Scour- and Erosion-Related Issues

The paper reports some recent advances in the mechanics and countermeasures of scour and internal erosion. It presents a concise review of the key issues on the tsunami/wave-seabed-structure interactions from geotechnical and hydrodynamic perspectives. I highlight here seepage erosion, effect of overflow and seepage coupling on scour of breakwaters, wave-induced liquefaction and scour protection around a monopile, internal erosion, cavity formation and collapse behind seawalls and quaywalls.

Scour often occurs around bridge piers and abutments, which are commonly supported by piles. This paper evaluates the effects of global scour and local scour on the behavior of laterally-loaded single piles and pile groups in sands and clays based on recent studies. The recent studies have been focused on stress history changes of remaining soils after scour, local scour-hole geometry and dimensions, and their effects on the behavior of laterally-loaded piles under scoured conditions. Experimental, numerical, and analytical studies have been conducted for these investigations and are discussed in this paper. Scour depth is identified as the most important influence factor, followed by scour width and slope. This paper also presents an integrated analysis of the effect of scour on the performance of pile-supported bridges that considered hydraulic, geotechnical, and structural aspects.

The paper presents an example of coastal scour and its mitigation and few examples of limitations of its prediction. An extremely serious case of continuous and rapid erosion of the coastline in the Gulf of Cambay (Khambat) in the state of Gujarat, was dealt with an innovative design using gabions and geosynthetics. The designed and installed measure included a global perspective of preventing erosion along the toe and the base with an apron. Scour prediction unfortunately as in several cases in Geotechnical Engineering presume a homogeneous soil profile which is rarely and ever met with in real life. The practice of ignoring this common occurrence leads to uneconomic and in some cases anomalous situations wherein the predicted depth extends to stiff and strong strata. Few examples illustrate these situations.

The coastal regions across the globe are more densely populated; this effect could be endorsed to urban development including the infrastructure for ports, inland waterways, trade and commerce, tourism, fisheries, hub for power plants, etc. Owing to the growing population demands many residential, industrial and commercial units have been constructed in the close vicinity/proximity of the shoreline. The developmental activities across the shore mandate the construction of facilities that promote industrial growth such as fishing harbours, ports, jetties, training walls, quays and wharfs as well as facilities for tourism development such as surfing friendly seabed slopes, added beach width for recreational activities and establishment of beach resorts, quays and wharfs. Predominant coastal hazards such as coastal flooding, coastal erosion, sea level rise, storm surge and seawater inundation are common threats to sensitive coastal communities. Alongside the prevailing natural causes of melting glaciers due to global warming and sea level rise, extreme events such as storms and cyclones; man-made artificial structures in the near shore regions/in between shallow and intermediate water depths can potentially destabilize the shoreline and lead to excessive coastal erosion if not addressed carefully. This paper details the vast experience gained in the field of coastal protection measures along the coastline of India.

Soil erosion caused by surface runoff poses serious problems to the stability of structures built in sloping ground. Erosion of soil from hill slopes result in soil slides with boulders/disintegration of soil mass and affects the foundations laid in the vicinity of hill slopes. Deep excavations in clayey silty sands for basement floor constructions are challenging as they appear to stand vertical in dry state and any saturation of soil due to rainfall or subsurface seepage and associated erosion result in sudden sliding of soil mass from the excavated surface. Also, the Coastal cities are getting vulnerable to erosion of beaches caused by sea erosion due to development of ports and improper nourishment activities undertaken by the port and local authorities. In the present paper, issues related to beach erosion, effect of soil erosion from surface runoff in sloping ground and deep excavation failures pertaining to Visakhapatnam are presented. The various remedial measures to check the erosion are discussed.

River engineering requires to start from the location and characteristic of a watercourse like in the mountains, or at the foothills (mid slope), or in the valley, close to the river outlet. Soil erosion in three different regimes varies due to different soils and different morphology of rivers. Soil erosion phenomena follow the principle that instability starts when shear stresses become critical generating soil particles to get to incipient motion with the flow. Gabions and mattresses made of double twisted wire mesh provide shear resistance due to confinement of stones and have been used as erosion protection for many years. Design approach of mattresses is based on a most recent research carried out at Colorado State University in 2019. This paper presents the new design approach along with performance limits of new type of mattress tested, as per ASTM D6460, based on observation of the mattress stability with respect to flow through mattress layer, combining the stability of the stone and its ability to control soil erosion underneath. Also, five case studies are presented covering different solutions which demonstrate performance and durability of mattresses.

Geotextile tube is used to encapsulate sandy soils to enable their use as flexible, erosion-resistant, mass-gravity structures in coastal protection and land reclamation applications. The paper introduces the various grade of geotextile tube units in use and discusses their range of applications. Important stability aspects of design are presented along with geotextile tube mechanical and hydraulic property requirements. Ways of enhancing geotextile tube durability are also discussed. Two case studies are presented that demonstrate the effective use of geotextile tubes for coastal protection and land reclamation project.