Step 1: Picture the flow path along the chute.
Water in a chute spillway moves fast, and where the slope of the bed changes, the flow line must bend to follow the new slope.
Step 2: Work out which way the curve must bend when slope flattens.
Going from steeper to milder slope means the bed surface turns upward relative to the flow direction, so the curve must be shaped like a valley, concave when viewed from above the water.
Step 3: Check why this shape is needed.
A concave curve keeps the water pressed onto the bed by centrifugal action, avoiding separation and the low pressure zones a convex bend would create at high velocity.
Step 4: Rule out the other named curves.
A convex curve fits a steepening slope, a reverse curve fits the toe connection to the stilling basin, and a converging transition is about narrowing width, none of which describe this slope change.
Final Answer:
A concave vertical curve is used for a steeper to milder bed slope change.
\[ \text{Answer: concave vertical curve} \]