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DR8902-SY 890310 GINN, INC. CONSUI,TING ENGINEERS March 10, 1989 Mr. Russell Doyle, P.E. City Engineer City of Coppell P.O. Box 478 Coppell, Texas 75019 Re: Storm Drainage Evaluation - Bethel School Road and Falcon Lane Intersection Project No. 280-241-055 and Storm Drainage Evaluation - Pelican Lane Project No. 280-241-056 Dear Mr. Doyle: Pursuant to your authorization, we have prepared and respectfully submit herewith our Preliminary Engineering Study and Report on the above referenced projects. Discussed in the report are: the existing conditions; engineering evaluations of the design conditions; preliminary analysis and evaluation of proposed design considerations; recommendations, with alternates, to help alleviate the problem; and cost estimates for proposed construction. It is our recommendation that the City initiate the necessary action which will result in the implementation of the report recommendations at the earliest date. Sincerely, H. Wayne Ginn, P.E. cc: John Karlsruher, P.E. Kevin Peiffer, P.E. Gabe Favre File 89405 17103 Preston Road · Suite 100 · LB 118 · Dallas. Texas 75248 · Phone214/248-4900 EXECUTIVE SUMI~RY STORM DRAINAGE EVALUATION OF BETHEL SCHOOL ROAD AND FALCON LANE INTERSECTION AND AT PELICAN LANE IN THE NORTHL~KE WOODLANDS EAST, PHASE VI SUBDIVISION. 1. Based upon a request and the approval of the city, an investigation was made, due to the reports of flooding during and after the July 12, 1988 storm at two locations in the Northlake Woodlands Bast, Phase VI subdivision. Specifically, those locations are 721, 725 & 726 E. Bethel School Road at Falcon Lane intersection and 763, 767 & 771 Pelican Lane. 2. Upon reviewing the development plans, it was found that the storm drainage system was designed in accordance with the standards and criteria that were in effect in the City of Coppell at the time of design. 3. It appears that some of the flooding problem may be attributable to a combination of unforeseeable circumstances, such as: inlet capacities, designed for 5-year storms, having to sustain storms of greater duration; provisions for positive overflow, between individual homes, being reduced; inadequate site grading and site drainage; and finished floor elevations of the homes in question not being high enough with respect to the street. 4. A recommendation to help alleviate some of the flooding problems involves a joint effort between the City and the homeowners. The City would be responsible for construction of the minimum recommendations in the report, while the homeowner would be responsible for the improvements to the site grading and implementation of removal of obstructions along the lot lines where positive overflow is essential. 5. The report outlines preliminary cost estimates for the construction of the recommended alternatives. For the above mentioned minimum recommendations, the following costs will apply: Bethel School Road / Falcon Lane Alternate No. 1 ....................... $ 22,263.00 Pelican Lane Alternate No. 1 ....................... $ 10,422.00 6. Prior to any City Council action on this matter, a recommendation is made that the City attorney be consulted. Respectfully submitted, H. Wayne Ginn, P.E. PRELIMINARY ENGINEERING STUDY AND REPORT FOR EVALUATION OF STORM DRAINAGE SYSTEM IN VICINITY OF INTERSECTION OF BETHEL SCHOOL ROAD AND FALCON LANE, NORTHLAKE WOODLANDS EAST, PHASE VI, DUE TO REPORTS OF FLOODING DURING AND AFTER THE JULY, 12, 1988 STORM AT 721, 725 AND 726 E. BETHEL SCHOOL ROAD. AND EVALUATION OF STORM DRAINAGE SYSTEM IN VICINITY OF PELICAN LANE, EAST OF MOCKINGBIRD LANE, NORTHLAKE WOODLANDS EAST, PHASE VI, AS A FOLLOW UP TO REPORTS OF FLOODING DURING AND AFTER THE JULY 12, 1988 STORM AT 763, 767, AND 771 PELICAN LANE. T&BLE OF CONTENTS I. GENERAL II. DATA COLLECTION/FIELD INVESTIGATION III. EXISTING CONDITIONS IV. OBJECTIVES V. METHODOLOGY VI. EVALUATION OF "AS-BUILT" CONDITIONS VII. PRELIMINARY DESIGN VIII. COST ESTIMATES IX. RECOMMENDATIONS X. SUMMARY XI. CONCLUSIONS XII. APPENDIX ~ENEI~L This Preliminary Engineering Study and Report will provide an evaluation of the storm drainage systems at two different locations in the Northlake Woodlands East, Phase VI subdivision of the City of Coppell. The first area of evaluation will be in the vicinity of the intersection of Bethel School Road and Falcon Lane (SITE 1), while the second area of concern is on Pelican Lane, west of Mockingbird Lane (SITE 2). During the July 12, 1988 storm, several homes in these two areas were flooded and damage to property occurred. These evaluations will report on the existing system's capacity to handle the storm water run-off, analysis and recommendations of suggested improvements, along with preliminary cost estimates for engineering and construction. II. DATA COLLECTION/FIELD INVESTIGATION The City of Coppell provided "as-built" development plans, which included street paving plans and profiles, water, sanitary, and storm sewer plans and profiles of the areas in question. Page 1 Individual lot grading or site plans were not available. The City's floodplain maps were utilized in the evaluations also. Additionally, a visual field investigation was made to verify the existing conditions and to attempt to determine possible causes for the flood damage. No field surveys of actual elevations were made nor were any measurements taken. III. EXISTING CONDITIONS SITE L - BETHEL SCHOOL ROAD / FALCON L~NE (See Appendix, Plates A, B, D, and F) Bethel School Road runs east and west from Moore Road to Mockingbird Lane. Falcon Lane makes a "tee" intersection with Bethel School Road from the north. From Pelican lane which is east of the intersection, Bethel School Road slopes at a 5% grade and as it approaches Falcon Lane, it is reduced to approximately a 1.07% slope, continuing to the inlets at station 17+00 with a 0.5% slope. Approximately 270' west of the inlets is a break point in grades with a beginning slope of 1.4%, and as it approaches the inlets it is reduced to 0.5%. Falcon Lane is shown to be constructed on a 5% slope. Page 2 The storm drainage system consists of two 10' standard curb inlets located along Falcon Lane and two 10' standard curb inlets located 70' west of the intersection in a sag point of Bethel School Road. All four inlets are connected to the main line by 18" diameter storm pipes. The storm drainage system consists of pipes ranging in size from 18" diameter to 30" diameter at the outfall. (See Plate D). The sag point inlets have a flowline elevation of 462.93 according to the plans. According to the maps in use at the time of construction of this project, Flood Insurance Rate Map (FIRM), Community Panel Number 480170 0005 C, dated February 15, 1984, as prepared by Federal Emergency Management Agency (FEMA),the 100 year water surface elevation at the outfall is approximate elevation 462.00. The areas, and their approximate sizes, contributing to the storm water run-off are shown on Plate F. Area 10A (0.9 acres), Area 13 (3.1 acres), Area 14 (3.2 acres), Area 15 (2.2 acres), Area 16 (3.4 acres). Total area amounts to approximately 12.8 acres. SITE 2 - PELICAN LANE (See Appendix, Plates A, C, E and G) Pelican Lane runs southeasterly from Bethel School Road and connects to Mockingbird at its east terminus. From its intersection with Bethel School Road, Pelican Lane begins with a 2.3% slope and eventually flattens out to a 0.5% slope as it Page 3 approaches the sag inlets. From Mockingbird, Pelican Lane is on a 5% slope to the curve in the road, then it flattens out to 0.5% as it approaches the sag inlets. There appears to be a discrepancy in the as-built plans as to the exact location of the sag inlets. The paving plans have the sag inlets located at station 7+48.64, (at approximate centerline of lot 19), while the storm drainage plans show the inlets to be located at station 7+09.09 (opposite line dividing lots 18 and 19). Also the paving plan indicates a flowline elevation of 466.79 for the inlets, while the storm drainage plan shows a flowline elevation of 463.67. The storm drainage system consists of two 10' standard curb inlets at the sag point. The east inlet is connected to the main line by an 18" diameter pipe. A 21" diameter pipe is shown to be from the inlet on the west. Where both inlets are tied to the main line, the pipe is 27" diameter, continuing to a 30" diameter pipe downstream. According to the FEMA map noted above, the 100- year water surface elevation at the outfall is shown to be at approximate elevation 460.00. The areas, and their approximate sizes, contributing to the storm water run-off is shown on Plate G. Area 17 (1.2 acres), Area 18 (2.7 acres), Area 19 (1.8 acres), Area 20 (1.9 acres), Area 21 (1.0 acres), Area 22 (1.0 acres), Area 23 (1.6 acres), and Area 24 (2.6 acres) for a cumulative area of approximately 13.8 acres. Page 4 IV. OBJECTIVES OF RESIDENTIAL STORM W~TER MANAGEMENT ~. B~SIC CONCEPTS Optimum design of storm water collection should strike a balance among capital costs, operation and maintenance costs, public convenience, risk of significant storm water-related damages, environmental protection and enhancement, and other community objectives. The optimum balance among these factors is dynamic, changing over time with changing physical conditions and value perceptions. A recognition that temporary ponding at various points in the system, including on the individual lot is a potential design solution rather than a problem in some situations. A continuing recognition that there is a balance of responsibilities and obligations for collection and storage of storm water to be shared by individual property owners and the City as a whole. A growing emphasis on the recognition that every site or situation presents a unique array of physical resources, requirements, conditions and environmental values. Page 5 Responsible solutions for individual developments in the absence of basin wide plans are more difficult to achieve particularly where current practices are based on traditional drainage concepts. B. DEFINITION OF STORM WATER RUN-OFF SYSTEM The components of a storm water run-off system consist of both natural and man-made elements, including not only those which contain and convey storm water, but also those which absorb, store and otherwise use storm water rather than dispose of it. The storm water run-off system is a single system having two purposes: (1) the control of storm water run-off to prevent or minimize damage to property and physical injury and loss of life which may occur during or after a very infrequent or unusual storm; and (2) the control of storm water to eliminate or minimize inconvenience or disruption of activity as a result of run-off from more frequently occurring, less significant storms. The City of Coppell drainage design criteria embraces this dual function approach. With a single system, there are components that are designed primarily to obtain convenience at the smallest scale of the system, at the individual site or intersection, during minor or Page 6 frequent storms. During an infrequent, or major storm, the capacities of many of the convenience-oriented components will be exceeded and flow capacity must be provided by other components designed to provide safety and minimize damage throughout the system, from the individual site to the discharge point of the drainage basin. It must be recognized and emphasized that a total storm water run-off system, subjected to an infrequent major storm, cannot be expected to prevent inconvenience and minor property damage. A design that would eliminate all such stress would be fundamentally unreasonable and almost certainly infeasible. C. OBJECTIVES Storm water run-off systems must fulfill two objectives: (1) they must prevent significant loss of life and property due to run-off from any foreseeable rainfall event; and (2) they must provide an acceptable degree of convenient access to property during and following frequent rainfall events. Both objectives must be accommodated in the design process with the understanding that some components of the system may have to operate only infrequently. It must also be understood that providing protection against a given event, e.g., against the worst storm of record, does not guarantee that a greater run-off event will not occur during the useful life of the property. Similarly, the Page 7 enlargement of storm water run-off system components providing access convenience is generally an infeasible approach to fulfillment of property protection objectives. V. METHODOLOGY The design of storm water systems can utilize various methodologies to arrive at solutions or alternatives for the drainage plans. The City of Coppell adheres to the criteria set forth in the "Design Manual for Storm Drainage Facilities", published by the City of Dallas. It appears that Technical Paper No. 25 was used to compute rainfall intensities for the development in question, since that was the criteria used by City of Dallas at that time. Analysis should be made considering the following factors: rainfall, drainage area characteristics, land use characteristics, design options, risk analysis and costs. The design technique currently used by the City of Coppell is based upon the Rational Method for rainfall run-off computation. Although the primary purpose of a residential street is to provide vehicular access to homes and community facilities, a secondary function is to collect and convey storm water run-off. In utilizing streets in this manner, careful consideration should Page 8 be given to the following: street slopes in relation to storm water capacity and velocity in gutters; location and sizing of inlets; location and sizing of culverts; the spread of water across streets; the flow of water across intersections; and coordination of street grades with lot drainage. The peak run-off flow contributed to a gutter is computed by the Rational Method. Inlets are usually sized so that a portion of the flow is bypassed; the actual flow in the next reach of gutter includes this by-pass flow. The low point or sag inlet catches the remaining flow from both directions and must be sized accordingly. Locations and required capacities of inlets are established by computing estimated flow rates, depth and velocity of flow, and spread across streets. The inlets are located as necessary to remove storm run-off flow based upon a five-year storm frequency. Under conditions prevailing during a major storm, the storm drainage system will be surcharged and the rest of the flow will be carried by lawns, streets, etc.. Inlet capacity in this case is hard to ascertain, but is probably somewhat less than the inlet would handle under sag conditions because of debris blockage and surcharge back pressures. It is probably safe to assume that the flow in the street would be the difference between the total run-off and the capacity of the storm water surcharged to the level of the gutter. Page 9 The most critical situation exists at a street intersection. Inlets should be provided which will intercept virtually all the flow from a minor storm before it enters the intersection. Full interception may be impossible for a major storm. Some ponding of water at sag points in the grade is inevitable, even during a minor storm; however, due to obvious hazards, ponding should be minimized. This is done best by intercepting most of the flow before it gets to the sag point. A curb opening inlet located at the sag point should be of ample size, and when in combination with positive overflow, be able to handle anticipated run-off for a major storm. The capacity of a curb inlet to intercept water flowing down a street depends to a large degree on the distribution and velocity of water in the gutter. On a continuous grade, an inlet will accept only that flow within its hydraulic reach. The width, length, and depth of the depressed section of the gutter in front of the opening is very important. VI. EVALUATION OF "AS-BUILT" CONDITIONS Considering the objectives and methodology outlined above, the following is an evaluation of the two sites under the existing conditions. Page 10 SITE 1 - Bethel School Road / Falcon Lane Upon reviewing the drainage area map, the drainage design data appears to be for a 100-year storm. Values included were C=0.45, I=7.52 in/hr., and time of concentration was 15 minutes. The 100-year intensity used is based upon TP-25. The intensity used for sizing of inlets is 4.5 in/hr. This correlates to a five-year storm frequency. In sizing inlets along Falcon Lane, design requirements dictate a minimum of 2 foot length of inlet opening for each CFS of gutter flow for street grades greater than 3.5% slope. Assuming the flows calculated are to be equally divided on each side of the street, then the inlet sizes are adequate to handle a five-year storm. Based upon visual inspection of the flows down Falcon Lane, the alley immediately north of Bethel School Road contributes a good portion of the flow, which when a major storm occurs, sheet flows across the street spreading to the west side of Falcon Lane, thereby contributing to the inlet downstream. Assuming the inlets on Falcon Lane can handle only 5 cfs each, with some flows bypassing to a downstream inlet, and the other contributing flows to the sag points, it appears the sag inlets are properly sized. By adding the flow from the east, the bypass Page 11 from Falcon Lane inlets, the flow from the west, and the bypass of the alley inlet west of the sag point, a total of 9.2 cfs is calculated to reach the sag point. Based upon sizing of 0.6 foot for each CFS of flow, it would require a 5.5' inlet. A 10' inlet was constructed on each side of the street. A quick check of the underground pipe system, assuming a 100-year storm design, indicates the sizes are adequate for the indicated flows. In the plans that we reviewed there appears to be adequate provisions for positive overflow should localized flooding occur. Looking at the completed construction, it appears that some of the provisions for positive overflow were reduced. FHA minimum property standards advises a minimum of 8" from the floor slab to the ground at the face of the structure and a minimum 2% slope away from the structure to the curb. For a 30' setback, the finished floor elevation should be approximately 1.25' higher than the curb. It appears that the finished floor elevations of the homes in question may be less than the minimum height above the curb elevation. An actual field survey would have to be made to determine the exact finished floor elevations, relative to curb elevation. Page 12 SITE 2 -Pelican Lane This site is similar to Site 1 above, except that the problem lies mainly with the sag point inlets capacity. Using the flows calculated, and using 0.6 foot length of inlet opening for each CFS of gutter flow, it appears the sag inlets are sized correctly to handle the five-year storm, but provisions for positive overflow appear to have been neglected during the home building phase. Due to the steep slopes of the pipes in this section of the system, capacities appear to be adequate, even though velocities are quite high. Again, adequate provisions for positive overflow should have been maintained by the developer/home builder. Consideration should have been given to higher finished floor elevations with respect to adjacent curb elevations. VII. PRELIMINARY DESIGN Using the latest design criteria as determined by the City of Coppell, (TP-40, 100-year storm frequency, 10-year design on the inlets), trying to minimize disruption to home owners in the area and allowing for the most economical solution, we propose the following alternates to attempt to help alleviate some of the flooding problems. Page 13 SITE i - Bethel School Road / Falcon Lane ALTERNATE NO. 1: In order to help intercept more of the surface run-off: 1. Add 10' inlets adjacent to existing inlets along Falcon Lane. One on the east side and one on the west side. 2. Add 2 - 10' inlets along north side of Bethel School Road adjacent to the exist 10' inlets. One on each side. 3. Add 2 - 5' inlets along south side of Bethel School Road adjacent to the existing 10' inlets. One each side. 4. Make provisions, as required, along lot lines between houses for positive over-flow to occur by constructing a more defined drainage swale to carry the positive overflow run-off. ALTERNATE NO. 2: In addition to items listed above in Alternate No. 1, these items may further reduce the run-off over-topping the curbs: 1. Add 10' inlet north side Bethel School Road just east of alley. Tie to existing drainage system with 18" pipe. 2. Add 10' inlet north side Bethel School Road east of Falcon Lane. Tie to existing drainage system with 18" pipe. Page 14 3. Add 5' inlet east side of Falcon Lane, south of alley, tie to existing inlet with 18" pipe. ALTERNATE NO. 3 In addition to construction of all of the Alternate No. 1 and 2 items above, a complete and detailed engineering review of the storm drainage system, with consideration of possible up-sizing of the existing pipes utilizing the latest City of Coppell criteria, to increase the capacity of storm water run-off would be advisable. This is the most drastic and also the most costly alternate. SITE Z -PELICAN LANE ALTERNATE NO. 1: In order to intercept more surface run-off: Add two 10' inlets, one each side of, and adjacent to the existing inlet on the east of the street at the sag point. On the west side of the street, add one 10' inlet adjacent to the existing inlet. Page 15 ~LTERNATE NO. 2: In addition to Alternate No. 1 above, add one 10' inlet, each side, in the curve of the street, prior to the sag point, to intercept gutter flow before it reaches the sag point. In addition to construction of either or both of these alternates, we are presuming that positive over-flow, should a major storm occur, is available. It appears that the construction of fences between houses interferes, to some degree, with positive over-flow. This interference could be resolved by causing removal of the fences which are encroaching a public drainage easement. VIII. COST ESTIMATES Cost estimates provided herein are for construction of alternates as outlined above. Costs do not include construction inspection or testing. These are estimates only, based upon information developed in this study, and the best and latest unit prices available. They are not to be construed as "bid prices". Estimates were developed from unit prices of recent projects in the City of Coppell. Due to nature of the work, prices could vary by as much as 15%-20% in either direction. Page 16 Costs include, but are not limited to: inlets; connections to existing inlets; saw and remove, and replace concrete where applicable; curbs and gutter; concrete pavement; water meter relocation; water valve relocation; sprinkler repair; landscape replacement; storm sewer pipes; water line relocation; and, all appurtenances for a complete project. SITE I - BETHEL SCHOOL ROAD / FALCON LANE ALTERNATE NO. 1 SURVEYING .............................. $ 1,500.00 CONSTRUCTION COSTS ..................... $ 15,700.00 CONTINGENCIES (15%) .................... $ 2,355.00 ENGINEERING ............................ $ 2,708.00 TOTAL .................................. $ 22,263.00 ALTERNATE NO. 2 SURVEYING .............................. $ 1,500.00 CONSTRUCTION COSTS ..................... $ 28,600.00 CONTINGENCIES (15%) .................... $ 4,290.00 ENGINEERING ............................ $ 4,934.00 TOTAL .................................. $ 39,324.00 ALTERNATE NO. 3 SURVEYING .............................. $ 2,250.00 CONSTRUCTION COSTS ..................... $ 59,395.00 CONTINGENCIES (15%) .................... $ 8,909.00 ENGINEERING ............................ $ 10,246.00 TOTAL .................................. $ 80,800.00 Page 17 SITE ~ - PELICAN L~.NE ALTERNATE NO.1 SURVEYING .............................. $ 900.00 CONSTRUCTION COSTS ..................... $ 7,200.00 CONTINGENCIES (15%) .................... $ 1,080.00 ENGINEERING ............................ $ 1,242.00 TOTAL .................................. $ 10,422.00 ALTERNATE NO. 2 SURVEYING .............................. $ 900.00 CONSTRUCTION COSTS ..................... $ 13,600.00 CONTINGENCIES (15%) .................... $ 2,040.00 ENGINEERING ............................ $ 2,346.00 TOTAL .................................. $ 18,886.00 IX. RECOMMENDATIONS SITE I - Bethel School Road / Falcon Lane We recommend construction of Alternate No. 1 as a minimum, to help intercept some of the flow by-passing the existing inlets on Falcon Lane. Construction of the inlets would intercept some of the flow running down Falcon Lane prior to its crossing Bethel School Road and overflowing the south curb. Additionally, by increasing the sag point capacity on Bethel School Road to accept more run-off, a possibility exists that the storm water drainage system may carry more surface run-off during a major storm. Provisions to allow a more positive over-flow situation, when run-off exceeds the top of the curb, should also be made. Page 18 Alternate No. 3, in our opinion, would possibly help alleviate the majority of the flooding in the area of concern. A further, more detailed engineering study and analysis would have to be made to be able to provide a better appraisal of the results of the suggested improvements. SITE Z - Pelican Lane We recommend construction of Alternate No. 2 to help alleviate the majority of ponding problems in this area. Alternate No. 1 would help somewhat, but not to the extent of Alternate No. 2. Both of the above minimum recommendations for Site 1 and Site 2 are relatively low cost, short term solutions to the hazard of flooding during a major storm occurrence, and are not to be construed as a cure-all. The recommendations are remedial solutions for the problem. By choosing either of the first two alternates for Site 1, or the first alternate for Site 2, there is no assurance that the problem will be alleviated entirely. Page 19 X. SUMMARY For Site 1, at this point in time, we feel that Alternate No. 3 would be most logical solution to satisfy current design standards of City of Coppell, would provide the greatest degree of safety to property, and would possibly eliminate the majority of the flooding problem during a major storm. The major drawback, obviously, is cost and inconvenience to residents in the area with the construction that would be required. For Site 2, Alternate No. 2 would provide the best potential flood mitigation to the site. Without adequate provisions for positive over-flow, the City would be limited in its solutions in this area. XI. CONCLUSIONS There is considerable potential for storm water damage associated with unusual siting and drainage practices, which are often overlooked by the developers and homebuilders and are often a source of residential flooding. The potential for residential flooding can be reduced if dwelling floors and openings into dwellings are higher than potential run- off overflow elevations. Page 20 One of the most common, but easily overlooked source of residential flooding occurs where storm water run-off is blocked in the drainage swale along the property line between the dwellings. There is always a potential for water-related damage, from storm water run-off, to structures improperly sited or improperly graded. Positive drainage of individual building sites should be essential. There are many possible solutions to drainage problems. A combination of adequate storm water system design, and proper site design is necessary to achieve proper drainage. It may be impractical or cost prohibitive to design for the ultimate storm occurrence. From an engineering point of view, the City and home owners must accept the fact that every piece of property involves storm water run-off in either a major or minor way and as both a contributor and recipient. Page 21 ·:...~ ......-,._ ..--~_.--,~_~~'- :,.<.~ .;...:..~.~ ,~, :t:~¢ "..~_.~,, ,-.;- .. ~ --~.~-~ ": i='-,-::,- F'-. '-.'. -'~ ."'~ 'i.'--':'~ "'.'-:'.. · - . - ....: - ..~..~ ,.; : (:R~,,~_ C- '".'-~- '- '": '.:'- · ' ''~ ' ' ' " " -:~\×/~ .... '"~-~'~' ' .... ' .. ~T~. 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