INTRODUCTION .1 Terminology

The run-off that is generated within a catchment through precipitation will depend on the:

• characteristics of the storm event;

• the response characteristics of the catchment;

• and the influence of temporal storage on the run-off.

The temporal distribution of the run-off is reflected in a hydrograph, such as plotted in Figure 3.1.

The flood peak (QP) is reached as soon as the entire catchment contributes to the flood, which is also

Figure 3.1: Typical flood hydrograph

The potential damage that a flood could cause to human and other activities may be related to one or more of the following parameters:

• High flood level (HFL) - the maximum water level reached at a given point during the flood.

• Peak discharge (QP) - the maximum flow rate during the flood.

• Maximum flow velocity - the maximum calculated flow velocity associated with a given flow rate.

• Flood volume - the volume of water that is released from a catchment, responding to a given storm event and catchment characteristics.

• Flood duration - the period of time during which the discharge does not drop below a given limit.

Peak discharge is the most useful parameter in the calculation of the required cross sectional area to convey a flood and to determine the backwater effect (upstream influence) of any structure that influences the normal flow conditions. The peak discharge is directly related to the characteristics and response of the contributing catchment area.

Although the peak discharge does not remain constant as the flood progresses along a watercourse, changes are fairly gradual where there are no tributaries or local temporary storage. It could, therefore, be postulated that the peak discharge is independent of local changes in the watercourse, such as bed slope and cross sectional shape. With the peak discharge having been determined, the high-flood level (flood line) and associated flow velocities may be determined by means of hydraulic calculations (uniform or gradually varied flow relationships). The flood volume and temporal variance of the flow rate can be derived from the hydrograph.

3.2.2 Different methods in use for the calculation of flood peaks

There are different hydrological calculation methods in use that can be applied to road drainage. The proven and most used methods in Southern Africa have been selected for inclusion in this Manual, and they are:

• Statistical methods

• Rational method

• Alternative Rational method

• Unit Hydrograph method

• Standard Design Flood (SDF) method

• Empirical methods.

These methods have been developed by various institutions, and are either based on measured data (statistical); or on a deterministic basis (Rational, Unit Hydrograph and SDF methods); or are empirical relationships. Except for the statistical method, the methods were “calibrated” for certain regions and flood events, and are limited in terms of the size of the catchment areas on which they could be applied. Table 3.2 lists the methods, input data requirements, maximum recommended catchment area for which each procedure can be used and references related to the procedures.

Table 3.2: Applications and limitation of flood calculation methods

Method Input data

Recommended maximum area

(km²)

Return period of floods that could be determined

(years)

Reference paragraph Statistical

method Historical flood peak record No limitation (larger areas)

2 – 200 (depending on the

record length)

3.4

Rational method

Catchment area, watercourse length, average slope, catchment characteristics,

rainfall intensity

< 15 2 – 100, PMF 3.5.1

Alternative rational method

Catchment area, watercourse length, average slope, catchment characteristics,

Catchment area, watercourse length, length to catchment centroid (centre), mean annual

rainfall, veld type and synthetic regional unit graphs

15 to 5000 2 – 100 3.5.3

Standard Design Flood

Catchment area, slope and

SDF basin number No limitation 2 – 200 3.5.4

Empirical methods

Catchment area, watercourse length, distance to catchment centroid, mean annual rainfall

No limitation

(larger areas) 10 – 100, RMF 3.6 Methods that are not included in this manual are the SCS and Run hydrograph methods (see Section 3.7).

It is good practice in the determination of design floods for bridges or large culverts to use more than one of the above methods, and historical records, if available, should also be evaluated. The procedures on which these methods are based are briefly discussed below.

Statistical methods (section 3.4) involve the use of historical data to determine the flood for a given return period. Their use is thus limited to catchments for which suitable flood records are available, or for which records from adjacent catchments are comparable and may be used, as described in detail in Section 3.4. Where accurate records covering a long period are available, statistical methods are very reliable. The method lends itself to extrapolation of data to determine flood magnitudes for longer return periods.

The Rational method (section 3.5.1) is based on a simplified representation of the law of conservation of mass. Rainfall intensity is an important input in the calculations. Because uniform aerial and time distributions of rainfall have to be assumed, the method is normally only recommended for catchments smaller than about 15 km². Only flood peaks and empirical hydrographs can be determined by means of the rational method. Judgment and experience on the part of the user are important in this method, but thanks to improved methods for determining run-off coefficients, subjective judgment is becoming less important.

The Alternative Rational method (section 3.5.2) is an adaptation of the standard rational method.

Where the rational method uses the depth-duration-return period diagram to determine the point precipitation, the alternative method uses the modified recalibrated Hershfield equation as proposed by Alexander ^(3.1) for storm durations up to 6 hours, and the Department of Water Affairs’ technical report TR102 for durations from 1 to 7 days.

The Unit Hydrograph method (section 3.5.3) is suitable for the determination of flood peaks as well as hydrographs for medium-sized rural catchments (15 to 5 000 km²). The method is based mainly on regional analyses of historical data, and is independent of personal judgment. The results are reliable, although some natural variability in the hydrological occurrences is lost through the broad regional divisions and the averaged form of the hydrographs. This is especially true in the case of catchments smaller than say 100 km² in size.

The Standard Design Flood (SDF) method (section 3.5.4) was developed by Alexander ^(3.14) to provide a uniform approach to flood calculations. The method is based on a calibrated discharge coefficient for a recurrence period of 2 and 100 years. Calibrated discharge parameters are based on historical data and were determined for 29 homogeneous basins in South Africa.

Empirical methods (section 3.6) alone are not suitable for the determination of design floods. They usually consist of a combination of experience, historical data and/or the results of other methods.

These methods are more suited to check the order of magnitude of the results obtained by means of the other methods. It should be possible to explain any significant differences analytically.