Contemporary subsidence and settlement of the Fraser River delta inferred from SqueeSAR(TM)-type InSAR data

The Fraser River delta in southwestern British Columbia formed over the past 10 000 years and currently supports a population of about 250 000 people. This research focuses on the urbanized and rapidly expanding Richmond area of the Fraser River delta. Dyking, which began in the early 1900s, has prevented flooding and sediment deposition, with the result that the delta plain is subsiding at an average rate of 1-2 mm/a due to the slow, natural consolidation of thick Holocene sediment. Localized higher rates of subsidence stem from anthropogenic sources, notably the application of loads in construction. InSAR data was used to relate load-induced settlement to geology and the spatial and temporal pattern of urbanization. All displacement rates 10 mm/a or more are associated with industrial or large commercial structures. Similar amounts of total settlement are observed from similar sized loads, yet rates of settlement differ, indicating that while load is important in determining total settlement, lithology is as important as load in determining rates of settlement. Holocene sediments underlying the delta are water-saturated, porous, fine sand, silty sand, silt. They are compressible to considerable depth and can experience significant settlement when subjected to structural loads, dewatering, or seismic shaking. No relationship between subsidence rate and surface geology was observed, although surface settlements are generally removed prior to construction. There is an insufficient amount of subsurface data to disentangle lithology and sediment thickness effects. A weak relationship between Holocene thickness and displacement rate has been observed for commercial sized buildings in eastern Richmond with an increase of 0.95 mm/a in the displacement rate for every 100 m of sediment thickness. A weak logarithmic relationship was also observed for the entire RADARSAT dataset.