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1 | P a g e HYDROLOGY STUDY REPORT PROJECT SITE: PATRICK SWEENEY POLICE HEADQUARTERS 1 & 1A GOLDEN GROVE CHRISTIANSTED, ST. CROIX, USVI 00820 PREPARED BY: 7-1 BONNE ESPERANCE P.O. BOX 8269 CHRISTIANSTED, ST. CROIX USVI 00823 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) .................................................................................................................... 6 RUNOFF (Q) AND STORAGE VOLUME (V) ..................................................................................................... 9 CONCLUSION ............................................................................................................................................... 11 APPENDICES ................................................................................................................................................ 14 3 | P a g e INTRODUCTION This site was previously developed and has a considerable amount of development that currently exist. The proposed improvements primarily encompass the replacement of the existing storm damaged multi- plex facility with a consolidated new state-of-the-art one-story administrative complex measuring approximately 22,500 gross square footage. The new facility will essentially fall within the same general footprint as the existing buildings and parking areas to be 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 split between Group B and 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 four (4) 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 parking lots, roofs, driveways, etc. (including right-of-way) – this accounts for the paved surfaces and buildings within the property. 6 | P a g e BASIN CHARACTERISTICS The disturbed area of the site is approximately 5.57 acres broken down into three (3) major ground cover types, broken up into five (5) categories. 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 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. 7 | P a g e  PRE-DEVELOPMENT In the pre-development, the site has one distinct flow path which is overland flow as follows:  A-B (Overland Flow) – 611 ft. travel distance @ average slope of 2.0% The flow pattern will be evaluated to determine the 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 one critical distinct flow path, which include a combination of overland and pipe flow as follows:  A-B (Overland) – 88 ft. travel distance @ average slope of 1.0%  B-C (Pipe Flow) – 203 ft. travel distance @ average slope of 0.5%  C-D (Pipe Flow) – 284 ft. travel distance @ average slope of 2.75% The flow pattern will be evaluated to determine the Time of Concentration (Tc) which will be used to determine the stormwater water runoff volume. 9 | 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 10 | P a g e  PRE-DEVELOPMENT 11 | P a g e  POST-DEVELOPMENT CONCLUSION The proposed improvements result in a small increase in building and impervious areas over the existing condition which generates additional runoff in the amount of 0.04 ac-ft (1.35 ac-ft – 1.31 ac-ft). In this case, 1-inch of runoff over the entire site controls as the minimum required storage to be provided at 0.46 ac-ft. Site layout allowed much larger storage areas to be created and as such it was thought prudent to maximize onsite storage resulting in three retention ponds totaling 1.66 ac-ft. 12 | P a g e 13 | P a g e STORAGE POND LAYOUT FOR A TOTAL VOLUME OF 1.66 AC-FT 14 | P a g e APPENDICES PRE-DEVELOPMENT SITE AREA BREAKDOWN 15 | P a g e POST-DEVELOPMENT SITE AREA BREAKDOWN