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HYDROLOGY STUDY REPORT 
PROJECT SITE: 
V.I. FIRE & EMERGENCY MEDICAL SERVICES (VIFEMS) 
GEORGE SCOTT FIRE STATION 
PLOT 390 ESTATE ANNA’S RETREAT, NEW QUARTER 
CHARLOTTE AMALIE, ST. THOMAS, USVI 00802 
 
 
 
 
PREPARED BY: 
 
 
7-1 BONNE ESPERANCE 
P.O. BOX 8269 
CHRISTIANSTED, ST. CROIX USVI 00823 
 
 
 
SITE LOCATION
TUTU PARK MALL 
FOUR WINDS PLAZA 
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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 ............................................................................................................................................... 13 
APPENDICES ................................................................................................................................................ 14 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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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 4,700 square feet 
antiquated fire station building with a new expanded state-of-the-art facility measuring approximately 
8,240 gross square footage.  The new facility will be constructed within the exact same footprint of the 
existing facility.  
The disturbed area is only 0.59 acres which is less than the 1-acre mandated threshold for a required 
hydrology study.  Nonetheless, a hydrology study has been undertaken to ascertain the impacts of the 
proposed improvements.  This hydrology study will focus on a pre-development vs. a post-development 
analysis. 
 
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. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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St. Croix Hydrologic Soils Classification (Source: https://usvi.mapgeo.io) 
 
Based on the above, the Hydrologic Soils Classification for the Site is Group D.  This soil classification 
type is synonymous with Group D in Table 3.6 below.     
 
 
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
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Based on the above, the site falls into two (2) distinct classifications listed as follows: 
Poor condition (grass cover < 50%) – this accounts for the grass cover within the property. 
Paved parking lots, roofs and driveways, etc. (excluding right-of-way) – this accounts for the paved 
surfaces and buildings within the property. 
 
 
 
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BASIN CHARACTERISTICS 
 
The disturbed area of the site is approximately 0.59 acres broken down into two (2) 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 
 
 
 
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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 one distinct flow path, which is overland flow as follows: 
 
A-B (Overland Flow) – 218 ft. travel distance @ average slope of 7.8% 
 
 
 
 
 
The flow pattern will be evaluated to determine the Time of Concentration (Tc) which will be used to 
determine the stormwater water runoff volume.   
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 POST-DEVELOPMENT 
In the post-development, the site has one distinct flow path, which is a combination of overland and 
pipe flow as follows: 
 
A-B (Overland Flow) – 30 ft. travel distance @ average slope of 2.0% 
 
B-C (Pipe Flow) – 98 ft. travel distance @ average slope of 0.5% 
 
C-D (Pipe Flow) – 52 ft. travel distance @ average slope of 0.5% 
 
D-E (Pipe Flow) – 40 ft. travel distance @ average slope of 0.5% 
 
 
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The flow pattern will be evaluated to determine the Time of Concentration (Tc) which will be used to 
determine the stormwater water runoff volume.   
 
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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; whichever is greater 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 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. 
 
 
 
 
 
 
 
 
 
 
 
 
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  POST-DEVELOPMENT 
               
 
As can be noted from the above calculations, the 2yr – 24 hr. storm event generates the most runoff in 
the post-development. 
 
 
 
 
 
 
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CONCLUSION 
 
The proposed improvements do not result in any adverse impacts over the existing condition which is 
evident in the pre vs. post runoff coefficient, having a negligible increase from 95 to 96.  Pre vs. Post runoff 
equates to zero net increase (0.06 ac-ft – 0.06 ac-ft).  
1-inch of runoff over the entire disturbed site equates to 0.05 ac-ft.  Even though the site is less than 1-
acre and per V.I. Code, the provision of onsite storage is not a requirement, the site layout does facilitate 
the installation of underground storage chambers to accommodate the 1-inch runoff volume. 
 
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APPENDICES 
 
PRE-DEVELOPMENT SITE AREA BREAKDOWN 
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POST-DEVELOPMENT SITE AREA BREAKDOWN