5 子計畫五：水庫洩洪排淤對下游河道環境影響之評估

本計畫採用顯式有限解析法(explicit finite analytic method)動床數值模式，模擬高 濃度含砂水流洩放後，探討各原水取水口處 6,800ppm上下，顯示排淤過程中水體之泥砂 濃度是處於高濃度之狀態。堆置之清淤泥砂

Abstract

Because great amount of rainfall occurred during the periods of July 2nd floods and the Aere typhoon, huge amount of sediment was eroded and flushed into the Shihmen Reservoir and the outlet flow with so high sediment concentration that water treatment plants could not function. Therefore, water supply in the southern Taoyuan area was forced to decrease for three weeks, and the social and industrial activities were seriously affected.

In this study, a mobile-bed numerical model using the explicit finite analytic model was adopted to simulate the sediment concentrations at the intakes and the deposition and scour situations of the downstream Ta-han Creek due to the flushing of

Reservoir. In addition, the feasibility evaluation on the deposition of the excavated material from the reservoir in the downstream reach was also executed.

According to the simulated results, significant deposition and scour occur in the back pond with small magnitude of bed change in the downstream reach under the high-concentrationed sediment flushing condition. The sediment concentrations at the intakes, such as the back pond weir and Yin-San weir, have about 6,800ppm during the flood release period. This high-concentrationed flow will force the treatment plant to shut down.

The excavated material from the reservoir is assumed to be deposited in the downstream reach Hou-Tsun weir. By considering the cohesive property of the excavated material, half of the deposition is estimated to flush away under the condition of Matsa typhoon flood.

The flushing of high-concentrationed flow from reservoir can increase storage capacity, but the sediment concentrations at the intakes keep high during flood release period so that treatment plant should find other plans to make sure water can supply normally. The deposition of the excavated material from the reservoir in the downstream reach may be feasible on the premise that it would not affect the flood-prevention work and some hydraulic laws should be modified. The cohesive property of the excavated material deposited on the channel leaves a lot for further discussions, and how much should be deposited is also another topic to study.

Keywords: reservoir release floodwater, channel aggradations and degradations, EFA model 3,000NTU)，亦因而被迫停止供水，使得南 桃園地區無法供應自來水長達三週，影響民

二、

雙曲線型方程式之特性，針對移流項之部份 (advection-reaction term)與擴散項(diffusion term)二部份，所以輸砂方程式之結合演算步 驟，首先將懸浮載質量守恆之移流及反應項 與作用層質量守恆方程式、整體河床輸砂質 量守恆方程式，利用 Newton-Raphson 疊代 聯立求解；所得各變數之結果再與懸浮載質 13,679ppm，其中 D1、D2 粒徑各佔 49.5%，

D3粒徑佔 1%。 modified method。

5.2 清淤泥砂堆置河道案例

置於第 53、52、51 三個斷面，如圖 5，堆高 化約在+0.4~-0.4m 之間；在河心距 16,000m 處有一明顯之沖淤變化，此處前、後斷面其 6,800ppm上下，顯示排淤過程中水體之泥砂 濃度是處於高濃度之狀態。圖 13 為後池堰

游約保持在 4,000ppm 上下，其餘粒徑之濃 度小於 1,000ppm，在堆置區段處可見 D1、

D2 粒徑之濃度有些許變大，這是由於堆置 13,679ppm 的高含砂濃度排砂，後池內 的沖淤約在+0.5~-1.2m 之間，後村堰前 的沖淤約在+0.4~-0.4m 之間，下游段的 沖淤變化不大。

2. 模擬時間內，後池堰與鳶山堰處 D1、

D2粒徑懸浮質濃度各保持在 6,800ppm 上下，D3~D6 粒徑濃度則都在 500ppm 以下。

3. 水庫排放之粒徑主要為 D1、D2，因此 unsteady Navier-Stokes equations using explicit finite analytic Scheme.” Ph.D. Interior, Bureau of Reclamation Technical Service Center, Denver Colorado.

[3] 經濟部水利署(1986)，「大漢溪治理規

納莉颱風石門排洪流量歷線

0 2000 4000 6000 8000 10000 12000

河心距(m)

大漢溪段底床高程沖淤變化

0 5000 10000 15000 20000 25000 30000 35000

河 心 距( m)

0 5000 10000 15000 20000 25000 30000 35000

河心距( m)

0 2000 4000 6000河心距(m)8000 10000 12000 14000

濃度(ppm) 粒徑D1

0 2000 4000 6000河心距(m)8000 10000 12000 14000

濃度(ppm)

大漢溪下游段4 hr後懸浮質濃度沿程變化

0 2000 4000 6000 8000 10000 12000 14000

河心距(m)

0 2000 4000 6000 8000 10000 12000 14000

河心距(m)

0 2000 4000 6000 8000 10000 12000 14000

河心距( m)

圖20大漢溪下游段t=8hr懸浮載源沿程變化

16 hr後懸浮載源沿程變化

0 2000 4000 6000 8000 10000 12000 14000

河心距( m)

0 2000 4000 6000 8000 10000 12000 14000

河心距(m)

0 2000 4000 6000 8000 10000 12000 14000

河心距

0 2000 4000 6000 8000 10000 12000

河心距(m)

0 2000 4000 6000河心距(m)8000 10000 12000 14000

濃度(ppm)

清淤泥砂堆置案例 4hr 懸移質濃度沿程變化

0 2000 4000 6000 8000 10000 12000 14000

河心距( m)

0 2000 4000 6000 8000 10000 12000 14000

河心距(m)

0 2000 4000 6000 8000 10000 12000 14000

河心距(m)

0 2000 4000 6000 8000 10000 12000 14000

河心距(m)

0 2000 4000 6000 8000 10000 12000 14000

河心距(m)