14 



BULLETIN 831, U. S. DEPARTMENT OF AGRICULTURE. 



more of these have been installed on the more recently constructed 

 spillway regulators than any of the other devices. Their popularity 

 is based on the possibility of using them where long span is essential. 

 The rolling dam is a patented type and consists of a large hollow 

 cylinder, the diameter of which is usually made to correspond to the 

 height to which it is desired to raise the pond level above the crest 

 upon which the cylinder rests. Its length is made equal to the width 



Cr<23/ofGa/e£L959. ^ 



?• -6a-/e Hanger P/n. 



When Wa/er in filler ae/s 

 \hfgh rnis Gare tarns c/onn, 

 x and Courier yye i<g/?/ raises 

 op-^o/A/npSeam /ipnirig 

 ^» accord'/n<g/y- 



C'fv . 



",r Knife Edge 

 •i 'Hinge. 



. 



'J% : \ ' Concrete r?oJ/yray &T$are. 



■V" 'P> • a : ■ 



"A 



Fig. 3. — Concrete dam 17 feet high with flood gates, Nashua, Iowa. This structure on 

 the Cedar River is equipped with automatic flood gates 7 feet high by 46 feet long. 



of the opening into which it is fitted. As a general thing the material 

 of which it is made is boiler plate riveted together and braced to 

 withstand the strain to which the dam is to be subjected. The me- 

 chanical operation is provided for by having a gear engage a rack 

 on an inclined abutment and by means of a sprocket chain wrapped 

 around one end of the cylinder and connecting with the operating 

 mechanism, the dam is rolled up or down the abutment as desired. 

 The power is generally furnished by an electric motor or by winches. 

 Various forms have been installed at Grand Junction, Colo., on one 



