River Mud Loads Measure Of Erosion

Press, Volume CI, Issue 29844, 9 June 1962, Page 7

River Mud Loads Measure Of Erosion

Mr A. P. Campbell, senior engineer (hydrologist), of the Ministry of Works, Wellington, today continues his discussion of the recent technical hydrology and land management symposium held at Lincoln College under the auspices of the Soil Conservation and Rivers Control Council.

The meeting started with explanations of the basis of work in New Zealand hydrology. A substantial reorganisation has been going on in this work to give a proper basis for data in the five main branches of water science water resources, channel stability, flood information, catchment condition, and suspended sediment. Explanations of the basic work covered also what was

being done in education and training, the field work on ‘'bench - mark" catchments and mapping to define the hydrological regions of New Zealand.

A knowledge of soil science is vital for the proper appreciation of shortterm and long-term changes in catchment condition and Mr K. S. Birrell, of the Soil Bureau, D.5.1.R., with a display of natural soil profiles described the types of soil Which may be of, most concern in erosion studies here. It became clear at this meeting that some concentration of work was required to establish by field measurement the hydrological characteristics of these soils so that measurements can be ob-

tained to define the best and the worst condition likely to be experienced with a particular kind of soil. Since the extent of the influence of catchment condition on erosion and flooding depends entirely on how much water can be absorbed and stored in the soil and on how much water can be temporarily stored on the surface in the vegetation and surface litter, measurement of these two quantities is a task on which hydrologists are concentrating. The data obtained are referred to as infiltration and surface detention and the results obtained by the analyses done so far on small catchments were described by Mr C. Toebes,

hydrologist, Ministry of Works. The hydrologists have now gone sufficiently far with the data obtained from the Taita experimental catchment, whicn has been set up and observed by the staff of the Soil Bureau, to separate the whole of the annual flow into different parts. There is a part which is surface runoff, a part which is interflow or water travelling through the soil, and another part which is base flow or flow passing through groundwater storage to reappear in the stream. With this data it is also possible to establish a water balance to check the agreement between total annual rainfall and total runoff plus plant transpiration. A good deal of future research will be concentrated on the study of infiltration. Mud Everybody has seen that in times of flood most rivers are muddy and the larger the flood the more muddy they are likely to become. Not many will appreciate, however, the way in which measurements of this suspended load of mud can be used to assess the severity of erosion. Not nearly enough measurements have yet been done in New Zealand, but fisures so far indicate that rates of erosion are severe by world standards. An account of work in the southern half of the North Island was given by Mr A C. Hopkins, Ministry of Works. Palmenston North who has been responsible for pioneering activities in this work in New Zealand. He compared the sediment loads of four rivers—two. moderately clear rivers, the Motu and the Upper Ngauroro. with two streams having high rates of erosion, the / whea in Wairarana and the Upper Waipaoa, Gisborne. Rate It is noted that the Waipaoa river in a fairly high flood has been recorded as transporting sediment at a rate of 70 tons per second when the proportion of solid material in the river flow was 4.5 per cent. To complete the description of scientific work on sediment. accounts were given by Mr A. P Campbell of the lines on which specialised work is being developed; by Messrs J. Hogg, and I. Simmers of the Ransitikei Catohment Board describing a special studv which is being started in the Porewa stream; and by Dr R. D. Northey. of the Soil Bureau, D.5.1.R., who described in more detail the various methods by which a more detailed study of suspended sediment particles could be made, leading possibly to further lines of investigations into forms of erosion.