Editor's Note — Ord. no. 2069 adopted April 15, 2003,
repealed ch. 507 and enacted the new provision set out herein. Former
ch. 507 derived from ord. no. 1929 §1, 1-16-2001 and ord. no.
2032 §1, 8-6-2002.
[Ord. No. 2069 §2, 4-15-2003]
Whenever homes, buildings or other structures are flooded downstream
from a proposed development in a 100-year or smaller flood, or where
flood damage areas have been identified, or an engineering study indicates
the proposed development would cause or increase such flooding, stormwater
detention facilities shall be constructed. Exceptions are when such
flooding occurs within the limits of the 100-year flood on lower reaches
of the main channel of major streams as established by the current
federal Flood Insurance Study and when such development is a single
family or multi-family residential lot of one-half (½) acre
or less and not part of a larger development.
[Ord. No. 2069 §3, 4-15-2003]
For the purpose of this Chapter, the following terms, phrases,
words, and their derivations shall have the meanings given herein:
An artificial barrier or embankment having greater than a
six (6) foot difference in elevation between the crest of the emergency
spillway and the lowest point of the cross section along the centerline
of the dam, which does or may impound water.
Any manmade area or structure which serves as a means of
temporarily storing storm water runoff.
The temporary detaining or storage of storm water on or beneath
the ground surface, on rooftops, parking lots, or by other means under
predetermined or controlled conditions.
Any change in land use or improvement on any parcel of land.
The rate of outflow of water from detention storage.
The geographical area contributing stormwater runoff to a
point under consideration, i.e., a watershed, tributary area or catchment
area.
A detention basin or facility not intended to have a permanent
pool.
A flood having a fifty percent (50%) chance of occurrence
in any given year. The 2-year flood is assumed to be caused by a 2-year
storm.
A rainstorm of a given duration and depth of precipitation
having a fifty percent (50%) chance of occurrence in any given year.
A flood having a twenty percent (20%) chance of occurrence
in any given year. The 5-year flood is assumed to be caused by a 5-year
storm.
A rainstorm of a given duration and depth of precipitation
having a twenty percent (20%) chance of occurrence in any given year.
A flood having a ten percent (10%) chance of occurrence in
any given year. The 10-year flood is assumed to be caused by a 10-year
storm.
A rainstorm of a given duration and depth of precipitation
having a ten percent (10%) chance of occurrence in any given year.
A flood having a four percent (4%) chance of occurrence in
any given year. The 25-year flood is assumed to be caused by a 25-year
storm.
A rainstorm of a given duration and depth of precipitation
having a four percent (4%) chance of occurrence in any given year.
A flood having a two percent (2%) chance of occurrence in
any given year. The 50-year flood is assumed to be cause by a 50-year
storm.
A rainstorm of a given duration and depth of precipitation
having a two percent (2%) chance of occurrence in any given year.
A flood having a one percent (1%) chance of occurrence in
any given year. The 100- year flood is assumed to be cause by a 100-year
storm.
A rainstorm of a given duration and depth of precipitation
having a one percent (1%) chance of occurrence in any given year.
The difference in elevation between design water surface
in the detention facility and the elevation at which uncontrolled
overtopping of the facility begins.
Intensity distribution of a rainfall event with respect to
time.
Any development involving the construction, reconstruction
or improvement of structures and/or grounds.
The waters derived from precipitation falling within a drainage
area, flowing over the surface of the ground or collected in channels
or conduits.
A detention basin intended to have a permanent pool.
[Ord. No. 2069 §3, 4-15-2003]
In general the maximum allowable release rate of stormwater
runoff originating within the proposed development shall be limited
to an amount that will not cause or increase downstream flooding within
that tributary in a 100-year or smaller flood. Such allowable release
rate shall be defined as the rate of runoff occurring from the proposed
development in its undeveloped condition and shall
be determined mathematically as the runoff resulting from the 2-,
5-, 10-, 25-, 50- and 100-year storm return frequency rainfall calculated
using the rational formula. The difference between the pre- and post-construction
values is the required detention and should be released through a
weir or orifice design at each respective storm level. Deviations
from this rainfall frequency in design calculations shall be only
where approved by the Director of Public Works. Actual rainfall intensity
shall be determined for the time of concentration of the tributary
area in its undeveloped and natural state. The release rate as identified
in the current edition of the American Public Works Association (APWA)
Standard Specifications and Design Criteria may be used on large developments
over ten (10) acres only if it determines a more restrictive release
rate. In no case shall the release rate exceed the existing "safe"
storm drainage capacity of the downstream drainage system or watercourse
as determined by the Director of Public Works based on a study of
the tributary area and the downstream drainage system. Where no study
exists but a downstream flooding problem has been identified, the
Director of Public Works may require a study of the downstream drainage
system and/or watercourse to determine the developments maximum release
rate. Detention Basin outflows shall discharge directly into the downstream
drainage system.
[Ord. No. 2069 §3, 4-15-2003]
The volume of storage provided in detention basins shall be
sufficient to store the storm water runoff generated by the development
during a 100-year flood, less the discharge that is permitted in this
Chapter.
[Ord. No. 2069 §3, 4-15-2003]
A.
Emergency
spillways shall be sized to carry an additional 100-year flood assuming
the detention basin is already filled to design storage capacity.
B.
Freeboard
for earthen detention basins shall be one (1) foot difference in elevation
between the top of the settled embankment and the water surface, with
the emergency spillway flowing at design depth.
[Ord. No. 2069 §3, 4-15-2003]
A.
A minimum
of the following detention design criteria will be required on plans,
however graphs and charts may be submitted separately from the plans
to the Director of Public Works for a developments proposed detention
basin:
1.
Detention
calculation summary table and other graphs containing the following
information:
a.
Total
site area (acres).
b.
Design
storm frequency as required in this Chapter.
c.
Runoff
coefficient (both pre- and post-development).
d.
Allowable
release rate as permitted in this Chapter (cubic feet per second (CFS)).
e.
Required
storage volume as required in this Chapter (cubic feet).
f.
Storage
volume at design pool elevation as required in this Chapter.
g.
Stage
storage curves for proposed detention facilities plotted in units
of detention facility water surface elevation (and depth) as ordinates
and cumulative volume of storage as abscissas.
h.
Stage
outflow curves for outlet works plotted in units of detention facility
water surface elevation (and depth) as ordinates and discharge rate
(cubic feet per second) as abscissas.
i.
Inflow
and outflow hydrographs plotted in units of cubic feet per second
of inflow and outflow as ordinates and time from start of storm as
abscissas.
j.
Inflow
and outflow mass curves plotted in units of cumulative volume as ordinates
and time from start of storm as abscissas. Curves shall be so arranged
that the vertical distance between the cumulative storage and the
cumulative discharge will indicate the net volume of storage at any
time. These curves may be omitted if the inflow and outflow hydrographs
are so plotted and labeled as to show total storage requirements and
time to empty the detention facility.
k.
Emergency
spillway flowline elevation, width and flow when passing the 100-year
storm (design depth) as required in this Chapter.
l.
Elevation
of top of settled embankment-must have one (1) foot freeboard.
m.
Provide
details of the detention basin emergency spillway and controlled outlet
pipe along with a section through the detention basin.
2.
If the
design engineer determines that detention will not be necessary for
a particular development, he shall submit a stormwater study and report
summarizing the details of the study and may request a waiver of the
detention requirement to the Director of Public Works.
[Ord. No. 2069 §3, 4-15-2003]
A.
A topographic
map of the project site and immediately adjacent areas of suitable
scale and contour interval which shall define the location of any
streams, extent of floodplains and calculated high water elevations,
and shorelines of lakes and ponds or swamps.
B.
The size,
location and flowline elevations of all existing sewage treatment
facilities which fall within the project limits and within the distance
of five hundred (500) feet beyond the boundaries of said project.
C.
A proposed
grading plan and/or site plan showing existing and proposed contours,
buildings, parking lots, platted lots and other development features.
D.
Drainage
area map showing upstream drainage areas tributary to the development
and to each proposed detention facility.
E.
Special
study to determine effects of development if required by the Director
of Public Works.
F.
Indicate
on the plan proposed areas and/or methods to be used for detention
facilities. The developer or builder shall coordinate with the Director
of Public Works on the location, size, shape or other desired design
features of proposed detention basins.
[Ord. No. 2069 §3, 4-15-2003]
A.
In addition
to the foregoing criteria, the following shall be applicable depending
on the detention alternative(s) selected.
1.
Wet bottom basins-for basins with permanent pools.
a.
Minimum depths. The minimum depth of water before introduction
of excess stormwater shall be four (4) feet. The side slopes of the
basin shall conform as closely as possible to regraded or natural
land contours.
b.
Multi-purpose features. These shall 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, if possible.
2.
Rooftop storage. Detention may be met in total or in part
by detention on roofs. Details of such designs shall include the depth
and volume of storage, details of outlet devices and down-drains,
elevations and details of overflow scuppers and emergency overflow
provisions. Connection of roof drains to sanitary sewers is prohibited.
Design loadings and special building and structural details shall
be subject to approval by the City Building Inspector.
3.
Parking lot storage. Paved parking lots may be designed
to provide temporary detention storage of stormwater on a portion
of their surfaces. Generally such detention areas shall be in the
more removed portions of such parking lots. Depths of storage shall
be limited to a maximum depth of seven (7) inches in parking areas.
Where storage is to be deeper than seven (7) inches, such areas shall
be precluded from parking use and shall be located so that access
to and from parking areas is not impaired.
4.
Other storage. All or a portion of the detention storage
may also be provided in underground or surface detention areas including,
but not limited to, oversized storm sewers, vaults, tanks, swales,
etc.
[Ord. No. 2069 §3, 4-15-2003]
Based on design criteria in this Chapter, plans and specifications
shall be submitted to the Director of Public Works for approval prior
to any building permit being issued, grading or land disturbance permit,
or the beginning of any construction.
[Ord. No. 2069 §3, 4-15-2003]
The rate of discharge from a detention facility and the rate of discharge of stormwater runoff from areas of the development not controlled by the detention facility shall not collectively exceed the maximum release rate established pursuant to the provisions of Section 507.020.
[Ord. No. 2069 §3, 4-15-2003]
A.
Standards
for construction of inlets, pipes, manholes, paved ditches, and other
detention appurtenances shall be in accordance with the most current
edition of APWA standards, unless otherwise designated by the Director
of Public Works.
B.
If slopes
for wet or dry detention basins exceed twenty percent (20%), both
erosion control and safety measures shall be provided. In no case
shall earthen slopes exceed thirty-three percent (33%). If vertical
walls are used, the basin must be fenced with steps, ramps or other
means of egress provided.
C.
Wet detention
facilities shall have an outlet works consisting of valves, gates,
pipes or other devices necessary to completely drain the facility
in seventy-two (72) hours or less when required for maintenance.
[Ord. No. 2069 §3, 4-15-2003]
A.
Maintenance
of private detention facilities shall be the responsibility of the
property owner and shall include debris removal and cleaning, cutting
of vegetation, repair of erosion, removal of silt and maintenance
of structural facilities.
B.
Maintenance
of public detention facilities shall be the responsibility of the
City of Parkville, and permanent easements shall be dedicated to the
City and sized according to the latest edition of APWA.
[Ord. No. 2069 §3, 4-15-2003]
Violation of any provision of this Chapter shall be a misdemeanor
punishable by a fine of not more than five hundred dollars ($500.00)
or by imprisonment of not more than thirty (30) days, or by both such
fine and imprisonment. Each day of violation shall constitute a separate
offense.
