Executive Summary

The purpose of this model report is to evaluate the flood stage implications of a one- foot increase in the crest elevation of several weirs in the Gator Slough canal system. The objective of this crest change is to store more water in the water table and canals during the wet season for use during the following dry season. The model was constructed using the Storm Water Management Model developed by XP-Software (XP-SWMM2000).

The modeling objectives are to run the 5, 25, and 100-year storm events and estimate the flood stages for these events with and without the crest elevation changes. The project area is located in Cape Coral City, between Pine Island Road (S.R. 78) and the northern edge of Lee County. Canals included in the model are Gator Slough, Shadroe, Horseshoe and Hermosa. The land area modeled in detail measures 25 square miles and is bounded by U.S. 41 on the east, Pine Island Road on the south, Matlacha Pass on the west and the Cape Coral city limits on the north. The contributing watershed to the east of U.S. 41 is approximately 33 square miles of low-density residential and undeveloped lands. This area was incorporated into the model through historical (USGS) and stochastically generated flow data sets. These input hydrographs provide boundary inflow to the modeled watershed of Gator Slough. The model was calibrated using a 24-day data set collected between 5^th and 28^th of September, 1996. This was considered to represent a one-year storm event. Another 24-day period (July 12^th to August 4^th, 2001) provided the verification data corresponding to a 25-year event. The calibrated/verified model was then run for three design events:

(i) 5-year, one-day rainfall event of 5.2 inches

(ii) 25-year, three-day rainfall event of 10.6 inches

(iii) 100-year, three-day rainfall event of 12.91 inches

These events were simulated first with the target weir crests (Weirs #11, 13, 14, 15, 19) set to their existing conditions. The model simulations were repeated with the weir crests raised by one foot. Interconnections between sub basins 6 and 7, sub basins 7 and 9, and sub basins 9 and 10 were not included in the model since preliminary modeling show low gradients between the areas. Thus inclusion of these interconnections would provide little additional model performance enhancements. Also not included in the model is a Transfer Pump Station, which conveys water from Basin 9 to Basin 4.

When compared to data collected by USGS corresponding to a 5-year rainfall event, model flow volumes were 14% less than measured flow values on Gator Slough. Only one rainfall station was available to represent the entire 25 square miles watershed. Agreement was very good (less than 8% difference) between XP-SWMM predicted peak flow (764 cfs) for Gator Slough at S.R. 765 and the Gumbel estimates generated from USGS measured data (826 cfs). Simulated water surface elevations were approximately 0.1 foot above USGS measured values. XP-SWMM water surface elevations were less than 0.5 feet below Johnson Engineering estimates [1]. The previous Johnson Engineering modeling study was conducted using the HEC-2 program. The Johnson Engineering model was not subject to calibration using measured data. Thus, it is not surprising that the new XP-SWMM dynamic routing model delivers results that show much better agreement with stage and flow data measured by the USGS.

When compared to the verification event data collected by USGS (very similar to the 25-year design event), model flow volumes were 17% less than measured data on Gator Slough. Simulated water surface elevations were approximately 0.5 feet above measured values. However when compared to Johnson Engineering modeling results, the 25-year XP-SWMM water surface elevations were 3 feet below Johnson Engineering estimates, derived from the HEC-2 model. Agreement was very good (less than 1.5% difference) between XP-SWMM predicted peak flow (1300 cfs) for Gator Slough at S.R. 765 and the Gumbel estimates generated from USGS measured data (1281 cfs)

When the model was extended to the 100-year event, XP-SWMM simulation generated water surface elevations 3 feet below Johnson Engineering’s estimates. When the target weir crests were raised, the 100-year XP-SWMM model generated water surface profiles that increase from a minimum of 0.0 feet (almost no change) at U.S. 41 to a maximum of 0.6 feet at Weir #11. However these increased water surface elevations were still below the pre-modification Johnson Engineering HEC-2 model results. Agreement was very good (less than 2.2% difference) between XP-SWMM predicted peak flow (1692 cfs) for Gator Slough at S.R. 765 and the Gumbel estimates generated from USGS measured data (1656 cfs).

Based upon results of the new XP-SWMM simulation of the Cape Coral canal system, very little additional flooding is expected to result from raising the target weirs. Furthermore, all design event flood stages associated with the raised weir conditions are still well below the previously predicted values. Therefore, if prior Johnson Engineering HEC-2 model estimated water surface profiles were deemed acceptable according to flood stage criteria, then the post-modification water surfaces as predicted by XP-SWMM should also be acceptable. The reliability of these XP-SWMM results is supported by favorable comparisons with USGS measurements. In fact XP-SWMM results are still conservative relative to City of Cape Coral measured stage data for a 25-year event documented in July 2001. The XP-SWMM predicted water surface elevations for this event were approximately 3 feet above measured stages over the middle and upper reaches of Gator Slough.

Results of the XP-SWMM model implementation on the Gator Slough, Horseshoe, Hermosa and Shadroe canals of Cape Coral show very little additional flooding due to increasing weir crest elevations by 1 foot at Weirs 11, 13, 14, 15 and 19. For the 100-year design simulations, water surface profiles increased by a maximum of 0.6 foot in a limited reach of Gator Slough between Burnt Store Road (S.R. 765) and Chiquita Boulevard. Horseshoe canal showed a slightly greater stage increase (0.8 foot) along the lower reach between Burnt Store Road and Chiquita Boulevard. Hermosa and Shadroe canals showed little change for flooding in their downstream reaches between Burnt Store Road and Chiquita Boulevard.

The modeling results presented in this report show a worst-case scenario. These conditions should not actually occur assuming the Operating Protocol Table procedures are followed, as developed and presented in the City of Cape Coral project permit application document. This table provides conditions for the City to add or remove 1-foot stop logs in response to monitored water levels in the canals.