Hello

It seems that this rough choice may be considered as reasonable based on the following rules:

- Ease of access to daily recorded data (interpreted as 24-hour data) in many areas.

- The availability of limited hourly and sub-hourly data in the past has given more confidence to development of many hydrology theories compared to daily data.

- Since the duration of rainfall must be equal to or greater than the time of concentration to produce the actual peak flow, continuous 24-hour data ensures that all durations less than 24 hours are included in the calculations.

- Availability of developed rainfall distributions (e.g., SCS 24-hr).

- The time scale of simulation in many developed hydrological models is daily, and this choice can be compatible with the research framework.

I second what Raoof said.

When data is available, and depending on the question i.e., intensity vs volume, you may want to consider 1-, 2-, 6-, or 72-hr duration. For large basins, longer duration rainfalls are controlling events while for catchment-scale studies, shorter duration events are usually critical. Duration is not a critical factor but the temporal distribution is. For instance, you can temporally distribute a 6-hr rainfall event with a 1- or 2-hr event nested within the 6-hr event.

Selecting a reasonable temporal distribution, either synthetic or based on observations, is always challenging.

Since I have been working on overseas H&H projects, there are countries that consider using 24hr precipitation as a common/good practice while CDA (2021) guideline strongly suggested to consider different rainfall durations with respect to the problem that needs to be addressed.

Here are some of the key reasons

the choice of a 24-hour duration is not a one-size-fits-all solution. In some cases, engineers and hydrologists may use different durations or customize design criteria based on local climate, terrain, and specific project requirements.

The preceeding responses answer the question. I would emphasize the widespread availability of 24-hr rainfall data as likely being the original reason that the 24-hr rainfall duration rainfall became a design standard. Also, long-term (multi-decade) rainfall data is typically available only for 24-hr and longer time periods--sub-daily logging of rainfall data has typically not been performed for multi-decade periods at most rainfall measurement stations.

Methods for determining design flows are often dictated by local or state zoning lows. In theory, however, the duration of a design storm really depends on the area of the watershed. Smaller watersheds are likely to have peak flows from more intense small area short duration storms only lasting a few hours. The duration that will likely cause the greatest peak flow from a given watershed is sometimes assumed to be equal to the time of concentration, that is the time that it takes a drop of water from the furthest reach of a watershed to its outlet. Peak flows determined by the rational method use the time of concentration for estimating the storm duration. The NRCS curve number method uses the 24-hr precipitation to estimate total storm runoff in a given day, and then uses the time of concentration to determine the peak flow. For larger gaged watersheds, historic flow records of peak flows are often used to estimate peak flows. As watershed modeling capabilities advance, peak flows may be estimated by physically based models run for 100 or more years of stochastic climate (https://wepp.cloud/weppcloud/). Just because "everybody" is using some method doesn't make it the best method. Have a look at Huffman, Rodney L., Delmar D. Fangmeier, William J. Elliot, and Stephen R. Workman. 2013. Chapter 5: Infiltration and Runoff. Pp. 81-113 in Soil and Water Conservation Engineering, 7th edition. St. Joseph, Michigan: ASABE. Copyright © American Society of Agricultural and Biological Engineers. ISBN 1-892769-86-7. DOI 10.13031/swce.2013.5. for a discussion and examples of these methods.

Dear Amir

Regarding this, I agree with your idea of selecting the time of concentration as the design storm duration, in designing drainage works for relatively small catchments where we can expect uniform storm distribution over the space and time. Normally such storms produce the highest inflow rates at the points of interest. Unit hydrograph methods are generally applicable to the small catchments. Even in small catchments, it is recommended to carry out several trials with different storm durations (beginning from the Tc) when sizing the spillways of reservoirs. In some cases, the larger durations produce the critical outflow depending on the detention capacity of the reservoir. In many parts of the world, I-D-F (intensity- duration-frequency) curves have been developed to facilitate such designs using hydrograph methods.

I don’t see any other reason to use 24 hr as the design duration except the availability of data. Historically, 24 hr was selected as the common time interval to collect rainfalls for practical reasons, mainly the convenience and the cost of collecting data. When you collect rainfall at 24 hr intervals, the real time of the storm may be much smaller and produce higher flow rates at the catchment outlet or your point of interest. Nowadays, since recording rain gauges are commonly used, I think it is better to select time of concentration as the design storm duration for the drainage works. You may refer into my publication https://www.researchgate.net/publication/328578090_Estimation_of_Direct_Runoff_and_Flow_Hydrographs_in_Small_Un-gauged_Catchmentsto find out more details. However, when you are dealing with large complex catchments it is advisable to use a suitable hydrological model, calibrated with observed data.

Thank you very much for your great responses, everybody.

As many of you cited, the utilization of 24-hour rainfall duration in the hydrologic modeling for design purposes may increase the drainage structure capacity; hence the project price, particularly when the contributing drainage area is not large and the time of concentration (Tc) is short. This situation may occur more significantly in rural drainage areas where the NRCS Curve Number (CN) is relatively smaller than the urban drainage areas.

Some design manuals cite that the appropriate storm duration for storm runoff calculations is dependent on the drainage area's hydrologic response. The selected storm duration should be sufficiently long that the entire drainage area contributes to discharge at the point of interest. Therefore, I assume that four or five times the time of concentration (4 or 5 x Tc) might be, in general, a good estimation as the design storm duration. However, this assumption needs to be validated and ascertained. It may increase the storm duration to more than 24 hours or decrease it to less than 24 hours depending on the time of concentration. This way the "Frequency Storm" method can be applied to the rainfall modeling instead of the "24-hour NRCS Hypothetical Storm" method. Again, as you folks noted in your comments, the availability of rainfall data with sub-daily increments is another factor that should be considered. Note that NOAA made the IDF curves available for the entire CONUS ranging from 5 min to 60 days and return periods of 2-year to 1000-year (https://www.weather.gov/media/owp/hdsc_documents/NA14_Sec5_PFDS.pdf).

According to my experience, the shorter the rainfall duration, the higher the intensity (you may examine some I-D-F curves developed by different agencies). The time of concentration (Tc) is defined as the time taken to reach runoff from the most remote part of the catchment to the outlet or the point of interest. As such the period Tc is sufficient to get the contribution (discharge) from the entire catchment resulting the highest flow rate. Therefore, I don’t see any reason for extending storm duration beyond the Tc. Somehow, I would like to hear the comments of other experts in this regard and discuss further.

Your question is very important, as it appeared from the comments of experts, in determining the design discharges of drainage works. Apart from the storm duration, another important factor influencing the design flow is temporal distribution of rainfall. Some design manuals assume uniform distribution while the others rearrange the storm distribution pattern, theoretically, to get the highest possible flow from the design rainfall amount. These methods generally overestimate the discharges, especially when the storm duration is long. Practically, we can see gaps (periods without rain), in such rains, reducing peak flows. I would like to have the opinion of the experts regarding this and some acceptable methods to get realistic estimates of design discharges.

Hydraulic structures such as detention and retention basins are designed to control "volume" of floodwater, thus requiring severe storms of 24h and high frequency to ensure sufficient design capacity. Stormwater drainage systems such as open channels, culverts, and storm sewers are required to deliver "peak" discharges from the watershed under specific frequencies. Here rainfall durations larger than the watershed time of concentration would not affect the design concept, assuming constant rainfall intensity uniformly distributed over the watershed

This is a little off topic but I think it's important to keep in mind, particularly when developing IFD estimates.

24 hour rainfall is different from daily rainfall in that daily rainfall is generally constrained by having the observation made a the same time each day, whereas 24 hour rainfall isn't but it is often constrained by being measured on particular hours, (rather than say minutes). In other words, an analyst will look at hourly data with a moving 24 hour window.

This issue makes a difference in the probabilities of events. For example if observing daily data, there is only a single chance of measuring an amount each day. Whereas for 24 hr data, there are 24 observation times each day. Measuring daily runs the risk of missing a 24 hour measurement that's larger or smaller than the daily amount.

Another example is with respect to record temperature amounts. Some time ago in the U.S., the highest or lowest temperature in each particular month is what was commonly referred to. More recently, records for each Julian day are what is publicized. The effect of this is to have 365 possible record amounts each year rather than only 12 possibilities.

A somewhat similar issue exists with respect to the resolution at which precip is measured. It is discussed in depth in two papers by Rock Durrans:

Durrans, S. & Burian, Steven & Pitt, Robert. (2004). Enhancement of precipitation data for small storm hydrologic prediction.

Durrans, S. (2023). Coping with the Coarse Resolution of Short-Duration U.S. Precipitation Data.

Agree with the valuable comments of the experts. However, still I wander whether the problem is adequately answered or not. As I understand, for designing the drains and culverts which are governed by the peak flows, the design duration equal to Tc is suitable. For detention and retention basins, designed to control flood volume, the longer durations should be considered. For such cases, ‘Guidelines for Design of Dams’ by ‘New York State Department of Environmental Conversation’ recommends the minimum storm duration of 6 hrs for small basins. For larger drainage areas for which the Tc exceeds 6 hrs, it says, the precipitation amounts must be increased by the applicable duration adjustment. As a practicing engineer, I followed this and hope it will be useful to Dr Amir also.

Prema Hettiarachchi As you said all the comments and responses from experts are great and extremely extremely valuable, but the problem hasn't been fully responded yet. The assumption that rainfall duration is equal to the Tc value is valid only for the drainage areas less than 200 acres (~0.81 sq. km.) where the Rational Method is used to calculate the peak flow rates. For the drainage areas over 200 acres (~0.81 sq. km.), in which the hydrograph method (i.e., the NRCS method) was recommended to use, the rainfall duration must be extended to a value greater than Tc; so that, the contribution of infiltration process to the peak flow values has been fully addressed.

Dr Amir

Yes. Rational Formula is only applicable for very small catchments. Unit Hydrograph methods (which take infiltration into account) are recommended for somewhat larger catchments. In cases of large complex catchments comprised of several sub-catchments, detention areas etc, hydrological modelling is recommended to estimate discharges.

Regarding the design storm duration, I think Tc is ok for drains and related structures for which the peak flow is important and longer duration (above Tc) for retention reservoirs and spillways (flow volume is also important). In such cases the design storm duration should be determined by several trials.

Rainfall (design storm) duration is a factor decided by the function of the hydraulic structure rather than the size of drainage area.

I can project 6 different reasons: