IMPORTANT TERMS USED FOR RIVER TRAINING WORKS AND BRIDGES
AFFLUX (h) is the rise in water level upstream of a bridge as a result of obstruction to natural flow caused by the construction of the bridge and its approaches. Affux is to be taken into consideration while providing for "free board" and "clearance".
CAUSE WAY or Irish bridge is a dip in the railway track which allows foods to pass over it.
CLEARANCE (C) is the vertical distance between the water level of the design discharge (Q) including affux and the point on the bridge superstructure where the clearance is required to be measured.
DEPTH OF SCOUR (D) is the depth of the eroded bed of the river, measured from the water level for the discharge considered.
DESIGN DISCHARGE (Q) is the estimated discharge for the design of the bridge and its appurtenances. Generally 50 years return period is considered for estimation of design discharge.
DESIGN DISCHARGE FOR FOUNDATION (Qf) is the estimated discharge for design of foundations and training/protection work which is higher than design discharge by certain percentage depending on catchment area.
FREE BOARD (F) is the vertical distance between the water level corresponding to the design discharge (Q) including afflux and the formation level of the approach banks or the top level of guide banks. Free board of the at least 1.0 m should be provided while constructing a bridge.
FULL SUPPLY LEVEL (FSL) in the case of canals is the water level corresponding to the full supply as designed by canal authorities.
HIGHEST FLOOD LEVEL (HFL) is the highest water level known to have occurred.
LOW WATER LEVEL (LWL) is the water level generally obtained during dry weather.
IMPORTANT BRIDGES are those having a lineal waterway of 300 m or a total waterway of 1000 sq m or more and those classified as important by the Chief Engineer/Chief Bridge Engineer, depending on considerations such as depth of waterway, extent of river training works and maintenance problems.
MAJOR BRIDGES are those which have either a total waterway of 18 m or more or which have a clear opening of 12 m or more in one span.
PROTECTION WORKS are works to protect the bridge and its approaches from damage by flood water such as marginal bund, launching aprons, etc.
TRAINING WORKS are works designed to guide the flow of a river such as guide bund, spurs, etc.
GUIDE BUNDS are meant to confine and guide the river flow through the structure without causing damage to it and its approaches. They also prevent out flanking of the structure. According to the form in plan they may be parallel or divergent or convergent and according to geometrical shape they may be straight or elliptical.
SPURS OR GROYNES: These are structures projecting outward from the bank into a stream for the purpose of protecting the bank from erosion, arresting sand movement along the bank, concentrating flow of a stream into a smaller channel. They also help to train the river along the desired course.
HYDROGRAPH: A graph showing discharge of water-flow past a section with respect to time is known as hydrograph.
UNIT HYDROGRAPH: The unit hydrograph of a drainage basin is defined as a hydrograph of direct surface run off resulting from 1 cm of effective rainfall generated uniformly over the basin area at a uniform rate through a specified duration known as unit period (generally 1 hour).
SYNTHETIC UNIT HYDROGRAPH: It is unit hydrograph derived for a catchment based on correlations that have already been developed between important parameters of representative unit hydrographs (RUH) and relevant catchment characteristics by gauging adequate number of sites from a homogeneous region.
REPRESENTATIVE UNIT HYDROGRAPHS: The average of all available unit hydrographs capable of reproducing the flood hydrographs when applied to corresponding effective rainfall.
LACEY'S FORMULAE FOR ALLUVIAL RIVERS : If 'W' and 'D' are respectively the width and depth of flow in metres :
D=0.43Qf1/3 or 1.338Q2f1/3
Where 'Q' is the discharge in m3/s, 'q' is the discharge intensity in m3/s per metre width, 'f' is the silt factor which is 1.76 (m) 0.5 and 'm' being the weighted mean diameter of bed material in mm. Though these are empirical formulae arrived at by Lacey based on his observations on some of the alluvial rivers, these are very important in the design of water way and protection works of the bridges.
Scour: Estimation of scour depth is an important aspect of design of protection works and bridge foundation. Lacey's formula for calculation of depth of flow 'D' is employed for calculation of scour depth at bridge piers or along guide bund adopting suitable multiplying factor. According to IR Substructure Code, the depth of scour is taken as follows
Nature of the river
Depth of Sour
* Straight reach
* Moderate bend condition e.g. along apron of Gide bund
* At a severe bend
* At a right angle bend or at the nose of pier
* Against mole head of guide bund and in severe swirls