A trapezoidal grass-lined swale is to carry a discharge of 1.5 m3/s at 1.5% slope. The soil is clayey sand of PI = 20 and e = 0.5. The swale is 1.25 m wide at the bottom and has 3:1 side slope. For a mixed-grass lining in very good condition, 75 mm thick, design the swale using the HEC-15 (FHWA, 2005) method.