Depending on the control points, head water and tail water conditions there can be six type of flow through a culvert. It is very important to understand the types of flow before a hydraulic analysis or design is undertaken.
Type 1 - Critical depth at inlet
Flow is controlled at the inlet of Culvert. The water surface remains below critical depth through out the culvert including outlet.
Type 2 - Critical depth at Outlet
Flow is controlled at Outlet. The flow inside the culvert is free surfaced. Here the tail water depth is less than critical depth and the slope of the culvert is less than critical slope
This is similar to type2, but here the tail water depth is greater than critical depth but less than culvert soffit. This is the condition to which the culvert should be designed.
Type 4 - Submerged outlet
Here the inlet as well as oulet are submerged and the flow is controlled somewhere downstream of the outlet due to which submergence is occuring.
Type 5 - Rapid flow at inlet
There are two variations for this type of flow. For both the variations the head water depth is high causing the inlet of the culvert to act like an orifice. The flow inside could be below critical depth in which case a hydraulic jump occurs downstream to outlet or it can be above critical depth in which case the hydraulic jump is created inside the culvert.
Type 6 - Free outlet and full flow.
Here the culvert flows full through out the length but the tailwater elevation is less than the culvert soffit. The control point in this case is either outlet or downstream to outlet.
A variation to this flow is when flow seperates from soffit inside the culvert and then flow free.
An ideal culvert design should have to take in to account many features. Some of them are,
1. Capcity in excess of design flow to allow free flow at all times.
2. Section should be large enough to carry any debris freely across.
3. Self cleansing velocity.
4. No change in direction unless unevitable.
5. Long design life and low maintenance construction.
6. Allow fish and wildlife migration.
Method for the design of a new culvert
Design will depend upon the condition of flow, (i) full free flow (ii) submerged flow.
An ideal design should always be carried out for full free flow condition. Design process shall be as follows.
1. Data Collection
2. Conceptual design
3. Flow rate calculation
4. Tail water level calculation
5. Free flow design (Surcharge design if unavoidable)
6. Culvert size selection
7. Detailed design.
a) Design flood flow - Flood flow need to be calculated for a given return period. A 100 year return period is common in urban area where consequence of flooding can be severe. A 50 year return period is sufficient for rural areas and can be as less as 10 in agricultural areas. Design flow should be determined based on a complete understanding of the hydrlogy of the upstream catchment and the consequence of flooding. Design data should the least include, size and nature of the upstream catchment area; survey of channel and flood plain with slope, size, alignment, bed material and flood plain extent; maximum flood level (based on local knowledge). A likely future development during the life of the culvert also should be considered while establishing design flow.
b) Minimum flow - Though a flood flow is the key factor to design of culvert, sometimes it is also essential to know the low flow conditions. Self cleansing velocity, fish migration and inspection access will depend on low flow regime.
c) Sediment and debris - A blockage of culvert can produce disastrous results sometimes. It is hence essential to carry out an assessment of sediment and debris loads. Nature of the catchment will give an indication of this, still a reconnaisance survey upstream and downstream of the site will give information on the amount of silt deposited and the type of material.
d) Engineering data - Ground conditions, practicalities of construction, access, temporary flow diversions and likely environmental impact.
Given below is a brief list of minimum data required.
upstream and downstream channel cross sections
length of culvert
location and level of road, railway or other structures/utilities
maximum permissble upstream water level
channel bed slope, soil type
channel description and roughness estimate
sediment transportation data
measured flows and water levels
imposed loads and foundation
The initial design should be identify all the constraints
1. Type of design - Free or surchrged
2. Alignment - skewed, 90o
3. Flow rate - exceeding flood flow, change in return period
4. Tail water level
5. Maximum head water level
6. Culvert type - pipe, box, arch; sigle/multiple
7. Freeboard - 0.3 to 0.5m
8. Trial size
9. Flow velocity - sedimentation/scour 0.75 to 2 m/s
10.Culvert Slope - assume bed slope
12.Low flow channel
13.Inlet and outlet type
15.Operation and maintenance