GATEWAY DRAINAGE SYSTEM 2017 WET SEASON/IRMA OVERVIEW

13 Year Drought Regional water elevations were very low coming into the end of 2017. We were in the end of a 13 year drought cycle. This resulted in historically low dry season water levels as much as 4 feet below control elevation throughout many of these communities. During this low water time, vegetation expanded along the lake banks and canals. The lack of drainage demand made the maintenance of the system a lower priority.

2017 Lake Bank Remediation Project During this time we started working on the 2017 Lake Bank Remediation project. This work was to address the five lakes in the community with the worst lake bank erosion issues. The work started when the water levels were low, which allowed of easy contractor access to set the toe of the remediation slope and regrade the lake banks.

2017 Wet Season Rainfall Events Wet season started strong in June this year with significant rainfall amounts bringing the regional water levels back up to and over the design control elevations. These above average rainfall months have continued to today. During August 24th to 28 this area received 9.50 INCHES of rainfall. This is the the equivalent of a 25 YEAR design storm for this area. The drainage systems are permitted and designed to: Keep roads dry for a 25 year storm event Keep homes dry for a 100 year storm event This all happened well in advance of Hurricane Irma.

Drainage System Conditions Before Irma High water levels were being experienced in many portions of the drainage system that did not draw down as designed. We tracked this as the lake remediation contractors were dealing with this issue daily. We worked with your staff to find the constrictions in the drainage system that were extending the time for these high water events.

Hurricane Irma Preparation - Inspection We worked with your staff to track the system constrictions and found that the final outfall structure and lake abutting Six Mile Cypress had issues. The outfall structure fish guard had been clogged with hydrilla and was broken in several places. The final out fall lake was also full of hydrilla. These combined to stack water back throughout the entire system.

Hurricane Irma Preparation - Repairs We coordinated with the lake bank remediation contractor and your staff to make localized repairs in anticipation of Irma. The damaged fish guard was removed and the outfall weir was cleared of debris. The final outfall lake was also sprayed to reduce the hydrilla and improve water flow capabilities. This improved the systems ability to draw down the upstream water levels and reduce flooding elevations and durations. Water levels in the system began to drop immediately.

Hurricane Irma Impacts Hurricane Irma made landfall on Marco Island and moved north through the Gateway Community. The storm eye wall hit at 7 PM on Sunday night as a Category 2 hurricane. Wind damage was limited primarily to landscaping, roofs and pool cages. The rainfall with the fast moving storm was found to be 11.5 INCHES in ONE DAY. This correlates to a 100 YEAR storm event. The drainage system did have a responsive rise in the water elevation, but the improved outfall allowed for a faster bleed down and lower peak water elevations.

Irma Drainage Issues

Recommendations Develop an aggressive maintenance program to check weirs and connector waterways during the dry season when access is easier and the system is not under flow stress. Insure that the weirs and connectors are clear of debris and vegetation. Evaluate the in-line and outfall weirs with respect to recorded tail waters to see if any of the weir sizes can be increased or if the weirs can be retrofitted with operable gates for pre-storm water level reduction. Consider revising existing drainage permits with SFMWD and Lee County to accommodate larger weirs and operable gates so water level bleed downs can be accommodated. Document and track rainfall and water level data to discover where there are constrictions in the system to prioritize maintenance work. Review perimeter berm and offsite inflows to protect against uncontrolled breaches. Develop a pre-storm protocol for emergency review of the drainage system constriction points and draw down of the water levels in anticipation of an event.