4. Revising the Guideline for Designing Gentle Slope-type Coastal Dikes
(1) Introduction
Most coastal dikes and revetments have steep slopes, and have suffered damages such as collapse of the structures by scouring of the front slopes, and impeded people from using beaches and coastal zones. A gentle slope of 1:3 reduces the runup height of waves and enables access to beaches. Thus, the Guideline for Designing Gentle Slope-type Dikes was issued in 1989, encouraging the construction of gentle slope-type dikes to promote shore protection and coast utilization.
However, gentle slopes were found to be sometimes ineffective for coasts with no sandy beaches. Thus, the range of constructing gentle slope-type banks was revised, and the guideline was revised jointly by the Coast Division of the River Bureau and the Coast Division, River Department, National Institute for Land and Infrastructure Management. In the revised guideline, performance-based standards were introduced to enable engineers in the field to devise their own measures, and newly acquired knowledge was added.
(2) Revising the range of application
Gentle slope-type dikes have been reported to cover large areas of beaches since they have long sectional bases, destroying habitats of marine organisms and impeding the use of the beach. Since most gentle slope- type dikes were constructed on the seaside of existing steep slope-type dikes, the footings are located under the sea and cause waves to runup higher than in steep slope-type dikes.
Thus, the revised guideline requires the footings to be constructed above the low-water level and advises reconsideration of the construction of gentle slope-type dikes when the width of the beach may become narrower than 20 m after the construction due to coastal erosion or other causes, and to use alternative measures, such as constructing steep-slope banks.
(3) Performance-based standards
In the revised guideline, performance-based standards were introduced to enable engineers in the field to devise measures. The performances to be met were decided to be the protection levels determined in the Coastal Protection Basic Plan and the necessary performances for utilization.@Safety performances are safe structures against assumed external forces, such as storm surges, tsunami and waves. To facilitate designing, the guideline summarizes points to note when designing structures. Since coordination among coastal protection, environment, and utilization was set as the goal of the Coast Law in 1999, points to note on environment are also stated.
(4) New knowledge
New knowledge, which has been acquired in the 10 years since the Guideline for Designing Gentle Slope- type Dikes was issued, was added to the new guideline.
For example, gentle slopes were believed to less reflect waves than steep slopes and thus control the erosion of beaches. However, it has been shown that the principal cause of beach erosion is drift sand along the coast and that gentle slope-type dikes do not control erosion but cause changes in the amount of drift sand at each position and may actually accelerate erosion depending on drift sand conditions. Thus, to assess whether a gentle-slope bank accelerates or controls erosion, safety and performance assessment methods were added, such as contour line models.
The revised Guideline for Designing Gentle Slope-type Dikes will effectively promote coast protection projects that also consider the environment and utilization.
The performances were decided with reference to the ICC Performance Code; the contents of "Purposes," "Functions," "Required Performances," and "Checking Items" of artificial reefs were explicitly defined; and "Checking Methods" were provided. Finally, the contents of "Structural Details," were revised under the heading of "Important Reminders regarding Structures" for ease of understanding when designing artificial reefs.
