Volume control BMPs are intended to maintain existing hydrologic conditions for small storm events by promoting groundwater recharge and/or evapotranspiration as described in this section. Runoff volume controls shall be implemented using the Design Storm Method described in §
315-32A below, or through continuous modeling approaches or other means as described in the BMP Manual. Small projects may use the method described in §
315-32B to design volume control BMPs.
A. The Design Storm Method is applicable to any size of regulated activity.
This method requires detailed modeling based on site conditions.
(1) Do not increase the post-development total runoff volume for all
storms equal to or less than the two-year twenty-four-hour storm event.
(2) For modeling purposes:
(a)
Existing (predevelopment) non-forested pervious areas must be
considered meadow in good condition.
(b)
When the existing development site contains impervious area,
20% of existing impervious area to be disturbed shall be considered
meadow in good condition in the model for existing conditions.
(c)
The maximum loading ratio for volume control facilities in karst
areas shall be 3:1 impervious drainage area to infiltration area and
5:1 total drainage area to infiltration area. The maximum loading
ratio for volume control facilities in non-karst areas shall be 5:1
impervious drainage area to infiltration area and 8:1 total drainage
area to infiltration area. A higher ratio may be approved by the Borough
if justification is provided. Hydraulic depth may be used as an alternative
to an area-based loading ratio if the design hydraulic depth is shown
to be less than the depth that could result from the maximum area
loading ratio.
B. Volume control for small projects.
(1) At least the first one inch of runoff from new impervious surfaces
or an equivalent volume shall be permanently removed from the runoff
flow, i.e., it shall not be released into the surface waters of this
commonwealth. Removal options include reuse, evaporation, transpiration
and infiltration.
C. A detailed geologic evaluation of the development site shall be performed in areas of carbonate geology to determine the design parameters of recharge facilities. A report shall be prepared in accordance with §
315-45A of this chapter.
(1) If the developer can prove through analysis that the development
site is in an area underlain by carbonate geology, and such geologic
conditions may result in sinkhole formations, then the development
site is exempt from volume control requirements as described in this
chapter. However, the development site shall still be subject to NPDES
and E&S requirements.
D. Storage facilities, including normally dry, open-top facilities,
shall completely drain the volume control storage over a period of
time not less than 24 hours and not more than 72 hours from the end
of the design storm. Any designed infiltration at such facilities
is exempt from the minimum twenty-four-hour standard, i.e., may infiltrate
in a shorter period of time, provided that none of this water will
be discharged into waters of this commonwealth.
E. Any portion of the volume control storage that meets the following
criteria may also be used as rate control storage:
(1) Volume control storage that depends on infiltration is designed according to the infiltration standards in §
315-31.
(2) The volume control storage which will be used for rate control is
that storage which is available within 24 hours from the end of the
design storm based on the stabilized infiltration rate and/or the
evapotranspiration rate.
F. Applicable worksheets from of the BMP Manual shall be used when establishing
volume controls.
G. The green
infrastructure and LID practices provided in the BMP Manual shall
be used for all regulated activities wherever possible.
[Added 9-27-2022 by Ord. No. 2022-04]
Rate control for large storms, up to the one-hundred-year event,
is essential to protect against immediate downstream erosion and flooding.
A. Match predevelopment hydrograph.
(1) Applicants shall provide infiltration facilities or utilize other techniques which will allow the post-development one-hundred-year hydrograph to match the predevelopment one-hundred-year hydrograph, along all parts of the hydrograph, for the development site. To match the predevelopment hydrograph, the post-development peak rate must be less than or equal to the predevelopment peak rate, and the post-development runoff volume must be less than or equal to the predevelopment volume for the same storm event. A shift in hydrograph peak time of up to five minutes and a rate variation of up to 5% at a given time may be allowable to account for the timing effect of BMPs used to manage the peak rate and runoff volume. "Volume control" volumes as given in §
315-32 above may be used as part of this option.
B. Where the predevelopment hydrograph cannot be matched, one of the
following shall apply:
(1) Within the Little Conestoga Creek watershed, post-development rates of runoff from any regulated activity shall not exceed 50% of the peak rates of runoff prior to the development for all design storms [two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year twenty-four-hour storms (NOTE: A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table 2 in §
315-35 or (NOAA) Atlas 14 data for the specific project site.)]. See the Release Rate Map in Appendix L for additional information.
(2) Within all remaining watersheds of Millersville Borough, the calculated post-development peak rate of stormwater runoff from any regulated activity shall not exceed the calculated predevelopment peak rate of runoff for all design storms [two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year twenty-four-hour storms (NOTE: A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table 2 in §
315-35 or (NOAA) Atlas 14 data for the specific project site.)]. See the Release Rate Map in Appendix L for additional information.
C. All basins not including groundwater recharge and/or water quality
storage shall include an outlet structure to allow for draining the
basin to a completely dry position within 24 hours following the end
of the design rainfall. All basins that include groundwater recharge
and/or water quality storage shall include an outlet structure to
allow draining the basin to the level of the groundwater recharge
and/or water quality storage within 24 hours following the end of
the design rainfall.
D. A variety of BMPs should be employed and tailored to suit the development
site. The following is a partial listing of BMPs which can be utilized
in SWM systems for rate control where appropriate:
(1) Decreased impervious surface coverage.
(3) Grassed channels and vegetated strips.
(4) Bio-retention areas (rain gardens).
(5) Concrete lattice block or permeable surfaces.
(6) Seepage pits, seepage trenches or other infiltration structures.
(9) Cisterns and underground reservoirs.
(13)
Other methods as may be found in the BMP Manual.
E. Small projects are not required to provide for rate control.
Aboveground storage facilities consist of all stormwater facilities
which store, infiltrate/evaporate/transpire, clean or otherwise affect
stormwater runoff, and the top of which is exposed to the natural
environment. Aboveground storage facilities are located above the
finished ground elevation. Aboveground storage facilities do not include
SWM facilities designed for conveyance or cisterns.
A. All basins shall be structurally sound and shall be constructed of
sound and durable materials. The completed structure and the foundation
of all basins shall be stable under all probable conditions of operation.
B. Design criteria. Aboveground storage facilities shall comply with
the design criteria in the following table:
|
Aboveground Storage Facility Design Criteria
|
---|
|
|
Facility Depth
|
---|
|
|
Less than 2 feet
|
2 feet to 6 feet
|
Greater than 6 feet
|
---|
|
(a)
|
Embankment geometry
|
|
|
[1]
|
Top width (minimum)
|
2 feet
|
5 feet
|
8 feet
|
|
|
[2]
|
Interior side slope (maximum)
|
2:1
|
5:1
|
5:1
|
|
|
[3]
|
Exterior side slope (maximum)
|
2:1
|
3:1
|
3:1
|
|
(b)
|
Embankment construction
|
|
|
[1]
|
Key trench
|
Not required
|
Required
|
Required
|
|
|
[2]
|
Pipe collar
|
Not required
|
Required
|
Required
|
|
|
[3]
|
Compaction density
|
Not required
|
Required
|
Required
|
|
(c)
|
Internal construction
|
|
|
[1]
|
Dewatering feature
|
N/A
|
Required
|
Required
|
|
|
[2]
|
Pretreatment elements
|
Not required*
|
Required
|
Required
|
|
(d)
|
Outlet structure
|
|
|
[1]
|
Pipe size (minimum)
|
6 inches
|
12 inches
|
15 inches
|
|
|
[2]
|
Pipe material
|
SLHDPE, PVC, RCP
|
RCP
|
RCP
|
|
|
[3]
|
Anticlogging devices
|
Required
|
Required
|
Required
|
|
|
[4]
|
Antivortex design
|
Not required
|
Required
|
Required
|
|
|
[5]
|
Watertight joints in piping?
|
No
|
Yes
|
Yes
|
|
(e)
|
Spillway requirements
|
|
|
[1]
|
Spillway freeboard (minimum)
|
Not required
|
6 inches
|
12 inches
|
|
|
[2]
|
Width (minimum)
|
Not required
|
10 feet
|
20 feet
|
|
|
[3]
|
Width (maximum)
|
Not required
|
50 feet
|
50 feet
|
|
|
[4]
|
Spillway channel design
|
Not required
|
Required
|
Required
|
|
|
[5]
|
Routing of 100 year storm
|
Permitted
|
Permitted
|
Permitted
|
|
*
|
Pretreatment required for infiltration BMPs unless shown to
be unnecessary
|
|
|
N/A = Not applicable
|
|
|
SLHDPE = Smooth lined high-density polyethylene pipe
|
|
|
PVC = Polyvinyl chloride
|
|
|
RCP = Reinforced concrete pipe
|
C. Facility depth.
(1) For the purposes of the design criteria, the facility depth is defined
to be the depth between the bottom invert of the lowest orifice and
the invert of the spillway. If there is no spillway, the top of the
berm shall be used. For basins with no orifices or outlet structure
at the bottom of the basin, the bottom elevation of the basin shall
be used.
(2) Facilities with a facility depth greater than six feet shall not
be permitted in residential areas.
(3) Facilities with a facility depth greater than 15 feet require a dam
permit from DEP.
(4) The maximum depth of water for aboveground storage facilities without
restricted access shall not exceed six feet unless approved by modification
or waiver by the Borough Council or its designee. Access to basins
with a maximum depth of water greater than six feet shall be restricted
by fencing that will discourage access.
D. Embankment construction.
(1) Impervious core/key trench. An impervious core/key trench, when required,
shall consist of a cutoff trench (below existing grade) and a core
trench (above existing grade). A key trench may not be required wherever
it can be shown that another design feature, such as the use of an
impermeable liner, accomplishes the same purpose.
(a)
Materials used for the core shall conform to the Unified Soil
Classification GC, SC, CH, or CL and must have at least 30% passing
the No. 200 sieve.
(b)
The dimensions of the core shall provide a minimum trench depth
of two feet below existing grade, minimum width of four feet and side
slope of 1H:1V or flatter.
(c)
The core should extend up both abutments to the ten-year water
surface elevation or six inches below the emergency spillway elevation,
whichever is lower.
(d)
The core shall extend four feet below any pipe penetrations
through the impervious core. The core shall be installed along or
parallel to the center line of the embankment.
(e)
The area under the embankment shall be cleared, grubbed and
stripped of topsoil to a depth of two feet prior to any placement
and compaction of earthfill.
(2) Compaction.
(a)
Compaction requirements shall be the same as those for the embankment
to assure maximum density and minimum permeability.
(b)
The core shall be constructed concurrently with the outer shell
of the embankment. Core and key trench shall be constructed to a minimum
of 95% standard Proctor density.
(c)
The trench shall be dewatered during backfilling and compaction
operations.
(3) Pipe collars. All pipe collars, when required, shall be designed
in accordance with Chapter 7 of the E&S Manual. The material shall
consist of concrete or otherwise nondegradable material around the
outfall barrel and shall be watertight.
(4) Embankment fill material. The embankment fill material shall be taken
from an appropriate borrow area which shall be free of roots, stumps,
wood, rubbish, stones greater than six inches, frozen or other objectionable
materials.
(5) Embankment compaction. When required, embankments shall be compacted
by sheepsfoot or pad roller. The loose lift thickness shall be nine
inches or less, depending on roller size, and the maximum particle
size is six inches or less (2/3 of the lift thickness). Five passes
of the compaction equipment over the entire surface of each lift is
required. Embankment compaction to visible nonmovement is also required.
E. Internal construction.
(1) Bottom slope. The minimum bottom slope of facilities not designed
for infiltration shall be 1%. A flatter slope may be used if an equivalent
dewatering mechanism is provided.
(2) Dewatering features. When required, dewatering shall be provided
through the use of underdrain, surface device, or alternate approved
by the Borough Engineer. If the facility is to be used for infiltration,
the dewatering device should be capable of being disconnected and
only be made operational if the basin is not dewatering within the
required time frame.
(3) Pretreatment elements. When required, pretreatment elements shall
consist of forebays, or alternate approved by the Borough Engineer,
to keep silt to a smaller portion of the facility for ease of maintenance.
(4) Infiltration basins. Within basins designed for infiltration, existing native vegetation shall be preserved, if possible. For existing unvegetated areas or for infiltration basins that require excavation, a planting plan shall be prepared in accordance with §
315-31L and the BMP Manual which is designed to promote infiltration.
F. Outlet configuration.
(1) For facilities with a depth of two feet or greater, a Type D-W endwall
or riser box outlet structure shall be provided.
(2) For facilities with a depth less than two feet, the designer must
specify a suitable outlet structure.
(3) All discharge control devices with appurtenances shall be made of
reinforced concrete and stainless steel. Bolts/fasteners shall be
stainless steel.
G. Spillway.
(1) Material. The spillway shall be designed to provide a nonerosive,
stable condition when the project is completed.
(2) Nonemergency use. Use of the spillway to convey flows greater than
the fifty-year design storm is permitted.
(3) Emergency use. The spillway shall be designed to convey the one-hundred-year
peak rate of runoff which enters the basin after development in a
manner which will not damage the integrity of the facility and will
not create a downstream hazard.
(4) When required, freeboard shall be measured from the top of the water
surface elevation for emergency use.
H. Breach analysis. The Borough may require a breach analysis based
on site-specific conditions and concern of threat for downstream property.
When required, the breach analysis shall be conducted in accordance
with the NRCS methodology, the United States Army Corps of Engineers
methodology (HEC-1) or other methodologies as approved by the Borough.
I. SWM facilities which qualify as a dam under DEP regulations or facilities
deemed a potential threat to the life, safety or welfare of the general
public shall be subject to the following requirements:
(1) Facilities which qualify as a dam under DEP regulation shall obtain
the required permit through DEP and design the facility in accordance
with DEP standards.
(2) Additional requirements and analysis may be required by the Borough
to prove that the proposed facility has been designed to limit the
potential risk to the life, safety or welfare of the general public.
Subsurface storage facilities consist of all stormwater facilities
which store, infiltrate/evaporate/transpire, clean or otherwise affect
stormwater runoff and the top of which is not exposed to the natural
environment. Subsurface facilities are located below the finished
ground elevation. Subsurface facilities do not include SWM facilities
designed for conveyance.
A. Subsurface storage facilities shall comply with the design criteria
in the following table:
|
Subsurface Storage Facility Design Criteria
|
---|
|
|
Facility Type
|
---|
|
|
Infiltration and Storage
|
Storage without Infiltration
|
---|
|
(a)
|
Facility geometry
|
|
|
[1]
|
Depth from surface (maximum)
|
2 feet less than limiting zone
|
N/A
|
|
|
[2]
|
Loading ratio (maximum)
|
Per BMP Manual*
|
N/A
|
|
(b)
|
Distribution system requirements
|
|
|
[1]
|
Pipe size (minimum)
|
4 inches
|
4 inches
|
|
|
[2]
|
Pretreatment
|
Required
|
Required
|
|
|
[3]
|
Loading/balancing
|
Required
|
Not required
|
|
|
[4]
|
Observation/access ports
|
Required
|
Required
|
|
*
|
Unless otherwise determined by professional geologic evaluation
|
B. Distribution system requirements.
(1) Pretreatment requirements. The facility shall be designed to provide
a method to eliminate solids, sediment, and other debris from entering
the subsurface facility.
(2) Loading/balancing. The facility shall be designed to provide a means
of evenly balancing the flow across the surface of the facility to
be used for infiltration.
(3) Observation/access ports.
(a)
For facilities with the bottom less than five feet below the
average grade of the ground surface, a cleanout shall be an acceptable
observation port.
(b)
For facilities with the bottom five feet or more below the average
grade of the ground surface, a manhole or other means acceptable to
the Borough shall be provided for access to and monitoring of the
facility.
(c)
The number of access points shall be sufficient to flush or
otherwise clean out the system.
C. Materials.
(1) Pipe material. Distribution system piping may be PVC, SLHDPE, or
RCP.
(2) Stone for infiltration beds. The stone used for infiltration beds
shall be clean washed, uniformly graded coarse aggregate. The void
ratio for design shall be assumed to be 40%.
(3) Backfill material. Material consistency and placement depths for
backfill shall be (at a minimum) per all applicable pipe manufacturer's
recommendations, further providing it should be free of large (not
exceeding six inches in any dimension) objectionable or detritus material.
Select non-aggregate material should be indigenous to the surrounding
soil material for non-vehicular areas. Backfill within vehicular areas
shall comply with this section unless otherwise specified in the Borough
Street or Subdivision and Land Development Ordinance or by the Borough
Engineer. Furthermore, if the design concept includes the migration
of runoff through the backfill to reach the infiltration facility,
the material shall be well drained, free of excess clay or clay-like
materials and generally uniform in gradation.
(4) Lining material. Nonwoven geotextiles shall be placed on the sides
and top of subsurface infiltration facilities. No geotextiles shall
be placed on the bottom of subsurface infiltration facilities.
D. Cover.
(1) When located under pavement, the top of the subsurface facility shall
be a minimum of three inches below the bottom of pavement subbase.
(2) Where located under vegetative cover, the top of the subsurface facility
shall be a minimum of 12 inches below the surface elevation or as
required to establish vegetation.
E. Subsurface facilities shall be designed to safely convey and/or bypass
flows from storms exceeding the design storm.
F. Infiltration facilities shall be designed with measures to protect
infiltration facilities from compaction and sedimentation during and
after construction.
Conveyance facilities consist of all stormwater facilities which
carry flow, which may be located either above or below the finished
grade. Conveyance facilities do not include SWM facilities which store,
infiltrate/evaporate/transpire, or clean stormwater runoff.
A. Design criteria. Conveyance facilities shall comply with the design
criteria in the following table:
|
Conveyance Facility Design Criteria
|
---|
|
Location
|
Within Public Street Right-of-Way
|
Outside Public Street Right-of-Way
|
---|
|
Loading
|
All
|
Vehicular Loading
|
Non-Vehicular Loading
|
---|
|
(a)
|
Pipe design
|
|
|
[1]
|
Material
|
RCP
|
PVC, SLHDPE, RCP
|
PVC, SLHDPE, RCP
|
|
|
[2]
|
Slope (minimum)
|
0.5%
|
0.5%
|
0.5%
|
|
|
[3]
|
Cover
|
1 foot to stone subgrade
|
1 foot to stone subgrade
|
1 foot to surface
|
|
|
[4]
|
Diameter (minimum)
|
15 inches
|
12 inches
|
8 inches
|
|
|
[5]
|
Street crossing angle
|
75° to 90°
|
N/A
|
N/A
|
|
|
[6]
|
Access/ maintenance port frequency (maximum)
|
400 feet
|
400 feet
|
600 feet
|
|
(b)
|
Inlet design
|
|
|
[1]
|
Material
|
Concrete
|
Concrete
|
N/A
|
|
|
[2]
|
Grate depression
|
1/2 inch or less
|
2 inches
|
1 inch minimum
|
|
(c)
|
Manhole design
|
|
|
[1]
|
Material
|
Concrete
|
Concrete
|
Concrete
|
|
(d)
|
Swale design
|
|
|
[1]
|
Freeboard (minimum)
|
6 inches
|
N/A
|
6 inches
|
|
|
[2]
|
Velocity (maximum)
|
Stability check
|
N/A
|
Stability check
|
|
|
[3]
|
Slope (minimum)
|
1%
|
N/A
|
1%
|
|
|
[4]
|
Side slopes (residential area)
|
4:1 max
|
N/A
|
4:1 max
|
|
|
[5]
|
Side slopes (nonresidential area)
|
4:1 max
|
N/A
|
3:1 max
|
|
|
[6]
|
Bottom width to flow depth ratio
|
12:1
|
N/A
|
12:1
|
|
(e)
|
Outlet design
|
|
|
[1]
|
End treatment
|
Headwall/endwall
|
N/A
|
Headwall/endwall or flared end section
|
|
|
[2]
|
Energy dissipater
|
Required
|
N/A
|
Required
|
|
N/A = Not applicable or no criteria specified
|
|
SLHDPE = Smooth lined high-density polyethylene pipe
|
|
PVC = Polyvinyl chloride
|
|
RCP = Reinforced concrete pipe
|
B. Conveyance pipes, culverts, manholes, inlets and endwalls within
the public street right-of-way or proposed for dedication shall conform
to the requirements of PennDOT Standards for Roadway Construction,
Publication No. 72M as directed by the Borough Engineer.
C. Conveyance pipes, culverts, manholes, inlets and endwalls outside
the public street right-of-way which are subject to vehicular loading
shall be designed for the HS-25 loading condition.
D. All material and workmanship for conveyance facilities shall conform
to current PennDOT Publication 408 specifications.
E. Conveyance pipes.
(1) Backfill requirements. Backfill material. Material consistency and
placement depths for backfill shall be (at a minimum) per all applicable
pipe manufacturer's recommendations, further providing it should be
free of large (not exceeding six inches in any dimension) objectionable
or detritus material. Select non-aggregate material should be indigenous
to the surrounding soil material for non-vehicular areas. Backfill
within vehicular areas shall comply with this section unless otherwise
specified in the Borough Street or Subdivision and Land Development
Ordinance or by the Borough Engineer.
(2) Inlets or manholes shall be placed at all points of changes in the
horizontal or vertical directions of conveyance pipes. Curved pipe
sections are prohibited.
(3) Access/maintenance ports. An access/maintenance port is required
and may either be an inlet or manhole.
(4) Watertight joints shall be provided where pipe sections are joined,
except for perforated pipe installed as pavement base drain.
(5) The street-crossing angle shall be measured between the pipe center
line and the street center line.
(6) Elliptical pipe of an equivalent cross-sectional area may be substituted
in lieu of circular pipe where cover or utility conflict conditions
exist.
(7) The roughness coefficient (Manning "n" values) used for conveyance
pipe capacity calculations should be determined in accordance with
Appendix E, or per the manufacturer's specifications.
F. Inlets.
(1) All pipes must enter inlets completely through one of the sides.
No corner entry of pipes is permitted.
(2) Within the public street right-of-way, the gutter spread based on
the twenty-five-year storm shall be no greater than 1/2 of the travel
lane and have a maximum depth of three inches at the curbline. A parking
lane shall not be considered as part of the travel lane. In the absence
of pavement markings separating a travel lane from the parking lane,
the parking lane shall be assumed to be seven feet wide if parking
is permitted on the street.
(3) Flow depth within intersections. Within intersections of streets,
the maximum depth of flow shall be 1.5 inches based on the twenty-five-year
storm.
(4) Curbed streets.
(a)
Inlets in streets shall be located along the curbline.
(b)
Top units shall be PennDOT Type "C." The hood shall be aligned
with the adjacent curb height.
(5) All inlets placed in paved areas shall have heavy-duty bicycle-safe
grating consistent with PennDOT Publication 72M, latest edition. A
note to this effect shall be added to the SWM site plan or inlet details
therein.
(6) Inlets, junction boxes, or manholes greater than five feet in depth
shall be equipped with ladder rungs and shall be detailed on the SWM
site plan.
G. Swales.
(1) A swale shall be considered as any man-made ditch designed to convey
stormwater directly to another stormwater management facility or surface
waters.
(2) Inlets within swales shall have PennDOT Type "M" top units or equivalent
approved by the Borough Engineer.
(3) Swale capacities and velocities shall be computed using the Manning
Equation using the following design parameters:
(a)
The first condition shall consider swale stability based upon
a low degree of retardance ("n" = 0.03);
(b)
The second condition shall consider swale capacity based upon
a higher degree of retardance ("n" = 0.05); and
(c)
All vegetated swales shall have a minimum slope of 1% unless
otherwise approved by the Borough Engineer.
(d)
The "n" factors to be used for paved or riprap swales or gutters
shall be based upon accepted engineering design practices, as approved
by the Borough Engineer.
(4) All swales shall be designed to maximize infiltration and concentrate
low flows to minimize siltation and meandering, unless geotechnical
conditions do not permit infiltration.
H. Culverts. In addition to the material requirements in this section,
culverts designed to convey waters of this commonwealth may be constructed
with either a corrugated metal arch or a precast concrete culvert.
I. Level spreaders.
(1) Shall discharge at existing grade onto undisturbed vegetation.
(2) Discharge at a depth not exceeding 3.0 inches for a fifty-year, twenty-four-hour
design storm.
J. Energy dissipaters. Energy dissipaters shall be designed in accordance
with the requirements in the E&S Manual.
K. End treatments.
(1) Where the connecting pipe has a diameter 18 inches or greater, headwalls
and endwalls shall be provided with a protective barrier device to
prevent entry of the storm sewer pipe by unauthorized persons. Such
protection devices shall be designed to be removable for cleaning.
(2) Headwalls and endwalls shall be constructed of concrete.
(3) Flared end sections shall be of the same material as the connecting
pipe and be designed for the size of the connecting pipe.