The purpose of the preliminary plans is primarily to allow the executive director to determine whether the project appears safe and is compatible with existing hydraulic conditions in the area. Preliminary plans should clearly reflect the design concept and indicate how the design was developed. Details of project construction are not required to be shown in the preliminary plans. It is the policy of the commission to evaluate the project from preliminary plans in order that the applicant may determine whether the project concept is to be approved prior to the incurring of large expenditures for a complete development of the final plans and specifications.