Hydraulics is the essential feature of bridge design. Fair assessment of flood levels, maximum flood discharge expected to occur at bridge site during design life of bridge, and maximum scour levels are essential aspects of bridge hydraulics.
Faulty determination of these parameters may lead to failure of structures. While doing hydraulic calculations attention should be paid to the following:
1. The river cross section should be truly representative. The cross section should not be vitiated by artificial cuts etc.
2. If the bridge site is along the existing natural crossing, the cross section for hydraulics should be across the nearby natural undisturbed channel. The cross section within 100 m U/S or D/S may be quite useful.
3. Spill channels should be properly located, marked and catered for.
4. Appropriate coefficient of rugosity should be used. The same rugosity coefficient should not be used for bed and banks, as the nature of stream changes according to properties of material and vegetation growth etc.
5. The reasonableness of computed velocity should be judged in relation to bed material for e.g existence of boulders in the stream and low velocity of flow do not generally go together.
6. In tidal creeks the possibility of high tides and floods coinciding should be kept in view. In such cases discharge by usual ways i.e. by Manning’s formula should be carefully worked out and tallied with Inglis discharge.
7. The adoption of either the observed H.F.L. obtained by local enquiry or the computed H.F.L. as design level should be done judiciously. The observed H.F.L. may be effected by obstructions like rice fields, bunds, blocking of spill channels etc. Higher of the two values be adopted as design H.F.L.
8. Details of various levels is explained as below.
- HFL (observed) Highest flood levels ever recorded. (50 years record)
- HFL(Inglis) Flood level giving Manning’s discharge equal to Inglis discharge.
- HFL(Modified Inglis) Flood level giving Manning’s discharge equal to Modified Inglis discharge.
- O.F.L. Ordinary flood level. This is level of flood when cleared by bridge (without submergence of bridge) that will not give more than permissible interruption to traffic during floods.
9. Maximum permissible interruptions for various standards of roads are as follows- National Highways – No interruptions.
- Bridges on SH, MDR – 6 times a year and for a period not exceeding 12 hr at a time.
- Bridges on ODR – 6 times a year and not exceeding 24 hours at a time.
- Bridges on VR – 6 times a year and not exceeding 72 hours at a time.