[CC 1990 §425.010; Ord. No. 98.002 §1, 1-12-1998]
The purpose of this Chapter is to lessen or avoid hazards to
persons and property caused by uncontrolled stormwater runoff or by
obstructions to drainage and to lessen the degradation of the quality
of surface runoff.
[CC 1990 §425.020; Ord. No. 98.002 §1, 1-12-1998]
Unless specifically defined, words or phrases shall be interpreted
so as to give them the meanings they have in common usage and to give
this Chapter its most reasonable application. The term "shall" is always mandatory and not discretionary; the word "may" is permissive.
The duly appointed City Administrator of the City or his/her
designated representative.
Any modifications, alterations or effects on a feature or
characteristic of surface waters, including their quality, quantity,
hydrodynamics, surface area, species composition, living resources,
aesthetics, or usefulness for human or natural uses which are or may
potentially be harmful or injurious to human health, welfare, safety
or property, to biological productivity, diversity or stability, or
which unreasonably interfere with the enjoyment of life or property,
including outdoor recreation. The term includes secondary and cumulative
as well as direct impacts.
The record owner or his/her authorized representative of
a tract of land that is the site of development or development activity
within the scope of this Chapter.
The elevation at any location delineating high waters having
a one percent (1%) probability of being equaled or exceeded in any
given year (also known as the 100-year flood).
The Board of Aldermen of the City of Desloge, Missouri.
Any structure built for the support, shelter or enclosure
of persons, animals, chattels or movable property of any kind.
The maximum ability of a storm drainage facility to convey,
detain or retain stormwater flows without causing substantial damage
to public or private property, and in the case of a pipe, without
surcharging.
A natural or artificial watercourse with definite bed and
banks which periodically or continuously contains moving water or
which forms a connecting link between two (2) bodies of water.
The City of Desloge, Missouri.
Any channel, pipe, sewer or culvert used for the conveyance
or movement of water, whether open or closed.
An element of a discharge structure which allows the gradual
release of water under controlled conditions.
Design elevation of a discharge structure at which, or below
which, water is contained behind the structure.
The delay of storm runoff prior to discharge into receiving
waters.
A facility constructed or modified to restrict the discharge
of stormwater to a prescribed maximum range and to concurrently detain
the excess waters that accumulate behind the outlet. A wet bottom
detention basin is a structure designed to retain a permanent pool
of water having provided its planned detention of runoff during a
storm event. A dry bottom detention basin is designed to be completely
de-watered after having provided its planned detention of runoff during
a storm event.
The temporary detaining or storage of stormwater in storage
basins, on rooftops, in streets, parking lots, school yards, parks,
open space or other areas under predetermined and controlled conditions,
with the rate of drainage therefrom regulated by appropriately installed
devices.
The volume of water equal to the difference between the control
elevation of a discharge structure and the maximum level at which
water can be accumulated prior to discharge times the average area
of open surface storage (between the control elevation and maximum
level) behind the discharge structure.
Any individual, firm, corporation, association, partnership
or other entity involved in commencing proceedings under this Chapter
to effect the development of land for himself/herself or another.
Any manmade change to improved or unimproved land including,
but not limited to, buildings or other structures, mining, dredging,
filling, grading, paving, excavating or drilling operations. Any subdivision
of land as defined in the City's subdivision regulations, excluding
minor subdivisions.
The difference in rate and volume of stormwater runoff from
a parcel or project in its undeveloped natural condition and its developed
condition is known as differential runoff.
The rate of outflow of water from detention storage.
Structural device, usually of concrete, metal, timber, etc.,
through which water is discharged from a project to the receiving
water.
The area from which water is carried off by a drainage system;
a watershed or catchment area.
An element in a drainage system which includes, but is not
limited to, any of the following:
Conduits and appurtenant features, canals, channels, detention
basins, ditches, streams, culverts, streets and pumping stations.
The surface or subsurface system which conveys water from
or over the land, including all watercourses, water bodies and wetlands
(also referred to as the stormwater management system).
A grant by a property owner to the public, a corporation
or a person of the use of land for a specific purpose.
Height in feet above mean sea level according to National
Geodetic Vertical Datum (NGVD).
Saddle or low point facility located around the rim of a
dammed impoundment such as detention or retention basin for providing
an efficient and safe means of conveying the maximum design flood
discharge through the impoundment, without overtopping the dam.
The volume and rate of flow of stormwater discharged from
a drainage area which is or will be in excess of that volume and rate
which is specified in this Chapter.
The special flood hazard land adjoining a watercourse, the
surface elevation of which is equal to or lower than the base flood
elevation (the 100-year flood), which is subject to periodic inundation
during floods.
The channel of a watercourse and those portions of the adjoining
floodplain which are reasonably required to carry and discharge the
base flood.
The higher portion of the floodplain, immediately adjacent
to and on either side of the floodway, occupied by quiescent or slow
moving water during floods.
The elevation of floodwaters of a flood of specified frequency
or occurrence at any given point.
A surface which has been compacted or covered with a layer
of materials so that it is highly resistant to infiltration of water;
the term includes most conventionally surfaced streets, roof, sidewalks,
parking lots and similar structures.
A structure on or other alteration of land on a given site,
including but not limited to grading, street paving, storm sewers,
sanitary sewers and utilities.
A feature or structure for temporarily storing excess stormwater
originating at two (2) or more sites, having devices for controlling
the rate of release of the stored water, and located downstream of
all sites where the runoff originates.
A feature or structure for temporarily storing excess stormwaters,
having devices for controlling the rate of release of the stored waters,
and located within the sites where the runoff originates.
Design elevation of a discharge structure at which point,
or above which point, water leaks out or bleeds out through a control
device down to the control elevation.
The maximum rate of flow of water at a given point in a channel
or conduit resulting from a predetermined storm or flood.
Those conditions which are expected to exist, or do exist,
after alteration, resulting from human activity, of the natural topography,
vegetation and rate, volume or direction of surface or subsurface
flow.
Those conditions which exist at the time this Chapter becomes
effective in terms of topography, vegetation and rate, volume or direction
of surface or subsurface flow, as indicated by the best available
historical data.
Includes major waterways and appurtenant structures or systems
whose total tributary area from origin to outfall exceeds or equals
fifty (50) acres.
An engineer or architect duly registered by the State to
practice engineering or architecture.
An empirical formula for calculating peak rates of runoff
resulting from rainfall. Expressed mathematically, the formula is:
|
Q = CIA
| |||
|
Where:
|
Q
|
=
|
Peak flow in cubic feet per second (cfs)
|
|
C
|
=
|
Runoff coefficient, or fraction of runoff to rainfall, dependent
on type of surface cover and topography
| |
|
I
|
=
|
Rainfall intensity, in inches per hour, for a specific storm
and duration
| |
|
A
|
=
|
Watershed area, in acres
| |
Any water body or stream into which surface waters flow.
The plan, usually in map form, prepared pursuant to the governing
body's zoning or other land development ordinances as a requirement
for the development of property.
The waters derived from rain falling or snow melting within
a tributary drainage basin, flowing over the surface of the ground
or collected in a stormwater drainage system, storm sewer or watercourse.
As defined in Chapter 410, Subdivisions, of the Code of Ordinances of the City of Desloge, Missouri.
The elapsed time for stormwater to flow from the most distant
point in a drainage basin to the outlet or point in question.
Any stream, creek, brook, branch; natural or artificial depression,
slough, gulch, reservoir, lake, pond or natural or manmade drainageway
in or into which stormwater runoff and floodwaters flow, either regularly
or intermittently.
Rainstorms of varying duration and intensity having a 6.67
percent probability of being equaled or exceeded in any given year.
Rainstorms of varying duration and intensities having a four
percent (4%) probability of being equaled or exceeded in any given
year.
Rainstorm of varying duration and intensity having a one
percent (1%) probability of recurring in any one (1) year.
[CC 1990 §425.030; Ord. No. 98.002 §1, 1-12-1998]
A.
Any person,
firm, corporation or business or government entity within the municipality
shall apply to the Administrative Officer for approval of a stormwater
management plan and obtain a permit before commencing any development
or development activity as defined herein.
B.
A stormwater
management plan shall be required of any platted subdivision for which
no approved improvement plans have been submitted at the time this
Chapter takes effect.
C.
Detention
requirements are directly related to permitted land use in the Zoning
Ordinance of the City of Desloge. The permitted densities and minimum
lot areas are important factors in the anticipated runoff. Table I
lists the zoning districts with major permitted use category, minimum
lot area and the corresponding percent of imperviousness.
D.
For permitted
land uses other than those listed in Table I such as schools, churches,
fire stations, etc., the rate of imperviousness shall be determined
by the same method as for commercial uses listed in Table 1. For conditional
land use in any zoning district, the same ratio determination criteria
shall apply.
|
Table I
| |||
|---|---|---|---|
|
Stormwater Data By Zoning Districts
| |||
|
Zone
|
Major Permitted Areas
|
Minimum Lot Area
(square feet)
|
Percent of Impervious Area for Design Standards in Table
II
|
|
R-1
|
Single-Family Residential
|
6,000 or less
|
45
|
|
R-1
|
Single-Family Residential
|
Greater than 6,000
|
35
|
|
R-2
|
Two-Family Residential
|
8,000
|
35
|
|
R-3
|
Multi-Family Residential
|
None
|
Determined Percentage of Impervious Area by Ration of Impervious
Surface Area to Total Area
|
|
R-4
|
Mobile Home
| ||
|
C-1
|
Commercial
| ||
|
C-2
|
Commercial
| ||
|
C-P
|
Commercial
| ||
|
M-1
|
Manufacturing
| ||
|
M-2
|
Manufacturing
| ||
|
M-P
|
Manufacturing
| ||
[CC 1990 §425.040; Ord. No. 98.002 §1, 1-12-1998]
A.
Development
within a subdivision shall not require approval of a stormwater management
plan if each of the following conditions have been met:
B.
Development
on isolated lots of record for all single-family and two-family dwelling
purposes shall not require approval of a stormwater management plan
nor a grading permit, unless the development involves altering, rerouting,
deepening, widening, obstructing or significantly changing any existing
drainage facilities, degrades the quality of water, adversely affects
any wetland or adversely affects any sinkhole, watercourse or water-body.
C.
Stormwater
management plans may be waived for building construction which will
not increase the amount of impervious area on the site and will not
adversely impact an existing drainage area or drainage on adjoining
properties.
D.
Written
notice of intent to commence development shall be delivered to the
Administrative Officer prior to undertaking any development activity,
whether exempt from plan provisions or not. Application for a building
permit may constitute notice of intent.
[CC 1990 §425.050; Ord. No. 98.002 §1, 1-12-1998]
A.
Purpose. The purpose of the stormwater management plan review
process is to provide an organized framework for evaluating and acting
upon proposals for development as they relate to stormwater management
issues.
B.
Required Information. It is the responsibility of an applicant
to include sufficient information in the stormwater management plan
to enable evaluation of the environmental quality of the affected
area, the potential and predicted impacts of the proposed activity
on affected waters, and the effectiveness and acceptability of the
measures proposed by the applicant for preventing or reducing adverse
impacts. The applicant shall furnish the Administrative Officer with
three (3) copies of the stormwater management plan application form
and stormwater management plan, sealed by a professional engineer
or architect registered in the State.
[CC 1990 §425.060; Ord. No. 98.002 §1, 1-12-1998]
A.
A stormwater
management plan is designed to safely manage the stormwater runoff
following the rainstorms which exceed the maximum allowable release
rate and the capacity of the stormwater drainage system and/or the
storm sewer system by detention of the excess stormwater runoff. The
plan shall provide or be accompanied by maps at a minimum scale of
one (1) inch less than or equal to two hundred (200) feet and other
descriptive material, including the basis of computation, showing
the following:
1.
Required
pre-development site information (map scales are minimums):
a.
Detailed
location sketch showing the parcel and major adjacent roads.
b.
Topographic
map of the site at a scale of one (1) inch less than or equal to two
hundred (200) feet with maximum five (5) foot contour intervals, except
in floodplains or other areas of low relief where a smaller interval
may be required.
c.
Where
percolation or infiltration systems are proposed, information as to
the location and type or vegetative cover and soil types and characteristics
representative of the design condition mapped at a scale of one (1)
inch less than or equal to two hundred (200) feet.
d.
Location
of streams and other floodwater runoff channels, their normal channels,
and the extent of the floodplains at the established high water elevations,
and the limits of the floodway mapped at a scale of one (1) inch less
than or equal to two hundred (200) feet.
e.
Location
of lakes, ponds, swamps and detention basins indicating their normal
shorelines, floodplains and lines of inflow and outflow mapped at
a scale of one (1) inch less than or equal to two hundred (200) feet.
f.
Location
of farm drains, inlets and outfalls, storm and sanitary sewers and
outfalls, septic tank systems, and outlets, if any, and seeps, springs,
and flowing and other wells mapped at a scale of one (1) inch less
than or equal to two hundred (200) feet.
g.
Location
and description of nearby existing off-site water management facilities
such as wells, lakes, drainageways, etc., which are potentially directly
affected by the proposed construction or development, mapped at a
scale of one (1) inch less than or equal to two hundred (200) feet.
2.
For
all existing drainage facilities which are to be maintained. altered
or enlarged as part of the stormwater management system, provide information
as to their size, slopes, depths, outfalls, receiving waters, elevations,
cross-sections, profiles, construction materials and other design
details as applicable.
3.
Location
of all new drainage facilities, including detention basins, to be
constructed. Provide design details on each facility as applicable.
4.
Location
and extent of existing and proposed impervious surfaces (roads, parking
lots, building, etc.) and their elevations. Provide grading and paving
plans and specifications.
5.
Location
and extent of rights-of-way and easements for the stormwater management
system, including all areas to be dedicated for water management purposes.
6.
Identification
and description of any special or required maintenance procedures
to keep the project functioning as designed.
7.
Provide
stormwater management system design calculations as follows:
a.
Design
storms used.
b.
Calculated
hydrographs of inflow and outflow of design storm runoff for the project
site under natural and undeveloped conditions.
c.
Hydrographs
of the runoff from the design storm runoff under developed conditions.
d.
For
all detention basins, a plot or tabulation of storage volumes with
corresponding water surface elevations and of the basin outflow rates
for those water surface elevations.
f.
Runoff
routing calculations showing discharge, elevations and volumes retained
and/or detained during applicable storm event.
g.
Calculations
required for determination of minimum building floor and road elevations.
8.
Identify
the entity responsible for operation and maintenance of the system.
9.
The
requirements for specific plan elements cited above may be waived
by the Administrative Officer upon written supportive recommendation
of the City Engineer. The stormwater management plan for minor development
activities may consist of a certification from a professional engineer
or architect that the differential runoff is not significant.
10.
The
Administrative Officer will ascertain the completeness of the stormwater
management plan application. The Administrative Officer will return
incomplete applications to the applicant for completion and resubmission.
After determining that the application is complete, the Administrative
Officer will distribute copies of the application for review and comment
to other persons with special technical expertise as deemed appropriate
by the City Administrator. Within a reasonable period after submission
of the complete stormwater management plan application by the applicant,
the Administrative Officer shall approve, approve with specified conditions,
or deny the application and shall notify the applicant accordingly.
If the stormwater management plan is denied or approved with conditions,
the Administrative Officer shall state the reasons for denial or the
specified conditions. If the applicant fees aggrieved due to the denial
or conditions, he/she may appeal the decision to the Board of Aldermen.
[CC 1990 §425.070; Ord. No. 98.002 §1, 1-12-1998]
All stormwater management plan projects shall be prepared by
and bear the seal of a professional engineer prior to the submission
of the stormwater management plan application to the City. All fees
for such review and evaluation shall be paid by the property owner
or developer directly to the engineer, and the engineer shall certify
that he/she has reviewed the proposed stormwater management plan and
that said plan conforms with all design requirements and other requirements
imposed by this Chapter. In addition, a permit fee of twenty-five
dollars ($25.00) shall be remitted to the City with the application
plan.
[CC 1990 §425.080; Ord. No. 98.002 §1, 1-12-1998]
A.
A stormwater
management system shall be provided for protecting lots, roads, streets
and buildings in the project area from the potential adverse impacts
of stormwater runoff. Streets, blocks, depths of lots, parks and other
public grounds and ultimate land usage shall be determined, located
and laid out in such a manner as to reduce the velocity of overland
flow and allow the maximum opportunity for infiltration of stormwaters
into the ground and to preserve and utilize natural streams, channels
and detention basins and, whenever possible, to include streams and
floodplains within parks or other public grounds.
B.
The maximum
allowable release rate of stormwater after development shall not exceed
the before development rate, based on a 15-year frequency storm. The
total drainage area must be used in calculating the allowable release
rate. The required storage volume will be based on the project area
only, with extraneous flows from upland areas being by-passed or discharged
via overflow spillway or other devices.
C.
The increased
stormwater runoff resulting from the proposed development shall be
detained on-site by appropriate detention basins; by storage on flat
roofs, parking lots or streets; or by other acceptable techniques.
The minimum volume of storage shall be sufficient to store the volume
from a 25-year storm (under developed conditions). Control devices
shall limit the discharge from storage to a rate no greater than that
prescribed by this Chapter. Downstream property, watercourses, channels
or conduits shall not receive stormwater runoff from proposed development
at a higher peak flow rate than that which existed prior to the development.
D.
Generally
acceptable locations of the stormwater runoff channels in the design
of the project area may include the following:
1.
In a
depressed median of a double roadway, street or parkway, provided
the median is wide enough to permit required roadway shoulders.
2.
Centered
on the rear lot lines in a block, or entirely within the rear yards
of a single row of lots or parcels, provided that in either case the
lots are deep enough to permit side slopes in the runoff channel and
a building site of the evaluation required by other Sections of this
Chapter.
3.
In a
depressed area along roadway, street or parkway, provided the area
is wide enough to permit side slopes as required in other Sections
of this Chapter.
4.
Alternative
systems such as curb and gutter and storm sewers that discharge into
an appropriate detention basin may be incorporated into the stormwater
management plan.
5.
The
smaller, uppermost reaches of drainage channels and swales forming
the natural drainage system near a watershed divide may be relocated
or eliminated and incorporated into the planned system of storm sewers
and open channels designed consistent with the requirements of this
Chapter.
E.
A continuous
easement may be provided abutting rear lot lines as shown in the stormwater
management plan within which utility lines and open drainage facilities
may be located and maintained, but on which no accessory buildings
may be constructed. The utility drainage easement shall be at least
twenty (20) feet wide. Within the area so designed for the passage
or storage of waters, no structure may be erected, no fences, shrubbery
or trees planted or changes made to the prescribed grades and contour
of the specified floodwater or stormwater runoff channels.
F.
All storm
sewer outfalls shall be so designed, by reason of elevation of the
invert, by a flap gate or by other features, that when the receiving
stream is in full flood, the storm sewers will continue to drain the
areas they are designed to serve unless the provision is made for
sewer backups into planned storage locations.
G.
All sanitary
sewer manholes constructed in a floodplain, in a street designed for
detention or in an area designed for the storage or passage of floodwater
or stormwater, shall be provided with either a watertight bolted manhole
cover, a watertight concealed pick-hole cover, or be constructed with
a rim elevation at or above the high water elevation of the 100-year
flood or the high water elevation of the design storm, whichever is
applicable to the specific area.
H.
Projects
that are to be developed in phases will normally require the submission
of a master plan of the applicant's contiguous land holdings. Applications
for individual project phases may be considered only when the phases
are totally independent of, or make sufficient provisions for, adjacent
lands.
[CC 1990 §425.090; Ord. No. 98.002 §1, 1-12-1998]
A.
Design Storms. Stormwater systems will be designed with
sufficient hydraulic capacity as a minimum for the following frequencies
and durations:
|
|
Type of Facility
|
Design Frequency
(Year)
| |
|---|---|---|---|
|
1.
|
Detention basins
|
25
| |
|
2.
|
Primary drainage systems
|
25
| |
|
3.
|
Bridges
|
50
| |
|
4.
|
Secondary drainage systems (i.e., cross-drains and ditches for
internal subdivision drainage)
|
15
| |
B.
The Administrative
Officer may require alternative designs or features to reduce the
cost of long-term maintenance.
C.
In critical
areas, the Administrative Officer may require additional hydraulic
capacity above the minimum set forth above, up to the 100-year frequency
design.
[CC 1990 §425.100; Ord. No. 98.002 §1, 1-12-1998]
A.
Method Of Evaluation. Differential runoff evaluation consists
of the determination of rates of runoff before and after development,
the determination of required volume of detention and verification
of adequacy of discharge and control structures.
B.
Differential Runoff Rates. Differential runoff rates for
all three (3) size classes of projects shall be evaluated by the rational
formula. The runoff coefficients in Table II shall be used. Differential
runoff rates shall be evaluated by equation:
|
|
R = (Rd-Ru)
|
(Equation 1)
| |||
|
|
Where
|
R
|
=
|
Differential runoff rate
| |
|
Rd
|
=
|
C I factor for developed conditions
| |||
|
Ru
|
=
|
C I factor for undeveloped conditions
| |||
C.
Volume Detention. Volumes of detention for the three (3)
classes of projects shall be evaluated according to the following
methods:
1.
Volume
of detention for projects of less than twenty-five (25) acres shall
be evaluated by the "simplified volume formula".
2.
Volume
of detention for projects of twenty-five (25) acres or greater, but
less than two hundred (200) acres, may be evaluated either by the
"simplified volume formula" or the "modified rational hydrograph method".
3.
For
projects larger than two hundred (200) acres, the owner's engineer
shall submit his/her proposed method of evaluation for the sizing
of the detention basin to the Administrative Officer. The method will
be evaluated for professional acceptance, applicability and reliability.
No detail review for projects larger than two hundred (200) acres
will be rendered before the method of evaluation of the detention
is approved.
4.
Other
analytical methods of evaluation of volume of detention will be considered,
provided that reliability, professional acceptance and proper application
are approved by the Administrative Officer.
D.
Formulae. Any of the following methods may be used for calculating
detention volume as applicable:
1.
Simplified volume formula. Total volume of detention shall
be computed by the equation:
2.
Graphic representation. For purposes of further analysis,
the simplified volume formula may be represented by a triangular synthetic
hydrograph as shown in Figure III with the following elements:
|
|
Tb
|
=
|
Base time of hydrograph for developed project without detention
|
|
|
Tb
|
=
|
Sixty (60) minutes
|
|
|
Tp
|
=
|
Time of peak runoff of developed project
|
|
|
Tp
|
=
|
Twenty (20) minutes
|
|
|
Qd
|
=
|
Total peak runoff of developed project in cfs
|
|
|
Qd
|
=
|
A x Rd (see equation 1)
|
|
|
Qu
|
=
|
Total peak runoff of unimproved project in cfs
|
|
|
Qu
|
=
|
A x Ru (see equation 1)
|
|
|
A
|
=
|
Total area of project in acres
|
|
|
Tq
|
=
|
Assumed time of peak differential for unimproved project
|
|
|
Tq
|
=
|
Qu/Qd x 40
|
|
|
Tr
|
=
|
Assumed recendence time differential for discharge at rates
no greater than unimproved condition
|
|
|
Tr
|
=
|
(30 Qd/Qu) - 60
|
|
|
V
|
=
|
Volume of detention
|
|
|
V
|
=
|
(Qd-Qu) x 30 (min) x 60 (sec/min)
|
3.
Modified rational hydrograph method. This is a modification
of the unit hydrograph method of hydrologic evaluation simplified
to reflect features of present practice and some elements of topographic
characteristics, concentration patterns and routing. Figure III illustrates
the elements of the modified hydrograph. Steps to develop the hydrograph
are as follows:
a.
Determine
the time of concentration for the project by use of Chart IV or similar
reference manual adopted by the City Engineer. Chart IV is based on
Overland Flow Time from Seeley Data Book for Civil Engineers. Enter
chart with overland travel distance; follow horizontally to slope
curve then vertically down to C or P factor listed in Table II and
obtain Tc. For analysis of large improved channels, time for travel
for overland flow and channel are to be analyzed to determine reasonable
(Tc) time of concentration.
b.
Determine
time of peaking by equation:
|
|
Where
|
Tp
|
=
|
D/2 + 0.6 Tc
|
(Equation 3)
|
|
Tp
|
=
|
Time of peak discharge of developed project in minutes
| |||
|
D
|
=
|
20 minutes = storm duration in minutes
|
c.
Determine
the base time of the hydrograph without detention, by equation:
|
|
Tb = 2.67 Tp
|
(Equation 4)
| |||
d.
Determine
the base time of the hydrograph with detention by equation:
|
|
Tr = Tb (Qd - 1)
|
(Equation 5)
| |||
|
Tr
|
=
|
Tb
|
(Qd - 1)
(Qu )
| ||
|
|
Where
|
Tr
|
=
|
Additional time required for discharge at a rate no greater
than that of the undeveloped condition.
| |
|
|
Qd = Total peak runoff of improved project in cfs
|
(Equation 6)
| |||
|---|---|---|---|---|---|
|
Qd
|
=
|
A x Rd (see equation 1)
| |||
|
|
Qu = Total runoff of unimproved project in cfs
|
(Equation 7)
| |||
|---|---|---|---|---|---|
|
Qu
|
=
|
A x Ru (see equation 1)
| |||
e.
Determine
the required volume of detention by equation:
|
|
V = ½ (Qd - Qu)Tb
|
(Equation 8)
| |||
E.
Control Structures. Detention facilities shall be provided
with obvious and effective control structures. Plan view and sections
of the structure with adequate detail shall be included in plans.
The design discharge (Q) for the low-flow weir shall not exceed the
15-year (frequency) with a duration equal to the time of concentration
for the watershed, runoff from the tributary area for five percent
(5%) imperviousness (1.7 cfs per acre). The maximum discharge shall
be designed to take place under total anticipated design-head conditions.
Sizing of the low-flow weir shall be by appropriate weir formulas.
The overflow opening or spillway shall be designed to accept the total
peak runoff of the improved tributary area. The low-flow channel invert
through dry detention facilities on other permanently flowing streams
shall be paved.
F.
Detention Design Criteria. Detention in the overall system, including swales, lakes, canals, greenways, etc., shall be provided as follows in Subsection (G).
G.
General.
1.
Storage volumes. The minimum volume of storage provided
in detention basins, together with such storage as may be authorized
in other on-site facilities, shall be sufficient to store the volume
from a 25-year storm (under developed conditions) with a discharge
no greater than the rate of flow from a 15-year storm (under undeveloped
conditions).
2.
Outlet control structures. Outlet control structures shall
be designed to operate simply and automatically. They will limit discharges
into existing or planned downstream channels or conduits so as not
to exceed predetermined maximum authorized peak flow rates.
3.
Emergency spillways. Emergency overflow facilities must
be provided in all instances so that stored waters will not exceed
the safe capacity of the basin. At a minimum, the emergency spillway
must be able to pass without damage the 100-year storm.
4.
Designs
should result in aesthetically pleasing configurations which will
enhance public acceptability.
H.
Dry-Detention Facilities.
1.
Perimeter
maintenance easements of fifteen (15) feet (minimum) width at slopes
no steeper than 3:1 (horizontal to vertical) shall be provided.
2.
Side
slopes of the facility shall not be steeper than 3:1 (horizontal to
vertical).
3.
Provisions
must be incorporated to facilitate complete interior drainage to dry
bottom basins, to include the provision of natural grades to outlet
structures, longitudinal and transverse grades to perimeter drainage
facilities, or the installation of subsurface drains.
4.
Multipurpose
features may be designed to serve secondary purposes for recreation,
open space or other types of use which will not be adversely affected
by occasional or intermittent flooding.
5.
The
limits of maximum ponding elevation shall not be closer than twenty-five
(25) feet horizontally from any building and less than two (2) feet
vertically below the lowest sill elevation unless approved by variance.
The entire reservoir area shall be seeded, fertilized and mulched,
sodded or paved.
I.
Wet Detention Facilities.
1.
Perimeter
maintenance and operation easements of fifteen (15) feet (minimum
preferable) width at slopes no steeper than 3:1 (horizontal to vertical)
shall be provided.
2.
Control
elevations should be no higher than two and one-half (2½) feet
below the minimum road centerline elevation in the area served by
the control device in order to protect the road subgrade when structures
are constructed near roads.
3.
Side
slopes shall not be steeper than 3:1 (horizontal to vertical) out
to a depth of two (2) feet below the control elevation, then as steep
as soil stability will allow.
4.
If fish
are to be used to help keep the basin clean, at least one-quarter
(¼) of an area of the permanent pool must be a minimum depth
of ten (10) feet.
5.
For
emergency purposes, cleaning or shoreline maintenance, facilities
shall be provided or plans prepared for the use of auxiliary equipment
to permit emptying and drainage.
6.
The
limits of maximum ponding elevations are to be no closer than twenty-five
(25) feet horizontally from any building and no less than two (2)
feet below the lowest sill elevation of any building.
J.
Impervious Areas. Paved parking lots may be designed to
provide detention storage of stormwaters on all or a portion of their
surfaces. Outlets will be designed so as to slowly empty the stored
waters, and depths of storage must be limited to a maximum depth of
twelve (12) inches so as to prevent damage to parked vehicles. Ponding
should be regulated to those portions of the parking lots farthest
from the area served.
K.
Rooftop Storage. Detention storage requirements may be met
in total or in part by detention of flat roofs. Details of such designs,
to be included in the stormwater management plan application, shall
include the depth and volume of storage, details of outlets devices
and down-drains, elevations of overflow scuppers, design loadings
for the roof structure and emergency overflow provisions.
L.
Underground Storage. All or a portion of the detention storage
may also be provided for in underground facilities, as long as all
applicable requirements of this Chapter are met. details of such designs
are to include type of facility, depth and volume of storage, details
of inlet and outlet devices and locations, emergency overflow provisions,
and measures to be used for surface and ground water pollution control.
M.
Design Alternatives. The above listing of design criteria
is not intended to preclude the use of other known state-of-the-art
methods and available best management practices and should not be
construed as a mechanism to discourage innovative design concepts.
N.
Developments Adjoining A Floodplain. Where a development
adjoins or encompasses a portion of a floodplain for a 100-year flood,
the following shall apply:
1.
The
applicant shall show the floodplain and floodway on the stormwater
management plan.
2.
The
applicant shall include in the stormwater management plan all other
plans, plats, specifications, etc., required by Federal, State, County
and/or municipal law or regulations detailing such provisions or restrictions
as are necessary to comply with the following:
O.
Alterations Affecting Flow. The applicant shall not alter
any channel from conveying, in its post-development state, the same
amount of flow, at the same or lower maximum water elevation, that
it conveyed in its pre-development state.
P.
Additional Information. The applicant shall furnish, for
the Administrative Officer's review and approval, the following information
pertaining to proposed channel modifications:
1.
Typical
cross-sections of the existing and proposed channel.
2.
Plan
view of the channel showing the location of existing constrictions,
obstructions and other non-typical areas.
3.
Hydrographs
and/or flood routing calculations and backwater curve profiles of
the proposed waterway corresponding to a storm recurrence interval
of one hundred (100) years.
4.
Engineering
evaluation of all potential increases in flood hazards to the adjacent
upstream or downstream private or public lands and facilities located
thereon, showing provisions for eliminating any and all adverse impacts
on such land and facilities at no public cost.
5.
Minimum
finished floor elevations which shall be set at or above the maximum
water surface elevation as determined by either or both of the following:
"Flood Insurance Rate Map" published by the Federal Emergency Management
Agency; or backwater curve profiles of the proposed waterway due to
a 100-year storm recurrence interval.
6.
Designation
on the final plan of all areas reserved for flood routing, detention
or storage, together with the required wording pertaining to restrictions,
dedications and maintenance responsibilities of such areas.
Q.
Credit For Detention Storage. If detention storage is provided
within a floodplain, only the net increase in storage volume above
that which naturally existed on the floodplain shall be credited to
the development. No credit will be granted for volumes below the elevation
of the regulatory flood at the location unless compensatory storage
is also provided.
R.
Verification Of Adequacy. Analysis of all elements of design
is to be verified by the engineer of record submitting the plan. The
following outline is provided to ascertain that certain critical elements
are in workable compliance with the aims of this Chapter.
For projects less than fifty (50) acres in area, there is no
need for submittal of routing calculations or tabulated proof of adequacy
of tributary runoff for detention; however, it is recommended that
verification be made of:
1.
Volume
of detention for the total project;
2.
Tributary
(Q) peak runoff to basin;
3.
Balanced
maximum outflow rate from the low-flow structure;
4.
Ratios
of inflow to outflow rates;
5.
Sizing
of the overflow facilities;
6.
Stability
of detention dikes;
7.
Safety
features;
8.
Maintenance
features.
|
|
For projects of fifty (50) acres or greater but less than two
hundred (200), routing calculations shall be submitted in legible
tabulated form. Proof of adequacy of volume detention and sizing computations
for low-flow structure shall also be submitted. Features of stability
and safety may also need to be documented if the scope of the project
requires special attention in this area of design.
|
|
|
Projects over two hundred (200) acres in area shall provide
documented verification of adequacy according to scope and complexity
of design.
|
|
Table II
Average CI (PI) Factors In CFS Per Acre
20-Minute Inlet Time
| |||||||
|---|---|---|---|---|---|---|---|
|
Impervious Area
(Percent)
|
C or P
(Factor)
|
15-Year
|
25-Year
|
50-Year
|
100-Year
| ||
|
5
|
0.37
|
1.7
|
2.00
|
2.18
|
2.37
| ||
|
10
|
0.39
|
1.79
|
2.11
|
2.30
|
2.50
| ||
|
15
|
0.41
|
1.89
|
2.21
|
2.42
|
2.62
| ||
|
20
|
0.435
|
2.0
|
2.35
|
2.57
|
2.78
| ||
|
25
|
0.455
|
2.09
|
2.46
|
2.68
|
2.91
| ||
|
30
|
0.475
|
2.10
|
2.57
|
2.80
|
3.04
| ||
|
35
|
0.495
|
2.28
|
2.68
|
2.92
|
3.17
| ||
|
40
|
0.52
|
2.39
|
2.81
|
3.07
|
3.33
| ||
|
45
|
0.54
|
2.48
|
2.92
|
3.19
|
3.46
| ||
|
50
|
0.56
|
2.58
|
3.02
|
3.30
|
3.58
| ||
|
55
|
0.58
|
2.68
|
3.13
|
3.42
|
3.71
| ||
|
60
|
0.60
|
2.76
|
3.24
|
3.54
|
3.84
| ||
|
65
|
0.625
|
2.88
|
3.38
|
3.69
|
4.00
| ||
|
70
|
0.645
|
2.97
|
3.48
|
3.81
|
4.13
| ||
|
75
|
0.665
|
3.06
|
3.59
|
3.92
|
4.26
| ||
|
80
|
0.685
|
3.15
|
3.70
|
4.04
|
4.38
| ||
|
85
|
0.705
|
3.24
|
3.81
|
4.16
|
4.51
| ||
|
90
|
0.73
|
3.36
|
3.94
|
4.31
|
4.67
| ||
|
95
|
0.75
|
3.45
|
4.05
|
4.43
|
4.80
| ||
|
100
|
0.77
|
3.54
|
4.16
|
4.54
|
4.93
| ||
|
Rainfall
|
4.60
|
5.40
|
5.90
|
6.40
| |||
|
From U.S. Weather Bureau Technical Paper No. 40: "Rainfall Frequency
Atlas of the United States for durations from 30 Minutes to 24 Hours
and Return Periods from 1 to 100 Years."
| |||||||
S.
Installation
Of Stormwater Runoff Control Measures. Positive stormwater runoff
control shall be provided for during development. Stormwater management
plans shall include a schedule for the installation, construction
or modification of all drainage facilities. Erosion control measures
and a schedule for their installation shall be shown on the stormwater
management plan. Installation of drainage facilities and erosion control
measures shall proceed as scheduled in the approved stormwater management
plan.
[CC 1990 §425.110; Ord. No. 98.002 §1, 1-12-1998]
A.
All stormwater
management facilities shall be constructed within an easement or right-of-way
dedicated for stormwater management use and connected to a public
road or other location from which operation and maintenance are legally
available. Minimum rights-of-way and maintenance easements shall be
provided by instrument or plat dedication for all waterways used to
convey or detain runoff. Where permanent maintenance will be provided
by the property owner, maintenance easements or rights-of-way may
not be required. The minimum widths of rights-of-way and easements
shall be as follows:
|
|
Facility
|
Maintenance Access Width
|
|---|---|---|
|
|
Open drainage channel or facility
|
Thirty (30) feet, fifteen (15) feet each side
|
|
|
Greenways
|
Width of greenway
|
|
|
Pipes and culverts
|
Fifteen (15) feet, centered
|
|
|
Facility
|
Maintenance Access Width
|
|---|---|---|
|
|
Detention areas
|
Fifteen (15) feet continuous around total area
|
|
|
Connecting access
|
Fifteen (15) feet
|
B.
Easements
must include the "top of the bank width" and the maintenance access
width.
C.
The maintenance
access width begins at the point of the bank or slope of the facility.
D.
The maintenance
access width may be required by the Administrative Officer in special
circumstances where more width on one (1) or both sides is necessary
for maintenance purposes.
[CC 1990 §425.120; Ord. No. 98.002 §1, 1-12-1998]
A.
Purpose. The purpose of this Section is to establish maintenance
standards to ensure that, after stormwater management facilities have
been properly designed and constructed, they continued to function
properly. Continued proper functioning is highly dependent upon the
proper maintenance of the facilities.
B.
Design Of Facilities. Maintenance costs over a period of
years will generally surpass the costs of initially constructing water
management facilities. The applicant and the Administrative Officer
should utilize all appropriate methods, approaches and techniques
to design and construct these facilities in such a manner that fulfills
the requirements of this Chapter, facilitates their inspection, and
minimizes future maintenance costs.
C.
Maintenance Responsibilities.
1.
The
growth of noxious weeds, the creation of conditions which support
the growth of mosquitoes and other insects, and the decrease in available
storage by accumulated sediments shall be controlled. The cleanup
of accumulated debris, flotsam and other materials after runoff events
have subsided shall be assured.
2.
Detention
facilities, when mandatory, are to be built in conjunction with the
storm sewer installation and/or grading. Since these facilities are
intended to control increased runoff, they must be partially or fully
operational soon after the clearing of the vegetation. Silt and debris
connected with early construction shall be removed periodically from
the detention area and control structure in order to maintain close
to full storage capacity.
3.
The
responsibility for maintenance of stormwater management facilities
in single-lot development projects shall remain with the owner, developer
and general contractor until final inspection of the development is
performed and approved and a legal occupancy permit, where applicable,
is issued. After legal occupancy of the project, the maintenance of
stormwater management facilities shall be vested with the owner of
the project.
4.
The
responsibility for maintenance of stormwater management facilities
in subdivision projects shall remain with the developer until such
time as responsibility is transferred, under appropriate legal arrangements,
to the private individual owner in the subdivision, the trustees of
the subdivision, the City or such other maintenance entity or authority
as may be proposed and approved by the Board of Aldermen.
5.
If responsibility
is to be transferred to the private individual owners in the subdivision,
the developer shall assure perpetual maintenance of the drainage management
facilities, including detention systems if any are included, through
the adoption of maintenance agreements or covenants for any facilities
that remain in private ownership. Such agreements or covenants shall
be subject to the approval of the Administrative Officer.
6.
If the
responsibility is to be transferred to the trustees of the subdivision,
the developer shall establish a suitable indenture of trust. Upon
release of escrows required for the subdivision development, the responsibility
for maintenance shall be vested in the trustees of the subdivision
by virtue of the trust indenture. The indenture of trust shall clearly
indicate resident responsibility for maintenance and shall be subject
to the approval of the Administrative Officer.
7.
If responsibility
for maintenance is to be transferred to the City, the system(s) to
be maintained by the City shall have adequate easements, dedicated
to the public for stormwater management use, to permit the City to
inspect and enter upon the property, as necessary, to maintain the
system(s).
D.
Performance And Maintenance Security. Security against defects
in workmanship shall be required for any portion of the drainage management
facilities dedicated to the public. The terms of the performance security
shall apply to any repairs needed and maintenance needed during the
two (2) year period following completion. Inspection and repair of
defects shall be required as a condition of release of the posted
security.
[CC 1990 §425.130; Ord. No. 98.002 §1, 1-12-1998]
The applicant shall be required to adhere strictly to the stormwater
management plan as approved. Any changes or amendments to the plan
must be approved by the Administrative Officer in accordance with
the procedures set forth in this chapter for obtaining stormwater
management plan approval. Enforcement officials shall be and are herein
granted inspection rights and right-of-entry privileges in order to
ensure compliance with the requirements of this Chapter.
[CC 1990 §425.140; Ord. No. 98.002 §1, 1-12-1998]
A.
Approved Projects. The Administrative Officer or his/her
designated representatives shall carry out periodic inspections of
the project site to ensure the applicant's compliance with this Chapter.
If it is determined that the project is not being carried out in accordance
with the approved stormwater management plan, the Administrative Officer
is authorized to:
1.
Written notice. Issue written notice to the applicant or
owner, specifying the nature and location of the alleged non-compliance,
with a description of the remedial actions necessary to bring the
project into compliance within a reasonable specified time.
2.
Stop-work order. Issue a stop-work order directing the applicant
or owner to cease and desist all or any portion of the work which
violates the provisions of this Chapter, if the remedial work identified
in the "written notice" is not completed within the specified time.
3.
Revocation of approval. Should the applicant or owner not bring the project into compliance with the written notice and stop-work order, he/she shall then be subject to immediate revocation of his/her stormwater management plan approval and to the penalties described in Section 415.150, Penalties for Violation.
4.
Appeal. Any notice, order or revocation issued pursuant to Subparagraphs (1), (2) or (3) of Subsection (A) above shall become final unless the person or persons named therein requests, in writing, no later than ten (10) days after the date such notice, order or revocation is served, a hearing before the Board of Aldermen.
B.
Unapproved Projects. With respect to any development or
development activity determined by the Administrative Officer or his/her
designated representative to be subject to this Chapter, and being
carried out without approval, the Administrative Officer is authorized
to:
1.
Written notice. Issue written notice to the owner, specifying
the nature and location of the alleged non-compliance, with a description
of the remedial actions necessary to bring the project into compliance
within a reasonable specified time.
2.
Stop-work order. Issue a stop-work order directing the owner
to cease and desist all or any portion of the work which violates
the provisions of this Chapter, if the remedial work identified in
the "written notice" is not completed within the specified time.
3.
Non-compliance with stop-work order. Should the owner not bring the project into compliance with the written notice and stop-work order, he/she shall then be subject to the penalties described in Section 415.150, Penalties for Violation.
4.
Appeal. Any notice, order or revocation issued pursuant to Subparagraphs (1), (2) or (3) of Subsection (B) above shall become final unless the person or persons named therein requests, in writing, no later than ten (10) days after the date of such notice, order or revocation is served, a hearing before the Board of Aldermen.
[CC 1990 §425.150; Ord. No. 98.002 §1, 1-12-1998]
A.
General. Violation of the provisions of this Chapter or
failure to comply with any of its requirements, including conditions
and safeguards established in connection with variances or special
use permits, shall constitute a misdemeanor. Any person who violates
this Chapter or fails to comply with any of its requirements shall
upon conviction thereof, be fined no more than five hundred dollars
($500.00) or imprisoned for not more than three (3) months, or both,
and in addition, shall pay all costs and expenses involved in the
case. Each day such violation continues shall be considered a separate
offense.
B.
Corrective Actions. Nothing herein contained shall prevent
the City from taking such other lawful action as is necessary to prevent
or remedy any violation. All such costs connected therewith shall
accrue to the person or persons responsible.
[CC 1990 §425.160; Ord. No. 98.002 §1, 1-12-1998]
This Chapter shall not in any way limit or modify the vested
rights of any person to complete any development or improvements to
lands based upon prior law, where a previous permit or authorization
has been granted or applied for and where such previous permit or
authorization remains in effect. The City may acknowledge vested rights
in other circumstances where it is equitable and just.
[CC 1990 §425.170; Ord. No. 98.002 §1, 1-12-1998]
In the case of conflict within this Chapter or any part thereof
and the whole or part of any other existing or future ordinance or
code of the City, the most restrictive in each case shall apply.
[CC 1990 §425.180; Ord. No. 98.002 §1, 1-12-1998]
Before starting any work regulated by this Chapter, an applicant
shall comply with the requirements set forth in all other applicable
ordinances with respect to the submission and approval of preliminary
and final subdivision plats, site plans for and building, construction
and rezoning improvement plans, grading and zoning permits, along
with those set forth in this Chapter and as may be required by State
Statutes and the regulations of any Department of the State.
[CC 1990 §425.190; Ord. No. 98.002 §1, 1-12-1998]
In the interpretation and application of this Chapter, the provisions
expressed herein shall be held to be the minimum requirements and
shall be liberally construed in favor of the City and shall not be
deemed a limitation or repeal of any other powers granted by the State
Statutes.
[CC 1990 §425.200; Ord. No. 98.002 §1, 1-12-1998]
A.
Separability. If any part of this Chapter is held to be
unconstitutional, it shall be construed to have been the legislative
intent to pass this Chapter without such unconstitutional part, and
the remainder of this Chapter, as to the exclusion of such part, shall
be deemed and held to be valid as if such part had not been included
herein. If this Chapter or any provision hereof is held to be inapplicable
to any persons, group of persons, property, kind of property, circumstances
or set of circumstances, such holding shall not affect the applicability
hereof to any other person, property or circumstances.
B.
Abrogation And Greater Restrictions. It is not intended
by this Chapter to repeal, abrogate or impair any existing easements,
covenants or deed restrictions. However, where this Chapter imposes
greater restrictions, the provision of this Chapter shall prevail.
[CC 1990 §425.210; Ord. No. 98.002 §1, 1-12-1998]
The performance standards and design criteria set forth herein
establish minimum requirements which must be implemented with good
engineering practice and workmanship. Use of the requirements contained
herein shall not constitute a representation, guarantee or warranty
of any kind by the municipality or its officers and employees of the
adequacy or safety of any drainage management structure or use of
land. Nor shall the approval of a stormwater management plan and the
issuance of a permit imply that land uses permitted will be free from
damages caused by stormwater runoff. The degree of protection required
by these regulations is considered reasonable for regulatory purposes
and is based on historical records, engineering and scientific methods
of study. Larger storms may occur or stormwater runoff heights may
be increased by manmade or natural causes. Enforcement of these provisions,
therefore, shall not create liability on the part of the municipality
or any officer of the municipality with respect to any legislative
or administrative decision lawfully made here under, nor shall compliance
relieve an owner, developer and/or permittee from responsibility under
any circumstances where liability would otherwise exist.
