FLOOD01888

• County: Statewide
• Title: River Hydraulics
• Date: 10/15/1993
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

EM 1110-2.1416
15 Oct 93

@ ~
i--=-- -=- - -=- - ~ EnergY-Grade Line - - ~ he

2
(X2V2
2g

~ Water Surface

2
(X,V,
2g

Flow

~

WS2

WS,

Figure 6-1. Open chennel energy re18tionehips

~y2 !XIy2
WS+ 2=w.o+ I h
2"""2g""" " I """2g""" + e

(6-2)

An equation for the energy head loss he can be written as
follows

2 2
- ~Y2 !XIYI
he = LSf + CI_ - -I
2g 2g

(6-3)

where

L = discharge weighted reach length
~ = representative friction slope for reach
C = expansion or con1raction loss coefficient

The solution of Equation 6-2 is the basis of water surface
profile computations in programs such as HEC-2. The
standard step method used to obtain a solution requires
successive approximations. A trial value of WS2 in
Equation 6-2 is assumed, and values for he and change in
velocity head are computed and summed to obtain ~WS.
This value is added to the known downstream water
surface elevation 10 compute WS2. The difference
between trial and computed values converges with suc-
cessive trials. The steps in this procedure are as follows:

6-2

a. Assume a water surface elevation at the upstream
cross section (or downstream cross section if a supercriti-
cal proftle is being calculated).

b. Based on the assumed water surface elevation,
determine the corresponding Iotal conveyance and veloc-
ity head.

c. With values from step 2, compute ~ and solve
Equation 6-2 for he'

d. With values from steps 2 and 3, solve Equa-
tion 6-2 for WS2.

e. Compare the computed value of WS2 with the
values assumed in step I; repeat steps I through 5 until
the values agree to within .01 feet (or .01 meters).

6-4. Range of Applicability

The assumptions of the method as described in sec-
tion 6-2 are the basis for determining applicability. Their
effects in modeling are as follows:

e

'I

.

1.

.

~

.1;,

e