C.8 - Stormwater Management.
C.8.1
General.
A.
Stormwater management (SWM) programs aimed at controlling increased urban runoff generated by development are a top priority in urban planning. More frequent flooding, increased rates and volumes of runoff, increased stream channel erosion and degradation, increased sedimentation and increased water pollution are all problems intensified by development. SWM facilities such as detention, retention, extended detention, infiltration, and sedimentation ponds have proven to significantly reduce downstream flooding, reduce sediment and pollutant loads, and provide debris removal which can benefit water quality.
B.
The basic concept of SWM for peak rates of runoff is to provide for a temporary storage of stormwater runoff. Runoff is then released at a controlled rate which cannot exceed the capacities of the existing downstream drainage systems, or the predeveloped peak runoff rate of the site, whichever is less.
C.
The solid lined hydrograph shown in Figure C-23 represents a storm runoff event without SWM, while the dashed line hydrograph depicts the same event with SWM. The peak flow of the undetained hydrograph could exceed the capacity of the downstream conveyance system and thereby cause surcharging and flooding problems. With the introduction of the SWM facility, the solid lined hydrograph is spread over a longer time period and its peak is reduced. The area between the two (2) curves to the left of their intersection represents the volume of runoff, temporarily stored or detained in the SWM facility.
Figure C-23 - Concept of Detention Pond
C.8.2
Stormwater Management Ponds.
A.
Safety criteria for SWM ponds. All ponds shall meet or exceed all specified safety criteria. Use of these criteria shall in no way relieve the engineer of the responsibility for the adequacy and safety of all aspects of the design of the SWM pond.
1.
The spillway, embankment, and appurtenant structures shall be designed to safely pass the design storm hydrograph with freeboard equivalent to one (1) foot or fifteen (15%) percent of design capacity, whichever is greater. Any orifice with a dimension smaller than or equal to twelve (12) inches shall be assumed to be fully blocked. For all spillways (especially enclosed conduits), the ability to adequately convey the design flows must take into account any submergence of the outlet, any existing or potential obstructions in the system and the capacity of the downstream system. For these reasons, enclosed conduit spillways connecting directly to other enclosed conduit systems are discouraged. If used, they must be justified by a rigorous analysis of all enclosed conduit systems connected to the spillway.
2.
If an embankment is classified as a dam pursuant to Title 30, Part 1, Chapter 299, Texas Administrative Code, V.T.C.A. all design criteria found in Title 30, Part 1, Chapter 299, Texas Administrative Code, V.T.C.A. must be met, as evidenced by certification by an engineer licensed in the State of Texas.
3.
All SWM ponds shall be designed using a hydrograph routing methodology.
4.
The constructed height of an earthen embankment shall be equal to the design height plus the amount necessary to ensure that the design height will be maintained once all settlement has taken place.
a.
This amount shall in no case be less than five (5) percent of the total fill height. All earthen embankments shall be compacted to ninety-five (95) percent of maximum density.
5.
Earthen embankment side slopes shall be no steeper than four (4) horizontal to one (1) vertical. Slopes must be designed to resist erosion to be stable in all conditions, and to be easily maintained.
6.
Detailed hydraulic design calculations shall be provided for all SWM ponds. Stage-discharge rating data shall be presented in tabular form with all discharge components, such as orifice, Weir, and outlet conduit flows, clearly indicated. Stage-storage table shall also be provided. In all cases, the effects of tailwater or other outlet control considerations should be included in the rating table calculations.
7.
When designing ponds in series (i.e., when the discharge of one (1) becomes the inflow of another), the engineer must submit a hydrologic analysis which demonstrates the system's adequacy. This analysis must incorporate the construction of hydrographs for all inflow and outflow components.
8.
No outlet structures from detention, filtration and/or sedimentation ponds, parking detention or other concentrating structures shall be designed to discharge concentrated flow directly onto arterial or collector streets. Such discharges shall be conveyed by a closed conduit to the nearest existing storm sewer. If there is no existing storm sewer within three hundred (300) feet, the outlet design shall provide for a change in the discharge pattern from concentrated flow back to sheet flow, following as near as possible the direction of the gutter.
9.
Storm runoff may be detained within parking lots. However, the engineer should be aware of the inconvenience to both pedestrians and traffic. The location of ponding areas in a parking lot should be planned so that this condition is minimized. Stormwater ponding depths (for the 100-year storm) in parking lots are limited to a maximum of eight (8) inches in depth.
10.
All pipes discharging into a public storm sewer system shall have a minimum diameter of eighteen (18) inches and shall be constructed of reinforced concrete. Other materials may be used with approval of City Engineer. In all cases, ease of maintenance and/or repair must be assured.
11.
All concentrated flows into a SWM pond shall be collected and conveyed into the pond in such a way as to prevent erosion of the side slopes. All outfalls into the pond shall be designed to be stable and nonerosive.
B.
Outlet structure design. There are two (2) basic types of outlet control structures: those incorporating orifice flow and those incorporating Weir flow. Rectangular and V-notch Weirs are the most common types.
1.
Generally, if the crest thickness is more than sixty (60) percent of the nappe thickness, the Weir should be considered broad-crested. The coefficients for sharp-crested and broad-crested Weirs vary. The respective Weir and orifice flow equations are as follows:
a.
Rectangular Weir flow equation (See Figure C-24).
Q = CLH 3/2 Equation C-55
n C - 55
Where:
Q = Weir discharge, cubic feet per second
C = Weir coefficient
L = horizontal length, feet
H = Head on Weir, feet
b.
V-notch Weir flow equation (See Figure C-24).
Q = C v tan (0/2)H 2.5 Equation C-56
Where:
Q = Weir flow, cubic feet per second
C v = Weir coefficient
O = Angle of the Weir notch at the apex (degrees)
H = Head on Weir, feet
c.
Orifice flow equation (See Figure C-15).
Q = C o A(2gH) 0.5 Equation C-57
Where:
Q = Orifice flow, cubic feet per second
C o = Orifice coefficient (use 0.6)
A = Orifice area, square feet
g = Gravitation constant, 32.2 feet/sec
H = Head on orifice measured from centerline, feet
d.
Analytical methods and equations for other types of structures shall be approved by the SWMD prior to use. In all cases, the effects of tailwater or other outlet control considerations should be included in the rating table calculations.
Figure C-24 - Weir and Orifice Flows
C.8.3
Detention Pond Storage Determination.
A.
A flow routing analysis using detailed hydrographs must be applied for all detention pond designs. The Soil Conservation Service hydrologic methods (available in TR-20, HEC-HMS) and the Hydrologic Engineering Center (HEC) hydrologic methods may be used. The engineer may use other methods but must have their acceptability approved by the city engineer.
(Ord. No. 2016-61, § 1(Exh. A), 10-13-16)