ABE 527
Computer Models in Environmental and Natural Resources Engineering
Flood hydrograph package developed by the Hydrologic Engineering Center (HEC)
Release 1, U.S. Army Corps of Engineers.
Model components
A river basin is subdivided into an interconnected system of stream network components.
Fig. 2.1-2.2
Steps in the analysis.
Watershed boundary delineation
Subbasins division based on study purpose and degree of heterogeneity. Each
subbasin represents a hydrologic unit that is homogeneous.
Each subbasin is represented by a combination of model components
a. Runoff
b. River routing
c. Reservoir
All are available in HEC-1
d. Diversion
e. Pump
Subbasins and their components are linked together to represent the
connectivity of the river basin.
Runoff component
Input to this component is a precipitation hyetograph. Precipitation excess is computed
based on (subtracting) infiltration and detention. Infiltration and detention are uniform
over the subbasin. The rainfall excesses are routed by using the unit hydrograph or
kinematic wave techniques to the outlet of the subbasin producing runoff hydrograph.
Base flow is also computed and measured.
River Routing
Simulation of flood wave movement in a river channel. Input to this component is
upstream hydrograph. This routing depends on the characteristic of the channel.
Combined use of river routing and subbasin runoff components
This is achieved via tree structure
Reservoir component
This component is used to represent the storage-outflow characteristics of a reservoir,
lake, detention pond, highway, culvert, etc. It takes an inflow hydrograph and routes it.
Reservoir outflow is a function of storage and not on downstream controls.
Diversion components
Used to represent channel diversions, stream bifurcations or any transfer of flow from
one point of a river basin to another point in or out of the basin. It receives an upstream
inflow and divides the flow according to a specified rating curve.
Characteristics and on-off elevations, number, capabilities, pumpflow can be retrieved in
the same manner as directed flow.
Hydrograph transformation
This includes ratio of ordinates and hydrograph balance
Differences between detention and retention in reservoir routing
Detention: temporary storage, released while mass balance is conserved
Retention: storage, may be released by main balance is not conserved
Typical cross section schematic of reservoir out flows
Derivation of flood flow routing
Continuity Equation:
Iavg  Oavg 
Inflow – Outflow = change of storage rate
I t-1  I t O t-1  O t St  St-1
‘I’s (Inflow info=I) are known from design storm runoff hydrograph ordinates.
Rearranging the above equation:
 2St
 2S
 O t   I t-1  I t   t-1  O t-1 
 t
 t
t-1 values are known; t=0 values are initial values.
The above equation is one equation with two unknowns, 
Storage (S) outflow (O) relationship is needed.
How? See figure 49 below:
Figure (E) is derived from figure (D). For a certain elevation, figure (C) provide total
flow where figure (E) provides the corresponding volume. These generate (F). The
example that follows on reservoir routing will demonstrate the use of Figure 49.
Principles of flood routing
Inflow = Outflow + Storage
idt = odt + sdt
i=inflow rate for small increment of time
o = outflow rate for same increment of time
s = storage rate for same increment of time
dt = time differential
i1  i 2
O  O2
 t 1
 S2  S1
1, 2 are subscripts denoting beginning and end of the time interval.
Problem Statement
Route the inflow hydrograph below through a reservoir structure with outflow rating.
Curve through weir and pipe flowing full shown in Figure 2. The stage storage
configuration of the reservoir is shown in Figure 3.
Example of reservoir routing
Figure 1. Inflow and outflow hydrographs
Figure 2. Outflow-stage relationship based on the outflow design
Figure 3. Storage-stage configuration of the reservoir.
Table 1. Computing  S    0 relationship.
 t 2 
Note 3.4 not 34 in column 2, row 2
Column 1 flow from figure 2
Column 2 corresponding storage (Figure 3) for each stage height of
Figure 2.
 S O
Figure 4.     O relationship; column 1 and 5 of Table 1.
 t 2 
Table 2. Flood routing through a reservoir computation
Column 2 is using inflow hydrograph of Figure 1.
Column 4 is flow from Figure 4.
Note that O2
 O2  
Weir flow formula used in the example problem
q = 0.55 CLh
q = discharge in m3/s
c = weir coefficient (3.0 typical)
L = weir length
h = depth of flow over the crest in m
Pipe flow formula used in the example problem
a 2 gH
1  K e  Kb  K c L
q = flow capacity L3/T
a = cross sectional area L2
H = head causing flow (L)
Ke = entrance cross coefficient
Kb = bend coefficient
Kc = friction loss coefficient
Pond Design criteria
100-year post development peak must not exceed the existing peak
The emergency spillway must not be used for the 100-year event or smaller
Pond is empty at the beginning of the storm
Max. depth is 6 ft. for safety
Outlet is 24” min diameter for easy access
How to find a pond that meets the requirements:
Compute the predevelopment design event hydrograph
Compute post development design hydrograph
Select a pond site and a relationship of water depth and storage
Select outlet configuration and develop a relationship of depth and outflow for
the configuration
Select routing interval ∆t (5-6 ordinates at least including inflow peak)
Construct storage outflow relationship by combining the relationship from
steps C and D.
Compute outflow hydrograph
Compare max outflow rate with target rate
Adjust pond and/or outlet structure configuration.
Repeat the procedure steps C-J for alternative design.