1987-08-24 Public Hearings and Regular Meeting

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e e 10M, BS thru JA to RTH, RE: Lomax CIP Drainage Improvements, Page 2 . As a first step, Flood Control agreed to fund two master watershed studies at a cost of about $150,000 each. The District entered into agreements with Landev Engineers (Lomax) and Espy-Huston (Little Cedar Bayou) in the summer of 1986. (The Little Cedar Bayou study is about three months behind the Lomax study.) Staff met with Flood Control and Landev in November 1986, March 1987, and July 1987 (twice) to rev iew prel iminary find ings, the ultimate improvements plan, and the interim improvements plan. Councilperson Betty Waters participated in the July 10, 1987 meeting. CITY ENGINEER COMMENTS Four alternative plans were considered in the study. A table comparing those alternatives is attached to this memorandum. . The master drainage plan chosen by Flood Control for Lomax departs from the city comprehensive plan approach of diverting the runoff from the western part of Lomax to Big Island Slough. The study contractor looked at (1) diverting the runoff from the land west of the north-south Exxon pipeline corridor which crosses the airport (AI ternat i ve III) and (2) divert ing the runoff from enough land in Lomax to keep from having to enlarge the culverts under Strang Yard (Alternative IV). Both these alternatives were studied under the restriction that Armand Bayou must not be harmed by drainage improvements to Big Island Slough. Consequently, a regional detention "lake" at the confluence of Armand Bayou and Big Island Slough would be required to capture excess runoff caused by these improvements. And, since Flood Control does not plan to update its model of the Armand Bayou watershed until 1988 or later, the cost of this detention facility is not available; nor is that cost included in the cost of Alternatives III and IV. Another problem with diversion of some of Lomax's runoff to Big Island Slough is that the land in that area of Lomax is several feet lower than it is at the Slough. This means the maximum water surface profile in the slough would have to be lowered to accept Lomax's runoff. Because Armand Bayou must be left alone, the Slough would have to be widened as much as 70 feet to increase its carrying capacity. This widening would require enormous amounts of excavation and concrete slope paving. It would also lead to a need to enlarge or replace the bridges at Red Bluff, Fairmont, Hillridge, Spencer, and North "L". In short, the Big Island Slough downstream costs generated by diversion of ~ of the Lomax runoff to the Slough are so prohibitive that city staff agrees with Flood Control that Alternatives III and IV are not viable alternatives. .