THE TDOT has a Design Drainage Manual (Chapter 5) in the Web. In page Appendix 5A-10 there is a capacity chart very useful.
In the chart you will find plotted QN/b**(8/3)*S**0.5 Vs d/b for trapezoidal channels.
With d/b = 5/20= 0.25 your value of QN/b**(8/3)*S**0.5 = 0.15, doing the math...