Green & Ampt or SCS? A Comparison of Hydrologic Methodologies for the City of Mesquite, Nevada

September 6, 2017Andrew Trelease, PE, CFM , Clark County Regional Flood Control District

Clark Barlow, PE, CFM, Atkins

Travis Anderson, PE, City of Mesquite

25 September 2017 1Contains sensitive information

2017 Flood Control Master Plan Update, City of Mesquite, NevadaProject Overview

2

Project Location

3

Master Plan Overview

• Nevada Revised Statute 543.596 requires that flood control master plans must be reviewed and updated at least every 5 years.

• Previous updates in 1991, 1995, 2001, 2007, 2012 for the City of Mesquite

• 2017 MPU facility recommendations are complete; full MPU being finalized

• The purpose of the update (MPU) is to add new information, assess progress, and recommend changes

4

Master Plan Scope

5

Recognize/ Identify

Risk

•Data collection and review•Fieldwork•Flood History

Define and Quantify

Risk

• Hydrologic analysis• Hydraulic analysis

Mitigate Risk

• Plan formulation• Cost estimates• Final document/plan and priorities

Study Area

• Town Wash• Abbott Wash• Pulsipher Wash• Western Washes• Additional Watersheds• Toquop Wash• Virgin River

6

2017 Hydrologic Model Comparison

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Model Background

• HEC-1 Model• 2007 Model used Green & Ampt Method• The rest of Clark County uses SCS method

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Overview of Methods

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SCS Curve Number Method Green & Ampt MethodBased on empirical formula Based on conceptual infiltration

modelRelatively easy to apply More difficult to applyDifficult to verify parameters Field verifiableCommonly used Less common

Goal of Model Comparison

10

Determine Most Appropriate Method for City of Mesquite

SCSUpdate models to

be more consistent with the rest of Clark County

Green & AmptUpdate models to

use prescribed methodology

(Mohave County)

Land Use Changes

11

2007 Hydrologic Soil Group• Data obtained from

2002/2003 NRCS Soil Survey

12

2017 Hydrologic Soil Group• Data obtained from

2014 NRCS Soil Survey

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Changed from C to B

Changed from C to A

Changed from B to C

2007 XKSAT Green & Ampt• Dark red = higher

infiltration (less runoff)• Yellow = low infiltration

(high runoff)

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2017 XKSAT Green & Ampt – Dense• Follows Mohave County

Method• Uses a uniform soil density

factor of 1.1 for all lad uses (No distinction between undeveloped and developed areas)

• Slightly lower infiltration than in 2007 for most of the area

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Lower infiltration rates

Lower infiltration rates

Higher infiltration rates

Model Comparison Strategy

Update 2007 Model to use Green & Ampt Method from neighboring Mohave County*

Develop SCS Model using 2017 data and compare

*for model comparison, SCS UH Transform was used for both models

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Results of Model Comparison

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2007 v. 2017 Green & Ampt – Hydrograph Comparisons

• 2007 infiltration values for the Lincoln soil group were higher than the dense infiltration for the 2017 soils causing the flows to be higher in this area.

• Infiltration rates were similar over the rest of the area, creating almost identical hydrographs to 2007.

180

1000

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0 200 400 600

Pulsipher Before DB07_G&A 17_G&A_D

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0 50 100 150 200

Abbott Before DB07_G&A 17_G&A_D

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Town Before DB07_G&A 17_G&A_D

SCS v. G&A

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Town Wash

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0 100 200 300 400 500 600

Green & Ampt vs. SCS 10T

07_G&A SCS 17_G&A_D

SCS v. G&A

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Town Wash

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0 100 200 300 400 500 600

Green & Ampt vs. SCS 40T (Town Before DB)

07_G&A SCS 17_G&A_D

SCS v. G&A

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Abbott Wash

0200400600800

10001200140016001800

0 50 100 150 200

Green & Ampt vs. SCS 10A

07_G&A SCS 17_G&A_D

SCS v. G&A

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Abbott Wash

0

500

1000

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2500

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3500

0 50 100 150 200

Green & Ampt vs. SCS Abbott 40A (Before DB)

07_G&A SCS 17_G&A_D

SCS v. G&A

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Pulsipher Wash

0

500

1000

1500

2000

2500

0 100 200 300 400 500 600

Green & Ampt vs. SCS 20P (Pulsipher Before DB)

07_G&A SCS 17_G&A_D

Observations

The more accurate method can only be determined with calibration data

The SCS Curve Number method produced generally lower peak flows than Green & Ampt but runoff volume was generally higher.*

Based on this comparison, it was decided to continue using the Green & Ampt method in Mesquite

*Using G&A Method with Clark UH (per Mohave County Manual) significantly lowered peak flowrates

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Goal of Model Comparison

25

Determine Most Appropriate Method for City of Mesquite

SCSUpdate models to

be more consistent with the rest of Clark County

Green & AmptUpdate models to

use prescribed methodology

(Mohave County)

G&A with Clark UH

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Town Wash

0

1000

2000

3000

4000

5000

6000

0 100 200 300 400 500 600

Green & Ampt vs. SCS 10T

07_G&A SCS 17_G&A_D 17_MPU

G&A with Clark UH

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Town Wash

0

1000

2000

3000

4000

5000

6000

0 100 200 300 400 500 600

Green & Ampt vs. SCS 40T (Town Before DB)

07_G&A SCS 17_G&A_D 17_MPU

G&A with Clark UH

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Abbott Wash

0200400600800

10001200140016001800

0 100 200 300 400 500

Green & Ampt vs. SCS 10A

07_G&A SCS 17_G&A_D 17_MPU

G&A with Clark UH

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Abbott Wash

0

500

1000

1500

2000

2500

3000

3500

0 100 200 300 400 500

Green & Ampt vs. SCS 40A (Abbott Before DB)

07_G&A SCS 17_G&A_D 17_MPU

G&A with Clark UH

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Pulsipher Wash

0

500

1000

1500

2000

2500

0 50 100 150 200 250 300 350 400 450

Green & Ampt vs. SCS 20P (Pulsipher Before DB)

07_G&A SCS 17_G&A_D 17_MPU

Master Plan Impact

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Master Plan Impacts

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Windmill Wash Detention Basin

0

1000

2000

3000

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5000

6000

7000

8000

0 50 100 150 200 250 300 350 400 450 500

Windmill Wash Detention Basin Inflow

2017 SCS G&A Normal FLO-2D* 2017 G&A

• 522 ac-ft existing• 887 ac-ft expansion proposed

Thank you

If you’d like to find out more visit:www.atkinsglobal.com© Atkins Limited except where stated otherwise.

The Atkins logo, ‘Carbon Critical Design’ and the strapline‘Plan Design Enable’ are trademarks of Atkins Limited.

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Supplemental Slides

Basin Specifics

25 September 2017 34

Town

The Dense Green and Amptand the SCS methods are overtopping the Detention Basin for Town wash.

25 September 2017 35

HEC-1Node

2007 MPU

2017 G&A

2017 G&A

Dense

2017 SCS G&A G&A Dense SCS

A W A W A WT1-A 1405 1280 1813 1169 -9% -19% 29% 63% -17% -37%T1-B 1863 1534 2172 1179 -18% -42% 17% 39% -37% -87%1T 3109 2658 3811 2241 -15% 23% -28%T2 2527 1889 2987 1969 -25% -84% 18% 61% -22% -74%T3 977 781 1096 1062 -20% -29% 12% 18% 9% 13%

10T 3943 2924 4779 3651 -26% 21% -7%T4 926 710 912 777 -23% -15% -2% -1% -16% -10%T5 2556 1490 2154 2134 -42% -139% -16% -53% -17% -55%

20T 3915 2266 4320 4188 -42% 10% 7%T6 2503 1793 2446 2314 -28% -99% -2% -8% -8% -26%

30T 4219 2363 4556 4876 -44% 8% 16%T7 1215 1167 1190 901 -4% -4% -2% -2% -26% -28%T8 946 850 924 742 -10% -10% -2% -2% -22% -21%

40T 4543 2685 4889 5426 -41% 8% 19%TWDB 602 496 1232 3300 -18% 105% 448%T9-A 775 747 762 618 -4% -2% -2% -1% -20% -11%51T 775 747 762 618 -4% -2% -20%T9-C 700 672 684 618 -4% -2% -2% -1% -12% -5%52T 1445 1389 1417 1202 -4% -2% -17%T9-B 1611 1537 1575 1367 -5% -5% -2% -2% -15% -16%

60T (I-15) 2870 2738 2804 2390 -5% -2% -17%T10 604 612 620 453 1% 1% 3% 1% -25% -12%

70T (Virgin River) 3394 3273 3344 2764 -4% -1% -19%T10A 246 260 261 187 6% 1% 6% 1% -24% -4%T11 104 124 128 69 19% 1% 23% 2% -34% -2%

Combined Differences -15% -21% 10% 5% 4% -17%

100-Year Flows for Town Wash

Abbott

25 September 2017 36

HEC-1 Node

2007 MPU

2017 G&A

2017 G&A

Dense

2017 SCS

G&A G&A Dense SCS

A W A W A WA1 1217 874 1217 1139 -28% -32% 0% 0% -6% -7%A2 744 517 726 1025 -31% -24% -2% -2% 38% 30%

10A 1566 1055 1547 1494 -33% -1% -5%A3 511 367 501 450 -28% -16% -2% -1% -12% -7%A4 287 220 285 257 -23% -7% -1% 0% -10% -3%

20A 1894 1259 1863 1851 -34% -2% -2%A5 730 722 742 566 -1% -1% 2% 1% -22% -11%

40A5 2033 1401 2008 1983 -31% -1% -2%A6 702 510 686 604 -27% -18% -2% -2% -14% -9%A7 367 278 358 298 -24% -7% -2% -1% -19% -5%

30A 1037 699 1012 881 -33% -2% -15%A8 413 422 428 329 2% 1% 4% 2% -20% -10%

40A 3082 2281 3081 2878 -26% 0% -7%AWDB 334 281 334 658 -16% 0% 97%A9-C 992 994 1019 831 0% 0% 3% 2% -16% -12%41A 992 994 1019 831 0% 3% -16%A9-B 942 887 896 612 -6% -5% -5% -4% -35% -29%

50A (I-15) 1820 1769 1802 1340 -3% -1% -26%A10 548 598 602 455 9% 5% 10% 5% -17% -9%

60A (Virgin River) 2206 2203 2240 1671 0% 2% -24%

Combined Difference -17% -15% 0% 0% -7% -11%

100-Year Flows for Abbott Wash

Pulsipher

25 September 2017 37

HEC-1Node

2007 MPU

2017 G&A

2017 G&A

Dense

2017 SCS G&A G&A Dense SCS

A W A W A WP1 944 845 935 745 -10% -9% -1% -1% -21% -18%P2 760 761 770 705 0% 0% 1% 1% -7% -4%10P 1333 1249 1333 1045 -6% 0% -22%P3 1061 1132 1164 951 7% 6% 10% 9% -10% -9%20P 1968 1978 2073 1652 1% 5% -16%

PWDB 148 285 464 209 93% 214% 41%P4 900 914 946 745 2% 1% 5% 4% -17% -14%30P 909 844 875 680 -7% -4% -25%P6 509 365 503 508 -28% -21% -1% -1% 0% 0%P7 622 585 629 532 -6% -4% 1% 1% -14% -9%

40P 1808 1576 1763 1510 -13% -2% -16%P8 479 458 468 413 -4% -1% -2% -1% -14% -4%

50P (I-15) 2105 1871 2058 1753 -11% -2% -17%P9 166 155 163 88 -7% -1% -2% 0% -47% -6%

60P (Virgin River) 2218 1987 2173 1815 -10% -2% -18%

Combined Difference 0% -6% 15% 2% -14% -13%

100-Year Flows for Pulsipher Wash

Additional

25 September 2017 38

HEC-1Node

2007 MPU

2017 G&A

2017 G&A

Dense

2017 SCS G&A G&A Dense SCS

A W A W A WAD-1 493 552 559 293 12% 4% 13% 4% -41% -13%A9a 176 170 172 149 -3% 0% -2% 0% -15% -1%AD-2 603 689 691 506 14% 8% 15% 8% -16% -9%10AD 741 821 825 618 11% 11% -17%AD-3 190 205 206 152 8% 1% 8% 1% -20% -3%AD-4 615 730 733 513 19% 11% 19% 12% -17% -10%AD-5 379 437 446 214 15% 6% 18% 7% -44% -17%

Combined Differences 11% 14% 12% 15% -24% -25%

100-Year Flows for Additional Washes

Western & Unnamed

25 September 2017 39

HEC-1Node

2007 MPU

2017 G&A

2017 G&A

Dense

2017 SCS G&A G&A Dense SCS

A W A W A WW1 1040 1046 1110 971 1% 1% 7% 8% -7% -8%W2 147 146 148 125 -1% 0% 1% 0% -15% -1%W3 564 525 542 405 -7% -3% -4% -2% -28% -11%W4 240 240 243 224 0% 0% 1% 0% -7% -1%W5 306 316 319 293 3% 1% 4% 1% -4% -1%

W6a 808 545 858 1063 -33% -46% 6% 9% 32% 45%W6b 1136 778 1091 1012 -32% -62% -4% -8% -11% -22%10W 1805 1208 1810 1947 -33% 0% 8%

Combined Difference -13% -21% 1% 2% -4% 0%

100-Year Flows for Western Wash

100-Year Flows for Unnamed Wash

HEC-1Node

2007 MPU

2017 G&A

2017 G&A

Dense

2017 SCS

G&AG&A Dense SCS

U1 (I-15) 740 733 733 581 -1% -1% -21%

Green & Ampt or SCS? �2017 Flood Control Master Plan Update, City of Mesquite, NevadaProject LocationMaster Plan OverviewMaster Plan ScopeStudy Area2017 Hydrologic Model ComparisonModel BackgroundOverview of MethodsGoal of Model ComparisonLand Use Changes2007 Hydrologic Soil Group�2017 Hydrologic Soil Group�2007 XKSAT �Green & Ampt�2017 XKSAT Green & Ampt – Dense�Model Comparison StrategyResults of Model Comparison2007 v. 2017 Green & Ampt – Hydrograph Comparisons�SCS v. G&A�SCS v. G&A�SCS v. G&A�SCS v. G&A�SCS v. G&A�ObservationsGoal of Model ComparisonG&A with Clark UH�G&A with Clark UH�G&A with Clark UH�G&A with Clark UH�G&A with Clark UH�Master Plan ImpactMaster Plan Impacts�Thank youSupplemental SlidesTownAbbottPulsipherAdditionalWestern & Unnamed