Rainfall Duration and Design Hydrograph
- Rhama Analysis
- 6 days ago
- 3 min read
Peak discharge and the design hydrograph are commonly used for the design of hydraulic structures. In most cases, streamflow measurements are unavailable, and the standard approach is to apply methods such as the Rational Method and design hydrographs derived from maximum rainfall data.
The Rational Method is typically applied to small watersheds of up to 8 km² and assumes a rainfall duration equal to the time of concentration. However, this criterion is not valid when a detention reservoir is involved. In such cases, the objective is to maximize the storage volume by considering variable rainfall durations. Tucci (2000) derives the equation that maximizes storage volume as a function of rainfall duration.
To illustrate these conditions, the 10-year design storm for Brasília was used, together with a detention reservoir having an outlet discharge of 24.4 L/(s·ha), an impervious area of 35%, and a watershed with a 15-minute time of concentration. What is the maximum storage volume (m³/ha) required to maintain this outlet discharge? Figure 1 shows the relationship between rainfall duration (min) and storage volume (m³/ha). When a rainfall duration of 15 minutes is considered, the maximum storage volume is 72.52 m³/ha. As clearly shown in Figure 1, however, this is not the maximum storage volume, which reaches 205.4 m³/ha at a rainfall duration of 60 minutes. The methodology described in the above-mentioned paper identifies and maximizes this storage volume.
This situation becomes even more critical in urban drainage systems because the time of concentration is short, meaning that only part of the rainfall event is considered. Under these conditions, the antecedent runoff may fill the detention reservoir before the period of highest rainfall intensity occurs.
For larger watersheds using a design hydrograph, it is not sufficient to adopt a rainfall duration only slightly greater than the time of concentration. In conventional event-based simulations, the reservoir is assumed to be empty at the beginning of the event, which is generally not the case in practice. For this reason, a rainfall duration of 24 hours is commonly recommended, although the appropriate duration ultimately depends on the storage volumes being analyzed.
Large storage volumes may require even longer rainfall durations. As the simulation duration increases, the event-based version of HEC-HMS (widely used for this type of analysis) becomes inadequate because it does not perform a continuous soil water balance, causing nearly all precipitation to be converted into runoff. Under these conditions, the continuous simulation version of HEC-HMS, or another continuous hydrologic model, should be used.
The hydrologic assumption that rainfall duration should equal the time of concentration cannot be applied to projects involving storage volumes.
The HEC-HMS model (also commonly referred to as the SCS method) has become widely used because of its simplicity in estimating parameters such as the Curve Number (CN), which separates runoff from infiltration. However, it tends to overestimate both peak discharges and hydrograph volumes.
More recently, I published a paper (Tucci, 2025) demonstrating how the Curve Number (CN) can be estimated using regionalized streamflow data combined with the Monte Carlo method, thereby reducing estimation errors. One important consideration is that the regionalization procedure must be representative of the watershed under study.
The referenced papers are available on the ABRHidro website: www.abrhidro.org.br. Select Publications, then access REGA – Latin American Journal of Water Management and search by publication year and article title, or access RBRH – Brazilian Journal of Water Resources and locate the publication following the instructions provided by ABRHidro.
References
TUCCI, C.E.M. (2000). Runoff Coefficient and Peak Discharge of Urban Watersheds. RBRH – Brazilian Journal of Water Resources, 5(1), 61–68.
TUCCI, C. E. M. (2025). Estimation of Peak Discharge and Design Hydrograph in Small Watersheds. Journal of Water Management of Latin America, 22, e12. https://doi.org/10.21168/rega.v22e12
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