STEP 10. Determine the travel time of each flow path and select the longest time at the basin TOC. Again, we
will use representative flow paths to estimate the travel time. Remember, we are trying to account for the time it
will take water to flow from the start of each path to the culvert inlet, so that we can determine the TOC.
To calculate the travel time we have to consider the two flow conditions that exist---sheet and ditch flow.
Sheet flow will be estimated using figure 2-2, page 2-7. Notice that on the graph there are a series of curves, each
with line and curvilinear portions. The slope of each curve indicates the velocity at a given point along the flow
path. In the curvilinear portion, the slope is initially zero and steepens until it becomes linear. This represents the
fact that initially water moves slowly and picks up speed as the depth of the water increases. It is initially slow-
flowing (sheet flow) and gradually it becomes faster (ditch flow) as it progresses. At some point along the path
the turbulent flowing water reaches a steady state velocity. Figure 2-5, page 2-11 shows that the slope and
"slickness" of the flow path dictates how quickly it transitions from sheet to ditch flow.
NOTE: If the slope exceeds the value of the 10 percent curve, the water will be at a steady state of velocity. Use
the 10-percent curve.
To use the graph, enter at the right hand vertical edge at the corner the type of the flow path, horizontally to the
left, until you reach the curved label labeled with the slope of the path. Follow it up or down until it intersects the
horizontal line that equals, the flow-path length. Finally, read travel time which is found by drawing a vertical
line down to the time in minutes axis on the bottom of the chart.
Using the chart below, calculate the ditch travel time using the tabulated format given in Step 7, page 2-12.
Table 2-4. Basin flow paths
Flow travel time in a ditch condition can be estimated using the ditch flow table. The table lists velocities in a
ditch for various slopes. Travel time can then be calculated using this simple formula: