丘陵流域における二つの流量ピークから成る流出現象とその生起条件 -とくに貯留量の閾値に着目して- Double-Peak Storm Runoff Observed at a Hilly Watershed in the Soya Hills, Northern Japan and the Conditions in Which It Occurs with Special Respect to Basin Water Storage

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The initial moisture conditions of a watershed immediately before a rainfall event begins affect the characteristics of the subsequent storm runoff. The authors found a drainage basin in the Soya Hills, northern Japan, where two different types of flood hydrograph are clearly distinguished for each flood event, and investigated the effect of the initial moisture conditions of the watershed and rainfall on the shape of the hydrograph.<br> The watershed studied (Fig. 1) is underlain by Neogene shale (Wakkanai formation, Wk). With joint planes about 10cm apart and fissure zones approximately 10m apart, Wk is permeable and does not constitute a hydrological basement. As a result of water level observation on an experimental watershed (drainage area A=0.69km<sup>2</sup>) in a Wk-hill, two types of flood event are recognized.<br> Type-1 floods (Fig. 2-a) have only one runoff peak corresponding to each rainfall event. This peak appears within 2 hours after the termination of rainfall.<br> Type-2 floods (Fig. 2-b) have two runoff peaks for one rainfall event. The time lag between the primary runoff peak and the termination of rainfall is similar to that of Type-1 flood events, while the secondary runoff peak occurs 7 to 26 hours after the primary one. This time lag varies for each event (Table 2).<br> The runoff ratio of Type-2 flood events are more than 10times greater than Type-1 flood events (Fig. 4). For Type-2 flood events, 90% of the total storm runoff is produced by the secondary peak runoff.<br> Type-1 floods and the primary peak runoff of Type-2 floods have similar runoff ratios (Fig. 5). These runoffs are explained as the direct rainfall runoff and shallow groundwater discharge from moist areas near the channel.<br> The conditions under which the secondary peak runoff occurs are expressed by the combination of initial discharge (<i>q</i><sub>0</sub>) and rainfall (<i>P</i><sub>1</sub>) before the termination of primary peak runoff (Fig. 6); the secondary peak runoff appears when either <i>q</i><sub>0</sub> or <i>P</i><sub>1</sub> is sufficiently large, while it is not detected when both <i>q</i><sub>o</sub> and <i>P</i><sub>1</sub> are small.<br> The authors defined the flood-controlling storage (<i>S</i>) as the amount of water storage in a basin directly contributing to storm runoff and attempted to express this value as a function of discharge (<i>q</i>). First, a two-component recession formula for one recession period is established as<br> <i>q</i>(<i>t</i>)=<i>e</i><sup>-0.0250<i>t</i>-1.78</sup>+1/(0.0103<i>t</i>+3.67)<sup>2</sup><br> where <i>t</i> is the time (h) after the beginning of recession and <i>q</i> (<i>t</i>) is discharge (mm/h) at <i>t</i>.<br> Then, using the theoretical relationship between <i>q</i> and <i>S</i> for each runoff component, we obtain<br> <i>S</i>(<i>t</i>)=<i>e</i><sup>-0.0250<i>t</i>-1.78</sup>+1/(0.0103<i>t</i>+3.67)<sup>2</sup><br> where <i>S</i>(<i>t</i>) is the flood-controlling storage (mm) at <i>t</i>. These two formulae implicitly represent the relationship between <i>q</i> and <i>S</i> (Fig. 7). Using this relationship, conditions under which Type-2 flood events occur are expressed as<br> <i>S</i><sub>0</sub>+<i>P</i><sub>1</sub>>32mm<br> where <i>S</i><sub>0</sub> is the initial water storage and <i>P</i><sub>1</sub> is rainfall before the termination of primary peak runoff (Fig. 8). There exists, however, an example when the secondary peak is not clearly detected when <i>S</i><sub>0</sub><20mm.<br> The value of <i>S</i><sub>0</sub>+<i>P</i><sub>1</sub> is also related to the time lag between two peaks of Type-2 flood events; the time lag <i>T</i><sub>L</sub> (h) is expressed as<br> <i>T</i><sub>L</sub>=39.5-0.512 (<i>S</i><sub>0</sub>+<i>P</i><sub>1</sub>) <i>R</i><sup>2</sup>=0.610 (Fig. 9).<br> This means that the response of secondary peak runoff is faster when the watershed is relatively wet.

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  • 地理学評論. Ser. A  

    地理学評論. Ser. A 70(12), 798-812, 1997-12-01 

    The Association of Japanese Geographers

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各種コード

  • NII論文ID(NAID)
    10002221338
  • NII書誌ID(NCID)
    AN1016484X
  • 本文言語コード
    JPN
  • 資料種別
    SHO
  • ISSN
    00167444
  • NDL 記事登録ID
    4350528
  • NDL 雑誌分類
    ZG1(歴史・地理)
  • NDL 請求記号
    Z8-571
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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