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2015-2016 Focused Drainage StudyCITY OF LA QUINTA FOCUSED AREA DRAINAGE STUDY Riverside County, California Prepared for City of La Quinta 78-495 Calle Tampico La Quinta, California 92253 Prepared by 14725 Alton Parkway Irvine, CA 92618 Contact Persons John McCarthy, RCE 47583 Zachary Snyder, EIT Tom Ryan, RCE 61701 (Dudek) February 2016 JN 146281 City of La Quinta Focus Area Drainage Study February 2016 Michael Baker International This page intentionally left blank. City of La Quinta Focused Area Drainage Study February 2016 i Michael Baker International Table of Contents Executive Summary ...................................................................................................................................... v 1 Introduction ......................................................................................................................................... 1-1 1.1 Introduction and Project Overview ............................................................................................ 1-1 1.2 Project Site Description and Location ....................................................................................... 1-2 1.3 Study Goals and Objectives ....................................................................................................... 1-2 2 Project Approach and Design Criteria ................................................................................................. 2-5 2.1 Background ................................................................................................................................ 2-5 2.2 Design Criteria ........................................................................................................................... 2-7 2.2.1 Focused Drainage Area Study Criteria for Alternative Selection ............................. 2-8 2.3 Data Research ............................................................................................................................ 2-8 2.4 Technical Software Description ................................................................................................ 2-9 3 Hydrology .......................................................................................................................................... 3-11 3.1 Methodology ............................................................................................................................ 3-11 3.2 Precipitation ............................................................................................................................. 3-12 3.3 Land Use .................................................................................................................................. 3-12 3.4 Soil Types ................................................................................................................................ 3-12 3.5 Watershed Descriptions ........................................................................................................... 3-12 4 Flood Routing Analyses .................................................................................................................... 4-19 4.1 Methodology ............................................................................................................................ 4-19 4.1.1 Topography ............................................................................................................. 4-19 4.1.2 Vertical Datum ........................................................................................................ 4-20 4.1.3 1-D Model Development ......................................................................................... 4-20 4.1.4 Manning’s “n” Value............................................................................................... 4-20 4.1.5 Grid Size .................................................................................................................. 4-20 4.1.6 Computational Time Step ........................................................................................ 4-21 4.2 Existing Conditions ................................................................................................................. 4-21 4.2.1 Downstream Water Surface Control Elevations ...................................................... 4-21 4.2.2 Existing Condition Results ...................................................................................... 4-22 4.3 Proposed Conditions ................................................................................................................ 4-29 4.4 Cost Estimates ......................................................................................................................... 4-29 5 Drainage Area Results ....................................................................................................................... 5-30 5.1 North Drainage Area ............................................................................................................... 5-30 5.1.1 Proposed Alternatives.............................................................................................. 5-30 5.1.2 Cost Estimates ......................................................................................................... 5-31 5.2 South Drainage Area ............................................................................................................... 5-37 5.2.1 Proposed Improvements .......................................................................................... 5-37 5.2.2 Cost Estimate ........................................................................................................... 5-37 5.3 Lake La Quinta Drainage Area ................................................................................................ 5-42 5.3.1 Proposed Improvements .......................................................................................... 5-42 5.3.2 Cost Estimate ........................................................................................................... 5-42 City of La Quinta Focused Area Drainage Study February 2016 ii Michael Baker International Figures Figure 1-1: September 8, 2014 flooding along Eisenhower Drive looking north to Avenue 50. .............. 1-1 Figure 1-2: Regional Vicinity Map ............................................................................................................ 1-3 Figure 1-3: Project Location Map .............................................................................................................. 1-4 Figure 2-1: Cumulative Peak Rainfall Plot ................................................................................................ 2-6 Figure 3-1: Hydrograph – Direct Rainfall Areas ..................................................................................... 3-15 Figure 3-2: Land Use Map ....................................................................................................................... 3-16 Figure 3-3: Model Boundary Map ........................................................................................................... 3-17 Figure 3-4: Offsite Hydrograph Watershed Map ..................................................................................... 3-18 Figure 4-1: Graphic Representation of 1D/2D Surface/Subsurface Model ............................................. 4-19 Figure 4-2: Eisenhower Drive and Avenue 50 Comparison .................................................................... 4-25 Figure 4-3: Eisenhower Bend Comparison .............................................................................................. 4-26 Figure 4-4: Calle Tampico Comparison ................................................................................................... 4-27 Figure 4-5: Washington Street at Lake La Quinta Drive ......................................................................... 4-28 Figure 5-1: North Alternative No. 1 Proposed Facilities ......................................................................... 5-34 Figure 5-2: North Alternative No. 2 Proposed Facilities ......................................................................... 5-35 Figure 5-3: North Alternative No. 3 Proposed Alternative ...................................................................... 5-36 Figure 5-4a: South Alternative No. 1 Proposed Alternative .................................................................... 5-39 Figure 5-4b: South Alternative No. 1 Proposed Alternative .................................................................... 5-40 Figure 5-4c: South Alternative No. 1 Proposed Alternative .................................................................... 5-41 Figure 5-5: Lake La Quinta Alternative No. 1 Proposed Alternative ...................................................... 5-43 Tables Table 2-1: Peak Rainfall Precipitation at Bear Creek during September 2014 storm event. ..................... 2-5 Table 3-1: NOAA Atlas 14 Precipitation Frequency Data at La Quinta Golf Estates ............................. 3-14 Table 5-1: North Drainage Area – Alternative 1 Cost Estimate .............................................................. 5-31 Table 5-2: North Drainage Area – Alternative 2 Cost Estimate .............................................................. 5-32 Table 5-3: North Drainage Area – Alternative 3 Cost Estimate .............................................................. 5-33 Table 5-4: South Drainage Area – Alternative 1 Cost Estimate .............................................................. 5-38 Table 5-5: Lake La Quinta Drainage Area – Alternative 1 Cost Estimate .............................................. 5-42 Exhibits Existing Condition Exhibit No. 1: North Area – 1% Annual Chance 6-Hour Storm Event Exhibit No. 2: North Area – 0.2% Annual Chance Storm Event, 1-hour duration Exhibit No. 3: South Area – 1% Annual Chance 6 Hour Storm Event Exhibit No. 4: South Area – 0.2% Annual Chance Storm Event, 1-hour duration Exhibit No. 5: Lake La Quinta Area – 1% Annual Chance 6-Hour Storm Event Exhibit No. 6: Lake La Quinta Area – 0.2% Annual Chance Storm Event, 1-hour duration Proposed Condition Exhibit No. 7 North Area Alternative 1 – 0.2% Annual Chance Exhibit No. 8 North Area Alternative 2 – 0.2% Annual Chance City of La Quinta Focused Area Drainage Study February 2016 iii Michael Baker International Exhibit No. 9 North Area Alternative 3 – 0.2% Annual Chance Exhibit No. 10 South Area Alternative 1 – 0.2% Annual Chance Exhibit No. 11 Lake La Quinta Area Alternative 1 – 0.2% Annual Chance City of La Quinta Focused Area Drainage Study February 2016 iv Michael Baker International This page intentionally left blank. City of La Quinta Focused Area Drainage Study February 2016 v Michael Baker International Executive Summary The purpose of this study is to assess whether the City of La Quinta (City) should consider and potentially spend limited public resources to augment the City’s flood control and drainage systems to a level that exceeds all generally accepted minimum standards for flood control and that goes beyond the minimum legal requirements as directed by Riverside County and the City, as well as federal and state standards, in order to provide the City’s residents an elevated level of flood control. The City experienced extreme storm events in two consecutive years that resulted in localized flooding and significant flood damage in parts of the City. The storm event on September 8, 2014 dropped almost 3 inches of rain in a one-hour period and was in excess of a 500-year storm event (meaning the probability of its occurrence is 0.2% in any given year). The “0.2-percent” storm event is an estimate of the long-term average recurrence interval. It is not an indication that an event of this size would happen once every 500-years. Rather, it is a representation that a major event (500-year) has a 0.2-percent chance of occurring in any given year. Consequently, it is possible, yet extremely unlikely, to have multiple extreme events over a short period of time. For example, the City of La Quinta experienced a similar extreme rainfall event just one year earlier on August 25, 2013 where over two inches of rainfall fell in a short duration, corresponding to an approximately 200-year event. While the City’s and surrounding area’s flood control facilities functioned consistently with industry-accepted design standards and requirements, which generally use a 100-year storm event (meaning the probability of an occurrence is 1% in any given year), these recent extreme storm events caused flooding and damage to certain parts of the City. This study includes a detailed drainage analysis for a focused area of the City that was hit the hardest by the recent extreme storm events. The study evaluated the existing flood risk, and identifies potential drainage improvements to reduce future flood damage and increase public safety beyond generally accepted legal requirements for handling a 100-year storm event. The focused area for the detailed drainage study is generally bounded by Avenue 48 to the north, Washington Street to the east, Eisenhower Drive to the west, and Calle Tampico to the south. Five (5) specific locations were initially identified by the City for this study area. This study focused on the following areas:  Calle Tampico/Avenida Bermudas  Eisenhower Drive/Avenue 50  Washington Street/Avenue 50  Eisenhower Drive between Coachella Drive and Vista Laguna  Washington Street/Lake La Quinta Drive XP Software’s XPSWMM 2-dimensional computer program was used to model the complex hydrology and hydraulic simulations of the study area. XPSWMM allows flows to be modelled within storm drains and on the surface using a 3D digital terrain model. The results of these City of La Quinta Focused Area Drainage Study February 2016 vi Michael Baker International advanced models were presented graphically to show the flooded depths throughout the study area. These advanced models provide a better understanding of how the existing storm drain system operates and how to improve it. Three (3) models were prepared for the study area: 1. North Drainage Area 2. South Drainage Area 3. Lake La Quinta Drainage Area For each area, the capacity of the existing street sections and storm drains were evaluated for the 100-year storm events with varying durations and the 500-year 1-hour storm event. It was important to evaluate the storm drain system in the 500-year storm event in order to correlate the flood routing model results with photographs and eyewitness descriptions of the extreme flooding that occurred during the September 2014 storm event. This correlation process was conducted by comparing flood depths in the model results to flood depths seen in the photographs and eyewitness accounts, and reasonably modifying model parameters until the flood depths in the model correlated to the depths seen in the photographs. Once the model was correlated to real flood events, the benefits of improvements to the drainage system for these extreme storm events were evaluated. This type of analysis and recommendations are beyond generally accepted and currently applicable standards to accommodate a 100-year storm event. The results of the analysis provided the following information:  The public streets and storm drain system met the applicable standards during the simulated 100-year storm events. Some private streets did exhibit localized flooding during the 100-year events.  500-year 1-hour analysis produced flood results consistent with the September 2014 storm event, which provided validation of the modeling methods and results.  A 500-year storm event exceeded the capacity of the existing regional and local drainage systems at various locations within the City. This event provided guidance for selecting proposed improvements that would exceed the generally accepted and minimal legal threshold of accommodating a 100-year storm event Using the results of the analysis, proposed improvements were identified to exceed the City’s 100-year standard storm event level of protection in the study area. Recommendations were developed for each of the three model areas. Atlas maps detailing the alternatives are shown in Figures 5-1 to 5-5. North Drainage Area Three alternatives were developed for the North Drainage Area. The alternatives include: Alternative 1  Estimated Cost: $11 million  Provides the highest level of additional improvements (500-year) at the largest cost  New storm drain lines in Eisenhower Drive  1 new detention basin along Eisenhower Drive  2 new retention basins in the La Quinta Golf Estates City of La Quinta Focused Area Drainage Study February 2016 vii Michael Baker International Alternative 2  Estimated Cost: $4.2 million  Lowest price alternative, provides near 500-year level of additional improvements  New storm drain lines in Eisenhower Drive  New retention basin on driving range in La Quinta Golf Estates Alternative 3  Estimated Cost: $6.5 million  Similar to Alternative 2, with an additional 1 acre retention basin along Eisenhower  Reduces flooding depths at the existing low point on Eisenhower Drive south of Avenue 50 compared to Alternative 2 South Drainage Area One alternative was developed for this area to improve the system to near 500-year storm event levels. The alternative includes:  Estimated Cost: $5.4 million  Expands existing storm drain system along Calle Tampico west of Washington Street  Expands existing storm drain line within Avenue 50 and Washington Street Lake La Quinta Area One alternative was developed in the Lake La Quinta area to improve the system to a 500 -year storm event level of protection:  Estimated Cost: $1.0 million  Construct new storm drain line from St. Francis of Assisi Catholic Church into the lake within Lake La Quinta  Construct new storm drain line along Washington Street from Avenue 50 into the lake within Lake La Quinta For each of the areas studied, alternatives were identified that could significantly reduce their flood hazard for storm events that exceed a 100-year storm event. The benefit of using the advanced model to complete the study is that it provided a more accurate evaluation of the impacts these proposed facilities would have on adjacent drainage patterns and floo d risk. The alternatives, and associated costs listed above, include cooperation with other stakeholders, such as Coachella Valley Water District, La Quinta Resort, home owners associations, and private land owners. City of La Quinta Focused Area Drainage Study February 2016 viii Michael Baker International This page intentionally left blank. City of La Quinta Focused Area Drainage Study February 2016 1-1 Michael Baker International 1 Introduction 1.1 Introduction and Project Overview The City of La Quinta (City) recently experienced two extreme storm events that have resulted in localized flooding and significant flood damage in parts of the City. The most recent event occurred on September 8, 2014, and produced rainfall depths of almost 3 inches in one-hour over parts of the City. This corresponds to a storm event in excess of a 500-year return frequency. A similar, short-duration high intensity storm in August of 2013 (>200-year return frequency) produced flooding and storm damage in the City. Located in the Coachella Valley of Riverside County, the City is subject to both winter and summer storm events. The summer thunderstorms can produce severely intense localized precipitation in a short period of time, producing flash- floods that have historically impacted the Coachella Valley. Figure 1-1: September 8, 2014 extreme-event flooding along Eisenhower Drive looking north to Avenue 50. The purpose of this study was to prepare a detailed drainage analysis for a focused area of the City that was hit the hardest by the recent extreme storm events. The study evaluated the flood risk associated with an extreme storm event, and identifies potential drainage improvements to reduce future flood damage and increase public safety if or when extreme storm events occur. City of La Quinta Focused Area Drainage Study February 2016 1-2 Michael Baker International 1.2 Project Site Description and Location The focused area for the detailed drainage study is bounded by Avenue 48 to the north, Washington Street to the east, Eisenhower Drive to the west, and Calle Tam pico to the south. Five (5) locations were initially identified by the City for this study area. Due to the relatively flat topographic relief of the urbanized area, it was recommended that all areas be analyzed together, to better understand how extreme flood events would bifurcate, or spill over to neighboring watershed areas. The five areas are listed below:  Calle Tampico/Avenida Bermudas;  Eisenhower Drive/Avenue 50;  Washington Street/Avenue 50;  Eisenhower Drive between Coachella Drive and Vista Laguna; and  Washington Street/Lake La Quinta Drive. Although this study will highlight these 5 areas, it will also identify other major flood issues and solutions within the overall focused study area. Figures 1-2 and 1-3 show the Regional Vicinity Map and Project Location Map for the City and the focused study area. 1.3 Study Goals and Objectives The overall goal of the study is to develop a detailed technical evaluation of the flood risk hazard in the focused drainage area when extreme storm events may occur and provide specific recommendations for storm drain facilities to reduce the risk during such extreme storm events. The technical analysis will include an inventory of existing storm drain systems, updated regional and local hydrology, and detailed flood routing analyses. Together, the in-depth understanding of the watershed and project site conditions will guide the development of appropriate, long-term solutions for improved flood protection in the focused drainage area that are above and beyond legal requirements and City and County standards. The detailed objectives of the report include the following: 1. Research, collect, and review previous studies and storm drain improvement plans completed in the watershed. 2. Establish the hydrologic design criteria, methodology and requirements to be used for the hydrologic and hydraulic studies and for the determination of the appropriate level of flood protection. 3. Prepare design hydrology to be used for the flood routing analysis. 4. Complete a flood routing analysis to determine flooding patterns and maximum flood depths in the study area. 5. Correlate the results of the flood routing analysis to known flooding from the September 8, 2014 storm event. 6. Evaluate existing system capacities and develop conceptual drainage solutions for identified flood hazard areas that exceed legally required and generally applied standards. 7. Prepare preliminary construction cost estimates for the recommended drainage improvements. 8. Prepare a detailed report to document the studies and support the recommended improvements. The technical studies have been completed in conformance with City, Coachella Valley Water District, and Riverside County standards. PACIFIC OCEAN SALTON SEA SAN BERNARDINO COUNTY RIVERSIDE COUNTY SAN DIEGO COUNTY MEXICO IMPERIAL COUNTY 10 30 247 18 62 62 18 330 38 215 15 10 10 60 111 243 371 79 79 74 74 86 15 15 8 8 5 5 805 111 76 78 78 86 86 98 94 94 163 125 56 79 79 67 52 54 Big BearLake Yucaipa Banning PalmSprings Idyllwild PalmDesert LaQuinta Indio Julian Ramona ChulaVista LemonGrove SanDiego MiraMesa ElCajon Santee Alpine Poway Coachella Brawley Westmorland Imperial Calipatria ElCentro DesertHot Springs YuccaValley TwentyninePalms CathedralCity Hemet MorenoValley Vista Escondido Encinitas SunCity Perris Temecula Campo Jamul Protrero Dulzura PineValley Ocotillo BorregoSprings WarnerSprings Mecca RanchoMirage Anza LakeArrowhead IndianWells Project Site Project Site 12/03/15 JN 146281-21421 MAS Figure 1-2 Vicinity Map LA QUINTA FOCUSED DRAINAGE STUDY010 14 miles APPROXIMATE BEAR CREEK CHANNELEVACUATION CHANNEL Bear CreekRain Gauge Calle Tampico BermudasEisenhower DrWashington StSinaloa Lake La Quinta Avenue 50 E is e n h o w er Dr 111 LEGEND City Border Study Area Focused Flood Site Rain Gauge 12/03/15 JN 146281-21421 MAS Figure 1-3 Project Location Map LA QUINTA FOCUSED DRAINAGE STUDY 0 2,000 feet APPROXIMATE City of La Quinta Focused Area Drainage Study February 2016 2-5 Michael Baker International 2 Project Approach and Design Criteria 2.1 Background In the past two years, historically rare, extreme storm events have caused extensive localized flooding within the City. Most recently, the September 8, 2014 summer thunderstorm dropped 2.79-inches of rain in a 1-hour period as measured by the Bear Creek Basin precipitation gage. On August 25, 2013, Monsoonal Storm Ivo dropped 2.25-inches of rain at the same precipitation gage. The Bear Creek Basin gage is located on the northwest side of the City in the Cove Area. The location of the gage is shown on Figure 1-3. The peak rainfall at the gage during the 2014 storm event is summarized in Table 2-1. The cumulative rainfall for the time period is also plotted in Figure 2-1. Table 2-1: Peak Rainfall Precipitation at Bear Creek during September 201 4 storm event. Time Incremental Rainfall (inches) Cumulative Rainfall (inches) 6:20 0 0 6:25 0.16 0.16 6:30 0.35 0.51 6:35 0.39 0.9 6:40 0.36 1.26 6:45 0.47 1.73 6:50 0.24 1.97 6:55 0.19 2.16 7:00 0.28 2.44 7:05 0.12 2.56 7:10 0.08 2.64 7:15 0.11 2.75 7:20 0.04 2.79 7:25 0.08 2.87 7:30 0.08 2.95 7:35 0.00 2.95 City of La Quinta Focused Area Drainage Study February 2016 2-6 Michael Baker International Figure 2-1: Cumulative Peak Rainfall Plot Statistically, the September 2014 storm event corresponds to an approximate 0.2-percent annual chance event for a 1-hour period (See Table 3-1 for NOAA Atlas 14 Precipitation Frequency data). This is commonly referred to as a “500-year/1-hour” storm event, or an event that would have a 1 in 500 chance of being equaled or exceeded in a given year. The “0.2-percent” storm event is an estimate of the long-term average recurrence interval. It is not an indication that an event of this size would happen once every 500-years. For example, if we had 1,000 years of rain gage data, a statistical analysis would reveal a 1-percent annual chance (or 100-year) event. A high probability exists that within this 1,000 years of data that 10 events would equal or exceed the calculated 1-percent chance (100-year) event rainfall depth. These 10 events would not occur once every 100-years, but would vary sporadically. A more realistic way to look at the likelihood of these events to occur, is to view them as percent -chance within a given year. For example, on a given year, there is a one-percent chance of a “100-year” event to occur. The chances of multiple large storm events occurring over a few years is highly unlikely, but possible. Moreover, climate change could factor into the possibility of more frequent extreme storm events. The City’s existing storm drain facilities reviewed in this study were based on rainfall data published in the National Oceanic and Atmospheric Administration’s (NOAA) Atlas 2. This document provided localized precipitation frequency data for the entire country. The document was first published in 1973 and included statistical analyses using rain gage data collected through the late 1960’s. Since the installation of the City’s existing storm drain facilities, NOAA has updated precipitation frequency data and may continue to do so as advancements in technology and data collection change. This study includes a full hydrologic and hydraulic analysis of the focused drainage study area for the 1% and 0.2% annual chance storm events. Evaluations of the existing flood hazard conditions were prepared to correlate flood depths to known flood elevations, based on City of La Quinta Focused Area Drainage Study February 2016 2-7 Michael Baker International photographs and watermarks. Once correlated, the models were used identify the most feasible solutions to improve drainage and reduce flood risk. To date, the detailed level of extreme flood study is unprecedented for any urban area in the Coachella Valley. 2.2 Design Criteria The primary goals of the study, and City’s use of it, include identifying possible improvements that consider: 1) Life and safety 2) Emergency vehicle access 3) Property protection Since December 19, 2006, the City’s hydrology and hydraulic criteria for the design of storm drain systems is defined in the City of La Quinta Engineering Bulletin #06-16, originally dated December 19, 2006 and revised October 28, 2015. Prior to December 19, 2006, criteria for design of storm drain systems refer to the Riverside County Flood Control and Water Conservation District’s (RCFC) Hydrology Manual and Engineering Bulletins #06-15 and #06- 16 guidelines and flood protection standards. For purposes of new flood control improvements, hydrology is to be prepared in accordance with the RCFCD Hydrology Manual guidelines. The design of retention basins and flood protection is to be based on the 1-percent annual chance (100-year) storm event. An analysis should be prepared for the 1-, 3-, 6-, and 24-hour storm durations. The storm duration that produces the worst flooding scenario is to govern the minimum requirements for the design of a new flood control improvement. Likewise, rainfall data for new improvements should be from the latest version of the National Oceanic and Atmospheric Administration (NOAA) atlas for precipitation frequency estimates. For new improvements, NOAA Atlas 14, adopted in 2006, would apply. Public streets are to be designed to contain a 10-year storm event within the top of curb of the street section. The 100-year 24-hour event is to be contained within the City’s right-of-way, with a minimum of one (1) foot of freeboard to the adjacent pad elevations. For retention basins, 1 foot of freeboard is to be provided above the 100-year maximum water surface elevation to the pad elevations. Additionally, La Quinta Municipal Code 8.11.030 references Federal Emergency Management Agency (FEMA) standards, which dictate that new structures have their lowest floor elevation at or above the 100-year base flood elevation. (See also La Quinta Municipal Code §§8.11.020(4) and (24); 8.11.050(A)(3)(a))The FEMA 100-year base flood has been a standard requirement since the enactment of the National Flood Insurance Act of 1968, the estimated rainfall used to generate the 100 -year (1% annual chance) base flood has changed as additional rain gage information has become available. City law requires that all improvements to structures and land in areas of special flood hazards be designed to accommodate at least a 1-percent annual chance (100-year) event (La Quinta Municipal Code §§8.16.020(4)(24); 8.11.030(C); 8.11.050(A)(3)(a); 9.140.030). The special flood hazard areas are identified by documents issued by the Federal Emergency Management Agency (FEMA) as amended from time to time (La Quinta Municipal Code §8.11.030(B)). City law, however, recognizes that larger floods can and will occur and that other areas of the City not in areas of special flood hazards may experience flooding (La Quinta Municipal Code City of La Quinta Focused Area Drainage Study February 2016 2-8 Michael Baker International §8.11.030(F)). Furthermore, City law expressly provides that no liability is created for the City or Federal Insurance Administration of FEMA for any flood damage that may occur in reliance on City flood hazard requirements (La Quinta Municipal Code §8.11.030(F)). 2.2.1 Focused Drainage Area Study Criteria The hydrology prepared for this study included the 1-percent annual chance (i.e., 100-year) and 0.2-percent annual chance (i.e., 500-year) storm events. The 1% annual chance event represents the typical design standard for flood protection within the City. The study criteria used the 1% annual chance storm event to evaluate the level of flood protection, with the goal of identifying proposed improvements to maintain one dry lane in each direction within major street arterials for emergency vehicle access. One dry lane in each direction is a higher standard than what current City, County, and Federal standards require. In order to evaluate the effectiveness of the existing storm drain system under the criteria that applied at the time of its design, hydrology for the 1% annual chance storm event was performed using NOAA Atlas 2 rainfall data published in 1973. The 0.2% annual chance storm event was evaluated for two reasons: to correlate the models to the September 8, 2014 storm event, and to evaluate opportunities to provide a higher level of flood protection in potentially flood prone areas. Industry standards and regulations do not require municipal drainage facilities to be designed to handle the 0.2% annual chance storm event. But due to the extensive flooding that o ccurred during the 2013 and 2014 storm events, the City requested this event be modeled to ensure the proposed facilities would reduce the potential flood risk for this larger storm event. A major benefit for modeling the 0.2% annual chance storm event was to correlate the existing condition models to the previous 2014 storm event. The thunderstorm that occurred in La Quinta on September 8, 2014, was similar to a 0.2% (1-hour) annual chance storm event, per the rain gage data recorded at the Bear Creek Basin. The 0.2% annual chance event was run for all of the flood routing models. The results of these models were used to compare flooded elevations as witnessed on September 8, 2014. Photographs were used to estimate approximate depths for major flooded areas. The results of these models were evaluated to verify the results of the models correlated to actual flooded locations and depths. 2.3 Data Research The data research and acquisition process included coordination with regional agencies, City staff and field review. The goal of the research was to identify available data, including previous drainage related studies, historical storm photos and articles, drainage related as-built plans, rain gage data, and information regarding previous flood issues. Items acquired included the following:  Storm Drain As-Builts  Street As-builts  Residential Development As-builts  Previous Drainage Studies (Regional and Focused Studies)  Historical Rain Gage Data  Historical Flood Photographs City of La Quinta Focused Area Drainage Study February 2016 2-9 Michael Baker International  Field Drainage Facility Investigations 2.4 Technical Software Description This study was performed using a state-of-the-art hydrologic and hydraulic approach as a result of the area’s drainage characteristics. XP Software’s XPSWMM model was used to model both the complex hydrology and hydraulic simulations. The XPSWMM software solves the full St. Venant Equations. In other words, the program solves the highest level of computations available for storm drain modeling. The model utilizes full rainstorm patterns, not just peak flows, to calculate expected runoff and storm drain capture and conveyance efficiencies. Surface flows are evaluated in two-dimensions (2D) based on a 3D surface or digital terrain model (DTM). Linking the surface to the subsurface storm drains, lakes or basins, and channels, produces a comprehensive and realistic analysis of the entire drainage system. City of La Quinta Focused Area Drainage Study February 2016 2-10 Michael Baker International This page intentionally left blank. City of La Quinta Focused Area Drainage Study February 2016 3-11 Michael Baker International 3 Hydrology 3.1 Methodology The 1-percent annual chance event was evaluated for the 1-hour, 3-hour, and 6-hour durations to identify the most severe flooding potential under applicable City and County standards for the existing flood systems. The event duration that created the worst flooding in the model was used as the “governing” storm event for the area’s proposed improvements. After coding the models with the proposed improvements, calculations were again performed to identify the impacts of these improvements on the study area. A major benefit to using XPSWMM is the identification of impacts to surrounding areas by way of bifurcation, or increasing flooding elsewhere as a result of improving a particular location. Each model area was divided into multiple subareas, delineated based on topographic and drainage patterns. Offsite areas, mainly hillside slopes, were evaluated using the Advanced Engineering Software’s (AES) program to develop Unit Hydrographs for each of the design storm events. All urbanized areas within the focused study area were evaluated in XPSWMM using the Direct Rainfall Method (DRM). DRM is the process of adding rainfall directly to the two-dimensional surface, or digital terrain model. This method allows the surface’s physical characteristics (i.e. topography, land use) to dictate the flow patterns, resulting in a more realistic rainfall-runoff modeling approach. Figure 3-1 shows the areas where Unit Hydrographs were developed and where DRM was used in the modeling. Loss rates for both the Unit Hydrograph method, and the DRM were calculated based on the Riverside County Hydrology Manual. Loss rates in XPSWMM can be incorporated in the rainfall or on the surface. To keep the two methods consistent, Loss rates were calculated and incorporated into the rainfall point precipitation depths based on Land Uses, Soil Type, and using Antecedent Moisture Condition 2 (AMC 2). For a given area, DRM will yield different peak flow values than the standard Unit Hydrograph method. This is a result of the response of the runoff on the 2D surface. Theoretically, the peak flows produced by the Unit Hydrograph are calculated based on time-of-concentration, which assumes all flows are concentrated and conveyed from one concentration point to the next, which has the tendency to reduce time of concentration. Reduced time of concentration yields higher peak flow rate estimates. In the absence of strea m gage information, the DRM runoff volumes were calibrated to the Unit Hydrograph volumes for selected locations. When using DRM, some of the rainfall volume will be attenuated on the 2D surface. Small depressions or “pits” located in an area can result in a reduction in runoff volume, when compared to the runoff volumes produced in the Unit Hydrograph model for a given area. These depressions can be residential backyards, or small low points in the model. Often these depressions contain small area drains, too small to identify in a regional model. To match the assumptions of the Unit Hydrograph approach, these depressions have to be “filled” prior to running a storm scenario. Although several methods exist to “fill” these minor depressions in the surface model, additional rainfall was added to the surface to “fill” the depressions prior to running the actual storm event City of La Quinta Focused Area Drainage Study February 2016 3-12 Michael Baker International models. Several iterations were performed to identify the appropriate added rainfall amount that would not impact the results of the design storm events. A total of 0.25 inches of rainfall over an hour long duration was dropped over all residential and commercial sites within the model before allowing the surface to drain for an additional 23 hours. This method ensured that the 0.25 inches of rainfall filled the small depression areas, but did not interfere with the design storm. 3.2 Precipitation DRM was prepared using the Riverside County Hydrology rainfall pattern to develop hyetographs. The point precipitation data for the 1% annual chance storm events were based on the NOAA Atlas 2 data for the La Quinta area. Precipitation data for the 0.2% annual chance storm event was based on the NOAA Atlas 14 data. The NOAA Atlas 14 data table is included as Table 3-1. For the purpose of this study, an Antecedent Moisture Condition value of 2 was used. 3.3 Land Use Land uses were categorized with the assistance of the City of La Quinta Official Zoning Map. Areas from the zoning map were assigned a land use value, as listed in the Riverside County Hydrology Manual. From the Hydrology Manual, Manning’s roughness values and rainfall loss rates were assigned based on land use. Land use types are shown on Figure 3-2. 3.4 Soil Types The hydrologic soil type is required in order to determine soil infiltration rates according to the Riverside County Hydrology Manual. The Natural Resources Conservation Service’s Web Soil Survey was referenced to determine the hydrologic soil type. The developed areas of the city near Calle Tampico and La Quinta Resort are predominantly composed of Type A soil, while the area within the La Quinta Golf Estates is composed of Type B soil. 3.5 Watershed Descriptions The study area was delineated into three models, based on flow patterns identified in a large initial course model. The areas were sub-divided to allow for more detailed resolution in the 2D surface calculations. A sensitivity analysis was performed to identify each model boundary and how the boundaries communicate with one another. Figure 3-3 shows the model boundaries for each of the three areas. Figure 3-4 illustrates the offsite hydrograph locations that were used in conjunction with the DRM areas to generate the hydrology for each of the models. The 3 models are identified as:  Lake La Quinta Model  North Model  South Model The Lake La Quinta model area extends from 48th Avenue north to approximately Highway 111. Most of the tributary drainage area to this model comes from the hillsides to the west and the residential and commercial areas to the north. The active model boundary extends just west of City of La Quinta Focused Area Drainage Study February 2016 3-13 Michael Baker International Caleo Bay Drive. This area drains into Lake La Quinta, and eventually out to the Whitewater River. No flows were found to travel south of 48th Avenue into the North Model. The North Model extends from 48th Avenue, south to the Evacuation Channel and from Washington Street west to the foothills. Where needed to better identify flooded areas, some of the hillside residential areas were included in the active area. Runoff tributary to the North Model comes primarily from the mountains and adjacent residential areas. The flows in this area are generally contained within the area of the North Model or drain to the Evacuation Channel. The South Model is bounded by the Evacuation Channel to the north and north east, Calle Sinaloa/Avenue 52 to the south, Calle Rondo to the east, and Bear Creek and the Oleander Basin to the west. This area consists of two watersheds, one that drains towards the Evacuation Channel, and one that drains east along Calle Tampico towards Calle Rondo and Avenue 52. The tributary drainage area to this model consists primarily of the urbanized areas within the model. The Cove Area is also tributary to this model, although most of it is serviced by storm drains that route flows to a CVWD maintained 60-inch reinforced concrete pipe. City of La Quinta Focused Area Drainage Study January 2016 3-14 Michael Baker International Table 3-1: NOAA Atlas 14 Precipitation Frequency Data at La Quinta Golf Estates PDS-based precipitation frequency estimates with 90% confidence intervals (in inches)1 Duration Average recurrence interval (years) 1 2 5 10 25 50 100 200 500 1000 5-min 0.07 0.107 0.165 0.218 0.305 0.383 0.474 0.581 0.755 0.917 (0.058-0.085) (0.089-0.130) (0.137-0.200) (0.180-0.268) (0.243-0.386) (0.298-0.496) (0.360-0.629) (0.430-0.795) (0.535-1.08) (0.627-1.36) 10-min 0.1 0.154 0.236 0.313 0.437 0.548 0.679 0.833 1.08 1.31 (0.084-0.121) (0.128-0.186) (0.196-0.287) (0.258-0.384) (0.348-0.554) (0.428-0.710) (0.516-0.902) (0.616-1.14) (0.766-1.54) (0.899-1.94) 15-min 0.121 0.186 0.285 0.379 0.528 0.663 0.821 1.01 1.31 1.59 (0.101-0.147) (0.155-0.225) (0.237-0.347) (0.312-0.464) (0.421-0.670) (0.517-0.859) (0.624-1.09) (0.745-1.38) (0.927-1.87) (1.09-2.35) 30-min 0.179 0.274 0.42 0.558 0.778 0.977 1.21 1.49 1.93 2.34 (0.149-0.216) (0.228-0.332) (0.350-0.511) (0.460-0.684) (0.620-0.987) (0.762-1.27) (0.920-1.61) (1.10-2.03) (1.37-2.75) (1.60-3.46) 60-min 0.253 0.388 0.595 0.79 1.1 1.38 1.71 2.1 2.73 3.32 (0.211-0.306) (0.323-0.470) (0.495-0.723) (0.651-0.968) (0.878-1.40) (1.08-1.79) (1.30-2.27) (1.55-2.88) (1.93-3.90) (2.27-4.90) 2-hr 0.351 0.515 0.766 0.999 1.37 1.7 2.07 2.52 3.21 3.84 (0.293-0.425) (0.429-0.624) (0.636-0.930) (0.823-1.22) (1.09-1.73) (1.32-2.20) (1.58-2.75) (1.86-3.44) (2.27-4.58) (2.62-5.67) 3-hr 0.418 0.606 0.889 1.15 1.57 1.93 2.35 2.84 3.6 4.28 (0.349-0.506) (0.505-0.734) (0.740-1.08) (0.951-1.41) (1.25-1.99) (1.51-2.50) (1.79-3.13) (2.10-3.88) (2.55-5.13) (2.92-6.32) 6-hr 0.546 0.785 1.14 1.47 1.98 2.43 2.94 3.52 4.42 5.21 (0.456-0.660) (0.654-0.951) (0.950-1.39) (1.21-1.80) (1.58-2.51) (1.89-3.15) (2.23-3.90) (2.60-4.81) (3.13-6.30) (3.56-7.70) *The highlighted cell indicates the rainfall depth used in the 0.2% annual chance (500-year) 1-hour scenario Calle Tampico Avenue 50 £¤111 Washington StBermudas Sinaloa 52 Eisenhower DrBear Creek Basin Tradition Golf Club #1 #2 #3 #8#6 #5 #4 #9 #10 #7 Calle Tampico BermudasEisenhower DrWashington StSinaloa Lake La Quinta Avenue 50 E is e n h o w er Dr 111 LEGEND City Boundary Direct Rainfall Area Hydrograph Area 12/03/15 JN 146281-21421 MAS Figure 3-1 Hydrograph-Direct Rainfall Areas LA QUINTA FOCUSED DRAINAGE STUDY 0 2,000 feet APPROXIMATE Calle Tampico BermudasEisenhower DrWashington StSinaloa LakeLa Quinta Avenue 50 E is e n h o w er Dr 111 LEGEND City Boundary Commercial High Density Residential Medium Density Residential Open Space Rock School 12/03/15 JN 146281-21421 MAS Figure 3-2 Land Use Map LA QUINTA FOCUSED DRAINAGE STUDY 0 3,000 feet APPROXIMATE BEAR CREEK CHANNELEVACUATION CHANNEL Calle Tampico BermudasEisenhower DrWashington StSinaloa Lake La Quinta Avenue 50 E is e n h o w er Dr 111 LEGEND City Boundary Lake La Quinta Model Boundary South Model Boundary North Model Boundary 12/03/15 JN 146281-21421 MAS Figure 3-3 Model Boundary Map LA QUINTA FOCUSED DRAINAGE STUDY 0 2,000 feet APPROXIMATE Calle Tampico BermudasEisenhower DrWashington StSinaloa Lake La Quinta Avenue 50 E is e n h o w er Dr 111 LEGEND City Boundary Offsite Hydrograph Location Offsite Hydrograph Watershed Bear CreekBasin #1 #9 #6 #7 #11 #5 #4 #3 #2 #12 #8 #13 #14 #10 TraditionGolf Course 12/03/15 JN 146281-21421 MAS Figure 3-4 Offsite Hydrograph Watershed Map LA QUINTA FOCUSED DRAINAGE STUDY 0 2,000 feet APPROXIMATE City of La Quinta Focused Area Drainage Study January 2016 4-19 Michael Baker International 4 Flood Routing Analyses 4.1 Methodology The study areas (North, South, and Lake La Quinta) were modeled using XP Software’s XPSWMM, which is an improved version of the U.S. EPA’s Storm Water Management Model (SWMM). As discussed previously, XPSWMM is a dynamic wave model that solves the full St. Venant Equations. Dynamic modeling allows the effects of storage and backwater in conduits and floodplains and the timing of the hydrographs to yield a true representation of the hydraulic conditions. XPSWMM can model the surface in 2-dimensions, while linking to the subsurface infrastructure, or storm drain system. The result is a comprehensive model that can communicate between the surface and subsurface facilities throughout the modeled design storm duration. Figure 4-1: Graphic Representation of 1D/2D Surface/Subsurface Model Due to the topographic and climatic characteristics of this region, this project study used an advanced surface model to identify flow quantity and direction as it moves through the urban area. Using these advanced modeling techniques, hydraulic analyses were completed for both existing and proposed conditions using a linked 2-dimensional surface model, and 1-dimensional subsurface model (1D/2D) in XPSWMM. The existing City storm drains were added to a 3 - dimensional surface terrain model to understand the level of flooding and to help identify what potential improvements could be implemented when extreme flooding (0.2-percent annual chance event) occurs. 4.1.1 Topography The topographic data used in this study was acquired from the Federal Emergency Management Agency. The topography was flown over a period of 25 days in February 2011. City of La Quinta Focused Area Drainage Study January 2016 4-20 Michael Baker International 4.1.2 Vertical Datum The study was performed using the North American Vertical Datum of 1988 (NAVD88). Many of the City as-built plans were based on NGVD29 and facility elevations had to be converted to NAVD88. The conversion used for this study was 2.24’. 4.1.3 1-D Model Development The existing storm drain systems were developed and modeled in XPSWMM as 1-dimensional (1D) elements. XPSWMM has the ability to create separate, yet linked, models: A surface model based on 2-dimentional topographic grid; and a subsurface model, based on a link-node 1- dimensional geometry. XPSWMM links and runs both models simultaneously. The geometry of the subsurface models were obtained from As-Built drawings and supplemented with field inspections. 4.1.4 Manning’s “n” Value The Manning’s value is a surface roughness coefficient used in hydraulic calculations. This value resembles the amount of resistance runoff will have as it flows through varying terrain. The Manning’s value generally ranges between 0.01 to 0.20. The smaller the value, the smoother the surface. A varying manning’s value was used to represent different land uses within each model. The manning’s designations were as follows: 0.035 – Open Space 0.015 – Residential 0.013 - Commercial 5.000 – buildings/obstructions To represent buildings in the models, a much higher than typical value was used (5.0). This was done to demark the buildings as “no-flow” zones within the surface 2D model. Manning’s values used in the subsurface, or 1D model, included the following: 0.013 – Reinforced Concrete Pipe 0.014 – Reinforced Concrete Boxes 4.1.5 Grid Size The grid cell resolution is an important consideration in two-dimensional modeling. Small grid cell sizes increase accuracy, but require additional computation times; while larger grid sizes compromise accuracy but decrease computation time. The determination of grid size requires a trade-off to ensure a workable model without compromising satisfactory accuracy. Multiple cell sizes can be specified within one model, allowing a larger grid size to be used in areas were high detail is not required and a smaller grid size to be used in primary areas of interest. Several scenarios were evaluated in this study and it was determined that for the South model, a 25-foot grid cell produced accurate models with reasonable simulation time and a 15- foot grid cell provided the detail required in sensitive urban areas with very flat grades. The Lake La Quinta Model required higher resolution, so a 15-foot grid cell was used for less sensitive areas, and 5-foot grid cells were used in the areas of interest. The north model utilized a 15-foot grid cell throughout the entire model. City of La Quinta Focused Area Drainage Study January 2016 4-21 Michael Baker International 4.1.6 Computational Time Step The computational time step is a critical variable in 2D modeling. At each time increment, the software computes a flow depth and velocity at each cell, as well as each cell boundary, and assigns flow direction accordingly, resulting in a new computation at the subsequent time step. Grid size is directly proportion to the computational time step. A time-step of 1 second was used for 15-foot grid cells, 2.5 seconds for 25-foot grid cells, and 0.35 seconds for 5-foot grid cells. 4.2 Existing Conditions The existing condition flood routing analyses were performed to identify existing street and surface conveyance and storm drain capacities and to acquire a benchmark for our proposed analyses. The 0.2% annual chance storm was used to correlate the hydrology and hydraulic model results to existing photos from the September 8, 2014 storm event. For a 1-hour duration, this storm event was approximately a 0.2 percent chance annual storm event. Once the existing condition 0.2% annual chance event model results were correlated to the storm photos, it provided a level of confidence that the rainfall-runoff relationship of the models was performing adequately. The 1% annual chance models were then calculated to develop a basis for the evaluation and development of potential drainage improvements. For the 1% annual chance events, multiple durations were run to identify which duration produced the maximum flooded depths. The results of the analysis indicated that the 6-hour storm duration produced the worst case conditions for each of the models. 4.2.1 Downstream Water Surface Control Elevations Most of the areas drain into regional storm drains that make their way to the Evacuation Channel. The depth of flow estimated in the channel can greatly impact the hydraulic performances of the local facilities connecting to them. The Evacuation Channel is downstream of Bear Creek and the Oleander Basin. Both systems were included in all the models to accurately model their hydraulic impacts on the rest of the local systems. For the 1-hour duration, it was assumed that Bear Creek Basin was not releasing flows into Bear Creek, based on eyewitness observations during the September 2014 event. For the 3 and 6-hour storm events, the full Bear Creek basin outlet hydrograph was added to the hydrology at the top of Bear Creek. This hydrograph was derived from the as-built plans and supporting calculations. Based on discussions with CVWD, during the September 8th storm event, no release from the Bear Creek Basin was observed. For this reason, the shorter duration storm events did not include the basin outlet hydrograph, and all the flows in the Evacuation Channels were from the Oleander basin and the local drainage pipes. For the Lake La Quinta model, the downstream control was located in the lake itself. Based on the lake as-built plans, a water surface elevation was used for the hydraulic models. This elevation was 55.0 feet, converted (NAVD88). The North Model had two downstream water surface control elevations, 1) the lake at the La Quinta Golf Estates, and the Evacuation lake at the Haciendas. As-built plans for the La Quinta City of La Quinta Focused Area Drainage Study January 2016 4-22 Michael Baker International Golf Estates could not be obtained, so a water surface of 42.0 feet (NAVD88) was used based off of LiDAR data. The water surface elevation for the lake at the Haciendas was set at 37.2 feet (NAVD88) based off of as-built plans The South Model had two main downstream controls, the Evacuation Channel and the open channel on the south side of Avenue 52 east of Silverrock Way. For the Evacuation Channel, Mission Drive West within the Rancho La Quinta homeowner’s association forms the downstream control, since it sits at approximately 6 feet above the invert of the Evacuation Channel. In order for flows to leave the model, water must pond up above the road and flow over into Rancho La Quinta. This creates a backwater effect within the channel. The storm drain line within Calle Tampico drains into the open channel near Silverrock Way. This was assumed to be an open outfall condition with no backwater effects. 4.2.2 Existing Condition Results The baseline existing condition models were developed and ran for the 0.2 percent annual chance event. The maximum water surface elevations for the event were plotted on an aerial of the focused study area. Four (4) flood event photographs were obtained from the City for different locations within the study area. The photographs that were chosen represent visual documentation at or near the peak flooding during the storm event. The 4 locations include: 1. Eisenhower Drive south of Avenue 50 2. Eisenhower Drive at Coachella Drive 3. Calle Tampico at Washington Street (City Hall) 1. Washington Street at Lake La Quinta Drive The peak flood depths on Eisenhower Drive south of Avenue 50 shows cars with water ponded to near the roof of a typical sedan vehicle. This is generally about 4-feet deep. The results of the flood modeling indicate maximum depths during the 0.2 percent annual chance event at approximately 4-feet deep. The Eisenhower Drive bend location at Coachella Drive shows flood depths of approximately 3-feet deep, which is consistent with the provided photograph. Along Calle Tampico at the City Hall, the storm photograph shows ponding to just below the front hood of a typical sedan. The flood model results at this location indicate anticipated flooding of about 2-feet, which is consistent with the observed conditions. The photograph along Washington Street near the Lake La Quinta Drive entrance indicated ponding depths that exceed the curb and gutter and the raised median area. The results of the flood routing analysis are consistent with the documented condition and include flooding over the raised median and maximum flood depths of up to 4-feet near the street right-of-way. A comparison of the observed flood and model results are illustrated on Figures 4-1 through 4-4. City of La Quinta Focused Area Drainage Study January 2016 4-23 Michael Baker International The full model results from the existing condition analyses are shown on the following exhibits:  Exhibit No. 1: North Area – 1% Annual Chance 6-Hour Storm Event  Exhibit No. 2: North Area – 0.2% Annual Chance Storm Event, 1-hour duration  Exhibit No. 3: South Area – 1% Annual Chance 6 Hour Storm Event  Exhibit No. 4: South Area – 0.2% Annual Chance Storm Event, 1-hour duration  Exhibit No. 5: Lake La Quinta Area – 1% Annual Chance 6-Hour Storm Event  Exhibit No. 6: Lake La Quinta Area – 0.2% Annual Chance Storm Event (1-hour duration) The exhibits illustrate the maximum flood depth over the duration of the storm event. Based on the results of the analysis, it was determined that the 6-hour duration provided the worst case flooding during the 1% annual chance storm event. This scenario, however, shows the public storm drain system is operating in accordance with Engineering Bulletin #6-16, Municipal Code 8.11.030, and the Riverside County Hydrology Manual. Therefore, only the 0.2% annual chance/1-hour storm event was used to develop a set of drainage alternatives to reduce the flood hazard in the study area to provide a higher level of flood protection than the City’s standard policy City of La Quinta Focused Area Drainage Study January 2016 4-24 Michael Baker International This page intentionally left blank. 12/03/15 JN 146281-21421 MAS Figure 4-2 Draft Eisenhower & Avenue 50 0.2% Annual Chance Comparison LA QUINTA FOCUSED DRAINAGE STUDY015075300 feet APPROXIMATE 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 LEGEND Photograph Location and Direction Existing Storm Drain Water Depth (feet) 12/03/15 JN 146281-21421 MAS Figure 4-3 Draft Calle Tampico 0.2% Annual Chance Comparison LA QUINTA FOCUSED DRAINAGE STUDY015075300 feet APPROXIMATE 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 LEGEND Photograph Location and Direction Existing Storm Drain Water Depth (feet) 12/03/15 JN 146281-21421 MAS Figure 4-4 Draft Eisenhower Bend 0.2% Annual Chance Comparison LA QUINTA FOCUSED DRAINAGE STUDY015075300 feet APPROXIMATE 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 LEGEND Photograph Location and Direction Existing Storm Drain Water Depth (feet) 12/03/15 JN 146281-21421 MAS Figure 4-5 Draft Lake La Quinta 0.2% Annual Chance Comparison LA QUINTA FOCUSED DRAINAGE STUDY015075300 feet APPROXIMATE 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 LEGEND Photograph Location and Direction Existing Storm Drain Water Depth (feet) City of La Quinta Focused Area Drainage Study January 2016 4-29 Michael Baker International 4.3 Proposed Conditions Using the design guidelines identified in Section 2.2.1, drainage improvement concepts were identified to alleviate flooding from an extreme 0.2% annual chance event. Preliminary alignments and sizes for conceptual improvement alternatives were estimated, and the concepts incorporated into the baseline exiting condition models. The models were re-run with the proposed improvements and the results were evaluated to determine the benefits of the conceptual alternative. Facility sizes and alignments were refined and re-run as necessary to optimize the flood hazard reduction associated with each alternative. The performance for each of the potential alternatives was tested for the 0.2% annual chance storm event. The results of the proposed condition analyses and a discussion of the conceptual alternatives identified for each sub-areas are discussed in Section 5. 4.4 Cost Estimates Cost estimates were created for each of the conceptual alternatives. The unit prices were developed in cooperation with the City and current market values. The calculated system costs estimates include costs for engineering, construction, SWPPP, surveying, construction management and contingencies. Any new storm drain construction within the City most likely will require utility relocation. This can be very costly especially considering the area is highly urbanized. The quantity and complexity of utility relocation is unknown and requires detailed site specific subsurface investigations. Pipe and box culvert costs are per linear foot and included costs for excavation, shoring, bedding, backfill, compaction, removal of excess material, and trench resurfacing. A unit price of $7 per inch of diameter was used for pipe sizes up to 36-inches in diameter. A unit price of $6 per inch of diameter was used for pipe sizes greater than 36-inches in diameter. Reinforced concrete box culvert facility costs were based on a unit price of concrete at $800 per cubic yard. Based on the individual size of the RCB, a volume of concrete was calculated in cubic yards per linear foot of culvert, and converted into a cost per foot. Due to the fact that construction will take place over a number of years, the total cost of drainage improvements will vary from the numbers provided in this study. It is recommended that any future implementation plans take into account future construction unit costs prior to creating a funding program for the proposed improvements. The Engineering Construction Cost Index is 10092 as of November 2015. City of La Quinta Focused Area Drainage Study January 2016 5-30 Michael Baker International 5 Drainage Area Results 5.1 North Drainage Area The north area generally extends from 48th Avenue southerly to the Evacuation Channel, and receives runoff from numerous hillslope areas north and west of the model area. The runoff from the hillslope areas is generally collected in local drainage systems constructed with the adjacent residential developments. Those developments include storm drain pipes and detention basin systems to convey and mitigate storm water runoff. Overflow from the developments is conveyed via surface flows to Eisenhower Drive. Runoff that is conveyed to Eisenhower Drive generally flows to the south towards an existing sump condition south of Avenue 50. The drainage systems in Eisenhower Drive and at the sump condition south of Avenue 50 meet city standards but improvements may be needed to convey the overflow during extreme storm events that is ultimately tributary to this area. 5.1.1 Proposed Alternatives The development of conceptual alternatives for this area focused on improving the drainage systems along Eisenhower Drive, and identifying opportunities for additional storm water detention basins to handle the 0.2-percent extreme storm water runoff volumes. Three (3) alternatives were identified for the north drainage area. The alternatives all include new storm drain systems in Eisenhower Drive, and the use of new or existing storm water detention areas. Alternative No. 1. This alternative includes a new storm drain culvert from Santa Ursula Street to south of Coachella Drive. A low height box culvert is proposed for this system due to the limited drainage facility slopes. A large storm water detention basin is proposed in the vacant parcel south of Coachella Drive and west of Eisenhower Drive. Outflow from the new basin would then be conveyed to an existing depression area on the La Quinta Country Club golf course driving range north of Avenue 50. A new storm drain system would also be included at the Avenue 50/Eisenhower Drive intersection to convey runoff to the proposed driving range detention basin. This alternative would provide the highest level of flood protection for the area, improving the area to a 0.2 percent annual chance level of protection for the majority of the study area. The proposed alternative is illustrated on Figure 5-1. Alternative No. 2. This alternative is similar to Alternative 1, except the new detention basin south of Coachella Drive is eliminated. The storm drain system in Eisenhower Drive would be extend south in Eisenhower Drive to proposed storage areas on the La Quinta Country Club golf course and driving range. This concept would provide 0.2 percent annual chance protection for most areas. Residual ponding at the sump condition on Eisenhower Drive south of Avenue 50 would remain during the 0.2 percent event, but there would still be adequate passage for emergency vehicles. The proposed alternative is illustrated on Figure 5-2. Alternative No. 3. Alternative 3 functions similar to Alternative 2, but with different storm drain alignments. Instead of extending the storm drain in Eisenhower Drive south of Coachella Drive, the system is routed through the existing residential development to the La Quinta Country Club storage areas. It also includes a small storm water detention basin in the vacant parcel south of Coachella Drive in an effort to eliminate ponding in the sump south of Avenue 50. The basin City of La Quinta Focused Area Drainage Study January 2016 5-31 Michael Baker International reduces the flooding at the sump location, but may still result in flooding depths from 1 to 2 feet during the 0.2 percent annual chance storm event. The proposed alternative is illustrated on Figure 5-3. 5.1.2 Cost Estimates Preliminary construction cost estimates were prepared for each of the alternatives. Alternative 1 provided the highest level of flood protection and had the highest cost. Alternative 2 was the lowest cost alternative. The detailed cost estimates are included in Table Nos. 5-1 through 5-3. Table 5-1: North Drainage Area – Alternative 1 Cost Estimate Item No. Item Description PROJECT TOTAL Unit of Measure Estimated Quantities Unit Price Item Total 1 Install 2'Hx4'W RCB FT 2736 $231 $632,016 2 Install 2'Hx8'W RCB FT 800 $400 $320,000 3 Install 2'Hx7'W RCB FT 1361 $384 $522,624 4 Install 2'Hx2'W RCB FT 1470 $128 $188,160 5 Basin Land Acquisition SF 261360 $14 $3,659,040 6 Basin Excavation and Grading CY 37760 $5 $188,800 7 Manhole EA 10 $4,700 $47,000 8 Catch Basin EA 19 $5,000 $95,000 9 Prepare Storm Water Pollution Prevention Plan LS 1 $5,000 $5,000 10 Miscellaneous Items (10%) LS 1 $565,800 $565,800 11 Mobilization/Bonding/Traffic Control (10%) LS 1 $622,400 $622,400 SUBTOTAL (CONSTRUCTION) $6,845,840 12 Administration (5%) LS 1 $342,300 $342,300 13 Engineering (10%) LS 1 $684,600 $684,600 14 Inspection (9.75%) LS 1 $667,500 $667,500 15 Professional (7.25%) LS 1 $496,400 $496,400 16 Relocate Utilities (2%) LS 1 $137,000 $137,000 SUBTOTAL (ENGINEERING AND CONSTRUCTION ADMINISTRATION) $2,327,800 SUBTOTAL COST $9,173,640 CONTINGENCY 20% $1,834,728 TOTAL PROJECT $11,008,000 City of La Quinta Focused Area Drainage Study January 2016 5-32 Michael Baker International Table 5-2: North Drainage Area – Alternative 2 Cost Estimate Item No. Item Description PROJECT TOTAL Unit of Measure Estimated Quantities Unit Price Item Total 1 Install 2'Hx4'W RCB FT 2221 $231 $513,051 2 Install 2'Hx8'W RCB FT 640 $400 $256,000 3 Install (2) 2'Hx6'W RCB FT 1620 $604 $978,480 4 Install 2'Hx2'W RCB FT 1026 $128 $131,328 5 Install 72" RCP FT 430 $432 $185,760 6 Manhole EA 2 $4,700 $9,400 7 Catch Basin EA 16 $5,000 $80,000 8 Junction Structure EA 6 $2,000 $12,000 9 Prepare Storm Water Pollution Prevention Plan LS 1 $5,000 $5,000 10 Miscellaneous Items (10%) LS 1 $217,200 $217,200 11 Mobilization/Bonding/Traffic Control (10%) LS 1 $238,900 $238,900 SUBTOTAL (CONSTRUCTION) $2,627,119 12 Administration (5%) LS 1 $131,400 $131,400 13 Engineering (10%) LS 1 $262,800 $262,800 14 Inspection (9.75%) LS 1 $256,200 $256,200 15 Professional (7.25%) LS 1 $190,500 $190,500 16 Relocate Utilities (2%) LS 1 $52,600 $52,600 SUBTOTAL (ENGINEERING AND CONSTRUCTION ADMINISTRATION) $893,500 SUBTOTAL COST $3,520,619 CONTINGENCY 20% $704,124 TOTAL PROJECT $4,225,000 City of La Quinta Focused Area Drainage Study January 2016 5-33 Michael Baker International Table 5-3: North Drainage Area – Alternative 3 Cost Estimate Item No. Item Description PROJECT TOTAL Unit of Measure Estimated Quantities Unit Price Item Total 1 Install 2'Hx4'W RCB FT 2311 $231 $533,841 2 Install 2'Hx8'W RCB FT 404 $400 $161,600 3 Install (2) 2'Hx6'W RCB FT 2200 $604 $1,328,800 4 Install 2'Hx2'W RCB FT 950 $128 $121,600 5 Install 18" RCP FT 280 $126 $35,280 6 Install 24" RCP FT 900 $168 $151,200 7 Install 36" RCP FT 230 $252 $57,960 8 Install 42" RCP FT 500 $252 $126,000 9 Basin Land Acquisition SF 43560 $14 $609,840 10 Basin Excavation and Grading CY 12906 $5 $64,530 11 Manhole EA 9 $4,700 $42,300 12 Catch Basin EA 20 $5,000 $100,000 13 Prepare Storm Water Pollution Prevention Plan LS 1 $5,000 $5,000 14 Miscellaneous Items (10%) LS 1 $333,800 $333,800 15 Mobilization/Bonding/Traffic Control (10%) LS 1 $367,200 $367,200 SUBTOTAL (CONSTRUCTION) $4,038,951 16 Administration (5%) LS 1 $202,000 $202,000 17 Engineering (10%) LS 1 $403,900 $403,900 18 Inspection (9.75%) LS 1 $393,800 $393,800 19 Professional (7.25%) LS 1 $292,900 $292,900 20 Relocate Utilities (2%) LS 1 $80,800 $80,800 SUBTOTAL (ENGINEERING AND CONSTRUCTION ADMINISTRATION) $1,373,400 SUBTOTAL COST $5,412,351 CONTINGENCY 20% $1,082,470 TOTAL PROJECT $6,495,000 Line NA1-3 Line NA1-4 Basin NA1-16 Acres7 Feet Deep 2'x4' RCB 2'x7' RCB2'x8' RCB2'x2' RCB 2'x4' RCB 2' x 4 ' R C B LA QUINTA FOCUSED DRAINAGE STUDYNorth Alternative 1 Proposed Facilities Figure 5-1°0 300 600150Feet12/7/2015 JN H:\pdata\146281\Admin\GIS\mxd\Proposed\Atlas Maps\NA1_Atlas.mxd <USER NAME>Source: Legend Proposed Catch Basin Proposed Storm Drain Proposed Basin Existing Storm Drain Parcel Boundary City BorderKEY MAP Line NA1-1 Line NA1-2 2'x4' RCB(2) 2'x6' RCB72" RCP2'x8' RCB2'x4' RCB(2) 2'x6' RCBLA QUINTA FOCUSED DRAINAGE STUDYNorth Alternative 2 Proposed Facilities Figure 5-2°0 300 600150Feet12/7/2015 JN H:\pdata\146281\Admin\GIS\mxd\Proposed\Atlas Maps\NA2_Atlas.mxd <USER NAME>Source: Legend Proposed Catch Basin Proposed Storm Drain Existing Storm Drain Parcel Boundary City BorderKEY MAP Line NA2-1 Line NA2-2 2'x4' RCB(2) 2'x 6 ' R C B 24" RCP4 2 " R C P 2'x4' RCB LA QUINTA FOCUSED DRAINAGE STUDYNorth Alternative 3 Proposed Facilities Figure 5-3°0 300 600150Feet12/7/2015 JN H:\pdata\146281\Admin\GIS\mxd\Proposed\Atlas Maps\NA3_Atlas.mxd <USER NAME>Source: Legend Proposed Catch Basin Proposed Storm Drain Existing Storm Drain Proposed Basin Parcel Boundary City BorderKEY MAP Line NA3-1 Line NA3-2 Line NA3-32'x4' RCB Line NA3-4 Basin NA3-11 Acre8 Feet Deep City of La Quinta Focused Area Drainage Study January 2016 5-37 Michael Baker International 5.2 South Drainage Area The south drainage area included the area south of the Evacuation Channel to approximately the Calle Sinaloa/Avenue 52 intersection, and extended east to Calle Rondo. This area includes a large offsite drainage tributary, which includes the Bear Creek watershed and channel system. The Bear Creek watershed discharges to an existing channel and detention basin system along the west side of the Cove which drains to the Oleander Basin and eventually to the Evacuation Channel. The primary drainage in this study area is the existing storm drain system in Calle Tampico. The main drainage system starts in the Cove on Avenida Montezuma and proceeds east along Calle Tampico towards City Hall. The system turns south, west of City Hall to Avenida La Fonda, then east to Calle Rondo and the existing channel system along the east side of the roadway. The results of the flood routing analysis indicated flooding during large storm events (such as a 0.2% annual chance event) at the cul-de-sacs on the north side of the Cove, along Calle Tampico, and at the intersection of Washington Street and Avenue 50. Each of these areas is serviced by a storm drain system that would need improvements to accomodate the additional runoff from extreme storm events. 5.2.1 Proposed Improvements The proposed improvements focused on increasing the capacity of the Calle Tampico and Washington/Avenue 50 storm drain systems to convey runoff from the larger storm events. The proposed systems functioned to meet current City standards for the 1 percent annual chance event. The proposed improvements were designed to increase the level of flood protection along the major arterial streets and critical facilities. Various sections of the Calle Tampico storm drain are proposed to be upsized. The improvements will increase the system capacity at critical locations resulting in higher overall system efficiency. The proposed improvements will generally parallel the existing system, with a new system along Calle Tampico at Washington Street. A parallel system was proposed to utilize the existing system capacity and to minimize improvement costs. Improvements to the Avenue 50/Washington Street system also include a proposed parallel facility. The proposed facilities increase the main system capacity and include secondary overflow inlets at the existing catch basins to prevent flooding as a result of sediment blockage at the inlets. The alternative is illustrated on Figures 5-4a through c. 5.2.2 Cost Estimate A preliminary construction cost estimate was prepared for each element of the alternative. The detailed cost estimate is included in Table No. 5-4. City of La Quinta Focused Area Drainage Study January 2016 5-38 Michael Baker International Table 5-4: South Drainage Area – Alternative 1 Cost Estimate Item No. Item Description PROJECT TOTAL Unit of Measure Estimated Quantities Unit Price Item Total 1 Install 36" RCP FT 250 $252 $63,000.00 2 Install 39" RCP FT 780 $234 $182,520.00 3 Install 54" RCP FT 212 $324 $68,688.00 4 Install 2'Hx6'W RCB FT 1700 $306 $520,200.00 5 Install 3'HX3'W RCB FT 741 $238 $176,358.00 6 Install 3'HX4'W RCB FT 687 $272 $186,864.00 7 Install 54" RCP FT 1662 $324 $538,488.00 8 Install 2.5'Hx6'W RCB FT 714 $370 $264,180.00 9 Install 3'HX6'W RCB FT 684 $388 $265,392.00 10 Install 1'Hx2'W RCB FT 442 $133 $58,786.00 11 Install 1'Hx4'W RCB FT 421 $202 $85,042.00 12 Install 1'Hx5'W RCB FT 791 $236 $186,676.00 13 Remove 18" RCP FT 1917 $19 $36,231.30 14 Remove 12" RCP FT 421 $13 $5,304.60 15 Manhole EA 7 $4,700 $32,900.00 16 Catch Basin EA 18 $5,000 $90,000.00 17 Prepare Storm Water Pollution Prevention Plan LS 1 $5,000 $5,000.00 18 Miscellaneous Items (10%) LS 1 $276,600 $276,600.00 19 Mobilization/Bonding/Traffic Control (10%) LS 1 $304,300 $304,300.00 SUBTOTAL (CONSTRUCTION) $3,346,530 20 Administration (5%) LS 1 $167,400 $167,400 21 Engineering (10%) LS 1 $334,700 $334,700 22 Inspection (9.75%) LS 1 $326,300 $326,300 23 Professional (7.25%) LS 1 $242,700 $242,700 24 Relocate Utilities (2%) LS 1 $67,000 $67,000 SUBTOTAL (ENGINEERING AND CONSTRUCTION ADMINISTRATION) $1,138,100 SUBTOTAL COST $4,484,630 CONTINGENCY 20% $896,926 TOTAL PROJECT $5,382,000 (1) 39" RCP Existing(2) 39" RCP Proposed (2) 2'x6' RCB Existing(3) 2'x6' RCB Proposed (1) 54" RCP Existing(2) 54" RCP Proposed(1) 36" RCP Existing(2) 36" RCP Proposed LA QUINTA FOCUSED DRAINAGE STUDYSouth Alternative 1 Proposed Facilities Figure 5-4a°0 250 500125Feet12/7/2015 JN H:\pdata\146281\Admin\GIS\mxd\Proposed\Atlas Maps\SA1-1_Atlas.mxd <USER NAME>Source: Legend Proposed Catch Basin Proposed Storm Drain Existing Storm Drain Parcel Boundary City BorderKEY MAP Line SA1-1 Line SA1-2 (1) 54" RCP Existing(2) 54" RCP Proposed18" RCP Existing3'x6' RCB Proposed2.5'x6' RCB (2) 3'x4' RCB Existing(3) 3'x4' RCB Proposed 2'x6' RCB (2) 3'x3' RCB Existing(4) 3'x3' RCB ProposedLA QUINTA FOCUSED DRAINAGE STUDYSouth Alternative 1 Proposed Facilities Figure 5-4b°0 250 500125Feet12/7/2015 JN H:\pdata\146281\Admin\GIS\mxd\Proposed\Atlas Maps\SA1-2_Atlas.mxd <USER NAME>Source: Legend Proposed Catch Basin Proposed Storm Drain Existing Storm Drain Parcel Boundary City BorderKEY MAP Line SA1-3 Line SA1-4 18" RCP Existing 1'x5' RCB Proposed 18" RCP Existing1'x2' RCB Proposed 12" RCP Existing1'x4' RCB Proposed LA QUINTA FOCUSED DRAINAGE STUDYSouth Alternative 1 Proposed Facilities Figure 5-4c°0 250 500125Feet12/7/2015 JN H:\pdata\146281\Admin\GIS\mxd\Proposed\Atlas Maps\SA1-3_Atlas.mxd <USER NAME>Source: Legend Proposed Catch Basin Proposed Storm Drain Existing Storm Drain Parcel Boundary City BorderKEY MAP Line SA1-5 City of La Quinta Focused Area Drainage Study January 2016 5-42 Michael Baker International 5.3 Lake La Quinta Drainage Area The Lake La Quinta study area focuses on the watershed tributary to the lake area north of 48th Avenue and west of Adams Street. The majority of the storm runoff to this area comes from the hillslope areas to the west of the drainage area. The drainage systems in this area predominately convey runoff directly to the lake or a few small storm water detention basins. The flood routing model results show some areas of ponding along Washington Street just north of 48th Avenue and near Lake La Quinta Drive. 5.3.1 Proposed Improvements The proposed improvements for this area focused on eliminating the flooded areas along Washington Street. A new storm drain is proposed at the Lake La Quinta Drive and Washington Avenue intersection to drain directly to the lake. A separate st orm drain line is proposed along Washington Street north of 48th Avenue to convey the roadway drainage to an existing detention basin along the west side of the road. The proposed improvements are illustrated on Figure 5-5. 5.3.2 Cost Estimate A preliminary construction cost estimate was prepared for each element of the alternative. The detailed cost estimate is included in Table No. 5-5. Table 5-5: Lake La Quinta Drainage Area – Alternative 1 Cost Estimate Item No. Item Description PROJECT TOTAL Unit of Measure Estimated Quantities Unit Price Item Total 1 Install 18" RCP FT 475 $126 $59,850 2 Install 2'Hx6'W RCB FT 452 $306 $138,312 3 Install 3'Hx6'W RCB FT 110 $388 $42,680 4 Catch Basin EA 5 $5,000 $25,000 5 Dry Well EA 5 $3,000 $15,000 6 BMP-CDS unit EA 1 $35,000 $35,000 7 Prepare Storm Water Pollution Prevention Plan LS 1 $10,000 $10,000 8 Miscellaneous Items (35%) LS 1 $114,100 $114,100 9 Mobilization/Bonding/Traffic Control (20%) LS 1 $88,000 $88,000 SUBTOTAL (CONSTRUCTION) $527,942 10 Administration (5%) LS 1 $26,400 $26,400 11 Engineering (20%) LS 1 $105,600 $105,600 12 Inspection (9.75%) LS 1 $51,500 $51,500 13 Professional (7.25%) LS 1 $38,300 $38,300 14 Relocate Utilities (10%) LS 1 $52,800 $52,800 SUBTOTAL (ENGINEERING AND CONSTRUCTION ADMINISTRATION) $274,600 SUBTOTAL COST $802,542 CONTINGENCY 20% $160,508 TOTAL PROJECT $963,000 18" RCP 2'x6' RCB LA QUINTA FOCUSED DRAINAGE STUDYLake La Quinta Proposed Facilities Figure 5-5°0 250 500125Feet12/22/2015 JN H:\pdata\146281\Admin\GIS\mxd\Proposed\Atlas Maps\LLQ2_Atlas.mxd <USER NAME>Source: Legend Proposed Catch Basin Proposed Storm Drain Existing Storm Drain Parcel Boundary City BorderKEY MAP Line LLQ-2-1 Line LLQ-2-2 City of La Quinta Focused Area Drainage Study January 2016 Michael Baker International This page intentionally left blank. City of La Quinta Focused Area Drainage Study January 2016 Michael Baker International Exhibits City of La Quinta Focused Area Drainage Study January 2016 Michael Baker International This page intentionally left blank. # # # # # ##5 #3 #2 #6 #4 #1 LA QUINTA FOCUSED DRAINAGE STUDY 100-YEAR (1% ANNUAL CHANCE) 6-HOUR NORTH MODEL Exhibit 1 °0 250 500125 Feet Source: Legend #Inflow Hydrograph Existing Storm Drain Model Boundary Water Depth (ft) 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 # # # # # ##5 #3 #2 #6 #4 #1 LA QUINTA FOCUSED DRAINAGE STUDY 500-YEAR (0.2% ANNUAL CHANCE) 1-HOUR NORTH MODEL Exhibit 2 °0 250 500125 Feet Source: Legend #Inflow Hydrograph Existing Storm Drain Model Boundary Water Depth (feet) 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 2/25/2016 JN H:\pdata\146281\Admin\GIS\mxd\NOAA Atlas 2\LQ6HRSouth_Atlas2.mxd <USER NAME> 2/25/2016 JN H:\pdata\146281\Admin\GIS\mxd\LQ500YRSouth_24x36.mxd <USER NAME> # # # Avenue 48 Avenue 47 £¤111 #14 #13 #12 LA QUINTA FOCUSED DRAINAGE STUDY 100-YEAR (1% ANNUAL CHANCE) 6-HOUR LAKE LA QUINTA MODEL Exhibit 5 °0 200 400100 Feet Source: Legend #Inflow Hydrograph Existing Storm Drain Water Depth (ft) 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 # # # Avenue 48 Avenue 47 £¤111 #14 #13 #12 LA QUINTA FOCUSED DRAINAGE STUDY 500-YEAR (0.2% ANNUAL CHANCE) 1-HOUR LAKE LA QUINTA MODEL - EXISTING CONDITION Exhibit 6 °0 200 400100 Feet Source: Legend #Inflow Hydrograph Existing Storm Drain Water Depth (feet) 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 # # # # # ##5 #3 #2 #6 #4 #1 LA QUINTA FOCUSED DRAINAGE STUDY 500-YEAR (0.2% ANUAL CHANCE) 1-HOUR MODEL - North Alternative 1 Exhibit 7 °0 250 500125 Feet Source: Legend #Inflow Hydrograph Proposed Storm Drain Existing Storm Drain Model Boundary Water Depth (ft) 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 # # # # # ##5 #3 #2 #6 #4 #1 LA QUINTA FOCUSED DRAINAGE STUDY 500-YEAR (0.2% ANNUAL CHANCE) 1-HOUR MODEL - North Alternative 2 Exhibit 8 °0 250 500125 Feet Source: Legend #Inflow Hydrograph Proposed Storm Drain Existing Storm Drain Model Boundary Water Depth (ft) 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 # # # # # ##5 #3 #2 #6 #4 #1 LA QUINTA FOCUSED DRAINAGE STUDY 500-YEAR (0.2% ANNUAL CHANCE) 1-HOUR MODEL - North Alternative 3 Exhibit 9 °0 250 500125 Feet Source: Legend #Inflow Hydrograph Proposed Storm Drain Existing Storm Drain Model Boundary Water Depth (ft) 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 2/25/2016 JN H:\pdata\146281\Admin\GIS\mxd\Proposed\LQ500YRSouthAlt1.mxd <USER NAME> # # # Avenue 48 Avenue 47 £¤111 #14 #13 #12 LA QUINTA FOCUSED DRAINAGE STUDY 500-YEAR (0.2% ANNUAL CHANCE) 1-HOUR LAKE LA QUINTA MODEL - ALTERNATIVE 1 Exhibit 11 °0 200 400100 Feet Source: Legend #Inflow Hydrograph Proposed Storm Drain Existing Storm Drain Water Depth (ft) 0.5-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10