Soil compaction can occur in a variety of situations – in agriculture, the urban environment and sports turf. In agriculture, the effects are usually deleterious affecting yield, urban compaction can lead to increased run-off and flash-flooding and foot traffic on sports turf makes for unsuitable playing conditions.

Here is an activity developed by the Field Studies Council to investigate soil hardness around your grounds. Think about places which see ‘regular traffic’ and compare and contrast the infiltration rates at different locations around your school grounds.

The activity below is adapted from material developed by the Field Studies Council, http://www.field-studies-council.org/outdoorscience/diy.htm

Measuring soil hardness

(a) Introduction

Soil hardness (or soil compaction) can be measured in the field by using a metal stake or pin. These can easily be made by using knitting needles. Hold the stake out at arm's length above the centre of the quadrat and let it fall through the fingers. Measure the depth of entry into the soil. This may seem unscientific, but it really does work if pupils take care to let the stake fall from the same height above the quadrat each time.

Alternatively, after a dry period when the soil is hard, ask pupils to apply uniform pressure to the stake at each measuring site and measure the depth it reaches into the soil. Clean the stake of soil before it is used for the next measurement.

(b) Make your own soil pin

A knitted needle is long enough to measure most soil depths which will be found. It may be useful to use indelible ink to mark depths on the side of the needle.

Infiltration rate

(a) Introduction

The infiltration rate is the velocity or speed at which water enters into the soil. It is usually measured by the depth (in mm) of the water layer that can enter the soil in one hour. An infiltration rate of 15 mm/hour means that a water layer of 15 mm on the soil surface, will take one hour to infiltrate.

The most common method to measure the infiltration rate is by a field test using a cylinder or ring infiltrometer. Infiltration tubes can also be easily constructed from simple materials, as suggested below. If more than one type of device is used, it would be interesting to compare them to see if they give consistent readings.

(b) Make your own equipment

Use a 30cm length of 10cm diameter plastic pipe. You will also need a 30cm ruler, a jug of water, a mallet and a piece of wood.

(c) Procedure

It is important to maintain the same 'head' of pressure when pouring water into the infiltration tube. Although there are several ways in which infiltration tubes can be used with students, one method that is active for students but still scientifically valid, is as follows.

Press the plastic tube down into the soil until it is buried by at least 10cm. You may need to use a mallet and a piece of wood to hammer it into the soil. Pour enough water into the plastic tube to reach the 10cm mark. Start the stopwatch. Stop the stopwatch when the water level has dropped to 9cm.

Alternatively use a tin can and push the ring into the ground to a depth of around 10cm, making sure that the ring is stable. Pour water into the ring until the water level is equal to the top of the ring.  Take a reading of the amount of time it takes for the water to disappear completely into the ground.

Background Information

In dry soil, water infiltrates rapidly. This is called the initial infiltration rate. As more water replaces the air in the pores, the water from the soil surface infiltrates more slowly and eventually reaches a steady rate. This is called the basic infiltration rate (below).

The infiltration rate depends on soil texture (the size of the soil particles) and soil structure (the arrangement of the soil particles) and is a useful way of categorizing soils from an irrigation point of view. It would be very rare to see a puddle form on the beach during a rain storm because sand has very high permeability (permeability is a term used to defined the rate of infiltration, a low permeability means the water flows through the soil slowly, and a high permeability means is flows fast). On the other hand it is very common to see puddles form on asphalt pavement because it has a very low permeability.

Basic Infiltration Rates for Various Soil Types

Soil type	Basic infiltration rate (mm/hour)
sand	less than 30
sandy loam	20 - 30
loam	10 - 20
clay loam	5 - 10
clay	1 - 5

Source: http://www.fao.org/docrep/S8684E/s8684e0a.htm#TopOfPage

Infiltration rates are also affected by degree of compaction, ground cover (plant roots help keep the soil structure loose, so the presence of vegetation promotes infiltration) and the topography of the land.

Urban compaction, the use of inappropriate materials in the urban environment and an increased tendency for the creation of hardstandings (usually for offstreet parking) has led to increased surface water run-off and related flood risk. It was estimated that around two-thirds of the 55,000 homes affected in the 2007 floods, were flooded from surface water run-off.

Climate change scientists predict that while winters will generally be wetter and summers drier, the number and intensity of extreme storms will increase. BY the 2080’s winter rainfall could increase by 10-30% and rainfall intensity by up to 20%. The Government’s Foresight report calculated that damages from surface water drainage flooding could increase from £270 million to £1-10 billion by the 2080s (Foresight (2004) Future Flooding, www.foresight.gov.uk/Previous_Projects/Flood_and_Coastal_Defence/index.html)

On 1^st October 2008 legislation was introduced that meant such future developments would require planning permission if they didn’t use permeable materials (http://www.communities.gov.uk/publications/planningandbuilding/pavingfrontgardens).

These measures were introduced a part of local governments’ flood attenuation strategy. Related to these developments, we describe below an activity that supports the dvd enclosed with the EcoBox about Sustainable Urban Drainage Systems, designed to deal more efficiently with large volumes of water produced as a result of surface run-off from sudden, flash flooding. Such flood water may contain an increased level of pollutants from oil, fuel and animal droppings.

SUDS Activity

Flash floods are often caused by heavy rain falling on surfaces where the water cannot drain away efficiently. This problem is being made worse as more and more areas of land are concreted over for building projects. In this activity, the effect on drainage of using different materials for garden surfaces is investigated.

Materials needed

1 large plastic tray

4 identical but smaller, shallow plastic trays

4 small polystyrene blocks

100ml measuring cylinder or jug

Gravel

Sand

Soil

Polyfilla

Piece of turf

Collection of small ‘Lego’ bricks

What to do

1. Set up the 4 small plastic trays as follows:

· Fill 1 with gravel

· Fill 2 half full of sand and arrange the Lego blocks on top as ‘brick paving’

· Fill 3 half full of soil and put a piece of turf on top as ‘lawn’

· Fill 4 with a thick paste of Polyfilla + water and leave to set as ‘concrete’

2. Place a polystyrene block ‘house’ on one end of each tray and stand the 4 trays in the large tray (see photograph)

3. Fill the measuring cylinder with water. Slowly pour the water over the gravel in tray 1 and observe what happens. Continue pouring until the water starts to overflow (you may need to refill the cylinder). Observe what happens to the house. Make a note of how much water the gravel was able to soak up.

4. Repeat step 3 with the other ‘brick’, ‘lawn’ and ‘concrete’ trays.

5. Which material would be best to use in the garden to reduce the risk of flash floods? What advice would you give to someone who planned to concrete over their front garden in order to be able to park their car off the road?

6. What would be the impact on others, if every house in your street did this with their gardens? Where does the water go? Would this water be suitable for rainwater harvesting? If not, why not.

The Environment Agency is the ministry which establishes government policy in the areas of flood attenuation and SUDS.

General information about floods and how to reduce them is at

http://www.environment-agency.gov.uk/homeandleisure/floods/31736.aspx

The best page to start for information on SUDS is at

http://www.environment-agency.gov.uk/business/sectors/36998.aspx

The Code of Practice for SUDS is at

http://www.environment-agency.gov.uk/static/documents/Business/icop_final_0704_872183.pdf