§ 152.71 POLICIES.

   (A)   Under most circumstances, channels designed and constructed in conjunction with new developments will be owned and maintained by the developer, HOA or individual property owner, unless they are within the town right-of-way.

   (B)   Channel improvements must conform to town design specifications whether or not the facilities are to be dedicated to the town for operation and maintenance.

   (C)   For constructed channels, the 100-year peak discharge shall be contained in a defined channel cross-section that includes a minimum freeboard per § 152.73(D).

   (D)   Drainage channels may not be constructed adjacent to roadways or within the town right-of-way without prior written permission from the Public Works Engineer.

   (E)   Unless flow leaving a site is entering a constructed drainageway, the flow must be returned, as nearly as feasible, to the pre-development condition (i.e. depth, velocity and width).

   (F)   Constructed channels shall follow the existing watercourse alignment as closely as is feasible.

   (G)   Where the expected velocity, depth of flow, or channel geometry would result in scour in an earthen channel, an appropriate type of channel stabilization shall be required.

   (H)   No valves, closures, transformers, standpipes, poles or other obstructions will be permitted in any surface drainage channel or roadside ditch.

   (I)   Channels shall not be designed with a Froude number greater than 0.86 for 100-year peak flow rates in excess of 50 cfs without special design consideration for the effects of inertial forces, and written approval from the Public Works Engineer.

(Res. 1637, passed 2-28-02)

§ 152.72 CHANNEL STABILIZATION METHODS.

   (A)   General. Channel stabilization is used to control the horizontal and/or vertical alignment of a watercourse, whether natural or constructed. The purpose of channel stabilization is to reduce flood hazards, erosion hazards and maintenance costs associated with the drainage of natural or urban runoff. Channel stabilization is generally required when flow velocities and soils conditions result in the potential for bank erosion. Guidance on permissible velocities for channels with erodible linings can be found in Design Charts for Open Channel Flow (1961), U.S. Department of Transportation. Some form of channel stabilization will generally be required in conjunction with new channel construction unless an analysis is performed, as described in §§ 152.80 - 152.83, that demonstrates an erosion hazard does not exist along the subject reach. A variety of stabilization methods are acceptable within their range of applicability. Stabilization methods, which have been found to be acceptable, include those described within divisions (B) - (H) of this section.

   (B)   Rock riprap. Rock riprap stabilization consists of either dumped rock, or rock held in place with wire mesh and rail piles. Rock may be placed on the banks only or on the bed and banks as the channel conditions dictate. Rock riprap is acceptable for use within the town if designed in accordance with the procedures presented within this chapter. Methods of riprap design other than the one presented herein may also be used, provided written approval is obtained from the Public Works Department.

      (1)   Riprap sizing.

         (a)   The chart provided on Figure 6.1 provides the median size of riprap, D50 for the average flow velocity with the riprap in place. A family of curves on Figure 6.1 is provided for channels with various curvatures. The angle of curvature specified on Figure 6.1 is defined as the angle made by the intersection of the centerline of the straight channel with a tangent to the outside bend (Figure 6.2). The chart provided on Figure 6.1 was developed under the assumption that the specific weight of the rock is 165 pounds per cubic foot. If rock of a substantially different specific weight is to be used, the D50 should be adjusted by use of Equation 6.1:

         kr = 102.96k/(wr - 62.4) (Eqn. 6.1)

   Where:

         k = D50 from Figure 6.1, in feet.

         kr= D50 for rock to be used, in feet.

         wr= Unit weight of rock to be used, in pounds per cubic foot.

         (b)   Figure 6.1 is also based on the assumption that riprap will be placed on channel banks having side-slopes no steeper than 3:1. Dumped rock riprap is not permitted as a method of bank protection on side-slopes steeper than 3:1 unless an approved engineering analysis is performed which demonstrates the stability of the steeper slope. Generally, if side-slopes steeper than 3:1 are required for rock bank protection, then the rock must be held in place with wire mesh, gabion baskets or grout.

      (2)   Riprap gradation, thickness, and rock shape.

         (a)   The gradation of rock riprap should follow a smooth curve. The ratio of the largest size rock to D50 should be about two, and the ratio of D20 to D50 should be about one-half. The riprap blanket shall have a minimum thickness of 2.0D50. Table 6.1 provides a recommended design gradation for riprap.

         (b)   The shape of the riprap rock should be "blocky" rather than elongated. More nearly cubical rocks "nest" together, and are more resistant to movement. Also, rocks with clean, sharp edges and relatively flat faces will form a riprap mass having an angle of internal friction greater than round stones, and will be less susceptible to slope failure. The use of river-run rock may acceptable for riprap. However, the Town Engineer may require that river-run rock be grouted in place.

   Table 6.1: Riprap Gradation Chart
   (From USDOT, FHWA, HEC-11, 1989)

 

Stone Size Range	Stone Weight Range	Percent of Gradation
1.5 d 50 to 1.7 d50	3.0 W50 to 5.0 W50	100
1.2 d50 to 1.4 d50	2.0 W50 to 2.75 W50	85
1.0 d50 to 1.15 d50	1.0 W50 to 1.5 W50	50
0.4 d50 to 0.6 d50	0.1 W50 to 0.2 W50	15

 

      (3)   Riprap filters. Filters are generally required underneath rock riprap to prevent the material from being leached out through the riprap. Two types of filter materials are commonly used: gravel filters and fabric filters. Gravel filters consist of a layer of well-graded sands and gravel. Generally the thickness of a gravel filter should not be less than nine inches. Fabric filters are more commonly used, generally are very effective and easier to install than gravel filters, although care must be exercised in placing large rocks on the fabrics to prevent damage to the fabric. Filter fabric use shall be designed in conformance with the manufacturer's recommendations.

   Figure 6.1: Riprap Sizing Chart

   Figure 6.2

   (C)   Concrete. Concrete lined channels, reinforced with rebar or wire mesh, are often used when flow velocities are high, or when there is limited width for the construction of a channel. Concrete lining of the entire channel (i.e. banks and bottom) is usually required for very high flow velocity and steep channel gradients. However, an earthen bottom and concrete-lined banks, with bank toe-downs and, in some instances, periodic grade-control structures are also practical in certain locations. Toe-downs refer to the vertical distance that the bank protection extends below the invert of the channel. Concrete or "shotcrete" channel lining without some form of rebar or mesh reinforcement is not permitted by the town. The minimum thickness for concrete lined channels is six inches, measured perpendicular to the face of the lining.

   (D)   Gabion baskets and mattresses. Gabion baskets and mattresses are specially designed wire mesh containers for rock riprap stabilization. Gabions are generally used when adequate rock size or gradation is unavailable for ordinary dumped riprap. Additionally, gabion structures can be constructed on steeper slopes than dumped riprap, and will therefore require less right-of-way. In general, the minimum thickness of a gabion basket should equal two-thirds of the D50 rock size determined from Figure 6.1. Additionally, an adequate gravel or fabric filter should always be installed with gabions. The maximum allowable side slope for construction of gabions is 2H:1V.

   (E)   Grouted rock. Grouted rock provides another bank stabilization option when adequate rock size or gradation is unavailable for dumped riprap. Grouted rock may be placed on slopes as steep as 1:1 provided the underlying soils have adequate strength. During construction of grouted rock bank protection, it is important that the grout is applied in a manner that ensures grout penetration to the bottom of the riprap blanket. The minimum thickness of grouted-rock bank stabilization shall be equal to two-thirds of the D50 rock size determined from Figure 6.1. The rock shall be free of fines that prevent full penetration of grout.

   (F)   Articulated revetment units. Articulated revetment units (ARU's) are a stabilization material that is composed of a system of interlocking concrete blocks that may be used to line drainageways. ARU's have limited application in this area, and are used primarily on small watercourses that have very flat side-slopes and very low velocities of flow. Certain manufacturers are producing ARU's that are suitable for larger watercourses. These particular products have the advantage of allowing vegetation to grow through the ARU mat. However, in many cases, cost is a limiting factor in utilizing these products. The design engineer should consult the manufacturer's design specifications in order to evaluate the suitability of using a particular ARU on a project.

   (G)   Soil cement. Soil cement, also termed Cement Stabilized Alluvium (CSA), is primarily used as channel bank stabilization on large alluvial watercourses in Arizona and elsewhere. It may also be used to line channel bottoms, as well as for use in the construction of larger scale grade-control structures, collector dikes, and spillways. Soil-cement bank stabilization is normally placed on 1:1 slopes, and consists of 6 to 8-inch vertical lifts, 8 to 10 feet in width, placed horizontally in a stair-step manner in order to attain the desired height of channel bank. Soil cement can also be placed on 3:1 (or flatter) slopes, at a minimum thickness of 8 to 12 inches where a lesser level of protection is acceptable. This latter technique is often termed soil-cement "slope paving". Soil cement design applications shall consider the effects of freeze-thaw cycles.

   (H)   Erosion control mats and vegetation lining. Erosion control mats may be used for bank and bed stabilization where appropriate. Biodegradable matting shall be used to establish grass-lined channels that do not require permanent mats. Permanent erosion control mats are allowable where design conditions warrant their use, and provided the manufacturers design guidelines are followed.

(Res. 1637, passed 2-28-02; Am. Res. 2871, passed 9-17-15; Am. Ord. 872, passed 9-17-15)

§ 152.73 DESIGN CONSIDERATIONS.

   (A)   General. The design of drainage channels can involve highly complex hydraulic analysis techniques and considerations. The existence of transitions, culverts, channel curves, changes in flow regime, etc., can produce hydraulic conditions significantly different from those determined from steady, uniform-flow analyses. This chapter provides certain considerations that are commonly encountered in open channel analysis and design. However the design engineer is referred to the following publications for detailed presentation of these and other procedures and considerations associated with the design of open channel facilities:

   Hydraulic Design of Flood Control Channel (1970), U.S. Army Corps of Engineers.

   Design Manual - Hydraulic (1982), Los Angeles County Flood Control District.

   Design of Small Canal Structures (1978), U.S. Department of the Interior, Bureau of Reclamation.

   (B)   Channel geometry.

      (1)   Open drainage channels shall be designed using trapezoidal, rectangular, or compound cross sections, unless prior written approval of an alternate design is provided from the Public Works Engineer. Side slopes for constructed earthen or dumped riprap channels shall be no steeper than 3:1, unless an approved soil analysis demonstrates that steeper side slopes are stable. Side slopes for lined channels may be steeper, depending upon the structural stability of the lining and the underlying soils. Reinforced concrete linings may have vertical side slopes, provided that the design is adequate to prevent failure from hydrostatic or earth pressures. Shotcrete may be placed on side slopes as steep as 1:1, provided this slope is not significantly steeper than the natural angle of repose of the soil. Soil cement lining may be placed on 1:1 side slopes, provided it is of sufficient thickness to be structurally stable. The minimum thickness of soil cement on a 1:1 side slope should be four feet, measured normal to its face. Where soil cement is used as slope paving, with a maximum thickness of one foot, the maximum allowable side slope should be 3:1.

      (2)   In the case where a channel outside of public right-of-way will be accepted for maintenance by the town, a minimum bottom width of 10 feet is required, and an access road, along at least one side of the channel bank, having a minimum width of 12 feet. Exceptions to the minimum requirements for public drainageways may be permitted if deemed appropriate by the Public Works Engineer. Privately maintained channels have no minimum bottom width, except as dictated by hydraulic considerations.

   (C)   Flow regime. Flow regime in an open channel can be either subcritical (tranquil) or supercritical (rapid). The Froude number for subcritical flow is less than 1.0 and the Froude number for supercritical flow is greater than 1.0. Critical flow is defined as having a Froude number of 1.0. Flow that is in the proximity of critical depth is generally unstable and excessive wave action or undulations of the water surface may occur. For this reason channel designs should avoid flow regimes that have Froude numbers in the range of 0.86 to 1.13.

   (D)   Freeboard.

      (1)   Freeboard is the additional depth required in a channel beyond the depth calculated for conveyance of the design discharge. The purpose of freeboard is to protect against hydraulic disturbances such as waves, unforeseen obstructions to flow, debris and inherent inaccuracies in assumptions and analyses techniques. Following are the minimum freeboard requirements for open channels, with a minimum freeboard of one foot for design depths of flow of three feet or more:

   Subcritical Flow (i.e. Froude number < 0.86): Minimum Freeboard = 1.0 feet

   Supercritical Flow (i.e. Froude number > 0.86): Minimum freeboard calculated from Equation 6.3. If the calculated minimum freeboard is less than one foot and the flow depth is three feet or greater the minimum freeboard shall be one foot.

      FB = 1/6(y + v2/2g) (Eqn. 6.3)

   Where:

      FB = Freeboard, in feet

      y = Maximum depth of flow, in feet

      v = Average velocity of flow, in feet per second

      g = Gravitational constant = 32.2 ft./sec

      (2)   The freeboard requirements described above are for uniform channel reaches where no unusual flow disturbances are anticipated. Additional freeboard is required at channel bends and junctions, where backwater effects or superelevation may occur, or where hydraulic jumps may occur. The engineer should consult the references provided at the beginning of this chapter for computing hydraulic conditions at such locations. The lining of protected channels shall extend to a height necessary to include the freeboard requirement, unless approval to the contrary is granted in writing from the Public Works Engineer.

   (E)   Hydraulic jump.

      (1)   A hydraulic jump occurs when flow changes from supercritical flow to subcritical flow. Hydraulic jumps can occur:

         (a)   When the slope of the channel changes from steep to mild;

         (b)   At sudden expansions or contractions in the channel section;

         (c)   At culverts or bridges in steep channels;

         (d)   At the downstream side of dip crossings;

         (e)   At channel junctions; and,

         (f)   Sharp bends.

      (2)   Hydraulic jumps are useful in dissipating energy, and consequently they are often purposely forced to occur at drainageway outlet structures in order to minimize hydraulic forces and erosion. However, because of the large amount of energy dissipated in hydraulic jumps, it is not advisable to allow them to occur except under controlled circumstances. Therefore, if during the design of a channel, a hydraulic jump is expected to occur, computations shall be made to determine the height, length and other characteristics of the jump. In addition, steps shall be taken to either eliminate the jump or contain it, in order to prevent damage to the channel or surrounding property.

      (3)   Procedures for analyzing the hydraulic jump are well documented in the references cited at the beginning of this chapter as well as in numerous other easily available hydraulic texts and manuals.

   (F)   Curved channels.

      (1)   Flow in a curved channel will create centrifugal forces that will cause a rise in the water surface along the outside of a bend. At the same time, a corresponding depression will be created in the water surface along the inside of the bend. In addition, spiral secondary currents tend to form within the bends. These currents can cause scour to occur along the outside of a bend, and deposition along the inside of a bend. Cross-channel waves that propagate downstream will also form, if the flow in the channel is supercritical.

      (2)   Although curves are inevitable in the design of most open channels, they should be minimized in order to avoid the special problems associated with their design. The design of channel bends must include considerations for superelevation, limiting curvature, bend scour, and special design curves.

   (G)   Transitions.

      (1)   Transition sections designed to collect and/or discharge flow between the natural floodplain and constructed channels can be located at either the upstream or downstream ends of constructed channels. They can also be located along a segment or segments of a constructed channel itself. In either case, it is necessary to design the flow transition to minimize the disturbance of flow. In the case where flow in a constructed channel is being transitioned back to the natural floodplain, sufficient distance must be allowed for the flow to adequately expand to the original width of the natural floodplain.

      (2)   Procedures for analyzing curved channels and transitions are well documented in the references cited at the beginning of this chapter as well as in other easily available hydraulic texts and manuals.

(Res. 1637, passed 2-28-02)

CULVERTS AND STORM DRAINS

§ 152.80 PURPOSE.

   This chapter provides policies and criteria for the analysis and design of culverts and storm drain systems. Analysis methodologies are provided mainly by reference to widely accepted and available design manuals that have been prepared by the Federal Highway Administration and other government agencies.

(Res. 1637, passed 2-28-02)

§ 152.81 POLICIES.

   (A)   All natural drainages crossing roadways will be culverted, unless otherwise approved by the Public Works Engineer.

   (B)   Street crossings shall be designed, as a minimum, to convey the post-development or future condition 10-year peak discharge under the road. The 24-hour duration storm shall be used for estimation of peak discharges for all watersheds greater than 160 acres in area. The peak discharges from the PDMP shall be used where available. Regardless of the size of the culvert, street crossings are to be designed to convey the post-development or future condition 100-year peak discharge under and/or over the road to an area downstream of the crossing to which the flow would have gone in the absence of the crossing. 100-year flow depths over the roadway shall not exceed 1 foot in depth. Flows up to or including the 100-year frequency shall not cause increased flooding of private land, developable lands or buildings, unless a drainage easement is obtained for those areas. The ponded headwater elevation shall be delineated on the site topography map, or delineated by field survey, as required.

   (C)   The minimum size for culverts draining roadways is 24 inches in diameter or arch equivalent, and for driveways is 18 inches in diameter, unless otherwise approved by the Pubic Works Engineer.

   (D)   Culverts shall have adequate end treatment at both ends.

   (E)   Outlet protection shall be evaluated for all culverts as described in this chapter.

   (F)   All culverts shall be placed in the natural flow line and channel whenever possible. A detail showing the proposed culvert(s) is required. The detail will include but shall not be limited to, invert elevations, top of road elevations, headwalls, inflow and outflow channel geometry, skew angle, and erosion protection.

   (G)   Minimum cover of fill over culverts must be provided to maintain the structural integrity of the pipe under anticipated loading conditions. Culvert manufacturers provide minimum cover requirements for prefabricated pipe. All street crossing culverts shall have a minimum of one foot of cover. Minimum cover shall be measured from the top of subgrade, which is the bottom of the pavement structural section.

   (H)   Storm drains shall be designed such that the flow depth during a 10-year 24-hour event shall not exceed the top of curb. Storm drains shall be designed such that the peak discharge during a 100-year 24-hour event is contained within the town right-of-way.

   (I)   The minimum pipe diameter allowable for public storm-drain systems is 18 inches, unless otherwise approved by the Public Works Engineer. In general main-line storm drains should be at least 24 inches in diameter.

   (J)   Public storm-drain systems should be designed for gravity flow whenever possible.

   (K)   The minimum flow velocity in a storm drain is three feet-per-second, for purposes of self- cleaning.

   (L)   The minimum allowable storm-drain slope for concrete or smooth metal pipe shall be 0.1%. However a minimum slope at 0.3% is desirable, whenever possible.

   (M)   Manholes shall be located at storm drain junctions, changes in pipe size, sharp curves, angle points in excess of ten degrees and at abrupt changes in grade. Manholes shall also be located at regular intervals as follows:

      (1)   300 feet: Pipe diameter 30"

      (2)   400 feet: 30" < Pipe diameter 48"

      (3)   500 feet: Pipe Diameter > 48"

(Res. 1637, passed 2-28-02)

§ 152.82 CULVERT DESIGN PROCEDURES.

   (A)   Culvert hydraulics.

      (1)   Culvert hydraulics shall be evaluated using the procedures established by the Federal Highway Administration (FHWA) as presented within the publication entitled Hydraulic Design of Highway Culverts (1985), often referred to as “HDS-5". Culverts shall be evaluated for both inlet control and outlet control, to ensure that the correct headwater elevation is determined. Use of computer programs, such as the FHWA “HY-8" (1999) program or equivalents, is also acceptable for culvert analysis and selection.

      (2)   In order to expedite review and approval of the hydraulic design of culverts by the Public Works Engineer, the Culvert Design Form within “HDS-5" should be used. This form is also provided as Figure 7.1. The computer reports produced by the “HY-8" or equivalent programs are also acceptable for presenting culvert analysis results.

   (B)   Debris grates.

      (1)   As part of the culvert design process, the engineer shall consider whether or not the upstream watershed would yield sufficient naturally produced or man-made debris to pose a potential blockage problem. If debris is considered a problem, then an appropriate grate shall be considered, or the culvert shall be enlarged to account for blockage. Because of the large number of combinations of culverts and types of debris possible, there is no single standard grate design. Rather, the engineer is advised to review the Federal Highway Administration manual entitled Debris-Control Structures (1971) to help aid in selecting an appropriate debris grate.

      (2)   It is the policy of the Public Works Department that debris grates on culverts be used only where necessary. The recommended method of accounting for expected debris problems is to increase the size of the culvert, whenever possible.

   (C)   Outlet protection.

      (1)   Outlet protection shall be evaluated for all culverts. The following guidelines, adapted from the Arizona Department of Transportation, are suggested for determining what type of outlet protection is required. Outlet protection shall be required as shown in Table 7.1 where erodible soil conditions exist in the downstream channel bed and/or banks.

   Table 7.1: Culvert Outlet Protection Requirements

 

Culvert Outlet Velocity	Required Outlet Protection
Less than 4 fps	No protection required
More than 4 fps and less than 10 fps	Dumped rock riprap (See § 152.72(B)(2))
More than 10 fps and less than 15 fps	Wire tied or grouted rock riprap
Greater than 15 fps	Energy dissipater

 

      (2)   For culverts with outlet velocities greater than 15 fps, an energy dissipater shall be considered. The objective of an energy dissipater is to return the flow to a condition that approximates the existing flow width, depth and velocity. The engineer designing energy dissipaters shall refer to the FHWA publication entitled Hydraulic Design of Energy Dissipater for Culverts and Channels, HEC No. 14, (1983).

(Res. 1637, passed 2-28-02)