This chapter presents criteria regulating the design of public
streets for storm flow drainage and allowable encroachment of these
flows within public street rights-of-way. Also included herein are
criteria for the selection and placement of storm drain inlets as
required to conform to maximum allowable street ponding. All design
submittals involving the utilization of public streets for the conveyance
of storm flows shall be reviewed based upon the criteria in this chapter.
(Res. 40-08 (§ 1101), 3-19-08)
The primary function of public streets is the movement of traffic.
Therefore, use of streets as part of the drainage system must be limited
in order to minimize interference with traffic functions. Street inundation
and ponding limits are specified in this chapter in order to limit
this potential interference. However, due to the nature of the storms
typical to Mesa County, and the resulting ephemeral interference with
traffic movement, these limits are not highly restrictive.
Streets typically convey runoff collected not only on the street
surface itself, but often from some portion of the surrounding area
as well. As an integral part of the drainage system, streets must
be capable of conveying some portion of said runoff to a primary drainage
conveyance facility such as a storm drain or open channel drainage
system. The maximum allowable capacity of a street is based upon its
cross-sectional configuration, longitudinal slope, and the maximum
allowed ponding depth. Where the calculated minor storm event flow
depth exceeds this maximum depth, inlets must be used to reduce street
flow. During a major storm event, streets become emergency runoff
channels, routing floodwaters away from structures as much as possible.
During such an event, many streets will be inundated to a degree such
that they are impassable to most vehicles.
(Res. 40-08 (§ 1102), 3-19-08)
A number of factors may affect traffic movement and the streets
themselves when used for drainage purposes. Five of these factors
are discussed in the following sections.
(Res. 40-08 (§ 1103), 3-19-08)
Rainfall on the paved surface of a street must flow overland
(sheet flow) until it reaches a channel. In this case, the channel
will be either a curb-and-gutter or a roadside ditch. Ignoring the
effects of street inundation due to upgradient runoff, the depth of
sheet flow will be near zero at the street crown, increasing in the
direction of the collection channel (i.e., the gutter).
Sheet flow can interfere with traffic movement by increasing
the risk of hydroplaning or splashing. Hydroplaning is a phenomenon
in which one or more of a vehicle’s tires lose significant contact
with the pavement and become supported by a thin layer of water. This
water acts as a lubricant between the pavement and the vehicle, and
can cause loss of vehicle control. The potential for hydroplaning
increases with vehicle speed and with the depth of water on the road
surface, so extra attention shall be paid to this issue during the
design of arterial and highway-type roadways. Sheet flow depths can
be decreased by increasing the street cross-slope.
Splashing is also dependent on vehicle speeds and water depth,
and can interfere with traffic movement by reducing driver visibility.
Again, increasing street cross-slope may help to reduce sheet flow
depths and splashing potential. In general, a two percent cross slope
is desirable to promote swift removal of runoff and reduce sheet flow
depths while minimizing potential vehicle side-slippage from ice buildup
during winter months.
(Res. 40-08 (§ 1103.1), 3-19-08)
Where curb-and-gutter is utilized, all runoff tributary to a street will be directed to, and flow in, a gutter until it reaches a storm drain inlet or other primary conveyance. Generally, flow width will increase in the downstream direction due to an ever-increasing tributary area (and thus higher flows), eventually spreading into the traffic lane(s) and to the street crown. It is important to take into account the gutter flow width and its effect upon traffic movement. For streets where on-street parking is allowed (e.g., residential streets), gutter flow may spread at least to the inner edge of the parking lane without interfering with moving traffic. However, where no street parking is allowed, flow spreading beyond the actual gutter almost immediately reaches a travel lane. Drivers tend to avoid even partially inundated lanes if possible, causing traffic congestion and occasionally vehicular collisions. However, some interference with traffic movement is allowable and expected, even during the minor storm. The duration of such interference is to be minimized wherever possible. Emergency vehicles must be able to travel all streets without encountering extensive (potentially vehicle-disabling) ponding depths. These depths are defined in GJMC §
28.44.090 as maximum total gutter depths for the minor storm event.
(Res. 40-08 (§ 1103.2), 3-19-08)
Grade changes and street crowns at intersections can cause storm runoff to flow at depths greater than the intended design depth and to flow at greater depths for longer durations than anticipated. This localized, temporary ponding poses a high safety risk, especially on roads with higher speed limits (e.g., arterials). Because the ponding is localized, drivers may not be aware of the obstruction and vehicles may enter the ponded area at high speeds (see reference to hydroplaning, GJMC §
28.44.040). Even if traffic is moving at low speeds, vehicles encountering a deeper ponded area may stop entirely to avoid entering such an area or may experience engine stalling while attempting to cross such an area. Both of these scenarios have the effect of reducing or halting traffic movement. Therefore, street and drainage design must incorporate measures to control the depths at locations where temporary ponding may occur through the use of grading changes or additional inlets. For streets with high traffic volumes, it may be necessary to take steps to effectively eliminate any temporary ponding in order to avoid extensive interference with traffic movement.
Cross flow occurs as a result of a number of scenarios that
cause significant flow depths other than that which is the result
of sheet flow to travel across traffic lanes. The most common location
to encounter this phenomenon is at intersections where gutter flow
from the cross-street spills across the intersection. Where allowable,
a concrete V-pan may be utilized to control these flows (see City
of Grand Junction Standard Details for Construction of Streets, Storm
Drains and Utilities, Revised July 2005, page C-12). Localized cross
flow can have the same negative effects on vehicular safety and traffic
movement as general temporary ponding. Therefore, careful consideration
shall be given to potential cross flow locations during design to
minimize its effects, especially for streets with higher speed limits
and/or higher traffic volume.
(Res. 40-08 (§ 1103.3), 3-19-08)
Utilization of streets for drainage of storm runoff can have
a significant effect on roadway maintenance and repair and can, in
certain cases, contribute to the structural failure of the roadway
pavement. The latter situation may occur if water is able to penetrate
the pavement and reach the subgrade. Subgrade saturation and/or material
washout will eventually cause subgrade failure, leading to pavement
failure and an unsafe/unstable road surface. These locations of distress
are caused by other factors such as weathering, overweight vehicles,
temperature and temperature changes, heavy traffic, machinery, pavement
quality, subgrade inconsistencies, and pavement age. Water flow across
undamaged pavement will not typically penetrate to the subgrade. Therefore,
consistent roadway maintenance not only affects the efficiency of
the street as a drainage system component, but can also reduce the
likelihood of roadway failure due to the effects of ponding water.
Although undamaged pavement surfaces for the most part keep
water from penetrating through to the subgrade, it is possible for
some water to seep through with time. Street designs shall incorporate
measures to decrease the duration for which a given section of pavement
is submerged to minimize this seepage.
A common practice used to reduce the problem of bituminous surface
deterioration is to seal-coat or overlay the existing pavement surface.
While this method does effectively reduce pavement deterioration,
it reduces the available street flow area with each layer added to
the surface. To minimize this effect, it is recommended that the surface
be scarified whenever possible before new layers of pavement are added.
(Res. 40-08 (§ 1103.4), 3-19-08)
Sediment and debris buildup may occur on streets in any area
where flow velocities tend to decrease such as near grade changes
and inlets. Sediment and debris buildup can have a significant impact
on the flow capacities of gutters and streets, causing increased flow
width and thus increased interference with traffic movement.
Locations where significant deposits may occur shall be identified
for maintenance purposes to include street sweeping and inlet cleanout
as necessary. Inlets shall be designed to function properly based
on expected sediment and debris clogging as specified later in this
chapter.
Localized sedimentation issues due to construction activities
shall be controlled per the criteria presented in Chapter 28.6 GJMC.
(Res. 40-08 (§ 1103.5), 3-19-08)
(a) Streets are classified according to estimated traffic volume and right-of-way width. The City of Grand Junction and Mesa County have adopted the Transportation, Engineering, and Design Standards (
TEDS) manual (GJMC Title
29), including standard drawings and details for the construction of streets and location for utilities. While these drawings and details were developed and are maintained by the Grand Junction Department of Public Works and Utilities (Engineering Division), they have been adopted by the Mesa County Board of Commissioners (
TEDS Chapter 1, p.1). Eight street classifications are specified therein, but some of these have been grouped together in this manual due to similar hydraulic characteristics. The rural roadway street type is not presented in this chapter since it utilizes a roadside ditch and culverts instead of curb-and-gutter. See Chapter
28.32 GJMC for open channel design criteria.
(b) The local jurisdictions within Mesa County have adopted the policy
that streets can be used to convey storm runoff subject to the limits
established by this manual. For the minor storm event:
(1) Flow depths must not exceed 0.5 feet at the gutter flowline;
(2) Must not exceed curb height (varies); and
(3) Flow velocities must not exceed 8.0 feet per second.
(c) Street flow depth from the major storm event must not exceed 1.0
foot from the gutter flowline and the flow velocity must not exceed
8.0 feet per second.
(d) Figures 28.44.090(a) and 28.44.090(b) show typical half-street sections
and two-year storm event inundation limits for each street classification
group. Note that the cross-sections shown include geometric assumptions
made for hydraulic computation purposes and do not precisely reflect
the approved standard street details.
(e) Calculations for flow capacity and velocity in a given street section
are based upon these maximum depths and the assumption that any area
not within the street right-of-way would not contribute to the capacity
of the street system (called “ineffective flow area”).
Therefore, for calculation purposes, it is assumed that an infinitely
high vertical wall of zero roughness exists at the right-of-way boundary,
and any flow area outside this boundary is not considered in analysis.
Due to the potential for a single street cross-section to have different
half-street cross-sections, all street capacity calculations are to
be completed on a half-street basis. Therefore, the same vertical-wall
assumption applies to the street centerline as to the right-of-way
where the calculated flow width exceeds the half-street width.
(f) At street sag locations, pipes and/or channels must be provided to
facilitate compliance with maximum ponding depths for both minor and
major storm events as described above. Maintenance access must be
provided for these facilities, including permanent easements.
(g) Not inclusive of median spill gutters, the standard concrete details
include two curb types, vertical and drive-over (mountable). The vertical
curb configuration may be constructed monolithically with or without
a sidewalk, and has a height of six inches. This configuration or
an approved alternative design must be used for streets with an A.D.T.
value of more than 1,000 (Standard Street Details, Page ST-05). A
cross-section detail of the vertical curb and gutter is found in Figure
28.44.090(c). The drive-over, or mountable, curb configuration may
only be used for residential streets with an A.D.T. value of less
than 1,000 (Standard Street Details, Page ST-05). The total vertical
height from the gutter flowline to the top of the curb is 4.5 inches,
resulting in a lower maximum minor storm event water surface elevation
for streets utilizing this gutter type. It does, however, reduce the
need for driveway ramps, which are often a major contributor to the
reduction of residential gutter capacity. Figure 28.44.090(d) is a
detail of the standard mountable curb.
(Res. 40-08 (§ 1104), 3-19-08)
(a) Gutter and street flow can generally be assumed to be uniform for
the purpose of hydraulic evaluation and design, but as street flow
depth increases, flow width increases at a much faster rate. This
wide, relatively shallow flow has the effect of decreasing the hydraulic
radius, rendering the standard Manning’s equation somewhat inaccurate.
The Federal Highway Administration (FHWA) presents a modified form
of the equation in Section 5.3.2 of Introduction to Highway
Hydraulics, taken from HEC-12:
Where:
QS
|
=
|
Street Flow Capacity (not including gutter) (cfs)
|
n
|
=
|
Manning’s Roughness Coefficient
|
Ku
|
=
|
0.56 for English units
|
Ku
|
=
|
0.376 for S.I. units
|
SX
|
=
|
Street Cross Slope (ft./ft.)
|
SL
|
=
|
Street Longitudinal Slope (ft./ft.)
|
TS
|
=
|
Flow Top Width (not including gutter) (ft.)
|
(b) For streets with a single cross slope for the gutter and street section,
Equation 28.44-1 will suffice for determining total gutter/street
flow capacity if the TS term
is replaced by T = Total Flow Top Width (including gutter,
inside curb). However, the standard concrete details indicate
the use of composite cross slopes for all streets – the
gutter has a steeper cross slope than the street. In these cases,
the above equation specifies capacity in the flow area between the
edge of pavement (not including the gutter itself) and the edge of
flow. Note that the result of the same equation as applied to any
flow area beyond the street centerline must be subtracted from the
total capacity (see Figure 28.44.090(a)). The total flow capacity
in the street and the gutter is calculated as:
Where:
SW
|
=
|
Gutter Cross Slope (ft./ft.)
|
SX
|
=
|
Street Cross Slope (ft./ft.)
|
T
|
=
|
Top Width (inside curb) (ft.)
|
W
|
=
|
Gutter Width (ft.)
|
(c) Figures 28.44.100(a) through 28.44.100(f) show calculated half-street
flow capacities for each street classification group based on longitudinal
street slope and flow depth. The HEC-12 equation (Equation 28.44-1)
was used to determine most flow capacities for these charts except
in cases where the flow width does not exceed gutter width. Where
gutter flow does not encroach on the street surface, it is assumed
that the width to depth ratio is not large enough to warrant the utilization
of the HEC-12 modified Manning’s equation. Instead, the standard
form is used in these cases:
Where:
Q
|
=
|
Flow Capacity (cfs)
|
SL
|
=
|
Street Longitudinal Slope (ft./ft.)
|
R
|
=
|
Hydraulic Radius (ft.) = A/P
|
A
|
=
|
Cross-Sectional Flow Area (sf)
|
P
|
=
|
Wetted Perimeter (ft.)
|
(d) Major storm event flow depth may exceed curb height, thus the flow
area behind the curb – between the curb and the right-of-way –
must also be considered in the calculation of total street capacity.
The flow in this area is found using Equation 28.44-1, replacing TS with TB = Flow Top Width (behind
curb). Note that the truncation procedure described above must also
be applied here if the calculated top width extends beyond the right-of-way
boundary. Therefore, the total flow between the curb and street centerline,
QB, is:
(e) Total street capacity, then, is the sum of the resulting values from
Equations 28.44-2 and 28.44-5. The HEC-12 procedure has certain limitations
and includes certain assumptions. The most applicable of these are
listed below.
(1) One value for Manning’s roughness n must be used for the entire
cross-section. This can be a composite value derived from multiple
roughness segments in the cross-section.
(2) The HEC-12 method ignores any roughness characteristics of the vertical
segment of the curb. The energy-dissipating effect of this portion
is considered to be negligible when compared to the wide bottom sections.
(3) The vertical curb shown in Figure 28.44.100(g) actually slopes back
away from the street by one inch while rising six inches. This slope
is ignored by the HEC-12 equations. The gutter width is assumed to
include this extra one inch with a vertical segment of six-inch height
at this point. See Figure 28.44.100(g) for detail.
(4) The standard concrete details indicate that the edge of the roadway
pavement shall be one-quarter inch to one-half inch above the edge
of the concrete gutter. The HEC-12 method is unable to account for
this directly, and the hydraulic effects are assumed to be negligible,
so this is ignored.
(f) Two of the provided standard street capacity charts, Figures 28.44.100(a)
and 28.44.100(f), were not prepared using the procedure described
above. Both residential streets with mountable curbs (Figure 28.44.100(a))
and principal arterials (Figure 28.44.100(f)) have a more complex
cross-section than the HEC-12 method can accurately model. The former
is due to the alternate curb-and-gutter configuration, and the latter
has a required median to which the flow can spread, adding another
flow-control surface to the cross-section. Therefore, Haestad Methods’
FlowMaster computer program was used to model these sections and prepare
capacity charts. The program uses the Cox open-channel weighting method
to determine composite roughness and utilizes the standard form of
Manning’s equation to compute discharge.
(g) Certain assumptions were made in the creation of the street capacity
charts to minimize the required number of figures and to simplify
the design process. However, it is the responsibility of the designer
to ensure that each assumption is valid for a specific design.
(1) A Manning’s roughness value of n = 0.016 was
assumed for all flow surfaces encountered within the street right-of-way
when using the HEC-12 method. Varying values of n were used to construct the FlowMaster cross-sections, but composite
roughness values were not significantly different from 0.016 for the
two special cases.
(2) A street cross slope of 2.0 percent was assumed for all streets.
(3) A gutter cross slope of 8.33 percent was assumed for all gutters.
(4) Velocity curves are provided on the capacity charts for reference
only – the designer is responsible for the calculation
of actual gutter velocities.
(h) In cases where these assumptions may not be valid, such as designs
incorporating the use of nonstandard street sections, the designer
shall utilize the equations presented above to determine allowable
street capacity.
(i) The maximum allowable gutter velocity is 8.0 feet per second. Velocities
exceeding this value can create safety issues, cause erosive damage
to the street and other surfaces, and reduce the effectiveness of
storm drain inlets.
(Res. 40-08 (§ 1105), 3-19-08)
(a) Wherever street and gutter capacity exceeds allowable values based
on flow spread, velocity, or depth, some or all of the runoff must
be intercepted and diverted to an alternate flow path such as a storm
drain or designated runoff channel. The most common method of interception
is by the utilization of storm drainage inlets, of which there are
four major types:
(3) Combination inlets (curb-opening and grate);
(b) The slotted drain inlet is not recommended due to its high susceptibility
to clogging, and may only be used where specifically approved by a
local jurisdiction. All inlets must be labeled with “NO DUMPING –
DRAINS TO RIVER.” Isometric-view examples of the first three
types are shown in Figure 28.44.110.
(c) Inlets may either be located on a continuous grade – where
flow not intercepted by the inlet will pass to another location –
or in a sag portion of a street’s vertical alignment (or any
other sump location). Computation of inlet capacity involves several
factors including type of inlet, location (on-grade or in a sump),
grate type (if applicable), inlet geometry, flow width and depth,
and both longitudinal and cross slopes.
(d) Methods and rationale utilized in this chapter for the calculation of inlet capacities are based on those presented in HEC-12 (1984). However, HEC-12 contains a nomograph for finding grate splash-over velocity, and here we use an empirical formula for this value (Guo, Storm Water System Design, CE 5803, University of Colorado at Denver, 1999). Additionally, clogging must be considered in the selection and location of inlets, and will be addressed in GJMC §
28.44.140.
(Res. 40-08 (§ 1106), 3-19-08)
Gutter velocities exceeding a “splash-over” velocity
and flow widths greater than grate widths typically result in interception
efficiencies of less than 100 percent for an on-grade inlet. Inlet
efficiency, E, is defined as:
Where:
Qi
|
=
|
Intercepted Flow Rate
|
Q
|
=
|
Gutter Total Flow Rate
|
Flow which is not intercepted by the inlet is called bypass
flow, Qb:
The interception efficiency of an inlet is affected by different
factors for different inlet types. Grate inlets are most sensitive
to the amount of water flowing directly over the grate (frontal flow)
and the velocity of flow in the gutter. Curb-opening inlets vary primarily
with inlet length, depth of flow, and both longitudinal and cross
slopes of the gutter and street. Combination inlets where the curb
opening and grate inlets are of similar length and are installed adjacently
have been shown to have essentially the same interception capacity
as the same grate configuration acting without a curb opening. However,
curb-opening inlets have greater debris-handling capabilities than
grates, and therefore combination inlets typically experience significantly
less grate clogging than would be experienced if the curb-opening
was excluded. A “sweeper inlet” is a type of combination
inlet that utilizes a segment of curb opening upstream from the grated
portion. This configuration has the highest debris-removal efficiency,
and therefore lowest clogging rate, of all types listed here. Total
capacity is the sum of the flow intercepted by the curb opening located
upstream of the grate(s) and that intercepted by the grated portion
of the combination inlet.
(a) Grate Inlets.
Grand Junction Standard Storm Drain Details
(Page D-05, 2005) lists storm drain grate types which have been approved
for use. The four primary types for use on streets are Types R, L,
V, and D in single, double, and triple-inlet configurations, and are
included in the inlet capacity charts provided herein. Page D-05 also
contains two important notes concerning the selection of grates:
(1) Use Type R or Type D grate where inlet is located in sump condition.
(2) Use Type V or Type L where gutter flow is from one direction only.
Type V and Type L grates are vane grates, which are more efficient
for on-grade locations, but are very inefficient for sump conditions.
Therefore, local jurisdictions do not allow the use of grate Types
V and L for locations where sump conditions may exist. However, these
types are recommended for on-grade locations where bicycle traffic
is expected.
|
The FHWA and others have extensively researched the hydraulic
characteristics of the seven grate types listed in Table 28.44.120.
The approved grate types were each matched to the FHWA grate type
most closely matching in geometry and apparent hydraulic functionality.
These assumed pairings are indicated in Table 28.44.120.
|
The total rate of intercepted flow by a grate inlet (or standard
combination inlet) is the sum of the intercepted frontal flow and
the intercepted side flow. Frontal flow, Qw is that portion of the total gutter flow which has a flow width
equal to or less than the grate width. Total frontal flow is found
using the following HEC-12 equation:
|
Where:
QW
|
=
|
Total Frontal Flow (cfs)
|
Q
|
=
|
Total Gutter Flow (no behind-curb flow) (cfs)
|
W
|
=
|
Width of Grate (ft.)
|
T
|
=
|
Total Top Width of Flow (ft.)
|
Total side flow is that portion of total gutter flow
which is between the inner edge of the grate(s) and the street centerline,
and is the remainder of the total flow between the curb and the centerline:
Where:
To find the ratio of intercepted frontal flow (Qwi) to total frontal flow (Qw),
Rf, one must first find the splash-over velocity,
Vo, for the selected grate type. This function
was developed empirically by Guo (1999):
Where:
Le
|
=
|
Effective Unit Grate Length (see Equation 28.44-24)
|
α, β, γ, η
|
=
|
Splash Velocity Constants (see Table 28.44.120)
|
Table 28.44.120: Splash Velocity Constants
|
---|
Type of Grate
|
Assumed Equivalent
|
α
|
β
|
γ
|
η
|
---|
Bar P-1-7/8
|
Type D
|
2.22
|
4.03
|
0.65
|
0.06
|
Bar P-1-1/8
|
Not used
|
1.76
|
3.12
|
0.45
|
0.03
|
Vane Grate
|
Type L,V
|
0.3
|
4.85
|
1.31
|
0.15
|
45-Degree Bar
|
Not used
|
0.99
|
2.64
|
0.36
|
0.03
|
Bar P-1-7/8-4
|
Not used
|
0.74
|
2.44
|
0.27
|
0.02
|
30-Degree Bar
|
Not used
|
0.51
|
2.34
|
0.2
|
0.01
|
Reticuline
|
Type R
|
0.28
|
2.28
|
0.18
|
0.01
|
Adapted from Guo, Storm Water System Design, 1999.
|
Splash-over velocity is an experimentally-derived value
at which frontal flow begins to bypass the grate essentially because
water does not spend enough time over the inlet to allow it to fall
through the grate. As previously mentioned, this value varies per
grate type – some grates tend to better capture higher
velocity flows. This allows the designer to calculate the HEC-12 frontal
flow interception ratio:
Due to the primarily longitudinally-flowing nature of
street and gutter flow, the interception ratio of side flow is typically
minimal, and in some jurisdictions is ignored altogether. This ratio,
Rs, is defined as:
Where:
Qsi
|
=
|
Intercepted Side Flow (cfs)
|
V
|
=
|
Gutter Flow Velocity (fps)
|
Sx
|
=
|
Street Cross Slope
|
Le
|
=
|
Effective Length of Grate(s) (ft.) (see Equation 28.44-24, GJMC § 28.44.140)
|
Total intercepted flow (Qi),
then, is found using Equation 28.44-13:
Total capture efficiency (E) for a grate inlet can be
found using:
Inlet clogging due to the buildup of debris and sediment
can adversely affect the interception capacity of an inlet. A clogging
factor must be applied to find the design value for Q
i. See GJMC §
28.44.140 for an explanation of the calculation of clogging factors.
Included in Figures 28.44.120(b) through 28.44.120(k) are inlet capacity charts for all street types and curb-and-gutter and inlet configurations considered by this manual. Figure 28.44.120(a) is a legend for these figures. The capacity curves are specific to grate type, inlet type, and whether the subject inlet is single, double, or triple length. All curves include appropriate clogging factors per GJMC §
28.44.140. Inlet capacity values calculated from the provided equations may be used if they do not exceed the values provided in the inlet capacity charts.
(b) Curb-Opening Inlets.
The interception capacity of a
curb-opening inlet on a continuous grade is dependent on inlet length,
flow depth, and the longitudinal and cross slopes of the street. To
determine the capture efficiency of a curb-opening inlet, one must
first calculate the length that would be required for 100 percent
interception of the gutter flow at that location (LT):
Where:
Q
|
=
|
Total Gutter Flow (cfs)
|
SL
|
=
|
Longitudinal Street Slope
|
n
|
=
|
Manning’s Roughness Coefficient
|
Se
|
=
|
Equivalent Cross Slope
|
SX
|
=
|
Street Cross Slope
|
a
|
=
|
Gutter Depression Below Street Slope (ft.)
|
W
|
=
|
Gutter Width (ft.)
|
Eo
|
=
|
Defined by Equation 28.44-3
|
The efficiency of a curb-opening inlet is calculated
using Equation 28.44-17, where effective length
Le is defined in Equation 28.44-24 (GJMC §
28.44.140):
Included in Figures 28.44.120(b) through 28.44.120(k) are inlet capacity charts for all street types, grate types, inlet types and curb-and-gutter configurations considered by this manual. Each figure is specific to the type of grate used. The capacity curves for all on-grade inlets are specific to a single inlet length of 5.0 feet for curb-opening-only inlets and 3.0 feet for both grate-only and combo inlets. Figure 28.44.120(e) is the inlet capacity chart for the Grand Junction Drive-Over inlet with its own legend. It assumes the curb height is 4.5 inches, the flow line of the frame and grate is one inch below normal gutter flow line, and the vertical opening is set at one inch in height. Curves are provided for single, double, or triple length. All curves include appropriate clogging factors per GJMC §
28.44.140. Inlet capacity values calculated from the provided equations may be used if they do not exceed the values provided in the inlet capacity charts.
(c) Combination Inlets.
Per the introduction to this section, combination inlets are assumed to have the capacity of the same inlet with the grate(s) acting alone except in the case of “sweeper inlets.” However, the designer shall note the reduced clogging susceptibility of the combination inlet when compared with a grate-only configuration noted in GJMC §
28.44.140.
(d) Slotted Drain Inlets.
Slotted drains are effective for
intercepting flow over a wide section, such as sheet flow across the
pavement of a street. However, these inlets are highly susceptible
to clogging, and are only allowed as specifically approved by a local
jurisdiction. Methods for the design of such inlets can be found in
HEC-12 and HEC-22.
(Res. 40-08 (§ 1106.1), 3-19-08)
A sag or sump condition occurs in a location where water that
flows into the area must pond to some depth before any of the flow
can escape the area via channel or overland flow. Unlike inlets on
a continuous grade, those in a sump condition are not designed to
bypass a portion of the flow incident to the inlet location. This
means that these inlets must have the capacity to effectively capture
all of the runoff that ponds in the sump and to maintain acceptable
ponding depths.
These requirements, along with an increased potential for inlet
clogging due to low flow velocities, necessitate special provisions
for the design of sump inlets. A secondary flow path must be provided
to maintain a reasonable ponding depth in the case of inlet failure
(near-complete clogging, for instance). The preferred secondary flow
path is a designated emergency overflow weir and channel, which must
be located within an accessible drainage easement and must be protected
from erosive effects as necessary by pavement or riprap. If no easement
is available at the inlet location, flanker inlets must be installed
in the same gutter on each side of the primary inlet. Flanker inlets
are located upgradient 10 to 50 feet from the primary sump inlet.
The two flanker inlets shall have a combined design capacity equal
to or greater than that of the primary inlet.
Local jurisdictions recommend the use of combination inlets
in sumps due to their higher capacity and lower clogging tendency.
Curb-opening inlets are also allowable, but grate-only inlets and
slotted-drain inlets are not allowed for use in sump conditions.
Per GJMC §
28.44.120(a), Type L and Type V grates are prohibited for use with inlets located in sumps. Mesa County and the City of Grand Junction have approved grate Types D and R for inlets in these locations.
The hydraulic capacity of an inlet in a sump condition is dependent
on the configuration of the inlet and the depth of the ponded water.
At small depths, the flow into the inlet is by weir flow, transitioning
to orifice flow at increasing depths. These depths are defined in
Table 28.44.130.
The gross capacity of an inlet operating as a weir is defined
by Equation 28.44-18:
Where:
Qi
|
=
|
Inlet Capacity (cfs)
|
LW
|
=
|
Weir Length (ft.)
|
d
|
=
|
Flow of Ponding Depth (ft.)
|
CW
|
=
|
Weir Discharge Coefficient (see Table 28.44.130)
|
The gross capacity of an inlet operating as an orifice is defined
by Equation 28.44-19:
Where:
Qi
|
=
|
Inlet Capacity (cfs)
|
Ao
|
=
|
Orifice Open Area (sf)
|
g
|
=
|
32.2 ft./s2
|
do
|
=
|
Depth to Orifice Centroid (ft.)
|
Co
|
=
|
Orifice Discharge Coefficient (see Table 28.44.130)
|
Table 28.44.130: Discharge Coefficients and Variable Definitions
for Inlets in a Sump Condition
|
---|
Type of Inlet
|
Cw
|
Co
|
Weir
|
Orifice
|
Lw
|
---|
Grate
|
3
|
0.67
|
d < 1.79(Ao /Lw)
|
d > 1.79(Ao /Lw)
|
2w+L
|
Curb-Opening
|
3
|
0.67
|
d < h
|
d > 1.4h
|
L
|
Depressed Curb-Opening
|
2.3
|
0.67
|
d < h + a
|
d > 1.4h
|
2w+L
|
Where:
Ao
|
=
|
Orifice Open Area (sf)
|
LW
|
=
|
Weir Length (ft.)
|
d
|
=
|
Flow or Ponding Depth
|
w
|
=
|
Grate Width (ft.)
|
L
|
=
|
Inlet Length (ft.)
|
It is important to note that the capacity of a combination inlet
in a sump is defined by the capacity of the grate portion only when
operating as a weir (the curb opening is ineffectual and thus ignored),
but is defined by the cumulative capacity of the grate and curb opening
when operating as an orifice. Any curb opening length extending beyond
the ends of the grates may be included in the weir length.
For any given inlet, a certain range of depths will result in
transitional flow, where neither the weir equation nor the orifice
equation accurately models flow through the inlet. Linsley (1992)
states that where transition conditions exist, “the capacity
is intermediate between that of an orifice and a weir.” For
design purposes, the capacity for depths in the transitional range
is based on the lesser of the results of Equations 28.44-18 and 28.44-19.
Local inlet depression increases the capacity of inlets, especially those in sumps, by increasing the depth over the inlet without increasing street flow depth. Local depression loses effectiveness for curb-opening inlets of 12-foot length or greater, so the “Curb-Opening” information from Table 28.44.130 shall be used for these. Figure 28.44.130 contains two tables with maximum inlet capacities for inlets with a two-inch local depression and without depression (level with curb flow line). The first capacity table is for the standard six-inch vertical curb configuration (applies to most streets in Mesa County) and the second table contains values for the 4.5-inch mountable (drive-over) curb. All capacities listed in Figure 28.44.130 include a clogging factor per GJMC §
28.44.140.
(Res. 40-08 (§ 1106.2), 3-19-08)
Inlets are always susceptible to clogging when water flows to
them. Low flows can carry leaves and finer sediments and deposit them
at the inlet, while higher flows can deposit larger debris. The latter
is of most concern since it is often too large to be washed down the
inlet, even with heavy storm flows. Proper inlet design and timely
maintenance including street sweeping, visual inspection of inlets,
and removal of large debris are essential to the proper operation
of inlets. The clogging factors listed in Table 28.44.140 are based
on the assumption that routine maintenance is performed on all drainage
structures.
Table 28.44.140 contains the assumed clogging factors for different
inlet types. A value of zero percent indicates that no clogging is
expected, and a value of 100 percent indicates that the inlet shall
be considered completely clogged or is not allowed in the specified
conditions.
Table 28.44.140: Clogging Factors for Single Inlets
|
---|
Type of Inlet
|
On-Grade
|
Sag/Sump
d ≤ 0.5 ft.
|
Sag/Sump
d > 0.5 ft.
|
---|
Grate
|
50%
|
100%
|
100%
|
Curb-Opening
|
20%
|
20%
|
20%
|
Combination (Grate Portion)
|
0%
|
0%
|
50%
|
Combination (Curb-Opening Portion)
|
100%
|
100%
|
0%
|
In the case of multiple (double or triple) inlets, the continuous
(linear) application of some of these factors would result in the
inability to capture 100 percent of the flow incident to the subject
inlet. Although on-grade inlets are rarely designed to capture 100
percent of gutter flow from the minor storm event, inlet design lengths
may become unnecessarily long to achieve the desired flow interception
rate. It is reasonable to assume that a majority of the sediment and
debris is deposited at the inlet by the first few minutes of significant
storm flow, possibly before the peak flow occurs at that inlet. Also,
the majority of clogging tends to more greatly affect the upgradient
segments of a multi-unit inlet. For instance, the first grate of a
triple inlet may become 50 percent clogged, while the third grate
experiences only 10 percent clogging. This phenomenon was generalized
by Guo in Design of Grate Inlets with a Clogging Factor (2000) using a decay equation and empirically-derived decay factors.
It may be applied to inlets on a continuous grade and inlets in sump
conditions.
Where:
C
|
=
|
Multiple-Unit Clogging Factor (decimal)
|
Co
|
=
|
Single-Unit Clogging Factor (decimal)
|
N
|
=
|
Number of Units
|
e
|
=
|
Decay Ratio
|
e
|
=
|
0.5 for grates, 0.25 for curb-opening inlets
|
This adjusted clogging factor can be applied to the sump-inlet
capacity equations (Equations 28.44-18 and 28.44-19) by replacing
the variables for weir length Lw with effective
weir length (Lwe) from Equation 28.44-21 and
orifice area Ao with effective opening area
(Aoe) from Equation 28.44-22. The clogging
factor may also be applied directly to the gross flow capacities from
Equations 28.44-18 and 28.44-19 using Equation 28.44-23.
|
(28.44-21)
|
|
(28.44-22)
|
|
(28.44-23)
|
To apply the clogging factor to an on-grade inlet, the designer
must first find the effective length (Le) – that portion of the inlet length which
is considered unclogged.
This value is used in the computation of splash-over velocity
in Equation 28.44-10, side-flow interception ratio in Equation 28.44-12,
and curb-opening efficiency in Equation 28.44-17.
(Res. 40-08 (§ 1106.3), 3-19-08)
Selection of grates for on-grade and sump inlets must be consistent with GJMC §
28.44.120(a) and the page titled “Approved Storm Drain Inlets” (D-05) of the City of Grand Junction Standard Details. The designer is urged to obtain any grate rating data available from the manufacturer of the selected grate. This data may be used to cross-check the values obtained using the methods presented in this manual and/or to obtain interception and capacity values. However, in the latter case, it is imperative that the designer still incorporate all clogging/safety factors that apply to the inlet design per local jurisdictions’ requirements.
(Res. 40-08 (§ 1106.4), 3-19-08)
Inlets shall be placed at any location where ponding water may encroach on street traffic beyond the allowable limits. These limits are defined by gutter flow depth during the minor storm event (GJMC §
28.44.090). An inlet location is determined using an iterative process:
(a) Determine a preliminary location for the inlet based on street configuration
and estimated runoff to the gutter.
(b) If the inlet is in a sump, location is essentially fixed during the
remainder of the design process. The inlet shall be sized to maintain
ponding depths smaller than those required by local jurisdictions.
If the required inlet size becomes excessively large, the designer
is urged to install additional inlets upgradient from the sump.
(c) For inlets on a grade, the designer must find the flow characteristics
at the selected preliminary inlet location to determine whether the
inlet needs to be placed further upstream or may be moved downstream
based on maximum flow depth for the minor storm event.
(d) The designer shall take into account the change in tributary area
to the inlet associated with any upstream or downstream movement.
(e) A typical design interception efficiency of an on-grade inlet is
70 to 80 percent. As mentioned previously, on-grade inlets designed
to capture 100 percent of the minor storm runoff tend to be significantly
less effective both hydraulically and economically.
(f) The designer shall include any carryover (bypass) flow from an upstream
inlet when calculating the flow at a downstream inlet. Although the
peak runoff to an inlet may not coincide with the peak carryover flow
from an upstream inlet, these two peak flows shall be added to find
the total peak flow to the downstream inlet.
Maximizing the use of sump inlets tends to increase the overall
efficiency of the inlet system, and inlets must be installed at all
street sags (vertical curve low points) and at all sumps formed by
intersections except where other drainage provisions have been made.
Therefore, it is suggested that sump inlets are located prior to the
placement of any on-grade inlets during the design process.
|
(Res. 40-08 (§ 1106.5), 3-19-08)