當河川挾帶大量土砂流入湖泊或水庫時，河川之含砂濃度會高於湖泊或水庫，進而潛入水庫底床，形成高密度異重流(hyperpycnal flow)。河流所挾帶的粗顆粒泥砂則會在河川與水庫水位線之交界處(shoreline)形成三角洲。Lai and Capart (2007；2009)曾提出異重流三角洲(hyperpycnal delta)具有自我相似(self-similarity)之實驗與理論分析。Muto and Steel (1992)及Muto (2001)曾提出在固定之水砂條件下，下游水位穩定抬升時，吉爾伯特三角洲(Gilbert delta)具有自體產生向陸撤退 (autoretreat)之現象，當三角洲成長初期shoreline會向下游推進，隨後停止向前轉而向上游後退。然而少有研究指出在水庫水位穩定抬升之條件下異重流三角洲是否也有相同現象。本研究之目的為透過小尺度模型實驗探究下游水位抬升對異重流三角洲之影響，觀測(1)三角洲之形貌、(2)河川－水面交點及(3)岩盤沖積層之交點(bedrock-alluvial transition)隨時間之演化過程。實驗中採用鹽水做為異重流(ρ_in=1.2 g⁄ml)，在上游提供穩定輸砂量(Q_s)及輸水量(Q_w)，維持固定底床坡度(S_b)，並控制下游水位以固定速率抬升(V_w)，分別控制各個參數比較分析。實驗透過縮時攝影(Time-lapse photograph)每5秒紀錄下三角洲之發展過程，並以數位影像處理定量測量出三角洲之底床高程隨時間之變化。實驗結果顯示在不同抬升速率下，異重流三角洲皆會出現autoretreat之現象，並在三角洲前緣(deltaic foreset)產生侵蝕，此現象與Lai and Capart(2008)一致。推測此侵蝕現象能沿著水庫底床產生水下之河道，讓河川之泥砂能快速經由此通道運移至庫底。

Hyperpycnal flows, a kind of density currents, appear when a sediment laden river submergs into a lake or a reservoir. Then these density currents leads the depositions at lake shore to form hyperpycnal deltas. Muto and Steel (1992) and Muto (2001) reported that Gilbert delta exists “autoretreat”, an autogeneic process for delta evolution in response to rising water level. They also showed that shoreline will retreat after a relatively brief period of delta progradation. However, little reserch have addressed whether hyperpycnal deltas exist autoretreat under a steadily rising water level. In this study, we use small-scale experiments to examine this process, and measured (1) delta morphology, (2) shoreline trajectory and (3) bedrock-alluvial transition. Upstream, we provided constant sediment (Q_s) and inflow (Q_in)(ρ_in=1.2 g⁄〖cm〗^3 ) supplies to investigate hyperpycnal deltas in response to different water rising speed (V_w) with a constant bed slope (S_b). We use time-lapse photograph to record the delta evolution every 5 seconds. Our results confirm that hyperpycnal deltas exist autoretreat with different rising water speeds. Moreover, most of the runs appear erosion along deltaic foreset. This observation also in agreement with Lai and Capart (2008). We speculated that this phenomenon can produce a subaqueous channel, so that sediment from the river can be quickly transported through this channel to deep reservoir.

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