河口帶潮汐效應對水庫排砂之懸浮泥砂運移及濃度時空變化的影響

Abstract

河口潮間帶是河川與海洋之間物質、能量交換的重要區域，具有高生產力的河口海岸生態系統，因同時受到河流流量及潮汐相互作用而形成複雜流場，並影響懸浮泥砂的傳輸行為。淡水河系是國內最大的河口生態系統之一，其上游包含三大支流及兩座水庫，為了緩解水庫淤積問題，颱洪期間水利單位透過空庫排砂、異重流排砂等排洪排砂操作方式，減緩水庫淤積速度以延長水庫使用壽命，但此舉勢必排放大量庫區泥砂進入下游河道，對水體濃濁度及生態系統有一定的影響，但相關研究仍相對較少。為了解水庫有排砂之颱風事件發生及過後，淡水河流域懸浮泥砂濃度變化以及潮汐對懸浮泥砂傳遞之作用，本研究使用EFDC模式的水動力和泥砂傳輸模式進行模擬，並收集2015年蘇迪勒及杜鵑颱風期間水理、泥砂數據，作為模式輸入邊界條件及模式率定和驗證之用。 以「脈衝排放」方式計算蘇迪勒颱風的平均停留時間（tm），結果發現因上游水庫泥砂排放時間不同導致tm有明顯的差異，河口TE02斷面的tm平均約5.3-36小時，此結果顯示停留時間與排放時的河川流量與潮汐流量綜合結果有關，在回復潮汐作用後，因新店溪流量、泥砂量影響使tm仍短暫，因此推測颱風過後河口懸浮泥砂濃度居高不下應受到上游泥砂量持續輸入所致。另以「連續排放」方式，評估各河段受到颱風期間產生高濃度泥砂的影響，不同斷面估計現況影響河口濃度泥砂的綜合性平均停留時間（tcm）約3天7小時至3天15小時、總停留時間（Tb）約15天至18天左右，顯示颱風後淡水河口濃度高於平時許久。比較現況和情境模擬結果發現（Δ=「現況」-「情境」）：（1）「TDL情境」，Δtcm = 0~17.70小時、ΔTb除TE02外無明顯差異，顯示潮汐效應主要的影響為提高泥砂濃度而非延長總停留時間，然漲潮期間出海口泥砂如無法傳遞回河口，淡水河下游河道受高濃度泥砂影響的程度將會降低許多；（2）「Tri情境」，Δtcm =3.02~7.64小時、ΔTb=0.5小時~1天3小時（HHB-GDB），表示新店溪集水區崩塌產生大量泥砂，導致下游河道泥砂停留時間延長；（3）「BdE情境」，Δtcm =0.63~3.13小時、ΔTb=1天2小時~2天6小時（HHB-GDB），研究顯示支流量較大產生底泥沖刷，也易導致河口受影響時間變長；（4）「SLR情境」， Δtcm=1.13~3.41小時、ΔTb=-0.50~15.50小時，此情境因海平面上升導致河口水體積增加而稀釋濃度，及因流速減緩而增加沉積，反而使整體受影響的時間縮短；（5）「FRI情境」，Δtcm=-0.48~-3.84小時、ΔTb=-4天11小時~-8天6小時，此情境結果顯示上游支流量增加導致入流泥砂通量變大、沖刷及挾砂能力提升，整體受影響時間變長。

Reservoir sedimentation effects on water resources, flood prevention, and environmental management problems are recognized to be vital worldwide. To mitigate the sediment accumulation in reservoirs, the Water Resources Agency in Taiwan attempts to enhance sediment scours by hydraulic flushing and density current venting during typhoon events. The effluent sediment from reservoirs usually results in higher suspended sediment concentrations (SSC) downstream and impacts the aquatic environment. Unfortunately, there are few pieces of research discussing this issue. The estuary is a high-productivity ecosystem and a critical zone that exchanges sediment and nutrients between rivers and the ocean. The complicated flow field of the interaction among tidal and river currents dominates suspended sediment transport behavior. In order to understand suspended sediment transport after a typhoon event with sediment flushing from the Shihmen reservoir in northern Taiwan, the EFDC model simulations were conducted in the Tanshuei river estuary. The model was calibrated and verified by comparing the modeling results with the field investigation of hydraulic and sediment data during typhoon Soudelor and typhoon Dujuan in 2015. The mean residence time (tm) was calculated using the pulsed input method during the Soudelor typhoon period. We found apparent differences in the tm due to the different discharge times of the reservoir sediments. The tm of the estuary TE02 section was about 5.3-36 hours, indicating that the mean residence time is related to the comprehensive results of river flow and tidal currents at the sediment effluent time. In addition, the tm was short because the river flow and sediment of the Hsindian Creek continued to influence after the typhoon. We concluded that the estuary remained at a high SSC because of the continuous tributary sediment inputs. Then, we evaluated the effect of high SSC at different sections during the typhoon in terms of continuous sediment discharge conditions. The findings showed that the comprehensive mean residence times (tcm) and the total residence times (Tb) were from 3d 7hr to 3d 15hr and from 15d to 18d, respectively. The differences in the current situation and each scenario revealed (Δ=current – scenario): (i) In the TDL scenario, Δtcm=0~17.70 hr, and there was no significant difference in ΔTb excepting for TE02. We found that the tidal effect mainly increased SSC rather than prolonging the total residence time. If no sediment was capable of moving upstream with the flood tide, the impact of sediments could be much lower. (ii) In the Tri scenario, Δtcm =3.02~7.64 hr and ΔTb=0.5h ~1d 3h (HHB-GDB). The result indicated that the residence times were longer owing to the collapse of the Hsintien Creek Watershed. (iii) In the BdE scenario, Δtcm =0.63~3.13 hr and ΔTb=1d 2h~2d 6h (HHB-GDB). The result showed that sediments from bed erosion could have much influence on the estuary region. (iv) In the SLR scenario, Δtcm =1.13~3.41 hr and ΔTb=-0.50~15.50 hr. The residence times were shorter along with dilution and sediment deposition due to the rising sea-level effect. (v) In the FRI scenario, Δtcm =-0.48~-3.84 hr and ΔTb=-4d 11hr~8d 6hr. Since upstream sediment fluxes were larger and the capability of flushing and entraining enhanced, the residence times were found to be longer in the increasing upstream flow condition.