Reducing the Cost of Dam Safety Upgrades

Abstract Only - Small earth dams in urban areas are often expensive to upgrade or rehabilitate to meet dam safety criteria. Frequently, this is because there is the presumption that any earth dam with habitable structures downstream must safely pass the probable maximum flood (PMF) without overtopping. In several cases HDR has been able to significantly reduce upgrade coda by investigating potential breach impacts in the downstream reach. The analysis established the magnitude of a critical flood, which is less than the PMF, above which downstream breach impacts decline to an insignificant level. The criteria employed to assess significance is quite simple. If the breach peak evaluated at any damage point downstream is 1sssthan one foot greater than the non-breach peak the impact is considered to be insignificant. Two cases in which this criteria was used are described. The first is Hurst Creek Dam No. 1 in Lakeway, Texas, for which the downstream analysis resulted in the dam being reduced from Klghhazard to low hazard classification. The second is Thompson Park Dam No. 3 in Amarillo, Texas, for which the critical flood was determined to be about 40 percent of the PMF. The paper describes the methodology used to evaluate the downstream impacts. In both cases, the dam breach and storage routing options in the HEC-1 package were used to compare breach and non-breach scenarios. The final recommended plan for Hurst Creek Dam No. 1 involved removing the original dam, which had been previously breached to prevent failure, and completely reconstructing the embankment. This was done at less than half of the estimated cost to upgrade the dam to pass the full PMF. In the case of the Thompson Park Dam No. 3, a deteriorated drop spillway will be replaced and a roller compacted concrete armor section placed on the earth embankment as an auxiliary spillway. The cost of this option is approximately 60 percent of the option to pass the full PMF. In both_ the cost to conduct the breach analysis was on the order of ten percent of the construction cost savings. Guidelines for assessing the potential for reducing inflow design flood are presented.