●Research Trends and Results

Trial calculating impacts on maintenance costs and riverenvironments at the river improvement stage and applying
the results to river course design
HATTORI Atsushi (Dr.), HeadFUKUSHIMA Masaki(Dr.), Senior Researcher
River Department, River Division(Key words) Maintenance, flood control safety, river environment
1. River improvement and maintenance and theimportance of harmony with the river environment
In the future, river improvement guidelines must beenacted nationwide and specific river improvement
methods such as river course excavations and tree
felling etc. must be established. When that is done, itwill be important to specify concrete improvement
considering harmony between flood control and theriver environment in addition to river improvement
and maintenance intended to ensure flood control
safety. The River Division has, by introducing theconcept of a margin as sediment deposition space,developed a method of uniformly evaluating the costof river improvement and maintenance, and proposeda method of minimizing the total cost of rivermanagement1). It will be a particularly useful method
on river courses where sediment tends to be depositedat segment change points and at the river mouth. As aresult, it was possible to deal with improvement andmaintenance as a single activity, but as pointed out
under Neo-Natural River Reconstruction, it is more
important to strive to improve a river to its form
several decades in the future than to its presentcondition. To do so, change of the river course must
be predicted to assess its indirect impact on the riverenvironment, then concrete improvement measures
must be established.
2. Evaluating the impact on living organisms ofchange of the physical infrastructureRiver improvement and maintenance directly alter
the physical infrastructure, so for this study, instead ofpreparing a complex ecosystem change model
including competition between living organisms, aliving organism response model in conformity withthe change of the physical infrastructure is preparedand its impact on the river environment is evaluated.
Specifically, the riverbed change calculation model
which considers the advance and retreat of vegetation,
and a living organism model which responds tochange of the physical infrastructure were linked toprepare a river ecosystem change prediction model
capable of evaluating the indirect impact after a flood
discharge in addition to the direct impact of river
improvements (below called the “change predictionmodel”). The concept of flow capacity margin wasadded to this to perform integrated evaluations of flow

capacity, impact on living organisms, and maintenancecosts under the guidelines to river management,
multiple drafts of which had been provided. Thechange prediction model was prepared for the Kita
River on the Gokase River System, where large-scaleriver course excavation had been executed about tenyears earlier forming a research field for river ecology
research. With the focus on the crab, Deiratonotus
japonicas, the fish, Sicyopterus japonicas, and theraccoon dog, which are typical life on the Kita River,
the response of these living organisms to the river
improvements were evaluated. For example, in a casewhere a high water channel has been excavated toconserve the underwater environment, it is relatively
difficult to evaluate its indirect impact on livingorganisms, and there are even cases where flood
discharge after the improvement greatly changed thewater route or riverbed materials, resulting in thesharp deterioration of the underwater environment.
3. Directions of studies to establish concrete
improvement methods
Calculations to reproduce the short-term change ofthe physical infrastructure, which was subjected toseveral flood discharge processes, were done toevaluate their impact on the decline of flow capacity
and the living organisms. To decide on concrete river
improvement methods, conditions for a study
extremely varying its impact on the target livingorganisms should be set. Long-term calculations aredone to totally evaluate the cost flow capacity, andimpacts on living organisms in a case where, forexample, excavation of dry land was the main
improvement, where underwater excavation was themain improvement and where the necessary river
section was ensured by combining excavation of dryland with underwater excavation. Therefore, the key topractically applying the change prediction model is
how precisely the indirect impacts of river
improvement on living organisms can be predicted.
[Reference]
1) Takeuchi et al.: Concepts and specific methods for
management to provide a margin to flow capacity,
NILIM Report 2011