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HomeMy WebLinkAboutDrainage Report UNIT 16II LLOYD AND ASSOCIATES ENGINEERS AND SURVEYORS REPORTS DESIGNS SUPERVISION APPRAISALS CONSULTATIONS ROBERT F. LLOYD REGISTERED CIVIL ENGINEER 3538 REGISTERED LAND SURVEYOR 944 (305) 562-4112 1 B 3 S 2 0 T H STREET VER❑ BEACH, FLORIDA 32960 DRAINAGE ANALYSIS SEBASTIAN HIGHLANDS - UNIT SIXTEEN 1970 VS. 1980 PROPOSED DRAINAGE SYSTEMS This report is an analysis of the inherent advantages vs, dis- advantages which would result from implementation of the revised 1980 Sebastian Highlands, Unit 16 drainage system in comparison with the 1970 Sebastian Highlands, Unit 16 drainage design. The proposed subdivision, Sebastian Highlands, Unit Sixteen, con- sists of approximately 450 acres. For clarity in accurately assessing the 1970 vs. 1980 drainage designs, Unit Sixteen should be separated, as it is physically, into the East and West components of Sebastian Highlands, Unit Sixteen. (See attached map illustrating these areas. ) The proposed changes to the West Component of Sebastian High- lands consists of the removal of rip -rap, revision of pipe sizes and the elimination of crosspipe, which was changed to standard outfall pipes. These revisions will create erosion problems in the areas where concentrated quantities of water discharge, the pipe size changes are simply equivalency areas and do not affect the capabilities of the storm water system. Removal of rip -rap headwalls will promote damage to pipe by traffic running off - 1 - shoulders and striking the unprotected pipe. For most practical purposes the 1970 design has not been altered significantly enough to impact the storm water management capabilities of the western portion of Sebastian Highlands, Unit 16. The proposed changes to the East Component of Sebastian Highlands, Unit 16, are much more extensive and thereby potentially capable of signifi- cantly affecting the quality of storm water management as it impacts the residents of Sebastian Highlands, and thereby the City of Sebastian. It should be mentioned that no design negligence as it relates to the overall capacity of the system is inferred, only the quality of storm water management as it affects the relationship between residents and their environment are discussed to aid the citizens of Sebastian and their elected officials in rationally deciding what design parameters are viable for them. The East Component changes and their effects are discussed below: 1. An overall grade change of a few inches to three (3) feet is pro- posed for the entire eastern subdivision. 2. Concrete curbs and valley gutters as opposed to swales with culverts at intersections. 3. Valley gutters as opposed to culverts at street intersections. 4. Soil cement ditch bottoms for rear lot drainage easements. 5. Elimination of the bulkhead for lots fronting on Lake Schumann is proposed. 6. Additional culverts as opposed to ditches is proposed, along with catch basins. - 2 - 7. Rip -rap is eliminated. 8. Culvert sizes changed to reflect new drainage design. 9. Grades for roads, swales and ditches have been changed to meet new design conditions. 10. Sidewalks have been added. Soil cement ditch bottoms historically create maintenance problems. At joints and with time, contraction cracks occur which sprout vegetation; sand, silt and debris deposits create unsightly surfaces difficult to clean and main- tain. Settling occurs which creates pools of stagnant water which is only disposed of by evaporation. Without the soil cement ditch bottom, percolation would be promoted both by direct contact with the soil and the delayed run-off caused by vegetative growth. The first flush, or 1", of rainfall contains 90%n of the pollutants from roads, driveways, roofs, lawns and other areas. With swaled right-of-ways and grassed ditch bottoms, storm waters infiltrate back into the soil and are cleansed in the infiltration process. By providing only impervious surface contact for all water falling on pavement and soil cement ditch bottoms, fertilizers, petroleum products, debris and other pollutants are discharged directly into Lake Schumann. In time this could kill plants and fish in the lake. It should be noted that many small cities experiencing rapid growth are adopting drainage requirements for developers, requiring retention of the - 3 - r first inch of rainfall. As noted earlier, 90%n of pollutants resulting from storm waters occur in the first inch of rainfall, and infiltration from storage ponds designed to retain the first inch help the natural hydrologic cycles, in addition to minimizing drainage structures usually maintained by City personnel. CONCLUSION A. The plans for Sebastian Highlands, Unit 16, Western section, as revised April, 1980, show a minimum change from those approved in June, 1970, and in effect have not lowered the standards except for the elimination of rip -rapped headwalls and ditch drops. It is believed that serious con- sideration should be given to reinstate their use for physical protection and safeguard against accidents. B. The plans for the Eastern section of Unit 16, have been upgraded to metropolitan standards worthy of complimentary approval. - 4 -