EFFECTS OF BED ARMORING ON THE OCCURRENCE OF EROSIONAL AVULSIONS: ANALYSIS OF THE SUNCOOK RIVER NEAR EPSOM, NEW HAMPSHIRE

Erosional avulsions, when compared to aggradational avulsions that commonly occur in deltas and floodplains, are infrequent. These avulsions take place when a sediment-limited river abandons a channel it cannot incise to carve a new one at a lower elevation. The greater steepness of the new channel allows it to more easily accommodate the discharge and sediment load of the river. The Suncook River near Epson, New Hampshire, provides an ideal setting for the study of this type of avulsions. The Suncook River avulsed during strong rains in May 2006. The Suncook River incised into glacial lake deposits, and the new channel exposes thick beds of clay and cross-bedded sands. Unmapped areas of boulders are exposed in the bed of the abandoned channel. A short reach of boulders also outcrops 100m upstream of the avulsion site.

We suggest that the mobility a river's substrate is an important factor for the occurrence of erosional avulsions. In the specific case of the Suncook River, we suggest that the boulders in the abandoned channel armored the bed against erosion, preventing it from incising. We also suggest that the boulders upstream of the avulsion site did not permit the migration of the incision further upstream. We will present the results of surveys of the abandoned and new channels of the Suncook River. We will also look at the distribution of boulders near the site of avulsion through detailed mapping of the substrate of the channel. Initial observations show that a knickpoint migrated upstream through the new channel and continued to the reach of boulders upstream of the channel diversion, lowering the bed to about 1.5m below its original level. Based on the time the river reached its maximum discharge of over 3000 cfs, and the reported time of avulsion, we estimate an incision rate for the new channel of 1.15 meters per minute until channel capture was completed. Early surveys suggest that the profile of the channel upstream of the bouldery reach corresponds to pre-avulsion conditions. This suggests the river was not able to mobilize these boulders and the migration of the knickpoint was stalled. The present evidence from the Suncook River confirms our hypothesis of the role of substrate mobility in erosional avulsions.