Basis of Design Report: EKAM Settlement Upgrade – Offsite Drainage Channel Improvement

The need for these improvements is based on observations and analysis at EKAM, where heavy rainfall events cause inundation with runoff from a banana plantation on the west side of Trou du Nord, a major route along the west side of EKAM. Tetra Tech Inc. has designed a stormwater mitigation channel around the west and north sides of EKAM to intercept and divert stormwater runoff coming from the banana plantation. The proposed channel is designed to intercept runoff and direct it to the northwest corner of EKAM, where it discharges into a 1600’ long grass swale and then through 2-48” culverts under Route 6. Observations during past storms have shown that the existing offsite channel is seriously under capacity and prone to flooding, which backs runoff up, flooding homes at the lower elevation of EKAM. The proposed improvements will increase the capacity of the existing channel along Route 6 from approximately 110 cfs to 630 cfs, a 5.7 times increase. In addition to the Offsite Drainage Improvements, the plans call for culvert replacement under Route 6. The current channel discharges under the road through 2-48” culverts, having a capacity of approximately 290 cfs. The proposed culvert will have 2- 3’x9’ box culverts, with a capacity of approximately 760 cfs. These improvements will dramatically improve the capacity of the channel to convey runoff away from EKAM and reduce the incidence of flooding. Precipitation data for the EKAM region is not available for use in a hydrologic evaluation. However, the "Caracol EKAM Residential Park – Storm Water Management/Flooding Risk Mitigation Assessment Draft Report" (V3 Northwater International, 20 March 2015) provided off-site stormwater flow to the site, based on rainfall curves for Miami. The table below shows approximate off-site runoff values for different storm frequencies. Table 2.1 – Runoff from the Banana Plantation Storm Frequency Event | Runoff from banana Plantation to EKAM Housing Village ------------------------|-------------------------------- 2 Year | 169.5 10 Year | 381.4 100 Year | 1225.4 The proposed EKAM channel will discharge into the existing channel along Route 6, while reducing flow in the secondary canal. The existing channel along Route 6 collects stormwater from the Primary Channel Along Route 6 and the Secondary Canal, along the east side of EKAM. The proposed improvements will include design and construction of the EKAM Channel, which will divert offsite generated stormwater around the EKAM site. The proposed channels will be lined with gabions to reduce the footprint of the channels and to minimize scouring through the channels.