Research) wrote that the experiments reported by Ackers and Harrison had provided very useful knowledge of the behaviour of flood waves in partly filled pipes, and marked another stage in the rapid advance now being made into a more scientific approach to the problems of surface water drainage. The Authors did not comment on what impact they thought their experiments might have on current development of new methods of designing surface water sewers, in particular the R.R.L. Hydrograph method. It could be inferred from 3 of the Paper that a recorded retension curve was essential when the R.R.L. Hydrograph method was employed in the examination of an existing system. This is not SO; with the aid of a digital computer the approximate retention relation can be readily calculated.40. To assess the relative accuracy of the Authors' method of calculation and of the reservoir retention technique used in the R.R.L. Hydrograph method the trapezoidal input hydrograph shown in Fig. 2 has been routed through pipe retention by the R.R.L. method, for the four positions along the pipe illustrated. The values of maximum instantaneous depth then obtained were compared in Table 1 with the values given by the characteristics solution. 41.The agreement between the two methods was remarkably good. It was problematical, however, whether the method of characteristics could be applied, possibly as modifications to the R.R.L. method, to the more complex case of a sewer *

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The Paper carries the basis of design of urban storm sewers as embodied in the TRRL approach a step further in order to deal with conditions which, in tropical countries, are such as to necessitate modification in certain aspects of the criteria deemed applicable in these latitudes. I use the word 'storm' because much progress on the approach to efficient design of storm (i.e. surface water) sewers has been made over the years, but I wonder whether there has been comparable progress in the .design of completely separate foul sewerage systems?59. It seems to me that the time lag factor, which is so important in the design of storm systems, could be examined more closely than I think has been the case in relation to the completely separate foul system, particularly for new systems (of a rather large nature). In essence, it seems to me that many may size pipes in a completely separate system on the basis of an upstream contribution consisting of the aggregate of a notional figure in gal/h per day, plus a figure for infiltration. CP 2 0 0 9 appears to offer little further guidance. 60.In fact, the approach referred to appears to me to tend towards a probability that pipes in the lower regions of the system may be oversized because the lag factor may not have been appreciated. Assuming a pipe velocity of 3 ft/s, the foul sewerage will travel through the system at a rate of 180 ft/min. In a system, say, 2 miles long the time of travel for foul sewerage from head to outfall would be almost one hour. 61.While this example is deliberately naive, perhaps it points to a necessity for a closer examination of the lag factor in the design of separate foul systems. Perhaps also further consideration should be given to the possibility of providing staged retention ponds on separate foul systems. They could serve the dual purpose of equalizing flow and providing partial treatment, thus reducing the purification loading on terminal treatment plants.Mr R. Hepworth, M It worries me that the Author continues to pursue his belief in the TRRL method in the face of criticism from other engineers well versed in surface water drainage design. The assumptions of the TRRL method are (U) time of entry is short (b) full bore velocities apply after the initial time of entry (c) a typical recession curve exists and this occurs on every critica1,rainfall event.

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