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November 25, 2025 Department of Planning and Natural Resources Charles W. Turnbull Regional Library - 4607 Tutu Park Mall St. Thomas, VI 00802 SITE DESCRIPTION The proposed development for the US Virgin Islands Department of Education (VIDE) St. Thomas Emanuel Benjamin Oliver PREK-8 School is located along Palmetto Road St. Thomas, US Virgin Islands. The existing site is occupied by an existing non-operational school, a central courtyard area and two asphalt parking lots. The site is bounded on the north by Palmetto Road and west, south, and east by hills that slope down away from the development area. The site has an approximate area of 7.99 acres out of which only 4.95 acres will be disturbed. See image below for the project Location Figure. Figure 1 – Site Location Figure Re: Stormwater Management Calculations Cover Letter US VIDE St. Thomas Emanuel Benjamin Oliver PREK-8 School 325 Palmetto Road, Anna’s Retreat, St. Thomas 00802, U.S. Virgin Islands Stormwater Management Calculations Cover Letter US VIDE - Emanuel Benjamin Oliver PREK-8 School November 25, 2025 EXISTING SITE CONDITIONS The topography of the project site varies between approximately elevations 205.0-ft to 250.00-ft however, the existing site is located at the top of a hill that is about 100-feet above the surrounding landscape. The existing grades on the center of the cluster buildings are relatively flat but rapidly slopes down away in the east, south and west direction. The existing site slopes up from the center of the building cluster towards the north. The elevations referenced are based on Army Corp Aerial Topography, per the topographic survey prepared by Marvin Berning & Associates (refer to Attachment A). Stormwater runoff currently drains offsite throughout the perimeter of the site, downhill. Tabulated below is a breakdown of the existing land use of the site. Surface Type Area (acres) Existing Buildings 1.15 Pervious 2.44 Impervious 1.36 Total 4.95 PROJECT DESCRIPTION The proposed project will consist of demolishing all the existing buildings and constructing a completely new PK-8 educational school, an admin center building, a gymnasium (FEMA safe room) facility, a dining and media-center building, and new driveways with drop-off areas and parking stalls. Figure 2 – Existing Buildings to be Removed Stormwater Management Calculations Cover Letter US VIDE - Emanuel Benjamin Oliver PREK-8 School November 25, 2025 Figure 3 – Proposed Site Plan Below is the breakdown for the post-development land use condition: Surface Type Area (acres) Proposed Buildings 1.09 Pervious 1.95 Impervious 1.91 Total 4.95 PROPOSED ON-SITE STORMWATER MANAGEMENT IMPROVEMENTS The proposed stormwater management system includes two separate systems. One of these includes catch basins and drainage pipes along the proposed northern driveway and discharges into the proposed dry detention area to the east of the proposed parking lot. This dry detention area overflows downhill along its full perimeter. This system also collects and discharges the upland runoff that comes from the hill to the north of the site. The second system consists of interconnected catch basins, drainage pipes, and a dry detention area located along the southern perimeter of the site. The runoff collected from the site discharges into the dry detention area Stormwater Management Calculations Cover Letter US VIDE - Emanuel Benjamin Oliver PREK-8 School November 25, 2025 and the runoff from the building roofs are conveyed to the proposed cistern under the new gym building to be treated and reused. The cistern has an overflow into the proposed dry detention area. This dry detention area also overflows downhill along its full perimeter. The top of bank elevation of the two dry detention areas has been set to provide sufficient volume to ensure that discharge occurs once the required stormwater quantity and quality requirements have been met. We have conducted a pre-development to post-development analysis for the 10-year and 100- year 24-hour storm events (type II distribution) per the VI Department of Planning and Natural Resources (DPNR), section 2.2 – Post-Construction Standards for Permanent Stormwater Control. The post-development and pre-development total runoff volumes will be calculated using the following design parameters and criteria: 1. Groundwater Table Elevation for the site is approximately elevation 211-ft per the Geotechnical Engineering Study prepared by Langan. The bottom elevation of the proposed dry detention areas will be designed to be at least 1 foot above the groundwater table elevation. 2. Using Table 3-6 Typical Curve Number Values for Urban Areas (SCS 1986) a weighted curve number value was calculated for the pre-development conditions and a uniform curve number was used for the post-development conditions based on land use type. The values used are tabulated below: CN (pervious) CN (impervious) CN (weighted) Pre-Development 80 95 88 Post-Development - 95 95 3. Precipitation (P) data will be obtained from NOAA (National Oceanic and Atmospheric Administration) Atlas 14. Refer to Attachment B. The rainfall amount used for the calculations is equivalent to 90% of the highest rainfall amount within the range provided in Atlas 14 for each of the 10-year 24-hour and 100-year 25- hour storm events, as per the requirements of the DPNR Environmental Protection Handbook. The pre-development and post-development will be calculated using the NRCS (Natural Resources Conservation Service) method. See below for formulas. S= 1000 𝐶𝑁-10 𝑄= (𝑃−0.2𝑆)2 (𝑃+0.8𝑆) (A) Stormwater Management Calculations Cover Letter US VIDE - Emanuel Benjamin Oliver PREK-8 School November 25, 2025 Q = total runoff volume (ac-ft) P = precipitation data (ft) A = area (acres) The table below summarizes the results of the pre-development and post-development runoff volume calculations. Storm Events Pre-Development Q (ac-ft) Post-Development Q (ac-ft) 10-year, 24-hour 3.13 3.50 100-year, 24-hour 6.51 6.91 As shown in the table below, the post-development condition generates more runoff volume than the pre-development conditions. To address this condition, we are proposing dry retention areas that will be providing additional storage volume on site of approximately 0.83 ac-ft. This volume is larger than the additional runoff volume generated in the post-development condition for the 10-year 24-hour event and the 100-year 24-hour event. Therefore, the post-development condition will discharge less total runoff volume offsite, compared to the pre-development condition for the 10-year 24-hour and 100-year 24-hour storm events. WATER QUALITY Based on the available information for the existing site, it does not appear that there are currently any water quality treatment practices in place. The proposed stormwater management system will provide the required water quality volume within the proposed dry detention areas. The required water quality treatment volume is being calculated using the following formula per the DPNR Environmental Protection Handbook, section 2.2 – Post-Construction Standards for Permanent Stormwater Control. WQ= (1") (𝐼) 12 , WQ = required water quality volume (ac-ft) I = Impervious area for proposed development (acres) WQ= (1") (1.91 𝑎𝑐𝑟𝑒𝑠) 12 WQ= 0.16𝑎𝑐−𝑓𝑡 W WQ (ac-ft) Required 0.16 *Provided 0.83 * The total water quality volume provided includes the volume in the proposed dry retention areas. Stormwater Management Calculations Cover Letter US VIDE - Emanuel Benjamin Oliver PREK-8 School November 25, 2025 SWPPP (STORMWATER POLLUTION PREVENTION PLAN) The following Best Management Practices (BMPs) are being implemented as part of the stormwater pollution prevention plan: - Silt fences and berms around the perimeter of the site - Stabilized construction entrances, - Inlet protection for existing and proposed drainage inlets - Sedimentation basins. The sedimentation basins are sized to hold the volume equivalent to 1” of runoff from the contributing area (on-site area disturbed during construction), as per the DPNR Environmental Protection Handbook, section 2.1 – Standards for Temporary Erosion and Sediment Control. FINISHED FLOOR ELEVATION Per FEMA panel 7800000028G the project site is located within FEMA flood zone X which means the project is not located in a flood zone area. Refer to Attachment C. The proposed finished floor elevations for the proposed buildings are 226.0-ft and 230.0-ft. If you have any questions, please do not hesitate to contact us at 786-264-7200. Sincerely, Langan Engineering and Environmental Services, LLC Eric Blaine Shwarz, PE Senior Principal USVI PE License No. 0-50150-3B Registration No. 1644-e Cc: Leonardo Rodriguez, PE, Kelvin Martinez, PE Enclosure(s): Attachment A – Topographic survey prepared by Marvin Berning & Associates Attachment B – NOAA Atlas 14. Attachment C – FEMA panel 7800000028G FL Certificate of Authorization No. 6601 \\langan.com\data\MIA\data8\300375805\Project Data\_Discipline\Site Civil\Reports\Stormwater Letter\SWM Report (Oliver).docx ATTACHMENT A TOPOGRAPHIC SURVEY ATTACHMENT B NOAA ATLAS 14 RAINFALL DATA