Longform

Damage Control

Page 5 of 9

In May 1989 and again the following month, violent thunderstorms flooded a total of 2,376 homes, nearly 400 of them along White Oak Bayou. During a routine postflood analysis, the district noticed that, based on rainfall estimates, the amount of flooding in the White Oak watershed seemed inordinately high. "We thought, 'What in the world is going on here?' " Fitzgerald recalls. "When we compared the real flood to the models we had on Brays Bayou, it was a pretty good correlation. But on White Oak, the flood levels were a lot bigger than our models were showing."

The district shelved construction of the phase one channel improvements and hired the engineering firm of Klotz Associates to update the assumptions for White Oak Bayou. Klotz gathered data for more than a year, then performed an analysis that found the hydraulic models used by the county in 1983 contained significant errors. The model, which had been approved by the Federal Emergency Management Agency, factored in large drainage areas that didn't exist and under-represented the amount of new development that had occurred. The model also did not accurately measure the capacity of the 11-mile stretch of the White Oak that had been channelized by the corps, nor did it correctly calculate the effect of bridge abutments and channel bends on water levels at different places along the bayou.

The upshot of Klotz's analysis was that the White Oak Bayou floodplain was much bigger than the Pate Plan had assumed. In a June 1991 letter to Art Storey, who had replaced Jim Green, Wayne Klotz of Klotz Associates informed the flood control district that that flood levels were much higher than assumed at the White Oak's confluence with Buffalo Bayou.

The corrected models also showed water surface levels two to four feet higher than Pate had assumed between West Little York and Alabonson roads. In flat, clay-soiled Houston, the impact on the floodplain of a two- to four-foot rise in water levels depends on where the rise occurs. If that much water spilled onto a natural, undeveloped floodplain, it might go 100 yards. If it spilled into a subdivision, it might spread for half a mile. Unfortunately, most of that stretch of the bayou is subdivisions.

Klotz recommended a "workable plan" that shifted the emphasis of the regional plan downstream to an area that included the Arbor Oaks and Inwood Forest subdivisions, where most of the 1989 flooding had occurred. The new plan consisted of widening the channel from Cole Creek upstream to North Houston-Rosslyn Road and the immediate construction of two detention ponds, one at North Houston-Rosslyn and the other at Tidwell Road. Klotz also recommended the district construct a sheet-pile "transition structure" to control the depth and velocity of the stream as it enters the wider channel.

"I recognize that this recommendation represents a radical departure from the previous plan," Klotz wrote to Storey. "I also recognize that a new set of plans will be required for White Oak Bayou. We are prepared to move quickly on your behalf…"


Commissioners Court approved the Klotz version of the White Oak regional plan in December 1993. A few months later, the flood control district awarded construction contracts for the channel improvements and excavation of the detention ponds.

Meanwhile, the district submitted a Letter of Map Revision, or LOMR, to FEMA that represented the latest floodplain conditions in the White Oak watershed. The LOMR corrected the hydraulic modeling errors found in the '83 flood hazard survey and, using updated assumptions from Klotz Associates, plotted a new floodplain.

At the time, the mapped floodplain on the south side of the bayou, downstream of Fairbanks-North Houston Road, looked like no other in Harris County or, perhaps, anywhere else. A 1990 insurance rate map shows odd fingerlike islands throughout the Woodland Trails West and Woodland Oaks subdivisions. The islands represent land that, topographically, was considered beyond a 100-year storm, even though they are surrounded by land that is deep in the floodplain.

The LOMR submitted by the district would have eliminated those islands, putting both subdivisions entirely in the floodplain. The new floodplain would have also gotten significantly larger upstream to Jersey Village. But before FEMA could complete its review of the map revision, the flood control district withdrew the LOMR at the request of the West Houston Association, a chamber of commerce-like lobbying group that represents real estate interests in the White Oak watershed.

The association had objected to some of the assumptions reached by Klotz and asked for an opportunity to study them more closely. The district authorized an "Ad Hoc Committee for the Resolution of White Oak Bayou Modeling Concerns," which consisted of three engineers representing the West Houston Association, two engineers from the district and a hydrologist from Klotz Associates. The committee met eight times between March and July 1994, sifting through reams of hydraulic and hydrologic data on White Oak Bayou.



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Brian Wallstin