Colonial Pipeline will begin constructing new crossings for its two product pipelines under the San Jacinto River near Houston to replace lines that ruptured and caught fire last October during heavy flooding on the river.

Nearly three dozen pipelines were idle or ordered shut, disrupting supply patterns from Gulf Coast refineries to the Northeast, when floodwater cut a new channel after days of heavy rain, exposing a number of pipelines.Noel Griese, Colonial spokesman, said the $5.5 million project will take at least two months. Brush-clearing work will begin this week, with heavy equipment moving in to handle the direction-drilling task in early August.

Mr. Griese said a transfer of product to the new lines may take place in late September or early October.

He said the lines would be shut "very briefly - eight or 16 hours" - at that time to complete welding and testing of the new lines.

Sunland Construction of Eunice, La., has been contracted to bore two 3,000- foot holes 30 feet under the riverbed to replace concrete-covered pipe, which was used to repair the ruptured lines after the flooding subsided, he said.

One hole will be used for a 40-inch pipe to carry gasoline, and the second will replace Colonial's 36-inch pipe carrying fuel oil and kerosene, Mr. Griese said.

Mr. Griese said the lines were previously buried 16 feet under the river bottom. The additional depth to 30 feet, and the extension of the buried line across the area where the new channel appeared, will give added security in case of future flooding, he said.

Although the flooding last fall was considered likely to occur only once every 100 years, "the new crossing will be more conservative," said Mr. Griese.