Needles do not require hooks, etc., which are liable to be broken or lost.
A needle regulator requires few piers, and is therefore cheap.
Water falling over planks throws a strain on the floor.
Regulation with needles is easy and rapid. A jammed plank, especially if low down and not horizontal, may give great trouble.
Advantages of Planks. Floating rubbish is not liable to collect above the Regulator because the water flows over the planks.
By means of double grooves and earth filling, leakage can be quite stopped.
For large works the advantages are generally with needles, but for small works, e.g. distributary heads and shallow water, with planks. Needles 14 feet long are not too long for trained men. Planks are more likely than needles to arrest rolling sand, and this can be taken into consideration in designing double regulators. See [number 8] of Kennedy’s rules, [Article 5]. When planks are used there should be two sets of grooves. Planks are very suitable for escape heads which have only occasionally to be opened, earth being filled in between the two sets of planks.
Fig. 16A
Regarding notched falls, in the case of small distributaries the notches are so narrow that they are extremely liable to be obstructed either accidentally by floating rubbish or wilfully by persons whose outlets are upstream of them. Weirs are not open to this objection, and are frequently adopted. There is not the least chance of their causing any silting worth mentioning. A simple weir if made of the proper height for the F.S. discharge, will cause a slight heading up with ³⁄₄ths of the F.S. discharge, and this unfairly benefits any outlets for a considerable distance upstream of the weir. This difficulty can be got over by making the weir as in [Fig. 16A].