What is used to slow the water speed and improve control?

Rip rap is effectively used to slow water speed and improve control because it consists of a variety of rock materials that are strategically placed along shorelines, riverbanks, and slopes. The irregular shape and size of the rocks create a rough surface that disrupts the flow of water, reducing its speed and minimizing erosion. This technique helps stabilize areas that are vulnerable to water movement, providing a natural barrier that absorbs energy from flowing water.

In contrast, the other options have different primary functions. Geotextiles are synthetic fabrics used primarily for soil reinforcement and filtration but do not directly slow water down. Retaining walls are structures designed to hold back soil and prevent erosion or landslides, but their primary function is structural support rather than water control. Sandbags are often used for flood control and can help redirect water flow temporarily, but they do not create the same long-term, stable environment as rip rap does. Therefore, rip rap is the most effective method for achieving a decrease in water speed and enhancing control over water movement.