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The Science Bulletin of the Faculty of Agriculture. University of the Ryukyus >
No.60 (2013/12) >

 
Title :琉球石灰岩域における降雨流出とドリーネの浸透排水に関する研究
Title alternative :Studies on Rainfall Runoff and Doline Infiltration Drainage in a Ryukyu Limestone Region
Authors :来間, 玄次
Authors alternative :Kurima, Genji
Issue Date :27-Dec-2013
Abstract :River systems are often primitive in "Ryukyu" coral limestone soil regions. Therefore, water is drained through underground infiltration from a doline. A lower infiltration capability coupled with typhoon and heavy rain often triggers flood damages, which jeopardizes agricultural crop production in these areas. Therefore, it is urgent to evaluate the effect of various measures on flood damage mitigation these areas and implement the effective measure. In order to evaluate the effectiveness of these measures, we need to clarify the relationship between rainfall-discharge and infiltration drainage through a doline in these regions. However, previous studies used a simple model assuming only penetration apertures at the bottom of infiltration ponds and were not able to sufficiently capture the roles of side apertures and limestone pipes for drainage which are characteristics to be seen in these areas.The objective of this study is to clarify and improve our understanding of the relationship between rainfall-discharge and infiltration drainage through a doline in these regions considering the complex drainage system in these areas. We considered rainfall-discharge and infiltration drainage as a series of phenomena within a model. First, we estimated the inflow to a do line based on the amount of discharge. Then, we evaluated the residues by computing the differences between the inflow to a doline and infiltration drainage through a doline. Finally, we developed a mathematical simulation model and simulates an input-output relationship of these drainage systems. In order to reflect this complex systems in the model, we varied the size of side apertures depending on the water level of a seepage basin, modeled the relationship between the cross sectional area of side apertures and the water level, and incorporated this mechanism into our simulation model.We applied this simulation model to two areas, 1) "Ashi-chaga" area where a tunnel drainage system has been introduced for mitigating flood damages, 2)"Makabe" area where several measures have been discussed for mitigating flood damages.Our simulation model was able to closely approximate the observed water level changes by explicitly modeling the effect of side apertures in "Ashi-chaga" area. Our simulation results indicate that the peak discharge for the December's rainfall event (the maximum rainfall per hour: 54.0mm, the total rainfall: 187.0mm) was 8.5lm^3/sec. We applied parameters estimated for this December's rainfall event to the heavy rain event of September, 1999 (the maximum rainfall per 24 hours: 477.0mm, the total rainfall: 555.0mm) and simulated changes in water levels. This simulation closely projected the peak water level of the heavy rain event (simulated water level was 6.8m, while the observed water level was 6.7m). Furthermore, this simulation results indicate that the peak discharge of this event was 25.91m^3/sec.
In this region, the tunnel had been constructed as a drainage measure. However, we have not experienced the heavy rain event like September, 1999 and the effectiveness of this tunnel has not been tested yet. So, we used our simulation model to examine the effect of the tunnel drainage on the degree and extent of flood damages. Our simulation model suggests that the tunnel contribute to the reduction of the maximum flood area by 65% and 82% of the total discharge.In "Makabe" area, we simulated infiltration of our study site by modeling two adjacent infiltration ponds with different infiltration capabilities, overflow among these two adjacent ponds through a subterranean drain system, and pipe flow drainage system. Our simulation model was able to closely approximate the observed water level changes by modeling this complex multiple drainage ponds system.We used the rain event data of August, 2007(rainfall per day: 457mm, the maximum rainfall per hour: 83.5mm), which caused the flood damage over 20 ha, and simulated the changes in water level. Our simulation results show that the peak water level was at the evaluation of 26.3 m, and flooding occurred on the section of approximately 11Om of prefectural road. Although, the rainoff-discharge relationship at each pond has not been quantified, our simulation results indicate that the peak discharge and the cumulative discharge at the 1'1 pond were 28.4m^3/sec and 445,029m^3 respectively, while the peak discharge and the cumulative discharge at the gth pond were 6.2m^3/sec and 113,706m^3 respectively. Furthermore, we identified the characteristics of the limestone-cave which has been observed at the downstream direction of the 151 pond thorough our simulation analysis. Our simulation model closely projected changes in water levels when the base height of this pipe is set at the elevation of 18.5m (the base height of this pond is EL=l6.5m),the diameter of this pipe is set at 80cm and the roughness coefficient (n) of 0.1 is used.Our simulation results indicate that this modeling approach is useful for evaluating proposed measures for mitigating flood damages in these regions. These results will help us improve our understanding of the complex relationship between rainfall-discharge and infiltration drainage through a doline on and will provide scientific knowledge for evaluating flood damage mitigation measures in these areas.
琉球石灰岩域では,河川が未発達な地域が多く,流域の多くは,すり鉢状の地形をなしているため,降雨はドリーネの浸透孔から,地下に排水される事例が多い.このような地域では,台風等の豪雨の度に,ドリーネの浸透能力の機能低下が原因で,周辺農地は深刻な湛水被害に見舞われ,農業経営上,大きな問題を引き起こしている.そのため,湛水被害軽減のために様々な対策を早急に検討し,実施する必要がある.これら対応策の効果を評価するためには,琉球石灰岩域における浸透排水メカニズムを解明する必要があるが,これまでの研究では浸透孔が浸透池の底部にあると仮定した単純なモデルを用いて浸透排水の分析を行ってきたため,この地域に特徴的にみられる横浸透孔や鍾乳洞のトンネル「パイプフロー」が浸透排水に果たす役割を十分に捉えることができなかった.本研究では,これらの複雑な役割を考慮し,琉球石灰岩域における降雨流出と,ドリーネによる浸透排水の関係を解明することを目的とした.解析手法としては,降雨流出と浸透排水を一連の現象として捉えた.降雨流出は特’性曲線法を用いた表面流モデルにより求め,浸透排水はトリチェリーの定理を用いてドリーネの浸透孔断面積と水位の関係を数式化して,解析モデルに組み込んだ.本研究では,異なる2つの事例を比較考察するため,この解析モデルを糸満市アシチャガー地区と真壁地区に応用した.アシチャガー地区では,湛水被害解消のためにすでにトンネルを導入している.一方,真壁地区では,湛水被害解消に向けた対策工法を現在検討中である.
アシチャガー地区では,横浸透孔を浸透排水のモデルに組み込むことで,実測水位の変動と近似するシミュレーション結果が得られた.また,このシミュレーション結果から,2000年12月降雨(最大時間雨量54.0mm,総雨量187.0mm)におけるピーク流出量は8.51mVsecであったことが明らかになった.さらに,このシミュレーションモデルで採用されたパラメータを用いて,1999年9月降雨(最大24時間雨量477.0mm,総雨量555.0mm)について水位変化をシミュレーションしたところ,ピーク水位は実測値の6.7mに対してシミュレーションでは6.8mとなり実測値に近似したシミュレーション結果となった.また,この時のピーク流出量が25.91mVsecであったことが明らかになった.現在,アシチヤガー周辺の湛水被害を解消するため,国営かんがい排水事業でトンネル(馬てい形断面2r型)による排水対策が取られているが,いまだ1999年に匹敵する大きな降雨は見られず,トンネル導入の効果が評価できていない.そこで,今回の研究では1999年の降雨データを用いて,初めてトンネル導入の効果をシミュレーションにより検証した.その結果,トンネル導入により最大湛水面積が65%程度減少し,総排出量の82%がトンネル排水によることが明らかになった.一方,真壁地区については,県道と隣接した1号浸透池と8号浸透池の2つの浸透池における異なる浸透能,暗渠による浸透池問の越流,県道のオーバーフローによる越流,パイプフローによる排水等の形状・寸法を解析モデルに組み込み,シミュレーションにより降雨流出と浸透排水の仕組みを再現することができた.真壁地区において20haが湛水し,施設園芸作物等に多大な被害を及ぼした,2007年8月の降雨(日雨量457mm,最大時間雨量83.5mm)について,水位変化をシミュレーションした結果,湛水のピーク水位が標高EL=26.3mであり,県道の道路長約llOmの区間で冠水が生じていたことが明らかになった.また,これまでは,各浸透池における降雨流出量が不明であったが,このシミュレーションの結果,1号浸透池におけるピーク流出量は28.4m^3/sec,累加流出量は445,029m^3,8号浸透池については,それぞれ,6.2m^3/sec,113,706m^3であったことが明らかになった.パイプフローについては,1号浸透池より下流方向に鍾乳洞の存在が確認されていたが,本研究では実測水位と整合するパイプの特徴を明らかにすることができた. パイプ底高は標高EL=18.5m(浸透池の底高EL=16.5m)であり,その直径がD=80cm,粗度係数n=0.1の時に実測水位と最も一致した.これらの研究成果は,今後の琉球石灰岩域における湛水被害解消に向けた対応策を検証するときに,新たな知見を提供し,応用されていくものと期待される.
Type Local :紀要論文
ISSN :0370-4246
Publisher :琉球大学農学部
URI :http://hdl.handle.net/20.500.12000/28934
Citation :琉球大学農学部学術報告 = The Science Bulletin of the Faculty of Agriculture. University of the Ryukyus no.60 p.1 -43
Appears in Collections:No.60 (2013/12)

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