DWR_2801666

• Title: WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 1, ISSUE 2, JULY 2013
• Author/Source: URS
• Keywords: WAVE RUNUP, DESIGN OF RIPRAP, SLOPE PROTECTION, WAVE ACTION, DESIGN, OUTLET WORKS AIR VENTS
• Document Type - Reference Library: Research, Thesis, Technical Publications
• Document Date: 7/31/2013
• Year: 2013
• Team/Office: Dam Safety

14
Fortunately, for small to medium size dams where air
vents are likely not nearly as costly as for large dams, a
conservative design approach summarized below can
be employed, wherein the air vent is oversized,
negating the need for rigorous hydraulic analysis or
model studies to account for all the variables. In cases
where cost is a more significant issue, such as for low
budget projects or for larger or more complex dams, a
number of references describing alternate
methodologies are provided below.
A Generalized, Conservative Design
Approach
For flow in gated closed conduits with free surface
open channel flow conditions (i.e., jet flow and air drag
flow), the following equation, obtained from the 1980
publication Air-Water Flow in Hydraulic Structures (See
references for full citation.) may be used to calculate
maximum theoretical airflow rate:
(

)

where:
(

) = Air Demand Ratio
Qa = Volume Flow Rate of Air
Qw = Volume Flow Rate of Water
Ad = Cross Sectional Area of Conduit
Awp = Maximum Cross Sectional Area of Water in
Conduit
Ideally, a conduit water surface profile should be
calculated for a range of gate opening heights to arrive
at Awp. Alternatively, Awp can be approximated from the
water surface profile corresponding to a gate opening
of 75 percent under maximum design head, as studies
have shown that maximum air demand typically occurs
at/near 75 percent gate opening and maximum design
head. As a rough check, the design engineer should
verify that the maximum volume flow rate of air is
approximately equal to the maximum flow rate of
water.
For cases where the water surface profile indicates
that a hydraulic jump will occur, the following equation
from Air-Water Flow in Hydraulic Structures may be
used:
(

) ( )
where:
Fr = Froude Number Upstream of the Hydraulic Jump
(Note: Fr is a dimensionless index of flow regime (i.e.,
subcritical or supercritical)).
In a circular pipe, Fr can be calculated from the flow
depth y by using the following equation:

( )
where:
V = Mean Flow Velocity
g = Gravitational Constant
ye = Effective Depth = A/T
A = Cross Sectional Area of the Water in the
Conduit
T = Top Width of Flow Passage = 2[y(D-y)]1/2
D = Conduit Diameter
Y = Flow Depth
After Qa is calculated, a maximum design air velocity
can be selected, and the cross sectional area and
diameter of the air vent can be calculated. An example
calculation using this design method is provided at the
end of this article.
As a side note, the Bureau of Reclamation
conservatively designs their outlet conduits so that a
hydraulic jump will theoretically never occur, while the
U.S. Army Corps of Engineers (USACE) allows hydraulic
jumps in outlet conduits at their dams.
Alternative Design Methodologies
The 1980 USACE Engineering Manual Hydraulic Design
of Reservoir Outlet Works (EM 1110-2-1602), together
with “Hydraulic Design Criteria” 050-1 and 050-2,
present a method of estimating air demand and sizing
the air vent based on an envelope design curve that
was developed from outlet works air demand data
from 5 different dams with heads ranging from 24 to