Гідравлічний розрахунок

Hydraulic calculation of drainage trays is carried out in accordance with DBN 2.04.03-85 (Sewerage. External networks and structures) for the calculated maximum second flow rate of sewage.

Longitudinal slopes of drainage trays should be determined taking into account the permissible water flow velocities and the slope of the surface covering. In this case, the water flow velocity in drainage trays should be not less than 0.5 √R_h, m/s (where R_h - hydraulic radius), and not more than 8 m/s.

Reduction of water flow velocity along the length of the calculated drainage trays is not allowed. Calculated rainwater flow rates Q_w, l/s, in the sections of drainage lines should be determined using the limiting intensity method by the formula:

Q_w = Q_s × F_w

Where Q_s - runoff amount, l/s per 1 ha:

Q_s = Ψ × φ = 166.7Δφ / t_j^π

F_w - water collection area for the calculated section, ha;
Ψ - calculated rainfall intensity, l/s per 1 ha;
Δ - parameter equal to the maximum rainfall intensity lasting 1 min with the accepted recurrence, mm/min:

Δ = 0.006 × 20ⁿΨ₂₀(1-ClgT)

φ- coefficient of rainwater runoff, determined according to table 1;
t_j - calculated duration of rainwater flow to the calculated cross-section, min. (see below);
n - exponent characterizing the change in calculated rainfall intensity over time;
Ψ₂₀ - rainfall intensity for the given area lasting 20 min at T=1 year, l/s per 1 ha;
C - coefficient taking into account the climatic features of the region;
T - recurrence period of the calculated rainfall intensity, year.
The values of n, Ψ₂₀, and C are established in accordance with the requirements of DBN 2.04.03-85.

Table 1

The calculated rainfall duration t_j, min, equal to the time of rainwater flow to the calculated cross-sections of collectors t_w, should be determined

Where τ_s is the time for rainwater to reach the gutter surface, seconds; τ_s is the time for rainwater to reach the gutter to the rain receiver, seconds; τ_k is the time for rainwater to flow through the collector to the design section, seconds.

The time for rainwater to reach the gutter surface τ_s, seconds, should be determined by the formula:

τ_s = (2.4ln_e L_s / Δ^0.72 φ^0.72 i_s^0.5) ^{(1/1.72-0.72π)}

Where L_s is the length of the slope participating in the formation of maximum runoff, meters; i_s is the slope gradient; n_e is the surface roughness coefficient of the slope, adopted from the table:

When the longitudinal and transverse slopes of coverings and shoulders i_t / i_s ≥ 0.5, the calculated slope i_d and the slope length L_d should be taken along the steepest line by the formula:

i_d = √(i_t² + i_c²); L_d = (L_s / i_c) √(i_t² + i_c)

For heterogeneous slope surfaces (covering plus soil shoulder), the time for rainwater to reach the stream τ_c is determined by the formula at the average weighted values of slopes, runoff coefficients, and roughness.

The time for rainwater to reach the stream τ_c, seconds, should be determined by the formula:

τ_c = L_c / 60V_c

Where L_c is the length of the gutter, meters; V_c is the speed of rainwater movement at the end of the gutter, m/s;

V_c = ( 1 / n_s )R^0.5+y i_b^0.5

Where R is the hydraulic radius, meters;

R = w / x

d_w is the flow depth at the end of the gutter (near rain receivers, talweg wells), meters;

W is the area of the "live" cross-section of the closed channel, m²;

X is the wetted perimeter, meters;

y = 4 ⁴√(n_s³);

n_s is the roughness coefficient of the gutter surface.

The flow depth d_w at the end of the gutter should be determined from the condition of equality of the calculated flow rate in this section to the capacity of the gutter at the accepted gutter depth, with the gutter capacity Q_c, m³/s, needing to be determined by the formula:

Q_c = (d_w² / i_w ) V_c

Where i_w is the slope of the gutter side.

The calculated flow rates of rainwater entering drainage systems from coverings or from coverings and soil shoulders should be determined without considering the minimum rainfall intensity.

The calculated water flow rate during spring snowmelt should be determined at the average values of maximum snowmelt runoff in the area.