The HP-40 statistical method is used to calculate the 20-year interval maximum daily flow. The method was validated for drainage basins whose area is greater than 150 km2. Thus, where the area of the basin covers between 60 and 150 km2, the result must be validated in the field by looking for signs indicating the water level reached by the floods of previous years or by establishing a relationship with basins that were measured on the same territory or near it.
STEPS IN THE CALCULATION
1. Delimitation of the drainage basin with a topographic map at a scale of 1:20 000;
2. Calculation of the drainage basin’s area;
3. Calculation of the “85-10” slope of the watercourse;
4. Calculation of the proportion of the basin covered by lakes and bare and semi-bare wetlands;
5. Calculation of the 20-year interval maximum daily flow.
The delimitation of a drainage basin is shown as an example in step 1 of Schedule 6. The calculation method for the “85-10” slope of the watercourse is the same as that used for drainage basins of 60 km2 or less (Schedule 6 — step 5). The 20-year interval maximum daily flow (Q1.20) is determined using the following formula:
|Q1.20 (m3/s)||=||0.7882 (Ab/100) 0.93 (Sc) 0.30|
| || ||St0.24|
Ab = area of the drainage basin (ha)
Sc = “85-10” slope of the watercourse (%)
St = percentage of the area of the drainage basin covered by lakes and bare and semi-bare wetlands (%)
|Ab = 75 km2||=||Q1.20 = 0.7882 (75) 0.93 (1) 0.30 = 29.7 m3/s|
| || ||(5) 0.24|
Sc = 1%
St = 5%
A weighted factor of at least 5% is then applied to the flow obtained in order to take into account exceptional climatic events.
i.e.: 29.7 m3/s X 1.05 = 31.2 m3/s