Harris County Flood Control District HEC-RAS 2D Modeling Guidelines (Standardizing HEC-RAS 2D Models for Submittal Within Harris County) Presented by: April 27, 2017 Matthew Zeve, P.E., CFM Harris County Flood Control District Lonnie Anderson, P.E., CFM Pape-Dawson Engineers, Inc. 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 MAIN GOALS Standardize Common Model Parameters Reduce Review Time Standardize Expected Model and Report Results 2 Agenda Adverse Impact Definition Difference between a 1D NAI and 2D NAI Example Applications MAIN GOALS Standardize Common Model Parameters Reduce Review Time Standardize Expected Model and Report Results 3 1
4 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. 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 5 6 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. 2
Elevational Data For Modeling 2001 LiDAR data 7 Elevational Data For Modeling 2008 LiDAR data 8 Elevational Data For Modeling 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 preproject to isolate impacts of the proposed project. 9 3
Elevational Data For Modeling 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 10 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. 11 Manning s n-value Recommendations 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. 12 4
13 Manning s n-value Recommendations 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. Manning s n-value Recommendations 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 Buildings 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. 0.33 feet < Flow Depth < 3.0 feet 14 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 Buildings Manning s n-value Recommendations 15 5
Manning s n-value Recommendations 16 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. 17 2D Flow Area Cell Size Recommendations 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. 18 6
2D Flow Area Cell Size Recommendations 100 x 100 Grid with 70 Minimum Cell Size Along Street Center Breakline 25 x 25 Grid Without Breaklines 19 Lateral Weir Recommendations Lateral weirs are to be located to avoid double counting of storage in the 1D and 2D portions of the models. 20 21 Lateral Weir Recommendations 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. Item Being Modeled with Lateral Structure Levee/roadway - 3 feet or higher above natural ground Levee/roadway - 1 to 3 feet above natural ground Natural high ground barrier - 1 to 3 feet high Non elevated overbank terrain, lateral structure not elevated above ground Description Broad crested weir shape, flow over levee/road acts like weir flow Broad crested weir shape, flow over levee/road acts like weir flow, but becomes submerged easily Does not really act like a weir, but water must flow over high ground to get into 2D flow area Overland flow escaping the main channel Range of Weir Coefficients 1.5 to 2.6 (2.0 default) 1.0 to 2.0 0.5 to 1.0 0.2 to 0.5 7
Lateral Weir Recommendations 22 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) 23 24 Precipitation on Grid Recommendations When 2D modeling includes the precipitation on grid boundary condition: 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. 8
Precipitation on Grid Recommendations When 2D modeling includes the precipitation on grid boundary condition: 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. 25 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. 26 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. 27 9
Presentation of Results 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. Value Classification RGB Color Code Windows 7 Color Name value <= 0.25 0.25 < value <= 0.5 0.50 < value <= 1.0 1.0 < value <= 1.5 1.5 < value <= 2.0 2.0 < value <= 2.5 2.5 < value <= 3.0 3.0 < value <= 5.0 value > 5.0 Label Color Red Green Blue 0.0 to 0.25 190 232 255 0.25 to 0.5 0 197 255 0.5 to 1.0 0 112 255 1.0 to 1.5 0 77 168 1.5 to 2.0 255 255 0 2.0 to 2.5 255 170 0 2.5 to 3.0 230 76 0 3.0 to 5.0 168 0 132 > 5.0 76 0 115 Apatite Blue Big Sky Blue Cretan Blue Ultra Blue Solar Yellow Electron Gold Flame Red Cattleya Orchid Ultramarine 28 Presentation of Results 29 Presentation of Results 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. 30 10
Presentation of Results 31 Presentation of Results 32 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. 33 11
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. 34 35 Using 2D to assist in mitigation and bridge location. 36 12
Using 2D to assist in mitigation and bridge location. 37 Using 2D to assist in mitigation and bridge location. 38 Using 2D to assist in defining drainage area divides. 39 13
Using 2D to assist in defining ponded areas. 40 Results compared to traditional hydrologic method. Offsite Catchment ID AREA (acres) Clark U.H. (cfs) 100-Year Peak Flows 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% 41 Prior to including the ponding areas Clark U.H. flows were 30% to 40% higher than HEC-RAS 2D flows. Comparing static map rasters is very useful in GIS for determining the effectiveness of proposed mitigation. 42 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. 14
By adding a berm around the basin and a second weir, structure impacts were fully mitigated with notable benefits within the project area. 43 44 Questions? Thank you! Presented by: Matthew Zeve, P.E., CFM Harris County Flood Control District Lonnie Anderson, P.E., CFM Pape-Dawson Engineers, Inc. 15