Article Written by:
David E. Winslow, P.E., LJA Engineering & Surveying, Inc.
Tropical Storm Allison made landfall near Galveston on June 5, and over the next four days produced record-setting rainfall and flooding in parts of southeast Texas. Rainfall amounts over a large area of Harris, Galveston and Montgomery Counties ranged from 10 inches to as much as 35 inches. Flooding in these areas damaged over 20,000 houses and businesses, shut down major facilities at the Texas Medical Center and inundated Houston's downtown tunnel system.
Although significant rainfall occurred on June 5 and 6 as the storm first passed through southeast Texas, the major flood-producing rain occurred during a period of approximately 12 hours on the evening of June 8 and the morning of June 9. Rainfall amounts averaging over 12 inches fell over almost half of Harris County, with generally only the western third of the County escaping the heavy rain. Rainfall totals for the twelve-hour period between 5:00 p.m. to 5:00 a.m. obtained from the Harris County Flood District's (HCFCD) rain gage network are shown on Exhibit 1. Recorded point rainfalls for the 12-hour period exceeded 14 inches at 13 different gaging stations in four major watersheds. Based on rainfall frequency curves published by the U.S. Weather Service, 11 inches is a 100-year rainfall for a 12-hour period. Extrapolating the Weather Service data, 12 inches in 12 hours is around a 200-year event. The point rainfalls exceeding 14 inches at the 13 sites could be expected to have a frequency over 1,000 years. At two gages on Greens Bayou, over 25 inches were measured during the 12-hour period. The graph on page 6 shows the time history of rainfall for one of these gages.
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12 HOUR RAINFALL
FRI, JUNE 8, 5:00 P.M. - SAT, JUNE 9, 5:00 A.M. |
With the intense rainfall that occurred over the area on June 8 and 9, combined with the fact that the soils were generally saturated from the prior rains earlier on June 5 - 7, widespread flooding occurred. The U.S. Geological Survey operates 53 stream flow measuring stations in the Harris County area. Peak flood heights for 22 of these stations exceeded the previous historical peak flood levels. For most of the gaging stations on the streams in the eastern half of Harris County, including stations on Greens, Halls, Whiteoak, Brays, Hunting, Vince, and Berry Bayou, record flood heights were measured. Although most of these stations have been in existence for 40 years or less, several have existed for 80 years or more.
As might be expected, major flood damages occurred along the affected streams. However, because of the intensive rainfall over large areas, flooding due to sheet flow that exceeded the capacity of local storm sewers was also a significant factor in causing flood damage away from the major streams.
Design Criteria
After a major flood like Allison, questions always arise concerning ways to avoid this type of disaster in the future. Could design criteria or building standards be changed? Currently, storm sewers are typically designed for rainfall frequencies ranging from two to five years, with the streets expected to store and convey larger rainfalls. Man-made drainage ditches are now generally designed to contain the 100-year flood. For larger natural channels, most regulatory agencies would like to provide 100-year protection, but in many cases funding limitations, federal cost-benefit criteria, and environmental issues limit the level of protection to 25 to 50 years. Finally, flood insurance requirements adopted in the mid-70's dictate that homes be built at or above the 100-year level.
So, what could be done to reduce damages in the future? Concerning local street flooding, storm sewers could be designed for a higher frequency event. However, the cost and downstream impact have been debated in the past numerous times and the conclusion has usually been to not change storm sewer design. What was decided by numerous governmental agencies over 15 years ago is that the street system should be designed to convey excess sheet flows during an "extreme event" without damage to structures. Although it's too early to tell for sure, preliminary assessments of areas developed under this relatively new criterion indicate that less flooding occurred than in areas developed without the criterion. Concerning the major drainage ditches and streams, it's difficult now to obtain funding for projects which provide less protection than a 100-year design, so to design for a 200-year event would be that much more difficult to fund.
Buy Flood Insurance
So what else can be done? Building higher off the ground is one option to be considered, with relatively minor economic impact. Through the FEMA buyout program, severely damaged structures that have been repetitively flooded are being bought by the HCFCD. And finally, there's flood insurance. Although it won't stop flooding, it certainly can get people back on their feet financially after a catastrophe. Local regulatory agencies, such as HCFCD, as well as FEMA, are constantly telling homeowners to buy flood insurance, even if they're not in a designated flood zone. As we've seen from Allison, an extreme flood event can happen, and sooner or later it will happen again.
