Napa County Updates

From Supervisor Brad Wagenknecht (First District):

FAQ and fact sheet on Napa Creek issues

Prepared for the Flood Control District Board’s 2-28-06 meeting

This fact sheet was prepared with input from the U.S. Army Corps of Engineers (COE), Flood Control District (FCD), and the City of Napa.


The Creek floods more often than the River, why not make improvements to protect the Creek area first?

The Napa River/Napa Creek Flood Protection Project (Project) must generally be built in a downstream to upstream sequence. If you make improvements upstream and send more water down the creek, you will make flooding worse for people downstream. You will not solve the problem, only relocate it. Taking actions upstream (such as at Behrens Bridge) before work has been done downstream to accommodate increased flow is against COE policy and could cause exposure to litigation for both the COE and FCD.  


The Behrens Bridge is the problem, it causes the flooding. Why not remove it now?

The Behrens Bridge does act as a blockage during high flows when floodwaters reach a certain level. Debris in the water can add to this blockage.  When high water flows occur, water backs up behind the bridge and spills out over the banks, but the Creek spills out many other places as well. In fact, it leaves its banks just downstream of the Bridge into Seminary Street before the water reaches a level high enough to spill upstream of the bridge. While the bridge can obstruct high flows during floods, estimates and field observations tell us that Napa Creek would have flooded on December 31 and in many storms in the past with or without the Behrens Bridge in place. The volume of water in these events is simply far too much for the Creek channel to handle.  The COE project will widen sections of the channel and provides bypass culverts in other areas to accommodate a far greater capacity. Without a bigger channel, there will continue to be flooding along the Creek.

What would happen if the Bridge was removed and the Creek reached flood level?

The COE has recently reviewed more current hydraulic information. Analysis indicates removal of the bridge would increase flow downstream during a flood by about 150 CFS[1] <#_ftn1> . This is equal to 67,500 gallons a minute. That amount of water along with debris would be added to the flow downstream of the bridge. It could increase the flooded area in some directions or deepen the floodwaters in areas that already flood. A complete analysis of these potential impacts must be performed before removal of the bridge can be considered.  

Why not make changes downstream to accommodate the additional water?

This is what the Project plans to do, with the installation of culverts, excavation of terraces, and removal of bridges. The total cost of the one mile of planned improvements on Napa Creek is estimated by the COE to be $7.1 million.



In past floods, the City used an excavator positioned on the bridge to remove debris. Why was that not done this time?

In recent floods prior to 12-31-05, City crews noticed instability on the bridge when the excavator was used. The City of Napa Public Works Department asked a consultant (EarthTech) to evaluate the 72-year-old bridge, and the final report was received 12-21-05, just 10 days before the flood. This report found the bridge to have a maximum allowable weight (limiting load) of 20.6 tons. Of greatest concern is the weight bearing capacity of the floor beams, with the condition of the upstream girder in question as well. The John Deere 595 excavator that the City has positioned on the bridge to clear debris in the past weighs just less than 19 tons. Factoring in the weight of debris and water that would be lifted by the excavator, and the torque on the floor beams and the forces on the exterior upstream girder when the equipment is in use, it was determined that using this excavator for this purpose was no longer safe. (Some residents stated that garbage trucks and other heavy vehicles were using the bridge in this time frame. This is most likely true, as the report on the strength of the bridge was just recently completed and had not yet been shared with other agencies. Napa Recycling and Waste Services/Napa County Recycling and Waste Services has since been informed and their trucks are no longer to use the bridge.)

Over the weeks since the flood, the City has been examining other options and has recently contracted for the services of a new piece of equipment with a longer reach that can safely remove the debris without being placed on the bridge.   

Debris is the problem. Clean out the creek and there won’t be a flood.

It is true that large amounts of debris are carried in floodwaters. Most of the debris that causes jams, however, is created by the storms and runoff from the full range of the tributary drainage area – in other words, this debris is not necessarily in the creek beds prior to the storm.

Much of the Napa Creek channel is privately owned, and property owners have the primary responsibility for maintenance of the creek banks on their property.  However, there is a program funded by the FCD using a property tax assessment to assist private property owners with creek maintenance. As a part of this program, debris or vegetation that represents a significant blockage threat is removed during the dry season under a permit that the FCD has with the Department of Fish and Game, but the various agencies involved do not want to remove vegetation or large woody debris too aggressively. Making the Creek too fast-flowing will cause additional erosion of the banks. More erosion will mean more trees will fall in, and property owners along the Creek will lose more of their land. The loss of vegetation and large woody debris and the subsequent erosion is also detrimental to plants and animal species that live in the Creek habitat. While debris jams can cause water to back up during floods, potentially adding to localized flooding, estimates and field observations tell us that Napa Creek would have flooded on December 31 and in many storms in the past regardless of debris loads. The volume of water in these events is simply far too much for the Creek to handle.

Project improvements on Napa Creek include new culverts. How will these culverts be built to allow floodwater to pass through and be kept clear of debris during floods?

When the COE designs these culverts, they will also determine the appropriate location and design for debris racks. Preliminary design calls for large vertical grates. They will also consider the best method for providing access to remove any debris that accumulates on these racks.  Removing debris from the racks will be one of the many maintenance tasks that will have to be performed after the Project is completed.

The current Project schedule shows improvements to Napa Creek to be made in 2009. Why can’t the Creek work be done sooner?

The most recent evaluations by the COE indicate that work on the Creek could be moved up to take place before the bypass excavation, or could be done at the same time, without creating negative impacts to properties downstream. However, revising the schedule to protect the Napa Creek area from flooding sooner would result in exposing properties affected by River flooding to the danger of damage for a longer period of time. There would be a trade-off of reducing threat in the Napa Creek area for prolonging threat in other areas. As the number of flood damage claims from the 12-31-05 event has not been finalized, the FCD has estimated the extent of flooding in the City of Napa in acres. The analysis shows (see attached map) that about 450 acres were inundated by Napa River flooding, about 50 acres were inundated by Napa Creek flooding, and about 30 acres were inundated by flooding from both the River and Napa Creek.

Are the improvements to Napa Creek adequate to handle the kind of flows that were seen on 12-31-05?

The Corps has designed improvements that will provide protection from flows even larger than seen in the recent flood, up to 4,280 CFS. Peak flow on the Creek in the 12-31-05 flood is estimated to have been 3,500-4,000 CFS.

What actions can be taken now to reduce the flood danger on Napa Creek?

1. The City will pursue a new“during the flood” debris removal plan for the area around Behrens Bridge, using the long reachexcavator as described above.
2. The COE agrees thatarchitectural treatments within the Napa Creek channel downstream of Pearl Street (nearMervyns) could be removed in advance of other planned Napa Creek FloodProject activities, provided adequate funding is forthcoming. This would resultin channel smoothing that would improve flows in this area,
3. The FCD and other stakeholders areseeking additional money for COE to accelerate the Project. The COEcapability for construction in FY 2006-2007 is $31 million, but thePresident’s budget has offered only $9 million. It is unclear howmuch money the Project will receive in the coming fiscal year.
4. The City and/or Flood Districtcan seek money from other sources, such as FEMA Hazard Mitigation funds,to contract for Creek work to be done. It is unknown if this is a viablesource of funds.
   

• 1986:              Rainfall atConn Dam was measured as 14.2”
  

• 2005:              Rainfall atConn Dam was measured at 6.6”  
  

• 1986:              The peakstage at the spillway over Conn Dam was 5.6 feet during the 1986 flood.
  
  

• 2005:              The peakstage at the spillway over Conn Dam was                            6.5 feet on December 31, 2005, which confirms the intensity of the 2005 storm as compared to 1986.
  
  
  Directly applicable to Napa Creek, rainfall on Mt. Veeder, which is the top of the watershed for Napa Creek, approached 10” in the 24 hours before the flooding began. This is about 30% of the total annual expected precipitation in that location. This is an unprecedented amount of rainfall, resulting in a flood of record[2] <#_ftn2>  for Napa Creek , which reached a record height of 18.8’ on 12-31-05 at 6:15 a.m., measured at Highway 29. This is eight feet above flood stage.
  
  
  
  Flooding, especially on creeks, is not necessarily caused by rainfall of long duration. Floods result from the interaction of rainfall intensity and time. Heavy rain in a short period of time can cause the same flooding as more moderate rain over a longer period of time.
  
  
  
  It is also a popular misconception that drainage from the Highway 29/Trancas interchange project causes flooding on Napa Creek . In fact, all the water that flows to Napa Creek from that area now has always drained to Napa Creek. Caltrans specifically designed the project to maintain the historic direction of flow and distribution of flow. The largest quantity of flow has always been routed through the intersection from west to east and continuing down Trancas Street to the Napa River near the Trancas Bridge. The same flow is now being carried under the interchange in a siphon (a closed pipe system) and then down Trancas Street in the same old pipe system to the Napa River. The rain that falls directly on the depressed portion of the interchange (only about 23 acres) is collected at the bottom of the interchange and then pumped southerly along Highway 29 to Napa Creek. This is also consistent with the direction and quantity of flow from the interchange area that historically has flowed to Napa Creek.  The only difference in drainage now is the delivery system and the addition of a minor amount of groundwater to the flow (about 32 CFS or 14,400 gallons per minute, compared to the peak flow in the 12-31-05 flood of about 4,000 CFS or 1,800,000 gallons per minute[3] <#_ftn3> ). It’s worth noting that the Trancas project was blamed for Napa Creek flooding in 2002 as well – at a time when the pump system was not yet functioning.
  
  
  
  It has also been stated that the Project should have provided more protection in the recent event because it is “three-quarters of the way completed.” In fact, the project is about 40% complete, with some of the most important elements that will reduce the extent of flooding yet to be built, such as the oxbow bypass and floodwalls along the river.
  
  
  
  [1] <#_ftnref1> Flows are measured in cubic feet per second (CFS). One cubic foot of water equals 7.5 gallons. A flow of 1 CFS is equal to 450 gallons a minute.
  
  
  
  [2] <#_ftnref2> Data on Napa Creek levels and floods has been recorded in only the last 47 years, from 1959 to the present.
  
  
  
  [3] <#_ftnref3> Existing Napa Creek channel capacity is about 2,300 cfs. Flow greater than this results in a flood. Peak flow in the 2002 flood was 3,000-3,500 cfs. Peak flow in the 2005 flood was 3,500-4,000 cfs. The Project improvements on the Creek are designed to accommodate 4,280 cfs split between the channel and the culverts.
  
  

This website is volunteer managed, designed and donated. Content is the sole responsibility of Democrats of Napa Valley Club.

Democrats of Napa Valley
P.O. Box 206, Napa, CA 94559
webmaster@democratsofnapavalley.org