Recharge areas of the Texas Coast are structured such that rainwater is introduced slowly into the aquifer systems. Unless the rain falls directly upon the recharge area of an aquifer, the storage capacity of the soil eventually becomes exhausted, and the water added by the storm can no longer be absorbed by the ground surface. The runoff is then diverted to the bayou system or a reservoir (natural or improved).
A major factor that influences the size of Houston streams and bayous is the size of the area they drain. Another factor is the ability of the land surface to absorb the precipitation that it receives. The initiation of large-scale urbanization within a watershed caused an unintended alteration of the natural hydrologic system. When urbanization renders soil impervious, the net result severely limits the storage capacity of the soil. Additionally, the volume of water stored in the drainage channels is more quickly concentrated – the same volume of water must drain out of the system, but in the urbanized system it will happen in a matter of hours instead of days or weeks. The quick response makes the watershed susceptible to flooding from relatively short intense rainfall events.
Despite several large flooding events that caused severe damage in the early 20th century, no flood control action was taken until 1937, when the Harris County Flood Control District (HCFCD) was created. The HCFCD did not become effectual until 1939 when a flood control master plan was developed. The 13,100-acre Barker Reservoir was completed in 1945, and the 11,600-acre Addicks Reservoir was completed later that decade to serve the Buffalo Bayou watershed. During this time, the eleven independent drainage districts in the Harris County area were merged under the HCFCD so that drainage issues would be handled in the best interest of the Houston area as a whole.
Brays Bayou was channelized and concrete lined from 1955 to 1960 after a destructive flood proved that Buffalo Bayou was not the only area of concern. In 1969, the Houston Post reported that during the previous month, Brays Bayou carried three times as much water as caused in the 1949 flood without resulting in serious damage, leading to the conclusion that this work was highly successful. After these channel improvements were made on Brays, White Oak Bayou was also lined, and was the last bayou in Harris County to be completely concrete-lined.
Flood control activities in Harris County in the 1960’s became more complex and had further reaching effects than ever before. Houston and Harris County experienced unprecedented growth during the 1950’s as the city doubled the area within its boundaries each decade. Substantial amounts of development occurred outside the city limits during the 1960’s, and the trend accelerated during the 1970’s. Measures were taken in 1973 to restrict development in the floodplain, which had previously gone unchecked.
In June 1976, approximately 10 inches of rain fell in a six-hour period in southern Harris County, in the Brays and Sims Bayou watersheds. Flooding and damage was extensive, but the most heavily damaged area was the Texas Medical Center, where damages exceeded $20 million dollars. Rice University, the University of Houston, and the Museum of Fine Arts suffered over one million dollars in damage. Flood waters backed up through sewer pipes and flowed overland through homes and institutions in the lower Brays Bayou watershed. Brays Bayou never overflowed its banks, and thus the cause of the flooding has been contributed to the lack of capacity in the storm sewer system to contain the resulting flow.
This storm identified a previously unrecognized problem regarding urbanization and channelization of streams. When the $26 million Brays Bayou channelization project was undertaken, certain assumptions were made with regard to urbanization in the watershed, and the stream was channeled to accommodate the flow predicted from a 100-year storm event falling on this urbanized watershed. However, the urbanization that occurred exceeded the design estimates, and Brays Bayou could only hold a 33-year return frequency storm as of 1979. Today, Brays Bayou will flood between a five and ten year rainfall event.
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