442 'Transact I o?is. 



As time goes on, phenomenal floods will occur again, and former records 

 will be beaten ; for as the hills become more denuded the floods will become 

 proportionately more destructive. It would be wise, therefore, for the 

 Napier people to take warning from past experience, and make more ample 

 provision for the egress of the water from the lagoon into which several 

 large rivers discharge themselves, as it is quite within the bounds of pos- 

 sibility that it may cut its way through the lower portion of the township, 

 or perhaps carry away the harbour-works at the Spit. 



(2.) Erosion and Silting. 



Erosion and silting generally go together, and either one or the other 

 happens according to the velocity of the current in a river-bed. When 

 the course of a river is steep, and the soil is of a soft or friable nature, the 

 Avater in the proportion of its volume and velocity scoops out the bottom, 

 and the excavation works back until it reaches a substance of sufficient 

 hardness to be resistant, when a waterfall or permanent rapid is formed. 

 But as the inclination of the bed becomes less the flow of the stream is 

 retarded, and the substance that has been brought down by the current 

 tends to settle in the bottom. In flood-time, however, large masses of stone 

 are swept down, and by grinding against the rocks in the sides and bot- 

 tom, as well as by mutual attrition, they are rounded into pebbles, becoming 

 smaller and smaller as they travel along, until they wear down into gravel, 

 and eventually into fine sand, which is carried in ripples along the bottom. 

 Meanwhile all soft rock, clays, and earthy matter are quickly resolved into 

 mud. When the bed approximates so nearly to a level that the rate of the 

 current is less than 6 ft. per second on the bottom, then the river is no 

 longer able to shift the solid material, and only the impalpable particles 

 of mud, which may be almost said to be held in solution, are carried along. 



This is the process known as " silting," and it is easy to see that the 

 quality of the silting must entirely depend on the character of the river- 

 bed and of the nature of the material brought down. 



When the bed is short and steep and the incline is continued to the 

 coast, the bulk of the silt is carried down to the sea, and no harm is done 

 unless the mouth of the ]"iver be situated in a harbour, when, of course, 

 trouble may arise from the shallowing of the water. 



It is when a flooded river traverses an alluvial plain that the silting 

 does most damage. The debris brought down by the head-waters must 

 find a lodgment somewhere, and, as the current loses its velocity on reaching 

 the level country, it is no longer able to bear its burden along. The silt 

 therefore lodges on the bottom, and the bed gradually rises until the water 

 is forced over the banks. Then the water breaks away and cuts a new 

 channel for itself, which in time fills up, and the same thing happens over 

 again. 



Numberless instances of this process are found in many parts of both 

 the North and the South Islands. Wherever, as in Hawke's Bay, Canter- 

 bury, &c., the alluvial plains are backed by a mountainous country the 

 surface is often torn away, the land is scored in every direction, and the 

 fertile soil covered with a deposit of stones, gravel, and slime. A notable 

 instance occurred during the great Napier flood of 1893, alre;idy men- 

 tioned, when the River Ngaruroro left its bed, and, joining with the Tutae- 

 kuri, cut its way through the road and railway to the sea. 



A foolish tradition has prompted local governing bodies and private 

 owners in many places to plant the river-banks Avith AA^llow-trees, AAdth 



