Creation of IWR-NavSym and Application

The Waterway System Simulation model was developed in response to a request from the U.S. Army Corps of Engineers, Galveston District to the Institute for Water Resources (IWR) for assistance associated with a series of Section 216 feasibility studies of navigation improvements to the Gulf Intracoastal Waterway in Texas. The initial study area was from High Island to the Brazos River, with subsequent portions of the waterway to be examined in later studies. The Galveston District desired a flexible model that could be applied to each of these studies as they were initiated. Work on model development was initiated in February of 1997, and the model was completed and applied to the High Island to Brazos River study area in April of 1998. PMCL then developed IWR-NavSym, which is an evolutionary outgrowth and enhancement of model development work done by IWR in cooperation with the Galveston District in 1997-1998.

IWR-NavSym is a Monte Carlo (risk-based) waterway simulation model created under contract to the Corps Institute for Water Resources. The model simulates movement of tows on a waterway system and is used to assess the economic impacts of navigation improvements to the waterway. Hourly operating cost data for tows is used to calculate costs as a function of transit time in the system. A given model run results in statistics on total operating cost based on tow travel time. By comparing different improvement scenarios with a baseline alternative, the reduction in operating cost (based on reduced system-wide travel times) can be determined, and alternative navigation improvement plans can be assessed in a risk-based economic framework. The network is described as a set of ports, nodes, and regular and lock reaches. Tow trips originate and end at ports, traveling through the reaches of the network. Each tow is tracked individually through the system as it moves from origin to destination port, with the model keeping track of the amount of time each tow spends in each reach. Tow transit time in a reach is based on a user-specified distribution of minimum, most likely, and maximum transit times. Congestion (slower speeds in high-traffic areas) and transit rules (when tows are allowed to pass each other) are applied to modify the tow transit time in each reach to reflect traffic conditions.

The purpose of the system is to investigate the improved route times from origin to destination for all commerce across an entire waterway system due to proposed changes in the physical structure. In some areas of a channel, tows must slow to safely navigate, while in others passing is restricted. These limitations incur additional operating costs and increased transit time. The model investigates the benefits of channel improvements by estimating the transportation cost savings associated with modifications to selected portions of the overall waterway system. Although there may be some added benefit by improving tow safety, the primary beneficial effect is the reduction in operating costs by reducing transit times.