Analogue modeling of transverse drainage mechanisms

Fifteen physical experiments on a stream table replicated four general mechanisms proposed to explain transverse drainage development, namely antecedence, superimposition, overflow, and piracy. The experimental design encompassed multiple strategies to effectively replicate the different mechanisms. These experiments roughly emulate natural fluvial processes including aggradation, lateral erosion, degradation, knickpoint retreat, ponding, and overflow. The experiments address landscape morphology and depositional field criteria associated with antecedence, overflow, and piracy. Antecedence experiments showed that defeat of streams by a tectonically growing structure could result from a combination of upstream ponding and diversion into a trunk drainage, insufficiently fast retreating knickpoints, and downstream deposition of eroded sediments. Piracy can result from stream aggradation until it overtops a low divide by headward erosion of a stream through a divide and by sapping processes. For piracy driven by headward erosion, asymmetrically sloped ridges allow the drainage divide across the ridge to retreat from headward erosion, but a symmetric divide hinders the ability of a stream to retreat across the ridge apex. Aggradationally driven piracy is potentially hindered because the slope across the divide needs to be sufficiently steep compared to the uncaptured stream. Otherwise, deposition occurs across the divide and a levee is formed which prevents capture. For transverse drainages produced by overflow, knickpoints retreat across the overflow outlet producing a high probability of downstream flooding. A positive feedback develops where added discharge to the drainage from lake water increases erosion and knickpoint retreat, further eroding and undermining the basin outlet and releasing even more lake water. The result is rapid incision and dramatic flooding downstream of the bedrock high during lake outlet erosion. Two superimposition experiments were unsuccessful. Most of the field criteria for antecedence, overflow, and piracy were either directly replicated or implied based on observations during each experimental run.