Which of the following is a common way to upgrade existing drainage systems?

Upgrading existing drainage systems often involves the need to improve their capacity and efficiency in managing water flow. Replacing existing pipes with larger ones is a practical and effective approach for enhancing drainage systems. Larger pipes can accommodate increased water volume, particularly during heavy rainfall or flooding events, thereby reducing the risk of backups and overflow. This upgrade can significantly enhance the overall performance of the system, ensuring better management of stormwater and helping to prevent issues like erosion and property damage.

The other options, while they may have aesthetic or minor functional aspects, do not effectively address the fundamental goal of improving drainage capacity. Adding decorative elements or changing the color of drainage covers may enhance appearance but does not impact the system's effectiveness. Restricting water flow during rains counteracts the purpose of a drainage system, which is intended to carry water away efficiently. Thus, replacing existing pipes with larger ones stands out as the most logical and beneficial method for upgrading drainage systems.