The 7.4-acre Lugar Dam is a size 1, class “B” (significant hazard) dam. Its 400-foot-long by 28-foot-high dam uses ungated principal and emergency spillways. In response to demands by the Kansas Department of Agriculture, the owner contracted Water Resources Solutions to first assess the dam in 2013. WRS conducted the dam inspection using checklist criteria of USDA’s Natural Resources Conservation Service.
Because the dam had not been properly permitted by the state when it was built in 1975, Kansas was requiring it either be permitted by completing all the statutory stability analyses or that it be removed from state jurisdiction by lowering its height to less than 25 feet. WRS was hired to perform the necessary calculations and documentation to support the owner’s preference to pursue the option of lowering the height of the dam.
WRS delineated the drainage area to the lake using Arc Map 10.1 and contours provided from Wyandotte County. Using soils data from the USDA Web Soil Survey and an aerial photo from the Data Access and Support Center, the composite curve number was calculated by the SCS Curve Number Method. The storm event runoff was analyzed using HEC-HMS. All storm events through the 100-year event were modeled using a 24-hour storm event. The 0.4-Probable Maximum Flood 6-hour storm was also determined per state requirements. For this storm event, the curve number was changed to the ARC III condition, as the state requires. The runoff was routed through the lake using the HEC-HMS model program.
Once WRS analyzed alternative design solutions, it became apparent to the owner that the alternative solution of lowering the dam height was too costly. Therefore, the selected improvements included regrading the existing auxiliary spillway to pass the 0.4-PMF event flows while providing 3 feet of freeboard.
WRS completed all proposed modification design reports, design and construction verification as required by the state, outlining the procedures and results of the hydrologic and hydraulic analysis. The new auxiliary spillway was modeled using the USACE’s HEC-RAS modeling program. The auxiliary spillway was sized to pass the 0.4- PMF storm event with 3 feet of freeboard from the top of the dam. WRS also conducted construction oversight on the final dam improvements.