Why can high tides combined with heavy rainfall cause flooding in estuaries?

In the context of civil engineering, estuaries serve as vital channels for the discharge of inland runoff into the sea. Flooding occurs in these areas when the capacity of the estuary to drain upland floodwaters is overwhelmed by the simultaneous occurrence of high tides and heavy rainfall.1

Under normal conditions, estuaries function as efficient drainage systems. However, when heavy rainfall increases the volume of inland runoff, the estuary must discharge this water into the ocean. If this peak discharge coincides with a high tide, the elevated sea level acts as a barrier, restricting the estuary's ability to release the excess water. This blockage causes the water level within the estuary to rise rapidly, frequently leading to disastrous flooding in inhabited areas located along the banks. Consequently, maritime engineering projects often focus on maintaining these estuaries to ensure they remain deep and efficient enough to handle such combined hydraulic pressures.1