crest line. Heavier snwfalls ccur at higher altitudes where slpes tend t be steeper and the snwpacks mre unstable. Therefre these heavier high altitude snwfalls cause a significant fractin f all the snw that falls in the U.I.B. t avalanche thrugh sme hundreds f meters (Hewitt, 1988a). Cnsequently, part f the snwmelt cmpnent is derived frm avalanche transprted snw.

19 A jint Canada-Pakistan hydrlgy prject f the Indus River System was established in 1985 t investigate high muntain snw and glacier resurces. Initially the prject was planned as a three-year venture but was then extended t 1989 t investigate the hydrlgy f the muntain headwaters f the Indus, and especially the snw and ice cnditins abve 2,500 m a.s.l. (Hewitt, 1990a). The prject has nw entered its secnd phase which will be cmpleted in This is a cllabrative prject funded jintly by the Water and Pwer Develpment Authrity (WAPDA) and the Canadian Internatinal Develpment Research Center (IDRC). In Pakistan, the prject is crdinated by the Hydrlgy and Research Directrate f WAPDA. In Canada, the prject crdinatin is undertaken by B.C. Hydr Internatinal Limited (BCHIL). The prject bjective is t upgrade the capability f WAPDA t manage the utflws frm the Upper Indus Basin. It has invlved field wrk, with applicatins f remte sensing, analyses f existing river discharge and meterlgical bservatins t prvide the basis fr imprving seasnal streamflw frecasting. This wrk will lead t imprved and mre effective water management thrugh effective use f water. Bth Pakistani and Canadian engineers are invlved in all phases f the wrk. After cmpletin f the prject in 1996, the inflw frecasts will be used by WAPDA f Pakistan t plan reservir releases fr irrigatin and pwer generatin. Field wrk has been cncentrated in tw main areas; the Biaf - Hisper and Barpu - Bualtar glacier areas f the Central Karakfam, and the Nanga Parbat / Kaghan Valley area f the Western Himalayan. In bth areas basic data are being cllected t imprve knwledge f the meterlgy, glacilgy, and snw hydrlgy. Part f the snw 2

20 hydrlgy studies invlves the investigatin f the rle f avalanche in seasnal snwmelt especially in the Kunhar River basin. The Kunhar River basin experiences intense and high magnitude avalanche activity abve 1850 m elevatin. Mst f the avalanche activity ccurs in the 4,000 t 2,000 m elevatin range, s that high elevatin snw is redistributed dwnwards, smetimes by as much as 2,000 m (De Scally, 1992). The avalanche activity tends t cncentrate the avalanche snw int a deeper and mre dense snwpack which retards its melting. This avalanching ccurs nt nly in the Kunhar River Basin but als in the surrunding regin especially the Neelum River Basin, and it is therefre imprtant t investigate the hydrlgical influence f this avalanching. The avalanche snw, n ne hand, may increase snw melting rates because f lwer elevatin & albed and higher temperatures etc., but n ther hand it delays snwmelt runff because f the large amunt f snw accumulatin. We nt nly need t understand the effects f avalanching in hydrlgic prcesses, but we als need methds t predict the streamflw respnse that results frm avalanching. The basis f the present research is t understand the behaviur f avalanches as a fundamental cmpnent in the calibratin f hydrlgical mdel f the Kunhar River basin, and t establish a snw avalanche and a flw frecasting system. 3

21 1.2 RESEARCH OBJECTIVES Estimatin f peak flws is necessary fr the design f any hydr technical prject. The flw estimatin can be achieved by using a hydrlgical mdel alng with meterlgical data frm a number f statins. Als, gd knwledge f the area and the hydrlgical prcesses is needed t simulate the runff generatin. Flw simulatin and frecasting becme very difficult in muntain areas mainly because f the lack f reliable databases with the necessary spatial reslutin. Furthermre, because f the limited accessibility f high muntain areas very little is knwn f the runff prcesses in high elevatin watersheds. Fr example, avalanches redistribute the snw accumulatin and can result in majr time redistributin f the river flw. The gals f the present research are t determine the significance f avalanche cntributin in Kunhar basin hydrgraphs by using a watershed mdel t study the avalanche effects. This wrk will assist in establishing a flw frecasting system which can take accunt f the snw avalanche effects. Within these brad bjectives, the fllwing specific bjectives were adpted; 1. Calibrate the U.B.C. Watershed Mdel fr Kunhar Basin with n avalanches and get the best pssible results; 2. Intrduce and investigate the redistributin f snw t simulate avalanching frm higher t lwer elevatins; 3. Analyze the significance f avalanching by cmparing the results f calibratin with and withut avalanches input n the basis f hydrgraphs. ttal flw vlumes and efficiencies f the analyses; 4

22 4. Examine results fr each year and see if there is a cnsistent pattern f avalanching; 5. Establish an avalanche frecasting system n the basis f snwpack cnditins in higher band/s; and 6. Use this system t imprve streamflw frecasting fr the Kunhar Basin. While analyzing and simulating avalanches the fllwing imprtant parameters were als given cncentratin, i) mst active elevatin bands regarding avalanche activities (higher elevatin band as starting zne and lwer ne as runut); ii) iii) iv) avalanche areal distributin pattern fr each elevatin band; avalanche magnitude distributin pattern fr each elevatin band; snwmelt patterns befre and after avalanching within the elevatin bands; 5

23 Chapter 2 THEORETICAL BACKGROUND AND LITERATURE REVIEW 2.1 INTRODUCTION An avalanche is a mass f snw transprted at high velcities dwn a muntain slpe. Cnsiderable amunts f snw are displaced by avalanches frm higher t lwer elevatins and are then depsited & cncentrated in reduced areas. Avalanche snw is denser, deeper and in much mre cmpact masses than direct snwfall. Avalanches frm when the snwpack resting n a slpe underges failure. They ccur especially in areas f steep slpes - generally between 25 and 60 - but their mmentum may carry them n t flatter slpes, particularly if they are channeled int a gully (Gudie, 1993). New dry snw can cling t 40 slpes, where wet slushy snw may slide even n 15 slpes. The critical angle f repse depends n the temperature and density f the snw, which determine its texture and wetness (Barry, 1992). An avalanche path (Terrain bundaries f knwn r suspected avalanches) cnsists f an upper starting zne, the track zne (part f the path between the starting and runut znes) which is ften clearly delimited by a swath f grassy r shrubby vegetatin running dwn belw tree line, and a lwer runut-depsitin zne which may have a mre r less well-marked debris fan at the ft f the slpe (Fig. 2.1). The failure prcess 6

24 begins in the starting zne, and then the develped energy and ther dynamic characteristics depend n the relief f the track and the amunt f material that can be entrained int the avalanche as it gains mmentum (Perla. 1978). Tw generic grups f avalanches may be identified: Direct actin and delayed actin (Armstrng, 1976). A direct actin avalanche ccurs during r immediately after a strm and is the result f the increased stress applied t the snwpack in the frm f new snw. This type f avalanche is the immediate cnsequence f rate f snw lading in the starting zne. The snw lading rate is a functin f many meterlgical parameters, amng which crystal habit, snwfall intensity, wind transprt and snw depsitin and temperature are imprtant (Fraser, 1966 and Tesche, 1988). A delayed actin avalanche is the result f gradual changes taking place within the snwcver ver a lnger perid f time due t ver burden snw lad. The thickness f the snwcver exerts a large influence n the prprtin f the genetic types f avalanches (Shcherbakv, 1973) and als determines their vlumes and t a large extent the mment f avalanche mvement. On the basis f starting zne appearance and general snw structure, snw avalanches are classified int tw categries: pint avalanches (als called as lse-snw avalanche) and slab avalanches (Fig. 2.2). The pint avalanche ccurs when snw crystals which adhere prly t each ther cllect in a slpe steeper than their angle f repse. Failure initiates within a chesinless layer lcated immediately belw the surface. As sn as it breaks lse, the unstable lump rll dwn the slpe, bulldzing ut a widening pattern. During strms, pint avalanches ccur frequently when the slpe angle is steeper than abut 45. 7

25 Slab release is characterized by an initial spectacular prpagatin f cracks fllwed by the crumbling f a slab-like regin f the slpe int numerus blcks with dimensins n the rder f abut 1 m n slpes f The stability f a slab depends n the stress state and fracture tughness f a large, chesive mass. The slab incrprates larger amunts f snw. After slab failure, sharply defined fracture surfaces which utline the slab bundaries remain at the starting zne. These fracture surfaces are designated as fllws: bed surface: main sliding surface f slab (shear type f failure) crwn surface: upslpe fracture surface (tensin fracture) stauchwall: dwnslpe bundary (shear fracture) flank surfaces: tw side bundaries f the slab (cmbinatin f tensin and shear fractures) Slab avalanches may invlve dry r wet snw, but bth are assciated with shear stresses in the snw exceeding the shear strength in sme underlying layer. Dry-snw avalanches are particularly assciated with high snwfall amunts ver the preceding fur days and with wind redistributin f the snw hurs befre the event (Barry, 1992). Wet-snw slides, which mainly ccur in spring, are assciated with the antecedent air temperature values. 8

26 Fig A typical sketch f lse-snw avalanche. Crwn / / ' Bed surface y^^yy^'' Stauchwall-^»> Fig Crss sectin f a typical snw slab. (Mears, 1979) 9

27 2.2 GOVERNING FACTORS & RELEASE MECHANISMS Avalanches are generated by structural weaknesses in the snw cver which give rise t structural instability (Vight, 1990). When snw lies n a slpe, the frce parallel t the slpe caused by its weight prduces shear stresses while the frce perpendicular t the slpe prduces cmpressive stresses (Fig. 2.3). Failure ccurs when the stress exceeds the strength at sme pint. The shear stress at any depth h, acting parallel t the bttm f the snwpack, is given by the relatinship; x = p g h sin a [2.1] where; p = average snw density at depth h, g - acceleratin due t gravity, a = surface slpe angle measured frm the hrizntal. The nrmal stress (t slpe) at any depth h in the snwpack is given by; a = p g h Cs a [2.2] These equatins shw a direct relatin f shear stress t the surface slpe steepness and thickness f snwpack. Shear stress will reach a maximum at the base f the snwcver and decline t zer at the surface. 10

28 Fig Stresses in a snwpack due t weight. (Schaerer, 1981) 11

29 The sequence f events preceding avalanche release is apt t vary cnsiderably depending n meterlgical cnditins, and als n the immediate trigger, which may be artificial explsive blast f enrmus energy r a subtle internal disturbance. The fllwing are a few f the wide variety f avalanche triggers: /- Precipitatin. There is an bserved high prbability f avalanche ccurrence during r immediately after a severe strm. Amunt, intensity and duratin f snwfall (r rainfall) are cllectively the prime factrs f avalanche frmatin (Armstrng, 1975 and Williams, 1981). Snw strength due t sintering 1 cannt keep pace with the increasing stress in an underlying stratum caused by lad f additinal snwfall (Fraser, 1966). Under a slw rate f precipitatin, the snw can absrb the lad by changing its shape with a slw defrmatin r cmpressin, acting like a flwing r viscus material. Under a rapid lad, n the ther hand, there is a less time fr the snw t absrb the weight by changing its shapes, it is much mre likely t crack under the strain and acts as a brittle r elastic material (Elmesn and Nastaev, 1973). Winter avalanches are, in general, directly cnnected with sustained heavy snwfalls; air temperature and ther meterlgical factrs seem t have nly a secndary cause (Pggi and Plas, 1966; Perla, 1978). They are widespread and frequent in all muntains n slpes where the snw cver is sufficiently thick (ver cm) in winter. 1 After the snw is depsited the particle shapes are mdified and dendritic (needle-like) crystals decmpse int fragments. Simultaneusly with the breakup f the dendritic assemblies f newlydepsited crystals is the frmatin f bnds at the pint f cntact between snw crystals (Langham, 1981). This prcess is knwn as sintering r age hardening which increases the strength f the snw (Adam, 1981). 12

30 2- Wet snw instability. Avalanches tend t ccur during thaw caused by rain r heating. The relatively high winter temperatures and heavy snwfall prduce a deep snwpack which is generally well settled and mechanically strng, except during and pssibly after perids f thawing. Wet slabs are triggered by the cmbined effects f water weight and bed surface lubricatin. 3- Rain and temperatures. Rain and high temperatures are als imprtant triggers f avalanching, reducing chesin in the snwpack until failure ccurs. The precise significance f each is nt clear since they frequently ccur tgether and, in additin, the rain ften accmpanies snwfalls. Rain r high temperatures fllwed by freezing can create ice-crusts, which when buried by subsequent snwfalls may prvide a significant surce f snwpack instability. The rise in temperature weakens the bnd between the crystals; and if the rise cntinues it will eventually surrund each crystal with a film f meltwater which lubricates and reduces the static frictin. The shear strength f the snw is then reduced t zer, and in this state wet snw avalanches cmmnly ccur. Rain brings abut a rapid rise in temperature and als prvides free water fr lubricatin. Strng temperature changes appear t be mre imprtant than abslute temperature values in creating instability (Williams, 1981). When the snwpack is already critically stressed, a large temperature change in a few hurs can increase stress in the tpmst layers and lead t failure. Thus, temperature change can be viewed as the ultimate trigger when large stress values already exist frm ther prcesses. 13

31 4- Snw drifting. Snw drifting is ne f the mst imprtant factrs f avalanche frmatin in the muntains (Ktlyakv, 1966). Drifting causes snw t be re-distributed and t be cncentrated in certain sectins f the slpes. The shearing frces exerted by airflw against the snw surface erde the snw frm regins f high wind stress. Erded snw is redepsited in sectins f lw wind stress that becme the main snw-cllecting basins fr avalanches. 5- Ski lads. A ski traverse acrss an unstable slab is ften an effective way t trigger instability. A strng dwnhill push is applied t the slab by the back f the skis t reinfrce fracturing. 6- Shcks. Examples f natural and artificial shck energy which cause avalanche release are: earthquakes, crnice falls, artillery bursts, and snic bms. 2.3 HYDROLOGICAL ROLE OF AVALANCHE The ablatin characteristics f avalanche snw differ significantly frm the surrunding undisturbed snw cver, particularly with respect t the undisturbed snw in the starting znes (De Scally and Gardner, 1988). The changes f the climatic envirnment and f the expsed area influence the ablatin cnditins in a cmplex manner (Martinec and Quervain, 1971). Dwnslpe transfer f snw frm permanently frzen areas brings it int warmer climates, with a lnger melting seasn. At a given altitude, it takes lnger t 14

32 melt due t reduced surface area and deeper pack, but the flw frm it is mre cncentrated and cnsistent (Hewitt, 1990b). The differences in ablatin are prduced by tw imprtant changes which ccur as the snw is avalanched t a lwer elevatin; the ambient air temperature is increased, and the snw is cmpacted and cncentrated by wind, thermdynamic stress and stress frm ver burden (Wyman, 1995). The physical prperties f avalanche snw is entirely different frm the prperties f fresh snw. The snw depsited in the runut zne is abut tw t three times denser than the starting-zne snw, and is much harder. The albed f avalanche snw is very lw as cmpared t the fresh and undisturbed snw. Whereas, almst 80 t 90% f incident shrt-wave radiatin is reflected by a clean, dry snw surface (Linsley et al, 1986). Albed f snw surface keeps changing with the age f snw and als with the variatin in free water cntent f the snwpack. As snw ages, its albed drps t 50% r less because f changes in crystalline structure, density, and amunt f dirt n the surface, which is further enhanced by avalanching. These changes in snw prperties are the basic features f the avalanches. Due t all f abve factrs the fllwing three imprtant behaviurs f snwmelt are bserved (Zalikhanv, 1975; Martinec, 1976; Perla and Martinelli, 1979; and Bell et al, 1990): 1- Snwmelt rates are increased due t increased ambient air temperature wing t transprt f the snw frm higher t lwer elevatins. 15

33 2- Snwmelt is delayed as a result f the frequent cnfinement f avalanche snw, increasing the snw density and reducing depsit surface area expsed t radiative and turbulent energy exchanges. 3- The lwer albed f avalanche-snw, caused by entrained debris, increases the absrptin f slar energy which further enhance the ablatin rates. The decrease in area and increase in snwdepth are the mst imprtant factrs, s that althugh the melt rate is higher, the melting cntinues fr a lnger time. The avalanche snw therefre represents water temprarily withdrawn frm snwmelt runff, prducing a decrease in spring runff but an augmentatin f flws during the summer and autumn (Lsssev, 1960; Ivernva, 1966; Ssedve and Seversky, 1966). The precise hydrlgical imprtance f avalanche snw is dependent n the changes in snwmelt factrs between the starting zne and runut zne in additin t the magnitude f avalanches. 2.4 RESEARCH ON HYDROLOGICAL ASPECT OF AVALANCHES Avalanches in muntain regins transprt millins f cubic metres f snw t valley bttms, and the extent t which the changes affect snwmelt runff frm a basin depends largely n the prprtin f the basin's snw cver which is avalanched. A number f Sviet researchers have reprted a favrable effect f avalanches n the runff regime. They all shwed a maximum ablatin rate f snw avalanches which is fllwed by a prlnged perid f gradual decrease. The delay in melting nted in Sviet literature 16

34 appears t result frm the significant cncentratin f the snw during avalanching, prducing a small runut zne. Ivernva (1966) carried ut regular bservatins ( ) f the snw avalanches at the Tien-Shan statin f the Tersky Alatau Ridge in USSR. She discvered that frm 3 t 30% f the snw supply is being remved by avalanches. These figures vary strngly frm year t year depending n the meterlgical features f the winter and particulary n the amunt f snw in winter, as well as the temperature cnditins during summer. Ivernva fund time delays f 2-3 mnths in melting f the avalanche depsits after the disappearance f the snw cver. The meltwater frm the avalanche depsits amunted t 3-11%, f the annual runff, whereas during the ablatin perid it amunted t 10 t 27% f the ttal runff. This shws that the avalanches can play an essential rle in the runff f rivers. Ssedve and Seversky (1966) als fund a clse relatin f avalanching with amunt f winter precipitatin and air temperature regime in Zailiysky Alatau range f USSR. They estimated 5 t 10% f the amunt f snw in 1961 was transprted by avalanches. During the snwy 1964 year this prprtin was increased and amunted t 20-28%. The avalanche depsits yielded 3-4% and 10-11% f the annual runff in 1961 and 1964 respectively. During thawing f the depsits their discharge was up t 10-20% f the ttal runff, and 30-35% f the ttal surface runff respectively. Shcherbakv (1973) studied the activity f avalanches in Kirghizia, Tian-Shan. He calculated the mdules f avalanche flw (1950 t 1953) fr the whle f Kirghizia territry based n bservatins n 45 river basins. Shcherbakv fund large reginal differences in the degree f intensity f avalanche activity. On an average, abut 88 x

35 m f snw traveled annually dwn the muntain slpes in the frm f avalanches. This crrespnded apprximately t 0.4% f the annual flw f Naryn River. The duratin f the avalanche perid varied frm 3 t 4 mnths in Western Tian-Shan t 6 t 8 mnths in the nrthern and Interir Tian-Shan. February-March is the mst active perid, the prprtin f avalanches in theses mnths being 25 and 39% respectively. In his studies f the rle f avalanche in feeding muntain rivers and their flw, Shcherbakv indicated the pssibilities f cntrlling the activity f avalanches in mdifying the hydrlgical prcesses by artificially explding the avalanches. Zalikhanv (1975) studied the rle f avalanches in the Caucasus, USSR. Accrding t his results, 30-64% f the snw accumulatin can be transprted by avalanche t the valley bttms f the Caucasus where the hydrlgical rle f avalanches is great. In the Kabardivian-Balkar Republic, fr example, apprximately 2,500 avalanches ccurred frm 1120 catchment area during the winter f These avalanches brught 12.5 x 10 6 m 3 f snw t the valley bttms in alpine and sub-alpine znes which resulted in lnger perid f snw melting. He btained a relatin between the mean vlume f avalanche snw and the catchment area f avalanches. This relatin was: V= 23(H)* [2.3] where V is the mean vlume in 1,000 m 3 f avalanches and H is the catchment area f avalanches in hectares. Similar relatins were btained fr the ther regins f the Caucasus which were valid fr certain ranges f the elevatins. 18

36 Martinec and de Quervain (1975) presented a mdel f daily meltwater prductin and shwed that avalanche activity can accelerate snwmelt if 'vertical fall' r 'runut area' f avalanche are large. They established varius equatins fr calculating, i) water equivalent befre and after the avalanche event; ii) ttal daily ablatin; and iii) time f disappearing f snw in avalanched and nn-avalanched areas. They fund increased runff in April and May by avalanches f the Dischma Valley, Davs and abut 10% f area affected by avalanches, calculated by their mdel. Martinec and de Quervain's mdel is unable t accunt fr the rapid melting f that prprtin f the depsit which is spread ut and expsed, and the slw melting f the remaining prtected prtin, since it is assumed that the avalanche snw is unifrmly distributed n entire area f runut zne and nt cncentrated in depressins. The field bservatins indicated that, in the real situatin, the depsits n larger paths in nrmal years generally stay 2 t 3 mnths after the undisturbed snw cver has melted frm the starting znes (de Scally, 1988). Schearer (1984, 1988) develped equatin fr estimating the annual avalanche masses M a (in tnes) by; M=/A5 a [2.4] where / is the yield cefficient r prprtin f snw n the avalanche path which avalanches (t m" 3 ), A is the area f the catchment (starting and track znes f the path, in m 2 ), and 5 a is the amunt f precipitatin (water equivalence f snwfall and rain int snw). He fund the area f the catchment (A) and the ttal avalanche-seasn precipitatin (S a ) t be the mst significant determinatin f avalanche mass. The yield 19

37 cefficient if) averaged m" at Rgers Pass fr all avalanche paths and years, but varies widely (f = m~ 3 ) frm year t year and path t path. Kunhar River Basin De Scally (1992) investigated the effect f avalanches n snwmelt runff in the Kunhar basin. The results f mdeling based n field measurements shw that, f the tw main changes ccurring during avalanching which affect the subsequent generatin f snwmelt runff - cncentratin f the avalanche snw and an ambient temperature increase resulting frm the avalanches' fall t a lwer elevatin. As a result, very high rates f surficial melting n avalanche snw were ver weighed by the small surface area f the depsits, decreasing the rate f meltwater prductin and delaying the disappearance f avalanche snw cmpared with undisturbed snw. Ablatin begins earlier in the runut znes but the rate f meltwater prductin remains lw. He fund that in year , undisturbed snw in the runut zne (i.e. withut avalanching) disappears much sner than undisturbed snw in the starting zne, as a result f greater winter precipitatin and lwer temperatures in the starting zne. The results frm his equatins give a delay f 163 days n average in ablatin, if all the snw in the starting zne is assumed t avalanche int the runut zne where it is added t the existing undisturbed snw cver. When the vlumes f the actual avalanche depsits are added t the existing undisturbed snw in the runut zne the delay reduced t 57 days n average, which is still high. 20

38 De Scally studied avalanche activities within nly 288 km 2 (14% f the Kunhar Basin) and he extraplated his results t larger area (1372 km 2, i.e. 58% f the ttal watershed area) ptentially affected by avalanching. His extraplatin f results give an estimatin f ttal vlume f avalanche snw (water equivalent) as 212 x 10 6 m 3 (1986) and 248 x 10 6 m 3 (1987). These represent n average 7.8% f the April t September runff and 6.6% f the annual runff f the Kunhar in these tw years. He estimated the percentages f ptential avalanche slpe areas t be 15% in mderate winters and 54% in snwy winters. Fllwing a severe winter (i.e. all ptential avalanche slpes are active) avalanche snw prduced 8% f snwmelt runff and 5-7% f annual runff. Fllwing a nrmal winter theses prprtins were estimated t be f the rder f 1-2%. De Scally and Gardner (1989) estimated annual avalanche masses in the Kunhar basin. The predicted masses were cmpared t measured avalanche-depsit masses prduced during tw winters ( , ). The predictive equatin was based n data frm western Canada (Schearer, 1988). The calculated and measured depsit masses at the end f each avalanche seasn shwed a wide range. The estimatin f annual avalanche masses using Schearer's equatin was successful n the largest paths but significant differences frm the measured masses ccurred n the ther paths. De Scally and Gardner attribute these variatins t the inaccuracies in the measurement f the catchment area A and winter precipitatin S a parameters in the equatin [2.2]. The predictin f avalanche mass is, hwever, difficult and uncertain, particularly in prly knwn muntain regin such as the Himalayas (De Scally and Gardner, 1989). Frm a hydrlgical pint f view, the equatin may be useful because it is the large 21

39 paths which are mst imprtant in terms f the area f snw cver affected and thus the amunt f water stred in the avalanche depsits at the beginning f the ablatin seasn. Ablatin f avalanched and undisturbed snw was studied by De Scally and Gardner (1990). They fund a much mre rapid rate f ablatin than in ther muntain regins. The higher ablatin rate f avalanche snw resulted frm its lw elevatin and high temperature envirnment. On the ther hand this higher amunt f ablatin rates is frequently ffset by reduced surface avalanche area and greater depths, which delay their ttal melt as cmpared with undisturbed snw. Air temperature was fund t be strngly crrelated with snwmelt but they further suggested that energy balance is required t explain this relatinship. De Scally and Gardner (1994) fund strng assciatin between snwfalls and avalanche events, with 64% f the 196 events recrded ccurring during r within 24 hurs f a snwfall. The percentage f snw cver which is transprted by avalanches was estimated, n average, t be abut 10% Summary Mst f the literature n avalanching describe their rle as a hazard t prperty & life and gegraphical pint f view and a little wrk is dne n the basis f their hydrlgical rle. Sme f the Sviet literature refers t the hydrlgical rle f avalanches and all shw a direct and favurable influence f avalanches n the river systems. Millins f cubic meters f high muntain snw is transprted and depsited frm higher t lwer elevatin 22

40 each year in the frm f avalanches, where they melt faster but fr a lnger perid f time and in a cnsistent manner due t high degree f cmpactin and larger depths. Avalanches play a significant rle nt nly in the feeding f muntain glaciers and rivers but als in the frmatin f the relief, denudatin f the muntain slpes, and grwth f vegetatin (Ivernva, 1961; Tushinskii, 1963; and Peev, 1965). In muntain slpes, avalanches are directly crrelated with the high altitude snwfall intensity and i magnitude. 23

41 Chapter 3 GEOGRAPHY AND CLIMATE 3.1 OVERVIEW Pakistan is an arid t semi-arid regin with surface waters derived mainly frm the River Indus and its tributaries. It has an extensive netwrk f irrigatin canals, the largest in the wrld (Abbas, 1967). This irrigatin system in the Indus plains is fed thrugh 16 diversin dams and 580 km f inter-river link canals and three majr strage reservirs at Mangla, Tarbela and Chashma (Tarar, 1982). The Indus River is Pakistan's main surce f water fr irrigatin, pwer generatin, and water supply fr urban and industrial units. The lives f abut 125 millin peple depend n the waters f this huge river system (Kick, 1978). The Indus River, with its 860,000 km 2 drainage basin and 2,880 km length, is ne f the largest f Suth Asian regin. The river and many f its tributaries riginate in the Himalayan, Karakram and Hindu Kush regins. Its drainage basin cvers almst the whle f Pakistan as well as parts f Nrth India, Eastern China and Afghanistan. The nrthern muntains f Pakistan prvide the nly areas f the cuntry with substantial precipitatin and an annual misture surplus. Due t the high elevatin, mst f the precipitatin ccurs in the dmain f snw and ice and therefre has resulted in extensive glaciatin. Mst f the remainder f the Indus Basin has lw precipitatin and a 24

42 water deficit in mst r all f the mnths f the year. Histrically, surface water supply in Pakistan depended mre upn the easterly Indus streams and rainfall; bth mainly derived frm the summer mnsn. With the independence and partitining f India in 1947, mst f the Indus Valley became the territry f Pakistan. The internatinal bundary between India and Pakistan cut the irrigatin system f the Bari Dab and Sutlej Valley prjects, riginally designed as ne scheme, int tw parts. The Indus Water Treaty, signed in 1960, is the basis f water sharing between the tw cuntries. The treaty gives India full cntrl f the eastern tributary streams, Ravi, Beas, and Sutlej; and allws Pakistan t utilize exclusively the flw f the Indus and mst f the waters f the Chenab and Jhelum. India has use f the Chenab and Jhelum fr needs within Jammu and Kashmir. Pakistan, thrugh this treaty, has becme increasingly dependent n water frm the snw and ice surces in the nrthern muntains which fed the Tarbela and Mangla Reservirs. Just at the time f greatest water need in summer, the supply f meltwater is mst plentiful. In recent decades, mainly under the directin f the WAPDA, huge prjects have been undertaken t harness the waters f the Indus. 25

43 3.2 GEOGRAPHY OF THE NORTHERN MOUNTAINS Reginal Setting The Himalayan regin has been cnsidered t encmpass the muntain area frm the Pamir regin adjining the Karakram-Hindukush-Zaskar ranges in the West-nrthwest, the Tibetan plateau at the center brdered by the Kunlun Shan in the Nrth and the Heng Tuan in the East and by the Great Himalayan range in the Suth. The Karakram Muntains are situated in the interir f Central Asia. The Karakram cnsists f a series f muntain ranges that extends ver 2,500 km frm the eastern Ladakh t the Hindu Kush. They are brdered by the Great Himalayan t the suth and suthwest, the Aghil Ranges and Kun Lun t the nrth and nrtheast, the Pamirs t the nrthwest, and the Hindu Kush t the west (see Fig. 3.1) The Upper Indus Basin The Upper Indus Basin (U.I.B.) serves as catchment area drained by the Indus River upstream f Tarbela reservir, the Jhelum River upstream f Mangla reservir and the Kabul River, and cmprises an area f apprximately 250,000 km 2 in a muntainus regin f the Western Himalayan, Karakram and Hindu Kush Ranges. Thirteen percent f it is cvered by perennial snw and ice. At the end f the winter seasn, an area f abut 200,000 km 2 in the muntainus regins f the U.I.B. is extensively snw-cvered. Fr the 26

44

45 Indus main stem, snw may cver mre than 90% f the catchment abve Tarbela Dam, and cmmnly mre than 70% (Hewitt, 1985a). The Indus, extends nearly 3,000 km, rises in the suthwestern part f the Tibetan Plateau and flws t the nrth f the Vale f Kashmir in arid valleys between the Himalayan and Karakram muntain ranges. Frm its rigin in Tibet t its terminus in the Arabian Sea, the Indus drains a ttal catchment f 933,632 km 2 (Ringenldus, 1975). The river descends suth twards the Arabian Sea with a ten year average vlume f 38.7 billin m 3 discharge past Tarbela dam. The Upper Indus Basin includes the areas upstream f Mangla Dam n the Jhelum River and Kabul River at its muth. In the nrth-western Himalayan, the ranges n bth sides f the Indus River are aligned frm west-nrth-west t east-suth-east. The crest f the Ladakh ranges abut m a.s.l. (Burbank and Frt, 1985). The parallel ridge crest f the nrth eastern flank f the Zaskar Range rises t similar heights n the suth-western side f the Indus. The surces f the Indus and its tributaries lie high up in the Himalayan, Karakram and Hindu Kush muntains. Mst f the misture cmes frm the river's Himalayan headwaters. The easterly tributaries f the Indus are fed mainly by mnsnal rains; the westerly nes by snw and ice meltwaters. Meltwater frm Himalayan snw and ice at elevatins ranging frm 2,000 m t 5,000 m dminates the flw f these western rivers (Quick, 1990), and is the largest fractin f annual yields, supplying 75% f the inflw f the Kabul at Warsak; 80% f Swat River; abut 70-80% f the inflw f the main Indus t Tarbela Reservir (Hewitt, 1988a and Wake, 1987); and 65% f the Jhelum at Mangla Reservir (Hewitt, 1985b and 1988b). The snw and ice melt 28

46 streamflw is imprtant because it ccurs in the perid April t June, befre the mnsn rains when it is needed fr irrigatin and pwer generatin (Quick, 1990). 3.3 CLIMATE This discussin fllws the summary f brad climatic cntrl and lcal climate mainly described by Barry (1981), Barry and Chrley (1976), Bucher (1975), Hewitt (1988a and 1988b), Lckwd (1974) and Yung (1981) Brad Climatic Cntrls The climate f Pakistan is classified as arid r semi-arid. Dense natural frests are present nly in sme areas at higher altitudes where rainfall is heavy r at a lcatin in clse prximity t rivers. The mnsn f Asia are caused primarily by the differential respnse f land and sea t incming slar radiatin. In the winter mnths, the Asiatic land mass gets much clder than the adjining seas and nrth-east Asia, and becmes the center f an intense high-pressure system. This Sub Trpical High Pressure (STHP) ver Asia extends frm Siberia t the uter fringes f the Himalayan massif. There are fur distinct climatic seasns; 1. Winter Seasn (December - March) 2. Summer Seasn (April - June) 3. Mnsn Seasn (July - September) and 29

47 4. Pst-mnsn Seasn (Octber - Nvember). Precipitatin in Pakistan falls during tw distinct perids. The first f these seasns is frm July t September. Rain during this perid is the majr cmpnent and related mainly t the mnsn depressins. The secnd rainfall seasn, riginating frm winter cyclnic strms frm the west, ccurs frm December t March. The regin is dminated by the influx f westerly air masses. During the winter seasn, the westerly jet stream lies ver suthern Asia, with its cre lcated at abut 12 km altitude (accrding t the WMO, any speed exceeding 30 m/s may be called a jet stream, Lydelph, 1985). It splits int tw currents in the regin f the Tibetan Plateau arund the high muntains, the strnger branch flwing eastsutheastward dwn the Ganges Plain, and the ther curving nrthward and eastward thrugh nrth China and Manglia. The tw branches reunite again t the east f the plateau and frm an immense upper cnvergence zne ver China. These tw branches have been attributed t the disruptive effect f the tpgraphic barrier n the airflw, but the nrthern jet may be lcated far frm the Tibetan Plateau. This subtrpical westerly jet stream steers depressins twards the Karakram and Nrthern India. These lws, which are nt usually frntal, appear t penetrate acrss the Middle East frm the Mediterranean. On an average six t seven western disturbances mve acrss the Himalayan regin every mnth in winter and are an imprtant surce f precipitatin fr the Karakram and nrthern India. Over mst f the nrthern muntains, snwfall prduces extensive winter snwpacks. These ccur abve 2,000 m a.s.l, and the great bulk f the misture is fund abve 3,000 m a.s.l. 30