def calcFallVelocity(rho_s, D_s):
	"""
	Calculates fall velocity of sediment.

	Paramters
	---------
	rho_s : sediment density [kg/m^3]
	D_s : sediment diameter [m]

	Returns
	-------
	w - fall velocity of sediment [m/s]

	Notes
	-----
	Equation used is from Ferguson and Church [2004]. C1 and C2
	values correspond to fits for natural grains.
	"""
	Sd = (rho_s - 999.97)/999.7
	C1 = 18.; C2 = 1.0;
	w = Sd*9.81*D_s**2 / ( C1*1e-6 + (0.75*C2*Sd*9.81*D_s**3)**0.5 )
	return w