• Proceedings of the Korea Water Resources Association Conference
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• 2012.05a
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• pp.534-534
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• 2012

최근 집중호우 등으로 인하여 유역에 생산된 토사가 하도에 유입되면서 하상 및 지형변동을 일으키고 있으며, 보 및 댐 상류에서 유사의 퇴적으로 인하여 홍수위 변화와 저수지 기능감소 등 많은 변화를 가져오고 있다. 이로 인하여 댐 및 보의 지속적인 유지관리를 위하여 유사관리 방안이 절실하게 요구되고 있다. 본 연구에서는 낙동강에 건설된 상주보와 구미보를 대상으로 CCHE2D모형을 이용하여 보의 상류에 퇴적된 토사의 토출(배출(排出), SPT)효과를 분석하였다. 토사 토출 효과를 예측하기 위하여 2차원 하상변동 수치모형인 CCHE2D를 이용한 하상변동 과정을 모의한 결과, 보 상류에서는 수문을 통과하여 흐르는 유속으로 인하여 하상이 저하 되었으며, 보 하류에서는 상류에서 유입되는 유사량과 지형적인 영향을 받아 세굴과 퇴적이 반복되는 경향을 보였다. 상류에서 2년빈도 유량, $1,500m^3/s$, $1,000m^3/s$, 풍수량으로 48시간 동안 상주보와 구미보로 유입될 때, 수문이 완전히 열린 상태로 있을 경우에, 상주보와 구미보에서 배사효과를 예측하기 위하여 유사 전달(Sediment delivery)을 분석하였다. 상주보와 구미보에서 배사효과를 예측하기 위하여 유사 전달을 분석한 결과, 상주보는 2년빈도 유량인 $3,857m^3/s$로 48시간 동안 상주보로 유입될 때, 보에서 하류로 전달되는 유사량은 약 4,400 tons 정도로 산정되었다. 유량인 $1,500m^3/s$일 때, 약 2,700 tons이 하류로 전달되지만, 최대 유사전달이 발생되는 지점은 보 하류에서 발생하였다. 풍수량 인 $207.4m^3/s$ 일 때, 1,357 tons이 하류로 전달되며, 최대 유사전달이 발생되는 지점은 상주보에서 발생하였다. 구미보는 2년빈도 유량인 $5,400m^3/s$로 48시간 동안 상주보로 유입될 때, 보에서 하류로 전달되는 유사량은 약 3,216 tons 정도로 산정되었다. 유량이 $1,500m^3/s$일 때, 약 73 tons이 하류로 전달되지만, 최대 유사전달이 발생되는 지점은 보에서 발생하였다. 유량인 $1,000m^3/s$일 때, 약 16 tons이 하류로 전달되며, 풍수량 인 $129.7m^3/s$일 때, 28 tons이 하류로 전달되었다. 상주보와 구미보 상류에서는 유사 전달이 감소하여, 유사가 약간 퇴적되는 경향을 보이지만, 전 구간에 걸쳐서 대체적으로 유사 전달이 크게 변하지 않고 있다. 보 상류를 제외하고는 전제적으로 하상고의 변화가 거의 없이 평형 상태를 유지하고 있는 것을 의미한다. 따라서, 상류에서 2년빈도의 홍수량이 유입될 때, 수문을 완전히 개방할 경우에 배사 효과가 크게 있는 것을 알 수 있다.

• Proceedings of the Korea Water Resources Association Conference
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• 2015.05a
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• pp.237-237
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• 2015

The district of Marlborough has had more than its share of river management projects over the past 150 years, each one uniquely affecting the geomorphology and flood hazard of the Wairau Plains. A major early project was to block the Opawa distributary channel at Conders Bend. The Opawa distributary channel took a third and more of Wairau River floodwaters and was a major increasing threat to Blenheim. The blocking of the Opawa required the Wairau and Lower Wairau rivers to carry greater flood flows more often. Consequently the Lower Wairau River was breaking out of its stopbanks approximately every seven years. The idea of diverting flood waters at Tuamarina by providing a direct diversion to the sea through the beach ridges was conceptualised back around the 1920s however, limits on resources and machinery meant the mission of excavating this diversion didn't become feasible until the 1960s. In 1964 a 10 m wide pilot channel was cut from the sea to Tuamarina with an initial capacity of $700m^3/s$. It was expected that floods would eventually scour this 'Wairau Diversion' to its design channel width of 150 m. This did take many more years than initially thought but after approximately 50 years with a little mechanical assistance the Wairau Diversion reached an adequate capacity. Using the power of the river to erode the channel out to its design width and depth was a brilliant idea that saved many thousands of dollars in construction costs and it is somewhat ironic that it is that very same concept that is now being used to deal with the aggradation problem that the Wairau Diversion has caused. The introduction of the Wairau Diversion did provide some flood relief to the lower reaches of the river but unfortunately as the Diversion channel was eroding and enlarging the Lower Wairau River was aggrading and reducing in capacity due to its inability to pass its sediment load with reduced flood flows. It is estimated that approximately $2,000,000m^3$ of sediment was deposited on the bed of the Lower Wairau River in the time between the Diversion's introduction in 1964 and 2010, raising the Lower Wairau's bed upwards of 1.5m in some locations. A numerical morphological model (MIKE-11 ST) was used to assess a number of options which led to the decision and resource consent to construct an erodible (fuse plug) bank at the head of the Wairau Diversion to divert more frequent scouring-flows ($+400m^3/s$)down the Lower Wairau River. Full control gates were ruled out on the grounds of expense. The initial construction of the erodible bank followed in late 2009 with the bank's level at the fuse location set to overtop and begin washing out at a combined Wairau flow of $1,400m^3/s$ which avoids berm flooding in the Lower Wairau. In the three years since the erodible bank was first constructed the Wairau River has sustained 14 events with recorded flows at Tuamarina above $1,000m^3/s$ and three of events in excess of $2,500m^3/s$. These freshes and floods have resulted in washout and rebuild of the erodible bank eight times with a combined rebuild expenditure of $80,000. Marlborough District Council's Rivers & Drainage Department maintains a regular monitoring program for the bed of the Lower Wairau River, which consists of recurrently surveying a series of standard cross sections and estimating the mean bed level (MBL) at each section as well as an overall MBL change over time. A survey was carried out just prior to the installation of the erodible bank and another survey was carried out earlier this year. The results from this latest survey show for the first time since construction of the Wairau Diversion the Lower Wairau River is enlarging. It is estimated that the entire bed of the Lower Wairau has eroded down by an overall average of 60 mm since the introduction of the erodible bank which equates to a total volume of $260,000m^3$. At a cost of $$0.30/m^3$ this represents excellent value compared to mechanical dredging which would likely be in excess of $$10/m^3$. This confirms that the idea of using the river to enlarge the channel is again working for the Wairau River system and that in time nature's "excavator" will provide a channel capacity that will continue to meet design requirements.