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1 | P a g e HYDROLOGY STUDY REPORT PROJECT SITE: V.I. DEPARTMENT OF PUBLIC WORKS (DPW) PLOT 73-B ESTATE CONCORDIA FREDERIKSTED, ST. CROIX, USVI 00840 PREPARED BY: 7-1 BONNE ESPERANCE P.O. BOX 8269 CHRISTIANSTED, ST. CROIX USVI 00820 Email: dcartwright@buildteceng.com SITE LOCATION 2 | P a g e TABLE OF CONTENTS INTRODUCTION ............................................................................................................................................. 3 SOILS CLASSIFICATION .................................................................................................................................. 3 CURVE NUMBER (CN) CLASSIFICATION ......................................................................................................... 4 BASIN CHARACTERISTICS .............................................................................................................................. 6 TIME OF CONCENTRATION (Tc) .................................................................................................................... 7 RUNOFF (Q) AND STORAGE VOLUME (V) ................................................................................................... 10 CONCLUSION ............................................................................................................................................... 12 APPENDICES ................................................................................................................................................ 14 3 | P a g e INTRODUCTION This site is currently developed with two main building structures and three smaller utility structures. Hurricanes Irma and Maria in 2017 rendered extensive damage leaving the facility uninhabitable in its aftermath. Since then, the site has been abandoned. The proposed improvements will result in the demolition of the three small utility structures and the main administrative building with adjoining equipment bays. A new approximately 4,150 SF administrative building will be constructed towards the entrance to the site and a new approximately 1,780 SF heavy equipment parking structure will be constructed at the rear, within the same footprint as the main structure being demolished. This hydrology study will focus on a pre-development vs. a post-development analysis to ensure that the proposed improvements do not adversely impact stormwater quality and quantity beyond the pre- existing condition. Any additional impacts will be quantified and properly mitigated in accordance with the Department of Planning and Natural Resources (DPNR) requirements. SOILS CLASSIFICATION For Curve Number (CN) determination, there are four (4) main types of Hydrologic Soils Classification which are as follows: Group A – Soils that have low runoff potential when thoroughly wet. These soils typically have less than 10% clays and more than 90% sand or gravel. Group B – Soils that have moderately low runoff potential when thoroughly wet. These soils typically have between 10% - 20% clays and 50% - 90% sand. Group C – Soils that have moderately high runoff potential when thoroughly wet. These soils typically have between 20% - 40% clays and less than 50% sand. Group D – Soils that have high runoff potential when thoroughly wet. These soils typically have greater than 40% clays and less than 50% sand. 4 | P a g e St. Croix Hydrologic Soils Classification (Source: https://usvi.mapgeo.io) Based on the above, the Hydrologic Soils Classification for the Site is Group C. CURVE NUMBER (CN) CLASSIFICATION The curve number classification is determined based on land use type and hydrologic soil type. The table below indicates CN values for various land uses and hydrologic soil types. SITE LOCATION 5 | P a g e Based on the above, the site falls into three (3) distinct classifications listed as follows: Poor Condition (grass cover < 50%) – this accounts for the grass cover within the property. Gravel (including right-of-way) – this accounts for the gravel ground cover within the property. Paved; open ditches (including right-of-way) – this accounts for the paved surfaces within the property. Commercial and business – this accounts for the buildings within the property. 6 | P a g e BASIN CHARACTERISTICS The disturbed area of the site is approximately 1.68 acres broken down into four (4) major ground cover types. The table below shows the breakdown along with a Weighted Curve Number for each category and ultimately for the entire site.  PRE-DEVELOPMENT  POST-DEVELOPMENT 7 | P a g e TIME OF CONCENTRATION (Tc) The time of concentration is necessary to estimate peak discharge and is dependent upon the watershed characteristics. To accurately determine Tc, the hydraulics of each part of the flow path must be considered separately.  PRE-DEVELOPMENT In the pre-development, the site has two distinct flow paths which are all overland flow as follows: • A-B (Overland Flow) – 216 ft. travel distance @ average slope of 3.2% • A-C (Overland Flow) – 120 ft. travel distance @ average slope of 4.2% Both flow patterns will be evaluated to determine which one produces the most critical (longest) Time of Concentration (Tc) which will be used to determine the stormwater water runoff volume. 8 | P a g e  POST-DEVELOPMENT In the post-development, the site has two distinct flow paths as follows: • A-B-C (Overland Flow & Pipe Flow) – 150 ft. overland travel distance @ average slope of 1.33% & 229 ft. of 15” ø pipe @ 0.25% slope • A-B-D (Overland Flow & Pipe Flow) – 150 ft. overland travel distance @ average slope of 1.33% & 79 ft. of 15” ø pipe @ 0.00% slope 9 | P a g e All three flow patterns will be evaluated to determine which one produces the most critical (longest) Time of Concentration (Tc) which will be used to determine the stormwater water runoff volume. 10 | P a g e RUNOFF (Q) AND STORAGE VOLUME (V) DPNR Runoff Onsite Storage Requirements as per the Territorial Pollutant Discharge Elimination System - General Permit Number VIGSA0000 is as follows: 1) 3600 cubic feet per acre (1” over 1 acre) or, 2) 2yr – 24 hours storm; which is greater 11 | P a g e  PRE-DEVELOPMENT As can be noted from the above calculations, the 2yr – 24 hr. storm event generates the most runoff in the pre-development/ current site condition. 12 | P a g e  POST-DEVELOPMENT As can be noted from the above calculations, the 2yr – 24 hr. storm event generates the most runoff in the post-development. CONCLUSION Given that the proposed improvements do not significantly increase the building and impervious areas over the existing condition, the additional runoff generated is only a negligible 0.02 ac-ft. However, rather than simply provide the required amount to offset this minimal impact, a decision was taken to further improve the existing condition by providing enough storage to cover at least 1-inch of runoff over the entire disturbed site which equates to 0.14 ac-ft. 13 | P a g e STORAGE POND LAYOUT FOR A TOTAL VOLUME OF 0.24 AC-FT 14 | P a g e APPENDICES PRE-DEVELOPMENT SITE AREA BREAKDOWN 15 | P a g e POST-DEVELOPMENT SITE AREA BREAKDOWN