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remaining weir boards must be set according to the installation guidelines presented in Section 2 . 3 . 2 . In addition, as also discussed in Section 2 . 3 . 2 , the contractor must use weir boards fabricated from nominal 6x6 timbers ( 5 . 5 in. x 5 . 5 in. finished dimensions) for the lowermost 4 ft of each weir stack. As dredging progresses and the level of ponded water approaches the initial weir crest elevation, the contractor must determine whether to begin the release of effluent . This decision must reflect the results of turbidity testing of the upper 2 ft of the ponded water immediately upstream of the weirs . A depth of 2 ft represents the estimated maximum depth of withdrawal , that is, the depth of the surface layer released over the weirs . 3. 3. 2 Maximum Ponding Depths and Required Inspections If testing determines that the ponded surface water at the weirs fails to meet permit requirements , the contractor must provide additional retention time, either by raising the weir crest elevation or shutting down the dredge until sufficient settling has occurred . If the contractor chooses to raise the weir crest by adding additional weir boards, he must proceed with caution. Experience has shown that increasing the water level too quickly or maintaining an excessive ponding depth can lead to excessive seepage through the dike, a condition that increases the potential for slope instability and, under extreme circumstances, dike failure . Considerations of dike safety require that mean ponding depths at the IR - 14 containment facility must not exceed 5 ft without notification and approval of the Project Engineer. Even with all required approvals, the contractor must not exceed the 5 -ft maximum, even to meet required effluent standards, without instituting a significantly more rigorous program of dike inspection. Chapter 4 . 0 outlines the basic requirements for the dike inspection program as well as the critical conditions that would trigger more intensive inspections. Inspections , conducted at least once a week under normal conditions, must increase to at least once a day immediately upon discovery of a critical condition. However, increasing the ponding depth (as measured at the weirs) beyond the recommended 5- ft operational maximum requires that the contractor further increase the inspection frequency and provide close, continual inspection of the entire dike perimeter at all times while ponding depths remain greater than the recommended 5 -ft maximum. All dike inspections must be performed by a qualified geotechnical engineer or engineering technician experienced in the inspection of earthen dams , reservoirs , or dredged material containment facilities . As part of the required preconstruction submittals, the contractor must provide the qualifications of all designated inspectors to the Project Engineer for review and approval . - 16 -