When the Westphalia Dam was deemed hydraulically inadequate, ACCRWD No. 1 hired BG Consultants to design modifications that would alter the dam and take it out of the Division of Water Resources’ (DWR) dam classification. However, the owner of the site property requested a pond remain on the property. BG Consultants delivered plans to meet the needs of ACCRWD No. 1, the needs of the property owner, and comply with the State of Kansas Obstructions in Streams Act.
The modifications included:
- Removing the existing primary spillway
- Removing the auxiliary spillway control section
- Lowering and widening the auxiliary spillway to create a channel through the embankment
- Modifying the top of the dam so a one percent rainfall event would result in water surface elevation of no more than 1080.75 feet and the resulting impounded storage less than 50 acre-feet.
The improvements consisted of constructing a channel to move un-detained water from the reservoir to the downstream Thomas Creek channel. The channel was built at the location of the existing auxiliary spillway. Water flows through the existing reservoir as subcritical flow and enters the constructed channel designed to maintain subcritical flow. The water flows over a weir control section at the downstream end of the constructed channel, transitioning from subcritical to supercritical flow as it discharges into the existing Thomas Creek channel.
These improvements resulted in a structure less than 25 feet tall that impounds less than 50 acre-feet of water. The structure was therefore no longer classified as a dam by the DWR. The site continues to retain water; when the reservoir is full, additional stormwater runoff entering the reservoir passes un-detained through the site.