[Amended 8-23-2023 by Ord. No. 2023-357; 10-23-2019 by Ord. No. 2019-446]
The provisions of Sections 13.34.220 through 13.34.250, inclusive,
shall apply to the following areas under development:
A. All residential development of two acres or more and all commercial
and industrial development or redevelopment which cumulatively creates
5,000 square feet or more of impervious area located within the Davenport
City limits.
1.
There is no statute of limitations that would exempt multiple
impervious area additions over 5,000 square feet, unless granted a
waiver per Section 13.34.100.
B. Any development which, in the opinion of the City Engineer, lacks
an adequate external or internal system for the passage of stormwaters
as to not exceed the capacity of the receiving storm sewer system.
C. For circumstances where an existing structure is demolished and the
area is graded and seeded, there shall be a three-year time frame
from the demolition date where any new development may be considered
as redevelopment. Projects within this three-year time frame may follow
the release rate as outlined in Section 13.34.220C.
D. The following activities are exempt from this chapter:
1.
Development or redevelopment projects that do not create more
than 5,000 square feet of impervious area, regardless of previous
condition, provided they are not part of a larger common development
plan that does create more than 5,000 square feet of impervious area,
or that do not meet the requirement of Subsection 13.34.300B.
2.
The subdivision of a property with the intent of removing excess
land from a homestead.
3.
The replatting of one or more properties with the intent of
constructing one single-family residence.
E. When a site development or redevelopment plan is submitted that qualifies
as a development or redevelopment as defined in Section 13.34.030
of this chapter, appropriate on-site management practices will be
required and shall be guided by the latest editions of the Iowa Stormwater
Management Manual, the Iowa Rain Garden Manual, the Davenport Stormwater
Manual, or approved equal. Final authorization of all development
and redevelopment projects will be determined after review and approval
by the City of Davenport.
F. Any development or redevelopment within the floodplain must comply
with Chapter 15.44, Flood Damage Prevention. Stormwater detention
is not permitted within the floodway or within special flood hazard
(100-year floodplain) areas as delineated as areas of special flood
hazard identified by the Federal Insurance Administration through
a scientific and engineering report entitled "The Flood Insurance
Study for Scott County, Iowa, No. 19163CV000A, dated February 18,
2011," with accompanying Flood Insurance Rate Maps, and any revision
thereto are adopted by reference as part of Chapter 15.44.
[Amended 8-23-2023 by Ord. No. 2023-357; 10-23-2019 by Ord. No. 2019-446]
A. The release rate of stormwater from any detention basin required
under this chapter for new development shall not exceed the stormwater
runoff rate from the drainage area from a pre-developed two-year frequency
storm using a runoff coefficient "c" of 0.15, when using the Modified
rational method for areas under twenty acres or a curve number of
sixty when using the S.C.S. TR-55 "Urban Hydrology for Small Watersheds"
and the "Iowa Users Guide and Supplement" for said TR-55 for areas
over twenty acres.
B. The City may require a lesser release rate when the downstream outlet
is insufficient to safely convey the release rate as determined in
Subsection A above.
C. The planning and design of drainage systems should be such that problems
are not transferred from one location to another. Outfall points and
velocities should be designed in such a manner that will not create
flooding hazards downstream; or deterioration of existing structures;
or degradation of biological functions or habitat; or increased stream
bank erosion.
D. Restrictor (orifice) plates to control release rates placed within
a pipe section or at the outlet of a pipe will not be accepted. Only
orifice plates integrated into a staged release design structure that
cannot be removed will be accepted.
E. The single-stage outlet (i.e., one culvert pipe) is not recommended
because of its inability to detain post-developed runoff from storms
less that the five-year interval (i.e., channel protection volume
storm event). In many cases, runoff from storm events less than the
five-year recurrence interval has created erosion and sedimentation
problems downstream of a detention basin.
F. Reserved.
[Deleted 8-23-2023 by Ord. No. 2023-357]
G. The minimum orifice size shall be 4 inches in diameter when located
on the outside wall of a structure, where one side of the orifice
is not accessible. Otherwise, the minimum orifice size may be smaller.
In both cases of a 4 inch or smaller orifice to meet release rate
requirements, the orifice shall possess protections to minimize plugging.
[Amended 8-23-2023 by Ord. No. 2023-357; 10-23-2019 by Ord. No. 2019-446]
A. Utilize the Unified Sizing Criteria (USC) or the Water Quality Volume
(WQv) up to Extreme Flood Protection (Qf) for the required volume
of stormwater detention. The WQv shall provide 100% retention or infiltration
of the 1.25-inch-in-a-twenty-four-hour rainfall event per Section
13.34.310.
B. City standards that are to be used in conjunction with the USC are:
1.
Rainfall greater than the WQv up to the
Qf, or 100-year event, shall be released at the two-year, undeveloped,
state (as opposed to the USC stated five-year).
2.
The recharge volume (Rev) need not be included in detention
calculations.
3.
The Modified Rational Method may only be used for small catchments
< 20 acres.
C. For sites greater than 20 acres, the method of sizing of the detention
storage volume must be approved by the City Engineer. The TR-55 Method
is an approved method.
1. For
drainage areas larger than 2,000 acres, or for situations where the
methods described above are not appropriate, TR-20, HEC-1, HEC-HMS,
or other approved alternatives may be used.
D. For redevelopment sites, utilize the USC detention requirements from
the WQv up to Qf unless a partial waiver or
general waiver has been granted per Section 13.34.100, Waivers.
E. Detention storage may be provided as a dry bottom or wet bottom storage
area.
1.
Dry bottom stormwater storage areas may be designed to serve
a secondary purpose for recreation, open space, parking, or other
types of uses that will not be adversely affected by intermittent
flooding.
a.
A method of carrying the low flow or a system of drains to prevent
soggy areas shall be provided.
b.
Concrete or otherwise paved swales to carry low flow shall only
be permitted if stormwater has already been passed through a water
quality treatment facility.
c.
Both outlet control structures and emergency overflow facilities
shall be designed and constructed to fully protect the public health,
safety and welfare. Stormwater runoff velocities shall be kept at
a minimum, and turbulent conditions at an outlet control structure
will not be permitted without complete protection for the public safety.
The use of fences shall be kept to a minimum and used only as a last
resort when no other method of protection is feasible.
d.
Paved surfaces thatare to serve as stormwater storage areas
and rooftop storage shall be designed with permanent-type control
outlets. Emergency overflow areas shall be provided.
2.
Wet bottom stormwater storage areas shall be designed with all
of the items required for dry bottom stormwater storage areas, except
that the provisions of Subsection E1a of this Section shall not be
required. Utilize the Iowa Storm Water Management Manual.
a.
Restrictor (orifice) plates to control release rates placed
within a pipe section or at the outlet of a pipe will not be accepted.
Only orifice plates integrated into a staged release design structure
that cannot be removed will be accepted.
[10-23-2019 by Ord. No.
2019-446]
A. Detention facilities for stormwater runoff control shall be constructed
as part of the first phase of construction of a project unless alternate
construction phasing is agreed upon, in writing, by the Natural Resources
Manager or City Engineer.
B. The construction of the stormwater management systems shall be accomplished
as part of the cost of land development. If the amount of storage
capacity can be increased to provide benefit to the City, negotiations
for public participation in the cost of development shall be initiated.
C. All flood-control items, such as earthen embankments, conduits, outlet
structures, flood-control structures, spillways, excess stormwater
passageway channels, etc., shall be built as permanent facilities,
and all materials and their manner of construction shall be assembled
to accomplish as much permanency as is possible.
D. Water quality practices may be constructed within detention areas
to allow owners to conserve space. However, below-grade volume for
infiltration in a detention basin shall not be counted as required
detention storage area unless detailed plans and calculations are
submitted, reviewed and approved, in writing, by the Natural Resources
Manager or City Engineer.
[10-23-2019 by Ord. No.
2019-446]
A. Plans, specifications, and all calculations for stormwater runoff
control(s) shall be submitted in accordance with the Davenport Stormwater
Manual for review and approval, prior to the approval of a final plat
(in the case of a subdivision or planned unit development), approval
of a final development plan, or issuance of a building permit (in
the case of commercial or industrial construction).
B. Comply with 13.34.080, Inspections.
C. No certificate of occupancy for any building in the development will
be issued until the stormwater facilities are constructed, inspected
and approved.