21. Floodway Zone Volume Two 21.3.2.4. A motor vehicle body, old machinery or scrap iron must not be used for bank protection works. 21.3.2.5. Placement of rock rip-rap in a tidal reach must not take place between 1 August and 30 November in any year. 21.3.3. Driving and construction of a piled retard. 21.3.3.1. A piled retard may be used in conjunction with tree planting as bank edge protection. 21.3.3.2. A piled retard must only be used where there is adequate channel width and a retard is likely to aggrade with sediment to form a new river bank. 21.3.3.3. A piled retard may be used as a debris arrestor in front of a culvert, provided that fish passage is not obstructed. 21.3.4. Lining of a drainage channel with timber or concrete for hydraulic efficiency or bank structural stability reasons where the channel is of limited width. 21.3.4.1. Concrete lining must not be constructed when there is flowing water in the channel. 21.3.5. Maintenance of a culvert or floodgate. 21.3.5.1. The maintenance must occur within the original location of a culvert or floodgate. 21.3.5.2. A temporary coffer dam constructed for the purposes of the maintenance must be removed at the completion of the maintenance. 21.3.5.3. A secondary flow path to enable overtopping floodwaters to return to the downstream channel without increasing the flood hazard to another person’s property must be constructed during maintenance. 21.3.5.4. The culvert must be placed below the level of the stream bed by a distance equating to the diameter of the pipe divided by 5 (i.e., 20% of the culvert pipe). 21.3.5.5. The culvert must be placed at the same slope as the existing bed of the river. 21.3.5.6. There must be no increase in the velocity of flow through or downstream of the culvert, at the river’s median flow, after the maintenance is completed. 21.3.5.7. The culvert installation must be designed and implemented to ensure there is no erosion or scour downstream of the culvert. 21.3.5.8. Fish passage must be provided for floodgates, where habitat exists upstream of the floodgate. 21.3.6. Shaping and beaching. 21.3.6.1. The shaping must be necessary for maintaining the flood carrying capacity of the floodway, or for the stability of the riverbank in the vicinity. 21.3.6.2. Shaping and beaching work must not be to a level higher than that of the natural river bank. 21.3.6.3. The channel shape and form both in cross section and longitudinal slope must be kept similar to the typical natural bedform of that river. 21.3.6.4. Works must not be undertaken in flowing water. 21.3.6.5. Any discharge of sediment into water associated with the shaping or beaching must not, after reasonable mixing, cause a change in colour of the 21 – 4