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20 |

Living with Dams: Extreme Rainfall Events | 2015

a risk based hydrologic hazard

analysis or site-specific probable

maximum precipitation analysis.

The use of one of the above

recommended site specific

approaches may result in a more

cost effective rehabilitation project

for the dam under review than

using just a prescriptive approach

to spillway design floods.

Extreme Storm Events As

Existing Standard

Of Care

Regardless of the dam’s size,

when the costs of conducting

one of these detailed analyses

is prohibitive for the owner, the

recommended design storm for

high hazard dams is the flood

event resulting from the Probable

Maximum Flood (PMF). The current

recommended standard of care

for dam spillway design to protect

the safety of individuals inhabiting

areas downstream of dams is

summarized in Table 2 from FEMA


Lawmakers may believe that

owners are right when they say

their state is making them follow

some arbitrary design standard

that doesn’t relate to their

situation. However in reality these

requirements are not created in a

vacuum nor are they developed


The worse-case of these extreme

rainfall events has been determined

by the National Weather Service

(NWS) to be the Probable

Maximum Precipitation (PMP) in the

USA. These PMP events are defined

by the NWS as, “the greatest depth

of precipitation for a given duration

that is physically possible over a

given size storm area at a particular

geographic location during a

certain time of year.” Combining

these storm events with the most

severe hydrologic conditions

that are reasonably possible in a

given drainage basin is the basis

of determining the Probable

Maximum Flood (PMF) that is the

national industry standard for high

hazard dam design.

Hazard Pot. Class. Definition of Hazard Potential Classification

Inflow Design Flood


Probable loss of life due to dam failure or misoperation


(economic loss, environmental damage, or disruption

of lifeline facilities may also be probable, but are not

necessary for this classification)


No Probable loss of human life but can cause economic

0.1% (1,000 - year) Annual Chance

loss, environmental damage, or disruption of lifeline

Exceedance Flood

facilities due to dam failure or misoperation


No probable loss of human life and minimal

1% Annual Chance Exceedance Flood

economic and/or environmental losses due to dam

(100-year Flood) or a smaller flood

failure or misoperation

justified by rationale

Table 2

Living with Dams:

Extreme Weather


How can one reduce the

chances of a dam failing from

an extreme rainfall event?