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Hydrology and Hydraulic (H&H) Modeling
Date
Watershed Reference

Hydrology is the scientific study of earth’s natural water movement with a focus on how rainfall and evaporation affect the amount of flow of water in streams and storm drains. 

Hydraulics represents the engineering analysis of the flow of water in streams and infrastructure, such as channels, pipes and other man-made structures. 

The combination of these two areas of study provides a scientific and engineer-based approach to planning, designing and constructing flood reduction and restoration projects.

What is Hydrologic and Hydraulic (H&H) Modeling?

Hydrologic and Hydraulic (H&H) Modeling, used since the 1970s, is computer software that simulates rainfall runoff flow to predict the extent of creek and river water levels and flooding and to test ways to reduce the flooding without actually constructing the project. 

What Can We Learn from H&H Modeling?

These models inform decisions about selecting and implementing flood reduction and restoration projects. H&H Modeling also satisfies regulatory requirements and ensures that natural, agricultural and social resources are not damaged by flooding induced by modifications to creeks, rivers and channels. 

H&H Modeling is just one of the many engineering tools used to develop and refine flood risk reduction projects.

How is H&H Modeling Used in Ross Valley?

The Ross Valley Watershed and Flood Risk Reduction Program (Program) uses a calibrated model developed by Stetson Engineers using software from the U.S. Army Corps of Engineers Hydrologic Engineer Center (HEC). Use of the HEC software has become the industry standard, though similar software is available from other sources. 

Two HEC computer programs are required to perform the computer modeling simulations: Hydrologic Engineer Center- Hydrologic Modeling System (HEC-HMS) and Hydrologic Engineer Center- River Analysis System (HEC-RAS). The HEC-HMS program uses the size, slope and vegetation of the land and the rainstorm to estimate the amount of creek flow. The HEC-RAS program uses the size and slope of the creek channel and adjacent floodplain and the amount of creek flow to estimate the water level, velocity and extent of flooding. A technical memorandum explaining the creation of the H&H model is currently under development and going through a peer review, along with the H&H Model, by the US Army Corp of Engineers and should be available to the public when complete in approximately Spring 2018. 

How is HEC-RAS Modeling Results Presented to the Community?

The HEC-RAS output can be presented in many different ways. To date, the HEC-RAS output for the Program and for specific projects within the program have been presented as:

2-D videos of aerial views of flooding over time. Each video depicts a specific condition (such as “existing condition” or after a specific project is constructed) and a specific rainfall event (such as a “10 Year Flow” or a “100 Year Flow”).

2-D change maps compare the maximum water level depths and extents of two different conditions for a specific rainfall event (such as a “10 Year Flow” or a “100 Year Flow”). The change maps show the change in depth between the two conditions, but not the magnitudes of the depth for either condition. The two conditions are usually the “existing condition” and after a project is completed, but the changes between the completion of two alternate projects has also been depicted. The change maps also allow the District to use GIS mapping to count the number of parcels that may benefit by a reduced water surface elevation of 1 inch or more. 

H&H Modeling Results By Project 

How to read and interpret the WSE Change Maps

Limits of flooding is shown as two colors (blue and pink crosshatching) on the WSE Change Map. The blue area is the flooding that occurs under the existing conditions. The pink crosshatching area is the flooding that occurs after if the project concept is constructed.

The blue area not overlaid by pink crosshatching will no longer flood during the identified flow on the map. The small numbers within the blue and pink crosshatching are the approximate reductions (in inches) of flood depth at that location.

How to read and interpret the 2-D Side-by-Side Videos:

The videos are shown side by side so you can see the existing condition versus the impact with the project in place. The two videos run simultaneously at the same time sequence, which is shown in the upper right corner of both videos, starting early on December 31, 2005.

The video on the left is the existing condition, scaled down to the corresponding 10- or 25-year flow (from ~6,900 cfs peak flow to ~4,400 or ~5,400 cfs peak flow, respectively) depending on which model run video you choose to watch.

The video on the right shows the flooding that will occur after the CMC Phase 1 Project is constructed. This uses the December 31, 2005 flow “scaled down” from a 100-Flow to a 10- or 25-Year Flow (same flow as the left video), depending on which model run video you choose to watch. The 100-year flow video is not scaled down.

PROJECT: Corte Madera Creek Flood Risk Management Project (CMC Phase 1)*

The proposed Phase 1 of the Corte Madera Creek Flood Risk Management Project for Ross Valley includes evaluating the options for Frederick Allen Park and removal of the fish ladder, a floodwall/berm along the Granton Park neighborhood, among other potential aspects. 

A detailed H&H model of the proposed Phase 1 project has been completed and there are three models to view, each based on different creek flow assessments:

10-Year Flow (10% chance of happening in any given year; ~4,400 cubic feet per second, cfs) 

Approximately 223 parcels would see at least 1" reduction in floodwater during this kind of flow event. The area that no longer floods with the proposed project is a large area covering residences on the north side of Corte Madera Creek across from Frederick Allen Park, the Ross Commons area, and the Granton Park neighborhood. 

In the side-by-side video, notice that the Frederick Allen Park improvements eliminate flooding in the Ross Commons area and in Sir Francis Drake Blvd. near the Lagunitas Road intersection in the right video. Notice also that the Granton Park floodwall (or flood berm) eliminates flooding in the Granton Park neighborhood in Kentfield.

25-Year Flow (4% chance of happening in any given year; ~5,400 cfs)

Approximately 321 parcels would see at least 1" reduction in floodwater level during this kind of flow event. The area that would no longer flood with the proposed project is smaller than for the 10-year flow, but still includes the residences on the north side of Corte Madera Creek across from Frederick Allen Park, a portion of the Ross Commons area, and the Granton Park neighborhood.

In the side-by-side video, notice that the Frederick Allen Park improvements reduce the extent and delay the flooding in the Ross Commons area and eliminate flooding in Sir Francis Drake Blvd. near the Lagunitas Road intersection in the right video. Notice also that the Granton Park floodwall (or flood berm) eliminates flooding in the Granton Park neighborhood in Kentfield.

100-Year Flow (1% chance of happening in any given year; ~6,900 cfs)

Approximately 329 parcels would see at least 1" reduction in floodwater level during this kind of flow event. Areas that still flood will experience reduced flood depths (shown in pink crosshatching). The Sylvan Lane area will experience flood depth reductions from 1” to 6”; Ross Commons area flood depth reductions from 3” to 6”; and the area across the creek from Granton Park flood depth reductions from 1” to 3”. The area that would no longer flood with the proposed project is smaller than the 25-year flow, but still includes the residences on the north side of Corte Madera Creek across from Frederick Allen Park and the Granton Park neighborhood.

In the side-by-side video, notice that the Frederick Allen Park improvements delay the flooding in the Ross Commons area and eliminate flooding in Sir Francis Drake Blvd. near the Lagnitas Road intersection in the right video. Notice also that the Granton Park floodwall (or flood berm) eliminates flooding in the Granton Park neighborhood in Kentfield.

*Water Surface Elevation (WSE) Change Maps are based on a hydrology and hydraulic (H&H) computer model simulation (simulation #45) of the Ross Valley. The model results and maps are based on computer model simulations and are undergoing peer review by USACE. They are subject to change and historical data has shown that the actual change in inundation extent and depth may vary depending on the characteristics of the rain storm and other factors. As additional modeling results for projects become available, they will be provided here.

Contact

The project manager for the Marin County Flood Control & Water Conservation District is Hugh Davis, P.E., Associate Civil Engineer (hdavis@marincounty.org). Receive notices about these and other meetings related to the Ross Valley Flood Protection & Watershed Program by providing us with your email address.


Accessibility

If you are a person with a disability and require this document in an alternate format (example: Braille, Large Print, Audiotape, CD-ROM), you may request an alternate format document by using the contact information below. If you require an accommodation (example: ASL Interpreter, reader, note taker) to participate in any county program, service or activity, you may request an accommodation by calling (415) 473-4381 (voice) / (415) 473-3232 (TTY) or by e-mail at disabilityaccess@co.marin.ca.us not less than four work days in advance of the event.