lEEIGATIOlsr IE" IsrOETHERN COLOEADO. 71 



The earth embankments are of various dimensions. Crests are 

 from 8 to 16 feet wide and usually carry a roadway. Outer slopes 

 range between 2 to 1 and 4 to 1. Inner slopes are steep or flat, de- 

 pending on whether they are well protected against wave action. 

 The slopes paved with concrete are usually 1 to 1 or 1^ to 1, while 

 slopes with no protection are often as flat as 4 or 5 to 1. The free- 

 board maintained on the dams ranges from 1 to 15 feet and is 

 usually a compromise between water requirements and safety. When 

 possible it is the custom to fill the reservoirs only partly full in the 

 early spring to permit them to pass through the period of high 

 winds with a safe freeboard. After the danger from high winds is 

 past they are topped out and the water is raised to a point on the dam 

 which would be decidedly unsafe under a continued high wind. 



The erosive action of waves on earth embankments, illustrated in 

 Plate XVI, figure 2, is so destructive that some sort of protection is 

 always provided if possible. A few of the smaller dams are protected 

 by brush laid on the slope and held by stakes and wire. Many are 

 protected by a loose rock riprap laid on the upper part of the slope, 

 where wave action is most destructive. On the whole, this protection 

 seems to give as much satisfaction as any. Its chief fault is that the 

 rock is continually settling and slipping down the slope, making it 

 necessary to add more rock until a condition of stability has been 

 reached. In the case of the Cache la Poudre Keservoir more or less 

 rock has been dumped on the slope every year for 20 or 25 years and 

 a condition of strict stability has not yet been attained. A few slopes 

 are protected by a rock riprap hand laid on a cushion of gravel. This 

 type is also subject to dislodging and settling, and requires con- 

 siderable repair work to keep it in good shape. A number of dams 

 are protected by concrete pavements about 6 inches in thickness and 

 reinforced with wire mesh or iron rods. Some of these are laid on in 

 sheets without joints, but the majority are in strips running from 

 toe to crest. A few are supported by ribs running up and down the 

 slope at intervals. Owing perhaps to the short slopes covered, there 

 have been no total failures of this type so far, but local failures are 

 common. These failures start with cracks opened by expansion and 

 contraction through which the water is able to dig a cavity in the 

 slope under the pavement. After these cavities are formed it is 

 supposed that the force of the waves either smashes in the unsup- 

 ported pavement over the cavity or compresses the air in the cavity 

 sufficiently to produce an outward bulge of the pavement and con- 

 sequent failure. Plate XVII, figure 1, shows a break in the pavement 

 of Terry Lake resulting from a crack. 



As the great majority of the reservoirs in the valley are supplied 

 through canals and with few exceptions the drainage area immedi- 

 ately above develops only a small amount of water, wasteways are 



