Culvert Design

[c2h2u2]

Hi Dr Quek,

1. Runoff coefficient for Rational Method stated in Figure 1.12 for Rural Catchment consists of Category A to Category F. How to determine the correct category to use as the descriptions stated is not very clear?

2. For relief culvert, normally RCP 1.5m dia. is adopted. How to determine the catchment area correctly as it is on the flat terrain? Is it by adjusting the catchment area by comparing the flow rate that can be sustained by the RCP 1.5m dia.?

Thanks.

Best regards,
c2h2u2

[drquek]

1. Runoff coefficient for Rational Method stated in Figure 1.12 for Rural Catchment consists of Category A to Category F. How to determine the correct category to use as the descriptions stated is not very clear?
Can you confirm it is Figure 1.12?
2. For relief culvert, normally RCP 1.5m dia. is adopted. How to determine the catchment area correctly as it is on the flat terrain? Is it by adjusting the catchment area by comparing the flow rate that can be sustained by the RCP 1.5m dia.? Sorry I am not sure I understand what you said.

[c2h2u2]

Hi, Dr Quek

Sorry, the runoff coefficient chart for rural catchment is in Chapter 14-Design Chart 14.4.

For relief culvert, I would like to know the correct way to determine the catchment area for it. The main culverts will be allocated first if there are any streams or rivers crossing the road. After that, in between the main culverts or every 300m to 900m distance interval, relief culverts can be allocated to permit the water to flow over to the other side if the water way to the main culverts is blocked or due to the different level before or after the main culvert. Inside the catchment area of main culvert might consists of some relief culverts, therefore some people said that it is over design as part of the catchment area of the main culvert is also the catchment area of the relief culverts. It means that why still need the main culverts as there are some relief culverts every 300m to 900m interval??

Thanks.

Best regards,
c2h2u2

[drquek]

Sorry, the runoff coefficient chart for rural catchment is in Chapter 14-Design Chart 14.4.
I do agree Chart 14.4 is not very clear on this. However, you can use this as a guide: Type A is for the most impervious ground conditions and Type F is for the most pervious ground conditions. Types B to E are intermediates with different combination of varying degrees of perviousness/imperviousness.

For relief culvert, I would like to know the correct way to determine the catchment area for it.
You do not need to compute the contributing area. You can just provide relief culverts at suitable intervals.
The main culverts will be allocated first if there are any streams or rivers crossing the road. After that, in between the main culverts or every 300m to 900m distance interval, relief culverts can be allocated to permit the water to flow over to the other side if the water way to the main culverts is blocked or due to the different level before or after the main culvert.

Inside the catchment area of main culvert might consists of some relief culverts, therefore some people said that it is over design as part of the catchment area of the main culvert is also the catchment area of the relief culverts. It means that why still need the main culverts as there are some relief culverts every 300m to 900m interval??
Like you said the main culverts and the relief culverts serve different purposes so there is nothing wrong to provide additional culverts in case of blockage etc.