ALTERNATIVE STRATEGIES FOR STORMWATER DETENTION

WATER RESOURCES BULLETINVOL. 24, NO. 3 AMERICAN WATER RESOURCES ASSOCIATION JUNE 1988

ALTERNATIVE STRATEGIES FOR STORMWATER DETENTION1

Stephan J. Nix and Ting-Kuei Tsay2

ABSTRACT: In a simulation experiment, stormwater flows arepartially diverted, at various levels, to a detention basin in order tocompare the recombined (i.e., undiverted flows and basin discharges)hydrograph to the response of the traditional, in-line design. The useof off-line detention basins is shown to be an effective technique forreducing peak flows from developed watersheds to pre-developmentlevels with lower storage requirements. In addition, the dischargehydrographs produced by off-line detention are significantly differentfrom those produced by the traditional design and may be more suitedto certain stormwater management situations.(KEY TERMS: storm water; detention basins; urban runoff; simula-tion; SWMM.)

INTRODUCTION

For a given rainstorm, a developed watershed will generallyproduce a runoff event with a greater, earlier-occurring peakflow and a larger volume than it would have in its un-developed (or lesser developed) state. The results are increasedflooding, erosion, and ecological damage. Most commonlythese problems have been addressed through the use of deten-tion basins. Essentially, the goal of detention is to captureand temporarily store the runoff from the developed water-shed before it reaches a sensitive watercourse. The capturedwater is then released at a more suitable rate.

The commonly accepted wisdom is to design the basinsuch that the peak flow from a design storm is reduced tothe pre-development level. The concept is fairly straightfor-ward and well understood and a fairly large body of litera-ture exists to aid practitioners. However, there are problemsassociated with the current design philosophy. Recognizingthat stormwater problems are site-specific, this paper evaluatesalternative detention basin designs in light of their controlcharacteristics. With a larger portfolio of design optionsstormwater management problems may be more successfullysolved.

CURRENT PRACTICE

The currently accepted goal of detention as a stormwatermanagement tool is summarized in Figure 1. A watershed inits pre-developed state produces hydrograph A for, say, the25-year, 24-hour rainstorm. After the watershed is developedthe response to the same storm may look similar to runoffhydrograph B. Routing hydrograph B through a typicallydesigned detention basin produces a hydrograph (labeled as C)in which the peak flow is reduced to its pre-developmentlevel, but relatively high flows persists for an extended periodof time. The routed peak flow also occurs after the peak ofthe pre-development hydrograph. The shaded area labeled Iis the volume of runoff temporarily stored for release at alater time (shaded area II) and represents the requiredstorage capacity of the basin.

a0

Figure 1. Typical Hydrographs for Pre- andPost-Development Conditions.

Typically, detention basins are designed as follows:1) All runoff collected by the drainage system is directed

to the basin. In other words, in-line storage is used.

'Paper No. 87114 of the Water Resources Bulletin. Discussions are open until February 1, 1989.2Associate Professors, Department of Civil Engineering, Syracuse University, 220 Hinds Hall, Syracuse, New York 13244.

609 WATER RESOURCES BULLETIN

4 • Pre-deoeIopnent ho9rcPoe- withoot detenton

C • Post- with de'ert'on

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2) An orifice or conduit is used to provide the design re-lease with a spiliway used to provide emergency relief.

3) The storage capacity (the volume found between theinvert of the orifice and the spiliway crest) is roughly equalto area I in Figure 1. The design release rate occurs whenthis capacity is reached.

Naturally, there are variations on this theme. Most noticeably,many basins are now being designed to handle several dif-ferent design storms. There are, of course, as many detaileddesigns as there are detention basins but the basic designremains relatively unchanged.

Unfortunately, the reduction in peak flow comes at a cost.Without some other management techniques, the total run-off volume will increase substantially after development asa result of the increase in impervious area. The typicaldetention basin wifi not be able to significantly reduce thevolume (i.e., by seepage, evaporation, etc.). Much of thisexcess volume is released after the peak of the dischargehydrograph, thus causing an extended period of relativelyhigh flow (see Figure 1). This phenomenon can lead toincreased flooding, erosion problems, and ecological stressat downstream locations (Lumb, et al., 1974; Smiley andHaan, 1976; Abt and Grigg, 1978; McCuen, 1979; Lakatosand Kropp, 1982; Traver and Chadderton, 1983; Whipple,etaL, 1983).

The problem of exacerbating a downstream flooding prob-1cm is particularly troublesome. This appears most likelyto occur when the detention basin is located in the lowerportions of a watershed (Whipple, et aL, 1983). The extendedperiod of high discharge from the outlet of the downstreamdetention basin is combined with flows from upstream por-tions of the watershed to create a peak flow that may exceedthe level experienced without the detention basin. Thisphenomenon is demonstrated in Figure 2. In this illustrationthe watershed is divided into three identical subwatersheds(i.e., the upper, middle, and lower subwatersheds) equallyspaced in terms of travel time. The response of the water-shed to an areally uniform rainstorm without a detentionbasin is shown in Figure 2(a) and the response with the run-off from the lower subwatershed routed through a detentionbasin appears in Figure 2(b). Although this scenario ishypothetical, the potential for the problem is probably wide-spread.

Several researchers have suggested a watershed-wide ap-proach to stormwater management problems and the use of acoordinated system of detention basins to better achieve storm-water management goals and to avoid problems such as in-creased downstream flooding (McCuen, 1979; Amandes andBedient, 1980; Flores, et a!., 1982; Lakatos and Kroop, 1982;Dendrou and Delleur, 1982; Bennett, 1983; Traver and Chad-derton, 1983). This is a laudable goal and it should be pur-sued. However, this requires considerable coordination onthe part of local governments and regulatory agencies, some-thing that may be difficult to achieve. In addition, one mustnot forget the value of on-site control (i.e., reduction of thepeak flow to the pre-development level at the site boundary).

Nix and Tsay

A detention basin can considerably reduce the peak runoffflows and provide a localized benefit. Alas, what may begood for one may not be for one's neighbor. However,localized rights must be addressed as well as the well-beingof the watershed as a whole. We believe that a little moreflexibility in design could help alleviate local and watershed-wide problems.

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014U.

Overview

Figure 2. Downstream Effect of Detention(after Whipple, et at., 1983).

ANALYSIS OF ALTERNATIVE DESIGNS

The best detention basin design is one in which the dis-charge hydrograph replicates the entire pre-developmenthydrograph. However, unless the runoff volume is also re-duced to its pre-development level, this cannot be achievedwithout a stormwater management plan that reduces thevolume produced by the watershed and/or some sort ofretention technology (e.g., recharge basins). The questionhere is how can detention basins be designed so that theyrepresent a more effective management technique.

A look at typical pre- and post-development hydrographs,with and without detention, provides some illumination. Referagain to Figure 1. Generally, the routed post-developmenthydrograph lags behind the pre-development hydrograph.This suggests that the storage capacity of the typically de-signed detention basin is used too early. In other words, ifusing storage could be avoided during the early portion of astorm it would be possible to increase the earlier flows tonearer their pre-development levels and reduce the later flows.As will be seen, much of this can be accomplished with


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Alternative Strategies for Stormwater Detention

off-line storage, i.e., diverting only a portion of the storm-water to the basin.

The effect of this proposed strategy on nonpoint-sourcepollution control is not addressed here. Obviously, a strategythat proposes to avoid the use of storage during the earlyportion of a runoff event runs counter to the notion thatearly runoff flows should be captured in order to take ad-vantage of the so.called "first-flush" effect. Nevertheless,we believe that the concept presented here can play a rolein a number of situations; the extent of that role dependson the objectives of the detention system.

What follows is a description of a series of experimentalsimulations designed to provide some insight into the rangeof options open to the engineer who plans and designs storm-water detention systems for flow control.

Experimental Set-up and Procedure

Pre. and post-development stormwater runoff hydrographswere developed for a hypothetical 640-acre watershed. Thesewere constructed by driving the Runoff Block of the StormWater Management Model (SWMM) (Huber, Ct al., 1984)with the 100-year, 24-hour rainstorm for Syracuse, New York.The total depth of this storm, 4.8 inches, was distributedover 24 hours according to the Soil Conservation ServiceType-Il storm (SCS, 1973; SCS, 1981). The characteristicsgiven the watershed under the assumed pre- and post-develop.ment conditions are shown in Table 1. A 15-minute timestep was used to approximate the rainfall hyetograph and toproduce the runoff flows. The resulting hydrographs areshown in Figure 3. For the purposes of this study it was

assumed that the minimum management goal is to reduce thepost-developed peak flow (960 cfs) to the pre-developmentlevel (496 cfs). Other goals will be discussed in light ofthe results appearing later.

Figure 3. Pre- and Post-Development Hydrographs forHypothetical Watershed, 100-Year, 24-Hour Storm.

In order to restrict the size of the study to a manageablelevel certain assumptions regarding the detention basin werenecessary:

1) A single orifice serves as the principal outlet structure(or control). The orifice was assumed to behave as follows:

TABLE 1. Input Data for SWMM Runoff Block, Hypothetical Watershed.

Parameter Value

Area 640 acres (259 ha)

Imperviousness Pre-development 25%Post-development 50%

Width of Overland Flow Pre-development 10,560 ft (3,219 m)Post-development 21,120 ft (6,437 m)

Ground Slope 0.01 ft/ft

Manning's nImpervious area 0.013Pervious area 0.25 0

Depression StorageImpervious area 0.062 in. (1.57 mm)Pervious area 0.250 in. (6.35 mm)

Horton's Equation ParametersMaximum infiltration rate 3.00 in./hr (76.2 mm/hr)Minimum infiltration rate 0.52 in./hr (13.2 mm/hr)Decay rate of infiltration 0.00115/s

Evaporation Rate 0.10 in./day (2.54 mm/day)ci


Elapsed lime, hours

Q C A(2oh*0 0 Gwhere:

Q0 = orifice discharge, ft3/s;

C0 = discharge coefficient (assumed to be 0.6);

A = orifice area, ft2;

g = acceleration due to gravity, 32.2 ft _2; and

h0 = head above the centerline of the orifice, ft.

2) A spiliway discharges flow when the storage capacity isexceeded.

3) The depth from the invert of the orifice to the crestof the spiliway is 10 feet. The storage capacity is also lo-cated between these two points. An additional 2 feet offreeboard is provided for storms larger than the design storm.

4) The basin has a rectangular base (i.e., at the invert ofthe orifice) with a length-to-width ratio of 2. The basin sideshave 1:1 slopes.

5) A flow divider capable of diverting a portion of therunoff from the drainage channel to the basin is present ator near the inlet. This is intended to allow for the possibilityof an off-line facility. The diverted flow is assumed to be afixed fraction of the channel flow (hereafter called the diver-sion fraction) or that which exceeds a specified level (here-after called the diversion flow leve). The basin dischargeand the undiverted flow are recombined downstream.

Figure 4 provides a schematic of the basic system.

The model used to simulate the behavior of a detentionbasin is a modified version of the Storage/Treatment Blockof SWMM (Huber, et al., 1984). SWMM is not unique in itshandling of the hydraulics of detention basins, i.e., it is basedon the modified Puls method of reservoir routing. It wasused here because of the authors' familiarity with it. How-ever, it was modified to better meet the needs of this study.

The SWMM user is normally asked to describe the hydraulicbehavior of a detention basin with a set of depth, storage,

Nix and Tsay

(1)discharge data triplets. This was modified to allow the inputof a storage capacity, the orifice diameter, and either a fixedfraction or flow level for diversion. With the assumptionslisted above, this information was converted into the formatnormally expected by SWMM.

The simulation effort was directed toward locating avariety of system designs (storage capacity, orifice diameter,and diversion fraction or level) that could meet the manage-ment goal. This was done by conducting a "trial-and-error"search for the most "efficient" combination of storage capa-city and orifice diameter for each of a number of differentdiversion fractions and diversion flow levels (see earlier dis-cussion). An "efficient" design is one in which the recom-bined flow (basin discharge and undiverted flow) achievesthe peak flow target while using all of the storage capacitybut not overflowing the spillway. The search was not blind,of course, as it is not difficult to roughly estimate the neededstorage capacity and orifice diameter.

The diversion fractions investigated include 0.5, 0.6, 0.7,0.8, 0.9, and 1.0. The value 0.5 represents an approximatelower limit in this example because allowing less to be divertedensures that the management goal would not be met. (Recallthat the pre- and post-development peaks are 496 and 960cfs, respectively.) The diversion flow levels investigated in-clude 450, 400, 300, 200, 100, and 0 cfs. A diversion levelof 496 cfs or above would require that the basin retain ratherthan detain (that is, no discharge), thus 450 cfs was chosenas the upper limit. The cases of a diversion fraction of 1 .0and a diversion level of 0 cfs represent the traditional, in-linedetention facility; that is, all flow is diverted to the basin.

RESULTS AND DISCUSSION

The points in Figure 5 depict the most efficient combina-tions of storage capacity and orifice diameter to produce thedesired peak recombined flow (basin discharge and undivertedflow) of 496 cfs for the different diversion fractions. Con-necting the points estimates a continuum of combinationsfor the range of diversion fractions considered. Figure 6 isanalogous to Figure 5 except that diversion flow levels wereused instead of diversion fractions. The approximate locationof the "traditional" in-line design (i.e., total diversion) is alsoshown in both figures. The results indicate that substantialreductions (up to approximately 10 and 40 percent in Fig.ures 5 and 6, respectively) in the required storage capacity areobtained when off-line storage is used. This is because in theearlier stages of the runoff event the recombined flow ratesare much closer to the inflow rates than in traditional sys-tems. As a result, the use of storage is delayed and, thus,less storage capacity is needed. (This becomes more evidentwhen examining the inflow and outflow hydrographs pre-sented later.)

An interesting feature of Figure 5 is that there appears tobe an optimal orifice diameter for minimizing the storagecapacity (approximately 4 to 5 feet). Accompanying thisorifice diameter is a diversion fraction of about 0.7. Figure 6,


Undiverted Flow

DRAINAGE CHANNEL

FLOW DIVIDERRunoff

CHANNELDRAINAGE

Dlved

FIow

Flow

Recombined Flow

DETENTIONBASIN

Basinischarge

S PILL WAY

& ORIFICE

Figure 4. Detention System Schematic.

Alternative Strategies for Stormwater Detention

which depicts the results for various diversion flow levels,seems to suggest that an "optimal" design exists at a fairlysmall orifice diameter (less than 2 feet) and a high diversionlevel (i.e., only flows beyond 450 cfs are diverted to thebasin). The difference between the two figures can be at-tributed to the different diversion strategies. Storage is usedearlier with the diversion fraction approach, and to a muchgreater extent eventually, because a fixed fraction of allchannel flows is diverted to the basin. On the other hand,when the diversion flow level strategy is used no flow entersthe basin until a certain flow level is exceeded. This resultsin a minimal use of storage even with a small discharge ori-fice.

2 3 4 5 6 7 8

Orifice diameter, ft

It should be noted that there are a large number of com-binations of storage capacity, orifice diameter, and diversion

configuration that will achieve the peak discharge target withan inefficient use of storage. There are, as well, a large num-ber of combinations that will produce peak discharges lowerthan the target level. The curves in Figures 5 and 6 reflecta specific goal and set of conditions; another managementgoal or set of design assumptions will produce differentcurves.

It has been shown that a reduction in peak flow from aparticular design storm may be achieved with a number ofdifferent system designs. However, each recombined hydro-graph is unique and may have properties which may be usefulin attaining certain goals (such as avoiding the increase inoff-site, downstream peak flows and the duration of highflows) in addition to peak flow reduction.

Figures 7 and 8 present the recombined-flow hydrographsfor some of the diversion fractions or levels in Figures 5 and 6,respectively. Also shown on each figure are the pre- and post-development runoff hydrographs. The most striking featureof the recombined-flow hydrographs in Figure 7 (i.e., whena flow diversion fraction is used) is that when less water isdiverted to the detention basin the hydrograph better repli-cates the pre-development situation (at least during the highflow periods). Of course, the larger volume of water createdby development is still there, it is just being discharged overlonger periods as the diversion to the detention basin de-creases. In other words, in the later stages of a runoff eventthe hydrograph associated with a diversion fraction of 0.5will remain at a higher level than that associated with, forexample, the diversion fraction of 1.0. One can see this inthe later hours depicted in Figure 7. The longer release timesmay or may not be of concern in a particular situation. Inthis study, the basin was emptied in approximately threedays after the storm ended in the worst case (a diversionfraction of 0.5) and typically it took much less (a matterof a few hours for the in-line case). Also evident is thereduced and delayed use of storage when compared to thetraditional, in-line design. In fact, the maximum use ofstorage occurs at later times as the diversion fraction de.creases. The recombined-flow hydrographs for different di-version flow levels (in Figure 8) display a pattern similarto those in Figure 7 except that the use of storage is delayedeven further.

The simulated results were obtained by making some verysimple assumptions about the hydraulics of diversion struc-tures. Nevertheless, diversion structures that closely mimicthe two diversion techniques are feasible. A structure thatcould divert a constant fraction of the inflow might consistof a flume with a dividing wall. The dividing wall splits thewidth of the flume in proportion to the desired divertedand undiverted fractions. The other diversion strategy, that ofallowing flow over a certain level to enter the basin, couldbe approximated with a flume containing a sideflow weir.The weir crest can be determined by the desired diversionlevel. Of course, other hydraulic structures can be designedto achieve the necessary diversion.

The results indicate that there is considerable flexibilityto tailor the detention system response to meet localized


3

0Z2Ca0

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-Troditiomln-tine desi

08

Diversion frocton

ii I .1 I I

0

3

r0

0a0

•1

0

U,

0

2 3 4 5 6 7 8

Orifice diameter, ft

Figure 5. Efficient Detention System Designsfor Various Diversion Fractions.

• Traditiurialdesign

450 Diversion flow level, cts

I IU

Figure 6. Efficient Detention System Designsfor Various Diversion Flow Levels.

ACKNOWLEDGMENTS

Several students in the Department of Civil Engineering at SyracuseUniversity participated in this study including John Judge, RobertCanham, and Kimberlee Valdes. The authors are grateful for theircontributions.

LITERATURE CITED

I I I I I

Post-development900 — without detention

800 — Diversion fraction

Elopsed tune hoar,

Figure 7. Recombined Hydrographs forVarious Diversion Fractions.

Figure 8. Recombined Hydrographs forVarious Diversion Flow Levels.

Abt, S. R. and N. S. Grigg, 1978. An Approximate Method for SizingDetention Reservoirs. Water Resources Bulletin 14(4) :956-965.

Amandes, C. B. and P. B. Bedient, 1980. Storm Water Detention inDeveloping Watersheds. Journal of the Environmental Engineer-ing Division, ASCE 106(EE2):403-409.

Bennett, A. B., 1983. Dynamic Programming Model for DeterminingOptimal Sizes and Locations of Detention Storage Facilities. In:Proc. National Symposium on Urban Hydrology, Hydraulics, andSediment Control. University of Kentucky, Lexington, Kentucky,pp. 461-468.

Dendrou, S. A. and J. W. Deileur, 1982. Watershed-Wide Planning ofDetention Basins. In: Proc. Conference on Stormwater DetentionFacilities. American Society of Civil Engineers, New York, NewYork, pp. 72-85.

Flores, A. C., P. B. Bedient, and W. L. Mays, 1982. Method for Opti-mizing Size and Location of Urban Detention Facilities. In: Proc.National Symposium on Urban Hydrology, Hydraulics, and Sedi-ment Control. University of Kentucky, Lexington, Kentucky, pp.35 7-366.

Huber, W. C., J. P. Heaney, S. J. Nix, R. E. Dickinson, and D. J. Pol-mann, 1984. Storm Water Management Model User's Manual —Version III. EPA.600/S2-84-109a&b, U.S. Environmental ProtectionAgency, Cincinnati, Ohio.

Lakatos, D. F. and R. H. Kropp, 1982. Stormwater Detention — Down-stream Effects on Peak Flow Rates. In: Proc. Conference onStormwater Detention Facilities. American Society of Civil En-gineers, New York, New York, pp. 105-120.

Lumb, A. M., J. K. Wallace, and D. L. James, 1974. Anaysis of UrbanLand Treatment Measures for Flood Peak Reduction. OWRR-14-0001-33599.

McCuen, R. H., 1979. Downstream Effects of Stormwater DetentionBasins. Journal of the Hydraulics Division, ASCE, 105(HY1 1):1343-1356.

Soil Conservation Service, 1973. Method for Estimating Volume andRate of Runoff in Small Watersheds. Technical Paper 149, SoilConservation Service, Washington, D.C.

Soil Conservation Service, 1981. Urban Hydrology for Small Water-sheds. Technical Release No. 55, Soil Conservation Service, Wash-ington, D.C.

Smiley, J. and C. T. Haan, 1976. The Dam Problem of Urban Hy-drology. In: Proc. National Symposium on Urban Hydrology, Hy-draulics, and Sediment Control. University of Kentucky, Lexing-ton, Kentucky, pp. 25-29.

Traver, R. G. and R. A. Chadderton, 1983. The Downstream Effectsof Storm Water Detention Basins. In: Proc. National Symposiumon Urban Hydrology, Hydraulics, and Sediment Control. Univer-sity of Kentucky, Lexington, Kentucky, pp. 455-460.

Whipple, W., N. S. Grigg, T. Grizzard, C. W. Randall, R. P. Shubinski,and L. S. Tucker, 1983. Stormwater Management in UrbanizingAreas. Prentice-Hall, Inc., Englewood Cliffs, New Jersey.


Nix and Tsay

needs. For example, if post-peak flows need to be lower inorder to abate potential downstream flooding problems thenperhaps the use of off-line storage with fractional diversionwould help. However, this configuration causes longer periodsof low discharge that for other reasons may not be accept-able. There are always trade-offs, it is simply a matter ofmatching system responses to the most important storm-water management needs. Also important is the ability ofoff-line storage (especially with flow level diversion) to re-duce the storage capacity needed to meet a specific peak dis-charge target. The reduction may be substantial and shouldnot be lightly dismissed.

certainly warranted. Nevertheless, the flexibility demon-strated here should encourage the use of more innovativedesigns that allow detention systems to meet local andregional needs.

3 II 2 13

CONCLUSIONS

It has been shown that off-line detention systems have thepotential for reducing peak discharges with lower storagerequirements and for tailoring the recombined hydrographs tomeet specific stormwater management goals. However, thestudy was simply a simulation exercise and further study is

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ALTERNATIVE STRATEGIES FOR STORMWATER DETENTION