Section 5: Time of Concentration
Anchor: #i1026124Description
Several common hydrologic methods require an estimation of the time of concentration. This section provides guidance on ways to estimate time of concentration and covers the following topics: description, flow components, and procedure to estimate time of concentration. For additional information on time of concentration, refer to the TR55. You may use other published methods at your discretion subject to the documented limitations of the methods.
Anchor: #i1026134Time of Concentration
The time of concentration (tc) is the time at which the entire watershed begins to contribute to runoff; this is calculated as the time taken for runoff to flow from the most hydraulically remote point of the drainage area to the point under investigation. Use of the rational formula requires the time of concentration for each design point within the drainage basin. The duration of rainfall is then set equal to the time of concentration and is used to estimate the design average rainfall intensity (I). There may be a number of possible paths to consider in determining the longest travel time. Identify the flow path along which the longest travel time is likely to occur. This is a trial and error process.
Generally, it is reasonable to consider three following components of flow that can characterize the progression of runoff along a travel path: overland flow (sheet flow), shallow concentrated flow, and conduit and open channel flow (or concentrated channel flow).
One way to estimate the overland flow time is to use Figure 5-4 through Figure 5-7 to estimate overland flow velocity for a chosen path length. The path length divided by the velocity yields a travel time. For design conditions that do not involve complex drainage conditions, use Figure 5-4 and Figure 5-5. This method is most appropriate for distances of up to 525 ft. (160 m) over open paved and grassed areas such as parking lots, roadways, verges, and landscaped areas.
For each drainage area, determine the distance (L) from the outlet of the drainage area to the most remote point. Determine the average slope (S) for the same distance. Refer to Section 6 for discussion of the runoff coefficient(C).
Figure 5-4. Velocities for Upland Method of Estimating Time of Concentration--English (Adapted from the National Engineering Handbook Volume 4)
Figure 5-5. Velocities for Upland Method of Estimating Time of Concentration--Metric (Adapted from the National Engineering Handbook Volume 4)
For simplicity, you might employ Figure 5-4 and Figure 5-5 for shallow flow in gutters and swales. Alternatively, you might employ the method outlined in the following paragraphs.