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Waterway Calculation

1) The document calculates the design flood discharge for a bridge located at chainage 18+300 based on the catchment area and longest stream. 2) The catchment area is 375.023 hectares and the longest stream is 2422m with a slope of 0.5%. Using this information, the time of concentration is calculated as 104.7 minutes. 3) Using the rational formula with a runoff coefficient of 0.3 and 100 year rainfall intensity of 68.22 mm/hr, the peak design flood discharge is calculated as 21.32 m3/s. 4) Checking the capacity of the existing bridge structure, it is found that the design discharge exceeds the capacity of the structure

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2K views5 pages

Waterway Calculation

1) The document calculates the design flood discharge for a bridge located at chainage 18+300 based on the catchment area and longest stream. 2) The catchment area is 375.023 hectares and the longest stream is 2422m with a slope of 0.5%. Using this information, the time of concentration is calculated as 104.7 minutes. 3) Using the rational formula with a runoff coefficient of 0.3 and 100 year rainfall intensity of 68.22 mm/hr, the peak design flood discharge is calculated as 21.32 m3/s. 4) Checking the capacity of the existing bridge structure, it is found that the design discharge exceeds the capacity of the structure

Uploaded by

mullahurt
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Waterway Calculation

Design Flood Calculation

Identification of Catchment

The catchment area was marked using the elevations in Google maps, to find the discharge at
the bridge location 18+300. The stream paths too was marked and measured. In this case the
channel carries water from two reservoir outlets; hence the two respective discharges from the
reservoirs at HFL should be added to the calculated Design flood resulted from surface runoff of
the catchment area.

Figure 1 Catchment area and streams

Area of the catchment =375.023 Hectare


Length of the longest Stream = 2422 m
Runoff coefficient = 0.3 (For lawns/meadows and unimproved areas; Handbook of civil engineering
calculations by Tyler G Hicks)

Slope= (45.72-33.53)/2422 =0.005 =0.5%

Table 1 Velocity of flow Vs Slope


Average Gradient

Average Velocity m/s

%
0-1
1-2
2-4
4-6
>6

0.45

0.6
0.9
1.2
1.5

Average flow velocity = 0.45m/s

Computation of time of concentration


Tc = L/ (60x V) + to (Design of small irrigation works for small catchments by Ponrajah)
Tc - Time of concentration (min)
L -Length of longest stream (m)
V - Velocity of flow (m/s)
to Overland Flow time ( about 15min depending on terrain)
Tc = 2422/ (60x0.45) + 15 = 104.7min

Establishing recurrence interval


The recurrence interval for the bridge will be taken as 100 years.

Computation of peak flow


Peak flow is calculated using rational formula.
Q = CIA/360
C = Runoff coefficient (dimensionless)
I = Rainfall intensity corresponding to a storm duration equal to time of concentration
(mm/hr)
A = Total catchment area (Ha)

Figure 2 IDF Curve for Hambanthota

I = 5618 [105+45] -0.88033 = 68.22mm/hr


Q= (0.3 x 68.22 x375.023)/360 = 21.32 m3/s

Q = 21.32 m3/s
Discharge of Meegas Wewa Tank = Unkown
Discharge of Yakabenda Wewa Tank = Unknown
Therefore Q Design (Design flood) =

Checking Adequacy of existing structure


Bed width (B)=11.65-0.9) = 10.75m

FB

Water depth (D) =3m


Free Board (FB) = 0.2m
Bed Slope (S) = 0.001 (Assumed)

Mannings roughness coefficient (n) = 0.025 (earth channel with some grass and weed/ Flow in open
channels by Subramanya)
Mannings equation;
V = 1/n xR2/3 x S1/2

Where R = Hydraulic Mean Depth (m)


n= Mannings roughness coefficient
V= Flow velocity m/s
S = Channel slope
R = A/P
Where A= Cross sectional area of flow m2
P = Wetted perimeter (m)
V = 1/n x [BD/ (B+2D)] 2/3 x S1/2
Continuity equation;
Q = A.V
Where A = Cross sectional area of flow m2
Q = Actual Discharge (m3/s)
V = Velocity of Flow (m/s)
V =Q/ B.D
By substituting to Mannings equation,
Q = 1/n x [BD/(B+2D)]2/3 x S1/2 xB.D

Table 2 Calculation of Discharge


Manning's Channel
roughness bed
coefficient slope (s)
(n)

Channel
Width
(b)

Channel
Depth
(d)

Hydraulic
Radius (R)

R^2/3 S^1/2 Velocity


(v)

0.025
0.025

0.001
0.001

10.75
10.75

2.5
3

1.706
1.925

1.428
1.548

0.032
0.032

1.806
1.958

48.542
63.134

0.025

0.001

10.75

3.2

2.006

1.590

0.032

2.012

69.207

Q
Design

Conclusion
According to calculations the Design discharge for a 100 year return period exceeds the capacity of
the structure. The stakeholder comments obtained during the site visit states that the, High flood
level reaches the above the soffit of the deck.

Condition

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