Research Trends and Results

Toward setting the frequency of weeding of levee
required for flood control function maintenance
HATTORI Atsushi (Ph.D., Engineering), HeadFUKUHARA Naoki, Researcher
River Division, River Department(keywords)

levee, vegetation management, erosion, penetration

1.

Issues in executing rational management
Weeding of river levee has a function to enable to get
the situation of the surface of dike body on the occasionof visual inspection and assist to surely detect damages ordeformations, and has an effect of maintaining yieldstrength of the slope against erosion and slide by
preventing tall vegetation to grow and the surface tobecome soil. Weeding is carried out twice a year inprinciple in case of dikes under national control. It will
possibly lead to further rationalization of maintenance if
an appropriate frequency of weeding is set up in
consideration of the flow rate and duration of action atflooding, the different dike structure and soil property, etc.
for each section of dikes. Depending on the location,
the frequency of weeding is forced to reduce, but in thatcase, it must be confirmed in advance that enough yieldstrength is maintained even if tall vegetation with thepossibility of transitions grows.
Although theknowledge about short vegetation represented by zoysia
and cogon grass has been accumulated in terms of theeffect to prevent erosion to what extent1), the knowledge
about tall vegetation represented by common reed andreed is not enough. Here is a technological issue which
is a bottleneck to judge whether it is possible torationalize management which evaluates yield strengthand secures the prescribed flood control function as
before, while allowing transitions of growing vegetationon the slope of dike from short vegetation to tall one.
2.

Full-scale test using specimen obtained from theactual dikes
River Division conducted a hydraulic test to check theerosion resistance against flood flows and the stability
against sliding rupture caused by penetration of rain falls,etc. with the cooperation of the head office of Ministry ofLand, Infrastructure, Transport and Tourism and each
Regional Development Bureau. The biggest feature ofthis test was that the test was conducted under simulated
conditions of actual flood flows and rain falls by
sampling in steel frames a part of the slope with growing
vegetation from the actual dike just as it was, andbringing them into NILIM.
In the test concerning erosion resistance, thespecifications of vegetation (growth density, plant’s
height and leaf area, etc.) were measured first and fall
over height of vegetation and the flow rate near theground surface reduced by vegetation and the progression

rate of erosion, etc. were measured by providing the flowrate nearly equal to the one at actual floods. In addition,in the test concerning the stability against sliding rupture,coefficient of permeability (the degree of waterpenetration) by rhizome mixing in with the soil and thechange of soil resistance were examined.
3.

Obtained knowledge and its utilization
It was found that even under the situation covered byplants, if they were cleared by the flow rate near theground surface, the progression rate of erosion has thesimilar relation with the bare ground not covered byplants. In other words, it was thought that the reductionof flow rate near the ground surface by stalks and leavesof plants was the mechanism to exert the erosion
resistance of tall plants(2).
Furthermore, it wasconfirmed, comparing to the test, that the flow rate near
the ground surface can be calculated using the hydraulicmodel which gives vegetation specifications andhydraulic conditions as parameters. Utilizing theseobservations, we propose a method to evaluate thepossibility to prevent erosion against how much flow rateby organizing the calculation results of variable numbers
such as kinds of plants, the height of plants and numberof growing plants per unit area.
Rivers throughout the country have different hydraulicconditions, etc. according to the location. Therefore, byassembling the above outcomes, it will be possible todetermine the place where the flood control function canbe maintained with a reduced frequency of weeding or
the place where the flood control function cannot be
maintained without an increased frequency of weeding.
It is scheduled to assemble the outcomes to be applied toin deciding an appropriate frequency of weeding at eachlocation.
[References](1) UDA et.al.: Nature-oriented river dike protection
works when flood flow occurs, cohesive soil andbehavior of vegetation, Public Works ResearchInstitute papers No. 3489, 1997
(2) Naoki FUKUHARA, Atsushi HATTORI : Test on
special characteristics of erosion of river dike slopes
where altherbosa grows, River Engineering journal
Vol. 18, 2012