2006 Greater Everglades Ecosystem Restoration Conference

Simplified Modeling of Canal Water Intrusion in the Arthur R.Marshall Loxahatchee National Wildlife Refuge

Michael G. Waldon

DOI Everglades Program Team, A.R.M. Loxahatchee National Wildlife Refuge, Boynton Beach, FL, USA

Simplified models can provide insight and quantitative predictions that develop our understanding of relevant process. Here, a simple model of canal water intrusion in the Refuge is explored. The two component models described here are (1) a model of stage at refuge gauging sites, and (2) a model of water flow in and out of the marsh from the perimeter canal based on a flat-pool assumption and using a volumetric balance. From these models some observations and testable predictions are derived.
From the stage model (1) it is observed that surface water flow between a marsh site and the surrounding marsh area and canal initiates only after water stage surpasses not only the local ground elevation, but also a slightly higher elevation, termed the “puddle elevation.” In the interior marsh, puddle elevation exceeds soil elevation by very roughly 0.8 feet, and must approach soil elevation near the edge of the canal. When stage is above the puddle elevation at a marsh gauge site, stage at the site stays close to canal stage. That is, above the puddle elevation, Refuge stage is approximately uniform, or flat. The flat-pool assumption should apply as an approximation to any location in the Refuge where/when canal stage is above puddle elevation. When canal stage is below puddle elevation at an interior marsh site, water stage typically exceeds canal stage.
The flow model (2) calculates water velocity and volumetric discharge moving toward or away from the interior marsh by using a volumetric balance based on stage rise, rainfall, and evapotranspiration (ET) loss. This model demonstrates that canal water intrusion may occur when canal stage rise exceeds the net value of rainfall minus ET. Further, it shows that intrusion velocity and distance of penetration is greatest when stage is near, but above, the puddle elevation, and is greatly reduced when stage greatly exceeds puddle elevation. This is because at higher stage the cross-sectional area of a flow transect is greater, and water velocity is proportionately reduced for the same inflow volume. Thus, the combination of models (1) and (2) demonstrate that there is a “window” of stage beginning just above the puddle elevation at which time the risk of canal water intrusion is greatest.
These models assist in quantifying intrusion events and provide testable predictions concerning when intrusion occurs. The flow model could also provide transect flow data in support of 1- dimensional modeling of mass transport and water quality constituent modeling.

Contact Information: Michael G. Waldon, EPT, A.R.M. Loxahatchee NWR, 10216 Lee Road, Boynton Beach, FL, 33437 USA, Phone: 561-735-6006, Fax: 561-735-6008, Email: mike@mwaldon.com