Sediment budgeting of short‐term backfilling processes: The erosional collapse of a Carolingian canal construction

Abstract

Sediment budgeting concepts serve as quantification tools to decipher the erosion and accumulation processes within a catchment and help to understand these relocation processes through time. While sediment budgets are widely used in geomorphological catchment-based studies, such quantification approaches are rarely applied in geoarchaeological studies. The case of Charlemagne's summit canal (also known as Fossa Carolina) and its erosional collapse provides an example for which we can use this geomorphological concept and understand the abandonment of the Carolingian construction site. The Fossa Carolina is one of the largest hydro-engineering projects in Medieval Europe. It is situated in Southern Franconia (48.9876°N, 10.9267°E; Bavaria, southern Germany) between the Altmühl and Swabian Rezat rivers. It should have bridged the Central European watershed and connected the Rhine-Main and Danube river systems. According to our dendrochronological analyses and historical sources, the excavation and construction of the Carolingian canal took place in AD 792 and 793. Contemporary written sources describe an intense backfill of excavated sediment in autumn AD 793. This short-term erosion event has been proposed as the principal reason for the collapse and abandonment of the hydro-engineering project. We use subsurface data (drillings, archaeological excavations, and direct-push sensing) and geospatial data (a LiDAR digital terrain model (DTM), a pre-modern DTM, and a 3D model of the Fossa Carolina] for the identification and sediment budgeting of the backfills. Dendrochronological findings and radiocarbon ages of macro remains within the backfills give clear evidence for the erosional collapse of the canal project during or directly after the construction period. Moreover, our quantification approach allows the detection of the major sedimentary collapse zone. The exceedance of the manpower tipping point may have caused the abandonment of the entire construction site. The spatial distribution of the dendrochronological results indicates a north-south direction of the early medieval construction progress.

Channel morphodynamics and sediment budget of the Lower Ganga River using a hydrogeomorphological approach

“…The range of uncertainty in extractable sediment volume for GW1, GW2, and GW3 is 7–8%, while for GW4 it is around 42% (estimated from heights interpolated from hydrological sediment budgeting). Several workers have used high‐resolution DEMs for sediment budgeting studies (Erwin et al, 2012; Fuller et al, 2003; Schmidt et al, 2020), but the application of UAV surveys presented in this work is novel.…”

Special issue: German Society for Geomorphology

“…Schmidt et al ( 2020 ) used subsurface data (drillings, archaeological excavations and direct‐push sensing) and geospatial data (a LiDAR digital terrain model [DTM], a pre‐modern DTM and a 3D model) for the identification and sediment budgeting of the erosional collapse of the Fossa Carolina in South Germany—a Carolingian canal construction which represents one of the largest hydro‐engineering projects in mediaeval Europe. The modelling approach shows that a total of 15% of the excavated material was eroded during strong rainfalls and was redeposited as backfill in the canal trench.…”

High-Resolution Direct Push Sensing in Wetland Geoarchaeology—First Traces of Off-Site Construction Activities at the Fossa Carolina

“…The canal is approximately 3 km in length and has a conspicuous s shape (Figure 1c) in order to minimise the work needed to complete the respective section [35]. According to dendroarchaeological findings, it is assumed that the work progressed from north to south towards the summit [36]. However, the southernmost part of the canal was never finished, as no archaeological remains were detected in the Altmühl floodplain [37].…”

“…The construction of the summit canal was designed as a stepped chain of ponds with approximately 2.5-5 m fairway width [4,29,30,39] and a northward shifted summit [36,39]. Large oak timbers were used for the stabilisation of the canal edges.…”