70 BULLETIN 1017, U. S. DEPARTMENT OF AGRICULTURE. 



point by the greater width and excess of depth available as noted 

 above. The continuous spoil banks would necessitate the use of 

 flood gates for the small drains entering this section of the main 

 channel, and the construction of spoil-bank levees along the lower 

 parts of the larger ditches that enter the outlet channel. Such an 

 arrangement would make possible the drainage of the low, wet areas 

 along the Bois de Sioux, and only under the most extreme conditions 

 of flow, which seldom occur, would there be any danger of flooding 

 the lands adjacent to the outlet channel. 



THE SECONDARY DITCH SYSTEM. 



The secondary ditch system is designed pirmarily to provide outlet 

 for the local drainage which will be necessary in the development 

 of the wet lands along the Bois de Sioux River. The ditches are 

 placed also with the view of relieving the upper part of the main 

 channel from the run-off from the adjacent land, leaving its entire 

 capacity free for the removal of the discharge from the reservoir. 

 The proposed locations of the seven laterals are shown on figure 3, 

 sheets 2, 3, and 4. All would have final outlet into the main channel, 

 with the exception of No. 7, which would discharge the water from 

 Big Slough into the reservoir above the dam, thus permitting it to 

 be controlled within the reservoir. These lateral drains are designed 

 with capacities great enough to carry any probable run-off from their 

 drainage areas and, because of construction limitations, are in most 

 cases oversized. 



BRIDGES. 



On account of the enlarged channel along the Bois de Sioux, it 

 will be necessary to replace most of the present bridges with larger 

 ones. Several of the steel spans can be used at their present loca- 

 tions by readjusting their elevations and constructing approaches, 

 and some of the present pile trestles can be utilized as approaches 

 for new steel spans at the same locations. As the design of highway 

 bridges is in most States controlled by the State highway depart- 

 ment, no consideration has been given' to the design of the bridges 

 necessary, but it is assumed that steel bridges with concrete abut- 

 ments will be used. It is estimated that six spans 60 feet to 80 feet 

 in length, and 14 spans 20 feet to 28 feet long, will be necessary. 



LOCAL DRAINAGE. 



To gain full benefit from the proposed improvements, it will be 

 necessary to supplement them by local drains in all parts of the af- 

 fected areas. The secondary ditch system as planned herein will, 

 to a certain extent, drain the land in the immediate vicinity of the 

 ditches, but the main object of these ditches is to provide outlet for 



