River Routing

As a flood hydrograph moves through a river reach the peak flow is both lagged (delayed) and attenuated (decreased).  The amount of lag is determined by the slope and roughness of the channel, which both affect the speed of the flood wave.  The attenuation is determined by how much the water spreads out.  For example, if the channel is concrete lined and the flows remain within the banks then there will be little or no attenuation.  On the other hand, if the flood spills out of the banks onto the floodplain, or is "absorbed" by the channel as bank storage, then the attenuation could be significant.  The following graphics identify how a hydrograph is lagged and/or attenuated under certain conditions.

In the next graphic the effects of lag and attenuation are shown simultaneously for a flood hydrograph passing through a natural channel reach.  As the flows increase beyond the capacity of the natural channel it spreads out onto the floodplain.  As the inflow recedes the flows in the floodplain move back into the channel.  The final effect on the outflow hydrograph is that it is spread out with a decreased peak and longer base time.  The area under each hydrograph would be the same except for infiltration losses, which for short flooding events are generally small.