Whenever a county legislative authority enters into a cooperative agreement with a conservation district as provided in chapter 89.08 RCW, the agreement may specify that the county will participate in the cost of any project which can be anticipated to result in a substantial reduction of the amount of soil deposited in a specifically described roadside ditch normally maintained by the county. The amount of participation by the county through the county road fund shall not exceed fifty percent of the project cost and shall be limited to those engineering and construction costs incurred during the initial construction or reconstruction of the project.