The study looked at how to control water levels in tidal rivers in urban areas. By analyzing factors like the slope of the river bottom and the tide's oscillation, researchers found that a gentler slope and larger tide amplitude can lead to higher water levels. They used a method called perturbative analysis and found that a third-order approximation is more accurate than a second-order one. This means that when deciding on the design high water level of a tidal river, the third-order solution is better. The study established a concept for designing high water levels in river mouths and discussed the factors that influence this concept. The results were applied to urban tidal river basins to confirm their validity.