T>T?AIN-AGE METHODS FOR COUNTY EOADS. 



25 



this purpose, and the culverts and inlets should be placed at such 

 intervals as to prevent the ditch from eroding or overflowing. The 

 distance apart depends on the capacity of the gutter as designed. If 

 the gutter be large enough it may be possible to use few cross drains 

 or culverts and space tliem at intervals of several hundred feet. 

 Under other conditions they may be required at intervals of as little 

 as 50 to 100 feet. 



The design should provide an inlet that will carry all that the cross 

 drain will, and usually there is little difficulty in securing this condi- 

 tion. It is not essential that 

 Ithe inlet and cross drain be ^ 

 designed to clear the gutter if 

 this be continuous, and the 

 inlet entrance consequently 

 may be a flat grating or open- 

 ing designed to admit onlj a 

 part of the flow. Usually, 

 however, if the expense of in- 

 lets is incurred it is best to 

 clear the ditches at each point, 

 and the capacity of the cul- 

 vert and inlet should be cal- 

 culated accordingly. It is 

 also good practice to make the 

 gutter continuous, so that if 

 the inlet becomes partly or 

 temporarily clogged the water 

 may continue along the gut- 

 ter. On very steep grades it is 

 well to design the inlet open- 

 ing with a baffle on the lower 

 side, so that all water flowing 

 in the gutter to that point 

 will be taken. In the baffle 

 there should be a depression 

 along the upper edge to pro- 



CONCRETE DROP INLET 



Scale |"=l'-0" 

 Fig. 8. 



vide relief in case the inlet becomes interrupted. 



Owing to the difficulty sometimes had in clearing such culverts and 

 inlets of obstructions it is well to give careful consideration to the 

 probable conditions under which they will operate. If they are 

 frequently under a head the inlet will run full but little difficulty 

 will be experienced in keeping the culvert clear even at a very low 

 slope; but if the prevailing storms are light and only a small flow 

 occurs ordinarily in the culvert the pipe should be given sufficient 



