近年來鹽水溪口河道內持續增生的紅樹林是否造成鹽水溪水位抬升而有河防安全的疑慮，成為在地與相關單位探討之議題。根據民國109年《鹽水溪河川環境管理規劃》，現況鹽水溪下游段在計畫流量下無溢堤風險，但在大雨或颱風時外水位太高以至於內水排水不及還是會導致周遭低窪地區淹水，紅樹林疏伐是否為降低洪災風險之選項，有其深入之必要。
本研究以鹽水溪下游段為研究區域，利用地理資訊系統對多光譜影像進行植生範圍分析，以2011、2012、2016、2019、2021和2022年6次水文條件相似之影像計算植被指數NDVI且進行紅樹林面積判釋，並利用HEC-RAS 2D水理模式設置不同模擬情境，探討紅樹林增生和河防安全間的關聯性，並納入生物調查資料進行討論，以建議合適之疏伐方案。
研究結果顯示，在2011年至2022年間，鹽水溪下游段紅樹林面積由28.58公頃至50.05公頃，增加了75%。隨著紅樹林面積增加，100年重現期流量情況下鹽水溪各斷面水位有抬升之情形。透過本研究之模擬，疏伐紅樹林可以降低洪水位，但各斷面所受影響的紅樹林皆來自不同區域，僅能依照河道各斷面決定紅樹林最佳疏伐位置。本研究建議優先疏伐區為下游段靠近大港觀海橋左右岸的紅樹林，一方面可以有效降低距離堤頂最近的大港觀海橋洪水位，另一方面對於下游段的生物影響相對較小。

In recent years, the continuous growth of the mangrove in the mouth of the Yanshui River has raised concerns about the potential increase in water levels and riverbank safety. This issue has become a topic of discussion among local communities and relevant authorities. According to the 109th year of the "Yanshui River Environmental Management Plan," the downstream section of the Yanshui River does not pose a risk of overflow under planned flow conditions. However, during heavy rain or typhoons, high external water levels combined with inadequate internal drainage can still cause flooding in the surrounding low-lying areas. It is necessary to thoroughly investigate whether mangrove removing could be an option for reducing flood risks.
This study focuses on the downstream section of the Yanshui River. Utilizing geographic information systems, vegetation coverage analysis was conducted on multispectral images from 2011, 2012, 2016, 2019, 2021, and 2022 to calculate the Normalized Difference Vegetation Index (NDVI) and interpret the mangrove area. The HEC-RAS 2D hydraulic model was employed to simulate different scenarios and examine the relationship between mangrove growth and riverbank safety. Biological survey data were also incorporated for further discussion and to propose suitable removing strategies.
The research results indicate that from 2011 to 2022, the mangrove area in the downstream section of the Yanshui River increased from 28.58 hectares to 50.05 hectares, marking a 75% growth. As the mangrove area expanded, water levels at various cross-sections of the Yanshui River under the 100-year return period flow showed an upward trend. Through simulations conducted in this study, it was found that removing the mangrove can help lower flood levels. However, different sections are affected by mangrove from various areas, so the optimal removing locations should be determined based on each section of the river. The study suggests prioritizing removing in the downstream section near the left and right banks of the Da Gang Guan Hai Bridge. This approach can effectively reduce flood levels near the bridge and have relatively minor impacts on the downstream biota.

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