Project 002-074 French Drains
Project Name: French Drains
Project Details
Reference Sponsoring Entity Commenced Status Budget
22-23 23-24 24-25 25-26
002-074 KIRRAutility (Ka) 18-11-2023 Complete $ - $ 300 $3,000 Unallocated
Project Overview
French Drains are easy to install and require minimal resources compared to other types of drains. They do however, have a distinct disadvantage as they can become clogged over time depending on the quality of the water that is collected with them. Soil heavy water will clog the drains quickly.
Aim
Improve water drainage and collection in the pond.
Method
STEP 1: Using the trenching bucket on the excavator, a 200mm wide trench was dug where the drain was going to be located. In this case the trench covered about 80 metres running down the side of the main living area and from the front gate. The drain is only 150 mm deep.
STEP 2: Using a spade, the trenches are cleaned up and leveled out to ensure a constant smooth drop towards the pond.
STEP 3: Once leveled the trench is tamped to smooth and compact the surface of the drain trench.
STEP 4: Once tamped and leveled. Polyethylene bags are laid in the trench to help reduce erosion.
STEP 5: Once the bags are laid. The trench is back-filled with 20 mm blue metal or any other course gravel material.
STEP 6: To prevent erosion when the water enters the pond, a pvc pipe is used that is positioned in a hole within a bucket. The bucket has had the side removed, so the water can easily flow from the french drain trench into the bucket and through the pipe.
Conclusion
Whilst this was hard work. The drain was completed in a weekend.
It is important to understand that the amount of rain received over a period of time, will determine the outcome of the water capture.
On two consecutive days there was 13.7 mm of rain. (See more below)
The first day, the rain was delivered over the course of many hours. (Images to the left indicate the rain fall, and the pattern of rain fall over the course of the day. It is clear that the maximum rain fall over the course of the day was just 3.2 mm.
The second day, the same rain fall was coincidentally delivered, but in a much tighter time frame. The whole rain fall pattern is reduced to just a couple of hours in the evening. With a peak of 6.7 mm in the third hour, 6.0 in the second and 1 mm in the first hour. Effectively delivering 12.7 mm over just a one hour period (+/- 30 minutes)
In all instances the rain fall is measured from 9 am the prior day until 9 am of the current day.
Cost-Benefit Analysis
The most expensive part of this project is the blue metal at $115 per cubic metre. To complete this french drain dimension of approx 150 mm deep took all of 2 cubic metres to fill.
Erosion and sediment fall into the pond is reduced by the use of the french drain which terminates in a pvc pipe. As displayed in the images below.
In a 3-day period we experienced rain to the tune of 82.4 mm. Day 1 = 13.7 mm, Day 2 = 13.7 mm, Day 3 = 55 mm.
The French Drain held up exceptionally well. It correctly diverted all the water to the pond, and more importantly, prevented pooling and puddles in the areas that are needed to be kept dry.
It is estimated that the pond holds approximately 35,000 litres and after the 55 mm of rain, it was full to overflowing.
