Presentation Information
[O12-P05]The Role of Leaf Mold as Riparian Soil in the Upper Reaches of a River
*Yua Makioka1, Tomoaki Shibata1, Nanoka Mishima1 (1. Ibaraki Prefectural Hitachi First High School)
Keywords:
Leaf Mold,Water permeability,Water retention
Introduction
This study focuses on the role of leaf mold as riparian soil in the upstream region of a river.
With the aim of mitigating river flooding, we selected the Miyata River, located near our school,
as a model system. Previous studies have shown that the Miyata River is equipped with well-
established flood control infrastructure. However, flooding occurred during a linear rainband event
associated with Typhoon No. 13 in 2023. This suggests that improvements in riverbank soil
properties may also be necessary for flood mitigation.
Based on previous literature, we hypothesized that humus (leaf mold) in the riverbank soil of the
Miyata River contributes to flood mitigation due to its hydrological properties, and we initiatedexperimental investigations.
Objective
The objective of this study is to evaluate the water retention properties of leaf mold in order to
test the above hypothesis.
In this study, materials with high water retention are defined as those exhibiting both high
permeability and high water-holding capacity. Permeability refers to the ability of the material to
allow water to pass through without surface accumulation when water is supplied rapidly. Water
retention refers to the ability of the material to store water within its pore spaces. The amount
of retained water is defined as the difference between the inflow and outflow water volumes.
Methods
The initial experimental setup is shown in Figure 1. A perforated plastic cup was used to simulate
rainfall infiltration into leaf mold. Two identical cups were prepared and labeled Cup 1 and Cup 2.
The input water volume was varied from 130 mL to 170 mL in 10 mL increments.
Water dripping through a coffee dripper and overflow occurrence were observed, with overflow
defined as a flooding condition. The discharged water collected in a graduated cylinder was
measured every 30 seconds for 6.5 minutes. Each condition was repeated three times.
Results
The results are shown in Table 1 and Figure 2. Although the retained water volume varied with input
water volume, no clear proportional relationship was observed.
Furthermore, estimated flow velocities from Cups 1 and 2 were 5907 mm/h and 2672 mm/h,
respectively, indicating that the experimental conditions corresponded to unrealistically high
rainfall intensities.
Improvement of Experimental System
To improve reproducibility and accuracy, the experimental apparatus was modified. The target
rainfall intensity was set to 20 mm/h, classified by the Japan Meteorological Agency as heavy rain.
A siphon-based system was introduced to control flow rate.
The schematic of the improved apparatus is shown in Figure 3. Flow rate was controlled by adjusting
the hydraulic head difference in the system.
The flow rate was calculated using Torricelli’s theorem:
Q = Av = ACv(2gh)^(1/2),
where Q is flow rate, A is the cross-sectional area of the tube (4.0π * 10^-4 m²), Cv is the
discharge coefficient, g is gravitational acceleration, and h is the water head difference.
The calculation indicated that a head difference of approximately 1.0 cm is required to reproduce a
rainfall intensity of 20 mm/h.
Results (Improved Experiment)
The results are shown in Table 2 and Figure 4.
Discussion
From Table 2, the permeability limit of humus was estimated to lie between 5.8266 and 5.8267 cm³/s.
As shown in Figure 4, approximately 10 cm³ of water permeated within 1 minute after the start of
irrigation, after which no significant change in water retention was observed. This suggests that
50 g of humus (approximately 308.24 cm³ in volume) can retain approximately 10 cm³ of water.
In addition, approximately 10 g of water was retained per 50 g of humus.
Overall, the results indicate that humus can retain approximately 20% of its own volume in water,
and the permeability limit corresponds to approximately 2670 mm/h.
Future Work
Future work will involve more precise experimental validation of permeability and water retention
using the improved apparatus.
This study focuses on the role of leaf mold as riparian soil in the upstream region of a river.
With the aim of mitigating river flooding, we selected the Miyata River, located near our school,
as a model system. Previous studies have shown that the Miyata River is equipped with well-
established flood control infrastructure. However, flooding occurred during a linear rainband event
associated with Typhoon No. 13 in 2023. This suggests that improvements in riverbank soil
properties may also be necessary for flood mitigation.
Based on previous literature, we hypothesized that humus (leaf mold) in the riverbank soil of the
Miyata River contributes to flood mitigation due to its hydrological properties, and we initiatedexperimental investigations.
Objective
The objective of this study is to evaluate the water retention properties of leaf mold in order to
test the above hypothesis.
In this study, materials with high water retention are defined as those exhibiting both high
permeability and high water-holding capacity. Permeability refers to the ability of the material to
allow water to pass through without surface accumulation when water is supplied rapidly. Water
retention refers to the ability of the material to store water within its pore spaces. The amount
of retained water is defined as the difference between the inflow and outflow water volumes.
Methods
The initial experimental setup is shown in Figure 1. A perforated plastic cup was used to simulate
rainfall infiltration into leaf mold. Two identical cups were prepared and labeled Cup 1 and Cup 2.
The input water volume was varied from 130 mL to 170 mL in 10 mL increments.
Water dripping through a coffee dripper and overflow occurrence were observed, with overflow
defined as a flooding condition. The discharged water collected in a graduated cylinder was
measured every 30 seconds for 6.5 minutes. Each condition was repeated three times.
Results
The results are shown in Table 1 and Figure 2. Although the retained water volume varied with input
water volume, no clear proportional relationship was observed.
Furthermore, estimated flow velocities from Cups 1 and 2 were 5907 mm/h and 2672 mm/h,
respectively, indicating that the experimental conditions corresponded to unrealistically high
rainfall intensities.
Improvement of Experimental System
To improve reproducibility and accuracy, the experimental apparatus was modified. The target
rainfall intensity was set to 20 mm/h, classified by the Japan Meteorological Agency as heavy rain.
A siphon-based system was introduced to control flow rate.
The schematic of the improved apparatus is shown in Figure 3. Flow rate was controlled by adjusting
the hydraulic head difference in the system.
The flow rate was calculated using Torricelli’s theorem:
Q = Av = ACv(2gh)^(1/2),
where Q is flow rate, A is the cross-sectional area of the tube (4.0π * 10^-4 m²), Cv is the
discharge coefficient, g is gravitational acceleration, and h is the water head difference.
The calculation indicated that a head difference of approximately 1.0 cm is required to reproduce a
rainfall intensity of 20 mm/h.
Results (Improved Experiment)
The results are shown in Table 2 and Figure 4.
Discussion
From Table 2, the permeability limit of humus was estimated to lie between 5.8266 and 5.8267 cm³/s.
As shown in Figure 4, approximately 10 cm³ of water permeated within 1 minute after the start of
irrigation, after which no significant change in water retention was observed. This suggests that
50 g of humus (approximately 308.24 cm³ in volume) can retain approximately 10 cm³ of water.
In addition, approximately 10 g of water was retained per 50 g of humus.
Overall, the results indicate that humus can retain approximately 20% of its own volume in water,
and the permeability limit corresponds to approximately 2670 mm/h.
Future Work
Future work will involve more precise experimental validation of permeability and water retention
using the improved apparatus.