If in the construction of such ditches the same shall cross any public road, it shall be the duty of the drainage commissioners to notify the board of supervisors of such county in which such public road is located, at some regular meeting of said board held prior to a day which is thirty days next before the time fixed in such notice for the time at which the proposed work shall be constructed across said public road, stating in such notice the width and depth of such proposed work. It shall be the duty of the board of supervisors to cause to be removed and constructed, at the expense of the county, all bridges necessary to be removed or constructed, same to be done at such time as is reasonable, with a view to the convenience of the public and without unreasonable delay to the prosecution of such work.
Contracts may be made by the board of supervisors for such removal and construction of such bridge or bridges, without first advertising for bids where the cost of any one bridge does not exceed one hundred dollars.