10.3.2 Vertical drop structures(a) Common (straight) drop
The common drop structure, in which the aerated free-falling nappe
(modular flow) hits the downstream basin floor, and with turbulent circu-
lation in the pool beneath the nappe contributing to energy dissipation, is
shown in Fig. 10.14.
The following equations fix the geometry of the structure in a suit-
able form for steep slopes:
drop number, Drq^2 /gd^3 (10.28)whereqis the discharge per metre width;
basin length, LB/d4.3Dr0.27Lj/d; (10.29)pool depth under nappe, YP/dDr0.22; (10.30)sequent depths, y 1 /d0.54D0.425r ; (10.31)y 2 /d1.66Dr0.27; (10.32)heredis the height of the drop crest above the basin floor and Ljthe
length of the jump.
A small upward step, h(around 0.5h/y 1 4), at the end of the
basin floor is desirable in order to localize the hydraulic jump formation.
Forster and Skrinde (1950) developed design charts for the provision of
such an abrupt rise.
The USBR (Kraatz and Mahajan, 1975) impact block type basin also
provides good energy dissipation under low heads, and is suitable if the
DROP STRUCTURES 449
Fig. 10.14 Common drop structure (after Bos, 1976)