232 



NATURE 



[July 5, 1894 



of the Indian Geslo^ical Survey, on ihe great landslip in 

 ihe Kamaon HilU. The landslip has been more than once re- 

 ferred to in these columns, but Mr. Holland's report, from which 

 the following extracts have been made, is the first detailed in- 

 formation that has reached us on the occurrence. 



Mr. Holland made a journey to Gohna last February, that 

 is, five months after the landslip, and when the lake, formed 

 by Ihe barrier fallen across the birahi Ganga valley, had risen 

 to within 290 feet of the top of the dam. His investigations 

 lead him to believe that the lake will be full and will overflow 

 the barrier about the middle of August. .Means for recording, 

 by instantaneous photographs, the effects of the water on the 

 dam have been arranged by the Government of the North- 

 western Province. 



Gohna in British Garhwal (lat. 30° 22' iS" N., and long. 

 79° 31' 40' E.)is a small village in the valley of the Birahi 

 Ganga, a river running westward and joining, at a point S 

 miles west of Gohna, the Alaknanda, one of the principal 

 tributaries of the Ganges. The village is about 130 miles north 

 of Naini Tal, and by the road which follows the valley of the 

 Alaknanda, it is 160 miles from Hardwar. 



The bed of the Birahi Ganga, sloping at about 2j°, is at 

 Gohna 4600 feet above sea-level, and is the bottom of a narrow 

 gorge with steep, and sometimes precipitous, sides. The gentler 

 slopes are grass-covered, and higher up clothed with evergreen 

 oak, fir, and rhododendron. In Ihe more open parts of the 

 valley, a small amount of cultivation is carried on by the few 

 inhabitants of the small groups of houses dignified by the 

 name o( villages. The river basin, which is twenty miles long 

 and nine miles wide, is bounded on the north and east by a snow- 

 clad ridge rising to 21,286 feet. A considerable portion of the 

 water of the river is therefore derived from Ihe melting snows, 

 and it consequently receives its greatest supply dnring the warmer 

 months. The area of the basin east of Gohna, and consequently 

 the area draining into the lake which has been formed by the 

 landslip, is about ninety square miles. 



From the account of the villagers there seem to have been 

 fields along the sloping portion of the gorge near Gohna on both 

 sides of the river, whilst the hill ihey speak of as Maithana — 

 the one which fell — rose almost vcrlically above the slope on 

 the north side of the river. Two years ago there was a small 

 slip between Mailh.ina and Gohna village. On the 6lh of last 

 September (1893), and towards the close of the rainy season, 

 two falls took place, damming back the river to form a lake. 

 Falling continued for three days wilh deafening noise and 

 clouds of dust which darkened the neighbourhood and fell for 

 miles around, whitening the ground and tree-branches like 

 snow. Further slips occurred at subsequent intervals after 

 heavy rain ; and at the time of Mr. Holland's visit, a day's rain 

 or fall of snow was always succeeded by falls. Blocks of several 

 Ions would bound from ledge to ledge for more than 3000 

 feet over the broken hill face wilh a low rumbling noise and Ihe 

 production of clouds of dust. The hill which fell was a spur of 

 over 11,109 f*c' ''igh ; but excejit on ihc edge of Ihe precipice, 

 where pieces could be pushed over wilh Ihc fool, Mr. Holland 

 found no cracks in the hill. The rocks exposed on Ihe cliff- 

 front are crumbled and faulted in a complicated manner and 

 with varying dip ; but on the west side of the slip Ihc dip is 

 towards the valley at a lower angle than that of the precipice, 

 the average inclination of which is 54°. The mass of broken 

 material which fell stretches for two miles along the river valley, 

 and rests against the cliff of similar rocks on the opposite side 

 a mde away. On the higher mounds, from which Ihe mud has 

 been wa.she<l away, large masses, sometimes weighing hundreds 

 of Ions, of ciumpled dulomitic limestones are seen pilched in 

 obliquely and shot out like a pack of cards. In the (irsl fall, at 

 any rale, the hill mu.sl have pilched forward and not have slipped 

 down in Ihe usu.il (ashion of smaller sli<les. Blocks hurled a 

 mile away against the opposite cliff have knocked down num- 

 bers of trees. The second main fall now stands as a heap of 

 irregularly piled blocks weighing from about thirty tons down 

 10 ordinary hand specimens. 



The iuifacc of the dam exposed in early March was about 

 423 acres ; but it was gradually being submerged on Ihe eastern 

 side by Ihc rising lake. The lake in Ihc beginning o( March 

 was i\ miles long, I mile wide al Ihe widest part, anil covered 

 37oacrct. It was then rising ai the rale of about six inches 

 per day; but wilh the melting of the snows in Ihe hot 

 season, Ihc rise must become more rapid. When fall it will, 

 unless a culling is made, overflow at a point 5850 feet 



NO. 1288, VOL. 50] 



above sea-level ; and Ihe stream, rushing down an incline 

 of ir, will rapidly cut with increasing he.ad a channel in the 

 mud and loose stones, which cover that portion of the dam, 

 until its speed is checked by the reduction of slope and the 

 exposure of large blocks of dolomite which must occur below at 

 no great depth. Mr. Holland found it impossible from mere 

 inspection to estimate the thickness of the sofi mud, but he 

 thinks that if the rapid erosion becomes arrested before 100 feet 

 h.os been cut, there will be preserved above a lake 3I miles long 

 and I J miles wide, the destruction of which by gradual erosion of 

 the dam and silling up of the basin, though a niatter of lime geo- 

 logically considered short, will be sulTieienlly slow for what 

 historically may be called a permanent lake. The lake view 

 from the dam is the crowning charm of scenery typically Hima- 

 layan and wild. The steep mountain slopes, partially clad with 

 fir, evergreen oak, and gorgeously- flowered rhododendron, 

 slope steeply down on either side to the blue-green waters of 

 the lake, whilst to the east Tirsid and two associated peaks, 

 rising over 20,000 feel, wilh snow-clad slopes and glaciers, form 

 the background of the picture (Fig. I). 



It is pointed out that at several places in the Himalayas, 

 lakes have in the recent past been formed by landslips, filled, 

 and afterwards cut through by their own streams. Mr. Old- 

 ham has described ihe very interesting case of Tur,ag Tal near 

 Gonain in .\lmora District, which was formed behind a barrier 

 of slipped limestone 250 feet high. The level of the alluvium 

 in the lake is now within 50 feet of top of the barrier, so the 

 age of the lake is measured by the lime required to deposit 

 alluvium to a thickness of 200 feet. (A'lV. Geol. Siirv. India, 

 vol. xvi. 1SS3, p. 164 ) 



Whilst the steep slopes of the mountains around the lake at 

 (iohna add greatly to ihe beauty of the view, they are unfor- 

 tunately a source of danger to Ihe lake itself on account of their 

 liability to follow the example of Maithana and slide down, dis- 

 placing proporlionalely large bodies of «aler. At one spot, a 

 little 10 Ihe soulheast of the dam and half-way to Durmi, where 

 the dolomites dip in the direction of Ihe sleep slope towards ihe 

 lake, the hill side may at any time slide into the lake. In 1S69, 

 higher up the same valley, a small lake, Gudyar Tal, having 

 been formed in the same way by a landslip, became suddenly 

 nearly filled wilh a second slip and displacement of a body of 

 waler, which flooded the valley of Ihe Alaknanda and washed 

 away part of Srinagar, 78 miles below. That this, sooner or later, 

 will take place, seems to be certain ; but when, it is impossible 

 to say. The very size of the lake, however, will be a safeguard 

 against high floods. Suppose, for instance, that Ihc permanent 

 lake had an area of 500 acres, and a slip of 12,500,000 

 cubic feet occurred — Ihe maximum possibility near ihe south- 

 east corner of the dam — the result would be that the water in 

 ihe Like would rise about 7 inches. There are, however, one 

 or two steep precipices on the north side of ihe lake, which Mr. 

 Holland could not examine, and which he thinks might probably 

 give larger slips. 



Fears have frecjuenlly been expressed concerning Ihe danger 

 of the dam bursting under the hydrostatic pressure of the water 

 accumulating in the lake above. The sections and map 

 .accompanying Mr. Holland's report should be suHiciently con- 

 vincing to any engineer ; but to remove any doubt concerning 

 the security of the barrier, the strength of the dam is con- 

 veniently compared by a simple calculation with the actual 

 hydrostatic pressure which it will have lo resist before overflow 

 occurs. 



The point relerred lo as 5850 feet above sei-level is ap- 

 proximately in the centre of the dam, and lies in its weakest 

 section. It is shown Ihal ihis weak seclion would weigh about 

 401,922 Ions. When the oveiflow is about to take place, the 

 horizontal hydrostatic pressure against the section will be 

 13,95010ns 



The weight of the seclion is thus nearly twenty-nine limes 

 Ihe horizontal pressure of ihc waler. But even supposing this 

 section lo be free of fiiclion from the sides, and only offers the 

 resistance cslimaled by its own cocflicient on a bed of ihe 

 same material, it would require about four-fifths of its own 

 weight lo move it ; that is to say, a pressure of 321,53'' Ions. 

 But as Ihc maximum horizontal pressure of ihe waler will only 

 be 13,950 Ions, Ihc weakest .section of Ihe dam is at leail 

 twenlythice limes Ihe necessary strength. This estimate 

 would, of course, be slill higher if ihe weight of ihe ihou- 

 sands of lens of dolomitic blocks which rise on cither side the 

 weak seclion and point of oveiflow were taken into consiilcra- 



