Olsson Associates

Articles

Olsson dry dam project approved

Wednesday, January 14, 2015

Linda Van Hoosen, Communications

Waverly’s dry dam plan, a project six years in the making, has finally begun to take shape.

The recently approved plan calls for a dry dam to be built near the town. The nearly $3 million earthen structure, which will be designed by Olsson Associates, will hold back potential flood waters from Ash Hollow Creek. The creek begins south of Waverly and runs through the town along the west side and empties into Salt Creek northwest of town.

The call for a plan to reduce flood potential began in 2008 when the Federal Emergency Management Agency (FEMA) updated the floodplain maps for the area. The new maps included more Waverly homes and businesses within the FEMA floodplain than were shown in the previous maps. Home and business owners within the new FEMA floodplain were now faced with a regulated floodplain on their property, which meant reduced property values, difficulty reselling the property, and costly flood insurance premiums.

Beyond property valuation concerns, such widespread flooding potential can also lead to economic stagnation, loss of tax base, and a dwindling population for the community.

Carter Hubbard, a senior engineer on Olsson’s Water Resources team, said the city and the Lower Platte South Natural Resources District (NRD) together developed a plan that weighed several options. The dry dam ended up being the most cost-effective option for flood reduction along Ash Hollow Creek.

A dry dam is similar to a conventional dam, except there is no permanent pond or lake upstream of the dam. Thus, except after rain events, the dam does not hold back water.

“The idea for the dam is to cut off the flow of Ash Hollow Creek before it gets to town and reduce the amount of flow in the creek, thereby reducing the floodplain in town by quite a bit. The proposed dam would take a lot of structures out of the floodplain,” Carter said.

Dry dams work by temporarily holding back water during heavy rains that cause creeks to rise rapidly. The water is contained in the flood pool area upstream of the dam for several days and slowly released back into the stream. The dam is approximately 30 feet tall at its highest point and a quarter-mile long. The plan also calls for an overflow path around the west end that would safely pass excess flows during events much larger than the 500-year event, thereby preventing the dam from overtopping and failing.

One of the project’s challenges has been addressing landowner concerns and objections regarding the property needed for the dry dam. The dam and the overflow would cover approximately 30 acres. The flood pool would extend over an additional 110 acres. The property involved is farmland, and the contrast between the needs of the community and property owner rights came into sharp focus during public meetings. Olsson Associates provided information about the project during the public meetings so the Waverly city council could make an informed decision. At the meeting in December, the city council voted unanimously to move forward with the project.

Carter said the next step in the project is to develop a plan for funding, decide who will own and maintain the property, and determine whether the NRD will participate in the project. Once these decisions are made, design, permitting, and construction of the dry dam will begin.

 

Connect with us