O339
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QFFICIAI. CITY
SECRETARY'S COPY
ORDINANCE N0.339
AN ORDINANCE AMENDING ARTICLE V OF CHAPTER 17 OF THE
KENNEDALE CITY CODE, THE SUBDIVISION ORDINANCE,
PROVIDING FOR UNIFORM REGULATIONS GOVERNING STORM
WATER DETENTION AND RUNOFF; PROVIDING FOR A REGIONAL
DETENTION POND PROGRAM; PROVIDING STANDARDS FOR
EROSION AND SEDIMENT CONTROL; PROVIDING STANDARDS FOR
REVIEW AND APPROVAL OF STORM WATER FACILITIES;
PROVIDING REGULATIONS GOVERNING DETENTION POND
DESIGN; PROVIDING CRITERIA FOR ON-SITE AND REGIONAL SDP
PONDS; PROVIDING SAFETY CRITERIA FOR SDP PONDS;
PROVIDING CRITERIA FOR OUTLET STRUCTURE DESIGN;
PROVIDING A METHODOLOGY FOR CALCULATING DETENTION
POND CAPACITY BASED ON STORM WATER FLOW; PROVIDING
FOR DETENTION POND MAINTENANCE AND ACCESS
REQUIREMENTS; PROVIDING THAT THIS ORDINANCE SHALL BE
CUMULATIVE OF ALL ORDINANCES; PROVIDING A SEVERABILITY
CLAUSE; PROVIDING A SAVINGS CLAUSE; PROVIDING A PENALTY
CLAUSE; PROVIDING FOR PUBLICATION; AND PROVIDING AN
EFFECTIVE DATE.
WHEREAS, the City of Kennedale, Texas is a home rule city acting under its
charter adopted by the electorate pursuant to Article XI, Section 5 of the Texas
Constitution and Chapter 9 of the Local Government Code; and
WHEREAS, land development projects and associated disturbance of vegetation
and soil and changes in land cover, including increases in impervious cover, alter the
hydrologic response of local watersheds and increase storm water runoff rates and
volumes; and
WHEREAS, inadequately or improperly managed storm water runoff can deplete
groundwater resources and increase flooding, erosion, and sediment transport and
deposition; and
WHEREAS, storm water runoff contributes to increased quantities of waterborne
pollutants; and
WHEREAS, increases of storm water runoff, soil erosion and nonpoint source
pollutants can contribute to the degradation of the water resources of the City of
Kennedale and neighboring municipalities; and
WHEREAS, in order to alleviate and prevent such harm the City Council desires
to adopt uniform storm water detention regulations to promote orderly development of
the City of Kennedale so as to preserve and protect the significant natural, ecological,
agricultural, scenic, cultural and recreational resources of the City.
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(d) It is the intention of the City to protect the health and safety of the citizens
and visitors of the community and to prevent damage to private property and public
facilities through the installation and use of temporary and permanent erosion control
practices that prevent or adequately reduce increases in erosion and siltation that may
otherwise increase the risk of flooding and the associated risk of public endangerment
and property damage by clogging and/or partial filling of constructed or natural
drainageways as well as drainage structures and detention ponds.
(e) These regulations shall apply to all property within the corporate city limits
and the extra-territorial jurisdiction (ETJ) of the City unless otherwise stated.
(f) Sections 17-331(b) and 17-333 of this Division shall not apply to:
(1) Single family or duplex residential lots, of subdivisions approved prior to
the adoption of these regulations, unless specifically required by prior
agreement between the City and the owners or developers of such
subdivisions, or to new one- or two-lot subdivisions for single family or
duplex residential lots, and these regulations are intended to be
implemented for entire subdivisions at the time of platting and construction
of street and drainage improvements and not on an individual lot basis for
single family and duplex residential subdivisions.
(2) Residential lots in the ETJ that create no more impervious ground cover
than twenty percent (20%) of the gross lot surface area exclusive of any
area within the one hundred (100) year flood plain.
(3) Multi-Family or Non-Residential lots less than one-half ('/z) acre.
(g) Notwithstanding anything to the contrary contained herein, each
development exaction required by this Division shall be subject to the rough
proportionality determination set forth in this Chapter 17.
Sec. 17-331. Standards and requirements for storm water detention.
(a) It is prohibited to place fill material or construct impervious cover or
construct or place any other structure on such person's property or perform any
excavation or grading in a manner which alters the flow of surface water across said
property in a manner which damages any adjacent property.
(1) No final subdivision plat, subdivision construction plan, site plan or
building permit shall be approved by the City unless it can be
demonstrated by the owner or developer of such property that the
proposed development will not result in damage to any adjacent or
downstream property. This will be certified by a professional engineer's
submittal of sufficient data and calculations.
(b) The above requirement shall be accomplished through one of the
following means:
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NOW, THEREFORE, BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY
OF KENNEDALE, TEXAS:
SECTION 1.
Section 17-246(2) of the Kennedale City Code, is hereby amended to read as
follows:
(2) Where a drainage study indicates that additional runoff from the developing
property will either exceed pre-development peak flood flows, or, overload
downstream drainage facilities and result in hazardous conditions, the city may
withhold approval of the development until appropriate provisions have been
made in accordance with the requirements of Division 15 hereof.
SECTION 2.
Article V of Chapter 17 of the Kennedale City Code is hereby amended by adding
a division, to be numbered Division 15, which division reads as follows:
DIVISION 15. STORM WATER DETENTION
Sec. 17-330. Purpose and applicability.
(a) The growth in and around the City of Kennedale and the associated
development and construction of buildings, paved surfaces, roads and other
improvements has altered in the past and continues to alter the natural flow of surface
waters on the land, which together with the construction of gutters, culverts, drains and
channels for the carrying off of surface waters has both increased the quantity of storm
water and amplified the peak flow rates of runoff, thus leading to present and potential
flooding of property and homes, dangerous flows within and over public roadways and
streets, and soil and channel erosion.
(b) It is the intention of the City to protect the health and safety of the citizens
and visitors of the community and to prevent damage to private property and public
facilities through the proper design and construction of both on-site and regional storm
water detention facilities that prevent or adequately reduce increases in peak flow rates
of runoff that may otherwise increase the risk of flooding and the associated risk of
public endangerment, property damage and erosion.
(c) It is the intention of the City, through these regulations, to establish a
regional storm water detention pond program for the design and construction of regional
storm water detention facilities so that, where practical, the most cost-effective
protection from flooding may be accomplished.
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(1) Design and construction of an on-site storm water detention facility, or
facilities, by the land owner or developer which limits the peak flood flows
from the proposed development to the pre-development peak flood flows
from the subject tract.
(2) Construction of, or participation in the construction of, off-site drainage
improvements, such as storm inlets, storm sewers, culverts, channel
modifications, land filling, and/or other drainage facilities such that the
peak flood flows for fully-developed watershed conditions .from the
watershed area in which the proposed development is located will be
sufficiently and safely passed without flooding of adjacent and
downstream property and roadways.
(3) Design and construction of the development by certified engineering data
and calculations utilizing limited impervious cover, infiltration of runoff from
impervious cover via flow through pervious areas, and/or grass-lined
swales or channels such that these measures result in a minimal increase
in peak flood flows from the development.
(c) Acceptance of requests from the land owner or developer to meet the
storm water detention requirements through measures listed in subsections (b)(2) and
(b)(3) of this Section is solely at the discretion of the City Council after consideration
and recommendation from the Planning and Zoning Commission and shall not relieve
the owner of responsibility under civil law to adjacent and downstream properties.
(d) Acceptance by the City of on-site storm water detention plans will be
based on the suitability and adequacy of the engineering and technical design of the
proposed storm water detention facility, as described in Section 17-334 below.
Sec. 17-332. Regional detention pond program.
The City hereby establishes a Regional Storm Water Detention Pond Program
whereby the City may design and direct construction of or otherwise facilitate
construction of regional storm water detention ponds in order to prevent increases in
and, if practicable, to reduce peak flows of storm water runoff.
Sec. 17-333. Standards and requirements for erosion/sediment controls.
(a) No final subdivision plat, subdivision construction plan, site plan or
building permit shall be approved by the City unless the plans for the proposed
development include temporary and permanent erosion and sedimentation control
measures such that siltation of downstream drainageways are minimized.
(b) The above requirement shall be accomplished through a combination of
the following practices:
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(1) Installation of silt fences and rock berms before and during construction in
order to reduce on-site soil erosion and provide temporary capture of
sediment.
(2) Temporary and/or permanent revegetation of bare ground in order to
stabilize disturbed soil at the earliest practicable date.
(3) Construction of on-site storm water detention facilities by the land owner
or developer in a manner such that detention ponds function as temporary
sedimentation basins until permanent revegetation of the subject tract is
accomplished.
(4) Other measures which may be necessary to control erosion and
sedimentation on a site by site basis.
Sec. 17-334. Additional standards for approval.
(a) A Registered Professional Engineer, licensed in the State of Texas and
qualified and experienced in the design and operation of storm water detention ponds
and related storm water management facilities, shall be engaged by the owner or
developer to perform the hydraulic and structural design of required storm water
detention ponds and related storm water management facilities, including the
development of engineering and technical information required for evaluation by the
City.
(b) All design and technical information necessary to thoroughly evaluate the
suitability and adequacy of the engineering and technical design of proposed on-site
storm water detention facilities and, if proposed, off-site facilities shall be provided to the
City for review. All detention and runoff calculations, including computer model
simulations in digital format, if used, shall be provided.
(c) All on-site storm water detention facilities shall be designed to adequately
and safely pass all storm water inflows, including flood flows and runoff from upstream
and adjacent properties that have natural and/or existing overland flows toward and
onto the subject tract. The on-site storm water detention facilities should not impound
storm water onto or cause backwater to inundate any upstream or adjacent properties in
excess of existing conditions.
(d) On-site storm water detention facilities shall not encroach into the
regulatory 100-year floodplain as established by the City, Tarrant County, and/or the
Federal Emergency Management Agency, unless it can be satisfactorily demonstrated
to the City through the use of hydraulic modeling that such encroachment will not cause
any rise in the 100-year flood level on other off-site properties or that the increase in the
100-year flood level caused by such encroachment will occur entirely onsite on the
owner's or developer's property.
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(e) Additional engineering and technical rules and guidance with respect to
the application and review of the storm water detention requirements of this Division are
provided in Section 1-05E of the City of Kennedale's Public Works Design Manual.
(f) All design and technical information necessary to thoroughly evaluate the
suitability and adequacy of proposed erosion and sedimentation control measures shall
be provided to the City for review.
(g) Additional rules, guidance and requirements with respect to the review
and acceptance of temporary and permanent erosion and sedimentation control plans
may be provided in Section 1-05E of the City of Kennedale's Public Works Design
Manual.
SECTION 3.
The Kennedale Public Works Design Manual is hereby amended by adding a
new section, to be numbered Section 1-05E, which section read as follows:
E. Storm Water Detention Pond Design
The basic concept underlying the use of storm water detention ponds (SDP) involves
providing temporary storage of storm water runoff so that peak rates of runoff can be
reduced. Runoff is released from storage at a controlled rate which cannot exceed the
capacities of the existing downstream drainage systems or the predevelopment peak
runoff rate of the site, whichever is less.
Storm water detention ponds may be of two (2) basic types: On-site and Regional. In
general, on-site ponds are those which are located off-channel and provide storm water
detention for a particular project or development. Regional ponds are designed to
provide storm water detention in conjunction with other improvements on a watershed-
wide basis. The performance and safety criteria in this section E. apply to all ponds
which provide management of peak rates of storm water runoff, regardless of type.
PERFORMANCE CRITERIA FOR ON-SITE SDP's
1. On-site SDP's are further classified as either small or large, as follows:
ON-SITE SDP
POND CLASS DRAINAGE AREA
Small <25 acres
Large 25-64 acres
For design purposes, any pond with a drainage area larger that 64 acres shall be
classified as a regional pond.
2. On-site SDP ponds shall be designed to reduce post-development peak rate of
discharge to existing pre-development peak rates of discharge for the 2-, 10-, 25-
and 100-year storm events at each point of discharge from the project or
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development site. However, the City reserves the right to reduce peak rate
discharge even further if existing discharge of the developing property is
damaging adjacent properties. In addition, the capacity of the existing
downstream systems must be considered in determining the need for managing
the 100-year storm event. For the post-development hydrologic analysis, any off-
site areas which drain to the SDP pond shall be assumed to remain in the
existing developed condition.
3. The Modified Rational Method (MRM) may be used for the design of small on-
site ponds only. The maximum contributing drainage area to a pond designed
with the MRM is 10 acres when using this equation.
PERFORMANCE CRITERIA FOR REGIONAL SDP's
1. Regional SDP's are classified as small or large, based on the following criteria:
REGIONAL IMPOUNDED
POND CLASS VOLUME, AC-FT
Small 0-150
Large >150 acres
Any regional pond with a height of dam over 15 feet shall be classified as a large
regional pond.
2. Performance criteria for regional detention ponds shall be determined by the City
on aproject-by-project basis. The determination shall be based on a preliminary
engineering study prepared by the project engineer.
SAFETY CRITERIA FOR SDP's
All ponds shall meet or exceed all specified safety criteria. Use of these criteria shall in
no way relieve the engineer of the responsibility for the adequacy and safety of all
aspects of the design of the SDP.
1. The spillway, embankment, and appurtenant structures shall be designed to
safely pass the design storm hydrograph with the freeboard shown in the table
below. All contributing drainage areas, including on-site and off-site area, shall be
assumed to be fully developed. Any orifice with a dimension smaller than or
equal to twelve (12) inches shall be assumed to be fully blocked.
DETENTION DESIGN STORM FREEBOARD TO TOP
POND CLASS EVENT OF EMBANKMENT, FT.
On-site: Small 100 year 0
Large 100 year 1.0
Regional: Small 100 Year 2.0
Large 100 year
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*Design storm event and required freeboard for large regional ponds shall be
determined in accordance with Chapter 299 of the Texas Administrative Code
(Dam Safety Rules of the Texas Commission on Environmental Quality).
2. All SDP's (except small on-site ponds) shall be designed using a hydrograph
routing methodology. The Modified Rational Method (MRM) may be used only for
contributing drainage areas less than ten (10) acres.
3. The minimum embankment top width of earthen embankments shall be as
follows:
TOTAL HEIGHT OF MINIMUM TOP
EMBANKMENT, FT. WIDTH, FT.
0-6 4
6-10 6
10-15 8
15-20 10
20-25 12
25-35 15
4. The constructed height of an earthen embankment shall be equal to the design
height plus the amount necessary to ensure that the design height will be
maintained once all settlement has taken place.
This amount shall in no case be less than five (5%) percent of the total fill height.
All earthen embankments shall be compacted to 95% of maximum density.
5. Earthen embankment side slopes shall be no steeper than three (3) horizontal to
one (1) vertical. Slopes must be designed to resist erosion, to be stable in all
conditions and to be easily maintained. Earthen side slopes for regional facilities
shall be designed on the basis of appropriate geotechnical analyses.
6. Detailed hydraulic design calculation shall be provided for all SDP's. Stage-
discharge rating data shall be presented in tabular form with all discharge
components, such as orifice, weir, and outlet conduit flows, clearly indicated. A
stage-storage table shall also be provided.
7. When designing SPD's in a series (i.e., when the discharge of one pond
becomes the inflow to another), the design engineer must submit a hydrologic
analysis which demonstrates the system's adequacy. This analysis must
incorporate the development of hydrographs for all inflow and outflow
components.
8. No outlet structures from SDP's, parking detention, or other concentrating
structures shall be designed to discharge concentrated flow directly onto arterial
or collector streets. Such discharges shall be conveyed by a closed conduit to
the nearest existing storm sewer. If there is no existing storm sewer within 300
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feet, the outlet design shall provide for a change in the discharge pattern from
concentrated flow back to sheet flow, following as near as possible the direction
of the gutter.
9. Storm water runoff may be detained within parking lots. However, the engineer
should be aware of the inconvenience to both pedestrians and traffic. The
location of ponding areas in a parking lot should be planned so that this condition
is minimized. Storm water ponding depths (for the 100-year storm) in parking lots
are limited to an average of eight (8") inches with a maximum of twelve (12")
inches.
10. All pipes discharging into a public storm sewer system shall have a minimum
diameter of twelve (12"). In determining the actual size required, ease of
maintenance and/or repair must be assured.
11. All concentrated flows into a SDP shall be collected and conveyed into the pond
in such a way as to prevent erosion of the side slopes. All outfalls into the pond
shall be designed to be stable and non-erosive.
OUTLET STRUCTURE DESIGN
There are two (2) basic types of outlet control structures: those incorporating orifice flow
and those incorporating weir flow. Weir flow is additionally broken down into two (2)
categories: rectangular and V-notch. In each type, the bottom edge of the weir over
which the water flows is called the crest. Sharp-crested and broad-crested weirs are the
most common types.
Generally, if the crest thickness is more than 60% of the nappe thickness, the weir
should be considered broad-crested. The coefficients for sharp-crested and broad-
crested weirs vary. The respective weir and orifice flow equations are as follows:
1. Rectangular Weir Flow Equation (See Figure K-1)
Q = CLH sit (Eq. K-1)
where
Q =Weir discharge, cubic feet per second
C =Weir coefficient
L =Horizontal length, feet
H =Head on weir, feet
2. V-notch Weir Flow Equation (See Figure K-1)
Q = Cv tan (O/2)H 2.s (Eq. K-2)
where
Q =Weir Flow, cubic feet per second
C„ =Weir Coefficient
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O =Angle of the Weir notch at the apex (degrees)
H =Head on Weir, feet
3. Orifice Flow Equation (See Figure K-1)
Q = Co A (2gH) 0.5
where
Q =Orifice Flow, cubic feet per second
Co =Orifice Coefficient (use 0.6)
A =Orifice Area, square feet
g =Gravitation constant, 32.2 feet/sec2
H =Head on orifice measured from centerline, feet
Analytical methods and equations for other types of structures shall be approved by the
City prior to use.
DETENTION POND STORAGE DETERMINATION
For contributing areas up to ten (10) acres, the Modified Rational Method (MRM) may
be used. For contributing areas greater than ten (10) acres, a flow routing analysis
using detailed hydrographs must be applied. The Soil Conservation Service hydrologic
methods (available in Pond Pack TR-20, HEC-HMS, HEC-1) can be used. The engineer
may use other methods but must have their acceptability approved by the City. These
methods may also be used for the smaller areas. The most noticeable difference
between the two (2) methods is that the MRM is essentially an approximation of the
dynamic routing procedures used in the flow routing analysis using detailed
hydrographs. The MRM is also limited in application by the restrictions and assumptions
associated with the Rational Method. The following subsections give a more detailed
description of these two (2) storage determination methods.
MODIFIED RATIONAL METHOD (MRM)
The MRM is derived from the Rational Method. This procedure determines the critical
storm duration which produces the largest pond storage requirement with respect to the
release rate values established for the pond. These release rates can be derived either
from pre-development conditions or from specified allowable release rate criteria. In
addition to the general criteria stated previously, the following MRM criteria must also be
followed:
a. Maximum contributing area to the pond is ten (10) acres.
b. Pond outflows calculated by the MRM may not be used as inflows to
another pond (i.e., cascaded ponds cannot be analyzed by the MRM).
c. If the critical storm duration produces a peak flow less than the allowable
peak release rate, the storm duration to be used in all subsequent
calculation shall be that which produces a peak flow equal to the allowable
peak release rate.
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d. On-site flows which do not enter a pond are referred to as "bypass" flow.
All such flows must be subtracted from the allowable peak flow release
rate for the development.
e. Only single-storm events can be analyzed with the MRM.
The MRM is based on the same assumptions as the Rational Method. The most
significant assumption is that the period of rainfall intensity averaging is equal to the
duration of the storm. This means that the rainfall and corresponding runoff which
occurs either before or after that averaging period are not considered in the storage
calculations. Comparison of storage volumes calculated by the MRM with volumes
calculated by the Hydrograph Method suggests that significant underestimation of
required storage volumes may result for areas larger than a few acres. This appears to
be a direct result of the assumption just stated. Therefore, a volume adjustment factor is
always applied to the storage calculated by this procedure.
The MRM also assumes that the outflow hydrograph can be approximated by either a
triangular or trapezoidal shape. This assumption is equivalent to assuming that the
effective contributing drainage area increases linearly with time. In other words, there is
a linear area-time relationship for the contributing drainage area. If the actual
relationship differs significantly from this assumption (e.g., a preponderance of either
quickly-arriving or greatly-delayed flows), the pond could be significantly oversized or
undersized.
The MRM was originally designed as a graphical procedure and is still widely used and
accepted in that format for storage volume calculations. An equation method which
duplicates the procedures of the graphical method also is available. Both procedures
are described in this Section E.
1. MRM -Graphical Method
Step 1 -Select the design storm frequency.
Step 2 -Determine the allowable release rate for the pond and plot as a horizontal line,
as shown in Figure K-2.
Step 3 -Determine the required hydrologic data for the design drainage area, including
the proposed-condition contributing area, runoff coefficient and time of concentration.
Step 4 - Draw a vertical line at the proposed-condition time of concentration, as shown
in Figure K-2.
Step 5 -Select storm duration based on five-minute increments added to the proposed-
condition time of concentration. A value less than the time of concentration should not
be chosen, and any calculated peak flow should be equal to or greater than the
allowable release rate. The maximum duration can be determined by combining the
Rational Method equation for the proposed conditions with the equation relating rainfall
intensity to storm duration and return period, as follows:
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where
Qp =Peak flow rate for proposed conditions and the specific storm duration,
cfs
Cp =Runoff coefficient for proposed conditions and the specific storm
duration
ip =Average rainfall intensity for the specific storm duration and return
period, in/hr
Ap =Contributing drainage area for proposed conditions, acres
and
ip = a/[(tp+b)°] (Eq. K-5)
where
to =Specific storm duration, minutes
and a, b and c are coefficients as follows:
STORM FREQUENCY a b c
2 -year 51.393 9 0.81144
5 -year 71.154 12 0.81423
10 -year 77.103 12 0.79952
25 -year 90.982 13 0.79381
50 -year 97.721 13 0.78265
100 -year 110.202 14 0.77982
Combining Eq. K-4 and Eq. K-5 produces the following equation for the maximum storm
duration:
tDmax = (aCpAp/Qp)~'~`~ - b (Eq. K-6)
Step 6 -For each storm duration selected in Step 5, calculate the peak flow rate with
the Rational Method equation and construct the corresponding hydrograph as shown in
Figure K-2.
Step 7 -For each storm duration, draw a line connecting the origin with the intersection
point of 1) the allowable release rate line and 2) the recession limb of the hydrograph for
that storm duration. The indicated storage for a specific storm duration is represented
by the difference in volume between the total inflow hydrograph and the outflow
hydrograph produced by the line just drawn and the recession limb of the inflow
hydrograph. This is depicted by the crosshatched area in Figure K-2. Tabulate required
storages for all storm durations, and identify the largest, which corresponds to the
critical storm duration.
Step 8 -Having determined the required storage volume, the engineer should proceed
to the actual pond design.
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2. MRM -Equation Method
Due to the number of repetitive calculations and hydrograph which must be produced to
determine the critical storm duration by the MRM, the Equation Method was developed.
Since the calculated storage volume is directly related to the critical storm duration,
differential calculus can be used to determine those values. In order to accomplish this,
the peak flow rates for proposed conditions and various storm durations must be
expressed as a function of storm duration. As in the Graphical Method, the Rational
Method equation and the equation relating rainfall intensity to storm duration and return
period (Equation K-5) can be combined to find those values, as follows:
= Cp(a/[(tp+b)°]) Ap (Eq. K-9)
As shown in Figure K-2, the volume calculated for a specific storm duration can be
arithmetically calculated as follows:
Inflow Hvdroaraph Volume
V; = 60(1/2)(Qp)[(tD-tc)+ (tD+tc)] (Eq. K-10)
where,
V; =Inflow Hydrograph volume, ft3
Qp =Peak flow rate for proposed conditions and the specific storm duration,
cfs
tp =Time of concentration for the specific storm duration, minutes
t~ =Time of concentration for proposed conditions, minutes
Outflow Hvdroaraph Volume
Vo = 60(1/2)(QA)(tp+t~) (Eq. K-11)
where,
Vo =Outflow hydrograph volume, ft3
QA =Allowable peak flow release rate, cfs
tp =Time of concentration for the specific storm duration, minutes
t~ =Time of concentration for proposed conditions, minutes
Calculated Storaae Volume (before adjustment)
= 60(1/2)(Qp) [(tD-tc)+ \tp+tc)J - 60(1/2)(QA)(tD+tc)
= 600Qptp - 30QA(tp+tc) (Eq. K-13)
Combining Equation K-10 and K-11 with Equation K-12 results in an equation for the
calculated storage volume as a function of storm duration and the parameters for the
proposed condition, as follows:
SD = 60Cp{a/[(tp+b)°]} Ap tp - 30QA(tp+tc) (Eq. K-14)
where,
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SD =Calculated storage volume, ft3
Differentiating equation K-14 with respect to tp and setting the result equal to zero
produces an equation in terms of the specified variables, as follows:
[(to+b)°+1]/[a(to+b-cto)] = 2 CpAp/Qa (Eq. K-15)
Although this equation must be solved iteratively for the critical storm duration, the
procedure is simple and converges to a sufficiently accurate answer very quickly. The
procedure is as follows:
Step 1 -Pick any initial value for tp. A nominal value of 30 minutes usually results in
satisfactory convergence.
Step 2 - Calculate a value for the left-hand side of the equation, using the current
assumed value for tp.
Step 3 -Divide the right-hand side of the equation by the value calculated in Step 2 to
obtain an adjustment factor to apply to tp.
Step 4 - If the adjustment factor is within 0.01 of unity, multiply the current assumed
value for to by the adjustment factor from Step 3 to obtain a final value for tp. This is the
critical storm duration value. Go directly to Step 6.
Step 5 - If the adjustment factor is not within 0.01 of unity,. multiply the current assumed
value for to by the adjustment factor from Step 3 to obtain a new assumed value for to.
Go back to Step 2 and repeat the procedure as often as necessary.
Step 6 -Solve for the calculated storage volume by using Equation K-14 and the final
value for tp from Step 4.
Step 7 -Having determined the required storage volume, the engineer should proceed
to the actual pond design.
DETENTION POND MAINTENANCE AND EQUIPMENT ACCESS REQUIREMENTS
1. Silt shall be removed by the owner or developer and the pond returned to original
lines and grades when standing water conditions occur, the pond storage volume
is reduced by more than 10%, or the outfall structures are more than 20%
blocked.
2. To limit erosion, no unvegetated area shall exceed 10 sq. ft in extent.
3. Accumulated paper, trash and debris shall be removed every six (6) months or
as necessary to maintain proper operation.
4. Ponds shall be mowed annually between the months of June and September.
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5. Corrective maintenance is required any time a pond does not drain completely
within 60 hours of cessation of inflow (i.e., no standing water is allowed).
6. Structural integrity of pond embankments shall be maintained at all times.
7. Detention ponds in private subdivisions will be maintained by the required
property owners association, mandated by Section 17-326 of the Kennedale City
Code.
SECTION 4.
This Ordinance shall be cumulative of all provisions of ordinances and of the
Code of Ordinances of the City of Kennedale, Texas, as amended, except where the
provisions of this ordinance are in direct conflict with the provisions of such ordinances
and such code, in which event the conflicting provisions of such ordinances and such
Code are hereby repealed.
SECTION 5.
It is hereby declared to be the intention of the City Council that the phrases,
clauses, sentences, paragraphs and sections of this Ordinance are severable, and if
any phrase, clause, sentence, paragraph or section of this Ordinance shall be declared
unconstitutional by the valid judgment or decree of any court of competent jurisdiction,
such unconstitutionality shall not affect any of the remaining, phrase, clauses,
sentences, paragraphs or sections of this Ordinance since the same would have been
enacted by the City Council without incorporation in this Ordinance of any such
unconstitutional phrase, clause, sentence, paragraph or section.
SECTION 6.
Any person, firm or corporation who violates, disobeys, omits, neglects or refuses
to comply with or who resists the enforcement of any of the provisions of this Ordinance
shall be fined no more than Two Thousand Dollars ($2,000.00) for all violations
involving zoning, fire safety or public health and sanitation, including dumping or refuse,
and shall be fined not more than Five Hundred Dollars ($500.00) for all other violations
of this Ordinance. Each day that a violation is permitted to exist shall constitute a
separate offense.
SECTION 7.
All rights and remedies of the City of Kennedale, Texas, are expressly saved as
to any and all violations of the provisions of the Kennedale City Code or other
ordinances relating to storm water management which have accrued at the time of the
effective date of this Ordinance; and, as to such accrued violations and all pending
litigation, both civil and criminal, whether pending in court or not, under such ordinances
same shall not be affected by this Ordinance but may be prosecuted until final
disposition by the courts.
W:\Kennedale\Ordinances\Kennedale.Subdivision.042406.doc Page 15
SECTION 8.
The City Secretary of the City of Kennedale is directed to publish the caption,
penalty clause, publication clause and effective date of this Ordinance as required by
Section 3.10 of the Charter of the City of Kennedale.
SECTION 9.
This Ordinance shall be in full force and effect form and after its passage and
publication as provided by law, and it is so ordained.
PASSED AND APPROVED ON THIS tS DAY OF ~/~~w , 2006.
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~~~~'''CITY SECRETARY
APPROVED AS TO FORM AND LEGALITY:
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City Attorney
W:UCennedale\Ordinances\Kennedale.Subdivision.042406.doc Page 16
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RETAN~GIJLAR WEIR
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V-NOTCFi WEIR
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F~ow
i
ORIFICE DESIGN
Source: Clty o-F Austln, Watershed Management Divlslon '
WEIR AND ORIFICE FLOWS
FIGURE K-1
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~ ~ STORAGE
~ fNFLOW HYDROGRAP'-(
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QA
W
-OUT~'L.0 HYDROGRAPH
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~1c fD
tc = TIME OF CONCENTRATION (MINUTES)
tp = TIME OF DURATION (MINUTES)
Op ° PEAK FLOwS (CFS)
OA ~ MAX1MtJM ALLOWABLE .RELEASE RATE (CFS)
Sources City ofi Austin, Ivatarshed Management Dlvlslon '
MODIFIED RA.TION~L METHOD SCHEME
FIGURE K-2