Combined sewer / gemengd rioolstelsel
50 m
Ø300 PVC
Ø500 concrete
Ø250 PVC
pump
P4 P3 P2
GL (ground level) +6.00 m
IL +4,00 m
IL +3,90 m
IL +3,73 m
Rain water
Waste water
Rain
Waste
+5,5 m
IL (Invert level) +3,53 m
P1
1

Length
Chezy formula
Chezy formula describes the mean velocity of uniform, turbulent flow
ΔH
𝑉 = 𝐶 ∙ 𝑅 ∙ 𝑆𝑓
𝑉 = Mean Fluid Velocity [m/s]
R = Hydraulic Radius [m]
𝑆𝑓 = Hydraulic gradient [1]
𝐶 =
8𝑔
𝜆
Chezy coefficient [m1/2/s]
𝑆𝑓 =
ΔH
𝐿
3
Total Head
Pressure Head

Head loss sewer pipe
𝑉 = 𝐶 ∙ 𝑅 ∙ 𝑆𝑓 𝑄 = 𝑉 ∙ 𝐴 𝑆𝑓 = 𝑖 =
∆𝐻
𝐿
Combine
∆𝐻 = 𝐿
𝑄2
𝐶2 ∙ 𝑅ℎ ∙ 𝐴 𝑠
2
∆𝐻 = Head Loss, energy loss [m]
Q = discharge pipe [m3/s]
L = length of the pipe [m]
C = Chezy coefficient [m1/2/s]
R = Hydraulic Radius [m]
A = Wetted Area, flow surface [m2]
Sf ,i = slope of hydraulic gradient [-]
3

Question 1
50 m
Ø300 PVC
Ø500 concrete
Ø250 PVC
Pump
P4 P3 P2
GL +6.00 m
IL +4,00 m
IL +3,90 m
IL +3,73 m
Rain
Waste
Rain
Waste
+5,5 m
IL +3,53 m
P1
5

Question 2
50 m
Ø300 PVC
Ø500 concrete
Ø250 PVC
Pump
P4 P3 P2
GL +6.00 m
IL +4,00 m
IL +3,90 m
IL +3,73 m
Rain=0
Waste=10l/s
Rain=0
Waste=10l/s
+5,5 m
IL +3,53 m
Q=20 l/s
Q=10 l/s
P1
5

Partially filled pipe
𝐼𝑛𝑝𝑢𝑡:
𝑄 𝑝𝑎𝑟𝑡
𝑄 𝑓𝑢𝑙𝑙
= 0,17
𝑂𝑢𝑡𝑝𝑢𝑡:
ℎ
𝐷
= 0,27
𝑂𝑢𝑡𝑝𝑢𝑡:
𝑢 𝑝𝑎𝑟𝑡
𝑢 𝑓𝑢𝑙𝑙
= 0,75
5

Table
5

Question 3
50 m
Ø300 PVC
Ø500 concrete
Ø250 PVC
Pump
P4 P3 P2
GL +6.00 m
IL +4,00 m
IL +3,90 m
IL +3,73 m
Rain=1,1 ha
Waste=10 l/s
Rain=3,75 ha
Waste=10 l/s
+5,5 m
IL +3,53 m
P1
5

Question 3c
50 m
Ø300 PVC
Ø500 beton
Ø250 PVC
Pump
P4 P3 P2
GL +6.00 m
Rain=66 l/s
Waste=10 l/s
Rain=225 l/s
Waste=10 l/s
+5,5 m
Q=66 l/s
Q=291 l/s
P1
In example m = 1,85
Q=0 l/s

s/m1/3

Strategy [situation with overflow]
Preparation
Information available for each pipe
- Diameter, R, L, k, C
- Discharge and Velocity
Information Overflow / weir
- Width, m
- Discharge
- Level crest in m N.A.P.
𝐶 = 18 ∙ 𝑙𝑜𝑔
12𝑅
𝑘
5

1. Calculate H at weir
2. Calculate ∆H each pipe
3. Water level at weir (P1) = level crest weir + H at weir
4. Water level at P2 = Water level at weir + ∆Hweir(p1) – p2
5. Water level at P3 = Water level at P2 + ∆H p2– p3
6. Water level at P4 = Water level at P3 + ∆H p3– p4
Strategy [situation with overflow]
Steps
All levels in m N.A.P.
𝑄 = 𝑚 ∙ 𝐵 ∙ 𝐻
3
2 ∆𝐻 = 𝐿
𝑄2
𝐶2 ∙ 𝑅ℎ ∙ 𝐴 𝑠
2
5

Little error or inaccuracy:
Distance between H en y is velocity head. At manhole
(bigger than sewer pipe) we assume velocity head is = 0