, Many. timber dams have; -been, rebuilt in -masonry, -including: thereat - dam! at 'Holycdte -.before. referred to. Modern' practice in power :!plants combines a number : of turbines on. ,a ;horizontal shaft connected -direct' to electric -generator.land this :demands, that the -floor, ;of the fore-bay and of -the :generator room, shall -be above -the back-water- of ,(hoods, and- that the !turbines, discharge. into: the
through draft-ittibes. -Recent • practice. .as . -at Keokuk, -Iowa, and the :Cedars of the Saint-Lawrence, rrhals installed- very large:single' tarbine' units of . the , vertical 'type and has thus: much sitimplified power •evelopment. . The exacting demands cf electrical service -require. uninterrupted Power. ;:ar that the -head Shall -be. sufficient- ftia•-1 give -effective iresultsin flood time. These- ctonslitites I-have-- -increased- the, height,. of ,daims to abort
the , good range,: with law-water, heads ;of ::ZO to 40.feet.; :Some
er :tamed crest-, withrn :reverse-Curve -extending • dowatstream asi an -to ;protect !the The 'crest, and. :upper • Alain' should be' worked out ' as -si 'gravity, eur,yo for the maxi - mute• good, thus •ateiding•theitendeneY of the frappe, to lift, which ;hest -hem credited Ias -the /source. • of tremor and. -vibration: The , gash •:board failure to remove -,,these• in floods has caused such vibrations the (•confined airats -to- shake off the tops of dares, tn one - concrete: dam: f or one-third the height. , r - -high and ayellfformed' roll-way dam may ! be called: a isperedrgentrator, ,and in high water the horse power ;Rimed •lato velocity is tre larleadonschand thiS is:lhot by wave faction Muter distend() downstream, • The I:betis banks: tuust„ therefore 'be defended, !and natterall•roch, sites ,atie sought. These are ;-not always; safe;- as shown r in, aim ff allure , of-the --Oft-feet dam-,at -Whew the cloWd (stream -toe ..was undermined : by-,reason oft-the 7 'I; brir.
FOC ..Aite$ :withnuti sackAbottomi I :int reach, [tat* ILsrineueesaryliittikimidinavale lobak nel below from erosion. For such sitesatehe Ituntblehbay*, is besiti adhptetki in:which; the ,:force of :the falling :water! is r dissipated- aytthin .;the confines:of :the- structure.( 'in! utithis -*ale,: the sheet,of water' is.- given, a tauter - being - well ventilated; ;behind, - into thel,Pool cr bay which has a-depth-of one-half to auto,thirds the height. of. the fall, and the water escapes _ from- the bay over-- a - breast wall or rollavaY - with little or no destructive mergy. . Some recent designs show an A-shaped plan of dam- *Ultra triangular tumble-bay closed by a low roll-stay, , with a- bridge of reinforced concrete: spaening the roll-way between. piers. The bcittom; of bay is a heavy bed of concrete defended by a •iimbeelkior beneath the sheet of falling water. an rock oss the Mississippi River opposite Kpoku,k,41. foot of the Des Moines Rapids, for a head 'feet is of the roll-way type divided -into panels by piers fitted for °stop locksatt the • High dare. without-rock- will, in many situations; require founcitionsof bear ing piles and a - of pilin\, The structure has naturally a road base which is an element of Further security may be had by sea ng the, riverbed •with. -silt or 'day puddle for- some distance • Up Ptreani, depending its character the head. Nature will perform- this in time, if a blow-cut.does not meantime occur, as ,strgains generally Carry sediment at some season.
The problem in such foundations is one of Pereotaticin, around hettearit': -net: history of the: - barrago Across the two outleti of 'the • Nile, near the of the delta, is instruetive in this 'connection.. masonry structure Was boat
!upon strata 'of:the traibilo charactet, arid '.has been Out. in 'successful operation by bed! - up . with impertneable 'material - for a. didtatice of 20 •tiroes -the on the &in. Thiel -experience shotild assure: -the Pratticability of founding a- !dam- on any smite, ':.1! I • '1 -: Movable Dam&-- In the- improvement. and • Canalization . Of -rivers 'the datavrare Usually -of :moderate 'height, and when of the 'ordinary type are (Jailedfixerlt) ditna; :The: Purpose :of canal iratiori is, to -maintain theordinary pool -level las !• high :As' the adjadentilandi will permit witheitit r -interfering Unduly 'Baud -regimen, and : or. did 'end several types:of: knovable -damsr hole been devised. • These arernntel rated- 'abroad and a great develottment 4C be expected in' -this Country:. Movable' dams' are of twif general , classes: (I) automatic ; claths, / and (2) thode ,:operated by eicternal periver.--In the dirst group - belting the eight typea of bear-trap demi: - in tile second are darns•;' shutter and wicket dams!; roller. dams; :fname, dams.; and curtain f i ; : ' v The: original Of the autetnatie.typeis the bear.trap !first tiSect on the "Lehigh ,Ritier naviga tion of Pennsylvania early in the last century. This: consisted: of timber.. pl at fertn -at -or inear cite katil ,:the 1 -With. two: leaves• overlapping -The top was hinged-sit its tip fstreatn edgq to- the!floorrof the,clam;.iThetutder -,leaf was !hinged at -its'' downstream By ruttrimitteing water, beneath. through ,a .Iregulatistg sluice-A/se 'the -bead .(ph water above until !they: reached, proper leight. I itt. abekititmiail(a ',flat : The, ruse boi these dzinsrlormaltagation disappeared! for a einan rbutIcotttiastetbitaaber lantber, regions i sera far rhoAdiug, a-rpboll of fwaller/ der, --stortag logs and then flushing the same down the stream. As thus used the bear-trap has been variously altered and made as long as 60 feet, beyond which trouble has occurred from the warping or twisting of the leaves. Various de signs have been proposed for waterway pur poses. The most notable application is the so-called (Chicago( bear-trap, 160 feet long, in stalled at Lockport, Ill., as a part of the (con trolling works' of the Chicago Drainage Canal. This is made of metal, is hinged at the top angle, the upper-leaf dropping in front of a breast-wall. It has a range of 15 feet and is so counter-weighted and controlled that the depth flowing over is practically uniform at any elevation, and it is easily maneuvered by one man. The other modifications of the bear-trap are the DuBois dam, in which the two leaves are hinged at the apex, and the downstream leaf hinged to the foundation, the upstream leaf sliding; the Carro dam, in which the leaves are hinged at the apex, and both slide in links on the foundation; the Girard dam, in which the leaves are hinged at the apex, and the down stream leaf in two parts hinged at its middle, folding up stream; the Parker dam, in which the leaves are hinged at the apex, and the up stream leaf in two sections, hinged at its mid dle, and protected by an idler leaf sliding on the foundation; the Lang dam, claimed to be an improvement on the Parker, in which the upper part of the latter's upstream leaf is omitted, and the lower part hinged to the idler; and the Marshall dam, in which the hinge joining the leaves is near the middle of the water leaf, on its downstream side, and the downstream leaf is in two parts, hinged at the middle, form ing a salient angle pointing down stream; the upstream part of the downstream leaf is con stantly horizontal, and its lower part is parallel with the water leaf, forming an exceedingly flexible dam, managed entirely without chains or stops.