D.—6

12

Reference No. 3. To ascertain the best method of providing for the control of the said rivers and their tributaries so as to safeguard the lands affected, and to provide for the effective control and improvement of the sa,id rivers and, their banks. The remedial measures recommended by your Commissioners under this reference may be divided into four headings, viz.— (a) Works to safeguard Christchurch from flooding ; (b) works to protect river-banks from erosion ; (c) works to prevent or mitigate ponding-up of flood-waters near the lower end of Coutt's Island.; and (d) works to prevent or mitigate damage by flooding from the Eyre and Cust. With respect to (a) we recommend the strengthening, and raising to a height above flood-level sufficient to give a good margin of safety, of the principaJ stop-banks safeguarding the intakes to the old river-channels running down to the Styx, the Avon, and Christchurch. It is advisable that a margin of at least 2 ft. should be allowed between the top of stop-bank and the level of the highest recorded flood ; also that the width at top should be not less than 4 ft. —preferably 6 ft. —with side slopes of not less than lj to 1 on the river side, and on the opposite side up to 2 to 1, dependent on. the height of bank. It would appear that the existing groynes are, so far, ample for the purpose intended, except that they should be raised to the same height as the stop-banks referred to immediately above ; but the necessity for and the position of any additional groynes and stop-banks for the further protection of the old channels leading to the Styx, the Avon, and Christchurch is only to be determined by observation and experience during future floods. With respect to (6), the works necessary for the protection of the river-banks from erosion lie chiefly on the north or left bank, and extend, from a point about one mile above McLean's Island for a distance of about eight miles down-stream, although possibly only half this length requires urgent attention at present. The class of work which we recommend should, be adopted for this north-bank protection, consists in the construction of permeable (not solid) groynes of short length but at fairly close intervals, jutting out from the bank, the intervening bank between the groynes being protected by fascine work, or with trees laid with their branches down-stream and having their trunk ends secured by wire to pegs driven, in the ground some little distance back from the edge. The groynes to be constructed of short piles (of Pinus insignis or other easily obtained timber) sunk a sufficient depth in the river-bed to allow for subsequent scour, spaced 5 ft. apart, with two walings —one at low-water level and the other at flood-level; the up-stream sides to be faced with branches of trees laid diagonally, with their trunk ends uppermost and secured by wire to the two walings. These groynes might be of varying length, extending from the bank, so that their outer ends would present a uniform line or curve to the river-current; they should average not more than, say, 30 ft. in length, and be spaced about 1 chain apart. The accompanying sketch (plan No. 7) shows the general design of work recommended; but the spacing and lengths of groynes necessary to ensure adequate protection, and the gradual aggrading or silting-up of the intervening spaces, is largely a matter to be decided by experiment. In addition to this, levees should be constructed some little distance back from the river at all places along the river-banks wherever the grade of the high flood when controlled would stand above the general level of the banks, and several rows of young willows should be planted in front thereof. It is anticipated that these permeable groynes placed at close intervals will, by reducing the current next the bank, have a greater and quicker effect as compared with solid groynes in inducing the deposit of sand and gravelly shoals in between, and so reducing the scour along the banks. When the silting-up between these temporary groynes has taken place the groynes may, if necessary, be slightly extended, and the intervening shoals should be planted with willows immediately in front of the river-bank. The entrance to the old north channel should be permanently closed by adequate stopbanking, and this channel should be used solely for drainage from the Eyre and Rangiora districts. With respect to (c), in order to prevent ponding-up of the flood-waters in the vicinity of the lower end of Coutt's Island, and provide for the quicker discharge