Delta fronts formed by bedload and turbid influxes into mountain reservoirs: Field survey and Experiments

本文藉由水工模型實驗與現地調查分析出三角洲水下部分形貌變化的重要參數。本文所討論的三角洲是由河川底床的泥沙與混濁的河川流體匯集進入小型的山區型水庫。本文以坐落於中台灣的霧社水庫為研究目標並對其進行資料的搜集與水下形貌的調查。為了與現地的環境條件能夠相配合，我們利用單寬的水工試驗水槽，選定粒徑不同的渥太華砂並且與可控制入流量的輸砂儀器進行了一系列的水工模型試驗。藉由電腦程式的分析可以得到三角洲的形貌變化。此外在本文中採用了影像分析的技術以取得在三角洲不同區塊粗細顆粒的分佈比例與在此區塊的顆粒密度。在同一組實驗條件中，三角洲的水下形貌有可能會因為匯流入水庫中的混濁流體直接與水庫混合，使得三角洲的形貌從有異重流存在的hyperpycnal三角洲型態轉變為沒有異重流存在的Gilbert三角洲型態。由能量的角度進行分析，將水庫動能的通量與位能的消散相結合。三角洲形貌的變化會在一個束縮的能量範圍內發生變化。此外，根據實驗條件的不同，三角洲水下淤積的形貌也會有不同的變化。In this thesis, laboratory experiments and field observation are performed to determine the factors which control the morphology of delta fronts. The deltas examined are formed by bedload and turbid influxes into small reservoir. In the field, we collected data and performed bathymetry measurements in Wushe reservoir of central Taiwan. Based on the field observations, we then conducted a series of laboratory experiments. In a flume of constant width, controlled influxes of coarse and fine sand were supplied, and the resulting delta profiles were measured In addition, image analysis was used to estimate the composition of deposits, comprised of pore space, fine sand, and coarse sand, in different regions of the delta. It is found that, in a single experiment, a delta front can undergo a transition from hyperpycnal to Gilbert type delta front, dependent on whether the turbid flux plunges or mixes with the reservoir. The transition is further found to occur within a narrow range of values for the ratio of kinetic energy influx to potential energy dissipation. Dependent on conditions, it may therefore be possible to obtain either a Gilbert- type or hyperpycnal delta front in a given reservoir