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4 Culvert Worked Examples

4.1 Worked example 1

Jack and Jill Jersey have a dairy farm a few kilometres north of Gisborne and are planning to culvert a stream crossing on their farm.

Using the culvert guidelines to size their culvert, they first check that there are no low lying buildings or residential areas upstream of their proposed crossing point. They also know that the amount of debris in the stream during floods is low.

Jack and Jill use a topographic map to calculate their catchment area (the area of land from which water runs into the stream). Knowing that 1 grid square on the map equals 100 hectares they estimate their catchment area at 380 hectares, and check that it fits the criteria for using the culvert guidelines.

Looking at the rainfall maps, the Jerseys can see that they are close to the boundary between the low-medium rainfall area and the medium rainfall area. To ensure the culvert is properly sized they select the medium rainfall band as this will give a larger culvert.

Jack and Jill now know the size of their catchment, and the rainfall area their catchment is within. They can now check the culvert tables to find the required culvert size. Their catchment at 380 hectares is closest to 400 hectares on the table, which shows that they require a 2550 mm diameter culvert.

Figure 4.1: Jack and Jill Jersey's crossing point and catchment

Map showing the extent of the catchment area and the stream crossing point.

4.2 Worked example 2

Fred and Fiona Friesian have a dairy farm near Oamaru and are putting in a stream crossing on their property. Like the Jerseys (Worked example 1), they also check that their crossing location and catchment is suitable to use the guidelines, by looking at possible areas of flooding and how the water build-up that will occur upstream of a culvert may affect any nearby buildings. They confirm that the other criteria listed in the guidelines apply to their catchment.

Fred and Fiona have calculated their catchment area as 350 hectares and have identified from the maps that they are in the low rainfall area. Looking at the tables, Fred and Fiona see that they need a 1600 mm diameter culvert. However, the stream that runs through Fred and Fiona's farm is wide and shallow and constructing a 1600 mm culvert will mean the stream crossing is over 1 metre above the surrounding ground. This would also require a large amount of fill to be placed in the stream.

Instead of using a single pipe, Fred and Fiona decide to look at using more than one pipe to get the same capacity as a single 1600 mm pipe but with less earthworks. Looking at the multiple pipe table in the guidelines, they can see that they have the option of using two 1200 mm pipes or three 975 mm pipes. They draw up a simple cross section of the stream (see Figure 4.2 below) to compare their options. The three 975 mm pipes look to be the best option for the Friesians, as this gives a crossing which least impedes the waterway at a high level and requires less fill material across the stream bed.

While a multiple pipe option was the best for Fred and Fiona it is preferable to use a single pipe culvert where possible, as multiple pipe culverts are more prone to blockage.

Figure 4.2: Fred and Fiona Friesian's stream crossing cross-section

Diagram showing a cross section of the stream with the different culvert options (described above).