VIII. - DESIGN OF STORM SEWERS
(1)
Design Frequency
(a)
New Development: The Design Storm Event for sizing storm sewers in newly developed areas shall be a 3-year rainfall. System shall be designed to convey extreme event (100 year rainfall) by overland sheet flow path that will not impact infrastructure or buildings.
(b)
Redevelopment or In-fill Development: The existing storm drain (sewer, ditch) shall be evaluated using a 3-year design storm, assuming not development takes place. The storm drain will then be evaluated for the 3-year design event with the Development in place.
(i)
If the proposed Redevelopment has a lower or equal impervious cover, no modifications to the existing storm drain area required.
(ii)
If the proposed Development results in the hydraulic gradient of the existing storm drain below the gutter line, no improvements to the existing storm drain area required. Detention shall comply with Section VI above. Flow discharged to the storm drain shall comply with Section VI above.
(iii)
If the analysis of the existing conditions finds that the existing storm drain is deficient (i.e., the hydraulic grade line is above the gutter line), Applicant should check with the City to see if a Capital Improvement Project is proposed that will require a capital contribution. If a Capital Improvement Project is not proposed for the subject system, then on-site detention will be required in accordance with Section VI above. Flow discharged to the storm drain shall be in compliance with Section VI above.
(c)
City Projects (CIP): Proposed City capital improvements may indicate a larger diameter storm sewer is planned in the area proposed for drainage improvements. The Public Works Department has information on proposed improvements and should be consulted for impact on New Development.
(d)
Private Drainage Systems: Storm sewers for private drainage systems shall conform to the City of Dickinson's Building Code for development within the City limits.
(2)
Velocity Considerations.
(a)
Storm sewers should be constructed to flow in subcritical hydraulic conditions if possible.
(b)
Minimum velocities should not be less than 3 feet per second with the pipe flowing full, under the design conditions.
(c)
Maximum velocities should not exceed 8 feet per second without use of energy dissipation downstream.
(d)
Maximum velocities should not exceed 12 feet per second.
(3)
Pipe Sizes and Placement.
(a)
Use culverts, storm sewer and inlet leads with at least 18-inch inside diameter or equivalent cross section. Box culverts shall be at least 2 feet by 2 feet. Closed Conduits; circular, elliptical, or box shall be selected based on hydraulic principals and economy of size and shape.
(b)
Larger pipes upstream should not flow into smaller pipes downstream unless construction constraints prohibit the use of a larger pipe downstream, or the improvements are outfalling into an existing system, or the upstream system is intended for use in detention.
(c)
Match crowns of pipes at any size change unless severe depth constraints prohibit and the City has approved an alternative.
(d)
Locate storm sewers in public street rights-of-way or in approved easements. Back lot easements are discouraged and will require a variance from the City design standards.
(e)
Follow the alignment of the right-of-way or easement when designing case in place concrete storm sewers.
(f)
A straight line shall be used for inlet leads and storm sewers.
(g)
Center culverts in side lot storm sewer easements.
(4)
Starting Water Surface and Hydraulic Gradient.
(a)
The hydraulic gradient shall be calculated assuming the top of the outfall pipe as the starting water surface.
(b)
At drops in pipe invert, should the upstream pipe be higher than the Hydraulic Grade Line, then the Hydraulic Grade Line shall be recalculated assuming the starting water surface to be at the top of pipe at that point.
(c)
For the Design Storm, the hydraulic gradient shall at all times be below the gutter line for all New Development.
(5)
Manhole Locations.
(a)
Use manholes at the following locations:
(i)
Size or cross section changes
(ii)
Inlet lead and Conduit intersections
(iii)
Changes in pipe grade
(iv)
Street intersections
(v)
A maximum spacing of 700 feet measured along the Conduit run.
(b)
Use manholes for existing monolithic-concrete storm sewers at the same locations as above except for intersections of inlet leads unless a manhole is needed to provide maintenance access at those intersections.
(c)
Do not place manholes in driveways or in the street in front of or immediately adjacent to a driveway.
(6)
Inlets
(a)
Locate inlets at low points in the gutter.
(b)
Valley gutters across intersections are not permitted.
(c)
Inlet spacing is a function of gutter slope. The minimum gutter slope shall be 0.30 percent. For minimum gutter slopes, the maximum spacing of inlets shall result from a gutter run of 500 feet from high point in pavement or the adjacent inlet on a continuously graded street section, with a maximum of 1000 feet of pavement draining towards any one inlet location.
(i)
Residential Development: Maximum spacing of inlets shall result from a gutter run of 500 feet from high point in pavement to the adjacent inlet on a continuously graded street section, with a maximum of 1000 feet of pavement draining towards any one inlet location.
(ii)
Commercial Development: Maximum spacing of inlets shall result from a gutter run of 400 feet from high point in pavement to the adjacent inlet on a continuously graded street section with a maximum of 600 feet of pavement draining towards any one inlet location.
(d)
Use only City of Dickinson standard inlets.
STANDARD STORM SEWER INLETS
INLET APPLICATION CAPACITY Type A Parking Lots/Small Areas 2.5 cfs Type B-B Residential/Commercial 5.0 cfs Type C Residential/Commercial 5.0 cfs Type C-1 Commercial 10.0 cfs Type C-2 Commercial 15.0 cfs Type C-2A Commercial 20.0 cfs Type E Roadside Ditches 20.0 cfs (e)
Do not use beehive grate inlets or other specialty inlets.
(f)
Do not use grate top inlets in unlined roadside ditches.
(g)
Do not place inlets in the circular portion of cul-de-sac streets unless justification based on special conditions can be provided and the City has approved such placement.
(h)
Place inlets at the end of proposed pavement, if drainage will enter or leave pavement.
(i)
Do not locate inlets adjacent to esplanade openings.
(j)
For new residential development, locate inlets at the center of lots and drainage system with lot site layout such that inlets are not located within the driveway between the radius end points as defined by the driveway radius intersection with the curb or edge of pavement.
(k)
Place inlets on side streets intersecting major streets, unless justification based on special conditions can be provided and the City has approved such placement.
(l)
For private development with internal site drainage, only one connection is permitted to any one inlet, and that connection (lead) shall be made to the back of the inlet. Connection shall not be made to the front face and to the short sides of the inlet unless approved by the City. Design the connection not to exceed the pipe capacity minus either the capacity listed in the Standard Storm Sewer Inlets Table above, or calculated inlet inflow.
(m)
For all new construction, convey public or private alleyway drainage to an inlet prior to entering the public street drainage system.
ATTACHMENT A
Rational Method 'C' ValuesLand Use or Land Cover Rational Coefficient 'C' Raw, undeveloped acreage 0.20 Improved, undeveloped acreage
(i.e., mowed, filled, graded, etc.)0.30 Park Land 0.40 Residential—1 acre lots or larger 0.40 Residential—½ to 1 acre lots 0.45 Residential—less than ½ acre lots 0.55 Multi-Family 0.75 Commercial/Industrial 0.90 ATTACHMENT B
Manning's 'n' ValuesChannel/Pipe Material Manning's 'n' Plastic Pipe (PVC & HDPE) 0.010 Clean Cast Iron 0.014 Concrete 0.013 Corrugated Metal 0.025 Smooth Bare Earth 0.018 Natural Channels (good condition) 0.025 Natural Channels (stones & weeds) 0.035 Natural Channels (poor condition) 0.060 Rip-rap 0.035 ATTACHMENT C
Design Intensity Values for Use in City of Dickinson
(Values calculated from TxDOT Intensity equations for Galveston County)3-Year Frequency Storm
Storm Duration Average Storm Intensity (in/hr) 15 min 5.87 30 min 4.15 45 min 3.27 1 hour 2.72 3 hour 1.26 6 hour 0.75 12 hour 1.44 24 hour 0.26 5-Year Frequency Storm
Storm Duration Average Storm Intensity (in/hr) 15 min 6.59 30 min 4.52 45 min 3.53 1 hour 2.93 3 hour 1.38 6 hour 0.84 12 hour 0.51 24 hour 0.30 25-Year Frequency Storm
Storm Duration Average Storm Intensity (in/hr) 15 min 8.81 30 min 6.08 45 min 4.77 1 hour 3.97 3 hour 1.89 6 hour 1.16 12 hour 0.71 24 hour 0.43 50-Year Frequency Storm
Storm Duration Average Storm Intensity (in/hr) 15 min 9.80 30 min 6.85 45 min 5.41 1 hour 4.53 3 hour 2.21 6 hour 1.38 12 hour 0.85 24 hour 0.52 100-Year Frequency Storm
Storm Duration Average Storm Intensity (in/hr) 15 min 9.83 30 min 6.93 45 min 5.51 1 hour 4.63 3 hour 2.29 6 hour 1.44 12 hour 0.90 24 hour 0.56 These values were calculated using the TxDOT Intensity equation in the form:
I = b/(T c + d) e
The intensity values for durations not shown in the tables above can be calculated directly using the following coefficients:
Storm
Frequencyb d e 3-year 77 11.9 0.782 5-year 66 7.6 0.739 25-year 85 7.6 0.727 50-year 88 7.6 0.704 100-year 85 7.8 0.690 ATTACHMENT D
City of Dickinson Signature BlockApproved by the City of Dickinson on the ____________ day of ____________ , 20 ____________ .
/s/
Chief Building Official or His or Her DesigneeATTACHMENT E
City of Dickinson NotesRequired Notes for Plats
Refer to Subdivision Ordinance.
Required Notes for Drainage Plans
1.
City must be notified in writing of intent to begin construction at least 48 hours prior to commencing construction and upon completion of improvements for the final inspection.
2.
Buildings, fences or other structures shall not be erected in City rights-of-way or drainage easements.
3.
The detention and drainage facilities are to be maintained by the property owner(s). (Use this note for private systems only)
4.
All drainage facilities shall have erosion control established upon completion. Contractor to provide the City with proposed grass type, application rate, and application method for approval prior to commencing work.
5.
Plantings, flower beds, other landscaping, or fill materials are not permitted in side lot drainage or detention easements.
ATTACHMENT F
