§ 21. Preparation contract, plans and estimates. The corporation shall make surveys and prepare plans and specifications for work in connection with the improvement, maintenance or repair of the canal system to be performed under contract. It shall ascertain with all practical accuracy the quantity and quality of all materials to be used and all other items of work to be included in the contract and shall make a detailed estimate of the cost of the same. The quantities contained in such estimate shall be used in determining the cost of the work according to the different proposals received.