HYDROLOGIC-HYDRAULIC STUDY BAYAMON SOUTH COMMERCIAL CENTER BAYAMON, PUERTO RICO. Casiano Ancalle, P.E. August, 2006 I.

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2 HYDROLOGIC-HYDRAULIC STUDY BAYAMON SOUTH COMMERCIAL CENTER BAYAMON, PUERTO RICO Casiano Ancalle, P.E. August, 2006 I. INTRODUCTION A commercial development project named Bayamon South Commercial Center is planned to be constructed in a 27-acre property located west of state road PR-167, at Buena Vista Ward, in the Municipality of Bayamón. Quebrada Cancel crosses the proposed development site from east to west. This storm watercourse will not be affected by the project. According to the regulatory flood maps, the project site is not considered floodable for the 100-year rainfall event. The development of the site is going to increase runoff. This increase has to be mitigated according to the stipulations of the Puerto Rico Planning Board Regulation No. 3. Purpose of Study The purpose of the study is to determine the 100-year water surface elevation at the watercourse, and to mitigate the increment in runoff discharge due to the development in accordance to the Puerto Rico Planning Board Regulation No. 3. Approach The following steps have been undertaken throughout the study: 1

3 Hydrologic Analysis: The following parameters were determined for the hydrologic analysis: drainage areas, average soil curve number and runoff lag time. Based on these parameters, discharges for 100, 50, 25, 10 and 2-yr frequencies storm were determined for existing and proposed conditions. HEC-1 model was used. Runoff Discharge Mitigation Analysis: A mitigation analysis was made in order to counteract the effect of the proposed development peak discharge onto the downstream areas. HEC-1 model was used for the mitigation analysis. Discharges for 2, 10, 25, 50 and 100-year frequencies were analyzed for runoff mitigation. Hydraulic Analysis: Hydraulic computations were made to determine the 100-yrs. water surface elevations at Quebrada Cancel. COE s HECRAS model as used. Conclusions and recommendations were elaborated. Authorization RAY Engineers P.S.C. on behalf of the owner authorized this study, under a contract signed with Eng. Casiano Ancalle, principal of CA Engineering. 2

4 II. PROJECT BACKGROUND Location The project site is located at state road PR-167, Buena Vista Ward, in the Municipality of Bayamon. The project site is bordered by the PRHTA Parcel A on the north, Las Quintas Development on the south, Francisco Gastambide Vega School and state road PR-167 Road on the east, and Succession Ramos Vásquez on the west. Figure 1 shows the approximate location of the site in the USGS quadrangle. Figure 2 shows a layout of the proposed project. Topography Topography of the site shows slopes towards Quebrada Cancel. Ground elevation of the site varies between 125 and 105 m. referred to a local vertical control. Figure 3 shows the topography of the site. Water Bodies Quebrada Cancel is the only water body related to the site. This creek crosses the site from east to west. The project will not intend to affect neither the banks nor the bed of this watercourse. The project will be constructed outside of the 100-year flood footprint of the creek. Flooding From the regulatory point of view, the project site is not classified as floodable for a 100-year rainfall event. Figure 7 shows a portion of the FEMA flood insurance rate map Panel 705, revised on April

5 Field Work Field data used in this study was taken by the drawings provided by RAY Engineers. This information was used for the hydraulic modeling. Results obtained in this study are strictly based on this information. Fieldwork is attached in a pocket at the end of this study, as Appendix G. Former Studies No previous studies were found for this site. This study is based on the information provided by the engineer and gathered through site inspection and interviews of neighboring residents. Study Level This study is intended as an aid to the design engineer in the preparation of the construction drawings for the recommended structures. Figures, schematics and drawings must not be used as construction drawings. The design engineer must elaborate the construction drawings in agreement with the recommendations of this study. 4

6 III. HYDROLOGIC ANALYSIS Methodology The computer program entitled Flood Hydrograph Package (HEC-1) developed by the U.S. Army Corps of Engineers [1990] was used for the hydrologic analysis. Using this program, the Unit Hydrograph method and the Runoff Curve Number (CN) method, both developed by the Soil Conservation Service (SCS), were applied to determine the design hydrograph. This was computed by a process of translating the rainfall excess into a runoff hydrograph known as convolution. Peak discharges ranging in frequencies from 2, 10, 25, 50 and 100-year were estimated for existing and proposed condition. Drainage Areas From the topography, the project site drains to the watercourse. The project area shows to have four independent drainage areas at existing condition: Drainage area P1 with 4.50 acres, P2 with acres, P3 with 5.34 acres and P4 with Area P1 drain to two storm watercourses located to the northwest of the site. Areas P2, P3 and P4 drain to Quebrada Cancel. The four drainage areas have been essentially kept at proposed condition but area P2 has been subdivided in two areas; Drainage area P1 has 3.90 acres, P2A has 5.98 acres, P2B 7.50 acres, P acres and P4 has 7.15 acres. Seven offsite drainage areas contribute to Quebrada Cancel: E1 with 3.25 acres, E2 with acres, E3 with acres, E4 with acre, E5 with acres, E6 with acres, and E7 with acres. Offsite areas E4, E5, E6 and E7 were subdivided for the hydrologic computations. Figure 4 shows the drainage areas at existing condition and Figure 5 at proposed condition. 5

7 Curve Numbers Curve numbers were computed using the NRCS methodology. Soil types and land uses were gotten from SCS [1978] soil maps. According to the Soil Survey of San Juan Area of Puerto Rico, most of the soil of the project site was classified as Mucara clay (MxE) 20 to 40% slopes, and Mucara clay (MxD) 12 to 20% slopes. Soils at the offsite areas are Mucara clay (MxE) 20 to 40% slopes, and Mucara clay (MxD) 12 to 20% slopes, Naranjito silty clay (NaE2). Daguey clay (DaD) and Rio Arriba clay (RoC2). Weighted curve numbers for these soils were estimated. Curve Number of 93 was adopted for the developed area in consideration of the type of proposed land use. Appendix A shows the weighted Curve Number (CN) estimations and other physiographic characteristics of the drainage areas. Figure 6 shows the portion of the soil map corresponding to the area of study. Lag Time The lag time was estimated as sixty percent of the time of concentration as estimated by the formula of Kirpitch. Tc = L 0.77 /S where: L = channel length (ft) S = channel slope Tc = Time of concentration in min Detailed Lag Time calculations are shown in Appendix A. Rainfall Data The variation of rainfall volume with time was required as part of the storm input for the SCS Curve Number method. Therefore, the development of a design storm with a rainfall frequency 6

8 and duration was necessary to compute the design hydrograph for the watershed. Rainfall data used in this study was obtained from the Technical Paper No. 42 (TP-42) [National Weather Service, 1961]. The rainfall depths for 100, 50, 25, 10 and 2-years frequency for several durations was used and are shown in Table 1. Rainfall depths for 5 and 15 minutes duration were extrapolated by a regression analysis of the IDF curves. Table 1 Rainfall for 2, 10, 25, 50 y 100 years Precipitation Duration Inches Hrs. 2-yr 10-yr 25-yr 50-yr 100-yr Depth-Area Adjustment Point rainfall estimates obtained from the TP-42 represent values for areas up to 10 mi 2 ; therefore, a depth-area adjustment should be applied to the rainfall data when the watershed area is greater. In this case, the largest watershed is approximate 0.07 mi 2. Hence, this adjustment was not applied. 7

9 Time Distribution of Rainfall The triangular type methodology was used to distribute the rainfall depth in time. This method is considered acceptable for small areas. Rainfall Extraction Rainfall extraction such as the vegetative interception, the depressional storage, and the infiltration were estimated using the SCS's Runoff Curve Number method. Though this method is used to predict runoff volume directly, the rainfall extraction is incorporated in the model as function of the curve number of the watershed. Hydrologic Results Following HEC-1 methodology, hydrographs were determined for existing and proposed condition. Input and output data for the HEC-1 model are included in Appendix B for existing condition and Appendix C for proposed condition. Table 2 shows the results of the hydrologic analysis. 8

10 Table 2 Peak Discharges for and 100-yr Frequencies Peak Flow (cfs) CONDITION BASIN 2-yr 10-yr 25-yr EXISTING PROPOSED 50- yr 100-yr E E E7+E6+E E E7+E6+E5+E E E7+E6+E5+E4+E P E P E1+P E7+E6+E5+E4+E2+E1+P E P E7+E6+E5+E4+E2+E1+P4+P2+E3+P P E E E7+E6+E E E7+E6+E5+E E E7+E6+E5+E4+E P P2A P4+P2A E P2B E1+P2B E7+E6+E5+E4+E2+E1+P4+P E P E7+E6+E5+E4+E2+E1+P4+P2+E3+P P

11 From the inspection of Table 2, the peak discharge of the project site at proposed condition is higher than that of the existing condition at drainage areas P1, P2, P3 and P5; also the accumulated discharge at Quebrada Cancel. The increment in discharge has to be mitigated as required by Section of the Puerto Rico Planning Board Regulation No

12 IV. RUNOFF MITIGATION ANALYSIS The development of the site will increase the runoff discharge. The Puerto Rico Planning Board Regulation No. 3 requires a flow mitigation structure wherever an increase in discharge is produced. Therefore, a flow detention structures will be included in the project. One (1) detention pond was used: The pond will be located at drainage area P3. Figure 8 shows the location of this pond. The design engineer must provide the means to assure that the entire runoff of these areas discharges into the corresponding detention structure. The computer program HEC-1 provides the means for routing hydrographs through detention structures. Depth-Volume Relations The base area of the pond Pond will be 1,450 square meters and the side slopes 1V:2H. The Depth-Volume calculations for the pond were made under the assumption of a square base. Appendix D includes a spreadsheet with the depth-volume relation computations and the curve for the pond. 11

13 Flow Depth Relations The control structure for the pond will consist on one (1) 18 diameter orifice at the pond bottom, and a 20-feet wide rectangular weir located 2.3 meter above the bottom of the pond. Flow Depth relations for these structures were estimated taking into consideration discharges through orifices. Flow through the orifices was computed using Torrecelli s formula. Appendix D includes a spreadsheet with the depth-flow relation computations and its respective curve. Results Input and output data for the HEC-1 mitigation model are included in Appendix E. Table 3 shows the comparison of the discharges for existing, and proposed condition. 12

14 Table 3 Mitigation Analysis Results Comparison Peak Flow (cfs) CONDITION BASIN 2-yr 10-yr 25-yr EXISTING PROPOSED PROPOSED W/MITIGATION 50- yr 100-yr E E E7+E6+E E E7+E6+E5+E E E7+E6+E5+E4+E P E P E1+P E7+E6+E5+E4+E2+E1+P E P E7+E6+E5+E4+E2+E1+P4+P2+E3+P P E E E7+E6+E E E7+E6+E5+E E E7+E6+E5+E4+E P P2A P4+P2A E P2B E1+P2B E7+E6+E5+E4+E2+E1+P4+P E P E7+E6+E5+E4+E2+E1+P4+P2+E3+P P E E

15 E7+E6+E E E7+E6+E5+E E E7+E6+E5+E4+E P P2A P4+P2A E P2B E1+P2B POND E7+E6+E5+E4+E2+E1+P4+P E P E7+E6+E5+E4+E2+E1+P4+P2+E3+P P The results of the detention analysis show that the proposed detention pond provides appropriate runoff mitigation for the 100-year frequency discharge. Mitigation for 2-, 10-, 25- and 50-yr frequency peak discharges was also verified. 14

16 Runoff Mitigation Structure Dimensions and Accessories The maximum water stage at the tank for the 100-yr peak will be 2.36 m (7.76 ft). Final dimensions for the pond will include a minimum free board of 0.60 meters. Then, the detention pond will have the characteristics shown in Table 4. Table 4 Detention Tank Characteristics Dimensions Outlet Control Description Pond P3 Base Area, m2 1,450 Side Slope 1: 2 bottom, inches one-18" Weir 2.3 m above bottom 6.09 m Discharge Pipe One-36 ø For the mitigation analysis, the bottom geometry of the pond has been considered square but another shape can be used as well if the magnitude of the area is maintained. The pond has been located discharging above the 10-years water surface elevation at Cancel Creek. Figure 9 shows the schematics of the detention pond and the outlet accessories. 15

17 V. HYDRAULIC ANALYSIS A hydraulic analysis of Quebrada Cancel crossing the site has been made in order to find the water surface elevations and the extent of the 100-year footprint. This creek will not be affected by the project. The proposed pipe crossing will be made above the existing box culvert, therefore, way above the 100-year water surface elevation. The analysis was made by using the mathematical model HEC-RAS developed by the US Corps of Engineers. The friction coefficient used in the modeling was obtained from visual inspection of the existing structures; and cross-checked with the typical values provided by Barnes (1967) and Chow (1959). Manning s coefficient used for modeling the drainage system was for the natural watercourse and for the concrete structures. Contraction and Expansion Coefficients Coefficients of contraction and expansion used are those recommended by the HEC-RAS user s manual. Thus, coefficients of 0.1 and 0.3 respectively were used for gradual transitions. Hydraulic Run The geometry for the existing condition includes the natural watercourse across the site. The hydraulic analysis was made mainly for a 100-year discharge; but discharges for more recurrent storm events were also analyzed. Location of the cross sections used shown in Figure 10. Appendix F includes the analysis computer output. Table No. 5 shows the summary of the results. 16

18 Table 5: Results of the Hydraulic Run Reach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude #Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs yrs

19 Flood Footprint With the results of the hydraulic analysis, a flood footprint has been drawn and is shown in Figure 11. The project will not affect the course of the creek. The toe of the fill, if any, will be located outside of the flood footprint. 18

20 VI. CONCLUSIONS AND RECOMMENDATIONS The following are the conclusions of this study: 1. According to the regulatory flood maps, the project site is not considered floodable for the 100-year rainfall event. 2. Proposed condition discharge is higher than that of the existing condition at areas P2 and P4. Runoff discharge mitigation is needed for each and all these areas. Different from them, discharge from area P1 does not increase; it can be discharge without a mitigation structure. 3. The proposed runoff mitigation pond reduces the local peak discharge to levels below the ones of the existing condition, as well as the overall discharge of Quebrada Cancel onto the downstream. The following are the recommendations of this study: 1. The project shall be constructed outside of the 100-year flood footprint. 2. Mitigation structure will have the dimensions and accessories indicated in Table 4 of this report. 3. The pipes must be installed to the elevations given in this study; any change found necessary must be subject to hydraulic verification. 4. It is very important to prepare a long-term maintenance plan, which should include the proposed pipes, the pond outlet structure and the receiving storm system inspection after each significant discharge events. Damages, if any, must be repaired promptly and properly. 19

21 Study Limits All the recommendations specified in this study must be considered to assure the optimum performance of the proposed discharge mitigation tank and receiving stream. The design engineer will be responsible for elaborating the drawings in conformance with the recommendations of this study. The results of this study are based on free flow conditions through the hydraulic structures. Proper maintenance must be developed to assure this condition. On the event of the occurrence of any severe obstruction to the flow, the results and recommendations may be impaired. Finally, results and recommendations included in this report must be used only and exclusively by the design engineer for the intended purposes as indicated in this study. 20

22 VII. BIBLIOGRAPHY National Weather Service, Generalized Estimates of Probable Maximum Precipitation and Rainfall Frequency Data for Puerto Rico and Virgin -Islands. Technical Paper No. 42. U.S. Department of Commerce. Washington, D.C.. P.R. Planning Board, Reglamento sobre Zonas Susceptibles a Inundaciones. Reglamento de Planificación Número 13. Estado Libre Asociado de Puerto Rico, Oficina del Gobernador. Soil Conservation Service, Soil Survey of San Juan Area of Puerto Rico. U.S. Department of Agriculture. U.S. Army Corps of Engineers, Flood Hydrograph Package (HEC-1), User's Manual. Hydrologic Engineering Center. Davis, California. 21

23 TABLE OF CONTENTS I. INTRODUCTION... 1 Purpose of Study... 1 Approach... 1 Authorization... 2 II. PROJECT BACKGROUND... 3 Location... 3 Topography... 3 Flooding... 3 Field Work... 4 Former Studies... 4 Study Level... 4 III. HYDROLOGIC ANALYSIS... 5 Methodology... 5 Drainage Areas... 5 Curve Numbers... 6 Lag Time... 6 Rainfall Data... 6 Depth-Area Adjustment... 7 Time Distribution of Rainfall... 8 Rainfall Extraction... 8 Hydrologic Results... 8 IV. RUNOFF MITIGATION ANALYSIS Depth-Volume Relations Flow Depth Relations Results Runoff Mitigation Structure Dimensions and Accessories i

24 V. HYDRAULIC ANALYSIS Contraction and Expansion Coefficients Hydraulic Run Flood Footprint VI. CONCLUSIONS AND RECOMMENDATIONS Study Limits VII. BIBLIOGRAPHY...21 ii

25 LIST OF FIGURES FIGURE 1. FIGURE 2. FIGURE 3. FIGURE 4. FIGURE 5. FIGURE 6. FIGURE 7. FIGURE 8. FIGURE 9. Location Map Proposed Development Layout Topography for Existing Condition Drainage Area at Existing Condition Drainage Area at Proposed Condition Soils Map FEMA Flood Insurance Rate Map Proposed Structures Layout Detention Pond Schematics FIGURE 10. Typical Detention Tank Schematics FIGURE 11. Location of the Cross Sections FIGURE years Flood Footprint iii

26 LIST OF APPENDIXES APPENDIX A. APPENDIX B. APPENDIX C. APPENDIX D. APPENDIX E. APPENDIX F. APPENDIX G. Hydrologic Parameters Estimation HEC-1 Results for Existing Condition HEC-1 Results or Proposed Condition Detention Structure Volume and Discharge to Depth Relations HEC-1 Results for Proposed Condition with Mitigation HEC-RAS Results for Existing Condition Field Work iv

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