DEPTH-AREA-DURATION (DAD) CURVES

2.8 DEPTH-AREA-DURATION (DAD) CURVES

Rainfall rarely occurs uniformly over a large area; variations in intensity and total depth of fall occur from the centres to the peripheries of storms. From Fig. 2.13 it can be seen that the average depth of rainfall decreases from the maximum as the area considered increases. The average depths of rainfall are plotted against the areas up to the encompassing isohyets. It may be necessary in some cases to study alternative isohyetal maps to establish maximum 1- day, 2-day, 3 day (even up to 5-day) rainfall for various sizes of areas. If there are adequate self-recording stations, the incremental isohyetal maps can be prepared for the selected (or standard) durations of storms, i.e., 6, 12, 18, 24, 30, 42, 48 hours etc.

Step-by-step procedure for drawing DAD curves: (i) Determine the day of greatest average rainfall, consecutive two days of greatest

average rainfall, and like that, up to consecutive five days. (ii) Plot a map of maximum 1-day rainfall and construct isohyets; similarly prepare

isohyetal maps for each of 2, 3, 4 and 5-day rainfall separately. (iii) The isohyetal map, say, for maximum 1-day rainfall, is divided into zones to repre-

sent the principal storm (rainfall) centres.

HYDROLOGY

(iv) Starting with the storm centre in each zone, the area enclosed by each isohyet is planimetered.

(v) The area between the two isohyets multiplied by the average of the two isohyetal values gives the incremental volume of rainfall. (vi) The incremental volume added with the previous accumulated volume gives the total volume of rainfall. (vii) The total volume of rainfall divided by the total area upto the encompassing isohyet gives the average depth of rainfall over that area. (viii) The computations are made for each zone and the zonal values are then combined for areas enclosed by the common (or extending) isohyets. (ix) The highest average depths for various areas are plotted and a smooth curve is drawn. This is DAD curve for maximum 1-day rainfall. (x) Similarly, DAD curves for other standard durations (of maximum 2, 3, 4 day etc. or

6, 12, 18, 24 hours etc.) of rainfall are prepared. Example 2.5 An isohyetal pattern of critical consecutive 4-day storm is shown in Fig. 2.13.

Prepare the DAD curve.

Rain-gauge stations cm cm

Storm centres

Fig. 2.13 Isohyetal pattern of a 4-day storm, Example 2.5

PRECIPITATION

Solution Computations to draw the DAD curves for a 4-day storm are made in Table 2.1.

Table 2.1. Computation of DAD curve (4-day critical storm)

Storm Encom Area

Incremen- Total Average centre

passing enclosed

between tal volume volume depth isohyet

range

isohyetal

(cm.km 2 ) (8) ÷ (3) (cm)

isohyets (cm.km 2 )

5022.8 21.3 Plot ‘col. (9) vs. col. (3)’ to get the DAD curve for the maximum 4-day critical storm, as

shown in Fig. 2.14.

4-day stor m

Area in (1000 km 2 ) Fig. 2.14 DAD-curve for 4-day storm, Example 2.5

Isohyetal patterns are drawn for the maximum 1-day, 2-day, 3-day and 4-day (consecu- tive) critical rainstorms that occurred during 13 to 16th July 1944 in the Narmada and Tapti catchments and the DAD curves are prepared as shown in Fig. 2.15. The characteristics of heavy rainstorms that have occurred during the period 1930–68 in the Narmada and Tapti basins are given below:

30 3-da (cm)

depth

y stor

2-da m depth

2-da y stor m

3-da

age 20 y stor

20 y stor

1-da m

age A y stor m

10 1-day storm

Storm area (1000 km )

Storm area (1000 km )

(a) Narmada basin

(b) Tapti basin

Fig. 2.15 DAD-curves for Narmada & Tapti Basin for rainstorm of 4-6 August 1968

Maximum depth of rainfall (cm) Year

River basin

3-day 4-day 13–16