Physical hydraulic model study of a bottom charged spillway: A new concept for water level control in canals

Abstract

The bottom charged side spillway is a new concept of spillway in which the excess water is spilled from the bottom level which is sediment rich water and the cleanest quality of water is sent to the turbine. It is different in a sense that it will remove sediments as well while getting rid of the excess water different than the conventional spillways which will spill the cleanest quality of water.The concept was developed by Professor Haakon Støle and so far not much work has been done concerning the concept.

The main objective of the present study is to prepare a design guideline for bottom charged side spillway.

Hydraulic Model Study

Different calculations have been done for the design of canal, intake, surface charged spillway and bottom charged spillway considering different considerations. Detail design drawings were prepared based on design and model was constructed at NTNU/SINTEF lab.The present model of bottom charged spillway consists of the following modules.

• Inlet arrangement.

• Upstream canal section.

• Surface charged side spillway section.

• Bottom charged side spillway section.

• Downstream canal section.

• Outlet arrangement.

• Instrumentation and measuring arrangements.

Since the study is of a new concept, there has been no real prototype for the model but for ease of calculation and making the study more realistic all the calculations has been started considering some imaginary values. The prototype dimensions were then converted to model dimensions using Froude’s model law.

Model Test Result

It is observed from the study that the bottom charged spillway can be an efficient alternative for effectively spilling the excess water and parallel flushing of the sediments as well. The design of the model was conducted using different approaches. Since not much literature was available about the concept, some assumptions were made to proceed the design. Different approaches of general spillway approach, submerged side orifice approach and side spillway approaches were applied with modifications for bottom entry losses. The attempt were made to figure the relation for more accurate calculation.Some relations concerning the calculation of bottom entry losses were derived based on the model study but these need to be refined for better results. The most favorable approach by the model test was found to be the approach of side spillway with additional effect of bottom orificelosses. The calculation with side spillway approach gave less variation with the actual observed values. Downstream spillway end point was considered as the control point for the design. The depth and the Froude number at this point is the starting data for the design.

The variation in the nature of flow among the surface charge spillway and bottom charge spillway was not observed. Hence it can be said that the bottom charge spillway will not add disturbances to the flow in canal. There is development of high turbulence at the spill pool due to which the flushing of the sediments may not be a problem.