4.1 Introduction
Various considerations are necessary for the implementation of the chosen detention tank option. This chapter will identify and provide analyses for such considerations.
4.2 Location Consideration
Figure 37: Detention Tank Design Location
The proposed design for the detention tank has dimensions of 35’ x 35’ which can be adjusted to accommodate the school’s site design. The design location should be close to the drain and combined sewer system at Cotter St. to minimize distance to the orifice drain. It should also be a non-pavement area before construction to mitigate the cost of removing existing pavement. The area may be paved after construction except for any maintenance access points which should be reachable. With these considerations in mind, a design location was selected as depicted in Figure 37. The location reflects the placement of the tank relative to the combined sewer system in the schematic in Figure 32.
This location is dependent on the final location of the Golden Bridges School. Communication with them is paramount.
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4.3 Construction Considerations
Figure 38: Open Trench Installation Reinforced Concrete Box Structure
SFPUC has designed many wet weather storage facilities. One such facility is a 0.8 MG reinforced concrete detention tank at the corner of 17th St. and Folsom St. Due to the relative similarities in the
project types, construction considerations were developed for the detention tank option at 203 Cotter St. using the Conceptual Engineering Report for the 17th & Folsom Storage Tank and additional
recommendations from SFPUC (Works).
An open trench construction method is considered the most economical method of construction for the detention tank (Works). Open trenching, which is depicted in Figure 38, consists of excavating a trench where construction can take place and then backfilled. This can be a costly method when excavation is on a pavement area as pavement repair is expensive. As the detention tank is to be constructed in a non-pavement area, the trench may be backfilled with soil and surface vegetation and the open trenching is most feasible (Sewer Lateral Methods).
Per Table 7, 1681 ft.3 of reinforced concrete is required for the construction of the detention
tank.
4.4 Environmental Considerations
The implementation of the detention tank will be subject to the California Environmental Quality Act (CEQA) prior to authorization and approval of the final design and construction by SFPUC.
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CEQA requires analysis, disclosure, and mitigation of the physical environmental effects of the project (Works). This section will review various potential environmental issues with accompanying mitigation strategies.
Land Use
Temporary construction may disrupt the residential area through loss of parking, interference of roadway access, and increased truck traffic. These may be mitigated through advanced notice of
construction, restriction of construction during peak traffic hours, and coordination with other agencies to minimize disruptions to other services. The site is not a high traffic area and so land use disruptions will likely be minimal (Works).
Noise
There will likely be increased noise levels during construction. This can be mitigated by restricting construction to daytime hours when residents are unlikely to be home. The contract specifications will also require contractors to comply with the San Francisco Noise Ordinance (Works).
Air Quality and Climate
There will likely be increased dust nuisance and airborne particulate matter during construction. There are dust control measures required by the Bay Area Air Quality Management District (BAAQMD) which include daily water of construction areas, covering haul trucks, and daily sweeping of streets at the project site (Works).
Utilities and Public Services
There will potentially be disruptions to underground utilities including the combined sewer, natural gas pipes, electrical conduits, telecommunications, and other water supply pipes. Mitigation strategies include performing a survey of local utilities, developing a construction plan to maintain local utility service, locating and notifying utilities prior to construction, and controlling leakage of raw sewage during pipe installation (Works).
Hazards
There will potentially be hazardous materials that workers may encounter during construction. There is also a potential for accidental spills during construction and water quality impacts from dewatering in areas with groundwater contamination. A Hazardous Materials Abatement Plan will be required in contract specifications that complies with applicable federal, state, and local hazardous materials regulations. This will include health and safety plans and adherence to appropriate soil and groundwater management procedures (Works).
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4.3 Scheduling Considerations
The wet weather storage facility at 17th and Folsom proposed a schedule in 5 major phases:
Planning, Design, Advertise and Award, Construction, and Closeout (Works). This technical
memorandum includes the Planning phase and preliminary work for the Design phase. The duration of these two phases were incorporated into the proposed schedule in Table 13. Future phase durations were determined using relative ratios between the durations of each phase in the 17th and Folsom
proposed schedule. This schedule also does not take into full account the proposed schedule of the Golden Bridges School which has not been disclosed in full detail. They must be included as stakeholders for final approval of this schedule.
TABLE 13: PROPOSED SCHEDULE PHASE DESCRIPTION Duration (months)
PLANNING PHASE 2
DESIGN PHASE 3
ADVERTISE AND AWARD 1
CONSTRUCTION 5
CLOSEOUT 1
4.4 Costing Considerations
Per discussions with Nicholas Birth and Bimayendra Shrestha of SFPUC, additional work items were added to the cost breakdown to account for orifice and outlet pipe construction. A Design and Estimation Factor of 30% was applied to account for design and estimation costs. A Contingency Factor of 10% was applied to account for estimation uncertainty. Labor costs are accounted for by the
excavation item (B. Shrestha). A final detailed costing breakdown is given in Table 14. The final total cost is ~$350,000.
TABLE 14: DETENTION TANK DETAILED COSTING BREAKDOWN
WORK ITEM DESCRIPTION Quantity Unit Unit Cost Total $
BASE SLAB (1 FT. THICKNESS) 1225 ft.^2 61 74725
PERIMETER WALL (1 FT. THICKNESS) 679 ft.^2 61 41419
TOP SLAB (1 FT. THICKNESS) 1225 ft.^2 61 74725
PILES (12"X12"X75FT.), 1 PER 100 FT.^2 4 Each 9000 36000
EXCAVATION 5281.65 ft.^3 0.93 4890
ORIFICE CONSTRUCTION 1 Each 10000 10000
OUTLET PIPE CONSTRUCTION 1 Each 10000 10000
DIRECT SUBTOTAL 251759
DESIGN AND ESTIMATION 30%
CONTINGENCY FACTOR 10%
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Chapter 5: Conclusion
The road area outside the suburban property of 203 Cotter St. in San Francisco has been subject to frequent flooding during heavy rainfall events. The San Francisco Public Utilities Commission
indicated a need for an intervention to mitigate flooding at this site. The flooding problem was identified and defined such that it may be resolved by engineering approaches. Five technical options were
developed and compared to each other using five qualitative criteria. One primary option, a detention tank, and two secondary options, green roofs and bio-retention planters, were selected. An
implementation plan was created for the primary option with considerations toward location, construction, environmental impacts, costing, and scheduling.
Appendix
TABLE 15: GREEN ROOF MODEL INPUTS
ITEM Units Value Description (Rossman) Value Justification
SURFACE
BERM HEIGHT In 4 When confining walls or berms are present this is the maximum
depth to which water can pond above the surface of the unit before overflow occurs.
Weight of water at capacity will roughly equal 20 lbs/ft2 which
falls within system weight guidelines (San Francisco Living Roof Manual)
VEGETATION
VOLUME FRACTION dimensionless 0 The fraction of the volume within the surface storage depth filled with vegetation. Normally this volume can be ignored. Assumed to be negligible per SWMM guidelines. (Rossman)
SURFACE
ROUGHNESS Mannings n 0.4 Manning's n for overland flow over surface soil cover, pavement, roof surface or a vegetative swale Suggested value for light underbrush. (Rossman)
SURFACE SLOPE % 4.75 Slope of a roof surface, pavement surface or vegetative swale. Same as the sub-catchment slope. (Rossman)
SOIL
THICKNESS In 2 The thickness of the soil layer. Weight of soil (p=2.65 g/cm3) and water (p=1g/cm3) at capacity
will roughly equal 30 lbs/ft.2 which falls within system weight
guidelines (San Francisco Living Roof Manual)
POROSITY volume fraction 0.395 The volume of pore space relative to total volume of soil. Suggested value for sand (Yu).
FIELD CAPACITY volume fraction 0.2 Volume of pore water relative to total volume after the soil has been
allowed to drain fully. Default value provided by SWMM. Does not affect short-term flooding impacts. (Chapter 2 - Soil and Water)
WILTING POINT volume fraction 0.1 Volume of pore water relative to total volume for a well dried soil
where only bound water remains. Default value provided by SWMM. Does not affect short-term flooding impacts. (Chapter 2 - Soil and Water)
CONDUCTIVITY in/hr. 25 Hydraulic conductivity for the fully saturated soil (in/hr.) Suggested value for sand (Yu).
CONDUCTIVITY
SLOPE dimensionless 45 Slope of the curve of log(conductivity) versus soil moisture content. Typical values range from 30 to 60. Average of the suggested range.
SUCTION HEAD In 4.8 The average value of soil capillary suction along the wetting front. Suggested value for sand (Yu).
DRAINAGE MAT
THICKNESS In 3 The thickness of the mat or plate. Design value. Sensitivity analysis suggested negligible impact in
increase.
VOID FRACTION voids/solids 0.55 The ratio of void volume to total volume in the mat. It typically ranges
from 0.5 to 0.6. Average of the suggested range.
ROUGHNESS Mannings n 0.25 This is the Manning's n constant used to compute the horizontal flow
rate of drained water through the mat. Previous modeling studies have suggested using a relatively high value such as from 0.1 to 0.4.
TABLE 16: BIO-RETENTION PLANTER MODEL INPUTS
ITEM Units Value Description (Rossman) Value Justification
SURFACE
BERM HEIGHT in 24 When confining walls or berms are present this is the maximum depth to
which water can pond above the surface of the unit before overflow occurs. Design value.
VEGETATION VOLUME
FRACTION dimensionless 0 The fraction of the volume within the surface storage depth filled with vegetation. Normally this volume can be ignored. Assumed to be negligible per SWMM guidelines. (Rossman)
SURFACE ROUGHNESS Mannings n 0.4 Manning's n for overland flow over surface soil cover, pavement, roof surface
or a vegetative swale Suggested value for light underbrush. (Rossman)
SURFACE SLOPE % 4.75 Slope of a roof surface, pavement surface or vegetative swale. Same as the sub-catchment slope.
SOIL
THICKNESS in 6 The thickness of the soil layer. Design value.
POROSITY volume fraction 0.25 The volume of pore space relative to total volume of soil. Average of range for gravel (Yu).
FIELD CAPACITY volume fraction 0.2 Volume of pore water relative to total volume after the soil has been allowed
to drain fully. Default value provided by SWMM. Does not affect short-term flooding impacts. (Chapter 2 - Soil and Water)
WILTING POINT volume fraction 0.1 Volume of pore water relative to total volume for a well dried soil where only
bound water remains. Default value provided by SWMM. Does not affect short-term flooding impacts. (Chapter 2 - Soil and Water)
CONDUCTIVITY in/hr. 25 Hydraulic conductivity for the fully saturated soil (in/hr.) Suggested value for crushed rock (Yu).
CONDUCTIVITY SLOPE dimensionless 45 Slope of the curve of log(conductivity) versus soil moisture content. Typical
values range from 30 to 60. Average of the suggested range.
SUCTION HEAD in 0 The average value of soil capillary suction along the wetting front. Crushed rock has negligible suction head (Yu).
STORAGE
THICKNESS in 18 This is the thickness of a gravel layer or the height of a rain barrel. Crushed
stone and gravel layers are typically 6 to 18 inches. Maximum suggested value.
VOIDS RATIO voids/solids 0.67 The volume of void space relative to the volume of solids in the layer. Typical
values range from 0.5 to 0.75 for gravel beds. Average of the suggested range.
SEEPAGE RATE in/hr. 0.2 The rate at which water seeps into the native soil below the layer. This would
62
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