technical memorandum: swmm modeling for … · 2017-03-28 · technical memorandum: swmm modeling...

TECHNICAL MEMORANDUM:

SWMM Modeling for Hydromodification Compliance of:

Honey Hill Ranch Road County of San Diego Project No. PDS2015-

STP-15-013

Prepared For:

Greg Hamman Family Trust

Prepared by: Luis Parra, PhD, CPSWQ, ToR, D.WRE. R.C.E. 66377

REC Consultants

2442 Second Avenue

San Diego, CA 92101 Telephone: (619) 232-9200

TECHNICAL MEMORANDUM

TO: Greg Hamman Family Trust

FROM: Luis Parra, PhD, PE, CPSWQ, ToR, D.WRE. David Edwards, PE.

DATE: November 24, 2015, Revised October 11, 2016.

RE: Summary of SWMM Modeling for Hydromodification Compliance for Honey Hill Ranch, Alpine, CA.

INTRODUCTION

This memorandum summarizes the approach used to model the proposed residential development project site in the City of Alpine using the Environmental Protection Agency (EPA) Storm Water Management Model 5.0 (SWMM). SWMM models were prepared for the pre and post‐developed conditions at the site in order to determine if the proposed LID bioretention facility has sufficient volume to meet Order R9‐2013‐001 requirements of the California Regional Water Quality Control Board San Diego Region (SDRWQCB), as explained in the Final Hydromodification Management Plan (HMP), dated March 2011, prepared for the County of San Diego by Brown and Caldwell.

SWMM MODEL DEVELOPMENT

The Honey Hill Ranch project comprises of detached single family residences and improvement of the existing adjacent Honey Hill Ranch Road. The project site drains to three (3) Points of Compliance; POC‐1 to the southern project boundary, POC‐2 located to the north‐west of the project site at the intersection of Suncrest Vista Lane and Honey Hill Ranch Road, and POC‐3 to the north‐east project boundary. Developed condition tributary areas to both POC‐1 and POC‐3 are reduced (while maintaining the same land use) from those experienced in the pre‐developed condition such that there is no increase in flows at these POC’s – thus no HMP analysis is required. Two (2) SWMM models were prepared for POC‐2: the first for the pre‐development and the second for the post‐developed conditions. The SWMM model was used since we have found it to be more comparable to San Diego area watersheds than the alternative San Diego Hydrology Model (SDHM) and also because it is a non‐proprietary model approved by the HMP document. For both SWMM models, flow duration curves were prepared to determine if the proposed HMP facility is sufficient to meet the current HMP requirements.

The inputs required to develop SWMM models include rainfall, watershed characteristics, and BMP configurations. The Flinn Gage from the Project Clean Water website was used for this study, since it is the most representative of the project site precipitation due to elevation and proximity to the project site.

Honey Hill Ranch HMP Memo October 11, 2016

2 W.O.872

In regards to evapotranspiration, per the California Irrigation Management Information System “Reference Evaporation Zones” (CIMIS ETo Zone Map), the project site is located within the Zone 9 Evapotranspiration Area. Thus evapotranspiration values for the site were modeled using Zone 9 average monthly values from Table G.1‐1 from the County of San Diego 2016 BMP Design Manual. The site was modeled with Type C hydrologic soil as this is the existing soil determined from the NRCS Soil Survey. Soils have been assumed to be compacted in the existing condition to represent the current land use of the site and fully compacted in the post developed conditions. Other SWMM inputs for the subareas are discussed in the appendices to this document, where the selection of the parameters is explained in detail.

HMP MODELING

UNDEVELOPED CONDITIONS The existing site is an existing residence and paddock for agricultural use. The project site drains to three (3) Points of Compliance; POC‐1 to the southern project boundary, POC‐2 located to the north‐west of the project site at the intersection of Suncrest Vista Lane and Honey Hill Ranch Road, POC‐3 to the north‐east project boundary. Developed condition tributary areas to both POC‐1 and POC‐3 are reduced (while maintaining the same land use) from those experienced in the pre‐developed condition such that there is no increase in flows at these POC’s.

TABLE 1 – SUMMARY OF EXISTING CONDITIONS

POC Tributary Area, A (Ac) Impervious Percentage, Ip(1)

POC‐1 1.559 0.00%

POC‐2 2.154 0.00%

0.065 100%

POC‐3 0.607 0.00%

TOTAL 4.385 ‐‐

Notes: (1) – Per the 2013 RWQCB permit, existing condition impervious surfaces are not to be accounted for in existing conditions analysis. The existing portion of Honey Hill Ranch Rd tributary to POC‐2 will remain as is in post developed conditions, as such this is a constant area in both pre and post models.

DEVELOPED CONDITIONS

Storm water runoff from the proposed project site is routed to three (3) POCs located to the south, northeast, and northwest of the project site. Runoff from the developed project site is drained to one (1) onsite receiving biofiltration LID BMP. Once flows are routed via the proposed LID BMP, developed onsite flows are then conveyed to the storm drain within Honey Hill Ranch Road. A small portion of proposed sidewalk improvement (less than the 250 ft de minimis) bypasses the basin and confluences directly at the POC.

It is assumed all storm water quality requirements for the project will be met by the biofiltration LID BMP. However, detailed water quality requirements are not discussed within this technical memo. For further information in regards to storm water quality requirements for the project, please refer to the site specific Storm Water Quality Management Plan (SWQMP).


3 W.O.872

TABLE 2 – SUMMARY OF DEVELOPED CONDITIONS

POC DMA Tributary Area, A (Ac) Impervious Percentage,

Ip

POC‐1 ‐‐ 0.583 0.00%

POC‐2 DMA‐2C 3.701 47.30%

DE‐MINIMIS‐1(1) 0.005 100.00%

POC‐3 ‐‐ 0.096 0.00%

TOTAL ‐‐ 4.385 ‐‐ Notes:

(1): Sidewalk are bypassing basin (less than 250 sq. ft).

TABLE 3 – SUMMARY OF PRE VS POST DEVELOPED CONDITION POC AREAS

POC Pre‐Developed

Tributary Area (Ac) Post‐Developed

Tributary Area (Ac) Difference

POC‐1 1.559 0.583 ‐ 0.976

POC‐2 2.219 3.706 +1.487

POC‐3 0.607 0.096 ‐ 0.511

TOTAL 4.385 4.385 ‐‐

One (1) LID biofiltration basin is located within the project site and is responsible for handling hydromodification requirements for the project site. In developed conditions, the basin (bifurcated by the project entrance though hydraulically connected via surface and subsurface drains) will have a surface depth of 4 feet and a riser spillway structure (see dimensions in Table 4). Flows will then discharge from the basin via the slotted weirs in the outlet structure or infiltrate through the base of the facility to the receiving soil. The riser structure will act as a spillway such that peak flows can be safely discharged to the receiving storm drain system. Beneath the basins’ invert lies the proposed LID biofiltration portion of the drainage facility. This portion of the basin is comprised of a 3‐inch layer of mulch and an 18‐inch layer of amended soil (a highly sandy, organic rich composite with an infiltration capacity of at least 5 inches/hr) and a 24‐inch layer of gravel. Due to the lack of infiltration indicated by the site specific geotechnical investigation, the basin will be lined. The biofiltration basin was modeled using the biofiltration LID module within SWMM. The biofiltration module can model the amended soil layer, and a surface storage pond up to the elevation of the invert of the spillway. It should be noted that detailed outlet structure location and elevations will be shown on the construction plans based on the recommendations of this study.

BMP MODELING FOR HMP PURPOSES

Modeling of dual purpose Water Quality/HMP BMP

One (1) LID BMP biofiltration basin is proposed for water quality treatment and hydromodification conformance for the project site. Table 4 illustrates the dimensions required for HMP compliance


4 W.O.872

according to the SWMM model that was undertaken for the project. Table 5 details the outlet structure for the basin surface.

TABLE 4 – SUMMARY OF DEVELOPED DUAL PURPOSE BMP

BMP Tributary Area (Ac)

DIMENSIONS

BMP Area(1), (ft2)

Low Flow Orifice (in)

Gravel Depth(5) (in)

Depth Riser Invert (in)(2)

Weir Perimeter Length(3) (ft)

Total Surface Depth(4) (in)

BMP‐1 3.744 3603 1.125 24 43‐in 8‐ft 48‐in

Notes: (1): Area of amended soil equal to area of amended soil. (2): Depth of ponding beneath riser structure’s surface spillway.(3): Overflow length, the internal perimeter of the riser is 8 ft (2 ft x 2 ft internal dimensions).

(4): Total surface depth of BMP from top crest elevation to surface invert.(5): Gravel depth needed to comply with hydromodification purposes, 21‐inches above French Drain and 3‐inches dead storage.

TABLE 5 – SUMMARY OF RISER DETAILS:

BMP Lower Slot Middle Slot Top Riser

Width (ft)

Height

(ft) Elevation(1)

(ft) Width (ft)

Height (ft)

Elevation(1) (ft)

Length(2) (ft) Elev.(1) (ft)

BMP‐1 1.5 0.083 1.17 0.5 0.083 1.67 8 3.5 Notes:

(1): Basin ground surface elevation assumed to be 0.00 ft elevation. (2): Overflow length is the internal perimeter of the riser structure.

FLOW DURATION CURVE COMPARISON

The Flow Duration Curve (FDC) for the site was compared at POC‐2 by exporting the hourly runoff time series results from SWMM to a spreadsheet. The FDC was compared between 10% of the existing condition Q2 up to the existing condition Q10 for POC‐2. The Q2 and Q10 were determined with a partial duration statistical analysis of the runoff time series in an Excel spreadsheet using the Cunnane plotting position method (which is the preferred plotting methodology in the HMP Permit). As the SWMM Model includes a statistical analysis based on the Weibull Plotting Position Method, the Weibull Method was also used within the spreadsheet to ensure that the results were similar to those obtained by the SWMM Model.

The range between 10% of Q2 and Q10 was divided into 100 equal time intervals; the number of hours that each flow rate was exceeded was counted from the hourly series. Additionally, the intermediate peaks with a return period “i” were obtained (Qi with i=3 to 9). For the purpose of the plot, the values were presented as percentage of time exceeded for each flow rate. FDC comparison at the POC is illustrated in Figure 1 in both normal and logarithmic scale. Attachment 5 provides a detailed drainage exhibit for the post‐developed condition.

As can be seen in Figure 1, the FDC for the proposed condition with the HMP BMP is within 110% of the curve for the existing condition in both peak flows and durations. The additional runoff volume generated from developing the site will be released to the existing point of discharge at a flow rate below the 10% Q2 lower threshold for POC‐2. Additionally, the project will also not increase peak flow rates between the Q2 and the Q10, as shown in the graphic and also in the peak flow tables in Attachment 1.


5 W.O.872

Discussion of the Manning’s coefficient (Pervious Areas) for Pre and Post‐Development Conditions Typically the Manning’s coefficient is selected as n = 0.10 for pervious areas and n = 0.012 for impervious areas. However, due to the impact that n has in the continuous simulation a more accurate value of the Manning’s coefficient has been chosen for pervious areas. Taken into consideration the study prepared by TRWE (Reference [6]) a value of n = 0.05 has been selected (see Table 1 of Reference [6] included in Attachment 7). An average n value between average grass plus pasture (0.04) and dense grass (0.06) has been selected per the reference cited, for light rain (<0.8 in/hr) as more than 99% of the rainfall has been measured with this intensity.

SUMMARY

This study has demonstrated that the proposed HMP BMP provided for the Honey Hills Ranch project site is sufficient to meet the current HMP criteria if the cross‐section areas and volumes recommended within this technical memorandum, and the respective orifice and outlet structure are incorporated as specified within the proposed project site.

KEY ASSUMPTIONS

1. Type C Soil is representative of the existing condition site.

ATTACHMENTS

1. Q2 to Q10 Comparison Tables

2. FDC Plots (log and natural “x” scale) and Flow Duration Table.

3. List of the “n” largest Peaks: Pre‐Development and Post‐Development Conditions

4. Elevations vs. Discharge & Stage‐ Storage Curves to be used in SWMM

5. Pre & Post Development Maps, Project plan and section sketches

6. SWMM Input Data in Input Format (Existing and Proposed Models)

7. SWMM Screens and Explanation of Significant Variables

8. Soil Maps

9. Summary files from the SWMM Model

10. 10. Response to Comments


6 W.O.872

REFERENCES

[1] – “Review and Analysis of San Diego County Hydromodification Management Plan (HMP): Assumptions, Criteria, Methods, & Modeling Tools – Prepared for the Cities of San Marcos, Oceanside & Vista”, May 2012, TRW Engineering.

[2] – “Final Hydromodification Management Plan (HMP) prepared for the County of San Diego”, March 2011, Brown and Caldwell.

[3] ‐ Order R9‐20013‐001, California Regional Water Quality Control Board San Diego Region (SDRWQCB).

[4] – “Handbook of Hydrology”, David R. Maidment, Editor in Chief. 1992, McGraw Hill. [5] – “County of San Diego BMP Design Manual”, February 2016.

[6] – “Improving Accuracy in Continuous Hydrologic Modeling: Guidance for Selecting Pervious Overland Flow Manning’s n Values in the San Diego Region”, TRWE, 2016.


7 W.O.872

Figure 1a and 1b. Flow Duration Curve Comparison (logarithmic and normal “x” scale)


8 W.O.872

ATTACHMENT 1.

Q2 to Q10 Comparison Table – POC 2

Return Period Existing Condition (cfs) Mitigated Condition (cfs) Reduction, Exist ‐ Mitigated (cfs)

2‐year 0.743 0.379 0.364

3‐year 0.857 0.486 0.371

4‐year 0.904 0.529 0.375

5‐year 0.991 0.587 0.404

6‐year 1.008 0.616 0.391

7‐year 1.050 0.688 0.363

8‐year 1.052 0.711 0.341

9‐year 1.096 0.725 0.371

10‐year 1.137 0.744 0.393

ATTACHMENT 2

FLOW DURATION CURVE ANALYSIS

1) Flow duration curve shall not exceed the existing conditions by more than 10%, neither in

peak flow nor duration.

The figures on the following pages illustrate that the flow duration curve in post‐development

conditions after the proposed BMP is below the existing flow duration curve. The flow duration

curve table following the curve shows that if the interval 0.10Q2 – Q10 is divided in 100 sub‐

intervals, then a) the post development divided by pre‐development durations are never larger

than 110% (the permit allows up to 110%); and b) there are no more than 10 intervals in the

range 101%‐110% which would imply an excess over 10% of the length of the curve (the permit

allows less than 10% of excesses measured as 101‐110%).

Consequently, the design passes the hydromodification test.

It is important to note that the flow duration curve can be expressed in the “x” axis as

percentage of time, hours per year, total number of hours, or any other similar time variable. As

those variables only differ by a multiplying constant, their plot in logarithmic scale is going to

look exactly the same, and compliance can be observed regardless of the variable selected.

However, in order to satisfy the County of San Diego HMP example, % of time exceeded is the

variable of choice in the flow duration curve. The selection of a logarithmic scale in lieu of the

normal scale is preferred, as differences between the pre‐development and post‐development

curves can be seen more clearly in the entire range of analysis. Both graphics are presented just

to prove the difference.

In terms of the “y” axis, the peak flow value is the variable of choice. As an additional analysis

performed by REC, not only the range of analysis is clearly depicted (10% of Q2 to Q10) but also

all intermediate flows are shown (Q2, Q3, Q4, Q5, Q6, Q7, Q8 and Q9) in order to demonstrate

compliance at any range Qx – Qx+1. It must be pointed out that one of the limitations of both the

SWMM and SDHM models is that the intermediate analysis is not performed (to obtain Qi from

i = 2 to 10). REC performed the analysis using the Cunnane Plotting position Method (the

preferred method in the HMP permit) from the “n” largest independent peak flows obtained

from the continuous time series.

The largest “n” peak flows are attached in this appendix, as well as the values of Qi with a

return period “i”, from i=2 to 10. The Qi values are also added into the flow‐duration plot.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

0.001 0.01 0.1

Q (cfs)

Percentage of time exceeded (%)

POC‐2 Honey Hill ‐ Flow Duration Curve

Existing

Proposed

Qx

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11

Q (cfs)

Percentage of time exceeded (%)

POC‐2 Honey Hill ‐ Flow Duration Curve

Existing

Proposed

Qx

Flow Duration Curve Data for Honey Hill POC‐2 , Alpine, CA

Q2 = 0.743 cfs Fraction 10 %

Q10 = 1.14 cfs

Step = 0.0107 cfs

Count = 394487 hours

45.00 years

Pass or

Q (cfs) Hours > Q % time Hours>Q % time Post/Pre Fail?

1 0.074 397 1.01E‐01 388 9.84E‐02 98% Pass

2 0.085 381 9.66E‐02 374 9.48E‐02 98% Pass

3 0.096 359 9.10E‐02 360 9.13E‐02 100% Pass

4 0.106 335 8.49E‐02 333 8.44E‐02 99% Pass

5 0.117 327 8.29E‐02 319 8.09E‐02 98% Pass

6 0.128 322 8.16E‐02 308 7.81E‐02 96% Pass

7 0.139 309 7.83E‐02 292 7.40E‐02 94% Pass

8 0.149 301 7.63E‐02 284 7.20E‐02 94% Pass

9 0.160 289 7.33E‐02 270 6.84E‐02 93% Pass

10 0.171 273 6.92E‐02 263 6.67E‐02 96% Pass

11 0.182 263 6.67E‐02 250 6.34E‐02 95% Pass

12 0.192 238 6.03E‐02 231 5.86E‐02 97% Pass

13 0.203 235 5.96E‐02 217 5.50E‐02 92% Pass

14 0.214 228 5.78E‐02 207 5.25E‐02 91% Pass

15 0.225 222 5.63E‐02 193 4.89E‐02 87% Pass

16 0.235 219 5.55E‐02 184 4.66E‐02 84% Pass

17 0.246 213 5.40E‐02 175 4.44E‐02 82% Pass

18 0.257 207 5.25E‐02 169 4.28E‐02 82% Pass

19 0.267 197 4.99E‐02 157 3.98E‐02 80% Pass

20 0.278 185 4.69E‐02 148 3.75E‐02 80% Pass

21 0.289 177 4.49E‐02 139 3.52E‐02 79% Pass

22 0.300 169 4.28E‐02 129 3.27E‐02 76% Pass

23 0.310 160 4.06E‐02 123 3.12E‐02 77% Pass

24 0.321 157 3.98E‐02 118 2.99E‐02 75% Pass

25 0.332 154 3.90E‐02 112 2.84E‐02 73% Pass

26 0.343 148 3.75E‐02 104 2.64E‐02 70% Pass

27 0.353 143 3.62E‐02 99 2.51E‐02 69% Pass

28 0.364 130 3.30E‐02 91 2.31E‐02 70% Pass

29 0.375 120 3.04E‐02 85 2.15E‐02 71% Pass

30 0.386 113 2.86E‐02 76 1.93E‐02 67% Pass

31 0.396 106 2.69E‐02 73 1.85E‐02 69% Pass

32 0.407 103 2.61E‐02 68 1.72E‐02 66% Pass

33 0.418 101 2.56E‐02 66 1.67E‐02 65% Pass

34 0.428 98 2.48E‐02 63 1.60E‐02 64% Pass

35 0.439 94 2.38E‐02 61 1.55E‐02 65% Pass

36 0.450 85 2.15E‐02 60 1.52E‐02 71% Pass

Detention Optimized

Interval

Existing Condition

Pass or


Detention Optimized

Interval

Existing Condition

37 0.461 79 2.00E‐02 58 1.47E‐02 73% Pass

38 0.471 76 1.93E‐02 56 1.42E‐02 74% Pass

39 0.482 76 1.93E‐02 52 1.32E‐02 68% Pass

40 0.493 73 1.85E‐02 51 1.29E‐02 70% Pass

41 0.504 71 1.80E‐02 47 1.19E‐02 66% Pass

42 0.514 70 1.77E‐02 45 1.14E‐02 64% Pass

43 0.525 68 1.72E‐02 42 1.06E‐02 62% Pass

44 0.536 67 1.70E‐02 40 1.01E‐02 60% Pass

45 0.547 61 1.55E‐02 37 9.38E‐03 61% Pass

46 0.557 61 1.55E‐02 37 9.38E‐03 61% Pass

47 0.568 58 1.47E‐02 34 8.62E‐03 59% Pass

48 0.579 56 1.42E‐02 34 8.62E‐03 61% Pass

49 0.589 54 1.37E‐02 29 7.35E‐03 54% Pass

50 0.600 51 1.29E‐02 27 6.84E‐03 53% Pass

51 0.611 49 1.24E‐02 27 6.84E‐03 55% Pass

52 0.622 48 1.22E‐02 26 6.59E‐03 54% Pass

53 0.632 47 1.19E‐02 26 6.59E‐03 55% Pass

54 0.643 46 1.17E‐02 26 6.59E‐03 57% Pass

55 0.654 46 1.17E‐02 26 6.59E‐03 57% Pass

56 0.665 45 1.14E‐02 24 6.08E‐03 53% Pass

57 0.675 44 1.12E‐02 24 6.08E‐03 55% Pass

58 0.686 44 1.12E‐02 22 5.58E‐03 50% Pass

59 0.697 43 1.09E‐02 21 5.32E‐03 49% Pass

60 0.707 41 1.04E‐02 19 4.82E‐03 46% Pass

61 0.718 36 9.13E‐03 18 4.56E‐03 50% Pass

62 0.729 30 7.60E‐03 18 4.56E‐03 60% Pass

63 0.740 29 7.35E‐03 16 4.06E‐03 55% Pass

64 0.750 25 6.34E‐03 15 3.80E‐03 60% Pass

65 0.761 23 5.83E‐03 14 3.55E‐03 61% Pass

66 0.772 22 5.58E‐03 14 3.55E‐03 64% Pass

67 0.783 21 5.32E‐03 14 3.55E‐03 67% Pass

68 0.793 20 5.07E‐03 13 3.30E‐03 65% Pass

69 0.804 20 5.07E‐03 13 3.30E‐03 65% Pass

70 0.815 19 4.82E‐03 13 3.30E‐03 68% Pass

71 0.826 19 4.82E‐03 13 3.30E‐03 68% Pass

72 0.836 19 4.82E‐03 12 3.04E‐03 63% Pass

73 0.847 19 4.82E‐03 11 2.79E‐03 58% Pass

74 0.858 18 4.56E‐03 10 2.53E‐03 56% Pass

75 0.868 17 4.31E‐03 7 1.77E‐03 41% Pass

76 0.879 17 4.31E‐03 7 1.77E‐03 41% Pass

77 0.890 17 4.31E‐03 6 1.52E‐03 35% Pass

78 0.901 14 3.55E‐03 6 1.52E‐03 43% Pass

79 0.911 14 3.55E‐03 5 1.27E‐03 36% Pass

80 0.922 14 3.55E‐03 5 1.27E‐03 36% Pass

81 0.933 12 3.04E‐03 5 1.27E‐03 42% Pass

Pass or


Detention Optimized

Interval

Existing Condition

82 0.944 11 2.79E‐03 4 1.01E‐03 36% Pass

83 0.954 11 2.79E‐03 4 1.01E‐03 36% Pass

84 0.965 11 2.79E‐03 4 1.01E‐03 36% Pass

85 0.976 11 2.79E‐03 4 1.01E‐03 36% Pass

86 0.987 11 2.79E‐03 4 1.01E‐03 36% Pass

87 0.997 9 2.28E‐03 4 1.01E‐03 44% Pass

88 1.008 8 2.03E‐03 4 1.01E‐03 50% Pass

89 1.019 8 2.03E‐03 3 7.60E‐04 38% Pass

90 1.029 8 2.03E‐03 3 7.60E‐04 38% Pass

91 1.040 8 2.03E‐03 3 7.60E‐04 38% Pass

92 1.051 7 1.77E‐03 3 7.60E‐04 43% Pass

93 1.062 6 1.52E‐03 1 2.53E‐04 17% Pass

94 1.072 6 1.52E‐03 1 2.53E‐04 17% Pass

95 1.083 6 1.52E‐03 1 2.53E‐04 17% Pass

96 1.094 6 1.52E‐03 1 2.53E‐04 17% Pass

97 1.105 6 1.52E‐03 1 2.53E‐04 17% Pass

98 1.115 6 1.52E‐03 1 2.53E‐04 17% Pass

99 1.126 6 1.52E‐03 1 2.53E‐04 17% Pass

100 1.137 5 1.27E‐03 1 2.53E‐04 20% Pass

Peak Flows calculated with Cunnane Plotting Position

Return Period

(years)Pre‐dev. Q (cfs)

Post‐Dev. Q

(cfs)

Reduction

(cfs)

10 1.137 0.744 0.393

9 1.096 0.725 0.371

8 1.052 0.711 0.341

7 1.050 0.688 0.363

6 1.008 0.616 0.391

5 0.991 0.587 0.404

4 0.904 0.529 0.375

3 0.857 0.486 0.371

2 0.743 0.379 0.364

ATTACHMENT 3

List of the “n” Largest Peaks: Pre & Post‐Developed Conditions

Basic Probabilistic Equation:

R = 1/P R: Return period (years).

P: Probability of a flow to be equaled or exceeded any given year (dimensionless).

Cunnane Equation: Weibull Equation:

P.

. P

i: Position of the peak whose probability is desired (sorted from large to small)

n: number of years analyzed.

Explanation of Variables for the Tables in this Attachment

Peak: Refers to the peak flow at the date given, taken from the continuous simulation hourly

results of the n year analyzed.

Posit: If all peaks are sorted from large to small, the position of the peak in a sorting analysis is

included under the variable Posit.

Date: Date of the occurrence of the peak at the outlet from the continuous simulation

Note: all peaks are not annual maxima; instead they are defined as event maxima, with a

threshold to separate peaks of at least 12 hours. In other words, any peak P in a time series is

defined as a value where dP/dt = 0, and the peak is the largest value in 25 hours (12 hours

before, the hour of occurrence and 12 hours after the occurrence, so it is in essence a daily

peak).

List of Peak events and Determination of P2 and P10 (Post‐Development)

Honey Hill POC 2 ‐ AlpineT

(Year)

Cunnane

(cfs)

Weibull

(cfs)

10 0.74 0.78 Date Posit Weibull Cunnane

9 0.73 0.73 0.196 1/5/1992 45 1.02 1.01

8 0.71 0.72 0.203 10/28/1974 44 1.05 1.04

7 0.69 0.70 0.205 1/15/1993 43 1.07 1.06

6 0.62 0.63 0.206 2/10/1982 42 1.10 1.09

5 0.59 0.59 0.208 1/11/2001 41 1.12 1.11

4 0.53 0.53 0.232 1/15/1978 40 1.15 1.14

3 0.49 0.49 0.236 3/17/1982 39 1.18 1.17

2 0.38 0.38 0.252 2/18/1980 38 1.21 1.20

0.269 2/8/1998 37 1.24 1.23

0.293 11/11/1985 36 1.28 1.27

Note: 0.297 2/20/1980 35 1.31 1.31

Cunnane is the preferred 0.298 1/27/2008 34 1.35 1.35

method by the HMP permit. 0.303 1/8/1993 33 1.39 1.39

0.311 3/5/1978 32 1.44 1.43

0.311 1/25/1995 31 1.48 1.48

0.322 12/18/1967 30 1.53 1.53

0.325 3/13/1996 29 1.59 1.58

0.337 10/27/2004 28 1.64 1.64

0.34 10/19/2004 27 1.70 1.70

0.354 2/16/1980 26 1.77 1.77

0.359 2/15/1986 25 1.84 1.84

0.377 3/2/1983 24 1.92 1.92

0.379 2/10/1978 23 2.00 2.00

0.379 3/1/1978 22 2.09 2.09

0.384 10/29/1974 21 2.19 2.19

0.39 1/16/1993 20 2.30 2.31

0.405 2/9/1976 19 2.42 2.430.433 3/1/1983 18 2.56 2.57

0.436 12/1/2007 17 2.71 2.72

0.476 12/5/1966 16 2.88 2.90

0.496 12/6/1966 15 3.07 3.10

0.502 11/30/1982 14 3.29 3.32

0.522 2/14/1995 13 3.54 3.59

0.522 2/22/2004 12 3.83 3.90

0.546 1/29/1980 11 4.18 4.26

0.583 11/29/1970 10 4.60 4.71

0.591 3/1/1991 9 5.11 5.26

0.612 1/7/1993 8 5.75 5.95

0.684 3/5/1995 7 6.57 6.85

0.713 1/14/1969 6 7.67 8.07

0.736 1/4/1995 5 9.20 9.83

0.862 11/23/1965 4 11.50 12.56

0.863 10/20/2004 3 15.33 17.38

1.059 1/31/1979 2 23.00 28.25

1.757 2/20/1980 1 46.00 75.33

Peaks (cfs)

Period of Return

(Years)

List of Peak events and Determination of P2 and P10 (Pre‐Development)Honey Hill POC 2 ‐ Alpine

T

(Year)

Cunnane

(cfs)

Weibull

(cfs)

10 1.14 1.14 Date Posit Weibull Cunnane

9 1.10 1.12 0.57 1/27/2008 45 1.02 1.01

8 1.05 1.07 0.58 3/8/1973 44 1.05 1.04

7 1.05 1.05 0.583 2/18/1980 43 1.07 1.06

6 1.01 1.02 0.591 11/14/1993 42 1.10 1.09

5 0.99 0.99 0.595 3/31/1992 41 1.12 1.11

4 0.90 0.91 0.605 3/14/1982 40 1.15 1.14

3 0.86 0.86 0.605 10/31/1987 39 1.18 1.17

2 0.74 0.74 0.615 11/11/1985 38 1.21 1.20

0.66 2/19/1980 37 1.24 1.23

0.668 4/1/1982 36 1.28 1.27

Note: 0.688 12/28/1977 35 1.31 1.31

Cunnane is the preferred 0.698 3/18/1983 34 1.35 1.35

method by the HMP permit. 0.705 8/17/1977 33 1.39 1.39

0.717 1/13/1993 32 1.44 1.43

0.718 12/18/1967 31 1.48 1.48

0.719 2/16/1980 30 1.53 1.53

0.721 1/4/1995 29 1.59 1.58

0.724 2/13/1973 28 1.64 1.64

0.724 2/10/1978 27 1.70 1.70

0.73 3/1/1991 26 1.77 1.77

0.74 2/14/1998 25 1.84 1.84

0.741 1/3/1977 24 1.92 1.92

0.743 3/4/1978 23 2.00 2.00

0.747 11/29/1970 22 2.09 2.09

0.758 1/30/1980 21 2.19 2.19

0.766 3/13/1996 20 2.30 2.31

0.778 2/28/1978 19 2.42 2.43

0.784 1/7/1993 18 2.56 2.57

0.811 11/20/1983 17 2.71 2.72

0.853 3/1/1983 16 2.88 2.90

0.861 1/14/1969 15 3.07 3.10

0.893 2/22/2004 14 3.29 3.32

0.896 3/3/1980 13 3.54 3.59

0.896 3/24/1983 12 3.83 3.90

0.925 2/3/1998 11 4.18 4.26

0.99 1/9/2005 10 4.60 4.71

0.992 2/8/1998 9 5.11 5.26

1.005 2/6/1969 8 5.75 5.95

1.05 2/14/1995 7 6.57 6.85

1.052 1/31/1979 6 7.67 8.07

1.135 11/30/2007 5 9.20 9.83

1.163 11/23/1965 4 11.50 12.56

1.343 10/20/2004 3 15.33 17.38

1.427 1/25/1995 2 23.00 28.25

2.179 2/20/1980 1 46.00 75.33

Peaks

(cfs)

Period of Return

(Years)

ATTACHMENT 4

AREA VS ELEVATION

The area vs. elevation curve in the model is calculated in Excel and imported into the model. It

should be noted that the first 1.17 feet of surface ponding is allocated to the LID Module of the SWMM

model. Please refer to Attachment 7 for further information. The Excel stage‐storage calculation is

provided on the following pages.

DISCHARGE VS ELEVATION

The orifice has been selected to maximize its size while still restricting flows to conform with

the required 10% of the Q2 event flow as mandated in the Final Hydromodification

Management Plan by Brown & Caldwell, dated March 2011. While REC acknowledges that the

orifice is small, to increase the size of the outlet would impact the basin’s ability to restrict

flows beneath the HMP thresholds, thus preventing the BMP from conformance with HMP

requirements.

In order to further reduce the risk of blockage of the orifices, regular maintenance of the riser

and orifice must be performed to ensure potential blockages are minimized. A detail of the

orifice and riser structure is provided in Attachment 5 of this memorandum.

A stage‐discharge relationship was developed to represent the outlet structure for the

detention basin and is provided on the following pages.

DRAWDOWN CALCULATIONS

Drawdown calculations are provided in the site specific Storm Water Quality Management Plan

(SWQMP), please refer to the aforementioned study.

Outlet structure for Discharge of Detention Basin (note: 0.0 elev = 1.17 ft actual elevation)

Discharge vs Elevation Table

Low orifice: 1 " Lower slot Emergency Weir

Number: 0 Invert: 0.00 ft Invert: 2.333 ft

Cg‐low: 0.62 B 1.25 ft B: 8 ft

Middle orifice: 1 " h 0.083 ft

number of orif: 0 Upper slot

Cg‐middle: 0.62 Invert: 0.500 ft

invert elev: 0.00 ft B: 0.50 ft

h 0.083 ft

h H/D‐low H/D‐mid Qlow‐orif Qlow‐weir Qtot‐low Qmid‐orif Qmid‐weir Qtot‐med Qslot‐low Qslot‐upp Qemer Qtot

(ft) ‐ ‐ (cfs) (cfs) (cfs) (cfs) (cfs) (cfs) (cfs) (cfs) (cfs) (cfs)

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.100 1.200 1.199 0.000 0.000 0.000 0.000 0.000 0.000 0.123 0.000 0.000 0.123

0.200 2.400 2.399 0.000 0.000 0.000 0.000 0.000 0.000 0.203 0.000 0.000 0.203

0.300 3.600 3.599 0.000 0.000 0.000 0.000 0.000 0.000 0.259 0.000 0.000 0.259

0.400 4.800 4.799 0.000 0.000 0.000 0.000 0.000 0.000 0.305 0.000 0.000 0.305

0.500 6.000 5.999 0.000 0.000 0.000 0.000 0.000 0.000 0.345 0.000 0.000 0.345

0.600 7.200 7.199 0.000 0.000 0.000 0.000 0.000 0.000 0.381 0.049 0.000 0.430

0.700 8.400 8.399 0.000 0.000 0.000 0.000 0.000 0.000 0.414 0.081 0.000 0.495

0.800 9.600 9.599 0.000 0.000 0.000 0.000 0.000 0.000 0.444 0.103 0.000 0.547

0.900 10.800 10.799 0.000 0.000 0.000 0.000 0.000 0.000 0.472 0.122 0.000 0.594

1.000 12.000 11.999 0.000 0.000 0.000 0.000 0.000 0.000 0.499 0.138 0.000 0.637

1.100 13.200 13.199 0.000 0.000 0.000 0.000 0.000 0.000 0.525 0.152 0.000 0.676

1.200 14.400 14.399 0.000 0.000 0.000 0.000 0.000 0.000 0.549 0.165 0.000 0.714

1.300 15.600 15.599 0.000 0.000 0.000 0.000 0.000 0.000 0.572 0.177 0.000 0.749

1.400 16.800 16.799 0.000 0.000 0.000 0.000 0.000 0.000 0.594 0.188 0.000 0.783

1.500 18.000 17.999 0.000 0.000 0.000 0.000 0.000 0.000 0.616 0.199 0.000 0.815

1.600 19.200 19.199 0.000 0.000 0.000 0.000 0.000 0.000 0.637 0.209 0.000 0.846

1.700 20.400 20.399 0.000 0.000 0.000 0.000 0.000 0.000 0.657 0.219 0.000 0.875

1.800 21.600 21.599 0.000 0.000 0.000 0.000 0.000 0.000 0.676 0.228 0.000 0.904

1.900 22.800 22.799 0.000 0.000 0.000 0.000 0.000 0.000 0.695 0.237 0.000 0.932

2.000 24.000 23.999 0.000 0.000 0.000 0.000 0.000 0.000 0.714 0.245 0.000 0.959

2.100 25.200 25.199 0.000 0.000 0.000 0.000 0.000 0.000 0.732 0.254 0.000 0.985

2.200 26.400 26.399 0.000 0.000 0.000 0.000 0.000 0.000 0.749 0.262 0.000 1.011

2.300 27.600 27.599 0.000 0.000 0.000 0.000 0.000 0.000 0.766 0.269 0.000 1.036

2.400 28.800 28.799 0.000 0.000 0.000 0.000 0.000 0.000 0.783 0.277 0.427 1.487

2.500 30.000 29.999 0.000 0.000 0.000 0.000 0.000 0.000 0.800 0.284 1.687 2.771

2.600 31.200 31.199 0.000 0.000 0.000 0.000 0.000 0.000 0.816 0.291 3.415 4.522

2.700 32.400 32.399 0.000 0.000 0.000 0.000 0.000 0.000 0.831 0.298 5.506 6.636

2.800 33.600 33.599 0.000 0.000 0.000 0.000 0.000 0.000 0.847 0.305 7.906 9.058

2.830 33.960 33.959 0.000 0.000 0.000 0.000 0.000 0.000 0.851 0.307 8.681 9.839

STAGE STORAGE CALCULATIONS

BASIN 1 BASIN 2

Elev (ft) Area (ft2) Volume (ft3) Elev (ft) Area (ft2) Volume (ft3)

0 3150 0.00 0 453 0.00

1 3364 3257.00 1 756.0 604.50

1.167 3390 3819.83 1.167 818.3 735.70

1.5 3452 4960.08 1.5 940.4 1028.81

2 3533 6706.24 2 1142 1549.41

3 3697 10321.24 3 1583 2911.91

4 3697 14018.24 4 1583 4494.91

BASIN TOTAL

Elev (ft) Area (ft2) Volume (ft3)

0 3603 0.00 LID AREA

1.00 4120 3861.50

1.17 4208 4556.91 FIRST SURFACE OUTLET

1.50 4392 5988.85

2.00 4675 8255.60

3.00 5280 13233.10

4.00 5280 18513.10

Effective Depth 15.177 inches

DISCHARGE EQUATIONS

1) Weir:

/ (1)

2) Slot:

As an orifice: 2 (2.a)

As a weir: / (2.b)

For H > hs slot works as weir until orifice equation provides a smaller discharge. The elevation such that

equation (2.a) = equation (2.b) is the elevation at which the behavior changes from weir to orifice.

3) Vertical Orifices

As an orifice: 0.25 2 (3.a)

As a weir: Critical depth and geometric family of circular sector must be solved to determined Q as a function of

H:

; 2

; 2 ; 8

;

1 0.5 (3.b.1, 3.b.2, 3.b.3, 3.b.4 and 3.b.5)

There is a value of H (approximately H = 110% D) from which orifices no longer work as weirs as critical depth is

not possible at the entrance of the orifice. This value of H is obtained equaling the discharge using critical

equations and equations (3.b).

A mathematical model is prepared with the previous equations depending on the type o discharge.

The following are the variables used above:

QW, Qs, QO = Discharge of weir, slot or orifice (cfs)

CW, cg : Coefficients of discharge of weir (typically 3.1) and orifice (0.61 to 0.62)

L, Bs, D, hs : Length of weir, width of slot, diameter of orifice and height of slot, respectively; (ft)

H: Level of water in the pond over the invert of slot, weir or orifice (ft)

Acr, Tcr, ycr, αcr: Critical variables for circular sector: area (sq‐ft), top width (ft), critical depth (ft), and angle to the center,

respectively.

ATTACHMENT 5

Pre & Post‐Developed Maps, Project Plan and Detention

Section Sketches

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LEGEND

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BIOFILTRATION DETAIL FOR BMP

BIOFILTRATION CO DETAIL

BIOFILTRATION OUTLET DETAIL

FLOW CONTROL ORIFICE PLATE

LEGEND

SAMPLE PROHIBITIVE SIGNAGE

ATTACHMENT 6

SWMM Input Data in Input Format (Existing & Proposed Models)

PRE_DEV

[TITLE] [OPTIONS] FLOW_UNITS CFS INFILTRATION GREEN_AMPT FLOW_ROUTING KINWAVE START_DATE 08/09/1963 START_TIME 00:00:00 REPORT_START_DATE 08/08/1963 REPORT_START_TIME 00:00:00 END_DATE 08/08/2008 END_TIME 23:00:00 SWEEP_START 01/01 SWEEP_END 12/31 DRY_DAYS 0 REPORT_STEP 01:00:00 WET_STEP 00:15:00 DRY_STEP 04:00:00 ROUTING_STEP 0:01:00 ALLOW_PONDING NO INERTIAL_DAMPING PARTIAL VARIABLE_STEP 0.75 LENGTHENING_STEP 0 MIN_SURFAREA 0 NORMAL_FLOW_LIMITED BOTH SKIP_STEADY_STATE NO FORCE_MAIN_EQUATION H-W LINK_OFFSETS DEPTH MIN_SLOPE 0 [EVAPORATION] ;;Type Parameters ;;---------- ---------- MONTHLY 0.041 0.076 0.118 0.192 0.237 0.318 0.308 0.286 0.217 0.14 0.067 0.041 DRY_ONLY NO [RAINGAGES] ;; Rain Time Snow Data ;;Name Type Intrvl Catch Source ;;-------------- --------- ------ ------ ---------- FLINN INTENSITY 1:00 1.0 TIMESERIES FLINN [SUBCATCHMENTS] ;; Total Pcnt. Pcnt. Curb Snow ;;Name Raingage Outlet Area Imperv Width Slope Length Pack ;;-------------- ---------------- ---------------- -------- -------- -------- -------- -------- -------- DMA-2-C FLINN POC-2 2.154 0 264 14.9 0 DMA-EX-RD FLINN POC-2 0.065 100 16 1.7 0 [SUBAREAS] ;;Subcatchment N-Imperv N-Perv S-Imperv S-Perv PctZero RouteTo PctRouted ;;-------------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- DMA-2-C 0.012 0.05 0.05 0.10 25 OUTLET DMA-EX-RD 0.012 0.05 0.05 0.10 25 OUTLET [INFILTRATION] ;;Subcatchment Suction HydCon IMDmax ;;-------------- ---------- ---------- ---------- DMA-2-C 6 0.075 0.32 DMA-EX-RD 6 0.075 0.32 [OUTFALLS]

PRE_DEV

;; Invert Outfall Stage/Table Tide ;;Name Elev. Type Time Series Gate ;;-------------- ---------- ---------- ---------------- ---- POC-2 0 FREE NO [TIMESERIES] ;;Name Date Time Value ;;-------------- ---------- ---------- ---------- FLINN FILE "Flinn.txt" [REPORT] INPUT NO CONTROLS NO SUBCATCHMENTS ALL NODES ALL LINKS ALL [TAGS] [MAP] DIMENSIONS 925.000 2485.000 2575.000 7215.000 Units None [COORDINATES] ;;Node X-Coord Y-Coord ;;-------------- ------------------ ------------------ POC-2 2500.000 2700.000 [VERTICES] ;;Link X-Coord Y-Coord ;;-------------- ------------------ ------------------ [Polygons] ;;Subcatchment X-Coord Y-Coord ;;-------------- ------------------ ------------------ DMA-2-C 2500.000 6000.000 DMA-2-C 2500.000 6000.000 DMA-EX-RD 1000.000 2700.000 [SYMBOLS] ;;Gage X-Coord Y-Coord ;;-------------- ------------------ ------------------ FLINN 2500.000 7000.000

POST_DEV

[TITLE] [OPTIONS] FLOW_UNITS CFS INFILTRATION GREEN_AMPT FLOW_ROUTING KINWAVE START_DATE 08/09/1963 START_TIME 00:00:00 REPORT_START_DATE 08/09/1963 REPORT_START_TIME 00:00:00 END_DATE 08/08/2008 END_TIME 23:00:00 SWEEP_START 01/01 SWEEP_END 12/31 DRY_DAYS 0 REPORT_STEP 01:00:00 WET_STEP 00:15:00 DRY_STEP 04:00:00 ROUTING_STEP 0:01:00 ALLOW_PONDING NO INERTIAL_DAMPING PARTIAL VARIABLE_STEP 0.75 LENGTHENING_STEP 0 MIN_SURFAREA 0 NORMAL_FLOW_LIMITED BOTH SKIP_STEADY_STATE NO FORCE_MAIN_EQUATION H-W LINK_OFFSETS DEPTH MIN_SLOPE 0 [EVAPORATION] ;;Type Parameters ;;---------- ---------- MONTHLY 0.041 0.076 0.118 0.192 0.237 0.318 0.308 0.286 0.217 0.14 0.067 0.041 DRY_ONLY NO [RAINGAGES] ;; Rain Time Snow Data ;;Name Type Intrvl Catch Source ;;-------------- --------- ------ ------ ---------- FLINN INTENSITY 1:00 1.0 TIMESERIES FLINN [SUBCATCHMENTS] ;; Total Pcnt. Pcnt. Curb Snow ;;Name Raingage Outlet Area Imperv Width Slope Length Pack ;;-------------- ---------------- ---------------- -------- -------- -------- -------- -------- -------- DMA-2C FLINN BMP-1 3.618 47.30 182 6 0 BMP-1 FLINN DIV-1 0.083104 0 10 0 0 DMA-BYPASS FLINN POC-2 0.005 100 10 1 0 [SUBAREAS] ;;Subcatchment N-Imperv N-Perv S-Imperv S-Perv PctZero RouteTo PctRouted ;;-------------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- DMA-2C 0.012 0.05 0.05 0.10 25 PERVIOUS 100 BMP-1 0.012 0.05 0.05 0.10 25 OUTLET DMA-BYPASS 0.012 0.05 0.05 0.10 25 OUTLET [INFILTRATION] ;;Subcatchment Suction HydCon IMDmax ;;-------------- ---------- ---------- ---------- DMA-2C 6 0.075 0.32 BMP-1 6 0.075 0.32

POST_DEV

DMA-BYPASS 6 0.075 0.32 [LID_CONTROLS] ;; Type/Layer Parameters ;;-------------- ---------- ---------- BMP-1 BC BMP-1 SURFACE 15.177 0.05 0 0 5 BMP-1 SOIL 18 0.4 0.2 0.1 5 5 1.5 BMP-1 STORAGE 24 0.67 0 0 BMP-1 DRAIN 0.1170 0.5 3 6 [LID_USAGE] ;;Subcatchment LID Process Number Area Width InitSatur FromImprv ToPerv Report File ;;-------------- ---------------- ------- ---------- ---------- ---------- ---------- ---------- ----------- BMP-1 BMP-1 1 3620 0 0 100 0 [OUTFALLS] ;; Invert Outfall Stage/Table Tide ;;Name Elev. Type Time Series Gate ;;-------------- ---------- ---------- ---------------- ---- POC-2 0 FREE NO [DIVIDERS] ;; Invert Diverted Divider ;;Name Elev. Link Type Parameters ;;-------------- ---------- ---------------- ---------- ---------- DIV-1 0 BYPASS CUTOFF 0.04454 0 0 0 0 [STORAGE] ;; Invert Max. Init. Storage Curve Ponded Evap. ;;Name Elev. Depth Depth Curve Params Area Frac. Infiltration Parameters ;;-------------- -------- -------- -------- ---------- -------- -------- -------- -------- -------- ----------------------- BASIN 0 2.83 0 TABULAR BASIN 5280 1 [CONDUITS] ;; Inlet Outlet Manning Inlet Outlet Init. Max. ;;Name Node Node Length N Offset Offset Flow Flow ;;-------------- ---------------- ---------------- ---------- ---------- ---------- ---------- ---------- ---------- BYPASS DIV-1 BASIN 10 0.01 0 0 0 0 U-DRAIN DIV-1 POC-2 10 0.01 0 0 0 0 [OUTLETS] ;; Inlet Outlet Outflow Outlet Qcoeff/ Flap ;;Name Node Node Height Type QTable Qexpon Gate ;;-------------- ---------------- ---------------- ---------- --------------- ---------------- ---------- ---- ORIFICE BASIN POC-2 0 TABULAR/DEPTH OUTLET NO [XSECTIONS] ;;Link Shape Geom1 Geom2 Geom3 Geom4 Barrels ;;-------------- ------------ ---------------- ---------- ---------- ---------- ---------- BYPASS DUMMY 0 0 0 0 1 U-DRAIN DUMMY 0 0 0 0 1 [LOSSES] ;;Link Inlet Outlet Average Flap Gate ;;-------------- ---------- ---------- ---------- ---------- [CURVES] ;;Name Type X-Value Y-Value ;;-------------- ---------- ---------- ----------

POST_DEV

OUTLET Rating 0.000 0.000 OUTLET 0.100 0.123 OUTLET 0.200 0.203 OUTLET 0.300 0.259 OUTLET 0.400 0.305 OUTLET 0.500 0.345 OUTLET 0.600 0.430 OUTLET 0.700 0.495 OUTLET 0.800 0.547 OUTLET 0.900 0.594 OUTLET 1.000 0.637 OUTLET 1.100 0.676 OUTLET 1.200 0.714 OUTLET 1.300 0.749 OUTLET 1.400 0.783 OUTLET 1.500 0.815 OUTLET 1.600 0.846 OUTLET 1.700 0.875 OUTLET 1.800 0.904 OUTLET 1.900 0.932 OUTLET 2.000 0.959 OUTLET 2.100 0.985 OUTLET 2.200 1.011 OUTLET 2.300 1.036 OUTLET 2.400 1.487 OUTLET 2.500 2.771 OUTLET 2.600 4.522 OUTLET 2.700 6.636 OUTLET 2.800 9.058 OUTLET 2.83 9.839 BASIN Storage 0.00 4208 BASIN 0.33 4392 BASIN 0.83 4675 BASIN 1.83 5280 BASIN 2.83 5280 [TIMESERIES] ;;Name Date Time Value ;;-------------- ---------- ---------- ---------- FLINN FILE "Flinn.txt" [REPORT] INPUT NO CONTROLS NO SUBCATCHMENTS ALL NODES ALL LINKS ALL [TAGS] [MAP] DIMENSIONS 2925.000 1750.000 4575.000 7250.000 Units None [COORDINATES] ;;Node X-Coord Y-Coord ;;-------------- ------------------ ------------------ POC-2 3500.000 2000.000 DIV-1 3500.000 4000.000 BASIN 4500.000 3000.000

POST_DEV

[VERTICES] ;;Link X-Coord Y-Coord ;;-------------- ------------------ ------------------ [Polygons] ;;Subcatchment X-Coord Y-Coord ;;-------------- ------------------ ------------------ DMA-2C 3500.000 6000.000 DMA-2C 3500.000 6000.000 BMP-1 3500.000 5000.000 DMA-BYPASS 2500.000 2000.000 [SYMBOLS] ;;Gage X-Coord Y-Coord ;;-------------- ------------------ ------------------ FLINN 3500.000 7000.000

ATTACHMENT 7

EPA SWMM FIGURES AND EXPLANATIONS

Per the attached, the reader can see the screens associated with the EPA‐SWMM Model in both

pre‐development and post‐development conditions. Each portion, i.e., sub‐catchments,

outfalls, storage units, weir as a discharge, and outfalls (point of compliance), are also shown.

Variables for modeling are associated with typical recommended values by the EPA‐SWMM

model, typical values found in technical literature (such as Maidment’s Handbook of

Hydrology). Recommended values for the SWMM model have been attained from the interim

Orange County criteria established for their SWMM calibration. Currently, no recommended

values have been established by the San Diego County HMP Permit for the SWMM Model.

Soil characteristics of the existing soils were determined from the NRCS Web Soil Survey Exhibit

(located in Attachment 8 of this report).

Some values incorporated within the SWMM model have been determined from the

professional experience of REC using conservative assumptions that have a tendency to

increase the size of the needed BMP and also generate a long‐term runoff as a percentage of

rainfall similar to those measured in gage stations in Southern California by the USGS.

A Technical document prepared by Tory R Walker Engineering for the Cities of San Marcos,

Oceanside and Vista (Reference [1]) can also be consulted for additional information regarding

typical values for SWMM parameters.

PRE‐DEVELOPED CONDITION

POST‐DEVELOPED CONDITION

EXPLANATION OF SELECTED VARIABLES

Sub Catchment Areas:

Please refer to the attached diagrams that indicate the DMA and Bio‐Retention BMP (BMP) sub areas

modeled within the project site at both the pre and post developed conditions draining to the POC.

Parameters for the pre‐ and post‐developed models include soil type C as determined from the site

specific Natural Resources Conservation Service (NRCS) geologic review (attached at the end of this

appendix). Suction head, conductivity and initial deficit corresponds to average values expected for

these soils types, according to sources consulted, professional experience, and approximate values

obtained by the interim Orange County modeling approach.

REC selected infiltration values, such that the percentage of total precipitation that becomes runoff, is

realistic for the soil types and slightly smaller than measured values for Southern California watersheds.

Selection of a Kinematic Approach: As the continuous model is based on hourly rainfall, and the time of

concentration for the pre‐development and post‐development conditions is significantly smaller than 60

minutes, precise routing of the flows through the impervious surfaces, the underdrain pipe system, and

the discharge pipe was considered unnecessary. The truncation error of the precipitation into hourly

steps is much more significant than the precise routing in a system where the time of concentration is

much smaller than 1 hour.

Sub‐catchment BMP:

The area of bio‐filtration must be equal to the area of the development tributary to the bioretention

facility (area that drains into the biofiltration, equal external area plus bio‐filtration itself). Five (5)

decimal places were given regarding the areas of the bio‐filtration to insure that the area used by the

program for the LID subroutine corresponds exactly with this tributary.

LID Control Editor: Explanation of Significant Variables

Storage Depth:

The storage depth variable within the SWMM model is representative of the storage volume

provided beneath the surface riser outlet and the engineered soil and mulch components of the

bioretention facility.

In those cases where the surface storage has a variable area that is also different to the area of

the gravel and amended soil, the SWMM model needs to be calibrated as the LID module will

use the storage depth multiplied by the BMP area as the amount of volume stored at the

surface.

Let ABMP be the area of the BMP (area of amended soil and area of gravel). The proper value of

the storage depth SD to be included in the LID module can be calculated by using geometric

properties of the surface volume. Let A0 be the surface area at the bottom of the surface pond,

and let Ai be the surface area at the elevation of the invert of the first row of orifices (or at the

invert of the riser if not surface orifices are included). Finally, let hi be the difference in

elevation between A0 and Ai. By volumetric definition:

(1)

Equation (1) allows the determination of SD to be included as Storage Depth in the LID module.

Porosity: A porosity value of 0.4 has been selected for the model. The amended soil is to be

highly sandy in content in order to have a saturated hydraulic conductivity of approximately 5

in/hr.

REC considers such a value to be slightly high; however, in order to comply with the HMP

Permit, the value recommended by the Copermittees for the porosity of amended soil is 0.4,

per Appendix A of the Final Hydromodification Management Plan by Brown & Caldwell, dated

March 2011. Such porosity is equal to the porosity of the gravel per the same document.

Void Ratio: The ratio of the void volume divided by the soil volume is directly related to

porosity as n/(1‐n). As the underdrain layer is composed of gravel, a porosity value of 0.4 has

been selected (also per Appendix A of the Final HMP document), which results in a void ratio of

0.4/(1‐0.4) = 0.67 for the gravel detention layer.

Conductivity: Due to the existing type C soils located on the project site, the bio‐filtration basin

will be unlined. An infiltration conductivity of 0.075 in/hr was used to represent compacted C

soils.

Clogging factor: A clogging factor was not used (0 indicates that there is no clogging assumed

within the model). The reason for this is related to the fairness of a comparison with the SDHM

model and the HMP sizing tables: a clogging factor was not considered, and instead, a

conservative value of infiltration was recommended.

Drain (Flow) coefficient: The flow coefficient C in the SWMM Model is the coefficient needed to

transform the orifice equation into a general power law equation of the form:

(2)

where q is the peak flow in in/hr, n is the exponent (typically 0.5 for orifice equation), HD is the

elevation of the centroid of the orifice in inches (assumed equal to the invert of the orifice for

small orifices and in our design equal to 0) and H is the depth of the water in inches.

The general orifice equation can be expressed as:

2 (3)

where Q is the peak flow in cfs, D is the diameter in inches, cg is the typical discharge coefficient

for orifices (0.61‐0.63 for thin walls and around 0.75‐0.8 for thick walls), g is the acceleration of

gravity in ft/s2, and H and HD are defined above and are also used in inches in Equation (3).

It is clear that:

(4)

Cut‐Off Flow: Q (cfs) and q (in/hr) are also the cutoff flow. For numerical reasons to insure the

LID is full, the model uses cut‐off = 1.01 Q.

Detention Basin

ATTACHMENT 8

Geotechnical Documentation

Hydrologic Soil Group—San Diego County Area, California

Natural ResourcesConservation Service

Web Soil SurveyNational Cooperative Soil Survey

4/14/2015Page 1 of 4

3632

740

3632

760

3632

780

3632

800

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840

3632

860

3632

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3632

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3632

740

3632

760

3632

780

3632

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3632

820

3632

840

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860

3632

880

3632

900

523410 523430 523450 523470 523490 523510 523530 523550 523570 523590 523610 523630 523650

523410 523430 523450 523470 523490 523510 523530 523550 523570 523590 523610 523630 523650

32° 50' 2'' N11

6° 4

4' 5

9'' W

32° 50' 2'' N

116°

44'

49'

' W

32° 49' 56'' N

116°

44'

59'

' W

32° 49' 56'' N

116°

44'

49'

' W

N

Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 11N WGS840 50 100 200 300

Feet0 15 30 60 90

MetersMap Scale: 1:1,210 if printed on A landscape (11" x 8.5") sheet.

MAP LEGEND MAP INFORMATION

Area of Interest (AOI)Area of Interest (AOI)

SoilsSoil Rating Polygons

A

A/D

B

B/D

C

C/D

D

Not rated or not available

Soil Rating LinesA

A/D

B

B/D

C

C/D

D


Soil Rating PointsA

A/D

B

B/D

C

C/D

D


Water FeaturesStreams and Canals

TransportationRails

Interstate Highways

US Routes

Major Roads

Local Roads

BackgroundAerial Photography

The soil surveys that comprise your AOI were mapped at 1:24,000.

Warning: Soil Map may not be valid at this scale.

Enlargement of maps beyond the scale of mapping can causemisunderstanding of the detail of mapping and accuracy of soil lineplacement. The maps do not show the small areas of contrastingsoils that could have been shown at a more detailed scale.

Please rely on the bar scale on each map sheet for mapmeasurements.

Source of Map: Natural Resources Conservation ServiceWeb Soil Survey URL: http://websoilsurvey.nrcs.usda.govCoordinate System: Web Mercator (EPSG:3857)

Maps from the Web Soil Survey are based on the Web Mercatorprojection, which preserves direction and shape but distortsdistance and area. A projection that preserves area, such as theAlbers equal-area conic projection, should be used if more accuratecalculations of distance or area are required.

This product is generated from the USDA-NRCS certified data as ofthe version date(s) listed below.

Soil Survey Area: San Diego County Area, CaliforniaSurvey Area Data: Version 8, Sep 17, 2014

Soil map units are labeled (as space allows) for map scales 1:50,000or larger.

Date(s) aerial images were photographed: May 2, 2010—May 6,2010

The orthophoto or other base map on which the soil lines werecompiled and digitized probably differs from the backgroundimagery displayed on these maps. As a result, some minor shiftingof map unit boundaries may be evident.




4/14/2015Page 2 of 4

Hydrologic Soil Group

Hydrologic Soil Group— Summary by Map Unit — San Diego County Area, California (CA638)

Map unit symbol Map unit name Rating Acres in AOI Percent of AOI

FaD2 Fallbrook sandy loam, 9to 15 percent slopes,eroded

C 1.9 43.0%

FaE2 Fallbrook sandy loam, 15to 30 percent slopes,eroded

C 2.5 57.0%

Totals for Area of Interest 4.5 100.0%

Description

Hydrologic soil groups are based on estimates of runoff potential. Soils areassigned to one of four groups according to the rate of water infiltration when thesoils are not protected by vegetation, are thoroughly wet, and receive precipitationfrom long-duration storms.

The soils in the United States are assigned to four groups (A, B, C, and D) andthree dual classes (A/D, B/D, and C/D). The groups are defined as follows:

Group A. Soils having a high infiltration rate (low runoff potential) when thoroughlywet. These consist mainly of deep, well drained to excessively drained sands orgravelly sands. These soils have a high rate of water transmission.

Group B. Soils having a moderate infiltration rate when thoroughly wet. Theseconsist chiefly of moderately deep or deep, moderately well drained or well drainedsoils that have moderately fine texture to moderately coarse texture. These soilshave a moderate rate of water transmission.

Group C. Soils having a slow infiltration rate when thoroughly wet. These consistchiefly of soils having a layer that impedes the downward movement of water orsoils of moderately fine texture or fine texture. These soils have a slow rate of watertransmission.

Group D. Soils having a very slow infiltration rate (high runoff potential) whenthoroughly wet. These consist chiefly of clays that have a high shrink-swellpotential, soils that have a high water table, soils that have a claypan or clay layerat or near the surface, and soils that are shallow over nearly impervious material.These soils have a very slow rate of water transmission.

If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter isfor drained areas and the second is for undrained areas. Only the soils that in theirnatural condition are in group D are assigned to dual classes.




4/14/2015Page 3 of 4

Rating Options

Aggregation Method: Dominant Condition

Component Percent Cutoff: None Specified

Tie-break Rule: Higher




4/14/2015Page 4 of 4

ATTACHMENT 9

Summary Files from the SWMM Model

PRE_DEV

EPA STORM WATER MANAGEMENT MODEL - VERSION 5.0 (Build 5.0.022) -------------------------------------------------------------- ********************************************************* NOTE: The summary statistics displayed in this report are based on results found at every computational time step, not just on results from each reporting time step. ********************************************************* **************** Analysis Options **************** Flow Units ............... CFS Process Models: Rainfall/Runoff ........ YES Snowmelt ............... NO Groundwater ............ NO Flow Routing ........... NO Water Quality .......... NO Infiltration Method ...... GREEN_AMPT Starting Date ............ AUG-09-1963 00:00:00 Ending Date .............. AUG-08-2008 23:00:00 Antecedent Dry Days ...... 0.0 Report Time Step ......... 01:00:00 Wet Time Step ............ 00:15:00 Dry Time Step ............ 04:00:00 ************************** Volume Depth Runoff Quantity Continuity acre-feet inches ************************** --------- ------- Total Precipitation ...... 110.144 595.640 Evaporation Loss ......... 1.627 8.800 Infiltration Loss ........ 97.872 529.275 Surface Runoff ........... 11.064 59.830 Final Surface Storage .... 0.000 0.000 Continuity Error (%) ..... -0.380 ************************** Volume Volume Flow Routing Continuity acre-feet 10^6 gal ************************** --------- --------- Dry Weather Inflow ....... 0.000 0.000 Wet Weather Inflow ....... 11.064 3.605 Groundwater Inflow ....... 0.000 0.000 RDII Inflow .............. 0.000 0.000 External Inflow .......... 0.000 0.000 External Outflow ......... 11.064 3.605 Internal Outflow ......... 0.000 0.000 Storage Losses ........... 0.000 0.000 Initial Stored Volume .... 0.000 0.000 Final Stored Volume ...... 0.000 0.000 Continuity Error (%) ..... 0.000 *************************** Subcatchment Runoff Summary *************************** --------------------------------------------------------------------------------------------------------

PRE_DEV

Total Total Total Total Total Total Peak Runoff Precip Runon Evap Infil Runoff Runoff Runoff Coeff Subcatchment in in in in in 10^6 gal CFS -------------------------------------------------------------------------------------------------------- DMA-2-C 595.64 0.00 5.97 545.25 46.55 2.72 2.11 0.078 DMA-EX-RD 595.64 0.00 102.63 0.00 499.96 0.88 0.07 0.839 Analysis begun on: Fri Jan 27 15:57:03 2017 Analysis ended on: Fri Jan 27 15:57:14 2017 Total elapsed time: 00:00:11

POST_DEV

EPA STORM WATER MANAGEMENT MODEL - VERSION 5.0 (Build 5.0.022) -------------------------------------------------------------- ********************************************************* NOTE: The summary statistics displayed in this report are based on results found at every computational time step, not just on results from each reporting time step. ********************************************************* **************** Analysis Options **************** Flow Units ............... CFS Process Models: Rainfall/Runoff ........ YES Snowmelt ............... NO Groundwater ............ NO Flow Routing ........... YES Ponding Allowed ........ NO Water Quality .......... NO Infiltration Method ...... GREEN_AMPT Flow Routing Method ...... KINWAVE Starting Date ............ AUG-09-1963 00:00:00 Ending Date .............. AUG-08-2008 23:00:00 Antecedent Dry Days ...... 0.0 Report Time Step ......... 01:00:00 Wet Time Step ............ 00:15:00 Dry Time Step ............ 04:00:00 Routing Time Step ........ 60.00 sec WARNING 04: minimum elevation drop used for Conduit BYPASS WARNING 04: minimum elevation drop used for Conduit U-DRAIN ************************** Volume Depth Runoff Quantity Continuity acre-feet inches ************************** --------- ------- Total Precipitation ...... 183.959 595.640 Evaporation Loss ......... 20.737 67.144 Infiltration Loss ........ 127.579 413.089 Surface Runoff ........... 37.566 121.635 Final Surface Storage .... 0.000 0.000 Continuity Error (%) ..... -1.046 ************************** Volume Volume Flow Routing Continuity acre-feet 10^6 gal ************************** --------- --------- Dry Weather Inflow ....... 0.000 0.000 Wet Weather Inflow ....... 37.564 12.241 Groundwater Inflow ....... 0.000 0.000 RDII Inflow .............. 0.000 0.000 External Inflow .......... 0.000 0.000 External Outflow ......... 37.536 12.232 Internal Outflow ......... 0.000 0.000 Storage Losses ........... 0.023 0.007 Initial Stored Volume .... 0.000 0.000 Final Stored Volume ...... 0.000 0.000 Continuity Error (%) ..... 0.013

POST_DEV

******************************** Highest Flow Instability Indexes ******************************** All links are stable. ************************* Routing Time Step Summary ************************* Minimum Time Step : 60.00 sec Average Time Step : 60.00 sec Maximum Time Step : 60.00 sec Percent in Steady State : 0.00 Average Iterations per Step : 1.00 *************************** Subcatchment Runoff Summary *************************** -------------------------------------------------------------------------------------------------------- Total Total Total Total Total Total Peak Runoff Precip Runon Evap Infil Runoff Runoff Runoff Coeff Subcatchment in in in in in 10^6 gal CFS -------------------------------------------------------------------------------------------------------- DMA-2C 595.64 0.00 55.62 423.15 122.33 12.02 3.67 0.205 BMP-1 595.64 5325.82 567.09 0.00 5393.90 12.17 3.73 0.911 DMA-BYPASS 595.64 0.00 98.41 0.00 507.29 0.07 0.01 0.852 *********************** LID Performance Summary *********************** ------------------------------------------------------------------------------------------------------------------ Total Evap Infil Surface Drain Init. Final Pcnt. Inflow Loss Loss Outflow Outflow Storage Storage Error Subcatchment LID Control in in in in in in in ------------------------------------------------------------------------------------------------------------------ BMP-1 BMP-1 5921.46 567.11 0.00 1138.10 4256.01 0.00 0.00 -0.67 ****************** Node Depth Summary ****************** --------------------------------------------------------------------- Average Maximum Maximum Time of Max Depth Depth HGL Occurrence Node Type Feet Feet Feet days hr:min --------------------------------------------------------------------- POC-2 OUTFALL 0.00 0.00 0.00 0 00:00 DIV-1 DIVIDER 0.00 0.00 0.00 0 00:00 BASIN STORAGE 0.00 2.49 2.49 6039 20:37 ******************* Node Inflow Summary ******************* -------------------------------------------------------------------------------------

POST_DEV

Maximum Maximum Lateral Total Lateral Total Time of Max Inflow Inflow Inflow Inflow Occurrence Volume Volume Node Type CFS CFS days hr:min 10^6 gal 10^6 gal ------------------------------------------------------------------------------------- POC-2 OUTFALL 0.01 2.74 6039 20:37 0.069 12.231 DIV-1 DIVIDER 3.73 3.73 6039 20:15 12.171 12.171 BASIN STORAGE 0.00 3.68 6039 20:15 0.000 2.569 ********************** Node Surcharge Summary ********************** Surcharging occurs when water rises above the top of the highest conduit. --------------------------------------------------------------------- Max. Height Min. Depth Hours Above Crown Below Rim Node Type Surcharged Feet Feet --------------------------------------------------------------------- DIV-1 DIVIDER 394487.02 0.000 0.000 BASIN STORAGE 394487.02 2.494 0.336 ********************* Node Flooding Summary ********************* No nodes were flooded. ********************** Storage Volume Summary ********************** -------------------------------------------------------------------------------------------- Average Avg E&I Maximum Max Time of Max Maximum Volume Pcnt Pcnt Volume Pcnt Occurrence Outflow Storage Unit 1000 ft3 Full Loss 1000 ft3 Full days hr:min CFS -------------------------------------------------------------------------------------------- BASIN 0.002 0 0 12.173 87 6039 20:36 2.69 *********************** Outfall Loading Summary *********************** ----------------------------------------------------------- Flow Avg. Max. Total Freq. Flow Flow Volume Outfall Node Pcnt. CFS CFS 10^6 gal ----------------------------------------------------------- POC-2 3.43 0.03 2.74 12.231 ----------------------------------------------------------- System 3.43 0.03 2.74 12.231 ******************** Link Flow Summary ******************** -----------------------------------------------------------------------------

POST_DEV

Maximum Time of Max Maximum Max/ Max/ |Flow| Occurrence |Veloc| Full Full Link Type CFS days hr:min ft/sec Flow Depth ----------------------------------------------------------------------------- BYPASS DUMMY 3.68 6039 20:15 U-DRAIN DUMMY 0.04 836 20:14 ORIFICE DUMMY 2.69 6039 20:37 ************************* Conduit Surcharge Summary ************************* ---------------------------------------------------------------------------- Hours Hours --------- Hours Full -------- Above Full Capacity Conduit Both Ends Upstream Dnstream Normal Flow Limited ---------------------------------------------------------------------------- BYPASS 0.01 0.01 0.01 394487.02 0.01 U-DRAIN 0.01 0.01 0.01 394487.02 0.01 Analysis begun on: Fri Jan 27 16:07:40 2017 Analysis ended on: Fri Jan 27 16:08:01 2017 Total elapsed time: 00:00:21

ATTACHMENT 10

Response to Comments

Response to HMP Comments for Honey Hill Ranch Dated 6/10/2016

Item No. 10‐18: DMA Map for Hydromodification Proposed Conditions: POC 1 and the end of the brow

ditches at the northeast property corner (POC 3) appears to discharge to areas of a steep natural terrain

in which there are not concentrated flows in the pre‐developed condition. Conveyance improvements,

such as brow ditches and pipes, with or without rip rap pads, cannot terminate at locations of natural

terrain where there are not pre‐developed concentrated flows (i.e. hillside locations without defined

channels) without also providing hydromodification calculations to demonstrate that flows will not

increase from the pre‐developed condition to the post‐developed condition at the location for the range

of flows of concern (0.1Q2 through Q10). The current design would cause channeling of the natural

terrain and new hydromodification issues downstream of the site. Outfalls must be located in areas with

defined drainage channels. Revise DMA Exhibit and plane, as applicable, to be in compliance with the

BMP DM hydromodification management requirements related to discrete outfalls (i.e. either provide

the appropriate analysis to demonstrate flows do not increase at these locations, which may not be

feasible, or route flows to the pre‐developed point of concentration location). Post‐project POC’s shall be

appropriately located, and pre‐development POC’s shall be matching locations so that flow comparisons

between the conditions can be clearly demonstrated in the PDP SWQMP.

Response:

The reviewer should note that the slope located to the adjacent northern lot is a graded 2:1 man‐made

slope – there are no “natural” conditions to maintain.

In regards to POC‐3, the discharge location is an existing man‐made 2:1 slope. This slope is currently not

adequately protected from run‐on flows from the Honey Hills project site (it is typical that a graded 2:1

slope have a brow ditch located at the top of the grading to ensure the slope is not eroded by run‐on

flows). As such, the Honey Hill project is implementing the brow ditch that should have been

constructed with the offsite 2:1 slope, thus preventing erosive conditions (and conforming with

hydromodfication principles). The area is reduced in post developed conditions while the land use is

maintained, such that it is evident that there will be no HMP impact at this location.

In regards to the southern POC‐1, in existing conditions, flows sheet flow from the southern slope of the

property draining directly into the rear lots of the adjacent residences (see photograph below). In order

to intercept these flows (and protect their houses), the residents of these lots have constructed an

earthen berm which collects these flows and discharges them to the south‐east corner. The post‐

developed project site proposes to include a brow ditch where the current earthen ditch is located to

maintain this existing drainage conveyance. As is the case with POC‐1, the area tributary to the POC is

reduced while the land use has been maintained such there is no HMP impact at POC‐3.

Item No. 10‐19: Please utilize applicable evapotranspiration values from Appendix G of the BMP DM

(Table G.1‐1) or provide explanation on the data currently used in the model.

Response:

The evapotranspiration values have been updated using values listed in the 2016 BMP Design Manual.

Per the ETo Zone Map, Zone 9 Evapotranspiration values have been assigned to the SWMM models

accordingly.

Item No. 10‐20: Input parameters are not consistent with BMP DM Appendix G guidance. Revise

predevelopment condition and post‐project condition model to have N‐Perv and Dstore‐Imperv values

consistent with the BMP DM. (For N‐Perv use default use 0.10 for undisturbed vegetated areas with the

unincorporated areas of the County, otherwise provide documentation of other surface consistent with

Table A.6 of SWMM Manual).

Response:

All SWMM input values have been updated to comply with the 2016 BMP Design Manual.

Manning’s values are consistent with the County approved “Improving Accuracy in Continuous

Hydrologic Modeling: Guidance for Selecting Pervious Overland Flow Manning’s n Values in the San

Diego Region”, TRWE, 2016. A discussion and reference to the aforementioned study has been included

within the revised HMP Memo.

Item No. 10‐21: For BMPs implemented with the County’s jurisdiction, for pre‐developed conditions a

25% reduction cannot be applied to Green‐Ampt Conductivity values listed in Table G.1‐4 of Appendix G

only for redevelopment areas that are currently concrete or asphalt but must be modeled to their

underlying characteristics. Revise pre‐development condition Green‐Ampt Conductivity values to be in

compliance with the current requirements (i.e. a 25% reduction in conductivity cannot be applied to the

entire site for this project).

Response:

The reviewer should note that the San Diego Regional Water Quality Control Board’s (SDWQCB) intent is

to allow for the existing condition to use compacted soil values where current soil compaction exists and

to not take credit for pre developed condition impervious areas. An email confirming this from the San

Diego Regional Water Quality Control Board is attached to this comment response for the County’s

understanding.

The project site is an existing residence and ranch – the soil has been compacted in the current

condition and has been modeled correctly in accordance with the SDRWQCB’s permit intent.

Item No. 10‐22: Berm height used in the Surface Tab of the LID Control Editor is 15.88 inches: however,

the details in the PDP SWQMP indicate 15 inches ponding depth. Revise the SWMM model parameter as

appropriate to be consistent with PDP SWQMP exhibits and plans.

Response:

The ponded depth is an effective depth (not actual depth) and is calculated as described in Attachment

7 of the previously submitted HMP. The SWQMP details are correct in that the elevation of the first

riser outlet is 15‐inches above the basin surface of the BMP.

Item No. 10‐23: A seepage rate of 0.075 inches per hour was using the Storage Tab of the LID Control

Editor, whereas the PDP SWQMP Attachment 1 includes a completed form I‐8 that states that infiltration

is not feasible for this project. Revise the SWMM model parameter as appropriate to be consistent with

the PDP SWQMP Attachment 1 or revise Form I‐8 as appropriate.

Response:

An onsite percolation test has been undertaken since the original submittal – these tests indicate that

the soil located beneath the proposed BMP is not feasible for infiltration. As such, the HMP and

SWQMP have been updated to illustrate these findings.

Item No. 10‐24: A cutoff flow rate of 0.047 cfs is used in the divider node, whereas based on a 1.125‐

inch diameter orifice with a head of 4.75 our calculations indicate that the cutoff rate should be 0.072

cfs. Please provide hand calculations defining the equation and each variable used to determine the

cutoff flow used in the model, and revise the SWMM model parameter if appropriate.

Response:

SWMM version 5.0 uses as a head the depth of the gravel layer above the orifice as the head of the

orifice because it assumes the amended soil friction losses are equivalent to the head gains and also

assumes that the column of water is discontinuous in the amended soil, while version 5.0 used as a head

the total depth of the water above the orifice and neglects friction losses and possible discontinuity in

the water column (air) as the water travel in the amended soil. The reality is that a more complex

scenario with the combined energy equation and Darcy’s law could be used to produce an intermediate

result, and such scenario is not included in SWMM. It is our expert opinion that for purposes of

continuous simulation version 5.0 represents reality better than version 5.1, and there is no indication in

the BMP Manual or in the permit that version 5.0 cannot be used. Therefore, we are not changing our

design as a consequence of this issue. Notice that the total head “h” is 1.75 – 1.125/24 = 1.703 ft, and

hence Q = 0.044 cfs (Q = 0.25∙πD2∙Cg∙(2gh)0.5); however, the value 0.047 cfs was erroneously calculated

with h = 2.0 – 1.125/24 = 1.953 ft as we forgot to eliminate the 3 inches of gravel under the invert of the

French Drain. Consequently, we are reducing the divided cutoff flow rate to 0.044 cfs. Notice that if the

runoff out of the BMP is plotted (area BR‐1), the value 0.044 cfs is the most common runoff out of the

LID other than 0 cfs, because it represents the discharge of the LID when the amended soil is saturated.

Item No. 10‐25: No drawdown calculations are provided for the proposed biofiltration BMPs. Include

drawdown calculations for the proposed biofiltration basin and ensure that the drawdown time meets

the criteria listed in the Appendix F Biofiltration checklist. (The water surface drains to at least 12 inches

below the media surface within 24‐hours from the end of the storm event flow to preserve plant health

and promote health soil structure.)

Response:

The BMP was sized in accordance with the County of San Diego’s Automated BMP sizing calculator B.5‐1.

Drawdown calculations are included within this spreadsheet and are provided in Attachment 1 of the

PDP SWQMP. Per this BMP sizing tool the biofiltration basin has been sized correctly to meet all criteria,

inclusive of drawdown requirements. Please refer to the PDP SWQMP.

technical memorandum: swmm modeling for … · 2017-03-28 · technical memorandum: swmm modeling...

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