Study on Parameters of Two Widely Used Cohesive Soils Erosion Models

The erosion rate of cohesive soils was typically modeled with the excess shear stress model and the Wilson model. Several kinds of research have been conducted to determine the erodibility parameters of the two models, but few attempts have been made hitherto to investigate the general trends and range of the erodibility parameter values obtained by the commonly used Erosion Function apparatus. This paper collected a database of 177 erosion function apparatus tests to indicate the variability of all erodibility parameters; the range of erodibility parameters is determined by data statistics and parameter theoretical value derivation. The critical shear stress (τc) and erodibility coefficient (Z0) in the over-shear stress model have a positive proportional relationship when the data samples are sufficient. However, there is no such relationship between the erodibility coefficient (b0) and erodibility coefficient (b1) in the Wilson model. It is necessary to express the soil erosion resistance by considering all erosion parameters in the erosion model. Equations relating erodibility parameters to water content, plasticity index, and median particle size were developed by regression analysis.

Field Jet Erosion Tests on Benbrook Dam, Texas

This report summarizes the results of eight field Jet Erosion Tests (JETs) performed on Benbrook Dam, TX. The results from these tests will be used by the U.S. Army Corps of Engineers, Fort Worth District, in assessments of the erosion resistance of the Benbrook Dam with regards to possible overtopping by extreme flooding. The JETs were performed at four different locations, i.e., two locations at the lowest crest elevation and two locations at the mid-slope face of the downstream embankment. Variations in estimated critical hydraulic shear stress and erosion rate values may have been caused by differences in soil composition, i.e., when the material changed from silt/sand to clay. The resulting values of the Erodibility Coefficient, Kd, and Critical Stress, τc, are very useful information in assessing the stability of Benbrook Dam during an overtopping event. Because of the observed natural variability of the materials, combining the erosion parameters presented in this report with the drilling logs and local geology will be imperative for assessing erosion-related failure modes of Benbrook Dam.

TINJAUAN RINGKAS BERBAGAI TEKNIK TERKINI UNTUK MENENTUKAN KUAT GESEK EROSIF TANAH

ABSTRACTStream bank erosion manifests in two main modes, namely mass failure and fluvial erosion. Fluvial erosion is a quasi-continuous entrainment of individual soil grains or grain flocs or chunks from the bank face due to the hydrodynamic shear flow action. It occurs under water with retreat lengths in millimeter or centimeter scale. The interaction between fluvial erosion and mass failure determines the shape and retreat rate of the stream bank (Rinaldi et al., 2008). It is not rare, fluvial erosion which occurs at the bank toe later triggers slumping of soil mass known as mass failure. Despite its importance, the mechanism of fluvial erosion is less understood. This is predominantly due to lack of technique for measuring two key bank soil parameters that determine fluvial erosion rate, e.g., erosion shear strength τcand the erodibility coefficient M (Papanicolaou et al., 2007, Grabowski et al., 2011, Sutarto et al., 2014a). The goal of this manuscript is to provide a short review on the various techniques for measuring τcand M proposed and applied in various studies. The advantages and the limitations of each technique are discussed to get a glimpse on where we stand and what challenge remains to overcome.Key words : Fluvial erosion, erosion shear strength, erodibility coefficientABSTRAKErosi tebing sungai terjadi dalam dua model utama, yakni erosi gerusan dan longsor atau keruntuhan. Erosi gerusan ditandai dengan tergerusnya butiran atau bongkahan tanah, secara hampir kontinu, dari permukaan tebing akibat daya gerus air sungai. Erosi gerusan berlangsung di bawah muka air dan magnitudo erosi sangat kecil dalam skala millimeter hingga centimeter saja. Interaksi antara erosi gerusan dan longsor membentuk profil permukaan tebing sungai dan mendikte laju pergerakan mundur tebing sungai (Rinaldi dkk., 2008). Tidak jarang erosi gerusan, yang umumnya lebih dominan terjadi di tumit tebing, memicu longsor tebing. Meskipun demikian pentingnya peran erosi gerusan, pemahaman tentang mekanisme erosi gerusan tidak sebaik pemahaman tentang longsor tebing. Hal ini disebabkan belum adanya metode akurat yang disepakati bersama untuk mengukur dua parameter kunci tanah yang menentukan laju erosi gerusan, yakni kuat gesek erosif τcdan koefisien erodibilitas M (Papanicolaou dkk., 2007, Grabowski dkk., 2011, Sutarto dkk., 2014a). Artikel ini memaparkan hasil tinjauan ringkas tentang teknik pengukuran parameter τcdan M yang ditawarkan dan sudah diaplikasikan dalam berbagai studi. Kelebihan dan kekurangan dari masing-masing teknik juga akan dibahas untuk memahami sejauh mana kemajuan yang telah dicapai dan permasalahan yang masih harus diatasi.Kata kunci : Erosi gerusan, kuat gesek erosif, koefisien erodibilitas

PREDICTING SOIL EROSION AND SEDIMENT YIELD IN THE TAPACURÁ CATCHMENT, BRAZIL

The EPM is a model for qualifying the erosion severity and estimating the total annual sediment yield. The EPM uses empirical coefficients (erodibility coefficient, protection coefficient and erosion coefficient) and a matrix of the basin physical characteristics. The EPM gives a quantitative estimation of erosion intensity as well as the estimation of sediment yield and transportation. To analyze the suitability of the Gavrilovic method for use with GIS techniques, we prepared cartographic data on geology, pedology, slope, temperature and land use in digital form. A raster-based Geographic Information System (GIS) was applied to generate the erosion-severity and sediment yield maps. In order to validate the EPM estimated erosion, data annual sediment yield were collected between 1999 and 2007. The results showed a mean sediment delivery ratio (SDR) of around 8% and a calculated mean sediment yield of 0.108 t/ha/year, which is close to the observed one, 0.169 t/ha/year. The obtained soil loss map could be considered as a useful tool for environmental monitoring and water resources management.

In situ jet-testing of the erosional resistance of cohesive streambeds

Fletcher’s Creek is located in an urbanizing basin near Toronto and has a bed and banks composed primarily of cohesive Halton Till. Critical shear stress and an erodibility coefficient for the till were determined using an in situ jet-tester that directs a submerged jet of water perpendicular to the sediment surface. The results from 10 jet-tests indicate that the till has a relatively low critical shear stress and relatively high erodibility coefficient and could be susceptible to bed scour during flood events. Many other streams in southern Ontario have urbanizing watersheds with cohesive till beds that may also be susceptible to erosion.Key words: critical stress, submerged jet, erodibility, cohesive soils.