SMALL DAMS PETITS BARRAGES

8.6 DAM BREAK AND INUNDATION MAP
The analytical requirements for an EAP include the information necessary to conduct dambreak
analyses and to prepare inundation maps. The process of developing a workable EAP must
necessarily begin with the knowledge of what areas will be flooded as a result of a dam failure,
so that the jurisdictions and agencies and individuals involved in the implementation of the EAP
can be identified.
The tools for identifying the areas flooded and developing the notification procedures are
usually dambreak analyses and inundation mapping for large dams. Considering, therefore, the
great number of small dams, such technology can be applied only for the small dams with
extremely high hazard potential rating, using a more simplified way for the others small dams,
as will be presented at the end of this item.
Several different inflow conditions may need to be investigated to determine the appropriate
condition prevailing at the time of a dam failure in order to ensure that the EAP includes all
communities that need to be notified. A “fair weather” dam failure, that means reservoir at
normal full elevation, normal stream flow prevailing, is generally considered to have the most
potential for human life loss, primarily due to the element surprise. However, a failure of a dam
during flood flow conditions will result in flooding downstream areas to higher elevations than
during “fair weather” failure.
Experience has shown that the emergency management agencies will use the inundation maps
to develop their evacuation procedures, using both the “fair weather” breach and also a failure
during a flood level approaching the inflow design flood. Usually basic or hydrological maps of
1:10 000 scale are used for inundation line depiction.
One of the most thoroughly documented failures is the Teton Dam Failure, that took place on
June 5th of 1977, and the work presented by Graham/2008 [9] presents detailed signs of its
consequences and the water level hit by the flood. Downstream from the Teton dam, the river
goes along a Canyon for approximately 8 kilometers (km), and then meanders through a
relatively flat plain and divides into two forks about 7.4 km downstream from the end of the
canyon.
Based on the data about this accident, Figure 8.4 presents the estimated water level
downstream of small dams with 5, 10 and 15 m high, after a dam break. Considering that the
Teton dam had a canyon in the first 8 km, it is possible to conclude that in cases where the
valley is not so narrow, the water level would be below those plots. Anyway, these data will be
valuable in estimating the maximum water level downstream, with some safety factor already
computed.
Thus for small dams with 5 to 15 m high the water level will be higher than 0.5 m, below that it
is possible to consider that the damages will be minimum at the following distances
downstream the dam:
� Dam 5 m high ……………………………. 4.7 km
� Dam 10 m high ………………………….. 7.0 km
� Dam 15 m high ………………………….. 7.0 km
These data are important in order to show us that in the case of small dams, we usually have to
take care of persons, structures, highways and industries located up to 5 to 7 km downstream,
in case of a dam break.
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