江戸城外濠における水質改善施策のモデル評価と費用便益分析

摘要

江戸城外濠におけるアオコの発生などによる悪臭，景観への影響を改善するための水質改善施策を，水量,水質，コスト，便益の多方面から定量的•総合的に評価し，その有効性を示した。まず，流出解析モデルにより主な水源であるCSOの流入量評価を行い，その結果と水位•水質の現地測定結果を基に，現状を再現できる水質モデルを構築した。次に，有効な施策としてCSO対策に加えて下水再生水の導水を考え，アオコの発生を防ぐことが可能な最低導水量4,653 m~3·日~(-1)を求めた。そして，最低導水量を基に施策に必要となるLCC：3.9億円•年~(-1)を求め，CVMにより算定した施策の便益7.5億円•年~(-1)と比較することにより，施策の有効を示した。%The Edo Castle is located in central Tokyo and its Outer Moat provides a precious water environment in the urban area. The water sources are limited to precipitation and combined sewer overflow (CSO); therefore, the hydraulic retention time (HRT) becomes very long (tens of days), resulting in the poor water quality. Particularly in summer, serious algal bloom is observed, and it causes the problems of bad landscape and undesirable odor. To develop effective countermeasures for water quality improvement, the estimation of CSO inflow and water-quality prediction by means of runoff and algal growth models were conducted. Furthermore, the actual water level and water quality were regularly measured through field surveys for checking the adequacy of models. The analysis with the models proved that the combination of CSO prevention and introduction of reclaimed water (4,653 m~3·day~(-1)) could prevent the algal blooms. The cost of the countermeasure was estimated to be 390 million yen·year~(-1). The benefit was evaluated by the contingent valuation method (CVM) and estimated to be 750 million yen·year~(-1).Finally, the cost and benefit were compared and the feasibility of the countermeasures was shown.