Phase 1 Completion of XP-SWMM modeling

¡ What is the current storm water design standard for sizing conveyances and ponds? ¡ What is Atlas 14 ¡ How does Atlas 14 impact MCWD? ¡ Some results of XP-SWMM modeling. ¡ What are next steps for MCWD?

¡ TP-40 is a statistical and frequency analysis of rainfall event records from 1910 thru 1958, using 200 hourly stations and approximately 8,000 daily locations with records < 20 years in the eastern ½ of the US.

¡ Published in 1961 ¡ Used as the current industry standard for the

design of storm water capacities in conveyances, ponds, pipes and channels and to afford a level of protection of flooding.

¡ Predicts the magnitude of rainfall events and the probability or expected frequency of return.

¡ Published by NOAA in 2013. ¡ Will replace TP 40. ¡ Estimates event magnitudes and frequency, with 90% confidence limits,

and provides supplementary statistical information on temporal distribution of heavy precipitation and analysis of seasonality and trends of annual maximum precipitation.

¡ Estimates rainfall depths for storm durations of 5 minutes up to 60 days at recurrence intervals of 1-year through 1000-years.

¡ Incorporates data collected from more than 16,000 precipitation recording stations.

¡ Supercedes the Weather Bureau’s Technical Paper No. 40 (1961) and NOAA Technical Memorandum NWS Hydro-35 (1977) as design standards. Hydro-35 updated TP-40 for short duration storms (< 1 hour) in Mpls-St Paul area. Virtually all public service projects, like storm sewers, storm water ponds, outlets, culverts, and flood controls have been designed in the last 30-40 years using TP-40 and Hydro-35

¡ Atlas 14 is not a climate change model and incorporates no assumptions whatsoever.

¡ Atlas 14 is strictly reporting hard-data statistics and rigorous frequency analysis of precipitation event records collected from more than 16,000 locations in the eastern ½ of the US.

The Atlas can be found at: http://www.nws.noaa.gov/oh/hdsc/PF_documents/Atlas14_Volume8.pdf Precipitation frequency estimates for any location in the MCWD are available in a variety of formats through the Precipitation Frequency Data Server (PFDS) at http://hdsc.nws.noaa.gov/hdsc/pfds

Project Designs – more storm water volume capacity Regulation and Permitting – Larger event designs, rule revision to define design storms Planning – infrastructure inadequacies to handle larger storms Flood Predictions and Magnitudes Education – public awareness

Atlas 14 100-year, 24-hour Total Precipitation - Peer Reviewed Version

Uniform 5.9 inches/hr

25% increase

over TP-40

PM Rule Language Modeling GIS Total

Panzer Panzer/Shoemaker Shoemaker Shuck Phase 1 - Modeling

1-1 Prepare Atlas 14, 100-year Precipitation Input File 1 2 $415 Using an SCS Type 2 Rainfall Distribution

1-2 Run XP-SWMM FEMA Version Model with Atlas 14 1 8 $1,135 Input File

1-3 Prepare Excel Table Comparing Peak 100-year 2 4 $830 Water Levels at Creek Crossings for Three Scenarios:

2004 FIS Study 100-year XP-SWMM FEMA Version 100-year XP-SWMM Atlas 14 Version 100-year

1-4 Prepare Excel Table Comparing Peak 100-year Discharge (Q) at Creek Crossings for Three Scenarios: 1 4 $655

2004 FIS Study 100-year XP-SWMM FEMA Version 100-year XP-SWMM Atlas 14 Version 100-year

Conduct Limited Municipal and Agency Survey

1-5 Prepare Table of Atlas 14 1-year, 10-year and 100-year Rainfall Depths for all Cities in MCWD 2 8 6 $1,940

1-6 Presentation to Board of Managers Total Phase 1: 7 0 26 6 $4,975

MCWD Staff

No Charge

Atlas 14 XP-SWMM Modeling and Rule Changes Budget Hours

Phase 2 - Rule Revision

2-1 Red-line Draft Rule Language Changes for Atlas 14 1 4 $775

2-2 Tech Memo or Assist with SONAR 2 8 $1,550

2-3 Public Hearing

2-4 Develop Draft Atlas 14 Guidance for Permit Applicants 8 4 $1,880

Total Phase 2: 11 12 4 0 $4,205

No Charge

Conduct Limited Municipal and Agency Survey MCWD Staff

The XP-SWMM model is far from perfect, but available. It was used to look at how Atlas 14 might impact major resources like Lake Minnetonka and Minnehaha Creek. A comparison of the 2004 FIS and XP-SWMM using TP-40:

4 foot drop in flood level

Versioning

XP-SWMM was sent to FEMA in Release 10. It was released for preliminary mapping in Release 12. The results of the two versions do not compare well. Release 12 is considered superior to Release 10. Running TP-40 and Atlas 14 in Release 12 should show how the increase in large event total precipitation will affect flood flows on Minnehaha Creek. Generally, a peak flow increase of 50% is predicted.

100 cfs ?

¡ Atlas 14 will be the official design standard adopted by the Minnesota Department of Transportation effective June 30, 2014.

¡ In MCWD, Atlas 14 100-year storm magnitudes are 22% to 27% greater than TP-40. This will increase runoff volumes and peak flows that need to be accommodated by public works.

¡ In MCWD, Atlas 14 10-year storm magnitudes are 0% to 2% greater than TP-40

¡ The Atlas 14, 100-year 24-hour precipitation for MCWD ranges from 7.2-7.5 inches across the District.

¡ The 100-year, 24-hour “Design Storm” as described in current MCWD rules, should be changed from 5.9” in 24 hours to 7.5 inches in 24 hours. The 10-year and 1-year, 24-hour “Design Storm” should be updated similarly.

¡ The change of the “Design Storm” definition should be advisory with respect to all new permit applications subject to the Storm Water Rule until such time as a new MCWD Storm Water rule becomes effective.

¡ FEMA Flood maps are not going to be updated with Atlas 14 precipitation.

b a-b c c-b d e e-d

100-year, 24 Hour Model 2004 FIS minus XP Version 10 TP-40

100-year, 24 Hour Model Version 12 minus 100-year, 24 Hour Model 100-year, 24 Hour Model Version 12 minus

a XP Version 10, XP Version 12, Version 10 XP Version 10, XP Version 12, Version 10

Location

2004 FIS 100-Year TP-40 TP-40 Atlas 14 Atlas 14 Atlas 14

Flow (cfs) Elevation (ft, MSL) Flow (cfs) Elevation

(ft, MSL) Flow (cfs) Elevation (ft, MSL) Flow (cfs) Elevation

(ft, MSL) Flow (cfs) Elevation (ft, MSL) Flow (cfs) Elevation

(ft, MSL) Flow (cfs) Elevation (ft, MSL) Flow (cfs) Elevation

(ft, MSL)

Interstate 494 865 930.7 374 929.1 491.0 1.6 455.3 929.6 81.3 0.5 1000 931.8 546.9 930.0 -453.1 -1.8

CR 73 980 912.1 374 910.1 606.0 2.0 457.0 910.4 83.0 0.3 1000.7 912.1 553.0 910.8 -447.7 -1.3

Minnetonka Blvd 1050 909.9 374 904.3 676.0 5.6 456.3 904.8 82.3 0.5 991.9 907.7 549.6 905.4 -442.3 -2.3

34th Street 1100 907.5 380 902.7 720.0 4.8 458.5 903.1 78.5 0.4 988.3 905.3 549.1 903.6 -439.2 -1.7

36th Street 1100 905.1 378 902.0 722.0 3.1 458.5 902.2 80.5 0.2 988.1 904.3 549.1 902.8 -439.0 -1.5

Hwy 7 1100 904.6 420 900.8 680.0 3.8 513.9 901.1 93.9 0.3 988 903.4 549.1 901.6 -438.9 -1.8

Blake Road 1100 903.8 392 900.6 708.0 3.2 458.4 900.9 66.4 0.3 988.5 903 549.0 901.3 -439.5 -1.7

Meadowbrook Rd 1100 894.8 376 890.1 724.0 4.7 458.5 890.3 82.5 0.2 991.6 892 547.3 890.6 -444.3 -1.4

Louisiana Ave 1100 893.7 400 888.9 700.0 4.8 458.4 888.9 58.4 0.0 985.1 891.6 549.3 889.4 -435.8 -2.2

Excelsior Blvd 1100 893.6 413 888.9 687.0 4.7 458.2 888.8 45.2 -0.1 976.5 891.5 548.3 889.4 -428.2 -2.1

Yosemite Ave 1100 891.6 484 888.7 616.0 2.9 456.5 888.4 -27.5 -0.3 925.2 890.7 545.2 888.9 -380.0 -1.8

44th Street 1100 890.5 480 888.0 620.0 2.5 456.5 887.7 -23.5 -0.3 925 890.3 545.2 888.3 -379.8 -2.0

Hwy 100 1100 889.4 483 887.4 617.0 2.0 456.5 887.2 -26.5 -0.2 924.7 889.4 545.1 887.7 -379.6 -1.7

Browndale Ave 1100 888.9 510 887.3 590.0 1.6 456.4 887.0 -53.6 -0.3 924.1 889.3 545.1 887.5 -379.0 -1.8

Wooddale Ave 1100 878.2 514 871.9 586.0 6.3 456.4 871.7 -57.6 -0.2 924.1 875 545.1 872.1 -379.0 -2.9

W 54th Street 1100 867.5 535 862.9 565.0 4.6 456.4 862.5 -78.6 -0.4 923.8 864.6 545.1 863.0 -378.7 -1.6

W 56th Street 1100 862 523 860.0 577.0 2.0 456.4 859.4 -66.6 -0.6 920.7 862.3 544.9 860.2 -375.8 -2.1

France Ave 1100 861.3 544 859.1 556.0 2.2 456.4 858.8 -87.6 -0.3 911.9 861.1 545.1 859.3 -366.8 -1.8

Xerxes Ave 1100 853.6 542 851.0 558.0 2.6 456.4 850.6 -85.6 -0.4 911.4 852.6 545.1 851.1 -366.3 -1.5

Upton Ave 1100 851.5 543 849.8 557.0 1.7 456.4 849.2 -86.6 -0.6 911.5 851.4 545.1 850.0 -366.4 -1.4

Penn Ave 1100 849.4 560 846.7 540.0 2.7 480.4 846.2 -79.6 -0.5 911.5 848.2 619.2 847.1 -292.3 -1.1

Logan Ave 1100 846.9 567 844.0 533.0 2.9 543.8 843.7 -23.2 -0.3 894.3 844.8 747.7 844.6 -146.6 -0.2

Humboldt Ave 1100 845.1 567 843.4 533.0 1.7 560.6 843.1 -6.4 -0.3 891.8 844.1 770.8 843.9 -121.0 -0.2

Dupont Footbridge 1100 844.4 723 842.2 377.0 2.2 657.3 841.9 -65.7 -0.3 887.8 843.1 874.8 843.0 -13.0 -0.1

Lyndale Ave 1100 843.9 722 841.4 378.0 2.5 670.2 841.1 -51.8 -0.3 900.1 842.4 885.2 842.5 -14.9 0.1

Nicollet Ave 1100 834.4 800 833.7 300.0 0.7 810.6 833.7 10.6 0.0 882.1 833.9 1036.2 834.5 154.1 0.6

E 50th Street 1100 830 835 828.6 265.0 1.4 846.7 828.7 11.7 0.1 883.5 828.2 1058.3 830.3 174.8 2.1

Chicago Ave 1100 825.6 1069 825.1 31.0 0.5 1076.8 825.1 7.8 0.0 879.3 824.4 1344.3 825.6 465.0 1.2

12th Ave 1100 824.7 867 823.9 233.0 0.8 884.6 823.9 17.6 0.0 877.6 823.4 1353.5 824.4 475.9 1.0

Bloomington Ave 1100 823 1095 822.6 5.0 0.4 1109.5 822.4 14.5 -0.2 877.01 821.9 1393.9 823.1 516.9 1.2

Cedar Ave 1150 820.1 1089 820.5 61.0 -0.4 1102.2 820.5 13.2 0.0 879.4 820.1 1428.6 820.9 549.2 0.8

Minnehaha Pkwy 1150 818.8 861 818.7 289.0 0.1 600.6 817.1 -260.4 -1.6 680.5 817.8 627.8 817.7 -52.7 -0.1

28th Ave 930 817.8 927 817.6 3.0 0.2 497.0 815.5 -430.0 -2.1 669.3 815.9 623.6 816.1 -45.7 0.2

Nokomis Ave 930 816.6 952 816.3 -22.0 0.3 497.3 814.4 -454.7 -1.9 674.1 815 623.8 814.9 -50.3 -0.1

34th Ave 930 814.7 957 814.5 -27.0 0.2 496.8 812.4 -460.2 -2.1 635.4 813 626.4 812.9 -9.0 -0.1

Hiawatha Ave 930 813.4 954 810.9 -24.0 2.5 496.8 808.3 -457.2 -2.6 635.4 809.1 627.4 809.1 -8.0 0.0

903

905

907

909

911

913

2004 FIS XP-10 TP-40 XP-10 Atlas 14 XP-12 Atlas 14

Minnetonka Blvd, 100-year

900

905

910

915

920

925

930

Inte

rsta

te 4

94

CR 7

3

Min

neto

nka

Blvd

34th

Str

eet

Minnetonka, 100-year

2004 FIS XP-10 TP-40 XP-10 Atlas 14

XP-12 Atlas 14 XP-12 TP-40

885

890

895

900

90536

th S

tree

t

Hwy

7

Blak

e Ro

ad

Mea

dow

broo

k Rd

Loui

siana

Ave

Exce

lsior

Blv

d

SLP _ Hopkins, 100-year

2004 FIS XP-10 TP-40 XP-10 Atlas 14 XP-12 Atlas 14 XP-12 TP-40

850

855

860

865

870

875

880

885

890Yo

sem

ite A

ve

44th

Str

eet

Hwy

100

Brow

ndal

e Av

e

Woo

ddal

e Av

e

W 5

4th

Stre

et

W 5

6th

Stre

et

Fran

ce A

ve

Xerx

es A

ve

Edina, 100-year

2004 FIS XP-10 TP-40 XP-10 Atlas 14 XP-12 Atlas 14 XP-12 TP-40

810

815

820

825

830

835

840

845

850

855

Minneapolis, 100-year

2004 FIS XP-10 TP-40 XP-10 Atlas 14 XP-12 Atlas 14 XP-12 TP-40

0

200

400

600

800

1000

1200

1400

1600

Inte

rsta

te 4

94

CR 7

3

Min

neto

nka

Blvd

34th

Str

eet

36th

Str

eet

Hwy

7

Blak

e Ro

ad

Mea

dow

broo

k Rd

Loui

siana

Ave

Exce

lsior

Blv

d

Yose

mite

Ave

44th

Str

eet

Hwy

100

Brow

ndal

e Av

e

Woo

ddal

e Av

e

W 5

4th

Stre

et

W 5

6th

Stre

et

Fran

ce A

ve

Xerx

es A

ve

Upt

on A

ve

Penn

Ave

Loga

n Av

e

Hum

bold

t Ave

Dupo

nt F

ootb

ridge

Lynd

ale

Ave

Nic

olle

t Ave

E 50

th S

tree

t

Chic

ago

Ave

12th

Ave

Bloo

min

gton

Ave

Ceda

r Ave

Min

neha

ha P

kwy

28th

Ave

Nok

omis

Ave

34th

Ave

Hiaw

atha

Ave

Peak

Dis

char

ge (c

fs)

Peak Discharge, Minnehaha Creek, 100-year, 24-hour event

2004 FIS XP-10 TP-40 XP-10 Atlas 14 XP-12 Atlas 14 XP-12 TP-40

¡ Use XP-SWMM Release 12 to Develop Guidance for Permit Applicants ¡ Check in with Municipalities on Policies ¡ Tech Memo or Assist with Rule Change

SONAR ¡ Red-line Draft Rule Language ¡ Public Hearing

1

MINNEHAHA CREEK WATERSHED DISTRICT BOARD OF MANAGERS

REVISIONS

PURSUANT TO MINNESOTA STATUTES § 103D.341

Adopted Effective

DRAFT REVISIONS – December 27, 2013XXXXX, 2014

KEY DEFINITIONS

· Abstraction means permanent retention of runoff on a site through structures and practices such as infiltration, evapotranspiration and capture and reuse. See also the Abstraction Credit Schedule in Appendix A of the Stormwater Management Rule.

· Access Corridor means a corridor equal to 30 percent of the total shoreline length to a maximum of 30 feet.

· Agricultural activity means the use of land for the production of agronomic, horticultural or silvicultural crops, including nursery stock, sod, fruits, vegetables, flowers, forages, cover crops, grains, and Christmas trees. Agricultural activity also includes grazing.

· Alteration or alter means any activity that will change or diminish the course, current, or cross-section of public waters and wetlands.

· BMPs (best management practices) are actions taken to prevent or reduce detrimental impacts to the environment while maintaining the natural characteristics of the environment.

· Beds of a waterbody means all portions of a waterbody located below the ordinary high-water level.

· Bioengineering Stabilization means the strategic installation of natural, vegetative, biologically active materials in conjunction with toe stabilization, riprap or other hard-armoring materials to stabilize shoreline or streambank areas and associated slopes and prevent erosion.

· Biological Stabilization means the strategic placement of natural, vegetation, biologically active materials – such as but not limited to brush mattresses, live stakes/plantings, brush layering, fiber rolls, root wads and willow wattles – to stabilize shoreline or streambank areas and prevent erosion.

· Design Storm refers to a storm magnitude with a return period (T) which has the probability (1/T) of being equaled or exceeded in any given year. For example, a “100-year” event at a given location has a chance of 1/100 or 0.01 or 1% of being equaled or exceeded in any given year. For MCWD regulatory purposes, the rainfall depths to be used must be calculated using a model utilizing the most recent applicable published National Weather Service reference data (e.g., Atlas 14).are as follows:

o 1-year event = 2.4” in 24 hours o 10-year event = 4.1” in 24 hours o 100-year event = 5.9” in 24 hours

2

All rainfall depths shall use the NRCS Type II rainfall distribution. · Development means any land-disturbing activity that creates impervious surface, with

the exception of Linear Transportation Projects. · Dredge means the removal of the sediment or other materials from the beds, banks or

shores of, a waterbody by means of hydraulic suction, mechanical excavation or any other means.

· Excavation means the displacement or removal of sediment or other material. · Fast Track Permit means a permit issued by staff for standard rip rap, sandblankets or

maintenance fill projects which are installed according to technical specifications provided by District engineers, or an erosion control permit issued by staff in accordance with the criteria in the Erosion Control Rule.

· Fill means any material placed or intended to be placed on the bed or bank of any protected water or wetland. Fill must be clean, inorganic material that is free of pollutants.

· Floodplain means the areas adjoining a watercourse or water basin which have been or hereafter may be covered by a 100-year regional flood.

· Impervious refers to surfaces that are compacted or covered with a layer of material such that it is highly resistant to infiltration of runoff including but not limited to gravel, rock, asphalt, concrete and non-pervious paver systems.

· Land-disturbing activity or land disturbance means any disturbance to the ground surface that exposes soil and, through the action of wind or water, may result in soil erosion or the movement of sediment into waters, wetlands or storm sewers or onto adjacent property. Land-disturbing activity includes but is not limited to the demolition of a structure or surface, soil stripping, clearing, grubbing, grading, excavating, filling and the storage of soil or earth materials, but does not include agricultural activity.

· Linear Reconstruction Project means rebuilding a road, sidewalk or trail within existing right-of-way, and may include an increase in the area of impervious surface. Rehabilitation, including mill and overlay, of a road, sidewalk or trail within existing right-of-way in a manner that disturbs and/or replaces only the existing pavement and does not involve the addition of impervious surface area or the exposure of underlying soils is not considered a Linear Reconstruction Project.

· Linear Transportation Project means construction of a new road, trail, or sidewalk or reconstruction of an existing road, trail, or sidewalk (see Linear Reconstruction Project).

· Meadow Condition shall be modeled using an event mean concentration of 0.04 mg/L of Total Phosphorus in runoff in accordance with the Minnesota Pollution Control Agency’s Minnesota Stormwater Manual and the MCWD Hydrologic, Hydraulic, and Pollutant Loading Study (HHPLS). For runoff rates, meadow condition shall be modeled using curve numbers corresponding to soil and cover types “meadow,” “brush,” or “woods,” as appropriate, provided in Table 2-2c of the Natural Resources Conservation Service’s (NRCS) Technical Release 55 (TR-55).

· Native Vegetation means plant species that are indigenous to Minnesota or that expand the range into Minnesota without being intentionally or unintentionally introduced by human activity and that are classified as native in the Minnesota Plant Database, Minnesota Department of Natural Resources, St. Paul, 2002.

Comment [MW1]: My understanding is that this should be deleted as inapplicable given Atlas 14 data.

3

· Natural State refers to a segment of shoreline or streambank displaying essentially no evidence of constructed stabilization or from which constructed stabilization measures have been removed by degradation or deconstruction.

· New Principal Residential Structure means a single-family residential building to be constructed on undeveloped property that has not been and will not be subdivided for purposes of the construction or on a property from which the principal building has been removed and that has not been and will not be subdivided for purposes of constructing a new single-family residential building.

· NPDES refers to Clean Water Act National Pollutant Discharge Elimination System. · NURP means Nationwide Urban Runoff Program developed by the Environmental

Protection Agency (EPA) to study stormwater runoff from urban development. · 100-year high water elevation means the highest water elevation associated with a

waterbody reached during a 24-hour precipitation event with a recurrence interval of 100 years, as specified by the District in a written guidance document or, if not so specified, as calculated by a permit applicant in accordance with the most recent applicable National Weather Service reference data (e.g., Atlas 14) determined by the District in order to act on a the permit application.

· Ordinary high water level (OHW) means the boundary of a waterbody and shall be an elevation delineating the highest water level which has been maintained for a sufficient period of time to leave evidence upon the landscape, commonly that point where the natural vegetation changes from predominantly aquatic to predominantly terrestrial. For watercourses, the ordinary high water level shall be the elevation of the top of the bank of the channel. For reservoirs and flowages, the ordinary high water level is the operating elevation of the normal summer pool.

· Parcel or Site means a contiguous area of land under common ownership, designated and described in official public records and separated from other lands. “Parcel” and “site” generally have equivalent meanings for purposes of these rules, and are used interchangeably in rule text.

· Person means any natural person, partnership, unincorporated association, corporation, municipal corporation or political subdivision of the State of Minnesota.

· Pervious refers to surfaces that are readily penetrated or permeated by rainfall or runoff resulting in infiltration and reduced runoff.

· PID means Property Tax Identification Number and is an abbreviated method to substitute for the legal description for a parcel of property (ex. 03-117-24 33 0004 is Section 3 of Township 117; Range 24; Quarter 33 and Parcel 4).

· Preserve Wetland is the highest of four management classifications assigned through use of either the District’s Functional Assessment of Wetlands (FAW) or the current version of the Minnesota Routine Assessment Method (MnRAM).

· Public waters means all waters identified as public waters under Minn. Stat.103G.005, Subd. 15.

· Public waters wetlands means all wetlands identified as public waters wetlands under Minn. Stat. 103G.005, Subd. 18.

· Redevelopment means land-disturbing activity that creates or replaces impervious surface on a parcel that is fully or partially occupied by buildings and/or impervious surface with the exception of Linear Transportation Projects.

4

· Regional flood means a flood which is representative of large floods known to have occurred generally in Minnesota and reasonably characteristic of what can be expected to occur on an average frequency in the magnitude of the 100-year recurrence interval.

· Riprap (for purposes of Floodplain Alteration 3(c)). · Shoreline means the ordinary high water level of a water basin and the area waterward

thereof. · Site – see Parcel · Stabilization Zone means the area of land paralleling the shoreline or streambank and

extending 20 feet inland from all points along the ordinary high-water mark of the shoreline.

· Structural Stabilization is the use of engineered systems – such as riprap, retaining walls, headwalls, groins, revetments, gabions – to stabilize shoreline or streambank areas and associated slope and prevent erosion.

· Subwatershed means one of the fifteen major subwatershed planning units within the Minnehaha Creek Watershed District.

· Top of bank means the ordinary high water level for a water basin or wetland, and the break in slope for a watercourse.

· Waterbasin means an enclosed natural depression with definable banks capable of containing water which may be partly filled with waters.

· Waterbody means all waterbasins, watercourses and wetlands as defined in these rules. · Watercourse means any channel having definable beds and banks capable of conducting

generally confined runoff from adjacent lands, or any channel included in the District’s inventory of first order streams. During floods water may leave the confining beds and banks but under low and normal flows water is confined within the channel. A watercourse may be perennial or intermittent. Watercourse does not include roadside ditches created by excavation or other human construction activity.

· Wetland buffer zone means an area of vegetated groundcover abutting or surrounding a wetland.

· Wetlands means all wetlands identified as wetlands under Minnesota Stat. 103G.005, Subd 19. The term does not include "public waters wetlands" as defined under Minnesota Statutes 103G.005, subdivision 15a.

-MORE-

MINNESOTA DEPARTMENT OF TRANSPORTATION

Engineering Services Division Technical Memorandum No. 13-08-B-04

May 28, 2013

To: Electronic Distribution Recipients From: Jon M. Chiglo, P.E. Division Director, Engineering Services Subject: Use of Atlas 14 Volume 8 Precipitation Frequency Estimates Expiration This is a new Technical Memorandum and shall remain in effect until May 28, 2018 unless superseded or included in the MnDOT Drainage Manual prior to that date. Implementation The guidelines contained in this Memorandum are effective immediately for trunk highway projects where feasible. Use the Atlas 14 precipitation data for hydraulic design on all trunk highway projects let after June 30, 2014. Local road authorities are encouraged to adopt these or similar guidelines. Introduction The National Oceanic and Atmospheric Administration (NOAA) published new precipitation frequency estimates for the Midwestern States in Atlas 14 Volume 8. This information supersedes Technical Paper (TP)-40 published in1961 and NOAA Technical Memorandum NWS Hydro 35 published in 1977 that are the sources of precipitation frequency data and Intensity-Duration-Frequency (IDF) curves recommended in the Drainage Manual. Purpose This Technical Memorandum updates MnDOT design precipitation criteria to use the precipitation frequency data from NOAA Atlas 14. This replaces the design rainfall data in the Drainage Manual (2000). Guidelines

Use Atlas 14 Precipitation Frequency Estimates when using rainfall-runoff models to compute hydrology for the design of hydraulic infrastructure. The data is obtained from NOAA’s Precipitation Frequency Data Server (PFDS) at http://hdsc.nws.noaa.gov/hdsc/pfds/ based on the project location. For rainfall-runoff models that use the Natural Resources Conservation Service (NRCS) rainfall distributions, if feasible, use a rainfall distribution based on the Atlas 14 data. Use the NRCS Type II rainfall distribution for NRCS peak flow methodology or for other projects where developing a rainfall distribution is not feasible. Atlas 14 precipitation data should be used immediately for trunk highway projects using rainfall-runoff models provided its application does not jeopardize letting dates of projects already in the design phase. Use the Atlas 14 precipitation data for the hydraulic design of all trunk highway projects let after June 30, 2014. Where use of Atlas 14 is not feasible, evaluate the impacts of using Atlas 14 and document the justification for using the criteria from the Drainage Manual (2000). Notify the State Hydraulics Engineer about projects designed with rainfall-runoff models let after June 30, 2014 that are not designed with Atlas 14 precipitation data.

Technical Memorandum No. 13-08-B-04 Use of Atlas 14 Volume 8 Precipitation Frequency Estimates May 28, 2013 Page 2

-END-

Questions Any questions regarding the technical provisions of this memorandum can be addressed to the following:

• Andrea Hendrickson, State Hydraulics Engineer, at (651) 366-4466 Any questions regarding publication of this Technical Memorandum should be referred to the Design Standards Unit, DesignStandards.DOT@state.mn.us. A link to all active and historical Technical Memoranda can be found at http://techmemos.dot.state.mn.us/techmemo.aspx. To add, remove, or change your name on the Technical Memoranda mailing list, please visit the web page http://techmemos.dot.state.mn.us/subscribe.aspx