A  =  Cross sectional area, or drainage area 
B  =  Bottom width of a channel 
C  =  Runoff coefficient in Rational Method 
D  =  Culvert diameter or sediment size 
D_{50}  =  Sediment size for which 50 percent by weight of the particles are smaller;
similarly D_{65}, D_{84}, D_{90} represent sizes for which 65, 84, and 90 percent of the
particles are smaller 
Fr  =  Froude Number 
f  =  Darcy Weisbach friction factor 
g  =  Acceleration due to gravity, 9.81 m/s^{2} (32.2 ft/s^{2}) 
H  =  Specific energy 
H_{f}  =  Friction loss of head 
h_{L}  =  Headloss 
i  =  Rainfall intensity, mm/h (in/hr) 
K  =  Conveyance 
k_{s}  =  Height of roughness channels 
K_{u}  =  Correction factor for units: SI (English) 
n  =  Manning's resistance coefficient 
P  =  Wetted perimeter of the flow 
Q  =  Discharge (flow rate) 
q  =  Unit discharge 
R  =  Hydraulic radius 
Re  =  Reynolds Number 
r_{c}  =  Radius of curvature at the center of the stream 
r_{i}, r_{o}  =  Radius of curvature for inner and outer banks in a bend 
S  =  Channel slope 
S_{f}  =  Friction slope (also called the energy slope) 
S_{o}  =  Bed slope 
S_{w}  =  Slope of water surface 
T  =  Top width 
V  =  Velocity 
V_{*}  = 

X  =  Coordinate axis 
y  =  Coordinate axis, depth of flow 
y_{c}  =  Critical depth of flow 
y_{s}  =  Scour depth 
Z  =  Elevation above arbitrary reference level (and Side slope of channel bank) 