Final answer:
The cross-sectional area A of a trapezoidal stream cross-section can be calculated using the formula A = w * d, which results in an area of 18 m² for a width of 6 m and a depth of 3 m, assuming parallel sides for simplicity.
Step-by-step explanation:
To calculate the cross-sectional area A for a trapezoidal stream cross-section, you need to know the top width, the bottom width, and the depth of the trapezoid. In water resources engineering, this calculation is fundamental for tasks like designing channels or analyzing flow. The formula for the area of a trapezoid is A = (1/2) * (b1 + b2) * h, where b1 and b2 are the lengths of the two parallel sides and h is the trapezoid's height. In this case, the width w is 6 m, and the depth d is 3 m, but as we do not have the width at the bottom of the stream (assuming it could differ from the top width), we would consider a rectangular cross-section, where A = w * d. Therefore, the cross-sectional area A would be 6 m * 3 m = 18 m² assuming parallel sides for simplicity.