Culverts built of lumber are usable in the TO. When used, an end area equal to the end area of CMP on the same
slope can be used for hydraulic capacity. When constructing a lumber culvert remember these three factors:
The cover should not be less than the top width of the box (figure 5-3).
Figure 5-3. Lumber culvert cover
Care must be taken in spiking and fastening the lumber so as to have solid construction (figure 5-4).
Figure 5-4. Spiking and fastening a lumber culvert