HomeMy WebLinkAbout2022 10 10 Regular 500 - Consideration of Ordinance to Increase the Minimum Levels of Service for Drainage and Stormwater related to New Development • REGULAR AGENDA ITEM 500
Incnrinreted CITY COMMISSION AGENDA I OCTOBER 10, 2022 REGULAR MEETING
1454
TITLE
Per the City Commission's direction at the October 3, 2022 meeting, consider
whether to direct City staff and the City's engineering consultants to pursue
study and preparation of an ordinance to increase the minimum levels of
service for drainage and stormwater related to new development.
SUMMARY
The City Commission held a special meeting on October 3, 2022 to discuss the
impacts of Hurricane Ian on the the City of Winter Springs. Among the topics
discussed was the intense rainfall (estimated to be a 500-year storm event)
experienced by the City of Winter Springs and many other portions of Florida.
The amount of rainfall caused serious flooding and drainage impacts on the
community.
As a result of these flooding and drainage impacts, the City Commission
directed that this item be placed on the Agenda.
The purpose of this Agenda Item is for the City Commission to consider
whether to direct City staff and the City's engineering consultants to pursue
study and preparation of an ordinance to propose an increase in the
minimum levels of service for drainage and stormwater related to new
development.
The City Commission has established minimum drainage and stormwater
levels of service in the City's Comprehensive Plan and City Code. A copy of
the relevant excerpts of the Comprehensive Plan and City Code are attached
for reference.
The City Commission had some discussion about the distinction between the
level of service standard for a 25-year and 100-year storm event. For
references purposes, the 25-year rainfall depth is 8.6 inches during a 24 hour
period, and the 100-year rainfall storm event is 11 inches during a 24 hour
period. By comparison, Hurricane Ian dropped approximately 15 inches of
rain on Winter Springs. The total rainfall amounts in various places in
Seminole County was published by the National Weather Service as follows:
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Seminole County
Sanford International Airport - 16.10 inches
Oviedo 2.2 W - 15.82 inches
Winter Springs 0.6 S - 15.56 inches
Winter Springs 2.3 E - 14.97 inches
Chuluota 0.9 N - 14.55 inches
1 .0 NE Sanford -13.67 inches
Chuluota 0.6 WSW - 13.40 inches
Longwood 2.8 NW - 13.21 inches
Winter Springs 1.8 SSE - 12.63 inches
Altamont Springs 1 .4 SE - 12.32 inches
Casselberry 2.3 ESE - 12.31 inches
Source: https://www.wesh.com/article/rainfall-totals-hurricane-ian/41494992
The amount of rainfall experienced in a 24-hour storm event isjust one factor
in establishing drainage and stormwater levels of service. Increasing the level
of service standards for drainage and stormwater is a highly technical and
complex endeavor which involves multiple variables and computations on a
case-by-case and property basis. The City's stormwater engineering
consultant would be required to assist City staff if it is the Commission's
desire to pursue the study and preparation of an ordinance.
The City Commission should provide direction to the City staff as deemed
appropriate and necessary.
RECOMMENDATION
The City Commission should provide direction to the City staff as deemed
appropriate and necessary.
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Winter Sprints City Code
Chapter 9—Land Development
Article IV.—Required Improvements
Division 4. -Drainage
Sec. 9-241. Stormwater management.
(a) The design concept for drainage systems in proposed developments shall be approved by
the city engineer. All drainage systems shall have a positive outlet or overflow unless
documented as being unnecessary by geohydrologic investigation based on a twenty-five-
year design storm.
(b) All drainage facilities shall be designed for a rainfall of a twenty-five-year/twenty-four-hour
return period using an SCS Type II Modified rainfall distribution. The twenty-five-
year/twenty-four-hour storm is the "design storm".
(c) The drainage system for each subdivision shall include a sufficient facility to remove
stormwater without flooding any lot in the proposed subdivision or in the surrounding
territory.
All residential floor elevations shall be elevated to a minimum of eighteen (18) inches above
base flood elevation.
All nonresidential finished floor elevation shall be elevated to a minimum of eighteen (18) inches
above the base flood elevation unless flood proofed in accordance with section 8-52(2) of the
Flood Damage Prevention Code.
(d) Retention/detention ponds.
(1) Dry bottom ponds. All dry bottom retention/detention ponds will have a pond bottom
elevation of no less than one (1) foot(twelve (12)inches) above the seasonal high
ground water elevation, which is to be determined by a registered professional engineer
in the State of Florida with an expertise in soils. The side slopes of all dry bottom
retention/detention ponds shall not be steeper than two and one-half(21/2) horizontal to
one (1)vertical. All dry bottom ponds with no exceptions, in which the side slopes are
steeper than four(4)horizontal to one (1)vertical shall require a green vinyl chainlink
fence with a minimum height of four(4)feet. All fences must meet setback
requirements in section 6-193, buildings and building regulations.
(2) Wet bottom ponds. All wet bottom ponds must have side slopes no steeper than four(4)
horizontal to one (1)vertical measured from the top of the berm down to at least three
(3) feet below the normal water level, measured vertically. This is the side slope
transition point. Side slopes may then transition,from the transition point, to no steeper
than two (2)horizontal to one (1)vertical down to the pond bottom. Wet bottom pond
side slopes steeper than these values are not allowed. If a fence is used, it must be of
the same specifications as the fence for dry bottom ponds.
(3) All ponds. Retention, detention, percolation, and treatment of stormwater are required
by the city. Stormwater regulations, criteria and requirements of the State of Florida,
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St. Johns River Water Management District and the Department of Environmental
Protection, as they may exist and be modified from time to time, are to be the
regulations, criteria, and requirements which the city shall utilize for review of
stormwater facilities. All city stormwater requirements shall also apply in addition to
any federal and state requirement. If there is a conflict between requirements, the
stricter requirement will apply. All ponds shall have a minimum ten (10) foot wide
stabilized maintenance berm capable of supporting a maintenance vehicle. There shall
be a minimum ten (10) foot wide easement to each pond dedicated to the city and the
homeowners' association (if planned). All fenced ponds must have a locked gate with
ten-foot wide gate opening.
(4) For new development or significant redevelopment, those stormwater management
systems that eventually discharge into the city's MS4 (municipal separate storm sewer
systems) should mitigate any increase in pollutant loads to the maximum extent
practicable. To meet this requirement, the city encourages the use of best management
practices (e.g., stormwater reuse and baffle boxes), as well as low impact development
technologies, including but not limited to: replacement of traditional paving materials
with porous concrete/pervious pavement, grass swales, bio-retention, and other
comparable methods.
(e) Flow quantities off-site are not to exceed that prior to development. The stormwater runoff
volume and rate from the "design storm", for the post development condition, shall not
exceed that for the pre-development condition.
(f) The city encourages the preservation of existing swamp areas, ponds (including intermittent
ponds), wetlands and wet areas, and bayheads for water storage and conservation purposes.
Existing flood storage quantities shall not be reduced by development from that which
existed on-site prior to development. Pre and post-development volume must be adhered to
with additional compensating storage for all floodwater displaced by development below
the elevation of the hundred-year flood as defined by the Federal Emergency Management
Agency on the latest official panel of the flood insurance rate map. Compensating storage is
to be calculated between the hundred-year flood elevation and the seasonal high water table.
The seasonal high water table shall be established by a registered professional engineer in
the State of Florida with expertise in geotechnical engineering based on recent soil borings
on the subject site. There must be at least one (1) soil boring per pond,performed at the
proposed location of the pond.
(g) The development of each phase of a development shall be capable of standing on its own if
subsequent areas planned for development are not developed.
(h) Inlet spacing. Inlets shall be spaced in such a manner as to accept one hundred(100)percent
of runoff. Typically, the maximum allowable gutter run will be seven hundred Fifty (750)
feet on streets with curbs, unless specifically excepted by the city engineer. On streets other
than streets with curbs, the actual required spacing will depend on the characteristics of each
particular site. Calculations will be provided to show the hydraulic grade line (water level)
in all inlets and storm water manholes when the pond reaches its maximum stage water
elevation during the design storm. At no time will the hydraulic grade line be allowed to be
higher than the edge-of-pavement at inlets and at no time be higher than the manhole rim at
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storm water manholes. See subsection 9-300 for standard curb inlet. Florida Department of
Transportation (FDOT) inlets may be used at the discretion of the city engineer.
(i) Lot grading. To facilitate proper stormwater surface runoff from residential lots, the
minimum residential lot line slope that can be accepted is one and one-quarter(1.25)
percent. This will only be allowed in areas that have good percolation of rainfall into the
soils. In areas that do not have good percolation of rainfall into the soils, the minimum lot
line slope that can be accepted is one and one-half(1.50)percent.
The determination of good percolation of soils will be made by the city engineer based on a
recent soils report by a registered professional engineer in the State of Florida with an expertise
in soils. The soils report may have to include the results of a recent soil permeability test if
required by the city engineer.
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Excerpts of Winter Sprints Comprehensive Plan
Infrastructure Element
IV-A GOALS, OBJECTIVES, AND POLICIES
DRAINAGE (Page IV-11-12)
GOAL 4: Provide a stormwater system of appropriate capacity to protect the life and property of
the citizens of the City, as well as decreasing adverse environmental impacts attributable to
stormwater runoff.
Objective 4.1: Flood Control. The City shall achieve and maintain the following adopted
stormwater management Level of Service (LOS) standards that shall meet or exceed state and
federal regulations for stormwater quality and quantity.
Policy 4.1.1: Adopt LOS standards as follows: Require new development and
redevelopment to meet the following water quality and quantity standards.
• Water Quantity -At a minimum, the peak post-development runoff rate shall not
exceed peak pre-development runoff rate for the 25-year, 24-hour storm event. In addition, if
downstream facilities (from the positive outfall of the development) are inadequate to convey the
peak discharge for the design storm event, the development shall be required to accommodate its
proportion of basin runoff rate above the downstream systems actual capacity.
• Water Quality - Stormwater treatment shall be required to serve the
development through a stormwater treatment system which is sitespecific; or serve sub-areas of
the City and, if applicable, Seminole County. Regardless of the area served,the stormwater
treatment system must provide a level of treatment which meets the requirements of the Florida
Administrative Code (F.A.C.).
• Roadway construction - All public roadways within a development, and required
as part of the development order, shall be designed and constructed to standards which do not
allow any amount of water above the roadway during the following storm events for the
following roadway types:
• Local Roadway—25-year, 24-hour design storm event(8.6 inches of rainfall).
• Collector Roadway —25-year, 24-hour design storm event(8.6 inches of
rainfall).
• Arterial Roadway— 100-year, 24-hour design storm event(10.6 inches of
rainfall).
Objective 4.2: Stormwater Master Plan. The City shall maintain, and utilize the Stormwater
Master Plan and its updates which establish high water elevations, addresses existing
deficiencies, and coordinates the construction of new and replacement facilities.
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Policy 42.3: Utilize the expertise of a professional engineer to run models of the City's
stormwater system based upon critical design storm events periodically on an as-needed basis
and when necessary update the Stormwater Master Plan.
IV—E DRAINAGE SUB-ELEMENT
B. STORMWATER FACILITIES INVENTORY
3. Capacity and Level of Service (Page IV-56
The areas of Winter Springs that have a paved infrastructure system under the control and
maintenance of the City and have a stormwater management system with a design capacity to
handle a 25-year, 24-hour storm event represent approximately 59% of the City's overall area.
Approximately 95 percent of the City's drainage facility capacity is apportioned to the City; the
remainder serves the cities of Oviedo and Casselberry.
The 10-, 25-, and 100-year, 24-hour storm events were considered in the analysis of the existing
drainage systems. These storm events were selected by the City's staff to evaluate the LOS of the
existing secondary drainage systems. The approximate LOS for the 6 secondary drainage
systems was determined as required by Chapter 9J-5, F.A.C. The LOS for the analysis of the
secondary basins was assigned as summarized below:
• LOS A —Protection from 100-year, 24-hour storm event
• LOS B -Protection from 25-year, 24-hour storm event
• LOS C -Protection from 10-year, 24-hour storm event
• LOS D -Flooding from 10-year, 24-hour storm event
The recommendations of the SJRWMD were utilized in the Plan and Supplement to evaluate the
LOS standards of drainage facilities in the City. The LOS for structures asserts that all existing
structures should be free from flooding during a 100-year storm event. The LOS for roadways
requires that drainage facilities for arterial roads will be designed to carry a 100-year storm
event; collector and neighborhood roads will be designed to carry a 25-year storm event, per City
Code.
Four flood levels for residential street systems were defined in the Stormwater Master Plan for
use in the evaluation of the City's stormwater management facilities as follows:
• Flood Level 1 —Water surface at or below street grade.
• Flood Level 2—Water surface above street grade, but below 6-inch depth of ponding.
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• Flood Level 3 —Water surface depth greater than 6 inches above street grade, but below
12 inches.
• Flood Level 4—Water surface depth greater than 12 inches above the street grade.
The recommendations of the SJRWMD were utilized in the Plan and Supplement to evaluate the
LOS standards of drainage facilities in the City. The LOS for structures asserts that all existing
structures should be free from flooding during a 100-year storm event. The LOS for roadways
requires that drainage facilities for arterial roads will be designed to carry a 100-year storm
event; collector and neighborhood roads will be designed to carry a 25-year storm event,per City
Code.
Four flood levels for residential street systems were defined in the Stormwater Master Plan for
use in the evaluation of the City's stormwater management facilities as follows:
• Flood Level 1 —Water surface at or below street grade.
• Flood Level 2—Water surface above street grade, but below 6-inch depth of ponding.
• Flood Level 3 —Water surface depth greater than 6 inches above street grade, but below
12 inches.
• Flood Level 4—Water surface depth greater than 12 inches above the street grade.
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