First, determine the head loss at entrance. This will vary between nothing and one velocity head, dependent on the arrangement at the entrance.[41] The length of straight pipe which will give this same loss can be computed from the expression _l_ = _h_⁄_S_, using for _S_ the assumed slope of the hydraulic gradient. Second, determine the head loss due to the bends, This is determined from the expression _h_ = (_fl_)⁄(_d_) (_V_^2)⁄(2_g_) in which _h_ = the head loss in the bend; _l_ = the length of the bend; _d_ = the diameter of the pipe; _v_ = the average velocity of flow; _g_ = the acceleration due to gravity; _f_ = a factor dependent on the radius (_R_) of the bend and _d_. The relation between _f_, _R_, and _d_, for 90° bends is shown as follows:[42] _R_⁄_d_ 24 16 10 6 4 2.4 _f_ 0.036 0.037 0.047 0.060 0.062 0.072 After the head loss has been determined, the equivalent length of straight pipe is determined as above. Third. The equivalent length of pipe will be the sum of the actual length of pipe and the equivalent lengths as found above. In the problem in hand the head lost at the entrance will be assumed as one-third of a velocity head, or 0.0324 foot. With the assumed slope of 0.001 this is equivalent to 32 feet of pipe. The radius of the bend is about 20 feet and the length for a 45° central angle is about 16 feet. The head loss for this angle will probably be a little more than one-half that for a 90° angle. The expression will therefore be taken as about 0.2(_V_^2)⁄(2_g_) and for two bends is equivalent to about 40 feet of pipe. The equivalent length of pipe is therefore 150 + 32 + 40 = 222 feet. The elevation at the lower end should therefore be: the elevation at the upper end, 92.07 − 222 × .001 = 91.85.