Lag and K routing
Lag and K routing provides a computerized solution to a procedure which was initially developed as a graphical routing technique.
Operationally, Lag and K has been and continutes to be a practical and widely-used method of storage routing between flow-points.
It is also a very flexible method of routing since both the Lag and K elements can be either constant or variable.
Examination of historical flood hydrographs of varying magnitude provides a basis for establishing Lag and K relationships within a reach.
The first process in a normal operation is to lag or delay in hours an inflow hydrograph in order to create what is called a lagged inflow hydrograph.
The K part of the operation is then used to attenuate the lagged inflow graph in order to create an outflow hydrograph at the downstream flow-point.
Though normally used together, Lag and K can also be used separately to account for lag with no attenuation or attenuation with negligible lag.
Storage routing with K attenuation is based on the continuity equation expressed as:
I - O = ds/dt
where
I = inflow rate
O = outflow rate
s = storage
t = time with t representing the routing interval
