§ 35-29. Design criteria.

(a)

Storm drainage improvements shall be designed for the following storm events as shown in table 35-29A:

Table 35-29A
Drainage Area	Design Storm Frequency
Less than 100 acres	25-year frequency design
100 acres to 640 acres	50-year frequency design
Over 640 acres	100-year frequency design

 

(b)

Computations to determine whether the lots, land, or subdivision will be flooded by rainfall and the sizes of drainage facilities adequate to prevent flooding shall be based upon the standards and specifications in NCTCOG-iSWM.

(c)

Enclosed storm sewers, open drainage ditches, bridges, and culverts of a permanent design, adequate to carry off rainfall, shall be installed by the owner throughout the entire length of the drainage area within the subdivision in accordance with plans and specifications approved by the city engineer.

(d)

Drainage channels shall consist of natural banks and slopes where possible. Reinforced concrete pipe or concrete lining is permitted only where the city engineer or city council finds that this treatment is needed to maintain sufficient land area to accommodate development on an infill site that is designated in the comprehensive plan adopted by the City of Irving, as amended, or where a channel or grass lined channel cannot withstand the runoff velocities created from offsite drainage.

(e)

Fully developed watershed conditions shall be used for determining runoff for the conveyance storm and the flood mitigation storm.

(f)

The owner shall show that the 100-year fully developed design storm event is contained within the right-of-way, drainage easements, and road low points.

(g)

Drainage from residential areas, such as roof tops, must be allowed to flow overland before joining the storm sewer system.

(h)

Seepage into basements or sub-surface structures that is pumped to ground level, seepage from springs, and runoff from roof drains on non-residential buildings that would flow onto or across driveways, sidewalks, or other areas commonly crossed by pedestrians must not create hazards or nuisances to pedestrians. If hazards or nuisances are, or would be created, the basement and rooftop drains shall be tied directly to the nearest storm sewer, provided that pumped lines from basements have backflow preventers and the water is uncontaminated.

(i)

All storm sewer systems must be installed in accordance with the plans and specifications approved by the city engineer. After acceptance of the drainage system by the city engineer, no additional fill shall be placed above the system nor shall structural alterations be made to the system without the prior written approval of the city engineer.

Table 35-29B Options for Design Focus Area
Design Focus Area	Required Downstream Assessment	Design Options
Water Quality Protection	No	Option 1: Use integrated site design practices for conserving natural features, reducing impervious cover, and using natural drainage systems. Option 2: Treat the Water Quality Protection Volume (WQv) by reducing total suspended solids from the development site for runoff resulting from rainfalls of up to 1.5 inches (85^th percentile storm). Option 3: Assist in implementing off-site community stormwater pollution prevention programs/activities as designated in an approved stormwater master plan or Texas Pollutant Discharge Elimination System (TPDES) Stormwater permit.
Streambank Protection	Yes	Option 1: Reinforce/stabilize downstream conditions. Option 2: Install stormwater controls to maintain or improve existing downstream conditions. Option 3: Provide on-site controlled release of the 1-year, 24 hour storm event over a period of 24 hours (Streambank Protection Volume, SPv)
Flood Mitigation and Conveyance	Yes	Flood Mitigation Option 1: Provide adequate downstream conveyance systems. Option 2: Install Stormwater controls on-site to maintain or improve existing downstream conditions. Option 3: In lieu of a downstream assessment, maintain existing on site runoff conditions. Conveyance: Minimize localized site flooding of streets, sidewalks, and properties by a combination of onsite stormwater controls and conveyance systems.

 

(Ord. No. 2017-10008, § II, 10-26-17)