Suspended sediment dynamics for June storm events in the urbanized River Tame, UK

Few studies of urban water quality storm-event dynamics have been conducted at the sub-catchment scale, though most models assume a "first-flush", positive hysteresis response. This paper reflects on the appropriateness of the first-flush model for summer (June) storms, using high-resolution flow, turbidity and water quality data from the highly urbanized River Tame headwater system, central England. Discharge responded very quickly to storm precipitation, and turbidity events exceed 500 FTU 60 times per annum. However, "first-flush" responses were rare, and turbidity normally peaked after the flow maximum, leading to anticlockwise hysteresis, reinforced by lengthy turbidity "tails". This suggests limited sediment exhaustion. Many responses probably relate to triggering of Combined Sewer Overflows and/or Waste Water Treatment Works, which occur late in storms when capacities are exceeded, and is consistent with concurrent ammonia increases and DO decreases. Prolonged turbidity responses can raise suspended solids loads above predicted levels, increase ecological stress, and lead to complications for sediment budget studies. A re-examination of the first-flush model is overdue.