Basis of Design Report for Terrier Rouge Housing Project, Drainage and Sanitation Improvements

The project focuses on drainage and sanitation improvements, with a 100% design package prepared by Tetra Tech for the United States Agency for International Development (USAID). The existing sanitary system at Terrier Rouge consists of 16 communal septic tanks and associated leach fields, designed to support 242 housing units. However, the original contractor failed to build the leaching systems according to the plans and specifications, resulting in non-compliance with the specified system. Remedial construction was undertaken, and percolation tests revealed that the percolation rates were above the 120 minutes per inch threshold, prohibiting infiltration of the effluent. The existing systems were subsequently replaced in accordance with the design plans and specifications. The existing drainage system at Terrier Rouge is characterized by a 7,400 cubic meter stormwater basin, which collects runoff from the site. Due to the tight soils and poor infiltration rates, the runoff remains in the basin, creating a safety hazard and promoting mosquito breeding. The site slopes downgradient in the northerly direction, with stormwater runoff collected by roadway gutters and channels and routed to a large pond. However, the original design plans did not include an outlet structure for the pond, and the collected stormwater does not infiltrate into the ground as intended. Tetra Tech conducted a topographic survey to verify the information on the design plans and obtain topographic information in the vicinity of the drainage pond. This additional information was required to develop design alternatives for the proposed drainage and sanitation systems improvements. The proposed improvements to the sanitary system at Terrier Rouge include the construction of a communal sand filter to treat the effluent to an acceptable standard in accordance with the International Private Sewage Disposal Code (IPSDC). The sand filter design was USAID's preferred option, and it will provide filtration, aeration, and biological treatment within the filter before being discharged. The proposed system upgrades will be connected to each of the 16 onsite septic tanks and will redirect the septic tank effluent from the existing chamber system to the proposed multi-baffled chamber and sand filter system. The proposed drainage system improvements include the construction of a new stormwater drainage system, which will collect and convey stormwater runoff from the site to a new outlet structure. The new system will include a network of pipes and channels to redirect stormwater runoff away from the existing pond and prevent standing water from accumulating. The proposed system will also include a new outlet structure to allow for the safe discharge of stormwater runoff from the site. The proposed improvements to the drainage and sanitation systems at Terrier Rouge are designed to improve the living conditions of residents and reduce the risk of waterborne diseases. The new systems will provide a safe and reliable means of disposing of wastewater and stormwater runoff, and will help to prevent the accumulation of standing water and mosquito breeding.