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i Chapter 4 - Best Management Practices for Erosion and Sediment Control Anti-vortex device and trash rack An anti-vortex device and trash rack shall be attached to the top of the principal spillway to improve the flow of water into the spillway and prevent floating debris from being carried out of the basin . The anti-vortex device shall be of the concentric type . (See Plate 4 . 26c) . See Appendix 1 . 26A of The Florida Development Manual for design procedures for the anti-vortex device and trash rack. Dewatering Shall be done in a way that removes the relatively clean water without removing any of the sediment that has settled out and without removing any appreciable quantities of floating debris . As a minimum , provisions shall be made to dewater the basin down to the sediment cleanout elevation . This can be accomplished by providing a hole at the maximum sediment retention elevation (See Plate 4 .26b) . The dewatering hole shall be i no larger than 4 inches (10 cm) in diameter. Other means of automatic dewatering are detailed in Appendix 1 . 26A of The Florida Development Manual . i It is also advantageous (but not required) to provide for dewatering of trapped sediment before clean out. Basin underdrains are generally installed for this purpose . Appendix 1 .26A contains details for the design of an underdrain system . Base i The base of the principal spillway must be firmly anchored to prevent its floating . If the riser of the spillway is greater than 10 feet (3 m) in height , computations must be made to determine the anchoring requirements . As a minimum , a factor of safety of 1 .25 shall be used (downward forces = 1 . 25 x upward forces) . For risers 10 feet (3 m) or less in height, the anchoring may be done in one of the two following ways: 1 . A concrete base 18 inches (45 cm) thick and twice the width of riser diameter shall be used and the riser embedded 6 inches (15 cm) into the concrete . (See Plate 4 . 26d and Appendix 1 . 26A of The Florida Development Manual for design details) . 2 . A square steel plate , a minimum of 114 inch (6 . 5 mm) thick and having a width equal to twice the diameter of the riser, shall be welded to the base of the riser. The plate shall then be covered with 2 .5 feet (76 cm) of stone, gravel , or compacted soil to prevent floatation . (See Plate 4 .26d and Appendix 1 . 26A for design details) . Barrel The barrel of the principal spillway, which extends through the embankment, shall be designed to carry the flow provided by the riser of the principal spillway with the water level at the crest of the emergency spillway. The connection between the riser and the barrel i must be watertight. The outlet of the barrel must be protected to prevent erosion or scour of downstream areas . See Appendix 1 .26A for design details . i 4-83