[Amended 3-31-2015 by Ord. No. O-15-03]
A.
Storm drainage systems shall be designed on the basis of ultimate
development of the tributary watershed. Where the nature of future
development is uncertain, the Engineering Division should be consulted
by the designer for the purpose of estimating the type of future development
to be expected.
B.
A "public drainage system" is generally defined as those pipes and
structures intercepting water from public streets. The City of Hagerstown
shall only maintain storm drain systems in the accepted public right-of-way
or accepted easements specifically for City maintenance of the storm
drain.
C.
Reinforced concrete pipe shall be used for all storm drain systems
and culverts in the public right-of-way. All pipe shall be a minimum
of Class IV.
D.
All materials and construction must be in accordance with the Public
Ways Construction Standards and the Maryland State Highway Administrations
Standard Specifications for Construction and Materials.
For drainage areas less than 100 acres, runoff may be estimated
by the Rational Formula (Q = CIA). For drainage areas in excess of
100 acres, runoff shall be estimated by the methods and procedures
as described in the United States Department of Agriculture, Soil
Conservation Service publication entitled "Urban Hydrology for Small
Watersheds" (Technical Release No. 55) or by other methods and procedures
acceptable to the Maryland Department of the Environment and the City
Engineer.
A.
The Rational Formula is as follows:
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Q = CIA
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Where:
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Q
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=
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Flow rate of runoff expressed in cubic feet per second.
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C
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=
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Runoff coefficient.
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I
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=
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Rainfall intensity expressed in inches per hour for a given
storm frequency.
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A
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=
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Drainage area expressed in acres.
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B.
The values for the coefficient of runoff (C) and the intensity of
rainfall (I) shall be selected from the Maryland State Highway Administration
Drainage Manual. Design method shall be per this manual and the following:
(1)
Runoff coefficient (C). The runoff coefficient, used in computing
flow to a point under consideration, shall be a composite of the C
factors for all the areas tributary to this point. For drainage areas
containing several different types of soil, cover or slopes, a weighted
value of C must be used.
(2)
Rainfall intensity (I). Rainfall intensity shall be determined from
the rainfall intensity curves provided in the Maryland State Highway
Administration Highway Drainage Manual.
(3)
Time of concentration (TC). The "time of concentration" shall generally
be defined as the total time required for runoff to flow from the
most remote point in the drainage area to the point of investigation.
It shall be the sum of the duration of flow for all types such as
overland flow (sheet and shallow concentrated flow), swale flow, ditch
flow and pipe flow (each if applicable). The selected flow path shall
be representative of the drainage area as a whole. Overland flow through
most areas will become channel flow within 400 feet. Stream or channel
flow time should be computed by estimating the average velocity in
each type of course and dividing it into the total length of flow
through that course. In no instance shall a time of concentration
of less than six minutes be used for any drainage computation.
(4)
Drainage area (A). Care should be taken to assure that all areas
delivering runoff to the point under consideration shall be included.
The extent of the drainage area should be determined from field observations
and one of the following sources (the highest order of information
available shall be used):
A.
All enclosed storm drain systems shall be designed for the ten-year
design storm frequency.
B.
All cross culverts shall be designed based on ultimate development
of the contributing watershed and the following design storms:
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City Roadway Classification
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Design Storm Frequency
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Local
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25
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Minor collector
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25
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Major collector
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25
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Arterial
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50
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C.
The spacing of storm sewer inlets on closed roadways shall be determined
by the use of a two-year storm design storm frequency.
A.
The minimum diameter of storm drain pipe shall be 15 inches (or the
equivalent pipe arch size). Where abrupt alignment changes are required,
an inlet, manhole or bend structure shall be placed at the point of
change.
B.
Storm drain shall typically be laid on a straight horizontal and
vertical alignment. Pipes may be deflected per the manufacturer's
recommendation.
C.
The minimum slope of pipe shall not be less than 0.40%, and the ten-year
design storm velocity in the pipe shall not be less than three feet
per second, unless otherwise approved by the City Engineer. The top
of all pipes shall be at least six inches below the subgrade elevation
of the pavement.
D.
Manning's Equation shall be used for determining storm sewer pipe
sizes.
E.
The procedure contained in the Hydraulic Engineering Circular No.
5, Hydraulic Charts for the Selection of Highway Culverts, as prepared
by the United States Department of Transportation, shall be used for
the design of culverts.
F.
Under entrance control conditions, the normal maximum relationship
of the headwater depth to the pipe diameter (HW/D) shall be 1.2. The
computed high-water elevation thus created must not cause damage to
existing properties and must be at least 0.5 foot below the edge of
the roadway shoulder. The value of HW/D may be greater than 1.2 only
with the approval of the City Engineer.
Storm drains and all closed systems passing longitudinal or
transverse to the roadway shall be designed based on the procedures
outlined in the Maryland State Highway Administration Drainage Manual.
The hydraulic gradient for the ten-year storm shall be a minimum of
one foot below the bottom of manhole covers and inlet grates. The
hydraulic gradient shall be determined for all storm drain systems.
Inlets may be spaced to intercept the portion of the gutter flow for
the two-year storm, but storm drain pipes shall be designed for the
total flow from the ten-year design storm at any location.
A.
Inlets. Inlets shall be designed based on the procedures in the Maryland
State Highway Administration Drainage Manual and the following criteria:
(1)
Inlets shall be spaced with runoff from the two-year storm. The allowable
spread of water in a curb or curb-and-gutter section shall be eight
feet. Inlets shall be located at the upgrade side of all public road
intersections and median breaks, and in superelevation transitions
approximately 50 feet ahead of the section where the cross slope is
level. Inlets on a continuous grade shall be spaced to intercept at
least 85% of the total gutter flow where practical. Flow bypassing
that inlet shall be included in the total gutter flow contributing
to the next inlet downstream unless it is carried off on a side street
or is otherwise intercepted.
(2)
For closed roadways (with curb), runoff from cut slopes and areas
off the right-of-way shall, wherever possible, be intercepted by channels
in order to reduce the amount of water flowing onto the street pavement,
particularly on the high side of superelevated roadways.
(3)
All inlets shall be City standard inlets as designated in the Public
Ways Construction Standards.
B.
Bend structures. The use of bend structures is discouraged and may
only be used on pipe sizes 24 inches in diameter and larger with the
approval of the City Engineer.
C.
Manholes and other structures.
(1)
Structures shall be as designated in the Public Ways Construction
Standards. Refer to this document for structures, flow channels and
field connections.
(2)
For pipe 24 inches in diameter and smaller, manholes or other structures
providing access to the pipe from the surface shall be placed at intervals
not exceeding 400 feet. On pipes 27 inches in diameter and larger,
access structures may be spaced at greater intervals.
(3)
The grade of the main line pipe should normally be carried through
the invert of structures unless greater drop is required by hydraulic
conditions. A minimum drop of 0.1 foot shall be provided through the
invert of any structure.
(4)
The use of field connections is discouraged and may only be used
with the approval of the City Engineer.
The hydraulic design of channels consists of determining whether
the proposed channel is sufficient to carry the design peak flow and
determining the type of lining necessary to prevent scour or undesirable
sedimentation in the channel. The second phase of the design should
also include the determination of the point where the ditch lining
must change from one erosion protective lining to another.
A.
Design. Open channels shall be designed based on procedures outlined
in the Maryland State Highway Administration Drainage Manual.
B.
Location and alignment. Drainage channels shall be located as to
change the original drainagecourse as little as possible. Design should
provide gradual horizontal alignment changes and be located near or
on a property line when feasible.
C.
Limiting velocities. The maximum velocity in ditches shall be taken
from the following table, according to the type of lining.
(1)
New construction.
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Type of Lining
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Allowable Velocity
(feet per second)
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|---|---|---|
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Seed and mulch
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2.0
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Solid sodding
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4.0
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Soil stabilization matting
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6.0 (or manufacturer's recommendation)
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Dumped or placed riprap (four-inch to seven-inch stone)
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Refer to SHA Chart
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Concrete or macadam paving
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No maximum
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Storm drain outlet protection
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Refer to Maryland Standards and Specifications for Soil Erosion
and Sediment Control
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(2)
Investigation of existing ditches/channels.
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Type of Lining
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Allowable Velocity
(feet per second)
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Earth, without vegetation
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1.0 to 3.0
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Grains, still-stemmed grasses
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2.0 to 3.0
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Stiff clay, or clay and gravel
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5.0
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Meadow-type grasses, short pliant blades; lawn grasses, short
pliant blades
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5.0
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Coarse gravel (shingled channels)
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5.0 to 8.0
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Shale and rock
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No maximum
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D.
Ditch lining. Concrete or macadam paved ditches are to be considered
as the final alternate or solution to be employed as ditch protection
and, as such, shall be avoided whenever possible.
A.
Pipes, manholes, inlets, etc., located under existing or proposed
paving or public right-of-way shall be designed for a minimum HS-20
truck load in accordance with the requirements set forth in the latest
applicable Maryland State Highway Regulations. Maximum and minimum
depths of cover allowable on pipes under various loading conditions
shall be obtained from the pipe loading tables published and certified
by the certified pipe manufacturer.
B.
Reinforced concrete box culverts or bridges shall be designed in
accordance with the latest applicable Maryland State Highway Regulations.
A.
No person or firm shall make any connection to any storm drain appurtenance
without first obtaining a written permit from the City Engineer. The
permit fee shall be $150 per connection.
B.
The owner or his agent shall make application on a form furnished
by the City of Hagerstown. The permit application shall be supplemented
by any plans, specifications or other information considered pertinent
in the judgment of the City Engineer.
Refer to Chapter 213, Stormwater Management, for criteria on the management of runoff caused by development or grading.
Except when specifically defined otherwise herein, any violation
of this article shall be considered a municipal infraction. Citations
shall be $200 for each day the violation occurs.
Any person, corporation, taxpayer or department of the City
aggrieved by any decision resulting in a disapproval of a properly
filed application for a permit, issuance of a written notice of violation
or any alleged failure to properly enforce the chapter in regard to
a specific application shall have the right to appeal the action to
the Board of Technical Appeals of the City of Hagerstown. Each appeal
to the Board of Technical Appeals shall be filed in writing on the
appropriate form accompanied by the appropriate fee within 20 days
of the date of official transmittal of the final decision or determination
to the applicant, and shall state clearly the grounds on which the
appeal is based, and shall be processed in the manner prescribed for
hearing administrative appeals under the Code of the City of Hagerstown.
