Sediments in urban stormwater drainage systems
Suspended sediment, transported in urban stormwater sewers, is examined in order to determine its source, size, mineralogy, form and surface texture characteristics. The transport history is studied in relation to the hydrological parameters of rainfall and discharge in one catchment. The catchment is situated in North West London where field sampling was carried out over the period from March 1980 to December 1981. A Coulter Counter is used for particle size determinations; methods of sampling and the choice of dispersant and electrolyte are discussed. Particle surface texture analysis employs Scanning Electron Microscopy and preparation methods are discussed. Elemental composition is examined by energy dispersive x-ray analysis. Particle textures are described and quantified using a detailed surface area method and the Fuzzy Technique is employed in the analysis of a large number of particles. Sediment sources in the catchment include roads, buildings, open spaces and airborne material. Sediment is washed off land surfaces during rainfall and transported along the storm sewer to the outfall. Suspended sediment sampled at the outfall is commonly in the size range 1 to 40 mm and predominantly consists of quartz particles from roads tone erosion which have undergone considerable alteration by abrasion, silica precipitation and solution during drain transport. Storms and their sediment load fall into four groups : I. Intense rainfall of short duration generates moderately high total rainfall and discharge. Sediment comprises fresh-faced, angular, particles rapidly entrained from the land surface and of unimodal size distribution. II. Long periods of rainfall of moderate intensity create high rainfall totals and moderately high discharge. Drain deposited aggregates and surface particles are transported first; silica precipitates develop later, leading to aggregation as the discharge falls: size distributions are bimodal. III. Moderate rainfall and discharge transport sediment of similar characteristics to Group II but of moderated form. IV. Low rainfall and discharge for short period transports severly altered drain sediment of bimodal size distributions. Progressive sediment alteration along the storm sewer was simulated in a flume.