ARBORICULTURE 



245 



Floods, Brid"^es and Dams — OF Interest to Engineers. 



Economy in construction demands that 

 a structure sliall be substantial and per- 

 manent. The recent floods again call 

 attention of the engineering world to 

 facts which have been oft repeated. 

 Water is a power which cannot be trifled 

 with, and when an extraordinary rainfall 

 occurs there must be ample room for its 

 speedy exit. 



In the early days of railway building, 

 bridges were made of shortest length 

 permissible. Abutments extended into 

 the natural waterways, while superstruc- 

 tures were low to avoid heavy fills. The 

 first cost was considered a much more 

 important matter thirty years ago than 

 it is at present, the result being that the 

 limit of safety was overlooked, and many 

 bridges were washed away 1\v very mod- 

 erate freshets. 



On one Colorado line, where, during 

 the greater part of the year, and for 

 many years in succession, the streams 

 are mostly aroyos, or dry beds, every 

 bridge was carried out within a few 

 years, and, in places, miles of track were 

 washed away by flood waters. 



One bridge, on first construction, had 

 a 60-foot span; it was destroyed and re- 

 placed with a 90-foot bridge, and when 

 that was, in turn, lost, an iron structure 

 having 120-foot span, raised several feet 

 higher, was found necessary to admit of 

 a free passage for extraordinary rain- 

 falls. 



In after years a new management be- 

 gan to fill in the approaches with earth 

 work, but. being warned by a more ex- 

 perienced friend, desisted. 



Upon one occasion, while I was resi- 

 dent engineer on location, a bridge was 

 to be erected over an important river ii 

 the South. The grade established was 

 loM'ered four feet, contrary to my judg- 

 ment. I had found unmistakable evi- 

 dence of a former freshet which reached 

 the point which I had established. In 

 two years the bridge was carried away. 



There is too great encroachment upon 

 the natural M'ater course of streams 

 which, at infrequent intervals, become 

 raging torrents. Some dozen years ago 

 almost every bridge and culvert in Kan- 



sas was washed out by a freshet similar 

 to the recent flood. Railway bridges 

 were only saved by loading with heavy 

 trains of iron rails. 



In the city of Denver are several 

 bridges over Cherry Creek which are both 

 too short and far too low, which arc lia- 

 ble at any time to be destroyed and cause 

 much damage by backing the water when 

 have here encroached beyond the limit of 

 safety. 



There are similar instances in other 

 localities. The flood at Johnstown was 

 aggravated by the low-arched stone 

 bridge, which soon became blocked with 

 floating debris, while the mills had in- 

 truded upon the already too narrow 

 water course. 



A month ago the Eio Grande was an 

 insignificant brook, through which the 

 Mexicans were wading, scarce ankle deep, 

 to save toll over the bridge. Since that 

 time the torrent has been four miles 

 wide, breaking levees and doing great 

 damage. It is the extraordinary rainfall 

 that must be provided against, not the 

 average precipitation. 



With the general indift'erence which 

 exists among all classes of Americans 

 regarding forest influences and the con- 

 tinuous destruction of the mountain for- 

 ests, these floods must become of more 

 frequent occurrence, and ample provision 

 should be made for the accommodation 

 of a much larger flow of water than here- 

 tofore. 



In the irrigation works which are be- 

 ing planned greater precaution should 

 be observed in the strength of dams and 

 size of spillways. 



One-third as much water fell in Colo- 

 rado during the first week in June, the 

 present year, as the average for an entire 

 vear. 



The editor of Arboriculture luis many 

 duties which call him to distant points. 

 Since beginning this work he has trav- 

 eled more than four thousand miles each 

 month in the performance of his duties 

 in connection with the Society. If cor- 

 respondence is not as prompt as it would 

 seem proper, this fact will explain it. 



