LTC 147-2008 Overview of the City's Stormwater Systemm MIAMIBEACH
OFFICE OF THE CITY MANAGER
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CITY CL~~di~'S G~FF1C~~
No. LTC # 14~-loos LETTER TO COMMISSION
TO: Mayor Matti Herrera Bower and Members of the City Commission
FROM: Jorge M. Gonzalez, City Manager ~~~~~~~
DATE: May 27, 2008
SUBJECT: OVERVIEW OF THE CITY'S STORMWATER SYSTEM
This Letter to Commission (LTC) is intended to provide you with an overview of the City's stormwater system
and its current improvement program. This LTC is a follow up to the May 5, 2008 LTC on Storm Drainage and
includes additional data and a comprehensive map of the drainage areas.
The stormwater Utility is responsible for protecting the waterways from pollution and removal of stormwater
from the roadways. It is also responsible for maintaining the stormwater conveyance system, relieving flooding
conditions, and complying with National Pollutant Discharge Elimination System (NPDES) permit.
Most of the City's stormwater system was built between 40 and 50 years ago. Although the system has been
relatively well maintained, intense development in recent years and changes in stormwater regulations have
shown the system needs to be upgraded to provide the required level of service. The management of
stormwater in the City is a difficult task because of its unique features: low ground elevation, high ground
water table, poor soil percolation rates, and the environmentally sensitive nature of Biscayne Bay.
The City's stormwater system has two (2) primary objectives; the first is flood protection or the removal of
stormwater and the second is water quality improvement or pollutant removal. Arguably the most important
objective is flood prevention.
To successfully accomplish the flood prevention goal, the stormwater management system is required to
prevent flooding of homes and businesses, maintain safe passage on roadways, and remove standing water
within a reasonable amount of time. The City meets this objective by the following methods: permitting
construction of new buildings and improvements only at elevations above the anticipated flood elevation; and
by the construction of stormwater collection and conveyance systems to remove stormwater from the land
surface and discharge it into the surrounding waterways or into groundwater.
Current building codes require all new construction to be completed with a finished floor elevation (FFE) above
the 100-year flood stage elevation. Although this is successful in protecting the new structures, significant
portions of the existing structures within the City are substandard with the FFE's at or below the necessary
levels. The condition is exacerbated as new structures are built near older, substandard structures. The new
structures shed stormwater and the resulting flow pools around or within the older, substandard buildings.
Another problem associated with increasing land elevations is that it accelerates the rate at which stormwater
runs off of the property and into the stormwater collection system. The accelerated stormwater runoff may
overload the collection system and create pools of standing water at the point of collection.
A canal network is also utilized as part of the City's stormwater management program. The canals are
designed in conjunction with the stormwater system and reportedly were sized to handle a 25-year storm
having a 24-hour duration. Although the canals provide discharge points for the City's outfalls, they do little to
lower groundwater elevations. Because the elevation of the island is so low, groundwater elevations virtually
mirror surface water elevations.
The final method of stormwater management is through the use of stormwater and other stormwater collection
and conveyance devices. A stormwater system serves to collect and dispose of excess water after a rainfall
event through containment and/ or rapid disposal by gravity driven outlets. A stormwater system is comprised
of a collection of devices; typically catch basins, pipes and outfalls that historically have collected, conveyed
and discharged stormwater runoff directly into surface water bodies. Seasonal and tidal influences can result
in higher than usual water levels in the receiving waters, which will limit efficient operations of the stormwater
system. These influences can cause localized flooding that usually abates after a short period of time,
depending on the percolation rates and tidal fluctuation. The City's low elevation and location present a
condition under which the capacity of even the most conservatively designed and well-maintained stormwater
system can be temporarily exceeded by runoff from severe storms.
Since new direct outfall systems are not permitted under County guidelines, on-site retention systems are
typically used. Various types of on-site retention systems, either alone or in combination are used by the City
to achieve an appropriate and acceptable drainage system configuration. These systems include surface
infiltration through grassed swales, underground seepage disposal, and drainage wells.
swales are generally defined as the strip of land in front of homes and adjacent to the street. Swales are
important because they provide not only storage capacity for street runoff, but they also help filter the
pollutants in the stormwater including oils and grease. Swales also allow for the water to percolate into the soil.
The use of swales is a relatively easy and inexpensive method of stormwater management that tends to
mitigate pollutant levels better than the others systems. Staff has also identified that the preservation and
reclamation of swales is a critical component to alleviate flooding.
Underground seepage disposal systems, which disperse stormwater directly into the groundwater, can be
used as drainage facilities. An example of these seepage systems are French drains. French drains are
trenches filled with pea gravel and ballast rock that filter stormwater runoff before it enters the groundwater.
This type of drainage is most commonly used for spot treatment of flooding areas.
The third type of on-site retention system is a drainage well. A drainage well typically consists of several
collection basins fitted with pollution control boxes that are connected to an open-ended, vertical, cased well
that discharges stormwater into the saltwater portion of the aquifer.
Drainage wells as previously described can operate under the influence of gravity or by a pressurized system.
As surface water ponds, the elevation difference (head differential) between the ponded water and the
groundwater table drives the stormwater into the pervious formations at the base of the well. Drainage well
operation is dependent upon a multitude of variables. To minimize the influence of these variables and to
improve well performance, the City is using pumps to pressurize drainage wells (injection wells).The use of
injection wells has the ability to provide an increased flood level of service, and also to reduce residual
standing water. The use of injection wells provides increased drainage capacity; however, it also greatly
increases the cost and annual maintenance expenses of the City's drainage system.
In 1997 the City published a Comprehensive stormwater Management Program Master Plan (Master Plan).
The purpose of the Master Plan was to comply with the requirements of the federally mandated National
Pollutant Discharge Elimination System (NPDES) program and the resultant Interlocal Agreement entered
between the City and the County. The Master Plan evaluated over 160 stormwater basins; a stormwater basin
is a defined surface area that drains to a common discharge point. Factors used to evaluate each basin were:
pollutant loading, pollutant concentration, flooding potential, citizen complaint and City staff ranking. 34 of the
160 plus stormwater basins were identified as "priority drainage basins". The Master Plan was presented to
the City Commission, and ultimately adopted by the City Commission. The Master Plan's 5-year Capital
Improvement Program (CIP) recommendations were updated in 1999 and a stormwater Revenue Bond was
issued in 2000 to fund stormwater improvements to these 34 priority drainage basins.
The thirty-four (34) high priority basins (Shown as cross hatched on the attachment.) were scheduled for
inclusion in the Neighborhood Right-of-Way (ROW) Improvement projects. Therefore, the Citytasked its ROW
consultants to design stormwater system improvements in the priority basins that addressed a 5-year/24-hour
design storm to meet the Miami-Dade drainage system design criteria which specify a 5-year storm level of
service for collector and local streets in residential and commercial areas. These designs require the streets to
be passable and typically allow flooding to the crown of the road or within fifteen (15) feet of an occupied
building when seven point five (7.5) inches of rain falls in one day.
2
It is important to understand that the 5 year design standard does not eliminate the possibility of flooding.
When an area is provided drainage to meet the 5 year standard, if a storm with greater intensity strikes the
area, it will flood and the standard of not having the road crown underwater may not be met. Likewise,
property that does not flood in a 5 year storm maybe exposed to flooding for more severe events. Typically in
areas that have a 5 year level of service for storm drainage, any flooding that does occur will recede more
rapidly as a result of the improvements. The City considers flooding problems to be addressed/solved when
the 5 year standard is achieved, even if flooding from more severe storms occurs. The 5 year level of service
standard is often an area of confusion in the community as many people believe that when the City constructs
storm improvements, no more flooding will occur.
The choice fora 5 year design represents a balance between cost and the possibility for damage. A higher
level of service can be achieved in most areas, however, the cost tends to be prohibitive and the actual
improvements may begin to become intrusive.
In non-priority basins, Stormwater Bond funding was not provided, and the City ROW consultants did not
design comprehensive stormwater improvements. Instead, where drainage problems were identified, the
consultants were tasked with designing spot improvements. These spot improvements typically consisted of
plans for installing exfiltration trenches, regrading swales, and adjusting street grades. No modeling was done
to predict the level of flooding in non-priority areas during certain design storms.
The Master Plan originally identified proposed projects for the 34 stormwater priority drainage basins. During
the development of the Stormwater Revenue Bond, Series 2000, capital improvements were grouped together
and reclassified by the neighborhood in which they are located.
The original cost estimates for the priority basins generally were based on upsizing the conveyance pipes in
the system with assumptions made that the existing outfall system of disposal would essentially be
maintained. In actual design and construction, these cost estimates have been shown to be insufficient to
address the adopted level of service. The need for additional facilities and capacity to address the 5-year
storm standard and construction cost increases over the years has caused the overall program to increase
from the original $50 million bond budget to an estimated amount of $163 million. The bond interest through
March 2008 provided approximately $18.8 million, leaving an approximate shortfall of $94 million.
The Master Plan recommended improvements were generally incorporated without alteration into each of the
neighborhood Basis of Design Reports. There were a few exceptions where significant stormwater problems
were identified by residents during the planning. process for each neighborhood and then confirmed by CIP
staff, the consulting engineer, and Public Works-staff. In these few instances (such as Stillwater) stormwater
funding was added to address the flooding issue, both CIP and Public Works agreed that itwas appropriate to
do so.
The attached Geographic Information System (GIS) map (attachment 1) was developed recently by a joint
effort of our Public Works Department Stormwater Division staff with an institutional knowledge of + 15 years
of experience in stormwater and the Capital Improvements Projects (CIP) office staff. The map depicts the
priority basins in cross hatching, the areas previously mentioned that are outside of the priority basins where
some flooding exists in blue and yellow highlights, and known hot spots in red, at which flooding has been
confirmed that is of a serious nature.
In 2004 the City added resources to the Storm Water system maintenance. The attached map illustrates in
blue and red dots, those locations that the City has or is responding to reports and or complaints about
flooding. These work order number approximately 1,025 over the past three year period. The work orders
came both from complaints as well as staff observation of the storm water system. Work ranged from
cleaning a basin or line to replacement of failed structures. The three year time frame is significant as the City
devoted new resources to the maintenance of the storm water system over that period of time. In that same
period the City has through its own and contracted crews spent approximately 2.1 million dollars on
preventative cleaning of storm lines and catch basins. The Public Works Department now has a systematic
preventative maintenance program for the entire stormwater system. The smaller pipes in the system (less
than 18 inches), including catch basins and manholes are cleaned every year and a half by City staff. Starting
in 2003, the larger pipes in the system (larger than 18 inches), including catch basins and manholes, are
cleaned by contractors on a five (5) year cycle. The City also provides more frequent cleaning of known
problem areas.
3
Attachment 1 illustrates the entire City and the priority drainage basins. The map also shows the areas to
which the City has responded for complaints, known nuisance flooding areas outside of the priority basins, and
known severe flooding areas or hot spots. Each is reviewed briefly in the following categories.
Map Hiphliphts
• Priority basins:
The 34 high-priority stormwater basins identified in the City's Comprehensive stormwater
Management Program Master Plan are shown in the GIS map, attachment 1 as cross hatched. It
must be noted that flooding areas within the high-priority stormwater basins will be corrected under
the City's Capital Improvement Program for the different neighborhoods.
• Flooding areas outside of the priority basins
There are two. types of flooding indicated on the map indicated in blue and in yellow highlighting
illustrating areas that are outside of a priority basin, yet experience some flooding. In these areas
flooding occurs but property is not impacted and the flooding is normally observed to happen during
high tides and/ or the flooding typically recedes after a few hours. For the areas shown in blue, no
solutions have yet been designed or funded. For the areas shown in yellow, spot improvements to
help mitigate the localized flooding have been incorporated into the CIP neighborhood improvement
project. In both cases, blue and yellow, nuisance flooding exists as illustrated; the only real distinction
is whether or not the area has been incorporated into a project. Examples of nuisance flooding are
seen in the photos labeled attachment 2.
A brief description of each of the flood areas designated in blue follows:
• Star Island: Star Island has roads with center crowns that drain half of the stormwater inward toward
the median and half outward toward the swales that abut the homes. Over the years, some residents
have builf up the swales with landscaping and raised curbs. These encroachments impede drainage
to the swales and cause some ponding on the roads.'
The Star Island plans propose spot treatments that consist of adjusting street grades. The City is
avoiding Swale re-grading in order to minimize impacts to the encroachments. Some residents are
concerned these current plans will not sufficiently alleviate drainage problems. As a result, the
Commission asked staff to review options for additional improvements.
The first option, which would be recommended by the Administration, adds inlets with drainage piping
at low points adjacent to the street. These improvements would be connected to the existing
stormwater system draining into the existing outfall. This option is estimated to cost between
$125,000 and $250,000.
Most residents would prefer a second, more extensive option. This option involves sloping the entire
street toward the median and leaving swale areas and encroachments undisturbed. Should the
Commission wish to pursue this option, the consultant would need to redesign the street and median.
This would require rebuilding the profile of the street and changing the slope to the median. The
median would also have to be re-graded to detain /retain more stormwater. Further, the median
contains a number of piping systems including water, sewer, gas, and irrigation that would have to be
considered by the consultant during design. Based on preliminary estimates, this option would require
the allocation of $500,000 to $600,000 in additional funding, including soft costs.
Biscayne Point: The Master Plan identified only the area generally to the north of 83rd Street as a
priority basin. For the area south of 83rd Street, CIP tasked the ROW consultant with developing a
spot treatment that addresses a drainage problem brought to light by residents. In particular, there
was a complaint at Biscayne Point Circle. CIP and the consultant concluded that the ponding is due to
street elevations below the elevation of a nearby catch basin. As a result, the street elevations will be
adjusted to drain to the existing catch basin. Overall funding for the project must still be established
upon completion of the design so that the project, with this additional improvement, can be placed into
construction.
4
Venetian Islands: In this case, the residents have expressed concerns with stormwater runoff from the
Venetian Causeway flowing into the side streets and causing flooding conditions. The Miami-Dade
County project manager for the Venetian Causeway project has indicated that the Causeway's new
drainage system will include the installation of drainage inlet structures at all corners of the side
streets to collect the water runoff from the Causeway. As part of the existing ROW project, the design
includes re-grading of swales, pavement, and select curbs to improve drainage to existing drainage
inlets. These solutions are expected to be sufficient to address the current conditions. However, this
area was not classified as a priority basin and therefore is not receiving full drainage improvements.
This flooding is referred to as nuisance flooding and clearly creates a level of problem or concern for
those in the vicinity, however, is not severe enough to have yet been addressed.
A description of the yellow highlighted areas follows:
CIP Neighborhood drainage (non-priority basins); Corrective spot drainage solutions are being provided by
the CIP construction program (shown in yellow) within non-priority basins. This effort is being done to
address spot drainage problems. The limited drainage improvements best address the identified spot
drainage problems, within the available stormwater bond funding for those neighborhoods.
Sunset Islands 1 & II: The Sunset Islands have low areas in the streets and many swales that have
been built up with encroachments. As a result, there is isolated flooding. The ROW consultant is
proposing exfiltration trenches in multiple locations on these islands to address these problem areas,
as it was determined that the swales would not be reclaimed. The plans are complete and in
permitting. Funding is being established for this project so that it can be placed into construction in
October 2008. At this time, several residents are questioning the level of service that will be provided
by this design. The Home Owners Association was advised that an increased level of service would
require additional funds as well as re-designing and re-permitting the plans. Staff continues to work
with the Home Owners Association to see if the design can be further refined.
• Sunset Islands III & IV: The ROW consultant is also proposing exfiltration trenches in multiple
locations on these islands to address isolated flooding resulting from built up swales with
encroachments. The plans are on-hold pending resolution of the undergrounding of electric, cable TV,
and telephone utilities.
The map illustrates three (3) areas in red, described below, and referred to as hot spots. These are areas
where City staff has observed or residents have complained about excessive flooding that impacts private
properties. The flooding in these areas is severe and does not meet the 5 year storm standard. A picture
of one of the hot spot areas illustrates the nature and magnitude of the problem (attachment 3). These
three (3) areas are:
• Hot spots
(a) Prairie Avenue from Dade Boulevard to West 23 Street: construction is underway to take
care of the flooding problems in this area.
(b) The second location on 44th Street in the Orchard Park neighborhood is currently being
evaluated by City staff as to the different options which could resolve the flooding problem.
Several solutions have been presented by the consultant and are planned to be presented at
the Finance and Citywide Projects Committee meeting on May 29, 2008.
(d) The third severe flooding area is located on North Bay Road north of 56th Street.
This area has yet to be evaluated by City staff as to potential engineering
solutions.
The City has been working to correct an area of flooding at 8th Street and Collins Avenue, which we
believe has been successfully completed.
stormwater systems have and continue to be important to our community. While the City has expended
significant resources to date in maintenance and capital construction, more work will be forthcoming. Upon
completion of the CIP neighborhood projects, all of the City's priority basins will have been addressed. Those
areas most affected by less than 5 year flood events are being addressed in part and within the resources
available.
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