§ 35-34. Culverts, bridges, flumes, and detention structures.

(a)

Culverts shall be designed for the flood mitigation storm or in accordance with TxDOT requirements, whichever is more stringent. Considerations when designing culverts include: roadway type, tailwater or depth of flow, structures, and property subject to flooding, emergency access, and road replacement costs. The flood mitigation storm shall be routed through all culverts to ensure structures are not flooded and damage does not occur to the highway or adjacent property for this design event.

(1)

Velocity limitations.

a.

The maximum velocity shall be consistent with channel stability requirements at the culvert outlet.

b.

The maximum allowable velocity for corrugated metal pipe (CMP) is fifteen (15) feet per second. The use of corrugated metal pipe is not allowed in public right-of-way or easements unless approved by the city engineer. There is no specified maximum allowable velocity for reinforced concrete pipe, but outlet protection shall be provided where discharge velocities will cause erosion conditions.

c.

To ensure self-cleaning during partial depth flow, a minimum velocity of two and a half (2.5) feet per second is required for the streambank protection storm when the culvert is flowing partially full.

(2)

Length and slope.

a.

The maximum slope using concrete pipe is ten (10) percent and for CMP is fourteen (14) percent before pipe-restraining methods must be taken.

b.

Maximum vertical distance from throat of intake to flowline in a drainage structure is ten (10) feet.

c.

Drops greater than four (4) feet will require additional structural design.

(3)

Headwater limitations.

a.

The allowable headwater is the depth of water that can be ponded at the upstream end of the culvert during the design flood, which will be limited by one (1) of the following constraints or conditions:

1.

Headwater must not damage upstream property.

2.

Culvert headwater plus twelve (12) inches of freeboard shall not exceed top of curb or pavement for low point of road over culvert, whichever is lower.

3.

Ponding depth will be no longer than the elevation where flow diverts around the culvert.

4.

Elevations will be established to delineate floodplain zoning.

b.

The headwater shall be checked for the flood mitigation storm elevation to ensure compliance with floodplain management criteria and the culvert shall be sized to maintain flood-free conditions on major thoroughfares with 12-inch freeboard at the low-point of the road.

c.

Either the headwater shall be set to produce acceptable velocities or stabilizing/energy dissipation shall be provided where these velocities are exceeded.

d.

In general, the constraint that gives the lowest allowable headwater elevation shall establish the criteria for the hydraulic calculations.

(4)

Tailwater considerations.

a.

If the culvert outlet is operating with a free outfall, the critical depth and equivalent hydraulic grade line shall be determined.

b.

For culverts that discharge to an open channel, the stage-discharge curve for the channel must be determined in accordance with the methods provided by Section 2.1.4 of the NCTCOG-iSWM Hydraulics Technical Manual.

c.

If an upstream culvert outlet is located near a downstream culvert inlet, the headwater elevation of the downstream culvert will establish the design tailwater depth for the upstream culvert.

d.

If the culvert discharges to a lake, pond, or other major water body, the expected high water elevation of the particular water body will establish the culvert tailwater.

(5)

Other criteria.

a.

In designing debris control structures, the most current edition of the U.S. Department of Transportation Federal Highway Administration Hydraulic Engineering Circular No. 9 entitled "Debris Control Structures" or other approved reference is required to be used.

b.

If storage is being assumed or will occur upstream of the culvert, then Section 2.0 of the NCTCOG-iSWM Hydraulics Technical Manual regarding storage routing shall be used in the culvert design.

c.

Reinforced concrete pipe (RCP), pre-cast and cast in place concrete boxes are recommended for use (1) under a roadway; (2) when pipe slopes are less than one (1) percent; or (3) for all flowing streams.

d.

Culvert skews shall not exceed forty-five (45) degrees as measured from a line perpendicular to the roadway centerline without approval from the city engineer.

e.

The minimum allowable pipe diameter shall be eighteen (18) inches for laterals and twenty-four (24) inches for the storm sewer main line.

f.

Erosion, sediment control, and velocity dissipation shall be designed in accordance with Section 4.0 of the NCTCOG-iSWM Hydraulics Technical Manual.

(b)

Bridges are cross drainage facilities with a span of twenty (20) feet or larger.

(1)

Design frequency.

a.

Flood mitigation storm for all bridges.

(2)

Design criteria.

a.

A freeboard of two feet shall be maintained between the computed design water surface and the low chord of all bridges.

b.

The contraction and expansion of water through the bridge opening creates hydraulic losses. These losses are accounted for through the use of loss coefficients. Table 35-34A gives recommended values for the Contraction (Kc) and Expansion (Ke) Coefficients.

Table 35-34A - Recommended Loss Coefficients for Bridges
Transition Type	Contraction (Kc)	Expansion (Ke)
No losses computed	0	0
Gradual transition	0.1	0.3
Typical bridge	0.3	0.5
Severe transition	0.6	0.8

 

(c)

The use of flumes is not recommended for widespread use. Flumes shall not be permitted when the purpose of a permanent flume is to carry runoff down the sides of earthen channels, unless approved by the city engineer. A flume may be used to direct overflow runoff along property lines until the runoff can be intercepted by streets or conduit flows. Flumes crossing sidewalks shall be covered or bridged such as to minimize danger to pedestrians.

(d)

Detention structures shall be designed for each of the three (3) storm events (streambank protection, conveyance, and flood mitigation) for the critical storm duration that results in the maximum (or near maximum) peak flow in accordance with the following design criteria:

(1)

Dry detention basins shall be sized to temporarily store the volume of runoff required to provide flood protection up to the flood mitigation storm, if required by the city engineer.

(2)

Extended detention dry basins shall be sized to provide extended detention of the streambank protection volume over twenty-four (24) hours and may also provide additional storage volume for normal detention (peak flow reduction) of the flood mitigation storm event.

(3)

Routing calculation must be used to demonstrate that the storage volume and outlet structure configuration are adequate. Procedures on the design of detention storage shall be followed in accordance with Section 2.0 of the NCTCOG-iSWM Hydraulics Technical Manual.

(4)

Detention Basins shall be designed with an eight (8) foot side maintenance access.

(5)

No earthen (grassed) embankment slopes shall exceed 4:1.

(6)

A freeboard of one (1) foot will be required for all detention ponds.

(7)

A calculation summary shall be provided on construction plans. For detailed calculations of unit hydrograph studies, a separate report shall be provided to the city engineer for review and referenced on the construction plans. Stage-storage-discharge values shall be tabulated and flow calculations for discharge structures shall be shown on the construction plans.

(8)

An emergency spillway shall be provided at the flood mitigation maximum storage elevation with sufficient capacity to convey the flood mitigation storm assuming blockage of the outlet works with six (6) inches freeboard. Spillway requirements must also meet all appropriate state and Federal criteria.

(9)

A landscape plan shall be provided for all detention ponds.

(10)

All detention basins shall be stabilized against significant erosion and include a maintenance plan.

(11)

Design calculations will be provided for all spillways and outlet structures.

(12)

Maintenance agreements shall be included for all detention structures.

(13)

Storage may be subject to the requirements of the Texas Dam Safety Program (see NCTCOG-iSWM Program Guidance: Dam Safety and Water Rights; see also Texas Water Code Ch. 12 and 30 TAC 299, as amended) based on the volume, dam height, and level of hazard.

(14)

Earthen embankments six (6) feet in height or greater shall be designed per the Texas Commission on Environmental Quality guidelines for dam safety (see NCTCOG iSWM Program Guidance: Dam Safety and Water Rights; see also Texas Water Code Ch. 12 and 30 TAC 299, as amended).

(15)

Vegetated slopes shall be less than twenty (20) feet in height and no steeper than 4:1 (horizontal to vertical). Rip-rap-protected slopes shall be no steeper than 2:1. Geotechnical slope stability analysis is recommended for slopes greater than ten (10) feet in height.

(16)

Areas above normal high water elevations of the detention facility should be sloped toward the basin to allow drainage and to prevent standing water. Finish grading shall be installed and maintained to avoid creation of upland surface depressions that may retain runoff. The bottom area of storage facilities should be graded toward the outlet to prevent standing water conditions. A low flow or pilot channel across the facility bottom from the inlet to the outlet (often constructed with riprap) is recommended to convey low flows and prevent standing water conditions.

(17)

Extended detention (ED) orifice sizing is required in design applications that provide extended detention for downstream streambank protection of the ED portion of the water quality protection volume. The release rate for both the WQv and SPv shall discharge the ED volume in a period of twenty-four (24) hours or longer. In both cases, an extended detention orifice or reverse slope pipe must be used for the outlet. For a structural control facility providing both WQv extended detention and SPv control (wet ED pond, micropool ED pond, and shallow ED wetland), there will be a need to design two (2) outlet orifices must be designed - one (1) for the water quality control outlet and one (1) for the streambank protection drawdown.

(Ord. No. 2017-10008, § II, 10-26-17)