The design storm

Rainfall usually varies significantly with respect to time and geographical location. A rainfall event is characterised by the duration and the intensity or total quantity of rainwater that is falling down. A design storm on which the design of a wastewater management facility is based is a particular combination of rainfall conditions for which runoff is estimated. The magnitude of a design storm may be expressed as a total quantity of precipitation such as millimetres of rainfall or as a short-term intensity such as millimetres per hour. Intensity can be calculated by dividing the total quantity of precipitation by the catchment area and the duration of the rainfall event.

The recurrence interval is a way of expressing the probability that a storm of a given size or intensity may occur at a specific site. Recurrence interval, or frequency, is the average time between storms of a given magnitude. A 5-year storm is large enough that is has recurred, on the average, in only one of every 5 years in the local rainfall record. The probability of occurrence in any one year is the reciprocal of the recurrence interval. This means, the 5-year storm has a 20 percent chance of occurring in any one year, the 100-year storm has a 1 percent chance.

When choosing the design storm for designing a stormwater facility one needs to balance the risks and the costs. The selection of a large, infrequent storm as design storm reduces the risk of failure of the facility but increases the costs for construction and, thus, can make the facility uneconomical. For culverts and detention basins that drain local streets and prevent local drainage problems a 2- to 25-year recurrence interval is common. On the other hand, in extremely sensitive situations, where people's homes would be seriously damaged by flooding or their lives would be endangered, the most appropriate storm is the maximum probable storm, which can be larger than the 100-year storm. Usually local regulations specify the recurrence interval to be used. In Germany a 1-year recurrence interval and a duration of 15 minutes is used for the design of conveyance facilities, whereas infiltration systems are designed using a 5-year recurrence interval. Therefore, more intensive precipitation is used as a basis for the design of infiltration facilities in order to avoid the frequent failure of the systems.

Figure 5: Example of an intensity-duration-frequency curve

Beside the recurrence interval, the duration of a storm is an important parameter. A storm that is short in duration can be very intense. As duration continues, high intensity is not maintained and therefore the average intensity decreases (See figure 5). In specifying a design storm, you must specify both recurrence interval and duration. Since every geographic location has a different rainfall pattern, local rainfall statistics can be useful to derive the absolute rainfall quantity of the design storm.

The drainage area

The drainage area, or watershed, is the land area that drains to the point at which you estimate runoff. Any rainfall runoff model requires you to identify the drainage area and to specify its size, soil, and condition. A drainage area is identified by defining its boundaries on a map. For this, you need to first identify exactly the location where you are going to estimate runoff. Then you need to draw the boundary starting from the outlet and moving uphill on each side by the shortest (steepest) possible path perpendicular to the contour lines. This procedure finally results in the delineation of the watershed. Having drawn a drainage area's boundary correctly, you are in a position to estimate its size and to characterise its land use and soils to meet the needs of the rainfall runoff model you are using.

Time of concentration

Runoff's travel time is one of the watershed characteristics that can strongly influence the rate of storm flow. If a given volume of runoff drains off a drainage area quickly, the peak rate of flow at the outlet is correspondingly high. Time of concentration is a special case of travel time. It is the maximum amount of time runoff from any point in a drainage area takes to flow to the outlet. Among a number of alternative paths that runoff could take from distant parts of a watershed, time of concentration is defined by the longest possible time, whether or not it involves the longest distance.