Project: Flood Control Improvements - Dam III Oyster Creek @ Lexington

Dam III is located in Sugar Land along Oyster Creek approximately 2.4 miles downstream of US Highway 59. It was constructed in the late 1960s in support of the Imperial Sugar Refinery of Sugar Land. The dam is owned by the City of Sugar Land and is operated by the Gulf Coast Water Authority.

In accordance with the Texas Commission on Environmental Quality (TCEQ) the City of Sugar Land conducted a hydrology and hydraulics study that included a dam safety inspection, probable maximum flood (PMF) analysis, and a dam breach analysis. As a result of the potential for property damage and loss of life in the event of a dam failure, Dam 3 was classified by the TCEQ as a high hazard dam. According to Rule 299.15 of the TAC, Dam 3 is a small size, high hazard structure and is required to pass 75% of the Probable Maximum Flood (PMF).

Image of Dam III

The Probable Maximum Precipitation (PMP) is theoretically the greatest depth of rainfall for a given duration that is physically possible over a given size storm area at a particular geographic location. As a small-size, high-hazard dam, Dam 3 is required to pass a minimum 75% of the PMF according to State criteria. However, Dam 3 is only able to pass 26.5% of the PMF classifying it as hydraulically inadequate. 

As a result, the city initiated design of improvements in 2015 to bring the dam into TCEQ compliance. The proposed modifications include: removing various trees from the embankment, installing a 90-ft wide concrete emergency spillway along the left (east) end of the dam, armoring both sides of the inlet and discharge channel of the dam with rock riprap, and installing a 3-ft wide drainage swale along the left abutment of the emergency spillway to divert localized drainage into Oyster Creek.