Hydel Designs Directorate, Punjab P.W.D., Irrigation Branch, Chandigarh.
During the simulation trials of a fixed-bed tidal model of the Solway Firth (Horizontal Scale 1/3168) (Vertical Scale 1/144) Nikuradse’s type roughness, consisting of hemispherical cement-mortar blocks were used to alter the bed roughness. The blocks were stuck to the model surface in staggered rows with centre to centre spacing of three diameters. In order to provide, as far as possible, equal roughness change in the different reaches of the model, the size of the blocks ivas varied according to the depth of flow; the deeper area’s received 3.8 cm diameter blocks, intermediate reach 2.5 cm and the shallow reach 1.9 cm diameter blocks. It was found that a reasonable degree of similitude between the observed tidal data and the prototype data, was attained when the whole of the model surface was provided with the friction blocks.
The relative friction value of the blocks was evaluated from the tests in a tilting flume under uniform flow condition. The curve so derived (n—R/k) was utilised to evaluate the coefficient of friction value of the model.
To select the prototype bed friction value, in the absence of the data for the Solway Firth estuary, data from the neighbouring estuaries was used. Comparison was made between the friction value of the model and the prototype value, taking into consideration the friction scale of the model (depending upon the model scales) and it was found that there is reasonable agreement. It is concluded that it is possible to attain dynamic similarity between a model and the prototype, if the friction of the former is adjusted to theoretically determined level (from the friction scale and the prototype value of friction); in the entire length of the model by providing Nikuradse’s type roughness blocks.
