Effect Of Exceptional Flow Conditions On River Diversion Barrages And Performance Improvement Using Depresed Secondary Aprons. Diversion Barrages Across Rivers Are An Important Class Of Hydraulic Structures Constructed For Supplying A Portion Of The River Flow Into Off-Taking Canals. The Recommendations And Design Procedures For Barrages Have Evolved Over Time Considering Mostly The Normal Operation Conditions. However, There Have Been Instances Where Unusual Or Emergency Situations Have Led To The Formation Of Exceptional Flow Conditions, Some Of Which Have Been Observed And Reported By The Engineers Responsible For The Operation And Management Of Barrages. This Thesis Investigates The Effect Of Two Such Exceptional Conditions, Not Considered In Normal Design Procedures For River Diversion Barrages. A Modification To The Shape Of The Apron Is Proposed For Minimisation Of Scour And Increase Of The Apron’s Stability. The Conditions Tested Are: I) Oblique Approaching Flow Towards Barrage Gates In The Barrage Pond, And Ii) Occurrence Of Low Tail Water Level On The Downstream At The Time Of Unusual High Flows Through A Barrage Gate. The First Condition Is Usually Noticed In The Pond Area Of Barrages Which Have Been Heavily Silted Up Resulting In Formation Of Sediment Mounds, Upstream Of The Barrage Gates That Have Been Operated Less Frequently. The Second Unusual Condition Tested Is That Of A Completely Collapsed Gate Or A Gate Wedged Between Its Guides At A Certain Position During A Time Of Low Tail Water Level In The River. These Conditions Have Been Found To Inflict Heavy Damage To The Apron Made Of Loose Concrete Blocks Provided On The Downstream Of Barrage Stilling Basins. Taking A Cue From The Design Of Some Prototype Barrages, Where The Downstream Apron Is Shaped In The Form Of A Depression, It Is Hypothesised That Such Depressed Aprons Would Be Able To Reduce Downstream Scour And Improve The Apron’s Stability Under Low Tail Water Level Conditions. The Investigations Show This To Be Indeed True And Several Depressed Shapes Of Apron Are Tested. The Variation Of The Flow Variables, Like Velocity And Turbulence Intensity Components, Are Also Observed For The Depressed Apron And Compared To Those For The Horizontal To Understand The Mechanism Behind The Reduction Of Scour By The Former. Keywords: Diversion Barrage; Concrete Block Apron; Downstream Scour; Oblique Approaching