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.
Figure 5-6. Overland Time of Flow--English (Adapted from Airport Drainage, Federal Aviation Administration, 1965)
Figure 5-7. Overland Time of Flow--Metric (Adapted from Airport Drainage, Federal Aviation Administration, 1965)
You can estimate pipe or open channel flow time from the hydraulic properties of the conduit or channel. Generally, for department application, it is reasonable to assume uniform flow and employ Manning’s Equation for Uniform Flow with the following. open channel and conduit flow considerations.
For open channel flow, consider the uniform flow velocity based on bank-full flow conditions. That is, the main channel is flowing full without flow in the overbanks. This assumption avoids the significant iteration associated with other methods that employ rainfall intensity or discharges (because rainfall intensity and discharge are dependent on time of concentration).
For conduit flow, in a proposed storm drain system, compute the velocity at uniform depth based on the computed discharge at the upstream. Otherwise, if the conduit is in existence, determine full capacity flow in the conduit, and determine the velocity at capacity flow. You may need to compare this velocity later with the velocity calculated during conduit analysis. If there is a significant difference and the conduit is a relatively large component of the total travel path, recompute the time of concentration using the latter velocity estimate.
Anchor: #i1026229Procedure to Estimate Time of Concentration
Use the following procedure for estimating time of concentration:
- Divide the flow path into reach lengths along which flow conditions remain reasonably consistent. Characterize the progression of runoff along a travel path as either overland (or sheet) flow, shallow concentrated flow, or concentrated channel or conduit using the table titled Characterizing Runoff Progression.
- For each identified reach length, estimate the travel time using a method that is appropriate for the flow conditions. The Flow Conditions and Travel Time Methods table provides general guidance. Compute the time for each component reach using Equation 5-1.
- Determine the total time. Add the individual travel times to determine the total time. The total time is given in Equation 5-2.
- Choose an alternate flow path and repeat steps 1 and 2, as necessary.
- Select the path that results in the longest time. This
is the time of concentration (tc), that is,
tc = T (max), but TxDOT recommends a minimum
time of concentration of 10 minutes. If tc is
less than 10 minutes, use 10; otherwise, use the actual tc.
Anchor: #i1010704Characterizing Runoff Progression Natural Drainage Areas
Flow Type
upper reaches
overland (or sheet) flow transitions to shallow concentrated
lower, larger reaches
concentrated flow in swales, ditches, creeks, and rivers
Anchor: #i1010717Flow Conditions and Travel Time Methods If flow is:
...and the drainage conditions are:
...then the suggested method is:
overland
simple drainage conditions like open paved, grassed areas
shallow concentrated
gutters and swales
Velocities for Upland Method of Estimating Time of Concentration--Metric (Adapted from the National Engineering Handbook Volume 4) Figure 5-4
conduit and open channel
assume uniform flow
Manning’s Equation with considerations
Equation 5-1.
where:
- tn = travel time over nth reach (min)
- Ln = length of nth reach along flow path (ft. or m)
- vn = estimated flow velocity for nth reach (fps or m/s)
Equation 5-2.
where:
- T = total time along flow path (min)
- M = number of reaches in flow path
- n = reach number
Peak Discharge Adjustments
In some cases, runoff from a portion of the drainage area that is highly impervious may result in a greater peak discharge than would occur if the entire area were considered. In these cases, you can adjust the drainage area and time of concentration by disregarding those areas where flow time is too slow to add to the peak discharge. Sometimes it is necessary to estimate several different contributing areas and associated times of concentration to determine the design flow that is critical for a particular application.
Anchor: #i1026337Overland Flow Path Selection
In drainage system design, the overland flow path is not necessarily perpendicular to the contours shown on available mapping. Often, the land will be graded and swales and streets will intercept the flow that reduces the time of concentration. Exercise care in selecting overland flow paths in excess of 200 ft. (60 m) in urban areas and 400 ft. (120 m) in rural areas.