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Home My WebLink About1998 - Authorization to proceed from CountyMASTELLER & MOLER, INC. October 1, 1998 Ms. Martha Campbell, P.E. City Engineer City of Sebastian Mr. James W. Davis, P.E. Public Works Director Indian River County Re: Indian River County Project # 9611 County Route 512 Roadway Improvements Blossom Ditch Diversion System Dear Martha: Indian River County and the City of Sebastian have authorized us to proceed with the proposed additional work requested by the City of Sebastian regarding the Blossom Ditch Diversion Project. The Scope of Work for the portion of work will be completed as described in our Proposal to the City of Sebastian dated June 3, 1998. The purpose of the Blossom Ditch Diversion project is to eliminate as best as possible discharges from the Sebastian Highlands Unit 5 area from entering the natural creek which passes through the San Sebastian Springs subdivision area. Under the City's agreement with Indian River County, we are initiating the design of a diversion system that will direct the contributing runoff reaching the south end of the Blossom Ditch to a new culvert to be constructed within the right- of-way of County Route 512. The new culvert system will be extended along the County Route 512 right-of-way in a westerly direction to the South Prong of the Sebastian River. The design will also include a baffle box to remove sediments from entering the South Prong of the Sebastian River for small storms. In order to initiate our services on this project, we have updated the Stormwater Management Report and "ADICPR" Model prepared for the County Route 512 Project to highlight the stormwater analysis completed for the Sebastian Highlands Unit 5 area. For the City's review, we are enclosing herewith portions of the Stormwater Report containing information related to the Sebastian Highlands Unit 5 stormwater Basin(s); Node(s); Link(s); and Modeling Input and Output information. Please note that the Sebastian Highlands Unit 5 area has been incorporated within the portion of the County Route 512 Stormwater Management Report entitled "System E". System E represents those drainage areas east of the South Prong of the Sebastian River having an influence on the County Route 512 design. Sebastian Highlands Unit 5's stormwater contributions do not traverse and/or enter any existing County Route 512 stormwater conveyance facilities. El 2205 loth Avenue ❑ 5211 Okeechobee Road Vero Beach, Florida 32960 Ft. Pierce, Florida 34947 Ph(561)567-5300 Ph(561)466-9200 Fax(561)794-1106 Fax(561)466-9230 Ms. Martha Campbell, P.E. Mr. James W. Davis, P.E. 09/03/98 Page 2 of 4 In general, the stormwater runoff generated by the Sebastian Highlands Unit 5 area is currently directed into a drainage ditch which runs initially parallel to Roseland Road and thence parallel and directly adjacent to the County Route 512 right-of-way. The drainage ditch serving the Unit 5 area is known as the Blossombitch. Although the Blossom Ditch does not directly contribute runoff to the County Route 512 right-of-way, its discharge does merge with an existing conveyance system that traverses County Route 512. The traversing system drains an area south of CR 512. The Blossom Ditch and the traversing system both enter a small creek which runs in a northwesterly direction through the San Sebastian Springs subdivision which creek discharges to the South Prong of the Sebastian River. The design of the Blossom Ditch diversion system will separate the contribution of the Sebastian Highlands Unit 5 area from the traversing works system. The traversing works system will continue to drain via the small creek through San Sebastian Springs. (Please note that the County Route 512 project design incorporates the diversion of runoff from the right-of-way area east of the traversing works to the County's proposed new System E stormwater management system. Thus, both the County Route 512 project and the additional diversion of contributions from the Sebastian Highlands Unit 5 area will remove contributing areas to the creek which passes through San Sebastian Springs). As stipulated between the City and County, the diversion of the contributing flow from the Unit 5 area will not be delivered to the County's proposed System E stormwater management system. It is intended that the Blossom Ditch diversion system be constructed to function independent of the County Route 512 planned stormwater conveyance, treatment and attenuation facilities. As a result of the incorporation of the Sebastian Highlands Unit 5 drainage basin into the County Route 512 stormwater model, the stormwater runoff values to be utilized for design of the diversion system are available for study. The Drainage Basins within the CR512 stormwater model within the Unit 5 area have been designated as BHIGHl; BHIGH2; BHIGH3; BHIGH4; and BHIGH5. The "Input" data utilized within the CR512 stormwater model relating to the above described basins is summarized in the attached information. The basins described above contribute runoff within the CR512 Model to Nodes which are connected by Links. For ease of locating and understanding the model as it relates to the Sebastian Highlands Unit 5 area, the following information regarding Nodes and Links is provided. BASIN NODE BHIGHI NRIVU BHIGH2 NBLOSSU B1-DGH3 NBLOSSU BHIGH4 NS44D BHIGH5 NBLOSSD LMA Ms. Martha Campbell, P.E. Mr. lames W. Davis, P.E. 09/03/98 Page 3 of 4 The Nodes within the CR512 stormwater model are connected by LINKS. A summary of the NODE / LINK relationship for the Sebastian Highlands Unit 5 area is listed below: UPSTREAM DOWNSTREAM NODE LINK NODE NRIVU RPRIV NRIVU NRIVD RCBLSI NBLOSSU NBLOSSU RCBLS2 NS44D NS44D RCBLS3 NS41) NS4D RCBLS4 NBLOSSD NBLOSSD RCSSSI NSSSIU It should be noted that NODE NBLOSSD receives contributing flow in the CR512 model from both the Sebastian Highlands Unit 5 area and the CR512 traversing works system at Station 318+11. The downstream LINK from NODE NBLOSSD is designated as RCSSSI (i.e. (R)each / (C)hannel / (S)an (S)ebastian (S)prings (1). It is the LINK which receives contributing flows from both the Sebastian Highlands Unit 5 Blossom Ditch system and CR512 traversing works at Station 318+11. Modeling of the Sebastian Highlands Unit 5 area within the CR512 project for the 24 HR storm duration model produced the following peak contributing flow results to NODE NBLOSSD for the modeled storms: Computed Storm Peak Event Flow Mean 34.60 cfs 25 YR 84.02 cfs 100 YR 113.67 cfs In order to not adversely increase the Peak Stages within the Sebastian Highlands Unit 5 area and specifically the Blossom Ditch system, it is recommended that the diversion system be designed for the 100 -Year Storm Event. Peak Stage of the upstream end of the diversion system should not exceed the Peak stage of the existing system for the 100 -YR event which has been estimated at 17.45 NGVD at NODE NS44D. HM Ms. Martha Campbell, P.E. Mr. James W. Davis, P.E. 09/03/98 Page 4 of 4 Our Preliminary review of the route topography between the current south end of the Blossom Ditch and the potential discharge point indicates an approximate 3.0' available fall. The invert of the upstream end of the System (at the south end of the Blossom Ditch) will be set at Elevation 11.50 NGVD. The discharge point located approximately 1550' downstream within an existing ditch connected to the South Prong of the Sebastian River is at approximately Elevation 8.50 NGVD. Thus, the physical slope of the conveyance system maybe designed at 0.00194'/'. It is recommended that the diversion system be developed through the use of a 60" diameter culvert (w/ Mannings "n" = 0.012) based on the following evaluation: Pipe Capacity Size a, 0.001947' 54" 93.86 cfs 60" 124.25 cfs The hydraulic slope above a 54" diameter culvert carrying 113.67 cfs has been determined to be 0.00285T. Assuming a 1550' culvert with a downstream hydraulic head elevation of 13.00 NGVD (8.50 + 54"), the upstream hydraulic head elevation required to yield 113.67 cfs would be approximately 17.42 NGVD (13.00 + 1550*0.00285/') which is below the allowable peak stage elevation 17.45 NGVD. Thus, a 54" diameter culvert system will meet the required design parameters. Should you have any questions regarding the information contained herein, please do not hesitate to contact our office. Sincerely yours, MASTELLER & MOLER, INC. 52 - Stephen E. Moler, P.E. Executive Vice President sem/vk enclosures File #9649le (mc100198.doc)