٨Research Trends and ResultsLevee Management
Project
—To clarify effectiveness of vegetation
management in maintaining flood control
functions of levees and reflect the findings inlevee management—
HATTORI Atsushi, Head FUKUHARA Naoki, Researcher
NOBUTSUNE Hirotsugu, Researcher
River Division, River Department

Research Trends and Results(Key words) Levee vegetation management, flood control functions of levees
1. Introduction
To more effectively and efficiently manage riverlevees in the future, it will probably be important toset the maintenance level according to external force
conditions, conditions of the hinterland, etc. While theeffectiveness of mowing, inspections, river patrols andother maintenance activities in maintaining the levee’s
flood control functions have been partly studied 1),
their effectiveness is still inadequately clarified, socompleting technologies to assess the relationship ofmaintenance activities with maintaining flood control
functions remains a challenge.
2. Levee Management Project
The Levee Management Project is a framework forthe joint surveying of levees and development oftechnologies by the head office of the Ministry ofLand, Infrastructure, Transport and Tourism, National
Institute for Land and Infrastructure Management, thePublic Works Institute, and the regional development
bureaus in order to clarify the effects of levee slope
vegetation management on erosion resistance to flood
flow and on slip stability under seepage, and topropose
necessary
maintenancelevelsandmaintenance intervals.
 The project was planned to meet the following threechallenges.
Challenge 1: Clarifying change over time ofvegetation transition (flourishing of tall herbaceousplants), thickness of the loose layer of the surface soil,and strength or permeability inside the loose layer oflevee slopes according to mowing frequency
Challenge 2: Quantitatively clarifying the bearing
strength (erosion resistance, seepage resistance,resistance to slope sliding) of levees according to thequality of the levee (loose level, types of vegetation
flourishing)
Challenge 3: Setting the management level at theminimum necessary based on the above.
Studying the frequency of mowing and removing
and replacing the loose soil layer of the slope in orderto maintain flood control functions by quantitatively
evaluating the levee’s bearing strength (challenge 2)
according to change over time (challenge 1) of a levee
according to differences in the management level(frequency of mowing). And also studying how toreflect the indices for plant communities or loose layer
thickness etc. in levee inspection items (Challenge 3).
3. Towards reflection in the field
As an example of the project, the following areinterim results of a large-scale water channel
experiment using specimens taken in undisturbed statefrom an actual levee (Challenge 2). Assuming thaterosion resistance capacity is mainly impacted by root
hairs on roots of vegetation, the roots were classified
as “vegetation consisting mainly of root hairs(Japanese lawn grass, cogon grass)”, “vegetationconsisting mainly of tap roots (thick roots) (tallgoldenrod)”, “vegetation consisting of mixed roothairs and tap roots (nezasa (2NGKQDNCUVWUEJKPQXCTXKTKFKU))” and then compared, and based on theresults, it was concluded that the erosion resistancecapacity of “vegetation consisting mainly of root
hairs” was greater than that of the other types 2).
The achievements of this project will be reflected insetting the maintenance level, and we wish to continuethis study in order to contribute to more efficientmaintenance and to the improvement of the quality ofriver levees.
[Sources]
1) Uta et al.: Hydraulic stability and erodibility ofrich-in-nature-type bank protection works, cohesive
soil and short vegetation., Technical Note of PWRI,
No. 3489, 1997
2) S. Ochiai, A. Hattori, H. Nobutsune, N. Fukuhara:
Test of erosion resistance according to differences invegetation, No. 65 Proceedings of annual conferenceof the Japan Society of Civil Engineers, 2010