Of course, overland flow may be occurring simultaneously, as shown in figure 11, 

 some of which is caused to seep vertically downward to reach the phreatic surface. 



Below the road surface, the top flow line probably appears as shown in the fill 

 section of figure 11. Represented is an equilibrium situation, wherein the ditch is 

 full of water. Water on the road surface and fill slope tends to seep vertically 

 downward to reach the flowing ground water. 



Situations can arise, however, where water accumulates in the road surface. In 

 such cases, active seepage throughout the section, with flow roughly parallel to the 

 fill slope, is possible. It is this extreme situation which was assumed in the 

 analysis of fill-slope failures. 



Where an embankment or through fill crosses a draw, the situation can be repre- 

 sented as shown in figure 12. The culvert installation may interrupt part of the 

 subsurface flow, thus causing some drawdown of the top flow line above the embankment. 

 Below the culvert inlet, subsurface flow continues, and at some location downslope may 

 again coincide with the ground surface. It is possible for the top flow line to rise 

 above the original ground surface and into the fill material immediately downslope 

 from the culvert inlet. Thus, a conservative assumption in such a case would be one 

 in which the embankment was considered as an earth dam; furthermore, this "dam" would 

 have a free water surface upslope at the elevation of the culvert entrance, and tail- 

 water elevation at the toe of the downstream slope. 



16 



Figure 12. — Subsurface flow conditions for through fills. 



