Nov 2, 2014

Mechanistic Linkage among Flow- Erosion- Suspended Sediment

Erosion is an action of fluid force which transport soil and rock from one location to another location. The driving forces for erosion are water, wind, ice and gravity. Soil loses its productivity as it loses the nutrient content, water storage capacity and organic content. Erosion due to hydraulic action is the most complex form of erosion. It occurs in land surface as well as riverbed and riverbanks. In the land surface, erosion initiates after fall of energized raindrop, which disrupts the soil integrity and cause loose soil. Both rain water and ice mass carry a lot of sediment and drains into the river. Runoff volume increases the amount of suspended sediment in the river. Based on river flow velocity, longitudinal slope, flow depth and bed roughness, riverbed also erodes and increases the amount of suspended sediment in the water.
Mechanistic linkage

Figure-1 illustrates the detailed linkage of flow, erosion and suspended sediment. Here flow is indexed as river discharge and suspended sediment is indexed as concentration. For river erosion, it is difficult to provide an index. River erosion is dependent on different physiological phenomena i.e. velocity, shear stress over bed material, bed roughness etc. To generalize river erosion, it is indexed as shear stress (τo) over bed material in this study.

River flow increases the concentration of suspended sediment. Normally SS particle tends to settle into riverbed due to its self-weight. Stream flow velocity is highest near the surface and it reduces with flow depth due to viscous action. SS Concentration is highest near the river bed and the concentration decreases near top surface (Maidment, 1993). Figure -2 illustrates flow depth profile of velocity, SS concentration, shear stress and sediment diffusion. SS concentration is dependent on shear stress, which is maximum near bed. Also SS concentration dependent on fall velocity and eddy motion (fall velocity is the settling velocity of a sediment particle and eddy motion is the upward force). Shear stress near bed surface is the cause of riverbed erosion. Sediment diffusion is maximum in the mid zone because in the mid zone there is sufficient sediment concentration and velocity, which helps sediment to diffuse.
Flow profile along depth (Mays, 2010)
Flow is a vast terminology in hydrology. It includes river flow, overland flow or runoff, groundwater flow, base flow, pressurize flow etc. Flow is the volume of water passing a certain point in unit time. Generally m3/s is used as flow unit. Erosion generally takes place in stream and overland due to hydraulic action. In river, high flow or velocity, longitudinal slope, bed roughness, water viscosity etc. cause erosion. Here high shear velocity scoop up bed material when the bed roughness is low. High water level expedites river bank erosion. When water level is raised above its normal level, comparatively less viscous water replaces high viscous sap in the soil pore space near the riverbank by osmosis process. This less adhesive water particle weakens the cohesion force among the soil particle and ultimately it break apart soil integrity. This is known as river bank erosion. High-energized raindrop initiates overland soil erosion. Erosion due to overland flow is dependent on many factors i.e. rainfall, hydraulic property of soil, tillage, cropping pattern etc.

Incipient motion of riverbed particle is commonly used as beginning of riverbed sediment particle motion or riverbed erosion. Normally sediment particles move when the fluid force is higher than the resistive force (Maidment, 1993). Shields (1936) determined experimentally that a minimum or critical Shields number (τc*) is required to initiate motion of the grains of a bed composed of non-cohesive particles. Parker et al. (2003) provide an equation of Shield’s diagram, which is used in this study.
Here Rep is the Reynold’s number regarding shear velocity (U*). For the simplicity of the analysis, sediment particles were assumed to be nearly uniform and cohesionless.

where τo is shear stress and γs, γf is specific weight of sediment and fluid and d is average particle size
Rep = U*.d/υ where U*= √( τo/ρ) and ρ is fluid density, υ is dynamic viscosity.

SS concentration is based on a principle of balance between the downward force of fall velocity and upward force of eddy motion. In the field it simply measured by physical manner.

River bank erosion generally occurs after flooding. When flood water rises, it replaces the adhesive sap in the nearby riverbank soil pore space. So, when flood water recedes, it weakens the riverbank integrity and chunk of soil fall into the river. Bank erosion has complex relationship river flow, because it does not occur when flow increases rather when flow recedes. River bank vegetation, soil type, protective measures are other factors that affect bank erosion. Bank erosion directly increases SS concentration. Some portion of sediment settles instantly and some portion is conveyed downstream. Bank erosion is an important morphological phenomenon. It provides sediment source to the riparian habitat (Florsheim, Mount, & Chin, 2008).
Watershed erosion is the indication of soil erosion due to energized rainfall, cropping practice, soil characteristics etc. It can be estimated using Revised Universal Soil Loss (RUSL) Equation. This equation says that, potential, long term average annual soil loss is,

A = RKLSCP

Where, R is the rainfall factor, K is the soil erodibility factor, L and S are the slope length and steepness factors, C is the cropping-management factor, P is the support practice factor. Here A is in tons per acre per year. The amount indicates the magnitude of erosion and eroded soil accumulates nearby water body or river and increase the concentration of SS (“Revised Universal Soil Loss Equation for Application in Canada Handbook,” 2013).

Feedback loop indicates the relationship balance among different processes. One process provides some result or feedback to another process, which ultimately balance the entire system. Feedback loop could be positive or negative. In our study, flow- erosion- SS concentration mechanistic linkage creates a positive feedback loop. The controlling element of this loop is river sediment capacity. When the sediment capacity of a river is reduced, flow is increased and bed roughness decreases. It causes riverbed and bank erosion, which increases the amount of sediment in the river. Again when new sediment is introduced, river sediment capacity is increased and flow is reduced. This whole feedback loop is illustrated below:

Feedback loop
References

Abu Hammad, A. (2011). Watershed erosion risk assessment and management utilizing revised universal soil loss equation-geographic information systems in the Mediterranean environments. Water and Environment Journal, 25(2), 149–162. doi:10.1111/j.1747-6593.2009.00202.x

Florsheim, J. L., Mount, J. F., & Chin, A. (2008). Bank Erosion as a Desirable Attribute of Rivers. BioScience, 58(6), 519. doi:10.1641/B580608

Ji, U., Velleux, M., Julien, P. Y., & Hwang, M. (2014). Risk assessment of watershed erosion at Naesung Stream, South Korea. Journal of Environmental Management, 136, 16–26. doi:10.1016/j.jenvman.2014.01.033

Maidment, D. R. (1993). Handbook of hydrology (p. 1424). McGraw-Hill. Retrieved from http://books.google.ca/books/about/Handbook_of_hydrology.html?id=4_9OAAAAMAAJ&pgis=1

Mays, L. W. (2010). Water Resources Engineering (p. 890). John Wiley & Sons. Retrieved from http://books.google.com/books?id=Nh8Y3vIjXK8C&pgis=1

Parker, G., Toro-Escobar, C. M., Ramey, M., & Beck, S. (2003). Effect of Floodwater Extraction on Mountain Stream Morphology. Retrieved from http://ascelibrary.org/doi/abs/10.1061/(ASCE)0733-9429(2003)129:11(885)

Revised Universal Soil Loss Equation for Application in Canada Handbook. (2013). Retrieved from http://sis.agr.gc.ca/cansis/publications/manuals/2002-92/index.html

Shi, Z. H., Ai, L., Fang, N. F., & Zhu, H. D. (2012). Modeling the impacts of integrated small watershed management on soil erosion and sediment delivery: A case study in the Three Gorges Area, China. Journal of Hydrology, 438-439, 156–167. doi:10.1016/j.jhydrol.2012.03.016

Shields, A. (1936). Anwendung der Ähnlichkeitsmechanik und der Turbulenzforschung auf die Geschiebebewegung.