ecological characterization atlas of coastal alabama - Data Center

Changes in Perdido Pass are among the most extensive identified in this
study . In 1867 the Perdido River channel (now called Old River) flowed
around the east end of Ono Peninsula (now Ono Island), then westward to enter
the gulf . By 1890-1892, this river channel had been partly abandoned and the
major flow from Perdido Bay entered the gulf through a channel in Ono
Peninsula excavated by local residents between 1867 and 1892 (U .S. Army Corps
of Engineers 1973a) . By 1918, water exchange occurred through two inlets
separated by an island . This configuration may have been caused by the
hurricane of 8 September 1917 . Gradually the accretion resulting from the
westward littoral drift closed these two inlets to form a single inlet by
1941 . Between 1941 and 1974, persistent littoral drift had caused the pass
to migrate westward until arrested by the construction of a seawall in the
1960's . The pass in its natural state was about 1 .8 m (6 ft) deep and
presented great hazard to navigation (Ryan 1969) . Safe navigation has been
assured by the construction of several seawalls, which stabilized the pass
and protected a bridge over the inlet . Further migration of the inlet is
unlikely ; however, past experience suggests that a severe disturbance, such
as a direct blow from a hurricane, could breach the island again . The most
recent severe hurricane was Frederic in September 1979 . While much of the
land area west of Perdido Pass east to the Florida state line was overwashed,
there was no permanent breach that would link the gulf with Old River to
create a new pass .
The U .S . Army Corps of Engineers (1971) classified the shores of Mobile
and Baldwin Counties, Alabama, into three categories : non-eroding, eroding,
and critically eroding . Their inventory reveals that of approximately 566 km
(352 mi) of Alabama shoreline, 182 .6 km (113 .5 mi) are experiencing noncritical
erosion and almost 53 km (33 mi) are experiencing critical erosion
(Figure 8) . Critical erosion is so defined either because of the speed of
shoreline recession or because of potential impact on people's structures .
This classification is based on observed responses to normal conditions and
cannot be used to predict changes occurring under abnormal conditions . As an
example, observation of Figure 8 identifies the southeast shore of Dauphin
Island as an area experiencing critical erosion . Between 1901 and 1917,
hurricane surge and associated waves breached Dauphin Island, dividing it
i nto two smal l i sl ands separated by 8 .5 km (5 .3 mi ) of open water, shoal s,
and scattered remnants of the former island . Between 1917 and 1942, the
inlet was filled by natural processes . A less severe breach occurred during
the 4 September 1948 hurricane . When Hurricane Frederic hit the island in
1979, an area to the west of the designated area of critical erosion was
washed over, but the island was not severed . To predict damage to coastal
areas, storm conditions, as well as normal sea and weather, must be taken
into account . The most reliable information available to predict the effect
of storms is historical data, but neither meteorological events nor their
effects can be predicted with much reliability (U .S. Army Corps of Engineers
et al . 1981) .
GEOLOGIC FAULTS
The faults that are sho wn on the 1 :100,000 maps are marked as to their upthrown
and downthrown sides . These terms refer to the block or mass of rock
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