The URL can be used to link to this page

Home My WebLink AboutPD 2019-0007; MADISON STREET APARTMENTS; DRAINAGE STUDY; 2019-06-20TABLE OF CONTENTS I. DISCUSSION Vicinity Map ........................................................ 3 Purpose and Scope ................................................ 4 Project Description ............................................... 4 Study Method ...................................................... 5 Conclusions ........................................................ 6 Declaration of Responsible Charge ........................... 7 II. EXHIBITS Existing Hydrology Map & Developed Hydrology Map ... 8 III. HYDROLOGY & HYDRAULIC CALCULATIONS IV. REFERENCES 3130 Madison Street Drainage Study Existing Hydrology Calculations -100 Year. .................. 9 Developed Hydrology Calculations -100 Year ............... 9 Precipitation Frequency Estimates ........................... 13 Web Soils Survey San Diego County Hydrology Manual bliA, Inc. land planning, civil engineering, surveyin~ ,, L PURPOSE AND SCOPE: The purpose of this report is to publish the results of a hydrology and hydraulic analysis for the proposed apartment project at 3130 Madison Street, City of Carlsbad. The proposed project is a 0.17-acre site. The scope is to study the existing and developed hydrology and hydraulics as it influences the surrounding properties during a 100-year frequency storm event, and make recommendations to intercept, contain and convey QlOO to the historic point of discharge. PROJECT DESCRIPTION: The project is located at 3130 Madison Street, between Oak Avenue and Pine Avenue. The proposed project is a 0.166 acre site. The project proposes the development of four Apartments, covered parking, and two additional stories. The disturbed area is 0.166 acres; the existing site is 20% impervious pre-development (1,470 square feet) and 65 % impervious post development (4,707 square feet). The developed drainage basin matches the existing drainage basin in terms of overall area and basin limits. Storm flows affecting the site are limited to the rainfall that lands directly on this property. The following table summarizes the existing condition runoff information from the site. Please refer to the Existing Condition Hydrology Map for existing drainage area. TABLE 1-Summary of Existing Condition Peak Flows AREA (ac) 0.166 PEAK FLOW (cfs) 0.44 The developed drainage pattern will be similar as the ex1stmg drainage pattern with some modifications to incorporate the Best Management Practices (BMPs) into the project design to mimic the impacts on storm water runoff and quality. The developed runoff from the project will gravity flow toward Madison Street. Prior to discharging from the project site, low impact development strategies (site design bmps) will be incorporated into the site design. Developed site runoff from impervious areas such as rooftops and walkways will be directed onto the impervious dispersion areas. The intent is to slow runoff discharges and reduce volumes while achieving incidental treatment. Runoff will then sudace flow to Madison Street and eventually to an existing curb inlet in Oak Avenue near Tyler Street. Impervious sudaces have been minimized where feasible. Developed drainage patterns will not alter the existing flow pattern and will discharge from the site to the historic discharge location. 3130 Madison Street Drainage Study b~A, Inc. land planning, civil engineering, surveyin3 4 L Table 2 summarizes the expected cumulative 100-year peak flow rate from the developed site. Refer to the Developed Condition Hydrology Map for drainage patterns and areas. TABLE 2-Summary of Developed Condition Peak Flows AREA (ac) 0.166 STUDY METHOD: PEAK FLOW (cfs) 0.77 The method of analysis was based on the Rational Method according to the San Diego County Hydrology Manual. Drainage basin areas were determined from the developed grades and elevations shown on the grading plan. Initial time of concentration of 5 minutes is used for P6 for 100 year storm, see References. Rainfall Intensity= I = 7.44*(P6)*(Tc) ~-0·645 Pdor 100 year storm= 2.70" In accordance with the County of San Diego standards, runoff coefficients were based on land use and soil type. An appropriate runoff coefficient (B) for each type of land use in the subarea was selected from Table 3-1 of San Diego Hydrology Manual multiplied by the percentage of total area (A) included in that class. The sum of the products for all land uses is the weighted runoff coefficient (2, [CA]). For the existing condition and developed conditions, a runoff coefficient of 0.25 was selected for all landscaped and pervious areas assuming 0% impervious. In the developed conditions, the concrete and roof areas were considered 95% impervious and assigned a runoff coefficient of 0.87. See the comparison of existing condition hydrology and developed condition hydrology below. TABLE 3-Summary of Existing and Developed Condition Peak Flows AREA (ac) 0.166 3130 Madison Street Drainage Study EXISTING PEAK FLOW (cfs) 0.44 DEVELOPED PEAK FLOW DIFFERENCE (cfs) (cfs} 0.77 0.33 bl-IA, Inc. land planning , civil engineering , surveyinn.----~ 5 CONCLUSIONS: As shown in Table 3, the development of the proposed 3130 Madison Street project site wiJJ result in a net increase of peak flow discharged from the project site by approximately 0.32 cfs. This increase is considered negligible to the overall drainage discharged into the public storm drain downstream where an existing curb inlet located along Oak Avenue between Roosevelt Street and Madison Street intercepts runoff. Existing City of Carlsbad Improvement Drawing 365-2 (See References) shows that the 50 year storm event for the storm drain is contained in the existing storm drain system. The approximate increase is approximately 1 % of the total runoff shown at the existing curb inlet along Oak Avenue. Landscape, permeable pavers and impervious dispersion areas will slow runoff discharges, and reduce runoff. These small collection techniques foster opportunities to maintain the existing hydrology and provide a much greater range of retention practices. The developed site wiJJ also implement source control and site design BMPs in accordance with the "Standard Project" stormwater requirements. Peak flow rates listed above were generated based on criteria set forth in "San Diego County Hydrology Manual" (methodology presented in Chapter 4 of this report). Rational method output is located in Chapter 3. The hydraulic calculations show that the proposed onsite storm drain facilities can sufficiently convey the anticipated 0 100 flowrate without any adverse effects. Based on this conclusion, runoff released from the proposed project site will be unlikely to cause any adverse impact to downstream water bodies or existing habitat integrity. Sediment will likely be reduced upon site development. 3130 Madison Street Drainage Study bl-IA, Inc. land planning, civil engineering, surveying,---6---. 3130 Madison Street Drainage Study II. EXHIBITS EXISTING HYDROLOGY MAP & DEVELOPED HYDROLOGY MAP bl-tA, Inc. land planning , civil engineering, surveyinr,,--8--, III. HYDRAULIC CALCULATIONS 3130 Madison Street bl-IA, Inc. Drainage Study land planning, civil engineering , surveyin~ 9 L SUMMARY OF DEVELOPED PEAKS FLOWS AREA (ac) PEAK FLOW (cfs) 0.166 0.77 COMPARISON OF PEAKS FLOWS AREA (ac) 0.166 3130 Madison Street Drainage Study EXISTING PEAK FLOW (cfs) 0.44 DEVELOPED PEAK FLOW DIFFERENCE (cfs) (cfs) 0.77 0.33 bJ.tA, Inc. land planning, civil engineering, surveyi n~ 12 L 3130 Madison Street Drainage Study IV. REFERENCES bl-IA, Inc. land planning, civil engineering, suNeyin~ L -1.3 Hydrologic Soil Group-San Diego County Area , California Hydrologic Soil Group Map unit symbol Ma.i> unit name Rating AcreslnAOI Percent of AOI MIC Marina loamy coarse B 0.2 sand, 2 to 9 percent slopes Totals for Area of Interest 0.2 Description Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms. The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (ND, BID, and C/D). The groups are defined as follows: Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission. If a soil is assigned to a dual hydrologic group (AID, BID, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. Rating Options Aggregation Method: Dominant Condition Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey bl-tA, Inc. 100.0% 100.0% 517/2018 Page 3 of 4 3130 Madison Street Drainage Study land planning, civil engineering, surveyinn,------. 20 Hydrologic Soil Group-San Diego County Area, California Component Percent Cutoff: None Specified Tie-break Rule: Higher Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey bl-IA, Inc. 517/2018 Page 4 of 4 3130 Madison Street Drainage Study land planning, civil engineering, surveyin~ 21 L 3130 Madison Street Drainage Study San Diego County Hydrology Manual Date: June 2003 Section: Page: 3 5 of26 C = 0.90 x (%Impervious)+ Cp x (1 -% Impervious) Where: Cp = Pervious Coefficie11t Runoff Value for the soil type (shown in Table 3-1 as Undisturbed Natural Terrain/Pennanent Open Space, 0% Impervious). Soil type can be detennined from the soil type map provided in Appendix A. The values in Table 3-1 are typical for most urban areas. However, if the basin contains .rural or agricultural land use, parks, golf courses, or other types of nonurban land use that are expected to be permanent, the appropriate value should be selected based upon the soil and cover and approved by the local agency. 3-5 bl-IA, Inc. land planning, civil engineering, surveyin~ 23 L 3130 Madison Street Drainage Study San Diego County Hydrology Manual Date: June 2003 3.2 DEVELOPING INPUT DATA FOR THE RATIONAL METHOD Section: Page: 3 20 of26 'Ibis section describes the development of the necessary data to perform RM calculations. Section 3.3 describes the RM calculation process. Input data for calculating peak flows and Tc's with the RM should be developed a~follows: I. On a topographic base map, outline the overall drainage area boundary, showi11:g adjacent drains, existing and proposed drains, and overland flow paths. 2. Verify the accuracy of the drainage map in the field. 3. Divide the drainage area into subareas by locating significant points of interest. These divisions should be based on topography, soil type, and land use. Ensure that an appropriate first subarea is delineated. For natural areas, the first subarea flow path length should be less than or equal to 4,000 feet plus the overland flow length (Table 3-2). For developed areas, the initial subarea flow path length should be consistent with Table 3-2. The topography and slope within the initial subarea should be generally uniform. 4. Working from upstream to downstream, assign a number representing each subarea in the drainage system to each point of interest. Figure 3-8 provides guidelines for node numbers for geographic i-nformation system (GIS)-based studies. 5. Measure each subarea in the drainage area to determine its size in acres (A). 6. Determine the length and effective slope of the flow path in each subarea. 7. Identify the soil type for each subarea. 3-20 bl-IA, Inc, land planning, civil engineering, surveying! '27 L 3130 Madison Street Drainage Study San Diego County Hydrology Manual Date: June 2003 Section: Page: 3 22of26 8. Determine the runoff coefficient (C) for each subarea based on Table 3-I. If the subarea contains more than one type of development classification, use a proportionate average for C. ln determining C for the subarea, use future land use taken from the applicable community plan, Multiple Species Conservation Plan, National Forest land use plan, etc. 9. Calculate the CA value for the subarea. IO. Calculate the 1:(CA) value(s) for the subareas upstream of the point(s) of interest. 11. Detennine P0 and P24 for the study using the isopluvial maps provided in Appendix B. If necessary, adjust the value for P6 to be within 45% to 65% of the value for P24• See Section 3.3 for a description of the RM calculation process. 3.3 PERFORMING RATIONAL METHOD CALCULATIONS This section describes the RM calculation process. Using the input data, calculation of peak flows and Tc's should be performed as follows: 1. Determine T; for the first subarea. Use Table 3-2 or Figure 3-3 as discussed in Section 3 .1.4. If the watershed is natural, the travel time to the downstream end of the first subarea can be added to T; to obtain the Tc. Refer to paragraph 3.1.4.2 (a). 2. Dete1mine I for the subarea using Figure 3-1. If T; was less than 5 minutes, use the 5 minute time to detennine intensity for calculating the flow. 3. Calculate the peak discharge flow rate for the subarea, where Qp = },;(CA) 1. In case that the downstream flow rate is less than the upstream flow rate, due to the long travel time that is not offset by the additional subarea runoff, usJ the upstream peak flow for design purposes until downstream flows increase again. 3-22 bliA, Inc. land planning, civil engineering, surveyinr,,----~ 29 3130 Madison Street Drainage Study San Diego County Hydrology Manual Date: June 2003 4. Estimate the Tt to the next point of interest. 5. Add the Tt to the previous Tc to obtain a new T0• Section: Page: 6. Continue with step 2, above, until the final point of interest is reached. 3 23 of26 Note: The MRM should be used to calculate the peak discharge when there is a junction from independent subareas into the drainage system. 3.4 MODI.F'IED RATIONAL METHOD (FOR JUNCTION ANALYSIS) The purpose of this section is to describe the steps necessary to develop a hydrology report for a small watershed using the MRM. It is necessary to use the MR.M if the watershed contains junctions of independent drainage systems. The process is based on the design manuals of the City/County of San Diego. The general process description for using this method, including an example of the application of this method, is described below. The engineer should only use the MRM for drainage areas up to approximately 1 square mile in size. Jf the watershed will significantly exceed l square mile then the NRCS method described in Section 4 should be used. 'f.he engineer may choose to use either the RM or the MRM for calculations for up to an approximately 1-square-mile area and then transition the study to the NRCS method for additional downstream areas that exceed approximately I square mile. The transition process is described in Section 4. 3.4.1 Modified Rational Method General Process Description The general process for the MR.\1 differs frorn the RM only when a junction of independent drainage systems is reached. The peak Q, Tc, and I for each of the independent drainage systems at the point of the junction are calculated by the RM. The independent drainage systems are then combined using the MRM procedure described below. The peak Q, T0., and I for each of the independent drainage systems at the point of the junction must be calculated prior to using the MRM procedure to combine the independent drainage systems, as these 3-23 bl-IA, Inc. land planning, civil engineering, surveyin3 30 L