GUIDELINE FOR DRIFTWOOD COUNTERMEASURES (PROPOSAL

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GUIDELINE FOR DRIFTWOOD COUNTERMEASURES (PROPOSAL
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  GUIDELINE FOR DRIFTWOOD COUNTERMEASURES (PROPOSAL) July, 2000 Sediment Control (Sabo) Division Sediment Control (Sabo) Department Ministry of Construction  Part I GUIDELINE FOR DRIFTWOOD COUNTERMEASURES (PROPOSAL) Planning  CONTENTS 1.   GENERAL..............................................................................................................................4   2.   BASICS OF DRIFTWOOD COUNTERMEASURES..........................................................5   2.1   Basics of Driftwood Countermeasures..............................................................................5   2.2   Basic Point for Planning....................................................................................................7   2.3   Quantity of Driftwood for Planning...................................................................................7   3.   SURVEYS FOR DRIFTWOOD COUNTERMEASURES...................................................8   3.1   General...............................................................................................................................8   3.2   Survey of Basin Conditions...............................................................................................8   3.3   Survey of Causes of Driftwood Development...................................................................9   3.4   Survey on Source area, Quantity, Length, Diameter of Driftwood.................................10   3.5   Estimation of Driftwood Outflows..................................................................................14   3.6   Types of Damage due to Driftwood Outflows.................................................................15   4.   FACILITIES FOR DRIFTWOOD COUNTERMEASURES..............................................16   4.1   Driftwood Control Methods.............................................................................................16   4.2   Driftwood Countermeasure Facilities..............................................................................16   4.3   Planning of Driftwood Restraint Works...........................................................................18   4.4   Planning of Driftwood Entrapment Works......................................................................19   5.   MAINTENANCE AND MANAGEMENT OF DRIFTWOOD COUNTERMEASURE FACILITIES.........................................................................................................................22   5.1   Inspection and Survey on Changes in Basin Conditions.................................................22   5.2   Inspection and Repair of Driftwood Countermeasure Facilities and Removal of Driftwood.........................................................................................................................22   References........................................................ !  1. GENERAL * Driftwood countermeasures are to be implemented in basins where there is believed to be the danger of driftwood flowing down with other debris and sediment, for the purposes of protecting human life and property as well as the natural environment. Comments In recent years much driftwood has combined with debris and sediment, due to heavy downpours over mountain rivers, to result in disastrous damage and loss of life in the lower reaches of rivers. Recent attention has therefore been focused on driftwood countermeasures aimed at driftwood transported by debris/sediment waterflows. In Japan, in general there are many cases where the catchment area of mountain rivers is covered by forest. In these areas there are cases where, due to slope failure, sediment and driftwood are washed out and are carried downstream. Such driftwood clogs narrows in the river course or bridge or culvert sites giving rise to flooding, bridge damage or destruction and sometimes resulting in considerable loss of life and property. Based on observations of past experiences it is clear that the greatest damage has occurred when the outflows have been accompanied by driftwood and the countermeasures should thus be aimed at the linking the measures for debris/ sediment and driftwood outflows.  2. BASICS OF DRIFTWOOD COUNTERMEASURES 2.1 Basics of Driftwood Countermeasures * The driftwood means standing trees, fallen trees, felled trees, etc. that may flow out when a hillside collapse, a riverbank collapse, or a debris flow occurs and the resulting sediment flows out. The basics of driftwood countermeasures are to prevent the outflow of driftwood that may give damage to the downstream area as well as to capture the flowed-out driftwood in the mountain streams which are designated for sabo works implementation. Comments Restraint of driftwood means to prevent the production of driftwood that occurs when the sediment is produced. Entrapment of driftwood means to prevent fallen trees and felled trees accumulated on the hillside slopes from flowing into the mountain streams using sabo facilities, as well as to stop driftwood in the mountain streams from flowing down further and let it deposit using sabo facilities. The process of driftwood countermeasures consists of surveys conducted on driftwood in the target basin, formulation of a countermeasure plan based on the survey results, and implementation of sabo works and management. The flow of driftwood countermeasures and points of note are shown in Figure 1.
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