• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2015년도 학술발표회
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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.

• 한국환경생태학회지
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• 제33권2호
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• pp.187-201
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• 2019

본 연구에서는 2008년 6월과 2009년 5월 조사기간 동안 금강의 상 중 하류부의 14개 지점을 선정해서 이 화학적 수질의 특성을 파악하였으며 어류군집의 분포 및 길드 분석과 어류생물통합지수(Fish Assessment Index, FAI)의 다변수 8개 메트릭을 이용해 금강 수계의 건강도를 평가하였다. 수질 변수 평균 변화를 분석해 본 결과 중상류지점은 수질변수의 변이양상이 크게 없었고 갑천과 미호천이 합류되는 지점부터 유기물질이 축적되어 수질 변수가 급격한 변이양상을 보였다. 이 화학적 수질 특성을 분석한 결과, 미호천과 갑천이 합류되는 하류지점부터 BOD, COD, TN, TP, Cond, Chl-a는 증가하는 경향을 보였고 DO는 감소하여 부영양화와 녹조현상이 진행되고 있는 것으로 사료된다. 어류 군집 분석 결과, 종 풍부도지수와 종 다양도 지수는 중상류지점에서는 증가한 양상을 보인 반면 하류구간에서는 감소하는 양상을 보여 상류는 생태적 안정성은 양호하고 하류는 생태적으로 비교적 불안정한 것으로 보였다. 어류종 분포 분석결과 우점종은 피라미(Zacco platypus)이며 전체 어종의 20.9%를 차지하였고 아우점종은 참갈겨니(Zacco koreanus)가 13.1%를 차지했다. 어류 내성도 및 섭식길드 특성 분석에 따르면, 중상류지점에는 민감종, 충식성 그리고 여울성 어종이 우점한 반면 미호천이 유입되면서 하수종말처리장 오염원으로 영향을 받는 하류구간부터 내성종, 잡식성 그리고 정수성 어종 우점에 결정적인 영향을 준 것으로 나타났다. 따라서 우리는 수질 상태를 개선하기 위해 특히 도심 오염원의 부하량을 줄이도록 노력해야 하며 수생태계 보존과 복원에 관심이 필요한 것으로 요구된다.