Channel routing is used to analyze the effects of a channel on a
hydrograph's peak flow and travel time. You may choose from either
the Convex or Modified Att-Kin methods to route a hydrograph
downstream.
The Modified Att-Kin (attenuation-kinematic) method is used in
the TR-20 and TR55 methodologies. It is a combination of the
storage indication method and the kinematic wave method to reflect
the reservoir and translation effects on natural flood waves. The
hydrograph is first routed through a reservoir assuming that the
outflow is proportional to the storage. The resulting outflow
hydrograph is then translated using kinematic wave principles. The
storage routing portion allows for attenuation and translation.
Kinematic routing translates the outflow hydrograph but doesn't
attenuate the peak.
The continuity equation is
Ii - (Oi+1 -
Oi) / 2 = (Si+1 - Si) / Δt
where: Ii
= Inflow hydrograph discharge at (i)th time
step
Oi = Outflow hydrograph
discharge at (i)th time step
Oi+1 = Outflow hydrograph discharge at
(i+1)th time step
Si = Storage at (i)th
time step
Si+1 = Storage at (i+1)th time
step
Δt = Routing time step
Substitution of S = KO leads to
Oi+1 = CmIi
+ (1 - Cm)Oi
where: Cm = 2Δt/(2K + Δt)
K = L / (60mV)
V = Q / A
A =
(Q / x)1/m
where: L = Channel length (ft or
meter)
V = Average velocity in the channel (ft/s or
m/s)
Q = Average channel flow, default value is the
peak discharge of the inflow hydrograph (cfs or cms)
A
= Wet cross section area of the channel (sq. ft or sq.
meter)
x = Flow equation multiplier, > 0.0
m = Flow equation exponent, default value is 5/3,
usually is 1.0 <= m <= 2.8
The coefficient x can be estimated using Manning's Formula:
x = S1/2 / nP2/3
where: S = Average slope of the channel
n = Manning's n value
P = Wetted perimeter of the flow (ft or m)
Select Modified Att-Kin Method from Watershed > Hydrograph
Routing > Channel Routing menu. Specify the inflow hydrograph
and enter the channel length, the routing coefficient x and the
time coefficient m. The routing interval is as same as the time
increment of the inflow hydrograph. Button Estimate Coefficients x
and m allows you to enter the parameters of the Channel to
determine the x and m coefficients. Once the parameters are
entered, click on the Calculate button to compute the coefficients.
OK button accepts the values and proceeds to the previous
dialog.
After entering all information, click Routed Hydrograph button to generate the routing hydrograph. A dialog is opened with the tabular and graphic hydrograph data, from there you can draw the hydrograph on screen and save it to a file.
Channel Routing Hydrograph - Modified Att-Kin Method |
Channel Routing Hydrograph - Estimate Coefficients Dialog |
Routed Hydrograph Dialog |