Show power held up by great dams california disaster focuses attention on our other great structures yew york once more disaster has dramatized the incalculable power hat lies behind the towering walls of our great dams and reservoirs impounded by steel and concrete a col clossal force of water relieves man of bait his burdens with the effortless ness of the commonplace but when it breaks those bonds it wrecks a de nothing can withstand and leaves an ugly trail of desolation ays the york times A few weeks ago several thousand people were living in a pleasant cal valley borne of them were ranchers others lived in the scattered villages and many worked in the large hydroelectric plant several miles below the great new st fran els dam rising high into the air above them at the mouth of the san fran canyon they were very proud of that dam because it had brought so much prosperity to the region completed less than two years before it was one of the keystones of the immense system that brought water across the desert to los an geles and stood in the front rank of the count rys largest dams before dawn on a tuesday nearly three hundred of those people were dead the ranches and the orchards were burled deep in silt and foaming water the massive hydroelectric plant had been smashed to fragments twelve billion gallons of water had burst that great dam like so much cardboard and thundering down to the distant sea in a wave fifty feet high carried destruction on its crest through the whole valley other dams and reservoirs judgment Is withheld by experts until a complete examination has been made meantime attention Is being centered on the dams and reservoirs throughout the country in california and the est ot course this interest Is acute for a great many of the corn out there are faced with con almost identical to those of the country below the st francis dam the development of hydroelectric power and the necessity for alon in regions of little rainfall with its eternal promise of making the desert bloom has led to the build ing of countless dams throughout the est canyons and arroyos through which a thin brackish stream of water trickles throughout most of the year are impounded and made to distribute their torrential spring flow further more the need of directing the flow of dangerously erratic streams has also led to the building of dams on a large scale the outstanding example Is the colorado aher and the proposal to build the great boulder dam which Is now a subject of national debate is hedged about with a coffus ing mass of issues the proposed boulder dam alone calls tor a structure more than feet high twice as high as any dam ever constructed before in any part of the world it would create a lake eighty six miles long holding enough water to submerge to a depth of one foot an area the alz of ew hampshire vermont massachusetts con isea jersey and the district of columbia some great dams contrasted to that the dam that recently flave way with such appalling consequences is small the st fran els dam was feet high with a span of some feet across the mouth of the canyon and an add dional dike nearly as long on the spur of the bill it bad a nominal capacity of cubic feet of water ba tore the heavy rains swelled it beyond that the exchequer dam in northern california Is feet high and ar dam in idaho is said to be feet high mhd famous roosevelt dam in arizona Is feet high the whole science of dam engineer ing Is centered upon making every dam as secure as possible engineers point out that a wide margin of safety beyond the greatest calculable strain Is allowed several times as much steel and concrete as seems re quiren Is used and the requirements are worked out with scientific precision based on long study long before the actual work of building the dam begins an echaus tire study of the locality Is made the advance guard of engineers first select the location they examine the rock or soil beneath the surface for dounda alons they study the flow and volume of the stream then they go back into the records and find out what the average rainfall has been over a peri od of years nothing is left to chance after this the best type of dam cpr the site selected Is built it Is as near ly perfect as it can be made it is meant to endure indefinitely to stand against anything nature can devise usually it does but there are occasional cas ional exceptions such as the recent tragic one there have been other dam dalsas the johnstown flood of 1880 when a broken dam swept more than 2000 people to their deaths and desolated the country Is still the most appalling american instance but with in the past year the breaking of mis levees and the floods in ver mont have shown how brittle the walls that men build against water can be there are catastrophes of lesser magnitude in various parts of the united states every year dwarfed to comparative insignificance by the cal disaster in the same papers which carried first accounts of that devastation there were dispatches from wisconsin stating that a bursting dam had inundated a section of that state the pressure of an ice jam and abnormally high water was the cause the result was that farms were under water communication was at a standstill and a great deal of damage was done earth type still good instances such as this one come periodically from all parts of the country still the great majority of dams continue to stand and do their work unfailingly engineers assert that there need be no fear that they will fall they are more than adequate to any emergency the best possible creations of engineering skill that skill Is being extended con scantly men have been building dams according to records for a good many centuries and they have gradually evolved special types for special pur poses and sites paradoxically enough the type of dam that was built first the simple pile of earth in the path of a stream Is still considered one of the best it Is used in the waterworks water works system that supplies new york for instance with certain rather important modifications chief of these Is the insertion of an impervious dike usually of concrete in the center to prevent seepage in effect one engineer points out water has as much trouble sweeping an earth dam away as it does any natural hill these dams are practical only in a shallow valley but impossible in a mountain gorge where either a gravity concrete dam or an arch dam has to be used depending it Is said largely upon the strength of the rock foundation and side walls available the gravity dam rests on a triangular base below the surface of the stream bed it Is of course also anchored to the side walls the st francis dam was of this type and it was arched as well tor additional safety arch type trusted the arch type Is one of the newer developments in dam building and though it Is still little beyond the experimental peri mental stage it Is held to be the most perfect type yet devised unlike most dams whether of brick or stone or wood the arch dam does not depend entirely upon weight to bold back the water it Is ule a cylinder in appearance its convex side taking the thrust of the water and passing it on to the walls of the canyon it Is said that no dam of this type has ever failed around new york city there are a dozen big dams one of the greatest of which Is the ashokam ashokan dam in the catskills some of them impound two or three times RS much water as the volume that swept down the gorge of the canyon of san in california but their imminence should cause no undue alarm among the thousands of people who live below their massive sides according to william W brush who Is chief engineer of the department of water supply mr brush recently pointed out that the dams in the croton and watersheds are of the gravity type built to withstand the greatest con cel vable pressure with ease the dams are so constructed mr brush said that the abutments are extended far into the hillsides hill sides to minimize the dan ger of seepage around the abutments |