Recent morphodynamics of the Indus delta shore and shelf

Abstract

In natural conditions, the Indus River had one of the largest sediment loads in the world, building an extensive delta on the high-energy coast of the Arabian Sea. However, water and sediment discharge have been drastically altered in the Indus since the early 1960s, when several barrages were built along the river to feed the world's largest irrigation system. A digital terrain model based on detailed 19th century surveys has been constructed to assess the morphology of the Indus shelf. Comparison of the digital terrain model to a 1950s Pakistani bathymetric survey allowed an estimation of the natural sedimentation regime before extensive human-induced changes. Digital analysis of the Indus delta coastline based on satellite imagery was used to explore the effects of the drastic decrease in sediment delivery following extensive dam building. The Indus Canyon is a dominant feature of the region dissecting the shelf to within 20 m water depth and 3.5 km of the coast. Theoretical considerations based on estimates of the relative importance of wave energy vs. fluvial sediment delivery suggest that the Indus delta should develop a mid-shelf subaqueous clinoform. Instead, the Indus shelf exhibits a compound clinoform morphology. A shallow delta front clinoform extends along the entire delta coast from the shoreline to the 10–25 m water depth. A mid-shelf clinoform developed probably as a prodelta clinoform between ∼30 and 90 m water depth. The advanced position of the mid-shelf clinoform east of the Indus Canyon might reflect either a prolonged sediment delivery from the Indus River in that area compared to the shelf west of the canyon or the presence of a relict pre-Holocene mid-shelf delta. A distinct lobe of the mid-shelf clinoform developed along the Kutch (Kachchh) coast probably as sediment advected alongshore was redeposited on the mid-shelf by strong offshore-directed tidal currents at the Gulf of Kutch mouth. Accumulation and erosion between 1895/96 and 1952/54 occurred primarily on the delta front clinoform, but also on the prodelta clinoform sector covered by both the surveys. During that time period, at the active Indus mouths, the delta front clinoform has built directly into the Indus Canyon, where sedimentation rates exceeded 50 cm/year. A sediment budget for the shelf for the 1895/96–1952/54 period suggests that the previous estimate of an Indus sediment discharge rate of 250 million tons per year in natural conditions is probably a minimum estimate. For the same time interval, the shoreline advanced along most of the delta coast. The progradation rate at the active mouths along the central delta coast surpassed 100 m/year. Following the 80% reduction in sediment discharge after the late 1950s, the deltaic shoreline along the central delta coast started to recede at average rates of ∼50 m/year. The abandoned delta shore (southeastern and northwestern sectors of the delta coast) remained largely progradational over the same period, with the southeastern sector prograding at an even greater rate than before. This differential behavior of the delta shoreline suggests a significant role for delta front sediment transfer processes in the evolution of abandoned deltaic coast.