内水浸水想定に用いる想定最大規模降雨のあり方

摘要

下水道事業における内水の浸水対策は，概ね5年から10年の確率年の降雨を計画規模として，管路施設などのハード整備が実施されてきたが，近年，前線性の大雨や大気不安定に伴う短時間の局所的な大雨が頻発し，浸水被害を軽減するためには，雨水貯留施設などのハード対策のみでなく，内水ハザードマツプの公表や避難経路の確認などのソフト対策を組み合せた公助，共助，自助の連携した対応が--層必要性を増している.ソフト対策を効果的に実行するためには，あらかじめ想定し得る最大の浸水状況を市民や民間企業などに周知し，それぞれが実施する避難路確保などの対応を事前に定めること，そして，最大の浸水を想定するにあたり，どのような降雨を適用するかが重要となる.そこで，本論文では内水ハザードマップ作成のための降雨のあり方について言及するため，全国の降雨実績を基に降雨の地域分布や成因を整理し，時間・空間的特性について分析した.この分析の結果，河川の「外水氾濫」に比べると，下水道の排水能力不足などが原因で生じる「内水氾濫」に適用する最大降雨は，降雨の時間•空間的特性を考慮する必要性が低いことが明らかとなった.%Inundation control measures of sewerage systems have been based on structural development targeted for rainfall of a scale with a return period of approximately 5 to 10 years. In order to mitigate flood damage by localized heavy rainfall, which has been frequently occurring in recent years, the necessity is growing for combining structural measures with non-structural measures, including the issuance of water hazard maps, etc. to the public, instead of relying only on the structural measures. For effective implementation of non-structural measures, it is essential to announce the assumed maximum inundation conditions in advance to the public and establish evacuation routes in advance. And it is important to make on the category of rainfall to be applied when assuming maximum inundation. This paper summarizes areal distribution and components of rainfall on the basis of national rainfall records, and analyzes temporal and special characteristics. It was learned that, compared with cases of river water inundation," the necessity of taking into account the temporal and special characteristics of rainfall is less for maximum rainfall applicable to "landside water inundation due to insufficient drainage capacity.