The Extention Basin as a Storm Water Control Device

By: Ralph G. Mastromonaco, P.E., President

Ralph G. Mastromonaco, P.E., P.C.
Consulting Engineers
Croton-on-Hudson, NY 10520

August 31, 2000

Abstract
This paper proposes an integrated approach to storm water management and storm water treatment. Today’s requirements for capturing and treating the first-flush of storm water can be met with a new device that also controls peak flows over a wide range of storms and uses a net storage volume that is substantially lower than the storage computed by traditional reservoir routing methods.

The Extention basin debuts here as the most efficient method of reducing peak storm water flows—being far more effective than the retention or detention basins in common use today.

The most effective, and possibly the only device for simply reducing or controlling storm water peak flow, is the storage basin¾ commonly known as a retention or detention basin. The term detention basin has come to be distinguished from a retention basin in that the latter is a storage device that has a normal pool of water such as a lake, pond or reservoir, while the detention basin is considered dedicated to its task and is normally empty. Both of these operate by the natural accumulation of storm water when a restriction, such as a weir or orifice, is placed on the flow.

These storage basins are typically used to mitigate storm water increases due to land development and are very effective when designed properly. For example, in a small watershed of 5 acres, for a shopping center that converts an existing wooded site to a land use consisting of pavement, the peak storm water flows can rise from 10 cfs to 20 cfs rather easily. In larger watersheds, proportional increases such as these could cause serious flooding and environmental damage.

The key criterion in storm water management is the limitation of after-development peak flows to rates equal to or less than the peak flows prior to development. In the example above, the developer of the shopping center would need to provide a storage basin to limit the after-development peak flows to 10 cfs. The developer may then need to provide substantial water quality treatment storage. Of course, the storage basin would occupy a significant portion of the site, typically ranging from five (5) to fifteen (15) per cent or more of the development land area.

Many state and local municipalities normally require either control of storm water through written codes or insist on peak flow controls during the approval process. Whether or not storm water control is required, it is usually prudent to control storm water flows that are destined for off-site areas, merely to reduce the liability for damages in case of downstream flooding.

The treatment of storm water to improve water quality has gained considerable interest. Federal and state regulations now require storm water treatment for large sites and new Federal NPDES rules will require treatment from small sites. Further, some local municipal codes or environmental concerns mandate some form of storm water treatment for all sites.

A key criterion of storm water treatment is the capture of the first one-half (1/2) inch of runoff from newly disturbed areas within the watershed. The great majority of pollutants from runoff are contained in the first-flush. To treat the first-flush, the flows must be conveyed to specially designed water quality treatment basins where a variety of treatment processes take place, culminating with infiltration to the soil and/or evaporation. The water quality basins are designed particularly to capture only the first-flush of runoff, and to avoid the later segments of the runoff that would mix with and wash out the captured flow.

Our firm developed a simple design for a first-flush control device in 1990 that we have been using since on various engineering projects. Essentially, the control works on a hydraulic balancing principle – diverting the low flows to a water quality basin and then permitting direct conveyance back to the drainage system or local stream when the water quality basin is full. The water quality basin is designed to store water for just a few days since an empty basin is necessary at the time of rainfall to fulfill the goal of water quality treatment.

The method of computation used to design storm water storage systems is the straightforward and familiar application of conservation of mass principles—the volume flowing out is equal to the volume flowing into a system. This is known as the reservoir routing method, and a wide range of information is available on the subject in engineering and hydrology texts. A brief summation of the method is given here, as follows:

It is assumed for the numerical solution, that we are given the flow "Q" at every time interval "t", being the series, Qin(t).

Given: Vol (out) = Vol (in):

If a volume is allowed to accumulate (S), the modified mass equation accounts for this as follows:

Vol (out) = Vol (in) – S

In a time interval t: Vol(out)/Δt = Vol(in)/Δt – ΔS/Δt

Since: Vol(out)/Δt = Qout(t)

And, since: Vol(in)/Δt = Qin(t) and ΔS = S(t)

Substituting Qout(t) = Qin(t) – ΔS/Δt

Rearranging: S(t) = (Qin(t) - Qout(t)) x Δt (Eq. 1)

As described in words, the change in volume of storage within any time interval is equal to the rate of inflow in minus the rate of outflow, multiplied by the interval of time.

The outflow of a storage basin can be modeled by a non-linear hydraulic function, "g" relating head, or height (stage) "H" in the basin, and various physical characteristics of the control device; e.g., length of a weir or diameter of a pipe, referred to as the set "n", and generally a constant "C".

For example: Qout = C x g(n, H) (Eq. 2)

If the outflow of a storm water storage basin is restricted by a weir, the outflow function is as follows:

Q=C x L x H^3/2 or Qout(t)=C x L x H(t)^3/2

Where: C is a factor (3.337)

L is the weir length (ft)

H is the flood stage in the basin in feet and H(t) is the height at any time

Further, there is a natural geometric relationship, or function "f" between height "H" and the volume "S" in the storage basin. This is often a tabular relationship between contour elevation and surface area that can readily be interpolated for storage volume at any height.

For example: H =f(S) or H(t)=f(S(t)) (Eq. 3)

Equations 1, 2 and 3, above fully define the mathematics of the storage process that occurs in a detention or retention basin. The equations are easily solved by iterative techniques. The mathematical method is generally referred to by the generic term, reservoir routing, and it describes a relationship between inflow and outflow that can be seen graphically below:

It is important to note that the area between the inflow and outflow hydrograph is the exact equivalent of the storage volume reached in the storm water basin. Further, in the descending phase of the inflow, the area representing the outflow volume leaving the storage system is the same as the inflow volume, unless some volume is captured within the system.

The hydrographs in Figure 1 represent the flow in and out of a typical storage basin whose flow paths are represented by the figure below:

To absolutely minimize the amount of storage volume needed, one must allow the outflow hydrograph to closely track the rise in the inflow hydrograph until a pre-determined flow is reached. In theory, the most efficient storage basin—one with the least storage for the same flow reduction, is one whose outflow follows this non-continuous route, as shown below:

Such an outflow function is difficult to replicate using standard reservoir routing, though it can be provided by using mechanical intervention. For example, to restrict outflows to, say 100 cfs, an operator can be stationed at a valve in the system. The operator would know when to open the valve and divert flows away or towards the design point.

This mechanical system is not acceptable in practice for a variety of reasons, least of which is the reliance on mechanical means in perpetuity as well as the monitoring of rainfall and runoff rates. Clearly, a fully non-mechanical method of performing the same task is our goal.

The Extention basin provides such an automatic function. It operates hydraulically and non-mechanically, by allowing the storm flow to bypass the storage basin during the ascending part of the storm then diverts flow into the storage basin only during the period of peak inflow. The Extention basin provides flow reductions through external control structures and external piping, and extends the functionality of the storage basin by adding water quality treatment, hence the given name.

A flow schematic of a simple Extention basin operation follows:

A simple Extention basin will control peak flows over a narrow range of storm frequencies. The following is a narrative of the operation and components of the simple Extention basin.

A water quality feature is added to the flow path by simply permitting the first low flows, up to the volume of inflow equal to the first-flush, to enter the water quality basin. When the desired level in the water quality basin is reached, further flow is inhibited due to the backwater effect from the developed head in the water quality basin. Hence, the operation is similar to the simple Extention basin noted above, except additional storage is added for water quality treatment.

An advanced layout places an additional control structure on the low flow bypass, as illustrated below:

A numerical proof of the improved operation of the Extention basin can be provided based on the earlier equations. However, a practical proof is more easily provided by modeling the Extention basin using a variety of sample cases and computing the results using readily available software.

While there are a number of software products that can be used to model the flows through the Extention basin system, we have used the Army Corps of Engineers HEC-1 program here. The HEC-1 software allows a number of the necessary and detailed techniques.

For example, the development of separate hydrographs is needed for the low flow bypass and the inflow to the storage basin. HEC-1 can create these hydrographs using the diversion card. Further, in the plan with storm water treatment, the diversion cards can also be used to track the filling of the water quality basin and the subsequent re-diversion to the low flow bypass.

Of course, HEC-1 provides hydrograph creation based on watershed characteristics of curve number, lag time and area, as well as hydrograph summation and basic graphing functions.

Since the design of the Extention basin is most practical by trial and error or iteration, we have developed a new Windows ™ interface to HEC-1 that greatly improves the program functionality and allows numerous trial runs to fine-tune the proposed hydraulic system design. It is necessary to adjust the diversion ratios, internal control structure dimensions and storage basin until the desired final design flows are met.

To test our theory that the Extention basin requires minimal storage while providing the required capture of the first-flush runoff, we have created a sample watershed system that undergoes development.

We assume the watershed is mildly developed in the present state with a composite SCS runoff curve number of 70.75.

We further assume that a large, new development site of about 0.20 square mile (125 acres) is contemplated, which would convert a portion of the wooded land use to essentially, all impervious areas, resulting in a new, composite curve number of 73.75.

The breakdown of existing and proposed land uses that comprise the SCS curve number is shown in Table A below:

Since the base criterion for storm water treatment is the capture of the first-flush of runoff from the newly disturbed area, the volume of capture is computed to be 5.33 acre feet from 0.200 square miles, (0.5"/12 x 0.200 x 640 ac/sm).

The first-flush flow does not directly re-enter the drainage system¾ it is infiltrated to the soil, evaporated, or slowly drained back to the drainage system over a period of days at rates well below design storm frequencies.

First-flush capture for storm water treatment is generally additive to any storage required by the peak flow control system. In other words, if 9 acre-feet are required for storm water management, one must add the additional 5.33 acre-feet regardless of the method of storm water storage. It is ordinarily difficult and sometimes impossible to use the storage required for water quality to offset the storage required for peak flow control, because the first-flush volume accumulates well before the time of peak runoff. In some limited applications, it is possible to offset the storage required for peak flow reduction in very small storms, when runoff is near to one-half (½) inch.

We seek a solution where the storage required for water quality can be credited fully in the process of storm water management and peak flow reductions.

For simplicity, we have assumed that the watershed lag is 1.0 hour. This is certainly in the order of magnitude of the watershed size of 1 square mile. In general, the analysis herein can be done with any assumed value of lag. To simplify comparisons, we further assume that the lag time remains the same in both the existing and proposed case, and is possible when the new development is not on any conceivable flow path where lag time might be measured. If a new situation develops where the lag changes in the proposed condition, adjustment to the model can be made, easily.

For simplicity, we have chosen 4.0 inches of rainfall as the design storm. This is a mid-range value since design storms range from 3.2 inches up to 7.2 inches, depending on the application. The analysis herein can be run with any design storm, but to be consistent the same rainfall is assumed in both the existing and proposed condition.

The rainfall distribution is assumed the SCS 24 hour, with Type 3 rainfall distribution and Type 2 antecedent moisture conditions. We have provided the synthetic rainfall ordinates in the computer input card file based on values commonly in use in our area.

The control structures are necessary to either divert or retard flow. In the storage basin, they are composed of a low-level pipe or orifice, a mid-level spillway weir and a high level weir to control overtopping. All elevations used are relative, and it assumed the designer would use proper techniques to design individual components.

Diversion structures are devices that split flows according to certain, desired proportions. This is accomplished with weirs or notches that direct flows to different directions.

Each sample case assumes that the watershed flow must be reduced to 278 cfs for the design storm. This is the peak flow of the watershed at existing conditions. To compare methods, the storage volume necessary to produce this reduction is compiled for each case.

The following sample watershed characteristics are used to determine the inflow hydrographs.

This case assumes a watershed without development. It is provided to illustrate actual conditions in a typical situation, with nominal values that may be encountered by design engineers.

Based on the sample input data, the following are the results of the computations:

In this case, we model the peak flows after development, where flows are left uncontrolled. The change in development is modeled by simply increasing the SCS runoff curve number of the undeveloped case, based on the addition of 125 acres of impervious area in the watershed. The remaining watershed characteristics are assumed to be unchanged by the development.

In this case, the after development flows are routed through a conventional detention basin system using reservoir routing techniques. The characteristics of the detention basin are as follows:

In this case, we attempt to control peak flows and provide the required water quality storage volume. The water quality basin is fed by a diversion of the main watershed flow until the value of 5.33 acre-feet is reached, thereafter, the remaining flow is detained in a conventional storage basin.

The flow path of this case is illustrated below:

In this case, the after development flows are routed through the simple Extention basin system with a portion of the flow diverted to a water quality basin. The diversions are set according to the following relationships:

The volume characteristics of the storage basin are as follows:

In sample case 2A, we used a conventional detention basin computation that brought the peak flow from 328 cfs to 278 cfs and required 17 acre-feet of storage. In contrast, sample cases 3 and 4 provide clear proof that the simple Extention basin can provide the same reduction in peak flows with about one-half the storage (9.0 acre-feet).

In a variation of Sample Case 2, Sample Case 2B ads 5.33 acre-feet of water quality storage to the required peak flow storage requirement of 16 acre-feet, totaling 21.33 acre-feet. This variation in Case 2 was provided here, to assess if simply adding first-flush storage alone is effective in reducing peak flows. The results indicate it was only slightly effective, reducing the net required storage by about 5 per cent (22.33 to 21.33 ac-ft). For comparison purposes, the simple Extention basin in Case 3 required only 9 acre-feet of storage plus the required 5.33 acre-feet, for 14.33 acre-feet, total.

This remarkable result is evident graphically (Fig. 9)—the outflow hydrograph follows the rising limb of the inflow hydrograph and the need for storage is minimized accordingly.

However, the practical need of storm water management is to control flows over a range of storms, say, from the 2 year to the 100-year storm event. The simple Extention basin would not be able to control flows much lower than its design because its inherent bypass system allows low flows out to the design point without control. It is, however, the most effective system to control a small, well-defined range of storm frequencies.

Given the need to capture the first flush, and remembering that the first-flush capture basin is really only effective in reducing peak flows when the main flows are small, we can integrate the storm water control and water quality control in our highly effective, Extention basin. This is illustrated in Sample Case 4, below:

In our final Sample Case 4, a water quality basin is added to the Extention basin system and we attempt to control a wide range of storm frequencies. Flows are diverted to the water quality basin until the pre-computed first-flush volume of ½ inch of runoff over the newly developed portion of the watershed, is reached.

A portion of the flow is conveyed to the water quality basin by imposing a new diversion structure on the low flow bypass of the simple Extention basin. The lowest flows are directed to the water quality basin, thereafter, when the basin is full, flows are naturally re-directed to the final design point by the principle of hydraulic balancing.

Our sample case requires that 5.33 acre-feet of first-flush runoff be stored in the water quality basin. This value is placed in field 2 of the DT input card file of our HEC-1 model.

Most importantly, this case examines a range of flows from 1.84 inches of rainfall, to 4.0 inches of rainfall. This is accomplished in HEC-1 by creating 6 plans as evidenced by the JR multiratio card. The ratios of each plan range from 0.46 to 1.00 and operate in HEC-1 by re-computing the entire model for each ratio times the design rainfall of 4.0 inches on the PB card.

For Case 4, we have assumed that the 100-year storm is 4.0 inches of rainfall in 24 hours, and have provided rainfalls for the 2, 5, 10, 25 and 50-year storms by the multiratio plans. In fact, 100-year storms are closer to 7 inches of rainfall in the northeast; however, we use the lower value to maintain consistency with our goal of using mid range flows whenever possible in the sample cases. Any reasonable value of rainfall can be used to compare the effectiveness of the Extention basin to the detention basin since the computations are always relative.

The following are the steps in the final computation over a range of flows:

It is clear from the summary Table 4-E, that the Extention basin system has reduced peak flows to almost match the original flows, and more importantly, it has done this over a wide range of flows.

For example, the 100-year storm runoff is 278 cfs both in the existing and proposed cases, even though the development in the watershed has increased to flows 328 cfs. The 2-year storm has been reduced from the proposed flow of 42 to 24 cfs—slightly below the existing peak flow of 27 cfs.

The graph of the results of the existing flows as compared to the final flows is indicated below:

A close-up comparison of the final results along with the proposed, after-development inflows for the 100-year storm is shown on the graph below:

It is immediately apparent from the graphs that the Extention basin accomplishes an additional task since the time of peak flow is nearly, if not, identical. In Figure 11, the initial, existing hydrograph is nearly identical to the Extention basin outflows.

The reduction of outflow lag is an added, environmental benefit of the Extention basin since any natural drainage system is less likely to be affected by the change in timing. Further, we have eliminated unknown flooding affects associated with timing of peak flows from other watersheds.

Each sample case performed the task of reducing the after development peak flow from 328 cfs to the design peak flow of 278 cfs using a storage basin. The conventional storage basin system using standard reservoir routing techniques computed the storage at 17 acre-feet (16 acre-feet for case 2B), to these values we must add 5.33 acre feet required for first-flush storage.

The Extention basin performed very much better, requiring only 9 acre feet of storage to control peak flows and 5.33 acre feet for storm water treatment for a total storage of 14.33 acre-feet.

The Table below summarizes the storage required for each sample case.

The Extention basin provides the control of peak flows using less storage than a conventional retention or detention basin. This phenomenon occurs because we have found a method to "tune" the system to minimize the storage requirement.

The Extention basin described in our sample case requires only about 67% of the storage of a conventional storage basin where water quality treatment is also required (Case 3 vs. Case 2-B), and controls flows over a very wide range of storm frequencies.

Similarly, when control is required over only a small range of storm frequencies and water quality treatment is not needed, the simple Extention basin requires only about 50% of the storage of a conventional storage basin (Case 2A - 100, 50, 25 year storm).

When the capture of the first-flush of storm water is required for water quality treatment and control of peak flows is required over a wide range of storm frequencies, the storage volume can be minimized by the use of an Extention basin that uses storage volumes close to the theoretical minimum storage volume (Case 4).

The technique for computing these detailed volumes is straightforward—and can be computed by trial and error methods. Since the expected savings of up to 50% in storage is so great, the additional design time required to fine-tune the computations using successive iteration is well worth the effort.

The following pages include shortened printouts of the HEC-1 computer program for each of the Sample cases. The printouts have been edited to reduce blank lines, headers, and repetitive or non-essential matter that accompany the HEC-1 program output.

The input cards have not been edited, therefore, the sample input data can be used independently to test or reproduce these results.

HEC-1 INPUT PAGE 1

LINE ID.......1.......2.......3.......4.......5.......6.......7.......8.......9......10

1 ID RALPH G. MASTROMONACO, P.E., P.C. - Extention BASIN RESEARCH

2 ID CASE 1A + CASE 1B - EXISTING AND PROPOSED CONDITION NO CONTROL

3 IO 5 5

*DIAGRAM

4 IT 10 200 2000

5 IN 6 000

6 KK BEFORE

7 KM BEFORE DEVELOPMENT

8 KO 5 21

9 PB 4

10 PC .00100 0.00200 0.00300 0.00400 0.00500 0.00600 0.00700 0.00800 0.00900 0.01000

11 PC .01100 0.01200 0.01300 0.01400 0.01500 0.01600 0.01700 0.01800 0.01900 0.02000

12 PC .02101 0.02203 0.02307 0.02412 0.02519 0.02627 0.02737 0.02848 0.02961 0.03075

13 PC .03191 0.03308 0.03427 0.03547 0.03669 0.03792 0.03917 0.04043 0.04171 0.04300

14 PC .04431 0.04563 0.04697 0.04832 0.04969 0.05107 0.05247 0.05388 0.05531 0.05675

15 PC .05821 0.05968 0.06117 0.06267 0.06419 0.06572 0.06727 0.06883 0.07041 0.07200

16 PC .07363 0.07530 0.07703 0.07880 0.08063 0.08250 0.08443 0.08640 0.08843 0.09050

17 PC .09263 0.09480 0.09703 0.09930 0.10163 0.10400 0.10643 0.10890 0.11143 0.11400

18 PC .11666 0.11943 0.12232 0.12532 0.12844 0.13167 0.13502 0.13846 0.14206 0.14575

19 PC .14956 0.15348 0.15752 0.16167 0.16594 0.17032 0.17482 0.17943 0.18416 0.18900

20 PC .19402 0.19928 0.20478 0.21052 0.21650 0.22272 0.22918 0.23588 0.24282 0.25000

21 PC .25776 0.26644 0.27604 0.28656 0.29800 0.31430 0.33940 0.37330 0.41600 0.50000

22 PC .58400 0.62670 0.66060 0.68570 0.70200 0.71344 0.72396 0.73356 0.74224 0.75000

23 PC .75718 0.76412 0.77082 0.77728 0.78350 0.78948 0.79522 0.80072 0.80598 0.81100

24 PC .81584 0.82057 0.82518 0.82968 0.83406 0.83833 0.84248 0.84652 0.85044 0.85425

25 PC .85794 0.86152 0.86498 0.86833 0.87156 0.87468 0.87768 0.88057 0.88334 0.88600

26 PC .88858 0.89110 0.89358 0.89600 0.89838 0.90070 0.90298 0.90520 0.90738 0.90950

27 PC .91158 0.91360 0.91558 0.91750 0.91938 0.92120 0.92298 0.92470 0.92638 0.92800

28 PC .92959 0.93117 0.93273 0.93428 0.93581 0.93733 0.93883 0.94032 0.94179 0.94325

29 PC .94469 0.94612 0.94753 0.94893 0.95031 0.95168 0.95303 0.95437 0.95569 0.95700

30 PC .95829 0.95958 0.96085 0.96211 0.96336 0.96460 0.96582 0.96704 0.96824 0.96944

31 PC .97062 0.97179 0.97295 0.97410 0.97523 0.97636 0.97747 0.97858 0.97967 0.98075

32 PC .98182 0.98288 0.98392 0.98496 0.98598 0.98700 0.98800 0.98899 0.98997 0.99094

33 PC .99189 0.99284 0.99377 0.99470 0.99561 0.99651 0.99740 0.99828 0.99914 1.00000

34 BA 1

35 LS 70.75

36 UD 1

37 KK AFTER

38 KM AFTER DEVELOPMENT

39 BA 1

40 LS 73.75

41 UD 1

42 ZZ

SCHEMATIC DIAGRAM OF STREAM NETWORK

INPUT

LINE (V) ROUTING (--->) DIVERSION OR PUMP FLOW

NO. (.) CONNECTOR (<---) RETURN OF DIVERTED OR PUMPED FLOW

6 BEFORE

.

.

37 . AFTER

RALPH G. MASTROMONACO, P.E., P.C. - Extention BASIN RESEARCH

CASE 1A + CASE 1B - EXISTING AND PROPOSED CONDITION NO CONTROL

RUNOFF SUMMARY

FLOW IN CUBIC FEET PER SECOND

TIME IN HOURS, AREA IN SQUARE MILES

PEAK TIME OF AVERAGE FLOW FOR MAXIMUM PERIOD BASIN MAXIMUM TIME OF

OPERATION STATION FLOW PEAK AREA STAGE MAX STAGE

+ 6-HOUR 24-HOUR 72-HOUR

*** NORMAL END OF HEC-1 ***

HEC-1 INPUT PAGE 1

LINE ID.......1.......2.......3.......4.......5.......6.......7.......8.......9......10

1 ID RALPH G. MASTROMONACO, P.E., P.C. - Extention BASIN RESEARCH

2 ID CASE 2 - AFTER DEVELOPMENT - CONVENTIONAL DETENTION BASIN

3 IO 1 2

*DIAGRAM

4 IT 10 200 2000

5 IN 6 000

6 KK MAIN

7 KO 5 5 21

8 KM WATERSHED 1

9 PB 4

10 PC .00100 0.00200 0.00300 0.00400 0.00500 0.00600 0.00700 0.00800 0.00900 0.01000

11 PC .01100 0.01200 0.01300 0.01400 0.01500 0.01600 0.01700 0.01800 0.01900 0.02000

12 PC .02101 0.02203 0.02307 0.02412 0.02519 0.02627 0.02737 0.02848 0.02961 0.03075

13 PC .03191 0.03308 0.03427 0.03547 0.03669 0.03792 0.03917 0.04043 0.04171 0.04300

14 PC .04431 0.04563 0.04697 0.04832 0.04969 0.05107 0.05247 0.05388 0.05531 0.05675

15 PC .05821 0.05968 0.06117 0.06267 0.06419 0.06572 0.06727 0.06883 0.07041 0.07200

16 PC .07363 0.07530 0.07703 0.07880 0.08063 0.08250 0.08443 0.08640 0.08843 0.09050

17 PC .09263 0.09480 0.09703 0.09930 0.10163 0.10400 0.10643 0.10890 0.11143 0.11400

18 PC .11666 0.11943 0.12232 0.12532 0.12844 0.13167 0.13502 0.13846 0.14206 0.14575

19 PC .14956 0.15348 0.15752 0.16167 0.16594 0.17032 0.17482 0.17943 0.18416 0.18900

20 PC .19402 0.19928 0.20478 0.21052 0.21650 0.22272 0.22918 0.23588 0.24282 0.25000

21 PC .25776 0.26644 0.27604 0.28656 0.29800 0.31430 0.33940 0.37330 0.41600 0.50000

22 PC .58400 0.62670 0.66060 0.68570 0.70200 0.71344 0.72396 0.73356 0.74224 0.75000

23 PC .75718 0.76412 0.77082 0.77728 0.78350 0.78948 0.79522 0.80072 0.80598 0.81100

24 PC .81584 0.82057 0.82518 0.82968 0.83406 0.83833 0.84248 0.84652 0.85044 0.85425

25 PC .85794 0.86152 0.86498 0.86833 0.87156 0.87468 0.87768 0.88057 0.88334 0.88600

26 PC .88858 0.89110 0.89358 0.89600 0.89838 0.90070 0.90298 0.90520 0.90738 0.90950

27 PC .91158 0.91360 0.91558 0.91750 0.91938 0.92120 0.92298 0.92470 0.92638 0.92800

28 PC .92959 0.93117 0.93273 0.93428 0.93581 0.93733 0.93883 0.94032 0.94179 0.94325

29 PC .94469 0.94612 0.94753 0.94893 0.95031 0.95168 0.95303 0.95437 0.95569 0.95700

30 PC .95829 0.95958 0.96085 0.96211 0.96336 0.96460 0.96582 0.96704 0.96824 0.96944

31 PC .97062 0.97179 0.97295 0.97410 0.97523 0.97636 0.97747 0.97858 0.97967 0.98075

32 PC .98182 0.98288 0.98392 0.98496 0.98598 0.98700 0.98800 0.98899 0.98997 0.99094

33 PC .99189 0.99284 0.99377 0.99470 0.99561 0.99651 0.99740 0.99828 0.99914 1.00000

34 BA 1

35 LS 73.75

36 UD 1

37 KK OUTFLW

38 KO 5 5 21

39 RS 1 ELEV 340

40 SA 0 0.83 2.17 2.45 2.74 3.04 3.36

41 SE 340 342 344 346 348 350 352

42 SL 340 2.4 .61 .5

43 SS 346.5 14.5 3.337 1.5

44 ST 351 10 3.1 1.5

45 ZZ

SCHEMATIC DIAGRAM OF STREAM NETWORK

INPUT

LINE (V) ROUTING (--->) DIVERSION OR PUMP FLOW

NO. (.) CONNECTOR (<---) RETURN OF DIVERTED OR PUMPED FLOW

6 MAIN

V

V

37 OUTFLW

(***) RUNOFF ALSO COMPUTED AT THIS LOCATION

RUNOFF SUMMARY

PEAK TIME OF AVERAGE FLOW FOR MAXIMUM PERIOD BASIN MAXIMUM TIME OF

OPERATION STATION FLOW PEAK AREA STAGE MAX STAGE

+ 6-HOUR 24-HOUR 72-HOUR

HYDROGRAPH AT

+ MAIN 328. 13.00 137. 42. 31. 1.00

ROUTED TO

+ OUTFLW 278. 13.50 119. 43. 31. 1.00

+ 349.43 13.50

1 SUMMARY OF DAM OVERTOPPING/BREACH ANALYSIS FOR STATION OUTFLW

(PEAKS SHOWN ARE FOR INTERNAL TIME STEP USED DURING BREACH FORMATION)

PLAN 1 ............... INITIAL VALUE SPILLWAY CREST TOP OF DAM

ELEVATION 340.00 346.50 351.00

STORAGE 0. 9. 22.

OUTFLOW 0. 30. 501.

RATIO MAXIMUM MAXIMUM MAXIMUM MAXIMUM DURATION TIME OF TIME OF

OF RESERVOIR DEPTH STORAGE OUTFLOW OVER TOP MAX OUTFLOW FAILURE

PMF W.S.ELEV OVER DAM AC-FT CFS HOURS HOURS HOURS

*** NORMAL END OF HEC-1 ***

HEC-1 INPUT PAGE 1

LINE ID.......1.......2.......3.......4.......5.......6.......7.......8.......9......10

1 ID RALPH G. MASTROMONACO, P.E., P.C. - Extention BASIN RESEARCH

2 ID CASE 2 AFTER DEVELOPMENT - CONVENTIONAL DETENTION BASIN

3 IO 1 2

*DIAGRAM

4 IT 10 200 2000

5 IN 6 000

6 JR PREC 0.46 0.60 0.69 0.79 0.88 1.00

7 KK MAIN

8 KO 5 5 21

9 KM WATERSHED 1

10 PB 4

11 PC .00100 0.00200 0.00300 0.00400 0.00500 0.00600 0.00700 0.00800 0.00900 0.01000

12 PC .01100 0.01200 0.01300 0.01400 0.01500 0.01600 0.01700 0.01800 0.01900 0.02000

13 PC .02101 0.02203 0.02307 0.02412 0.02519 0.02627 0.02737 0.02848 0.02961 0.03075

14 PC .03191 0.03308 0.03427 0.03547 0.03669 0.03792 0.03917 0.04043 0.04171 0.04300

15 PC .04431 0.04563 0.04697 0.04832 0.04969 0.05107 0.05247 0.05388 0.05531 0.05675

16 PC .05821 0.05968 0.06117 0.06267 0.06419 0.06572 0.06727 0.06883 0.07041 0.07200

17 PC .07363 0.07530 0.07703 0.07880 0.08063 0.08250 0.08443 0.08640 0.08843 0.09050

18 PC .09263 0.09480 0.09703 0.09930 0.10163 0.10400 0.10643 0.10890 0.11143 0.11400

19 PC .11666 0.11943 0.12232 0.12532 0.12844 0.13167 0.13502 0.13846 0.14206 0.14575

20 PC .14956 0.15348 0.15752 0.16167 0.16594 0.17032 0.17482 0.17943 0.18416 0.18900

21 PC .19402 0.19928 0.20478 0.21052 0.21650 0.22272 0.22918 0.23588 0.24282 0.25000

22 PC .25776 0.26644 0.27604 0.28656 0.29800 0.31430 0.33940 0.37330 0.41600 0.50000

23 PC .58400 0.62670 0.66060 0.68570 0.70200 0.71344 0.72396 0.73356 0.74224 0.75000

24 PC .75718 0.76412 0.77082 0.77728 0.78350 0.78948 0.79522 0.80072 0.80598 0.81100

25 PC .81584 0.82057 0.82518 0.82968 0.83406 0.83833 0.84248 0.84652 0.85044 0.85425

26 PC .85794 0.86152 0.86498 0.86833 0.87156 0.87468 0.87768 0.88057 0.88334 0.88600

27 PC .88858 0.89110 0.89358 0.89600 0.89838 0.90070 0.90298 0.90520 0.90738 0.90950

28 PC .91158 0.91360 0.91558 0.91750 0.91938 0.92120 0.92298 0.92470 0.92638 0.92800

29 PC .92959 0.93117 0.93273 0.93428 0.93581 0.93733 0.93883 0.94032 0.94179 0.94325

30 PC .94469 0.94612 0.94753 0.94893 0.95031 0.95168 0.95303 0.95437 0.95569 0.95700

31 PC .95829 0.95958 0.96085 0.96211 0.96336 0.96460 0.96582 0.96704 0.96824 0.96944

32 PC .97062 0.97179 0.97295 0.97410 0.97523 0.97636 0.97747 0.97858 0.97967 0.98075

33 PC .98182 0.98288 0.98392 0.98496 0.98598 0.98700 0.98800 0.98899 0.98997 0.99094

34 PC .99189 0.99284 0.99377 0.99470 0.99561 0.99651 0.99740 0.99828 0.99914 1.00000

35 BA 1

36 LS 73.75

37 UD 1

38 KK OUTFLW

39 KO 5 5 21

40 RS 1 ELEV 340

41 SA 0 0.83 2.17 2.45 2.74 3.04 3.36

42 SE 340 342 344 346 348 350 352

43 SL 340 2.4 .61 .5

44 SS 346.5 14.5 3.337 1.5

45 ST 351 10 3.1 1.5

46 ZZ

SCHEMATIC DIAGRAM OF STREAM NETWORK

INPUT

LINE (V) ROUTING (--->) DIVERSION OR PUMP FLOW

NO. (.) CONNECTOR (<---) RETURN OF DIVERTED OR PUMPED FLOW

7 MAIN

V

V

38 OUTFLW

(***) RUNOFF ALSO COMPUTED AT THIS LOCATION

RATIOS APPLIED TO PRECIPITATION

OPERATION STATION AREA PLAN RATIO 1 RATIO 2 RATIO 3 RATIO 4 RATIO 5 RATIO 6

0.46 0.60 0.69 0.79 0.88 1.00

HYDROGRAPH AT

1 SUMMARY OF DAM OVERTOPPING/BREACH ANALYSIS FOR STATION OUTFLW

(PEAKS SHOWN ARE FOR INTERNAL TIME STEP USED DURING BREACH FORMATION)

PLAN 1 ............... INITIAL VALUE SPILLWAY CREST TOP OF DAM

ELEVATION 340.00 346.50 351.00

STORAGE 0. 9. 22.

OUTFLOW 0. 30. 501.

RATIO MAXIMUM MAXIMUM MAXIMUM MAXIMUM DURATION TIME OF TIME OF

OF RESERVOIR DEPTH STORAGE OUTFLOW OVER TOP MAX OUTFLOW FAILURE

PMF W.S.ELEV OVER DAM AC-FT CFS HOURS HOURS HOURS

0.46 343.57 0.00 3. 22. 0.00 15.00 0.00

0.60 346.73 0.00 10. 36. 0.00 15.50 0.00

0.69 347.50 0.00 12. 80. 0.00 14.33 0.00

0.79 348.18 0.00 14. 139. 0.00 13.83 0.00

0.88 348.74 0.00 15. 197. 0.00 13.67 0.00

1.00 349.43 0.00 17. 278. 0.00 13.50 0.00

*** NORMAL END OF HEC-1 ***

HEC-1 INPUT PAGE 1

LINE ID.......1.......2.......3.......4.......5.......6.......7.......8.......9......10

1 ID RALPH G. MASTROMONACO, P.E., P.C. - Extention BASIN RESEARCH

2 ID CASE 3 - SIMPLE Extention BASIN CONTROL SYSTEM - AFTER DEVELOPMENT

3 IO 5 5

*DIAGRAM

4 IT 10 200 2000

5 IN 6 000

6 JR PREC 0.46 0.60 0.69 0.79 0.88 1.00

7 KK MAIN

8 KO 5 5 21

9 KM WATERSHED 1

10 PB 4

11 PC .00100 0.00200 0.00300 0.00400 0.00500 0.00600 0.00700 0.00800 0.00900 0.01000

12 PC .01100 0.01200 0.01300 0.01400 0.01500 0.01600 0.01700 0.01800 0.01900 0.02000

13 PC .02101 0.02203 0.02307 0.02412 0.02519 0.02627 0.02737 0.02848 0.02961 0.03075

14 PC .03191 0.03308 0.03427 0.03547 0.03669 0.03792 0.03917 0.04043 0.04171 0.04300

15 PC .04431 0.04563 0.04697 0.04832 0.04969 0.05107 0.05247 0.05388 0.05531 0.05675

16 PC .05821 0.05968 0.06117 0.06267 0.06419 0.06572 0.06727 0.06883 0.07041 0.07200

17 PC .07363 0.07530 0.07703 0.07880 0.08063 0.08250 0.08443 0.08640 0.08843 0.09050

18 PC .09263 0.09480 0.09703 0.09930 0.10163 0.10400 0.10643 0.10890 0.11143 0.11400

19 PC .11666 0.11943 0.12232 0.12532 0.12844 0.13167 0.13502 0.13846 0.14206 0.14575

20 PC .14956 0.15348 0.15752 0.16167 0.16594 0.17032 0.17482 0.17943 0.18416 0.18900

21 PC .19402 0.19928 0.20478 0.21052 0.21650 0.22272 0.22918 0.23588 0.24282 0.25000

22 PC .25776 0.26644 0.27604 0.28656 0.29800 0.31430 0.33940 0.37330 0.41600 0.50000

23 PC .58400 0.62670 0.66060 0.68570 0.70200 0.71344 0.72396 0.73356 0.74224 0.75000

24 PC .75718 0.76412 0.77082 0.77728 0.78350 0.78948 0.79522 0.80072 0.80598 0.81100

25 PC .81584 0.82057 0.82518 0.82968 0.83406 0.83833 0.84248 0.84652 0.85044 0.85425

26 PC .85794 0.86152 0.86498 0.86833 0.87156 0.87468 0.87768 0.88057 0.88334 0.88600

27 PC .88858 0.89110 0.89358 0.89600 0.89838 0.90070 0.90298 0.90520 0.90738 0.90950

28 PC .91158 0.91360 0.91558 0.91750 0.91938 0.92120 0.92298 0.92470 0.92638 0.92800

29 PC .92959 0.93117 0.93273 0.93428 0.93581 0.93733 0.93883 0.94032 0.94179 0.94325

30 PC .94469 0.94612 0.94753 0.94893 0.95031 0.95168 0.95303 0.95437 0.95569 0.95700

31 PC .95829 0.95958 0.96085 0.96211 0.96336 0.96460 0.96582 0.96704 0.96824 0.96944

32 PC .97062 0.97179 0.97295 0.97410 0.97523 0.97636 0.97747 0.97858 0.97967 0.98075

33 PC .98182 0.98288 0.98392 0.98496 0.98598 0.98700 0.98800 0.98899 0.98997 0.99094

34 PC .99189 0.99284 0.99377 0.99470 0.99561 0.99651 0.99740 0.99828 0.99914 1.00000

35 BA 1

36 LS 73.75

37 UD 1

38 KK INFLOW

39 KM FLOWS TO STORAGE BASIN

40 KO 5 5 21

41 DT BYPASS

42 DI 0 10 20 50 80 100 180 300

43 DQ 0 10 20 40 55 65 120 230

44 KK OUTFLW

45 KO 5 5 21

46 RS 1 ELEV 340

47 SA 0 0.83 2.17 2.45 2.74 3.04 3.36

48 SE 340 342 344 346 348 350 352

49 SL 340 2.34 .61 .5

50 SS 348 10 3.337 1.5

51 ST 351 10 3.1 1.5

52 KK RETURN

53 KO 5 5 21

54 DR BYPASS

55 KK SUM

56 KO 5 5 21

57 HC 2

58 ZZ

SCHEMATIC DIAGRAM OF STREAM NETWORK

INPUT

LINE (V) ROUTING (--->) DIVERSION OR PUMP FLOW

NO. (.) CONNECTOR (<---) RETURN OF DIVERTED OR PUMPED FLOW

7 MAIN

.

.

41 .-------> BYPASS

38 INFLOW

V

V

44 OUTFLW

.

.

54 . .<------- BYPASS

52 . RETURN

. .

. .

55 SUM............

(***) RUNOFF ALSO COMPUTED AT THIS LOCATION

RATIOS APPLIED TO PRECIPITATION

OPERATION STATION AREA PLAN RATIO 1 RATIO 2 RATIO 3 RATIO 4 RATIO 5 RATIO 6

1 SUMMARY OF DAM OVERTOPPING/BREACH ANALYSIS FOR STATION OUTFLW

(PEAKS SHOWN ARE FOR INTERNAL TIME STEP USED DURING BREACH FORMATION)

PLAN 1 ............... INITIAL VALUE SPILLWAY CREST TOP OF DAM

ELEVATION 340.00 348.00 351.00

STORAGE 0. 13. 22.

OUTFLOW 0. 32. 211.

RATIO MAXIMUM MAXIMUM MAXIMUM MAXIMUM DURATION TIME OF TIME OF

OF RESERVOIR DEPTH STORAGE OUTFLOW OVER TOP MAX OUTFLOW FAILURE

PMF W.S.ELEV OVER DAM AC-FT CFS HOURS HOURS HOURS

0.46 340.40 0.00 0. 7. 0.00 13.33 0.00

0.60 342.74 0.00 1. 19. 0.00 14.00 0.00

0.69 343.79 0.00 3. 22. 0.00 14.33 0.00

0.79 344.75 0.00 5. 25. 0.00 14.83 0.00

0.88 345.41 0.00 7. 27. 0.00 15.17 0.00

1.00 346.19 0.00 9. 28. 0.00 15.50 0.00

*** NORMAL END OF HEC-1 ***

HEC-1 INPUT PAGE 1

LINE ID.......1.......2.......3.......4.......5.......6.......7.......8.......9......10

1 ID RALPH G. MASTROMONACO, P.E., P.C. - Extention BASIN RESEARCH

2 ID CASE 4 - Extention BASIN CONTROL SYSTEM - AFTER DEVELOPMENT

3 IO 5 5

*DIAGRAM

4 IT 10 200 2000

5 IN 6 000

6 JR PREC 0.46 0.60 0.69 0.79 0.88 1.00

7 KK MAIN

8 KO 5 5 21

9 KM WATERSHED 1

10 PB 4

11 PC .00100 0.00200 0.00300 0.00400 0.00500 0.00600 0.00700 0.00800 0.00900 0.01000

12 PC .01100 0.01200 0.01300 0.01400 0.01500 0.01600 0.01700 0.01800 0.01900 0.02000

13 PC .02101 0.02203 0.02307 0.02412 0.02519 0.02627 0.02737 0.02848 0.02961 0.03075

14 PC .03191 0.03308 0.03427 0.03547 0.03669 0.03792 0.03917 0.04043 0.04171 0.04300

15 PC .04431 0.04563 0.04697 0.04832 0.04969 0.05107 0.05247 0.05388 0.05531 0.05675

16 PC .05821 0.05968 0.06117 0.06267 0.06419 0.06572 0.06727 0.06883 0.07041 0.07200

17 PC .07363 0.07530 0.07703 0.07880 0.08063 0.08250 0.08443 0.08640 0.08843 0.09050

18 PC .09263 0.09480 0.09703 0.09930 0.10163 0.10400 0.10643 0.10890 0.11143 0.11400

19 PC .11666 0.11943 0.12232 0.12532 0.12844 0.13167 0.13502 0.13846 0.14206 0.14575

20 PC .14956 0.15348 0.15752 0.16167 0.16594 0.17032 0.17482 0.17943 0.18416 0.18900

21 PC .19402 0.19928 0.20478 0.21052 0.21650 0.22272 0.22918 0.23588 0.24282 0.25000

22 PC .25776 0.26644 0.27604 0.28656 0.29800 0.31430 0.33940 0.37330 0.41600 0.50000

23 PC .58400 0.62670 0.66060 0.68570 0.70200 0.71344 0.72396 0.73356 0.74224 0.75000

24 PC .75718 0.76412 0.77082 0.77728 0.78350 0.78948 0.79522 0.80072 0.80598 0.81100

25 PC .81584 0.82057 0.82518 0.82968 0.83406 0.83833 0.84248 0.84652 0.85044 0.85425

26 PC .85794 0.86152 0.86498 0.86833 0.87156 0.87468 0.87768 0.88057 0.88334 0.88600

27 PC .88858 0.89110 0.89358 0.89600 0.89838 0.90070 0.90298 0.90520 0.90738 0.90950

28 PC .91158 0.91360 0.91558 0.91750 0.91938 0.92120 0.92298 0.92470 0.92638 0.92800

29 PC .92959 0.93117 0.93273 0.93428 0.93581 0.93733 0.93883 0.94032 0.94179 0.94325

30 PC .94469 0.94612 0.94753 0.94893 0.95031 0.95168 0.95303 0.95437 0.95569 0.95700

31 PC .95829 0.95958 0.96085 0.96211 0.96336 0.96460 0.96582 0.96704 0.96824 0.96944

32 PC .97062 0.97179 0.97295 0.97410 0.97523 0.97636 0.97747 0.97858 0.97967 0.98075

33 PC .98182 0.98288 0.98392 0.98496 0.98598 0.98700 0.98800 0.98899 0.98997 0.99094

34 PC .99189 0.99284 0.99377 0.99470 0.99561 0.99651 0.99740 0.99828 0.99914 1.00000

35 BA 1

36 LS 73.75

37 UD 1

38 KK LOFLOW

39 KM FLOWS TO STORAGE BASIN

40 KO 5 5 21

41 DT BYPASS

42 DI 0 10 20 50 80 100 180 300

43 DQ 0 10 20 40 55 65 120 237

44 KK OUTFLW

45 KO 5 5 21

46 RS 1 ELEV 340

47 SA 0 0.83 2.17 2.45 2.74 3.04 3.36

48 SE 340 342 344 346 348 350 352

49 SL 340 1.4 .61 .5

50 SS 345.5 8 3.337 1.5

51 ST 351 10 3.1 1.5

52 KK RETURN

53 KM RETURN DIVERTED FLOWS

54 KO 5 5 21

55 DR BYPASS

56 KK THRU

57 KM DIVERT FIRST FLUSH TO-WQ BASIN - USE MAX. VOLUME TO LIMIT DIVERSION

58 KO 5 2 21

59 DT TO-WQ 5.33

60 DI 0 10 20 50 80 100 180 300

61 DQ 0 10 20 50 80 100 180 300

62 KK SUM

63 KO 5 5 21

64 HC 2

65 KK EXMAIN

66 KO 5 21

67 KM WATERSHED 1 - PRE-DEVE LOPMENT

68 BA 1

69 LS 70.75

70 UD 1

71 ZZ

SCHEMATIC DIAGRAM OF STREAM NETWORK

INPUT

LINE (V) ROUTING (--->) DIVERSION OR PUMP FLOW

NO. (.) CONNECTOR (<---) RETURN OF DIVERTED OR PUMPED FLOW

7 MAIN

.

.

41 .-------> BYPASS

38 LOFLOW

V

V

44 OUTFLW

.

.

55 . .<------- BYPASS

52 . RETURN

. .

. .

59 . .-------> TO-WQ

56 . THRU

. .

. .

62 SUM............

65 . EXMAIN

JR MULTI-RATIO OPTION

RATIOS OF PRECIPITATION

0.46 0.60 0.69 0.79 0.88 1.00

PEAK FLOW AND STAGE (END-OF-PERIOD) SUMMARY FOR MULTIPLE PLAN-RATIO ECONOMIC COMPUTATIONS

FLOWS IN CUBIC FEET PER SECOND, AREA IN SQUARE MILES

TIME TO PEAK IN HOURS

RATIOS APPLIED TO PRECIPITATION

1 SUMMARY OF DAM OVERTOPPING/BREACH ANALYSIS FOR STATION OUTFLW

(PEAKS SHOWN ARE FOR INTERNAL TIME STEP USED DURING BREACH FORMATION)

PLAN 1 ............... INITIAL VALUE SPILLWAY CREST TOP OF DAM

ELEVATION 340.00 345.50 351.00

STORAGE 0. 7. 22.

OUTFLOW 0. 16. 367.

RATIO MAXIMUM MAXIMUM MAXIMUM MAXIMUM DURATION TIME OF TIME OF

OF RESERVOIR DEPTH STORAGE OUTFLOW OVER TOP MAX OUTFLOW FAILURE

PMF W.S.ELEV OVER DAM AC-FT CFS HOURS HOURS HOURS

0.46 340.86 0.00 0. 6. 0.00 13.67 0.00

0.60 343.31 0.00 2. 12. 0.00 14.33 0.00

0.69 344.41 0.00 4. 14. 0.00 14.83 0.00

0.79 345.54 0.00 7. 16. 0.00 15.33 0.00

0.88 345.99 0.00 8. 26. 0.00 15.17 0.00

1.00 346.30 0.00 9. 36. 0.00 15.17 0.00

*** NORMAL END OF HEC-1 ***