EFFECTS OF STROMS AND STORM SEQUENCING ON SEDIMENT TRANSPORT

The physical hydrology and trunk channel discharge characteristics for small watersheds is well documented with the bankfull concept strongly influencing management strategies. In contrast, much less is well known about the geomorphic effects, such as the flux of suspended sediment, during the rising and falling limbs of a discharge hydrograph in the context of multiple storm events. Preliminary data collected from Saucon Creek, a mixed-use, instrumented watershed of 141.10 km2 in east central Pennsylvania indicate that suspended sediment both anticipates and follows peak discharge with the flux being strongly influenced by the specific sequencing of storm events. The sediment peak coinciding with the rising limb of the discharge hydrograph is interpreted as the mobilization of stored sediment in the channel near the sampling site whereas the peak coinciding with the falling limb is thought to be generated by overland flow and bank failure upstream of the sampling site. The interactions of local and distal sediment sources are reproduced in a series of small sand bed flume experiments which are scaled to mimic real discharges and storm sequencing. Similar to the real watershed, we observed sediment transport in the flume to occur out of phase with peak discharge. Scaled sediment fluxes of 2.8% saturation consistently resulted in storage in the form of flood plains or terraces. In contrast lower scaled sediment flux or higher discharges consistently resulted in erosion. In summary, the modeling and the instrumented watershed results together show that bankfull discharge may not be the dominant channel shaping discharge. Further investigations to the geomorphic work being done during the rising and falling limb and storm sequencing are necessary for effective management strategies.