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 non- 

 critical 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 

 into two small islands separated by 8.5 km (5.3 mi) of open water, shoals, 

 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 shown on the 1:100,000 maps are marked as to their up- 

 thrown and downthrown sides. These terms refer to the block or mass of rock 



98 



