HYDROLOGIC-HYDRAULIC STUDY - PR.gov structure. Sizes of the required storage and hydraulic...

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HYDROLOGIC-HYDRAULIC STUDY “ISABELLA OCEAN RESIDENCES”

ISLA VERDE, CAROLINA, PR

1 INTRODUCTION

1.1 Project Description and Location Isabella Ocean Residences is a residential development to be constructed on a 0.925-

acre parcel located on Isla Verde Avenue in the municipality of Carolina. Figure 1 shows

the project location on the San Juan topographic quadrangle. The project will consist of a

multi-story building with common areas, recreational spaces and a multi-level

underground parking lot. The site is presently occupied by a single-family residence

and a parking lot that will be demolished to develop the residential project.

1.2 Scope and Purpose of Study This document constitutes the hydrologic-hydraulic study for the proposed project.

It has been undertaken to determine the required stormwater detention volume for the

proposed development to comply with Puerto Rico Planning Board Regulation #3,

section 15, which requires that peak site runoff under the proposed condition not exceed

the existing condition peak discharge. It also provides the hydraulic dimensions for the

stormwater control structure. This report should be used as the basis for civil design of

the project.

1.3 Report Limitations and Warnings It shall be the responsibility of the site engineer or the project’s geotechnical

consultant to adapt the hydraulic design recommendations to the soil and other

conditions at the site in any matters concerning slope stability, conflicts with other

infrastructure, etc.

1.4 Authorization Mr. Joel Katz, President of IJK Inc., has authorized preparation of this report, in

representation of the owner, through an agreement with Gregory Morris Engineering.

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1.5 Personnel Involved in Project The professionals of Gregory Morris Engineering involved in the realization of this

study were:

Gregory L. Morris P.E.

José D. Miranda P.E.

The following parties were involved in the project:

Designer: LRA Architects

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2 STUDY AREA DESCRIPTION

2.1 Topography and Water Bodies Site elevations range between 2.67 m and 3.24 m (msl). Due to topographic

characteristics runoff generated on the site drains superficially toward Isla Verde

Avenue and enters the storm sewer system though a curb inlet adjacent to the site’s

existing entrance. No offsite runoff enters the property. There are no water bodies or

sinkholes on the site, but the Atlantic Ocean borders the site on the north. Figure 2

shows the topographic features of the site.

2.2 Prior Studies The Isabella Ocean Residences site is classified as Zone -X in the FEMA Flood

Insurance Rate Map (FIRM). A copy of the FEMA Floodway map (Panel 52, revised June

2, 1999) is presented as Figure 3. The project area is not classified as floodable by the

Puerto Rico Planning Board Flood Zoning Map (see Figure 4). The regulatory flood

elevation presented on the maps is 2.40 m-msl due to storm surge. There are no prior

hydrologic-hydraulic studies for the site.

2.3 Field Data This study uses topographic survey data provided by Surveyor Eugenio López

Encarnación. The survey was performed during the month of June 2003 and is

referenced to mean sea level. A copy of the topographic data is included in the back

pocket of this report.

2.4 Field Visit The site was visited during the month of November, 2003. Observed conditions are

summarized below.

• The site consists of two properties. The east property consists of a single residence

with significant green area. The west property is a paved parking lot.

• There is no storm sewer system on the site. The ground slopes to the south and

runoff generated at the site drains superficially toward Isla Verde Avenue. This

runoff reaches the Isla Verde Avenue storm sewer system through a curb inlet

located close to the site’s entrance.

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3 STUDY APPROACH

3.1 Conceptualization of the Hydraulic System The hydraulic system for the existing condition was conceptualized as one on-site

basin (representing the project site) draining to a node (representing the curb inlet at Isla

Verde Avenue) where the peak discharge produced by the basin was determined. For

the post-development condition, land use parameters and drainage patterns were

altered to account for the proposed site plan and change in hydrologic parameters at the

site, plus the proposed detention structures.

3.2 Study Approach and Methodology A detention analysis requires the generation of storm hydrographs showing the

variation of runoff over time. The runoff hydrographs in this analysis were generated

using the Soil Conservation Service unit hydrograph methodology as implemented in

the ICPR model.

Detention volume temporarily stores runoff and releases it more slowly through a

hydraulic structure. Sizes of the required storage and hydraulic configuration of the

outlet structure were determined by hydrologic and hydraulic modeling, comparing

existing and proposed condition runoff hydrographs at the point of analysis.

The study has been performed using the ICPR unsteady flow hydrologic-hydraulic

modeling system (Streamline Technologies v3.0, Winter Park, Florida). The ICPR model

dynamically routes stormwater through open channels, closed conduits and detention

ponds. The program’s solution algorithm allows it to simulate a variety of complex

conveyance systems. Each node in ICPR represents a control volume. Change in

storage for each node is calculated based on the difference between inflows and

outflows at each time step during the simulation period. The change in storage is used

to determine elevations at each node at the end of each time step. Flow through each

link is calculated from the known elevations at each end of the link and the hydraulic

properties of the link.

The hydrologic and hydraulic analysis was performed to determine the discharge

hydrographs for 2-, 10-, and 100- year return intervals storms from the area under

existing and proposed land use conditions.

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4 HYDROLOGIC ANALYSIS

4.1 Curve Number and Time of Concentration Soils in the area were not surveyed in the Soil Survey of San Juan Area of Puerto

Rico (Boccheciamp, 1977) because it is an urban area. Figure 5 shows a partial

reproduction of the soil survey where the project is located. Soil classification was

adopted from the geotechnical exploration performed at the site area by Geo-

Engineering and site inspection. Soils in the project area consist of beach sands and are

in hydrologic class A which indicates they have a low runoff potential. Antecedent

Moisture Condition II was used. Table 1 and Table 3 show the Curve Numbers used in

this analysis for existing and proposed conditions, respectively.

Table 1: Curve Number Calculations for Project Site under Existing Conditions Soil Type Hydrologic

Soil Group Land Use Area (m2) Curve Number

Tropopsamments A Grass, brush 1,315 68

Tropopsamments A Paved Area 2,429 98

Weighted Curve Number 88

Table 2: Curve Number Calculations for Drainage Areas under Proposed Conditions Drainage Area Land Use Area (m2) Curve Number

Roof 1 Paved 654.5 98

Roof 2 Paved 654.5 98

Remainder Site Paved/some grass 2,435 95

The time of concentration is the time required for a drop of water falling on the most

distant point of the watershed to influence discharge at the watershed exit. The time of

concentration was calculated using Soil Conservation method (TR-55). For sheet flow

calculation the following equation was used:

tc = (0.007*n0.8*L0.8) / (P20.5*S0.4)

where:

tc = time of concentration (minutes)

n = Manning’s roughness coefficient

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L = flow length (ft)

P2 = 2-year, 24-hour rainfall (in)

S = slope of hydraulic grade line (land slope, ft/ft)

The time of concentration for the entire project site under existing conditions is 5

minutes. Table 3 shows the results of the time of concentration calculations for the three

different drainage areas used in the proposed condition model.

Table 3: Time of Concentration Calculations for Drainage Areas under Proposed Conditions

Drainage Area Time of Concentration (minutes)

Roof 1 3

Roof 2 3

Site other than roof 5

4.2 Rainfall The study used the 24-hour rainfall depths reported by the US Department of

Commerce (1961) in Technical Paper-42, and the Soil Conservation Service Type-II 24-

hour rain distribution. The rainfall depths at the project site are shown in Table 4.

Table 4: 2-, 10- and 100- year Rainfall Depths (TP-42) at Project Site Return Interval Rainfall Depth

2-years 5 inches

10-years 7 inches

100-years 10 inches

4.3 Results of Hydrologic Analysis The peak discharges in this analysis were computed by the Natural Resources

Conservation Service’s unit hydrograph methodology with a peaking factor of 484.

Table 5 presents peak discharges reaching Isla Verde Avenue for existing and proposed

land use conditions, without detention storage.

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Table 5: Existing and Proposed Condition (without detention storage) Peak Discharges Peak Discharge (ft3/s)

Condition 2-year 10-year 100-year

Existing 4.7 7.0 10.3

Proposed 5.3 7.4 10.7

4.4 Verification of Hydrology Peak discharges for basins with areas less than 150 acres were verified using the

rational method:

Q = C*I*A

where:

Q = peak discharge (ft3/s)

C = runoff coefficient

I = rainfall intensity (in/hr)

A = drainage area (acres)

Parameter values were obtained from “Normas de Diseño para Sistemas de

Alcantarillado Pluvial” (Puerto Rico Planning Board, 1975). Table 6 shows the

parameters used and results obtained with the Rational Method.

Table 6: Parameters used for Hydrology Verification with Rational Method Condition Rainfall Intensity (in/hr) Runoff Coefficient Peak Discharge (ft3/s)

Existing 16 0.70 10.4

Proposed 16 0.75 11.1

Table 7 compares 100-year peak discharges obtained by ICPR and those obtained

with the rational method. Based on verification computations, the peak discharges

produced by ICPR were accepted as reasonable.

Table 7: Verification of Hydrology Results 100-year Peak Discharge (ft3/s)

Condition ICPR Rational Method

Existing 10.3 10.4

Proposed 10.7 11.1

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5 HYDRAULIC ANALYSIS

Three ICPR models were prepared to study the effects of the proposed project, one

for the existing condition and two for the proposed condition. The existing condition

model consists of the onsite basin draining into the Isla Verde Avenue curb inlet. Under

proposed conditions runoff will drain from the roofs into the proposed detention

system, and the rest of the site drains directly into the street.

Two different detention structure configurations were analyzed. Figure 6 shows a

schematic node-link configuration for existing conditions. Figure 7 and Figure 8 show

detention structure Alternative 1 and Alternative 2, respectively.

5.1 Existing Condition Model The existing condition model simulates existing hydrologic conditions on the project

site. The model consists of the project site basin discharging into the Isla Verde Avenue

curb inlet located in front of the property. Appendix A shows input data and modeling

results for the existing condition.

5.2 Proposed Condition Models The purpose of the proposed condition model is to size the detention storage

necessary to insure that the post-development discharge downstream the site does not

exceed the existing condition discharge. Roof runoff is directed into the detention

structure. Figure 9 shows the proposed project layout.

The total roof area was divided into two identical drainage areas, each discharging

into its own identical detention structure. Each detention structure (for both Alternative

1 and Alternative 2) consists of flow diversion box and detention tank. All discharge

generated by the roof should be collected in the flow diversion box. A portion of this

flow will discharge through the floor orifice in the flow diversion box and excess flow

will pass over the weir and into the detention tank. The total discharge exiting the

diversion box and detention tank for each portion of the roof, plus the total discharge

exiting the rest of the site, will not exceed the existing condition discharge. The

detention structures were modeled with the parameters presented in Section 6.1 of this

report. Appendix B and C shows input data and modeling results of the proposed

condition model for Alternative 1 and Alternative 2, respectively.

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Table 8 presents the existing and proposed condition discharges produced by the

project site.

Table 8: Existing and Proposed Conditions Peak Discharge

Peak Discharge (ft3/s) Condition 2-year 10-year 100-year

Existing 4.7 7.0 10.3

Proposed Alternative 1 4.7 6.6 10.0

Proposed Alternative 2 4.5 6.9 10.3

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6 HYDRAULIC DESIGN RECOMMENDATIONS

6.1 Detention Structure Design Parameters The detention system for the proposed project was modeled as a combination of

diversion boxes and detention tanks. The roof will be divided into two halves, each

draining into its own detention structure. Runoff from the roof will enter a flow

diversion box with and orifice in the floor and a rectangular weir at one end. The orifice

passes flows less than the 2-year discharge directly to the street drainage, and portions

of higher flow are passed into the detention structure via a rectangular weir.

The detention tank outlet structure consists of a standpipe and a two orifice to

control the flow rate and comply with the Puerto Rico Planning Board Regulation #3.

Two orifices will be placed at different elevations to mitigate the rainfall events of

different recurrences. The pond’s outlet structure will drain into an 8-inch pipe that will

discharge into the street.

The diversion box and detention tank for Alternative 1 will be placed one next to the

other, and share the same top elevation. The diversion box and detention tank for

Alternative 2 will not be placed one next to the other, and will be connected to each

other through a 10-inch pipe. Figure 10 and Figure 11 show schematic drawings of the

proposed detention systems Alternative 1 and Alternative 2, respectively.

The parameters shown in Table 9 and Table 10 should be used as the basis of design

for the detention tanks and diversion boxes for Alternative 1, respectively. The

parameters shown in Table 11 and Table 12 should be used as the basis of design for the

detention tanks and diversion boxes for Alternative 2, respectively.

The entire detention system for the two alternatives consists of two identical

detention tanks and diversion boxes, one for each half of the roof area.

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Table 9: Alternative 1 Proposed Detention Tank Parameters Parameter Value

DETENTION TANK

Depth 2 meters

Top/Bottom Area 2 meters2

Bottom Elevation ≥ 2.5 m-msl

Maximum 100-year Water Depth 1.9 m

DETENTION TANK OUTLET STRUCTURE

2-YEAR ORIFICE

Diameter 3 inches

Invert Elevation above bottom 0 meter

10-YEAR ORIFICE

Diameter 3 inches

Invert Elevation above bottom 1 meter

100-YEAR STANDPIPE

Diameter 6 inches

Invert Elevation above bottom 1.8 meters

OUTLET PIPE

Diameter 8 inches

Invert Elevation above bottom 0 meters

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Table 10: Alternative 1 Proposed Diversion Box Parameters Parameter Value

DIVERSION BOX

Depth 1 meters


Bottom Elevation above Detention Tank 1 m


RECTANGULAR WEIR TO TANK

Span 0.5 meters


OUTLET TO STREET

ORIFICE

Diameter 3 inches


OUTLET PIPE

Diameter 6 inches

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Table 11: Alternative 2 Proposed Detention Tank Parameters Parameter Value

DETENTION TANK

Depth 2 meters


Bottom Elevation ≥ 2.5 m-msl


DETENTION TANK OUTLET STRUCTURE

2-YEAR ORIFICE

Diameter 2.5 inches


10-YEAR ORIFICE

Diameter 4 inches


100-YEAR STANDPIPE

Diameter 6 inches


OUTLET PIPE

Diameter 8 inches


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Table 12: Alternative 2 Proposed Diversion Box Parameters Parameter Value

DIVERSION BOX

Depth 1 meters


Bottom Elevation As desired by architect


RECTANGULAR WEIR TO TANK

Span 0.5 meters


PIPE BETWEEN BOX AND TANK

Diameter 10 inches

Upstream Invert Elevation above box bottom 0 meters

Downstream Invert Elevation above tank bottom 1.8 meters

OUTLET TO STREET

ORIFICE

Diameter 3 inches


OUTLET PIPE

Diameter 6 inches

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7 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

1. The detention systems for Alternative 1 and Alternative 2 have been sized for this

project so that the existing condition peak discharges for the 2-, 10- and 100-year

events are not exceeded under proposed conditions.

2. It is the responsibility of the site engineer to direct all of the stormwater from the

roof area into the proposed system.

3. The detention outlet structures must be inspected periodically to avoid obstruction

with debris. The boxes and tanks should also be maintained periodically to remove

accumulated sediment.

4. Hydraulic design parameters and recommendations are provided in Section 6 of this

report.

I hereby certify that the document “Hydrologic-Hydraulic Study, Isabella Ocean

Residences, Isla Verde, Carolina, Puerto Rico” has been prepared in accordance with the

best hydrologic and hydraulic practices as described in this document and that, based on

the studies and field measurements provided by other parties, results are true and

correct.

Certified today October 21, 2004

Gregory L. Morris,P.E.,Ph.D.

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8 REFERENCES

Boccheciamp, Rafael A. 1977. "Soil Survey of San Juan Area of Puerto Rico." U.S. Soil

Conservation Service. San Juan PR.

Maidment David R. 1993. “Handbook of Hydrology”. McGraw-Hill, Inc. New York

Puerto Rico Planning Board, 1975. “Normas de Diseño para Sistemas de Alcantarillado

Pluvial”. San Juan PR.

Singhofen P.J., and L.M. Eaglin. 1995. “ICPR Advanced: User’s Manual”. Streamline

Technologies Inc., Winter Park, FL

US Department of Agriculture, Soil Conservation Service. 1986. “Technical Release 55

(Urban Hydrology for Small Watersheds)”. Washington DC.

US Department of Commerce. 1961. “Technical Paper #42”. Washington DC

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TABLE OF CONTENTS

1 INTRODUCTION ...................................................................................................... 1

1.1 Project Description and Location .................................................................................. 1 1.2 Scope and Purpose of Study .......................................................................................... 1 1.3 Report Limitations and Warnings................................................................................. 1 1.4 Authorization................................................................................................................... 1 1.5 Personnel Involved in Project ........................................................................................ 2

2 STUDY AREA DESCRIPTION ................................................................................ 3

2.1 Topography and Water Bodies...................................................................................... 3 2.2 Prior Studies ..................................................................................................................... 3 2.3 Field Data.......................................................................................................................... 3 2.4 Field Visit.......................................................................................................................... 3

3 STUDY APPROACH................................................................................................. 4

3.1 Conceptualization of the Hydraulic System................................................................ 4 3.2 Study Approach and Methodology .............................................................................. 4

4 HYDROLOGIC ANALYSIS ..................................................................................... 5

4.1 Curve Number and Time of Concentration................................................................. 5 4.2 Rainfall .............................................................................................................................. 6 4.3 Results of Hydrologic Analysis ..................................................................................... 6 4.4 Verification of Hydrology .............................................................................................. 7

5 HYDRAULIC ANALYSIS ........................................................................................ 8

5.1 Existing Condition Model .............................................................................................. 8 5.2 Proposed Condition Models .......................................................................................... 8

6 HYDRAULIC DESIGN RECOMMENDATIONS.................................................. 10

6.1 Detention Structure Design Parameters..................................................................... 10

7 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS............................. 15

8 REFERENCES ......................................................................................................... 16

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List of Figures and Appendix

Figure 1: Location Map

Figure 2: Topographic Features of Site

Figure 3: FEMA FIRM Map Panel 224B, dated July 19, 1982

Figure 4: PR Planning Board Flood Zone Map Sheet 39D, dated February 11, 1988

Figure 5: Soil Map of Puerto Rico Area of Southern Puerto Rico, Sheet 23

Figure 6: Existing Condition Node/Link Diagram

Figure 7: Alternative 1: Proposed Condition Node/Link Diagram

Figure 8: Alternative 2: Proposed Condition Node/Link Diagram

Figure 9: Proposed Project Layout

Figure 10: Schematic Drawing of Alternative 1 Proposed Detention Pond

Figure 11: Schematic Drawing of Alternative 2 Proposed Detention Pond

Appendix A: Input and Results of ICPR Model for Existing Condition

Appendix B: Input and Results of ICPR Model for Proposed Condition Alternative 1

Appendix C: Input and Results of ICPR Model for Proposed Condition Alternative 2

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List of Tables

Table 1: Curve Number Calculations for Project Site under Existing Conditions

Table 2: Curve Number Calculations for Drainage Areas under Proposed Conditions

Table 3: Time of Concentration Calculations for Drainage Areas under Proposed Conditions

Table 4: 2-, 10- and 100- year Rainfall Depths (TP-42) at Project Site

Table 5: Existing and Proposed Condition (without detention storage) Peak Discharges

Table 6: Parameters used for Hydrology Verification with Rational Method

Table 7: Verification of Hydrology Results

Table 8: Existing and Proposed Conditions Peak Discharge

Table 9: Alternative 1 Proposed Detention Tank Parameters

Table 10: Alternative 1 Proposed Diversion Box Parameters

Table 11: Alternative 2 Proposed Detention Tank Parameters

Table 12: Alternative 2 Proposed Diversion Box Parameters

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FIGURES

Figure 1: Isabella Ocean Residences Site Location in USGS Topographic Quadrangle of San Juan.

N

SITE

Figure 3: FEMA Floodway Map, Panel 720000 52 (June 2, 1999)

N

SITE

Figure 4: Puerto Rico Planning Board Floodway Map(9-B: June 1, 1999)

N

SITE

N

SITE

Figure 4: Isabella Ocean Residences Watershed Limits.

Figure 5: Soil Survey of San Juan Area of Eastern Puerto Rico.US Department of Agriculture, Soil Conservation Service

Rafael A. Boccheciamp, 1977.

N

SITE

NOT SURVEYEDNOT SURVEYED

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Appendix A

Input and Results of ICPR Model for

Existing Condition

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Appendix B


Proposed Conditions: Alternative 1

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Appendix C


Proposed Condition: Alternative 2

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