Houston County Lake H&H Analysis, EAP

In a world of extreme weather catastrophes, dam safety ranks at the top of the hazard mitigation list. Weather patterns that produce extreme rainfall and runoff result in flooding that tests the structural integrity of a dam. Dam safety is regulated by Title 30, Part 1, Chapter 299 of the Texas Administrative Code (TAC 299); which requires an existing, high-hazard dam pass 75% of a probable maximum flood (PMF) without overtopping. The PMF is a theoretical occurrence of the largest flood that would result from a combination of severe meteorological and hydrologic conditions. PMF is an estimation of runoff from the probable maximum precipitation (PMP) that is temporally and spatially distributed. PMP depths are calculated based on statistical data from historic rainfall events.
 
TAC 299 also requires an emergency action plan (EAP) and breach analysis be developed by a professional engineer. This plan identifies the probable inundation area and defines the procedures to be followed to minimize the loss of life and property, should the dam fail. Houston County WCID No. 1 hired KSA to determine if the dam met the minimum hydrologic criteria and develop a viable EAP that outlines the plan of action to take depending on the condition of the dam.
 
Houston County Lake Dam is an intermediate size, high hazard dam that impounds 40,000 acre-feet of water, is 50-feet tall, and has a 1,700-foot-long dam. After an extensive data collection process that included a site visit, KSA’s talented engineers developed a hydrologic and hydraulic (H&H). The hydrologic model encompassed the contributing watershed to determine how much water flowed to the lake. The hydraulic model determined the water surface elevation in Little Elkhart Creek and spanned 19 miles downstream where the flow was 75% contained within the Trinity River. The H&H model was used to develop the breach analysis and EAP. KSA’s PMF analysis determined that Houston County Lake Dam passed 75% of the PMF, is hydraulically adequate, and does not require future modifications to increase the spillway’s capacity. 
 
Houston County was pleased with the findings, not only because it fulfilled regulatory requirements and no construction was necessary, but also because it was conducted in a timely, cost-efficient manner. KSA’s relationship with Houston County began back in the mid-1990s when KSA initially became their engineer of record. Over the years, by successfully completing several projects (including two major water plant improvements) KSA is proudly regarded as their trusted partner and advocate.