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UW Chapter 4 - Best Management Practices for Erosion and Sediment Control fabric and sediment particle size when specifying fabric permeability . Sediment which has been deflected and settled out by the curtain may be removed if so directed by the on-site inspector or the permitting agency. However, consideration must be given to the probable outcome of the procedure - will it create more of a sediment problem by resuspension of particles and by accidental dumping of the material by the equipment involved ? It is , therefore , recommended that the soil particles trapped by a turbidity curtain only be removed if there has been a significant change in the original contours of the effected area in the watercourse . Regardless of the decision made , soil particles should always be allowed to settle for a minimum of 6- 12 hours before their removal by equipment or before removal of a turbidity curtain . It is imperative that the intended function of the other controls in this chapter, tokeep sediment out of the watercourse, be the strategy used in every erosion control plan . However, when proximityto thewatercourse makes successfully mitigating sediment loss impossible , the use ,r,,, of the turbidity curtain during land disturbance is essential . Under no circumstances shall permitted land disturbing activities create violations of water quality standards) Design Criteria 1 . Type I configuration (see Plate 4 .45a) should be used in protected areas where there .r is no current and the area is sheltered from wind and waves . 2 . Type II configuration (see Plate 4 .45a ) should be used in areas where there may be or small to moderate current running (up to 2 knots or 3 . 5 feet ( 1 m ) per second ) and/or wind and wave action can affect the curtain . ..� 3 . Type III configuration (see Plate 4 .45b) should be used in areas where considerable current (up to 3 knots or 5 feet ( 1 .5 m) per second ) maybe present, where tidal action maybe present , and/or where the curtain is potentially subject to wind and wave action . 4W 4 . Turbidity curtains should extend the entire depth of the watercourse whenever the watercourse in question is not subject to tidal action and/or significant wind and wave forces . This prevents silt laden water from escaping under the barrier, scouring and resuspending additional sediments . 5 . In tidal and/or wind and wave action situations , the curtain should never be so long a to . touch the bottom . A minimum 1 foot (30 cm ) "gap" should exist between the weighted lower end of the skirt and the bottom at "mean" low water. Movement of the lower skirt over the bottom due to tidal reverses or wind and wave action on the flotation system may fan and stir sediments already settled out. 6 .- In tidal and/orwind and wave action situations , it is seldom practical to extend a turbidity curtain depth lower than 10 to 12 feet (3 to 4 m ) below the surface, even in deep water . Curtains which are installed deeper than this will be subject to very large loads with " consequent strain on curtain materials and the mooring system . In addition , a curtain installed in such a manner can "billow up" toward the surface under the pressure of the moving water, which will result in an effective depth which is significantly less than the aw 4- 117