PowerPoint Presentation...5/25/2017 6 Manning’s n-Value Recommendations 16 L 2D Flow Area Cell...

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Harris County Flood Control District HEC-RAS

2D Modeling Guidelines(Standardizing HEC-RAS 2D Models for Submittal Within Harris County) Presented by:

Matthew Zeve, P.E., CFMHarris County Flood Control District

Lonnie Anderson, P.E., CFMPape-Dawson Engineers, Inc.

April 27, 2017

Agenda• HCFCD’s Need for 2D Guidance

Manual

• Recommended Standardized Parameters– Elevational Data

– Manning’s n-Value Selection

– Cell Size Guidance

– Manning’s n-Value Selection

– Lateral Weirs

– Precipitation on Grid

– Calculation Options and Tolerances

– Presentation of Results

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Standardize

Common Model

Parameters

Reduce Review Time

Standardize Expected Model

and Report Results

MAIN GOALS

Agenda

• Adverse Impact Definition

– Difference between a 1D NAI and 2D NAI

• Example Applications

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Standardize

Common Model

Parameters

Reduce Review Time

Standardize Expected Model

and Report Results

MAIN GOALS

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Introduction

Why the need for a 2D Guidance Manual:

• HEC-RAS 2D is new to consultants and HCFCD’s review staff.

• Outside of the HEC-RAS v5 2D Modeling User’s Manual, there is little reference material currently available.

• HCFCD sees more and more 2D submittals with a wide range of modeling approaches (some good, some not so good) and with widely varied input parameters.

• We need to standardize model deliverables and to a large extent modeling approaches/parameters.

• By standardizing what HCFCD expects in submitted models, review times are reduced and consultants know what is expected.

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Introduction

At the project initiation phase, HCFCD highly recommends a meeting be held with HCFCD Watershed Coordination Department. At this meeting, HCFCD will indicate when a 2D analysis must be included to support NAI drainage reports.

Examples where 2D may be required:

• Linear projects, such as roadway or canals that may obstruct sheet flow

• Land Developments located in areas where sheet flow may be impacted

• Stream crossings where complex flow patterns must be evaluated

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Elevational Data For Modeling

Various types of supporting maps and datasets are required for the development, update, use, and proper understanding of H&H models:

Topography: Effective FEMA 1D modeling for Harris County is based on LiDAR data collected in 2001. In 2008 a new LiDAR data set was acquired.

• 2001 LiDAR has a 15’ x 15’ resolution

• 2008 LiDAR has a 5’ x 5’ resolution

2008 LiDAR is to be used for 2D modeling due to the additional detail it represents. The higher resolution picks up small ditches, roadway embankments, etc. that are not well represented in the 2001 LiDAR.

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Elevational Data For Modeling2001 LiDAR data

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Elevational Data For Modeling2008 LiDAR data

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• Post-project or proposed conditions terrain can be developed using Civil 3D, ARCGIS, or other design software.

• Topographic changes to areas not within the modeler’s project and not reflected in the 2008 LiDAR should be considered as pre-project to isolate impacts of the proposed project.

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• Use of terrain other than the 2008 LiDAR for pre-project or base conditions requires special permission from HCFCD. Sources for other terrain data includes:

– Site Survey

– As-built plans

– New aerial mapping

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Manning’s n-Value Recommendations

• There is limited n-value reference material when applied to 2D modeling.

• Engineers/modelers are well versed in applying n-values to riverine flooding. However, for more shallow flow areas where 2D modeling is likely to be used the most, riverine n-values are not necessarily the best choice and are often much higher than typically used in the past.

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• Recommended values were interpreted by referencing the “Guide for Selecting Manning’s Roughness Coefficients for Natural Channels and Flood Plains,” which was produced by the U.S. Department of Transportation, Report No. FHWA-TS-84-204.

• Recommended values were derived from the Guide by using recommended values for calculating n value in floodplains and with shallow flow depths in mind.

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• The recommended n-values were developed to provide uniformity between models.

• To handle current HEC-RAS limitations of a single n-value per cell face, generalized values were developed for various land use categories as well as defined land cover.

• Recommendations were also developed based on depth of flow.

• Variation from the recommended values requires prior approval from HCFCD unless gage data exists for which calibrations can be made.

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0.0 feet < Flow Depth < 0.33 feet

Manning's n Description

0.02 Streets, Paved Areas

0.03 Industrial/Commercial/Multi Family

0.08 Grassed Swale

0.18 Generic Residential (Lots < ½ Acre)

0.16 Generic Residential (Lots ≥ ½ Acre)

0.15 Dense Grassed Areas (Lawns/Parks)

0.17 Agriculture

0.2 Generic Undeveloped/Open Area

0.22 Pastures

0.25 Woods

1 Bui ldings

0.33 feet < Flow Depth < 3.0 feet

Manning's n Description

0.016 Streets, Paved Areas

0.021 Industrial/Commercial/Multi Family

0.04 Grassed Swale

0.12 Generic Residential (Lots < ½ Acre)

0.08 Generic Residential (Lots ≥ ½ Acre)

0.07 Agriculture

0.05 Dense Grassed Areas (Lawns/Parks)

0.11 Generic Undeveloped/Open Area

0.14 Pastures

0.22 Woods

1 Bui ldings

Refer to the HCFCD H&H Guidance

Manual for help in selecting Manning’s

n values for concentrated flow areas

with flow depths greater than 3 feet.


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2D Flow Area Cell Size Recommendations

• Mesh size selection is largely dependent on land use and the level of detail the study requires. Cell sizes must be at a scale accounting for the differences in n values within urban areas that the modeler intends to evaluate due to current HEC-RAS limitation of single n-value per cell face.

• In urban areas where detailed results are required, a minimum 100’ x 100’ cell size is recommended to pick up changes in n-values.

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• In urban areas where less detailed results are required or in rural areas, a minimum 200’ x 200’ cell size is recommended and generic n-value’s can be applied.

• Smaller cell sizes can be used as necessary.

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100’ x 100’ Grid with 70’ Minimum

Cell Size Along Street Center

Breakline

25’ x 25’ Grid Without Breaklines

Lateral Weir Recommendations

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Lateral weirs are to be located to avoid “double counting” of storage in the 1D and 2D portions of the models.


• Use of 2D equation recommended for flow leaving channel and entering floodplain.

• For areas where true weir flow exist, modeler may use standard weir equation following recommendations found in the HEC-RAS v5.0 User Manual.

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Item Being

Modeled with Lateral

Structure

Description

Range of

Weir Coefficients

Levee/roadway -

3 feet or higher above natural

ground

Broad crested weir

shape, flow over levee/road acts like

weir flow

1.5 to 2.6

(2.0 default)

Levee/roadway -

1 to 3 feet above natural ground

Broad crested weir

shape, flow over levee/road acts like

weir flow, but becomes submerged easily

1.0 to 2.0

Natural high

ground barrier -1 to 3 feet high

Does not really act like

a weir, but water must flow over high ground

to get into 2D flow area

0.5 to 1.0

Non elevated

overbank terrain, lateral structure

not elevated above ground

Overland flow escaping

the main channel0.2 to 0.5

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Precipitation on Grid Recommendations

• When 2D modeling includes the precipitation on grid boundary condition:

– Terrain data should be evaluated to determine if large initial abstraction may occur due to flow being trapped within ditches or sinks within the terrain.

– Where significant initial abstraction is possible, rainfall data not considering losses may need to be applied. (HEC-HMS Precip-Inc)

– In more “well drained” terrains the use of precipitation that has loss rates accounted for may be more applicable. (HEC-HMS Precip-Excess)

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– Flow rates should be validated to the extent possible using traditional methods.

– Flow rates are highly influenced by Manning’s n-value selection.

– Shallow (less than 0.33’) n-values are recommend to be used across watershed, outside of concentrated flow areas like floodplains and channels when precipitation on grid is being used.

– Flow depth results from initial shallow n-value run should be reviewed to identify intermediate flow depth areas where lower n-values could be applied.24

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– To avoid potential conflicts with existing or future development adjacent to the proposed project (whose design may be based on traditional methods presented in the Policy Criteria & Procedure Manual (PCPM), the design of flow conveyance through the project area must be based on current PCPM criteria.

– Variance from use of PCPM methodology for designs using flows predicted by a precipitation on grid based 2D model must receive prior approval by HCFCD.

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Computation Options and Tolerance Recommendations

• Recommend using the Diffusion Wave standard 2D equation: If Saint Venant equation is used, it is to be documented as to reason selected over Diffusion Wave equation.

• HEC-RAS default options and tolerances are to be used: If values modified, the modeler must describe what defaults were modified, the reason for the modification, and the impact of the change on the model.

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Presentation of Results

• Model output and deliverables were standardized to aid reviewers in deciphering model results:

– Runtime Messages are to be included in the report. If errors and/or warnings are present the modeler must either adjust model accordingly to clear these messages or provide a clear explanation as to the cause for the messages and their potential impact on model results.

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– Pre-Project Depth Grid must be provided for 1- and 10-percent events. A symbology was developed to provide the reviewers with a standard color and depth scheme to quickly identify areas of concern.

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Classification RGB Color Code Windows 7Color NameValue

Label Color Red Green Bluevalue <= 0.25

0.0 to 0.25 190 232 255Apatite Blue

0.25 < value <= 0.50.25 to 0.5 0 197 255

Big Sky Blue

0.50 < value <= 1.00.5 to 1.0 0 112 255

Cretan Blue

1.0 < value <= 1.51.0 to 1.5 0 77 168

Ultra Blue

1.5 < value <= 2.01.5 to 2.0 255 255 0

Solar Yellow

2.0 < value <= 2.52.0 to 2.5 255 170 0

Electron Gold

2.5 < value <= 3.02.5 to 3.0 230 76 0

Flame Red

3.0 < value <= 5.03.0 to 5.0 168 0 132

Cattleya Orchid

value > 5.0> 5.0 76 0 115

Ultramarine


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– Pre- and Post-Project Water Surface Elevation Grid Comparison must be provided for 1- and 10-percent events. A symbology was developed to provide the reviewers with a standard color and depth scheme to quickly identify areas of concern.

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1D vs 2D NAI:

Harris County Flood Control District has a 0.00 feet increase WSE tolerance that is applied to 1D modeling.

• For 2D modeling, given the large number of cells, it is conceivable that some may show a slight increase or decrease based upon computational accuracy. In 2D modeling, much more detail is provided that often results in various areas of the model showing impacts and designing projects with a 0.00 feet or less increase is often impractical if not impossible.

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1D vs 2D NAI:

• If 2D modeling is used to help guide 1D modeling, minimal increases noted in the 2D modeling may be acceptable if the 1D modeling provides for 0.00 feet increases and the engineer justifies and explains the increases noted in 2D do not represent a change in flood risk.

• When the HEC-RAS 2D model is to be used as the impact analysis by itself, the HEC-RAS 2D model must show no adverse impact which is defined as 0.00 feet.

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Examples Using 2D to Aid 1D Model Setup:

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Using 2D to assist in mitigation and bridge location.

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Using 2D to assist in mitigation and bridge location.

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• Using 2D to assist in mitigation and bridge location.


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• Using 2D to assist in defining drainage area divides.

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• Using 2D to assist in defining ponded areas.


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• Results compared to traditional hydrologic method.

• Prior to including the ponding areas Clark U.H. flows were 30% to 40% higher than HEC-RAS 2D flows.

Offsite Catchment ID

AREA

(acres)

Clark

U.H. (cfs)

HEC-RAS

2D (cfs)

Percent

Difference

Catchment 1 1549 598.000 644.00 7%

Catchment 2 689 250.000 243.00 -3%

Catchment 3 943 313.000 290.00 -8%

100-Year Peak Flows


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• Comparing static map rasters is very useful in GIS for determining the effectiveness of proposed mitigation.

Example: Initial basin and floodplain fill impact evaluated by subtracting existing WSE grid from the proposed condition WSE grid. The red colors are increased due to the impact of fill on conveyance and basin filling too soon.

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– By adding a berm around the basin and a second weir, structure impacts were fully mitigated with notable benefits within the project area.


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Questions?Thank you!

Presented by:

Matthew Zeve, P.E., CFMHarris County Flood Control District

Lonnie Anderson, P.E., CFMPape-Dawson Engineers, Inc.

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