Hydraulic Design of Highway Culverts - DOT On-Line Publications

A. The Routing Concept
V. STORAGE ROUTING
Storage routing is the calculation of the change in shape of a flood wave over time. A
pronounced shape change occurs in a flood wave when a significant storage volume such as a
pond or a reservoir is encountered. The storage concept can be visualized by means of a
hypothetical situation (27). In this situation, a spigot discharges water into an empty barrel
which has an orifice (hole) at the bottom (Figure V-1). A plot of the inflow and the outflow
reveals some important characteristics of the storage routing process.
Figure V-1--Hole in the barrel analogy
Figure V-2--Inflow and outflow
hydrographs
The spigot is turned on at t=0 and discharges a
constant flow rate, Qi, until t=ti, at which time
the spigot is turned off. The flow rate entering
the barrel exceeds the discharge capacity of
the hole. This results in the storage of water in
the barrel. As the depth increases, the
discharge of water through the hole increases
due to the rising head on the hole. The
maximum outflow is reached at a time when the
depth is at a maximum. This peak outflow
occurs when the spigot is turned off since there
is no additional inflow after that time. Figure V-2
is a schematic representation of the inflow and
outflow hydrographs.
Additional information about the storage routing
concept may be obtained by examining Figure
V-2 more closely. An area on a graph of
discharge versus time represents a volume;
that is, a discharge increment multiplied by a
time increment. The area under the inflow
hydrograph depicts the volume of water
entering the barrel. The area under the outflow
hydrograph depicts the volume of water leaving
the barrel. The area between the two curves is
the volume stored in the barrel. This volume
(area) reaches a maximum when the spigot is
closed. From that point on, the area under the
outflow hydrograph represents the discharge of
the volume stored in the barrel. This equals the
maximum storage area previously defined. The
total area under the inflow and outflow curves
should be equal since the volume of water
entering and the volume of water leaving the
barrel are the same.
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