河口位處海陸交界，潮汐、風浪與逕流等營力在此競逐，敏感的河口形態在不同時距具有不同的變動特性。本研究選擇台灣西北部潮汐主導型的沙質河口—中港溪河口為研究區域，藉由實地測量並配合研究期間各項營力資料的方式，分析突發事件前後和短期年內時距的河口地形變動特性，後者分別針對日內、月內與月間等時距加以探討；同時，利用地圖與歷年營力比對的途徑，釐清長期時距的河口地形變動特性。綜合研究成果，有以下幾點結論。
1) 潮汐主導型河口在突發事件期間，逕流營力會暫時主控河口地形的動態機制；而本研究的三個颱風個案皆顯示，右岸侵蝕、左岸堆積，且整體灘沙流失量超過增加量的特性。2) 日內的垂直堆沙變動具有時空差異，以冬季、臨海側灘沙堆積的效率最高；水平變動主要與兩岸的灘面形態有關，漲、落潮流在左岸灘面上的作用力最強。3) 月內變動具有大潮堆沙超過小潮的特性，但冬季沿岸漂沙量的增加造成大、小潮的前後時序在右岸成為地形變動的關鍵。4) 由於冬季沿岸漂沙與夏季逕流營力的干擾，潮汐在月間地形變動的影響力降低；冬季堆積作用旺盛，一年之內可回復30%的堆沙，夏季的降雨事件則造成5月至10月間地形波動的現象。5) 長期時距以堆積、向外擴張為主，近30年來尤其明顯，但右岸河口沙洲仍存有侵蝕和堆積互現的變動特性；沙洲受洪水沖毀的界檻值為300公釐單日降雨量及500CMS日逕流量，回復週期約3-4年。
在一個潮汐主導的環境裡，中港溪河口地形的變動特性與人為活動的因子關係甚微，潮汐起沙、堆積的過程持續進行，風浪、逕流營力與沿岸漂沙間侵、淤作用的強弱差異則影響了潮汐營力在河口的作用效能。研究成果可提供政府或民間在河口地區進行規劃、開發時的參考依據。

Sensitive landform of estuary possesses changeable characteristics in different time span due to the interaction of tide, wave and run-off on the interface of sea and river. We choose tide-dominated sandy estuary in Chung-Kang River in northwestern Taiwan as a research area. The research not only analyzes the characteristics of geomorphological change of the estuary during typhoon/climate events and short-term time span based on in situ data, but also clarifies them by comparing clues on the maps with meteorological data. Some conclusions are reached as follow.
1) Fluvial process would temporarily dominate the dynamical mechanics during typhoon events. Three catastrophic cases the study discussed demonstrate some common properties, such as degradation unrolling on right bank and overall sediments loss exceeding sediments increase. 2) Vertical change of estuarine landform within a day presents time/space diversities. And the sedimentation is found to work most efficiently by the sea in winter. Horizontal change of estuarine landform works strongly on left bank. 3) Geomorphological change within a month demonstrates that there are more sediments input during spring tide than that during neap tide. However, long-shore sand drift would be a critical variable in the system. 4) Sedimentation works actively from November to March because of more long-shore sand drift but that would be interfered by abruptly fluvial process in summer. 5) In the past 30 years, estuarine sandbar presents extending seaward, while the destructive threshold of that could be 300 mm rainfall or 500 CMS runoff per day. Geomorphological cycle of this estuary is believed 3-4 years or so.
In a tide-dominated estuarine environment, the landform change is slightly related to man-made projects. The tide continues the process of lifting/piling and the wave, fluvial energy and sand drift are factors believed to modify the morphology at estuary. The results we get could provide insights to schemes and developments at estuaries by the government and the public.