水文水質モデルを用いた釜房ダム流域における豪雨による高濁度化の浄水処理システム影響評価

摘要

気候変動による頻発が懸念される集中豪雨は水道原水の濁度上昇を招き、浄水システムに負荷を与える。本研究では、仙台市水道局の水源のひとつである釜房ダム流域を対象として、降雨とダム湖内濁度の関係を定量化し、濁度増加を通じた、降水強度増加の浄水コストに与える影響の評価法を例示することを目的とした。まず、レーダー•アメダス解析雨量によって見積もられる流域降雨を入力として、ダムへの総河川流入量及びダム湖内濁度を十分に予測しうる準分布型水文水質モデルをSWAT(Soil and Water Assessment Tool)上に構築した。また、濁度上昇の水道システム負荷の代表的な例として、浄水薬品(ポリ塩化アルミニウムと石灰)購入費及び浄水発生土の運搬と処分にかかる費用を取り上げ、釜房ダム湖水を原水とする仙台市水道局茂庭浄水場の実測値をもととして、濁度とこれらの費用の関係を推算した。具体的には期間中の3つの降水イベントを対象とし、各々について回帰式にて近似的に表現した。さらに、さきのモデル計算結果とこれらの式を組み合わせて、降水強度の増加と浄水薬品費及び浄水発生土処分費増加の関係を明らかとした。今回提示したこの一連の手法は、気候変動を踏まえた 長期的な視点に立った水道計画の立案の一助になり得る。%Heavy rains, possibly caused by climate change, increase the turbidity of the raw watar, and therefore increase the burden on the water supply system. This study was performed to quantify the relationships between rains in a watershed and the resultant turbidity changes, and to determine a method to estimate the increase in water treatment costs due to the increase in precipitation intensity through turbidity change, with a drinking water resource (Kamafusa Dam reservoir, which is one of the water resources of the Sendai City Waterworks Bureau, Miyagi, Japan) as the target. First, a semi-distributed hydrological model was developed on SWAT (Soil and Water Assessment Tool). With reference to the rains tn the watershed calculated with the Radar-AMeDAS data as input, the developed model yielded acceptable predictions of the total water inflow and the turbidity of the reservoir. Equations describing the quantitative relationships between turbidity and representative burdens on water purification (i.e., the purchase costs of polyaluminum chloride and lime, and transportation and processing costs of water-purifying sludge) were derived based on actual observations at the Mohiwa Water Purification Plant (Sendai City Waterworks Bureau), which uses the Kamafusa Dam as its water resource. The effects of heavy rains on these costs were calculated using the model and equations. The techniques discussed here are available for use in the design of long-term water supply plans taking climate change into consideration.