Waterway Calculation

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

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

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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)

D

B

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

Where R = Hydraulic Mean Depth (m) n= Manning’s 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 Manning’s 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)

Q

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