HomeMy WebLinkAbout03/26/2002, B 2 - STORM DRAIN MASTER PLAN-AIRPORT AREA AND RELATED AREAS council M=tinD� 3-26-0
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CITY OF SAN LUIS OBISPO _ T-�
FROM: Michael McCluskey,Director of Public Work.
SUBJECT: Storm Drain Master Plan—Airport Area and related areas
CAO RECOMMENDATION: Receive and file report and consider specific projects as they
come forward in future fmancial plans.
DISCUSSION
Several years ago, in conjunction with the development of the Airport Area Specific Plan, the City of San
Luis Obispo initiated the development of a Storm Drain Master Plan (SDMP). Boyle Engineering
Corporation prepared the SDMP through a subcontract with Wallace, Roberts, and Todd (the lead
consultant of the Airport Area Specific Plan). The overall purpose of the SDMP was to evaluate the
existing capacity of storm drainage facilities that were needed to service development in the Airport Area
and make needed project recommendations to allow that development to take place.
However, the creeks and waterways that service the Airport Area are part of an extensive system that
likewise provides service to the Margarita Area, the Orcutt Area, developed portions of the City, and
areas in the County which are both upstream and downstream of the Airport Area. Therefore in order to
truly provide a SDMP, all the areas needed to be evaluated and all improvements needed to be
incorporated into one document in order to provide adequate facilities for development in the Airport
Area. This Plan will therefore provide the basis for all future storm drainage related improvements in the
greater southern San Luis Obispo area as development takes place therein.
Underlying Principles—the foundation of the Storm Drainage Master Plan
Before the consultant began work, key criteria were established as goals that any plan prepared were to
address as much as possible. They were:
1. Whatever system was created could not impact San Luis Obispo Creek adversely. In essence, this
meant that storm flow (measured in cubic meters per second) could not increase due to
development in the Airport or related areas.
This criterion basically set in place the need for detention basins, or retention basins.
Detention basins"detain-water" on its way to the ocean and have the effect of slowly metering
out storm water runoff to the creek after the major flows have passed and thus creek capacity
is available. Retention basins totally "retain water" and never. contribute flow to the creek;
they empty by evaporation or percolation into the ground.
The City did not want to add additional flows to the creek due to the limited capacity of the
creek downstream; the significant environmental studies that would be needed if even a small
increase occurred; and the arduous Corp of Engineers permitting process that would be
necessary if any modifications to the creek were found to be necessary.
2. If detention basins were recommended, they would be minor in number and controlled and
managed by the City.
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Council Agenda Report—Storm Drain Master Plan—Airport Area and related areas
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Current experience with existing storm water detention basins in the City has shown that
private ownership does not guarantee adequate maintenance. Eventually, storm water is no
longer detained and its original purpose is lost: additional flows downstream occur and
environmental damage is a very real potential. Multiple small basins do not lend themselves
to efficient maintenance, while large basins lend themselves to multiple public or private uses.
For example, cities such as Visalia, Turlock, Stockton, Tracy, and Ceres use detention basins
recreation areas about 95% of the year and such use has gained legal, liability and public
acceptance.
3. All land should be able to fully develop to its potential as described by the City's land use and
zone regulations.
All the City's regulations are based upon the ability to use 100% of the property in property
development. Although setbacks for buildings, floor area ratios, and landscaping
requirements vary, they all assume that 100% of the property is available to calculate those
requirements. Commercial and industrial land carries high land values and property owners
like to make use of the entire site. Although current county regulations allow on-site storm
water detention basins where no storm drainage system is available and certain developers
have employed these approaches, it is simply a matter of time until development pressures to
maximize land use will necessitate the elimination of the detention basin. Therefore to
provide for the ultimate build-out of areas the Master Storm Drainage Plan needed to assume
that all existing detention basins had been converted and all land built out to its maximum
potential.
4. The series of existing creeks within the study area would be the means of storm water transport
but that there be little (none if possible) negative effects upon the creek system due to development.
In essence, this criterion meant that wherever existing creeks did not have sufficient capacity to
carry the larger flows created by development, the additional capacity necessary would be
created by secondary by-pass channels located adjacent to the creek.
5. All improvements and calculations would be based upon the 100—year storm event.
Using the 100-year event assured the maximum degree of protection to all land uses and
building occupants, decreased or eliminated the need for flood hazard insurance, and assured
full emergency access to all streets and citizens during most any storm related event.
Study existing storm drainage systems and recommend improvement alternatives
The consultant then proceeded to map out the entire watershed servicing the Airport Area to its outfall
and connection with San Luis Obispo Creek. The watershed boundary was quite large and is shown on
Plate 2.1 of the SDMP. Then using topographical data from the City's existing GIS system, a
hydrological analysis was performed to establish base level conditions — i.e. what storm water is
currently being produced. Due to the criteria established above, no additional flows could be added to
San Luis Obispo Creek over the existing levels.
With existing flows known, the next step was to analyze the capacity of the existing system to identify
existing deficiencies. Numerous portions of the system were found to be deficient under existing
conditions. Next,future flows were developed based upon the eventual full development of all lands and
a new deficiency analysis performed. As expected, the number of deficient system components and
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Council Agenda Report—Storm Drain Master Plan—Airport Area and related areas
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degree of deficiency increased. .
Then the real meat of the project began—identifying alternative storm drainage systems not only for the
Council's preferred land use plan but also for all the alternative land use patterns. A series of iterations
were completed and likewise a series of cost estimates. Because of the tremendous 'size of the storm
drainage basin (Plate 2.1) the changes in potential land had relatively influence on the number and degree
of improvements recommended. In the end, five alternatives were developed; one preferred (Plate 4.3)
and four alternates (Plates B.1 through B.4).
Cost then became the major factor by which the preferred system was chosen. Below is a table showing
the preferred alternative and the four alternates.
Preferred $9,086,095
Alternate No. 1 $12,956,000
Alternate No. 2 $13,194,000
Alternate No. 3 $14,001,000
Alternate No. 4 $11,415,000
The Preferred System—What's included?
The preferred system, having the lowest cost, consists of multiple creek/channel improvements, culvert
replacement/enlargements and a single large detention basin. As shown in Plate 4.3 the drainage basin
has been divided into two basic components- a) Tank Farm Creek: a creek/channel system servicing the
majority of the Margarita Area and the Airport Area; and b) East Fork of San Luis Obispo Creek: a
creek/channel and detention basin servicing the easterly portion the Airport Area, the Orcutt Area, the
Damon-Garcia athletic fields, and portions of the City of San Luis Obispo which are already fully
developed as well as undeveloped areas in the county.
Tank Farm Creek
The name "Tank Farm Creek " was been assigned to a series of formally unnamed creeks by the
consultant in order to describe the waterways the associated improvements. The major elements
of this system include the installation of dual 6' x 10' Box Culverts at Buckley Road; replacing
existing culverts at Tank Farm Road with a new bridge, and creek/channel modifications the
entire length of the project from Prado Road southerly to Buckley Road. Storm flows from this
area would pass directly to San Luis Obispo Creek without detention. The impact to San Luis
Obispo Creek would be neutral as the increase in flows, due to new development in this area,
would be offset by decreased flows from the East Fork of San Luis Obispo Creek.
East Fork of San Luis Obispo Creek
This creek services a rather large area and a number of creeks drain into it such as: Orcutt Creek,
Acacia Creek, and the Upper, Middle, and Lower Forks of San Luis Obispo Creek. A single
detention basin is proposed to be located adjacent to Buckley Road at the downstream end of the
system in order to be able to regulate flows into San Luis Obispo Creek and offset the increased
flows of Tank Farm Creek. Although the location is schematic, it must be located somewhere
near the end of the drainage system. The farther it moves toward the main creek the more costs
are incurred as more creek/channel enhancement must take place. Therefore it's currently
location is shown because it produces the least cost option.
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Council Agenda Report—Storm Drain Master Plan—Airport Area and related areas
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It is not envisioned that this detention basin will be put to a joint recreational use as in numerous
other cities but instead be used in joint use with agricultural uses. Since flooding would only
occur, on average, every ten years the land can remain in agricultural production nearly year
around. When flooding would occur, the period of inundation would most likely be 24 hours or
less. After subsidence, City personnel would be responsible for debris clean-up and removal.
The SDMP assumes that the City will purchase the land. No assumptions have been made about
how the area would be managed. Whether the City owns the land and leases it out for some
nominal amount or obtains an easement for storm drainage purposes will be determined during
implementation actions.
Upstream of the detention basin, bridges at Santa Fe Road and Tank Farm Road will be replaced
with larger structures; and creek/channel improvements made to Acacia Creek and the East Fork.
It is important to note that the SDMP was long prepared prior to Army Corps of Engineers
permitting requirements of the Damon-Garcia athletic fields. As a condition of that permit,
certain improvements recommended by the SDMP will need re-study and modification prior to
implementation. Specifically, the SDMP recommends that Orcutt Creek be connected to Acacia
Creek in the vicinity of the Damon-Garcia athletic fields. While this recommendation would
have provided better year around habitat for Acacia Creek, the Corps has determined that the two
creeks should remain separated. Thus the exact sizing of bridge replacement at Tank Farm Road,
the need to replace the bridge at Tank Farm Road associated with Orcutt Creek, the exact degree
of creek/channel enhancements for both Acacia and Orcutt Creeks will need further evaluation
prior to implementation. The estimated costs of these changes are unknown as some costs will be
far less while some new projects will be necessary.
What are the proposed creek/channel enhancements?
As shown on Plate 4.2, little to no improvements are planned for existing creeks. In essence, the current
very wide flooded area would be reduced to a more manageable area by creating long sloping, shallow
channels adjacent to the existing creeks. .As the creeks overflow during large storms the water would be
contained within the larger channel and as flows decrease the floodwaters would return to the creeks.
Not shown in the SDMP but described in the Airport Area Specific Plan, the very outside edge of the
new channels would be used for dual-purpose activities: City maintenance vehicle access and
pedestrian/bicycle access. The SDMP assumes that all channels and creeks will be purchased and
become the responsibility of the City. Included in the cost estimates above are the financial impacts
associated with City maintenance. The channel areas would receive periodic mowing and kept free of
growth that would impede flood flows. They would be natural areas and not paved with asphalt or
concrete. City maintenance would assure that adequate maintenance would be provided and all areas
developed would enjoy protection from the 100-yr storm event.
Are Changes to the Storm Drain Master Plan possible?
Yes. The SDMP is not formulated to last forever as a rigid document. It provides a good set of
guidelines to allow development to take place while assuring excellent flood protection. However, as
noted above, we already know that some elements will need another look and analysis prior to
implementation. We know the Orcutt Area may propose an entirely different idea about storm drainage
with their plan. Should grant funding become available for open space acquisition in the County where
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Council Agenda Report—Storm Drain MasterPlan—Airport Area and related areas
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SDMP Alternates call for detention basins, the cost for Alternate systems could decrease to the point of
being preferred over the current recommendation. Thus, with study and further analysis, the SDMP is
open to change.
How will it be implemented?
Shown in Table 5`4 of the SDMP, the consultant has recommended a nine year phased implementation
schedule. This assumes that the entire area will not develop the minute the Airport Area Specific Plan is
approved and that some form off steady state development does occur. Since full improvements will not
be needed everywhere right away, the plan starts off fairly small, providing the basics of flood protection
and builds from there. Development that takes place near a fully completed facility will be able to tie
directly to the system. For development in other areas, guidelines will allow only modest development
to occur and interim on-site detention basins will be necessary to contain flows above drainage system
capacities. Storm drainage fees will be collected with each development and these fees will accumulate
and then be used to finance the construction of Master facilities identified in the phasing plan.
What's next?
On March 20, 2002 a town hall meeting will be held to review the Airport Area Specific Plan. Planning
Commission hearings will begin thereafter. A brief review of the SDMP was presented to the AASP
Focus Group and preliminary contact has been made with the property owner of the proposed storm
drain basin. Additional question and answer sessions are planned with property owners as needed.
Eventually, the Council will be asked to formally adopt the Airport Area Specific Plan. Assuming the
Plan is adopted, the Council will then be asked to adopt the Storm Drainage Master Plan for the Airport
Area and its related areas. Once adopted, implementation of the Plan will begin as development takes
place.
FISCAL IMPACT
There is no immediate fiscal impact associated with this report and the recommended action. The costs
of all improvements are proportioned based upon contributing watershed. The Airport Area is
apportioned 17.2% of all costs or $1,563,000; the Margarita Area is apportioned 40.8% or $3,707,000
and "areas east of Broad Street" are apportioned 42% or $3,816,000. This last area is a combination of
the Orcutt Area and areas already developed. Therefore, 58% of all costs will be borne by the combined
Airport Area and Margarita Area and the City/Orcutt Area will be responsible for funding 42%. Either
the General Fund or the proposed Creek and Flood Protection Fund will provide City funding.
Attachments
A copy of the SDMP for each Council Member, and one is available for public review in the City Clerk's
office
1:\Council Agenda Reports\storm drain master plan-airport area.doc