FLOOD01647

• County: Statewide
• Title: Optimal Sizing of Urban Flood Control Systems
• Date: 3/1/1974
• Prepared For: US
• Prepared By: COE
• Floodplain - Doc Type: Educational/Technical/Reference Information

the proposed system) exceedence frequency relationships at all damage centers.
The expected value of annual damages is automatically computed within the
simulation. Fig, 2 contains a diagram showing this procedure which is explained
in detail in Addendum 3 of Ref. 1.

FLOOD-CoNTRoL SVSTEM COMPONENTS

The components whose sizes may be automatically determined include detention
storage reservoirs, pumping plants, and diversions, Fixed facilities, e.g., existing
reservoirs, can be included without being considered components to be optimized.
Storage Reservoir Characterization.- The detention storage reservoirs that may
be considered variable in size are those for which it is possible to define the
operating characteristics as a unique function of the storage content within
the reservoir. A reservoir with uncontrolled outlet works, such as an overflow
spillway, exactly meets this requirement, To provide capability for automatic
adjustment of operating characteristics, a reservoir is characterized by the
following:

I. The outflow characteristics of a low level outlet, which is defined by the
center line elevation of the outlet and an orifice equation of the form

Q = Ka (2g) 1/2 (H) exp . . , , . . , . . , , , . , . . . . . . . . . . . . , . , (2)

in which K = orifice discharge coefficient; a = outlet flow area; H = head
on low level outlet; and exp = exponent dependent on tailwater conditions,
0.5 if no tailwater.
2. The overflow characteristics of a spillway which is defined by a weir
equation of the form

Q = K. LH~/2 .........

, . , (3)

in which K. = weir discharge coefficient; L = length of spillway; and H.
= head on spillway.
3. The site storage characteristics which are defined by an elevation-storage
capacity relationship.

.

For an index storage level to be optimized. which is the storage at the elevation
of the spillway crest, the foregoing relationships are merged to define the
reservoir's outflow as a function of the storage level in the reservoir (storage
outflow function), The storage outflow function is subsequently used in the
simulation to route flows through the reservoir by modified Puis procedure,
Two alternative optimization modes are possible for a reservoir. In the usual
mode a reservoir that can be characterized by a low level outlet and an overflow
weir as aforementioned will be automatically adjusted in its index storage capacity,
along with all other system components, to achieve the minimum value of the
objective function. The cost function for the reservoir in the usual mode consists
of a capital cost function and an associated capital recovery factor for converting
the capital cost to annual cost, and the annual cost of operation, maintenance,
and replacement expressed as a proportion of the capital cost. The capital cost
function land acquisition and construction costs, interest during construction,
etc., expressed as a function of the index storage size of the reservoir. The