What is the new and safe levee protection solution “air permeable waterproof sheet” instead of impervious sheets?

Due to the extreme weather conditions of recent years, flood damage has been occurring in unprecedented forms, sizes, and frequencies. The damage to local communities caused by river bank failures is particularly devastating, and new solutions are needed to replace conventional construction methods in order to strengthen the national land. Levee failures due to water overtopping occur when the back slope collapses due to erosion. This article introduces a new solution, “air-permeable tarpaulin. Air-permeable waterproof sheets prevent the collapse of the back slope, which could not be prevented by conventional sheet materials. It has three major functions, and its effectiveness has been demonstrated in a 1/10 scale model experiment of an actual levee at the Disaster Prevention Research Institute of Kyoto University.

• waterproof
• breathability
• Waterproofing of joints

 

Please read this book if you are involved in river engineering projects.

Recent climate change and the safety of river levees

The “embankment principle” is stipulated in the River Management Facilities Structure Ordinance, which states that “embankments shall be constructed by embankment”. Embankments are characterized by the ease of obtaining materials, relatively low construction costs, resistance to deterioration as a structure, and ease of repair, raising, and widening. However, earthen embankments are vulnerable to deterioration in strength due to seepage into the embankment caused by prolonged water pressure during floods, scouring by running water, and erosion caused by overflow.

 

In recent years, due to significant climate change, localized heavy rains such as “guerrilla rains” and “linear precipitation zones” that were never expected in the past are now overflowing rivers over a period of several days. Floods that exceed the high water mark or exceed the planned flood level are occurring with unprecedented frequency, and the risk of overtopping river levees is increasing year by year. New measures are now required to ensure the safety of river levees.

Cause of levee failure: collapse of back slope due to erosion

一般的には堤防決壊（破堤）の原因は大きく、越水、浸透、侵食・洗掘に分けられます。また、これらが複合的な要因となって堤防が決壊することもあります。

直轄河川における堤防決壊の原因について昭和22（1947）年～44（1969）年までの事例分析を見ると、その80％以上が越水によるものであるとされています。

また、1945年以降の直轄及び県管理河川堤防の破堤事例（全678事例）の調査結果では、越水が原因となるものが50％を占めるなど、破堤原因としては越水に起因するものが多いと判断されます。

 

「令和元年台風19号の被災を踏まえた河川堤防に関する技術検討会」でも台風19号による堤防決壊原因について同様の報告がされています。

近年、甚大な被害をもたらしている堤防の決壊は侵食による裏法面の崩壊が原因です。これまで経験したことのない未曾有の超過洪水に対して、河川堤防を越水した場合であっても、決壊しにくく、堤防が決壊するまでの時間を少しでも長くするなどの防災効果を発揮する粘り強い構造の河川堤防(越水した場合であっても「粘り強い河川堤防」)の整備に必要な堤防設計のためには、裏法面の越流侵食対策が要であるということを認識する必要があります。

ところが、従来のシート材を用いた対策には課題があります。

Challenges of conventional sheet materials in back slope protection

The most common methods of protecting the slope of river embankments have been the use of “impervious sheets” and “anti-seepage sheets”. However, these methods have the following problems, and their functionality has been found to be insufficient to cope with excess flooding caused by recent extreme weather events.

Challenges with impervious sheets

Although impervious sheets can prevent water from flooding into the embankment, they cannot vent air, so the pore air pressure may increase when the water level in the embankment rises, and they are insufficient to deal with local failure (air blow phenomenon) of the embankment. In addition, the joints are vulnerable to flooding from the joints, and the joints can become the starting point for the levee to be eroded by the overflow of water.

Issues with anti-absorption sheets

吸い出し防止シートは堤体の土砂の流出を防ぎますが、遮水性がないため雨や越流水の浸透は防止できません。越流の発生時には、裏のり面を侵食する可能性があります。

対策には、従来の遮水シートや吸い出し防止シートに代わるシートが必要です。

New technology to replace conventional sheet materials: Breathable Sheet, an air-permeable waterproof sheet

• waterproof
• breathability
• Waterproofing of joints

Breathable Sheet” (air-permeable waterproof sheet) is the only sheet that solves the problems of conventional sheet materials and provides all of the above functions required to protect the back slope of river embankments. Breathable Sheet” is a three-layered sheet consisting of a water-permeable sheet sandwiched between protective mats made of long-fiber non-woven fabrics, which prevents rainwater and river water from permeating into the embankment while allowing pore air to permeate out of the embankment. It has the following excellent performance and has been registered with NETIS as a new technology.

effect

従来の遮水シートは間隙空気を透過しないため、豪雨などで河川水位が上がって浸潤線が上昇した際に、堤体内の間隙空気圧の上昇を抑制することができませんでした。

高水位時や堤防越流時になると、遮水シートに揚圧力が作用して、シートが剥がれる際に堤体に損傷を与える可能性がありました。

『ブリーザブルシート』を下図のように裏法面に使用することで、裏法面全体からの透気性能により間隙空気が排出されます。堤体とブリーザブルシートとの密着性が確保できると同時に、シート下部の堤体を保護することができます。裏法面からの降雨浸透を抑制し、越流時の侵食防止に効果があります。

 

これにより越流しても決壊しない堤防を構築でき、地域で万一の水害が発生した場合にもその被害を最小限に抑えられます。

scope (of a document)

The “Breathable Sheet” is mainly applied in the following four situations, including the protection of back slopes.

• (1) Control rainwater infiltration into the levee from the ground surface due to rainfall
• (2) Control of river water infiltration into the levee
• (iii) Prevention of pore air pressure increase
• (4) Prevention of erosion of the back slope during flooding in excess of flood levels (when water overflows the levee)

 

(1) Control rainwater infiltration into the levee from the ground surface due to rainfall

 

(2) Control of river water infiltration into the levee

 

(iii) Prevention of pore air pressure increase

 

(4) Prevention of erosion of the back slope during flooding in excess of flood levels (when water overflows the levee)

physical property

Breathable Sheet” is not only highly effective in water interception and air permeability, but also lightweight and durable. The main physical properties are shown in the table below.

Experiment at Kyoto University: Demonstration of waterproofing and air permeability effect on back slope

The effectiveness of the Breathable Sheet has been demonstrated in a 1/10 scale model test of an actual levee at the Disaster Prevention Research Institute of Kyoto University. Two small levee models were constructed with and without “Breathable Sheet” on the back slope, and water overtopping tests showed a clear difference in erosion conditions.

 

The results of the experiment demonstrated the high effectiveness of the “Breathable Sheet” in protecting the back slope. A detailed report of the experiment can be found in the following report.

> “Geosynthetics Papers”

 

summary

In order to protect river embankments from heavy rains caused by recent climate change, conventional covering methods using sheet materials are not sufficient, and new solutions are required. Breathable Sheet” is the only product that has all the necessary functions to protect the back slope: “waterproofness,” “air permeability,” and “waterproofness at joints. Please feel free to contact us for further information at the following contact point. Please feel free to contact us.

> Contact for “Breathable Sheet”, Taiyo Kogyo Corporation

> Breathable Sheet official page