The vertical drop is a vertical drop into the embankment with a horizontal 90 degree connection to the outlet. Also a 1.5 foot of elevation difference is required between the intake and outlet for a efficient flow of water. The outlet of the trickle tubes can discharge onto the ground surface, into an open ditch or into an existing tile. The major concern is that the slope be very gradual so that erosion is minimized. The need to release excess water from a wetland after high rainfall or due snow melt runoff is met by the principal and emergency spillway.
Larger projects require a principal spillway through the dike and a vegetated emergency spillway.
The design limitations and requirements of the principle spillway are similar to that of the trickle tube discussed earlier. The same principles are used at larger capacities to relieve more water pressure. The capacity of the principal spillway should be able to handle the peak flow of runoff expected every ten years over a 24-hour period. Although the drop structure concept is rather simple, many different types and combinations of drop structures are used. The function of a drop structure is to remove excess water from the wetland and transport it through the mbankment to an outlet at a lower elevation.