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12 <br />M.-F. O'Grady and J. Curtin <br />sections in all sub-catchments with a <br />particular gradient and substrate type <br />for detailed study, in the knowledge <br />that the areas examined are likely to <br />reflect the situation over long sections <br />of channel where the same physical <br />conditions apply. Such a strategy <br />provides one with a good overview <br />provided there are no significant <br />changes in the nature of the riparian <br />zone within areas of specific grade <br />and substrate type. The author <br />(O'Grady) has noted that stands of <br />deciduous trees which excessively <br />shade channels, reducing the in- <br />stream aquatic flora, can significantly <br />reduce the standing crop of salmonids <br />at least in the summer/autumn period <br />Feet O.D. iom <br />Substrate Composition <br />at different siles <br />along the channel. <br />zrlW 2aloa tMOO tome elW aN0 <br />?---? nix ®sa ®ieo¢ <br />®s+e ?w ?N ?+.e <br />Suhslrete Diameter Renge(mm) <br />(O'Grady, 1993). One therefore needs <br />to take account of such change in de- <br />signing the sampling programme. The <br />length of channel sampled within any <br />sub-section will ultimately depend on <br />channel morphology. For example if <br />the gradient is such that there is a <br />riffle/glide/pool sequence then the <br />minimum sampling unit should include <br />at least one such sequence. The <br />desirable type and frequency of <br />measurements required within each <br />sampling reach have been described <br />by Binns (1982), Hermansen and <br />Kregs (1984), Petts and Foster <br />(1985), Brooks (1957) and others. Es- <br />sentially such records enable one to <br />describe each site quantitatively in <br /> <br />+ z a ? s e > e a +o ,i +z a +e +e i? +a ie zo zi zz za za zs ze v m zs sa a+ <br />0 yards 500 Horizontal scale <br />Fig. 3. - An illustration of the type of information compiled by Irish drainage engineers at the <br />pre-drainage planning phase.