The Effect Of Stones And Gravels On Runoff And Erosion Module 556 Organiser: Mr P. D. Smith BY: Ali Alharrasi

Definitions 1: Erosion: the wearing away of land surface by wind or water. Erosion occurs naturally from weather or runoff but can be intensified by land-clearing practices like human activities

Definitions 2: Runoff: water from precipitation or irrigation that flows over the ground surface and returns to the sea or streams. During runoff, it can collect pollutants from the air or land and carry them to the receiving water.

Definition 3: Stoniness: when we can call the soil be is stony ? The soil that contains more than 20 – 50% coarse fragments are considered to be stony. So, gravels, stones and rocks increase the cost and difficulty of excavation for underground services and increase the difficulty of cultivation. In general, gravels, stones and rocks occupy soil volume, reducing plant exploitable moisture and nutrients. Surface stones can have mixed effects on water infiltration, soil erodibility and moisture loss through evaporation.

Effect of Soil – Gravels on erosion The capability of a soil to erosion is known as erodibility. Soils that contain a high proportion of silt and very fine sand are the most rodible. Clayey soils have a very high water holding capacity compared to sands and gravels, but poor infiltration characteristics. In this respect, the clayey soils are vulnerable to erosion because they tend to have a higher rate of runoff. “Well-graded’ soils are those which contain a wide range of particle sizes. Well-drained and well-graded gravels and gravel-sand mixtures with little or no silt have low erodibility to sheet flow, but erode easily under concentrated flow. Coarse, granular soils also have high permeability and sufficiently good infiltration capacity to reduce runoff.

The Studies of Effect of Stones and Gravels on Runoff and Erosion There are many studies and researches showing the effect of stones and gravels on runoff and erosion when the soil was covered by the rocks fragments (stones and gravels). This paper mentions some of them:

Effect of stone content on soil properties: This research was public by Potato Research Centre, Agriculture and Agri-Food Canada. Soil used for potato production in Atlantic Canada and northeast USA are mostly derived from glacial tills containing abundance of coarse fragments. Excessive stoniness is a major limitation in virtually all potato fields in Atlantic Canada, with till soils under potato production in New Bruswick having estimated 5-25% coarse fragments (gravels, cobbles and stones) in the Ap horizone

The objective The objective of this research is identifying the impacts of various concentrations and size of coarse fragments on soil properties of the plow layer. There are for replicates of the following seven treatments: control (<1% coarse fragments); 10%, 1-2cm diameter gravels; 20% 1-2cm diameter gravels; 30%1-2cm diameter gravels; 10% 3-8 cm diameter gravels; 20%, 3-8cm diameter gravels; and 30%, 3-8 cm diameter gravels.

The results 1- stone removal significantly lower infiltration rate and increases soil compaction, which, in turn, increases surface runoff and soil erosion. Stones removal has also been found to reduce soil temperature and soil moisture content.

2- The effect of rock fragments on post fire erosion characteristics This research was done by laboratory of soil and agricultural chemistry, Agricultural University of Athens

The objective The objective of this study to investigate the influence of rock fragments on soil properties and erosion. Also to investigate the effects of different sizes, amounts, and positions of rock fragments on soil properties and erosion after a fire installed on a hill slope.

Description of the study site: The most important land and climate characteristics are the following: the experimental site is a) Located on a sloping site (17%) b) The distance 30km in the east of Athens. c) The attitude from the sea level is 140m. d) It consisted of 30 experimental plots each one 2m x5m. e) Thermo-Mediterranean climate (average air temperature of 17.8 º C and annual precipitation of 496mm. f) The soil is well-structured, dark, stonycarcalous clay loam, with sand stone.

The results and conclusions 1 The data concluded from the experiment clearly indicate that the runoff and the sediment yield were higher in plot containing cobbles than those containing coarse gravels. Concerning the case of the partially embedded gravels of 17.9% cover of the soil surface the sediment yield was higher than the soil without gravels in all rainfall. Also the result showed in the soil with appreciable amount of coarse gravels on the surface of the soil under rainfall of long duration and low intensity the runoff is higher than in rainfall with grater intensities

The results and conclusions 2 The largest amounts of runoff were from bare soil containing abundant rock fragments, either partially embedded on the surface or incorporated in the upper part of the soil. Stoneless soils give smaller amounts. Generally, large rock fragments, cobble resulted in greater runoff than smaller fragments, coarse gravels

The results and conclusions 3 Sediments yield loss was greater from soils with cobbles than from soils containing coarse grave

3-Effect of Surface Stones on Erosion, Evaporation, Soil Temperature, and Soil Moisture. This study was done by Lamb and Chapman (1943). To determine the effect of surface stones on soil erosion and soil moisture.

Method:- *They were used three types of plots, large field plots, small field plots and small weighed boxes. * Soil and water runoff from fields plots were collected and weighed, and soil moisture and soil temperature conditions were noted. * Soil in special boxes was weighed to determine the water loss by evaporation from the surface

Method:- *They were covered the plots soil surface in the small field boxes by different percent of stones (18%, 65%, 100% and /or Oat Straw). * The stones were removed from plot-7 and no stones were removed from plot-5 and plot-8. * Figures 1 & 2 shows two sites of research

The result: - - The removal of stones above 2 inches largest dimension on field plots approximately doubled the water runoff and increased soil loss as much as six fold. - A 65% stone cover compared to the normal 18% stones cover over the soil in weighed boxes slightly reduced the loss of soil water by evaporation, increased water absorption, decreased soil loss, and maintained a relatively high water- holding capacity.

Result 2 - A 6-ton per acre straw mulch cover over the soil in weighed boxes reduced the loss of water by evaporation, greatly increased water absorption, prevented soil loss, and maintained a high water- holding capacity. - A 65% stones cover on field plots increased soil temperatures and maintained a higher content of soil moisture than the 18% stone cover. - A straw mulch of 6 tons per acre gave soil temperatures at 1-inch depth as much as 24ºF lower than at similar depths under the 18% stone cover.

The conclusion The stones and gravels are increased infiltration rate and soil temperature. The stones and gravels decreased runoff and soil erosion. Removal stones from the soil surface increases runoff and the erosion, where the infiltration decreases.

The references 1-Ress,H.W, Chow.T.L, and Monteith. J. O. Vol. 2, No.1. May. 1, 200. Effect of stone content on soil properties. Potato Research Centre, Agriculture and Agri-Food Canada. ( 2- Panagea. N, Kosmas. C.S, Gerontidis The Effect of rok fragements on post fire erosion characteristics. Laboratory of soils and Agricultural chemistry, Agricultural University of Athens. ( 3-Lamb.J, and Chapman. J.E, Effect of Surface Stones on Erosion, Evaporation, Soil Temperature, and Soil Moisture. Journal of the American Society of Agronomy. Volume 35. P 567 to Moustakas. N, Kosmas. C, Danalatos. N, Yassoglou. N, Rock FRAGMENTS 1. THEIR Effect on runoff, erosion and soil properties under field conditions. Soil Use and MANAGEMENT 11:115 – Epstein.E, Grant.W.J, and Struchtemeyer Effect of stones on Runoff, Erosion, and Soil Moisture. Journal, Soil SCI. SOC. AMER. PROC., VOL. 30, 1966.