FMA Challenge 3

Introduction

If you haven't already completed FMA Challenge 2, please see this FMA Challenge 2 page. This page is for TUFLOW users of intermediate to advanced ability.

In this challenge, a typical non-urban stream of the California Central Valley will be explored. The stream originates near the Sierra Nevada Foothills and conveys runoff to the West into the Central Valley Flood Management System. The streams are natural, with large amounts of conveyance in their upper reaches. As the streams progress downstream, their conveyance capacity gradually reduces. At some point nearing the valley floor, typical levees have been constructed to contain normal floods within the conveyance systems.

From this challenge, it is expected you will develop skills in:

• Understanding of the influence of levees on flood behaviour;
• Understanding of the influence of infiltration on flood behaviour;
• Nested 1D/2D models; and
• Understanding the Green and Ampt infiltration method and USDR soil types.
  
  

Relevant Tutorials

It may be useful to revisit some of the following tutorials:

• 1D-2D Linking- Tutorial Module 2
• Running Events and Scenarios- http://www.tuflow.com/forum/index.php?showtopic=1149&hl=scenario

Information to Modelers

Assignment of all parameters for this analysis are at the discretion of the modeler. A high resolution aerial image is provided so that land cover details can be inspected. Information on the problem definition can be found on FMA_Challenge_Models/FMA_Scenario3/FMA_Challenge_Model_Introductions.

Relevant Files

It is up to the modeler's discretion to determine what GIS package to use, what text editor to use, and how to run their models (batch mode or within the text editor).All relevant files can be found on FMA_Challenge_Models/FMA_Scenario3/

Solution to Challenge 3

A sample solution has been presented, courtesy of BMT WBM.

Cross Section Spacing, Grid Size, Mesh Element Size, and Effects of Hydraulic Structures

The model was set up as a TUFLOW 1D/2D model. Two different resolutions for the floodplain were simulated using 100ft and 200ft cell sizes.

1D sections spacing were generally between 500ft and 1000ft (except at structures). Structures were modeled with additional energy loss, to simplify the model a value of 1.0 V2/2g was used for all structures in the main creek.

Linking between 1D/2D was along the top of the levees.

All outflow from the model was assumed to be only via the main creek (ie. there was no water level boundary applied to the 2D overbank domain).

The number of cross sections, grids, or mesh elements used were as follows:

2D Cell Size	Active 2D Cells
100ft	338, 020
200ft	84, 818

Computational Domain Assembly and Execution

The provided DEM via GMG format was too coarse for hydraulic modeling as key hydraulic features (such as levee crests) were not adequately represented. The first step was therefore to process the 20Gb of LiDAR data into a higher resolution DEM. A 10ft DEM was created, a small part of which is illustrated below.

Cross-sections were extracted from the 10ft DEM, with Manning’s n values extracted from the provided GIS land use layer shown in the image below.

The Manning’s n values adopted were as provided and are tabulated below with comments on their applicability.

Apparent Land Use	Manning's n	Colour (in image above)	Comment
Urban Areas	0.04	Red	Acceptable value when representing the combination of roads (0.02) and buildings, gardens and fences (0.08 to 0.30), especially if the roads are wide.
Cropping/Cultivation	0.10	Green	Representative of established medium density crops.
Cropping/Cultivation	0.12	Blue	Representative of established medium to high density crops.
Cropping/Cultivation	0.15	Magenta	Representative of established high density crops.
Creek	0.20	Yellow	Would consider this value very high. A quick inspection of the creek photos taken near structures would indicate that maybe in the upper reaches where there seems to be reasonably dense vegetation in the creek that this value is representative, but even so we still consider this value to be high in this case. In the lower reaches the creek seems quite open with little vegetation (more representative of an n value from 0.04 to 0.08).
Creek	0.25	Cyan	See comment above.

The 1d/2d linking was along the top of the levee. The elevations along these levees were extracted from the 10ft DEM, these and other significant features were included in the TUFLOW model as 3D GIS breakline layers, ensuring the hydraulic control is represented in the grid regardless of cell size.

The flood extent from the 100ft 2D grid model is shown below.

===Sensitivity Creek Manning's n Test (Scenario 100ft n0.1) As discussed in the Manning’s n table above, the main creek n value of 0.20 is considered very high, especially in the lower reaches of the study area. A sensitivity analysis was carried out by lowering all the Manning’s n values in the main channel (modeled as 1D cross-sections) to 0.10.

The image below shows the flood depths and extent, and the outflow is included in the ftp download. Of particular interest is that reducing the n value to 0.1 has a significant effect on the arrival time of the flood waters at the model outlet (much earlier), and reduces the volume of water flowing onto the floodplain by around 20% due to the higher conveyance of the creek. Also of interest is that for the n=0.2 scenario, some overbank floodwaters return to the main creek near the model outlet causing a delayed second rise in the outlet flow hydrographs as illustrated in the chart further below. This effect does not occur for the n=0.1 scenario, with all overbank floodwaters remaining on the floodplain.

Chart of flow out of the model for the different scenarios simulated. Note that “100ft GA” is the infiltration scenario modeled for Challenge 3+.

The sections of levee that were overtopped were post-processed using GIS for the 100ft with n=0.2 scenario as illustrated by the magenta lines in the image below. The GIS layer showing the overtopped sections is provided as part of the ftp download.

Solution to Challenge 3+

Where unstated, the same parameters were used as per Challenge 3.

Computational Domain Assembly and Execution

The 100ft and 200ft models were used from Challenge 3 with the Creek Manning’s n value set to 0.2.

The soils layers provided were roughly classified into two soil types for the purposes of demonstrating the Green-Ampt infiltration feature in TUFLOW. The soils are shown in the image below where:

1. Red indicates a Sandy Loam.
2. Green is Sand.
3. The blue cross-hatched areas are the Urban land use zones modeled in Challenge 3. These areas were assigned a 90% impervious parameter that restricts the rate of water entering the underlying soils.
The Green-Ampt parameters used were the USDR soil type classifications which are built into TUFLOW’s internal soils database. The parameters for the two soils are tabulated below.

USDR Soil Type	Suction (inches)	Hydraulic Conductivity (inches/h)	Porosity (fraction)
Sandy Loam	4.3	0.43	0.412
Sand	1.95	4.6	0.417


The underlying soil was assumed to have an infinite depth to saturation (saturation depths/levels can be specified as 3D surfaces in TUFLOW if known). The soils were also set to have an initial moisture content of zero.



The 100ft and 200ft simulations were re-run with the Green-Ampt infiltration switched on. The resulting peak flood depths map for the 100ft simulation is shown in the image below. As can be seen, the extent of inundation is significantly reduced with floodwaters infiltrating before reaching the model extents or returning to the main channel.

The mass balance reporting from TUFLOW indicates over half (58%) of the water infiltrates into the ground during the 171 hour simulation. For the flow out of the model see “100ft GA” in the chart of flow out of the model presented in Challenge 3.



Challenges

The soils GIS layers were difficult to work with. There is a lot of detail, but no clear way to correlate the different soils with Green-Ampt parameters (porosity, hydraulic conductivity, suction, etc).

Conclusion

In this challenge, we explored typical non-urban stream of the California Central Valley, with scenarios of various infiltration and flood levees adopted. From this, we gained a better understanding of the influence of flood levees on surface water behaviour, understanding of the Green Ampt infiltration method and USDR soil types, and a better understanding of nested 1D/2D models.

Congratulations on finishing Challenge 3!