Computes hydraulic elements for a Trapezoidal Channel using:
Area = y*B + y*y*m_L/2 + y*y*m_R/2
Topwidth = B + y*m_L + y*m_R
Wetted Perimeter = B + SQRT(y^2+(y*m_L)^2) + SQRT(y^2+(y*m_R)^2)
Hydraulic Radius = Area/(Wetted Perimeter)
Hydraulic Depth = Area/Topwidth