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Home My WebLink AboutLetter; No Impact & No-Rise CertificateCoastal Waterways 601 2P, Street Suite 300 Vero Beach, FL 32960 wu v.coastahvde.com September 25, 2019 RECEIVED Plans Examiner/Building Inspector Coastal and Floodplain Construction Inspector DEC 10 9-019 St Lucie County Code Compliance Div. sr. Lucie county, Permitting RE: FEMA NO RISE CERTIFICATION AND SUPPORTING DATA FOR THE PROPOSED IMPROVEMENTS AT 11298 MULLER ROAD, FT. PIERCE, FL Dear Plans Examiner/Building Inspector: In response to the requirement for a No Impact/No-Rise Certificate for the above referenced property relative to the site improvements, we offer the following, attached: • No Rise Certification (signed and sealed) • Minor Project Narrative/Supporting Documentation (signed and sealed) The attached are submitted in accordance with 44CFR60.3(d) as required by the NFIP Program, referenced in the St. Lucie County Code of Ordinances, Land Development Code, 6.05.05.C. Site plans and construction documents, which states, "the applicant shall have the following analyses signed and sealed by a Florida licensed engineer for submission with the site plan and construction documehts:1.For development activities proposed to be located in a regulatory floodway, a floodway encroachment analysis that demonstrates that the encroachment of the proposed development will not cause any increase in base flood elevations" In summary, the proposed improvement would not result in an increase in flood levels within the community because (a) it is within the shadow of conveyance outside of the base flood discharge associated with the adjacent flooding source, Tenmile Creek, and (b) the small amount of fill associated with the improvement below the BFE (18.0 feet NAVD) is de minimis. If you have any questions regarding the attached, please contact me at (772) 473-4099. REVIEWED FOR Sincerely, CODE CO11 PLIA14CE COASTAL WATERWAYS ST, LUC`pc BOCC v ' • Stephen W hning, P.E., CFM President / Principal Engineer cc c: Rebecca Percy, 11298 Muller Road, Ft. Pierce, FL 34945r'L�®�� Engineering, Floodplain & EmetgencyManagementSolutions within our Coastal and Marine Environment NO -IMPACT CERTIFICATION This is to certify that I am a duly qualified, registered professional engineer licensed to practice in the State of Florida. It is further to certify that the attached technical data supports the fact that site improvements, a 1- story structure, pole barn, and kennel, at the Rebecca Percy Residence - Parcel No. 233 313 30001000 5, St. Lucie County, Florida will not impact the 100-year flood elevations, floodway elevations, or floodway widths on Tenmile Creek at published sections in the Flood Insurance Study for St. Lucie County, Florida dated February 16, 2012 and will not impact the 100-year flood elevations, floodway elevations, or floodway widths at unpublished cross -sections in the vicinity of the proposed project. Refer to the attached supporting documentation, titled, "No -Rise Certification Narrative in Support for a No -Impact Certification for Site Structures for Rebecca Percy - Parcel No. 233 313 30001000 5, St. Lucie County, Florida", dated 9/25/2019. 9t$. X�oz \ o� ', uze� � enwuen in^��ue,nemNlb Sn Een 1- : W � ,�eN.venv �mpe4,muk,YrtWUWW[o+v[!IS - Stephen W. Boehning, P.E. Date FL P.E. No.: 52201 No -Rise Certification Narrative in Support for a No -Impact Certification for Site Structures for Rebecca Percy - Parcel No. 233 313 30001 000 5, St. Lucie County, Florida Prepared for Rebecca Percy 11298 Muller Road Ft. Pierce, Florida 34945 Submitted by Coastal Waterways Design & Engineering, LLC 601 21 st Street, Suite 300 Vero Beach, Florida 32960 Coastal`, A0 Waterways CA 28071 September 25, 2019 No M., cf Stephen W. Boehning, P.E., CFM FL. P.E. No.: 52201 No -Rise Certification Narrative In Support for a No -Impact Certification for Site Structures For Rebecca Percy— Parcel No. 233 313 30001000 5, St. Lucie County, Florida This No -Rise Certification Narrative will demonstrate that the site structures for Rebecca Percy - Parcel No. 233 313 30001000 5, St. Lucie County, Florida meets (a) local, (b) state, and (c) federal floodplain requirements for the site structures identified in this narrative. BACKGROUND (a) Local: Applicable St. Lucie County floodplain code requirements 6.05.05. - Site plans and construction documents 6.05.05.C. Additional Analyses and Certifications. As applicable to the location and nature of the proposed development activity, and in addition to the requirements of this section, the applicant shall have the following analyses signed and sealed by a Florida licensed engineer for submission with the site plan and construction documents:1.For development activities proposed to be located in a regulatory floodway, a floodway encroachment analysis that demonstrates that the encroachment of the proposed development will not cause any increase in base flood elevations. The above is satisfied in the following way: (a) A registered PE certifies that this standard is met by demonstrating the structure is outside of the floodway, in the conveyance shadow downstream of the bridge. 6.05.09.C. Site Improvements, Utilities and Limitations. 41imitations on Sites in Regulatory Floodways. No development, including but not limited to site improvements, and land disturbing activity involving fill or regrading, shall be authorized in the regulatory floodway unless the floodway encroachment analysis required in Section 6.05.05(C)(1) of this ordinance demonstrates that the proposed development or land disturbing activity will not result in any increase in the base flood elevation.S.Limitations on Placement of Fill. Subject to the limitations of this ordinance, fill shall be designed to be stable under conditions of flooding including rapid rise and rapid drawdown of floodwaters, prolonged inundation, and protection against flood -related erosion and scour. In addition to these requirements, if intended to support buildings and structures (Zone A only), fill shall comply with the requirements of the Florida Building Code. The above is satisfied in the following way: (a) A registered PE certifies that this standard is met by demonstrating the structure is outside of the floodway, in the conveyance shadow downstream of the bridge. (b) The minor amount of leveling material for the surface under the pole barn is insignificant, not intended to support the structure, and not susceptible to erosion. 6.05.09.G. Other Development. 1. General Requirements for Other Development. All development, including man-made changes to improved or unimproved real estate for which specific provisions are not specified in this ordinance or the Florida Building Code, shall: (b)Meet the limitations of Section 6.05.09(C)(4) of this ordinance if located in a regulated floodway; Page 1 of 7 4%�& Coastal . Waterways No -Rise Certification Narrative In Support for a No -Impact Certification for Site Structures For Rebecca Percy —Parcel No. 233 313 30001000 5, St. Lucie County, Florida The above is satisfied in the following way: (a) A registered PE certifies that this standard is met by demonstrating the structure is outside of the floodway, in the conveyance shadow downstream of the bridge. (b) The minor amount of leveling material for the surface under the pole barn is insignificant, not intended to support the structure, and not susceptible to erosion. 6.05.09.G. Other Development. 2. Fences in Regulated Floodways. Fences in regulated floodways that have the potential to block the passage of floodwaters, such as stockade fences and wire mesh fences, shall meet the limitations of Section 6.05.09(C)(4) of this ordinance.3.Retaining Walls, Sidewalks and Driveways in Regulated Floodways. Retaining walls and sidewalks and driveways that involve the placement of fill in regulated floodways shall meet the limitations of Section 6.05.09(C)(4) of this ordinance. The above is satisfied in the following way: (a) A registered PE certifies that this standard is met by demonstrating the structure is outside of the floodway, in the conveyance shadow downstream of the bridge. (b) The minor amount of leveling material for the surface under the pole barn is insignificant, not intended to support the structure, and not susceptible to erosion. (b) State: Applicable Florida Building Code (FBC) FBC 1612.4 Design and construction. The design and construction of buildings and structures located in flood hazard areas, including coastal high hazard areas and Coastal A Zones, shall be in accordance with Chapter 5 of ASCE 7 and with ASCE 24 (NOTE: ASCE 7 is only applicable to calculating loads, therefore, not investigated forthis report). ASCE 24-14: 2.1 SCOPE. The requirements of this section shall apply to new construction and substantial improvements in Flood Hazard Areas, including High Risk Flood Hazard Areas that are not identified as Coastal High Hazard Areas and Coastal A Zones. 2.2 DEVELOPMENT IN FLOODWAYS. Structures and fill shall not be constructed or placed in floodways unless it is demonstrated that those structures and fill will not (1) increase the flood level during occurrence of the base flood discharge, and (2) reduce the conveyance of the floodway. If the design flood elevation has been determined and a floodway has not been designated, structures and fill shall not be constructed or placed unless it has been demonstrated that the cumulative effect of proposed structures and fill, combined with all other existing and anticipated development, will not increase the base flood elevation more than 1 ft. The above is satisfied in two ways: (a) A registered PE certifies that this standard is met by demonstrating the structure is outside of the floodway, in the conveyance shadow downstream of the bridge, and (b) the proposed structure, to be constructed above the BFE using fill, will include excavation of an equivalent volume of material as compensatory storage, as necessary. Page 2 of 7 �S Coastal, Waterways No -Rise Certification Narrative In Support for a No -Impact Certification for Site Structures For Rebecca Percy— Parcel No. 233 313 30001000 5, St. Lucie County, Florida (c) Federal — FEMA NFIP Floodplain Regulations (CFR 44§60.31 Section 60.3 (d) (3) of the National Flood Insurance Program (NFIP) regulations states that a community shall "prohibit encroachments, including fill, new construction, substantial improvements, and other development within the adopted regulatoryfloodway unless it has been demonstrated through hydrologic and hydraulic analyses performed in accordance with standard engineering practice that the proposed encroachment would not result in any increase in flood levels within the community during the occurrence of the base (100-year) flood discharge." Priorto issuing any building, grading, or development permits involving activities in a regulatory floodway, the community must obtain an engineering certification stating the proposed development will not impact the pre -project base flood elevations (mandatory), floodway elevations (optional), or floodway data widths (optional). The certification should be obtained from the permittee and be signed and sealed by a professional engineer. The Engineering or "no -rise" certification must be supported by technical data or an explanation of why a hydraulic analysis is not required. The Federal Emergency Management Agency has established regulatory floodways, which are the channel of a river or other watercourse and the channel's adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height. All projects in the regulatory floodway must undergo an encroachment review to determine their effect on flood flows and to ensure that they do not cause problems. Communities are required to prohibit encroachments including fill, new construction, substantial improvements, and other development within the adopted regulatory floodway unless it has been demonstrated that the proposed encroachment would not result in any increase in flood levels. The above is satisfied in two ways: (a) A registered PE certifies that this standard is met by demonstrating the structure is outside of the floodway, in the conveyance shadow downstream of the bridge, and (b) the proposed structure, to be constructed above the BFE using fill, will include excavation of an equivalent volume of material as compensatory storage, as necessary. Demonstration that the structure is outside of the floodway, in the conveyance shadow downstream of the bridge, can be made through a No -Impact Certification, which must be submitted with the permit application. The No -Impact Certification must be signed and sealed by a Professional Engineer who is licensed in the State of Florida. There are two approaches for the No -Impact Certification: (a) Narrative certifications for minor projects and (b) technical certifications for projects that require modeling. This report will present a Narrative Certification for Minor Projects, with the reasoning for this approach and the supporting logic demonstrating that the project would not result in any increase in flood levels within the community during the occurrence of the base (100-year) flood discharge. The No -Impact Certification for minor projects too small to warrant an engineering study may be determined using logic and common sense without a hydrologic and hydraulic analyses. Typically, these may include erection of fences, pool enclosures, sheds and other small accessory structures in developed areas, as well as docks, at -grade improvements, such as a driveway, or improvements to existing sidewalks, entry areas and parking areas. Page 3 of 7 4§Coastal Waterways No -Rise Certification Narrative In Support for a No -Impact Certification for Site Structures For Rebecca Percy— Parcel No. 233 313 30001000 5, St. Lucie County, Florida Then a rrative review forth e minor project, the proposed accessory barn structure, that is the subject of this report, will consider the conveyance shadow and the area outside of the effective floodway as described in further detail below: Conveyance Shadow Location in the conveyance shadow may allow this narrative review for accessory buildings and similar small projects. The conveyance shadow is the area upstream and downstream of an existing building or other obstruction to flood flows (i.e., bridges). Flood water is already flowing around the larger obstruction, so the addition of a new structure will not change existing flood flow. Typically, determining the limits of the conveyance shadow is illustrated in the figure 1, below. Small structures located completely within the shadow can be permitted without extensive engineering analysis needed. Upstream of the existing obstruction: dmr Gees at a 1:1 r6o. Domstream: draw fires at a4:1 ratio Figure 1. Determining Conveyance Shadow Limits Another application of the shadow conveyance occurs in riverine systems where bridges represent an obstruction to channel and floodplain flow. The subject property, located immediately downstream of the 11 Mile Road bridge, is within the conveyance shadow of the bridge and the road leading to the bridge. The bridge, and the associated elevated road section leading to the bridge, creates a conveyance shadow both upstream and downstream of the bridge (referto Figure 2, below). Structures located completely within the conveyance shadow can be permitted without extensive engineering analysis needed. Page 4 of 7 Coastal. 1O Waterways No -Rise Certification Narrative In Support for a No -Impact Certification for Site Structures For Rebecca Percy — Parcel No. 233 313 30001000 5, St. Lucie County, Florida Coninulon React Lc I _ _��,--' 1 ' Bridge Conveyance Shadow (typ.) ' , y 7 rR Expamioa Reach I` .J r' mrm\�1111i �iim�dnen ER 4.1 ER ratio t ' .......... ..1. �.. �.. CCaA /.`a Figure 2. Determining Conveyance Shadow Limits at Bridge Abutments Effective Floodway Located outside of the effective area of the floodway are the "ineffective flow areas" which also may allow this narrative review for building additions, accessory buildings, and similar small projects. Ineffective flow areas are used in HEC-RAS modeling to represent areas where flow is not being conveyed. Modeling generally requires ineffective flow areas because the profile of the bridge is typically an obstruction to flow in the overbanks. In these cases, ineffective flow areas are defined to ignore, for conveyance calculations, the areas where water is being stored, but not conveyed. Per U.S. Army Corps of Engineers, HEC-RAS Reference Manual, "Ineffective flow areas are often used to describe portions of cross section in which water will pond, but the velocity of that water, in downstream direction, is close to zero. This water is included in the storage calculations and other wetted cross section parameters, but it is not included as part of the active flow area". Flood water is flowing beside the area of interest (the subject property) and not through this property due to large obstructions upstream (the adjacent bridge and roadway), so the addition of a new structure on the subject property within the conveyance shadow will not change existing flood flow (conveyance). SITE CONDITIONS The site structures for the Rebecca Percy property (Parcel No. 233 313 30001000 5) in St. Lucie County, Florida is the subject of this analysis. This property exists within the FEMA mapped floodway associated with Tenmile Creek as delineated by the FEMA National Flood Hazard Layer (NFHL) exhibit for St. Lucie County (figure 3, attached). Existing site structures are located on the subject property. The attached figure 4 "No -Rise Certification Narrative Exhibit" shows the location of the site structures. The exhibit shows both the conveyance shadow associated with the existing residential structure (green colored triangular area), and the conveyance shadow associated with the east side of the downstream section of the bridge (yellow area). The conveyance shadow is shown for the 4:1 Expansion Ratio accepted by FEMA and the more conservative 2:1 Expansion Ratio. Page 5 of 7 Coastal, �� Waterways No -Rise Certification Narrative In Support for a No -Impact Certification for Site Structures For Rebecca Percy— Parcel No. 233 313 30001000 5, St. Lucie County, Florida The results of the floodway 1% annual chance storm are provided in the St. Lucie County Flood Insurance Study (FIS). Figure 5 shows the Tenmile Creek Flood profile from this FIS study depicting the BFE (1& Annual Chance Flood) within the vicinity of the subject site. This figure demonstrates that the BFE of Elevation 18.3 feet NAVD applies to the area immediately downstream of the 11 Mile Road bridge, including the subject property. The floodway width, cross-section area, and associated velocity in the vicinity of, and most applicable to, the subject property are shown in the FIS table (table 1), below: FLOODING SOURCE FLOODWAY BASE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE WIDTH SECTION AREA MEAN VELOCITY REGULATORY FLITHOUT LOODWT WtrFl FLO WITH INCREASE (FEET) (SQUARE FEET) (FEET NAVD) (FEET NAVD) (FEET NAVD) IFEETI S(FECONO TENMILE CREEK AF 24.15' 702 3,399 1.7 183 18.3 19.3 1A IFEETABOvEN0UR1 2R18LS ABOVE COUNTY BOUNDARY Table 1. Excerpt from St. Lucie County FIS Floodway Data Table — Section AF is Upstream of the Bridge As shown on figure 4, the 1-story structures is within the conveyance shadow of the bridge and has a finished floor elevation (FFE) of 19.3 ft. NAVD. The regulatory Base Flood Water Surface Elevation for the 1% annual chance storm is 18 feet NAVD. According to the site survey data provided for the site, and shown on figure 4, the existing elevation at the location of the 1-story structure are above the BFE. Additionally, the pole barn structure is within the conveyance shadow of the bridge and has no walls. The minimal amount of leveling material placed below the pole barn is an insignificant amount. Finally, the kennel structure is within the conveyance shadow of the bridge and no fill is associated with this structure. The minor amount of fill mentioned above can be considered de minimus when compared to the entire floodplain volume available. CONCLUSION In conclusion, the entirety of the subject property is within the expansion shadow of the southeast (downstream) side of the bridge when using an Expansion Ratio of 4:1 typically accepted by FEMA. Any proposed improvements built within the expansion shadow as shown on the attached Figure 4 would not increase the conveyance calculations of the Base Flood Elevation (BFE). Although the floodway would not experience a rise due to the location of proposed improvements within the Expansion Shadow (conveyance calculation), an argument can be made that the improvements, if they reduce the capacity of the volume of the flood zone, could result in an increase in the flood zone (compensatory storage). This report demonstrates that the resulting volume is considered de minimus. Page 6 of 7 � Coastal; Waterways No -Rise Certification Narrative In Support for a No -Impact Certification for Site Structures For Rebecca Percy— Parcel No. 233 313 30001000 5, St. Lucie County, Florida Therefore, as demonstrated in the above narrative, using the FEMA approach and the supporting documentation, the impacts of the improvements proposed to be constructed within the Expansion Shadow would not result in any increase in flood levels within the community during the occurrence of the base (100-year) flood discharge. This analysis is provided in support of a No -Impact Certification for the site structures at the Rebecca Percy Residence - Parcel No. 233 313 30001000 5, St. Lucie County, Florida. Page 7 of 7 ` Coastal, Waterways {; ; i 9•Y'da4d� N' Y � � i ,;, lewfr, • J •i '0OD - 01. "'7'„g �y4,d,If4 Wb CJ "�JC��' IK5 �4�f�'� ;T/,.�• J r i � 1 'rl � b %4��i� t i'' •r4fr Q{4 1 i 4yi"� is �/'��,�v7.�}�]� ,` i .4 %jr,•� .� 7 f,S' ��.yl��i'1 M+/4I.�� A� .. �.."-- wry-...ni..e. wf � -.r�� irk 'PpNNSHIP ]! %OIIiN. RANG% 39 EAST 2�w� J THWEST 1/4 9.9 `lgS 1 "' ��m¢ uo H P 35 CREEK W+ a wA1FA OFF WAY AND pt�' O`AP� - 4:4 i mmm IRS IS 9 3435 C/L it MITE ROAD 4ry j (: l� �1 2E5 +/- ^�!; �! 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