(a) 
On-site detention of runoff is an alternative to other methods of urban stormwater management. Storage, which involves collecting excess runoff before it enters the main drainage system, can often be an effective and economical means of reducing peak flow rates and mitigating problems of flooding, pollution, soil erosion, and siltation.
(b) 
Detention facilities can be used to lessen the impact of peak flows on downstream property, and for the improvement of water quality. Large regional facilities serving a number of developments are generally preferable to small on-site facilities serving only one subdivision or office complex.
(c) 
The detention basin is the most widely used measure for controlling peak discharges from urbanizing areas. Basins can be designed to fit a variety of sites and can incorporate multiple-outlet spillways to meet requirements for multi-frequency control of flow. Measures other than a detention basin, such as infiltration trenches or porous pavement, may be preferred in some locations. Any device selected, however, should be assessed as to its cost, function, maintenance requirements (frequency and type), and impact on downstream peak flows.
(d) 
Storage is a means to mitigate problems associated with increased runoff caused by development. It is preferable to avoid causing the problems in the first place by minimizing the increase in runoff volumes or rates. This article outlines strategies to achieve those goals.
(Ordinance 392-2005, sec. 1(1), adopted 8/9/05)
(a) 
Generally.
(1) 
The Rational Formula shall be used for detention basin design only for small areas (20 acres or less) as described in section 3.08.004. Methods which include a runoff hydrograph, such as the SCS Tabular Method, HEC-1, or TR-20, shall be used for watersheds larger than 20 acres. Runoff hydrographs must be developed as part of the evaluation of drainage system performance during design and major storm events. Computations of runoff hydrographs which do not rely on a continuous accounting of antecedent moisture conditions shall assume antecedent moisture condition II.
(2) 
Detention basins shall be designed to protect the safety of any children or adults coming in contact with the system during runoff events. Safety of stormwater drainage system components is always a principal design criteria. The use of fencing around detention basins may be avoided by incorporating safety features into the design of the facilities. However, certain extreme cases may require the use of fences to protect the public. The shorelines of all detention basins at 100-year capacity shall be as level as practicable to prevent accidental falls into the basin and for stability and ease of maintenance.
(3) 
The side slopes of the banks of detention basins shall not be steeper than 4H:1V. All detention basins shall have a level safety ledge extending three (3) feet into the basin from the shoreline and two (2) feet below the normal water depth. Velocities throughout the drainage system shall be controlled to safe levels taking into consideration rates and depths of flow.
(b) 
Design storm.
(1) 
Detention basins shall be designed to limit the peak rate of discharge from the basin for the 10-year and 100-year events to the predevelopment rate or a rate which will not cause an increase in flooding or channel instability downstream when considered in aggregate with ultimate watershed development and downstream drainage capacities.
(2) 
A minimum of one (1) foot of freeboard shall be added to the design water surface elevation. Backwater computations for runoff entering detention basins shall assume a starting elevation based on a compatible storm. It is the responsibility of the developer to determine if additional freeboard is necessary. The city engineer or director of public works reserves the right to require additional freeboard if deemed necessary for safety or maintenance considerations. The design storm shall pass through the outlet without overtopping the structure.
(c) 
Principal outlet works.
(1) 
Where a single pipe outlet is to be used to discharge, it shall have a minimum inside diameter of 18 inches. Maintenance of outlets smaller than 18 inches is likely to be a problem. If design release rates call for outlets smaller than this, release structures such as perforated risers or flow control orifices shall be incorporated.
(2) 
Depending on the geometry of the outlet structure, discharge for various headwater depths can be controlled by the inlet crest (weir control), the riser or barrel opening (orifice control), or the riser or barrel pipe (pipe control). Each of these flow controls shall be evaluated when determining the rating curve of the principal outlet. The following weir, orifice and pipe flow equations can be used to evaluate a single opening outlet structure. Multiple openings required a more rigorous analysis which is beyond the scope of this article. Solicit a hydraulics textbook for procedure.
(3) 
Weir flow may be computed by the following equation:
Q = C L Hw 3/2 (1)
Where:
Q
=
Discharge, in cubic feet per second
C
=
Weir coefficient
L
=
Length of the weir, in feet; for circular riser pipes, L is the pipe circumference
Hw
The depth of flow over the weir crest, in feet
(4) 
Orifice flow may be computed by the following equation:
Q = C A (2gHo)0.5 (2)
Where:
Q
=
Discharge, in cubic feet per second
C
=
Orifice coefficient
A
=
Cross-sectional area of the pipe, in square feet
g
=
Acceleration of gravity, 32.2 feet per second squared
Ho
=
Head above the centerline of the pipe, in feet
The weir and orifice coefficients are a function of various hydraulic properties and dimensional characteristics. The designer is urged to solicit hydraulic textbooks such as Handbook of Hydraulics by Brater and King and use engineering judgment.
(5) 
Pipe flow may be computed by the following equation:
Where:
Q
=
Discharge, in cubic feet per second
A
=
Cross-sectional area of the pipe, in square feet
g
=
Acceleration of gravity, 32.2 feet per second squared
H
=
The difference between headwater and tailwater elevations, in feet
kb
=
Bend loss coefficient, use 0.6
ke
=
Entrance loss coefficient, use 0.5
kf
=
Friction loss coefficient = 185n2/D4/3
n
=
Manning’s roughness coefficient
D
=
Diameter of pipe, in feet
L
=
Length of pipe, in feet
(d) 
Emergency spillways.
(1) 
The designer is responsible to determine if an emergency spillway or a spillway feature is needed for an embankment type detention facility. The city engineer or director of public works may require the designer to evaluate a more stringent design requirement including a breach analysis if there is a potential for loss of life. In addition, certain embankments are classified as dams and are required to meet rules established by the state commission on environmental quality (TCEQ).
(2) 
The position, profile, and length of the spillway are influenced by geologic and topographic features of the site. The cross-section dimensions are governed by hydraulic elements and are determined by acceptable reservoir routing of the design storm. Most emergency spillways for detention ponds may be designed as grass-lined open channels. Table 1 presents acceptable grasses for vegetative spillways.
Table 1. Acceptable Grasses for Vegetated Spillways
Western Wheatgrass
Buffalograss
Bermudagrass
Tall Fescue
Blue Grama
(3) 
Discharge from the emergency spillways shall be directed to the main channel without causing erosion along the downstream toe of the dam. Emergency spillways proposed for the protection of earthen embankments shall be in full cut undisturbed soil, if possible, to avoid flows against constructed fill. The side slopes of the excavated channel in earth shall be no steeper than 4H:1V for ease of maintenance. Where the site limitations prevent a full channel cut, a wing dike shall be provided to direct spillway flows away from the downstream toe of the dam. Ready access to the emergency spillway system shall also be provided.
(4) 
The configuration of the entrance channel from the reservoir to the control section of the emergency spillway shall be a smooth transition to avoid turbulent flow over the spillway crest. The outlet channel of the emergency spillway shall convey flow to the channel below the structure with a minimum of erosion. The slope of the exit channel usually follows the configuration of the abutment. Slopes, however, should not exceed 10 percent. In cases of highly erodible soils, it may be necessary to use other means of protection such as riprap, grouted rock or concrete paving to form the exit channel. As an alternative, detention storage can be increased to reduce the frequency or duration of use of the emergency spillway and thereby reduce erosion problems.
(Ordinance 392-2005, sec. 1(2), adopted 8/9/05)
(a) 
Generally.
Potential advantages and disadvantages of on-site detention basins should be considered by the designer in the early stages of development. Discharge rates and outflow velocities are regulated to conform to the capacities and physical characteristics of downstream drainage systems. Energy dissipation and flow attenuation resulting from on-site storage can reduce soil erosion and pollutant loading. By controlling release flows, the impacts of the pollutant loading of stored runoff on receiving water quality can be minimized.
(b) 
Parking lots and streets.
(1) 
There are two general types of stormwater detention on parking lot surfaces. One type involves the storage of runoff in depressions constructed at drain locations. The stored water is drained into the storm sewer system slowly, using restrictions such as orifice plates in the drain. Proper design of such paved areas will restrict ponding to areas which will cause the least amount of inconvenience to the users of the parking areas.
(2) 
For example, the parking lot of a shopping center will have the ponding areas located in the least-used portions of the lot, allowing customers to walk to their vehicles in areas of no ponding, except when the entire lot is filled with vehicles. Drainage of ponded water would be fairly rapid to prevent customer inconvenience. In most cases, the water should pond to a depth not to exceed 7 inches and the ponding area should be drained within 30 minutes or less after the rainfall. Computation of the amount of storage needed would be similar to the analysis used in designing detention basins on ground surfaces.
(3) 
Another type of stormwater detention on parking lots consists of using the paved areas of the lot to channel the runoff to grassed areas or gravel-filled seepage pits (figure 1). Water from pavement should run through at least 30 feet of grass before entering an infiltration swale, trench or basin. The flow then infiltrates into the ground. Soil conditions and the effects of siltation in reducing infiltration must be considered.
(4) 
Minimum slopes of one (1) percent are recommended in parking lot detention areas. Maximum slopes should not exceed four (4) percent to avoid gasoline spillage from tanks and to minimize vehicle traction problems on icy pavement.
(c) 
Recreational areas.
(1) 
Generally, recreational areas such as outdoor athletic fields have a substantial area of grass cover which often has a high infiltration rate. Generally, storm runoff from such fields is minimal. Grassed recreational fields can be utilized for the temporary detention of storm runoff without adversely affecting their primary function.
(2) 
The city contains many parks, both the neighborhood type and the large, central type. Parks, like recreational areas, create little runoff of their own; however, parks provide excellent detention storage potential for runoff from adjacent areas.
(d) 
Property line swales.
(1) 
Subdivision planning and layout requires adequate surface drainage away from buildings. This is obtained by sloping the finished grade away from the buildings. When possible, the layout should call for a swale to be located along the back and/or side property line which then drains through the block (figure 2). Such drainage should be guided away from storm sewers and towards natural channels. If storm sewers are the only point of discharge, the route should be as long as possible to allow infiltration. The final grading plan for the lot layout can be finalized to allow up to six (6) inches of temporary ponding along the property line.
(2) 
Temporary ponding facilities along lot lines may include small controlled discharges or, if the subsoil conditions are favorable, such water may be percolated into the ground.
(e) 
Road embankments.
(1) 
The use of road embankments for temporary storage is an efficient method of attenuating the peak flows from a drainage basin.
(2) 
The design criteria to be used for the temporary detention of water behind road embankments shall include consideration of the major storm runoff. The use of roadway embankments to help reduce downstream peak flows is encouraged. Planning for the usage of embankments must be done with thorough consideration to avoid damage to the embankment, the structure, and adjacent property.
(f) 
On-site ponds.
(1) 
The construction of on-site ponds provides significant detention benefits when properly planned and designed. The use of such ponds is particularly encouraged in planned unit developments where large areas of grass and open space are common.
(2) 
Controlled outlets for the surcharge storage can be used, and it is suggested that such outlets be designed to release at a rate that does not exceed the rate estimated for natural conditions or downstream channel capacity (whichever is smaller).
(g) 
Combinations.
In many instances, one on-site detention method cannot conveniently or economically satisfy the required amount of stormwater storage. Limitations in storage capacities, site development conditions, soils limitations, and other related constraints may require that more than one method be utilized. For example, parking lot and surface pond storage might all be required to compensate for increases in runoff due to development of a particular site. Whichever combinations are suitable should be incorporated into the site development plan.
(Ordinance 392-2005, sec. 1(3), adopted 8/9/05)
(a) 
Modified Rational Method Analysis.
(1) 
The term “Modified Rational Method Analysis” is a procedure for manipulating the basic Rational Method to reflect the fact that storms with durations greater than the normal time of concentration for a basin will result in a larger volume of runoff even though the peak discharge is reduced. This greater volume of runoff produced by longer storm durations must be analyzed to determine the correct sizing of detention facilities.
(2) 
The approach becomes more valid on progressively smaller basins, eventually reaching a size so small that watershed modeling is approached. The procedure should, therefore, be limited to relatively small areas such as rooftops, parking lots, or other upstream areas with tributary basins less than 20 acres. This would minimize major damage which could result from overtopping or failure of the proposed detention facility.
(3) 
Figure 3 presents a family of curves for a theoretical basin described in the following example. These hydrographs are developed by using the basic Rational Method assumptions of constant rainfall intensity, time of concentration (tc) for the longest flow path, and the coefficient of runoff. The typical Rational Method hydrograph with the peak discharge coinciding with the time of concentration for the basin is first calculated using the formula, Q = CCfiA. Following this, a family of hydrographs representing storms of greater duration are developed. The rising limb and falling limb of the hydrograph are, in each case, equal to tc for the basin. The area under the hydrograph is also equal to the peak discharge rate for that particular rainfall multiplied by the duration of the rainfall.
 
Example 1. Modified Rational Method
Given:
Area: A = 2.0 acres
 
Type of development: commercial parking lot, fully paved, C = 0.88
 
Time of concentration: tc = 8 minutes
 
Design frequency = 25 years, Cf = 1.10
 
Use intensity-duration-frequency curves, figure 2-1.
Find:
Develop family of curves representing Modified Rational Method hydrographs for the 8-, 10-, 15-, 20-, 30- and 40-minute rainfall durations.
Solution:
Qp = C Cf i A
 
Example (.88)(1.1)(7.97)(2.0) = 15.4 cfs
Rainfall Duration
(min)
Rainfall Intensity
(in/hr)
Peak Runoff Rate
(cfs)
8
7.97
15.4
10
7.44
14.4
15
6.36
12.3
20
5.88
11.4
30
4.92
9.5
40
4.40
8.5
The resulting storm hydrographs are depicted in figure 3.
(4) 
The next step in determining the necessary storage volume for the detention facility is to set a release rate and determine the volume of storage necessary to accomplish this release rate.
(5) 
To determine the storage volume required, a reservoir routing procedure should be computed for each of the hydrographs, with the critical storm duration and required volume being determined. The importance of the particular project should govern the type of routing utilized. For small areas requiring repetitive calculations, such as parking lot bays, an assumed release curve is normally satisfactory. For larger areas, such as a pond in a small park with 20 acres or more of tributary area, a reservoir routing procedure would be more appropriate.
(6) 
Figure 3 represents a method for small area detention analyses. The assumed release curve approximates a formal reservoir routing in much the same way the Rational Method hydrograph approximates a true storm hydrograph. The curve allows for the low release rate at the beginning of a storm and an increasing release rate as the storage volume increases.
(7) 
In normal flood routing, the maximum release rate will always occur at the point where the outflow hydrograph crosses the receding limb of the inflow hydrograph. For this reason, the design release rate is forced to coincide with that point on the falling limb of the hydrograph resulting from the storm of duration equal to the time of concentration for the basin. The release rate is held constant past this point. The storage volume is then found by determining the area between the inflow and release hydrographs. Example 2 continues the calculations initiated in Example 1 to determine the required storage volume.
(8) 
The equation for the storm runoff volume, Vr, can be simplified as:
Vr = 60 D Qp (4)
Where:
Vr
=
Storm runoff volume, in cubic feet
D
=
Storm duration, in minutes
Qp
=
Peak runoff rate of the inflow hydrograph, in cubic feet per second
The equation for the required storage volume, Vs, can also be simplified as:
Vs = 60 D (Qp - Qo) (5)
Where:
Vs
=
Required storage volume, in cubic feet
D
=
Storm duration, in minutes
Qp
=
Peak runoff rate of the inflow hydrograph, in cubic feet per second
Qo
=
Maximum release rate, in cubic feet per second
Example 2. Critical Storage Volume
Given:
Drainage basin and other hydrologic information presented in Example 1.
 
Allowable release rate: Qo = 6.0 cfs
Find:
Determine the critical storage volume.
Solution:
V = 60 D Qp
Vs = 60 D (Qp - Qo)
Storm Duration
(min)
Storm Runoff Volume
(ft3)
Storage Volume
(ft3)
8
7,392
4,512
10
8,640
5,040
15
11,070
5,670
20
13,680
6,480 Maximum
30
17,100
6,300
40
20,400
6,000
Ex: (60)(8)(15.4 - 6.0) = 4,512
The critical storage volume is 6,480 cubic feet occurring for a 20-minute rainfall duration. The limitations in the assumptions behind this method are evident. The approach becomes more valid on progressively smaller basins. The procedures should, therefore, be limited to relatively small areas where no major damage would result from overtopping or failure of the proposed detention facility. Care should be used when applying this method to areas in excess of 20 acres.
(b) 
Hydrograph procedure for storage analysis.
(1) 
The unit hydrograph procedure develops a hydrograph which provides a reliable solution for detention storage effects. The unit hydrograph procedure provides the engineer/designer greater flexibility for the representation of actual conditions to be modeled. The unit hydrograph procedure can be used for any size drainage area. For detention basin design, a minimum design storm duration of 24 hours should be used.
(2) 
A storm runoff hydrograph is presented in figure 4, which represents inflow to a reservoir. The analysis for the reservoir storage must take into consideration the characteristics of the outlet pipe, the discharge of which is shown in figure 4 as a solid line. The shape of the solid line reflects the carrying capacity of the outlet works with various headwater elevations. The higher the elevation of the water surface in the reservoir the greater the discharge through the outlet works. The area between the dashed line and the hydrograph of storm outflow can be planimetered to determine the volume of storage required to reduce channel flow from 200 cubic feet per second to 100 cubic feet per second.
(3) 
For off-stream storage the basic approach to the analysis is presented in figure 4. In this case, the peak of the storm hydrograph is routed over a side channel spillway into a ponding area adjacent to the channel. The water removed from the channel, represented by the shaded area of the hydrograph, provides for a reduction in the peak channel flow from 200 cubic feet per second to about 100 cubic feet per second.
(4) 
For off-stream storage the basic approach to the analysis is presented in figure 4. In this case, the peak of the storm hydrograph is routed over a side channel spillway into a ponding area adjacent to the channel. The water removed from the channel, represented by the shaded area of the hydrograph, provides for a reduction in the peak channel flow from 200 cubic feet per second to about 100 cubic feet per second.
(c) 
Modified Puls routing procedure.
(1) 
A flood routing procedure may be used to determine the required volume of the detention basin. Several flood routing procedures are available in published texts. One commonly used procedure is the Modified Puls. The data needed for this routing procedure are the inflow hydrograph, the physical dimensions of the storage basin, the maximum outflow allowed, and the hydraulic characteristics of the outlet structure or spillway.
(2) 
To perform the Modified Puls procedure, the inflow hydrograph, depth-storage relationship, and depth-outflow relationship must be determined. They are then combined in a routing routine. The results of the routing are the ordinate of the outflow hydrograph, the depth of storage, and the volume of storage at each point in time of the flood duration.
(3) 
The routing period, or time interval, ?t, is selected small enough so that there is a good definition of the hydrograph and the variation in the hydrograph during the period At is approximately linear. This can usually be accomplished by setting ?t = 5 minutes.
(4) 
Several assumptions are made in this procedure and include the following:
(A) 
The entire inflow hydrograph is known.
(B) 
The storage volume is known at the beginning of the routing.
(C) 
The outflow rate is known at the beginning of the routing.
(D) 
The outlet structures are such that the outflow is uncontrolled and the outflow rate is dependent only on the structure’s hydraulic characteristics.
(5) 
The derivation of the routing equation begins with the conservation of mass which states that the difference between the average inflow and average outflow during some time period ?t is equal to the change in storage during that time period. This can be written in equation form as:
I - O = ?S/?T (6)
Where:
I
=
Average inflow rate
O
=
Average outflow rate
DS
=
Change in storage volume
?t
=
Routing period
If inflow during the period is greater than outflow, then DS is positive and the pond gets deeper. If inflow is less than outflow during the period, then DS is negative and the pond gets shallower. Using the assumptions made previously, this equation can be rewritten as:
Where:
I1
=
Inflow rate at time interval 1
I2
=
Inflow rate at time interval 2
O1
=
Outflow rate at time interval 1
O2
=
Outflow rate at time interval 2
S1
=
Storage volume at time interval 1
S2
=
Storage volume at time interval 2
?t
=
Routing period
Multiplying both sides by two and separating the right-hand side yields:
Rearranging so that all the known terms are on the left-hand side and all the unknown terms on the right-hand side yields the final routing equation:
However, Equation 9 has two unknowns, S2 and O2. A second equation is needed which relates storage and outflow. If outflow is a direct function of reservoir depth (as it is with uncontrolled outflow), there is a direct relationship that exists between reservoir elevation, reservoir storage, and outflow. Therefore, for a particular elevation, there is an answer for storage and outflow (S and O). A relationship between O and (2S/?t) + O is determined for several elevations and plotted on logarithmic graph paper. The routing equation is solved by adding all the known terms on the left-hand side. This yields a value for (2S2/?t) + O2. This value is found on the log-log plot of (2S/?t) + O versus O.
The (2S/?t) + O versus O relationship is derived by combining the depth-storage relationship and the depth-outflow relationship, as previously discussed. This is shown in table 2. Columns 1, 2, and 3 are tabulations of the depth-storage and depth-outflow relationships for a specific detention facility.
In column 4, the units of 2S/?t and O must be the same. If O is in cubic feet per second, then 2S/?t must be changed to cubic feet per second. For a routing time interval of 5 minutes:
Thus,
Where: S has units of acre-feet, O has units of cfs, and 2S/?t has units of cfs.
Table 2. Development of a (2S/?t) + O versus O Relationship
Depth
Storage, S
Outflow, O
(2S/?t) + O
(ft) (1)
(acre-feet) (2)
(cfs) (3)
(cfs) (4)
0
0.0
0
0
2
0.1
40
69
4
0.6
138
313
6
3.0
274
1,147
8
11.0
426
3,627
10
32.0
560
9,872
12
72.0
671
21,623
14
131.0
765
38,886
Figure 1
Figure 2
Figure 3
Figure 4
(Ordinance 392-2005 adopted 8/9/05; Ordinance 392-2005, sec. 1 (4), adopted 8/9/05)