1. Why doesn’t the County dredge Ten Mile Creek?

Keeping the stream free of debris (trash, tree branches, etc.) that accumulates around bridges and culverts increase flow capacity, reduce flood impacts, and is generally a good maintenance practice. However, extensive channel dredging with the purpose of lowering the bed elevation over  extended lengths of creek is not a sound management practice. The increase in channel capacity is typically short-lived as silt, sand, gravel and rock is transported into the reach and fills the dredged area as flood waters go down.

2. Why does the County not put in rip rap or retaining walls to stop the water from leaving the creek bed?

Constructing levees above the natural streambank to allow more water in the channel is not a viable, long-term solution for flood protection. Because Tenmile Creek is on an alluvial fan (learn about alluvial fans in next question), the floodplain is poorly defined and lacks effective flood carrying capacity. Putting levees along significant portions of the channel would only make flood impacts worse by further raising the channel in relation to the surrounding land. Flow capacity at bridges and culverts could also be reduced because of potential long-term raises in bed elevation.

3. What is an alluvial fan?

An alluvial fan is a fan-shaped deposit of gravel, sand, and silt.  An alluvial fan occurs where a fast-moving mountain stream empties out onto a relatively flat plain. When this happens, sediment that was being carried by the stream deposits itself in the stream channel as the water velocity slows. This causes a build-up of alluvial (stream-transported) sediments in the area where the stream slope abruptly changes from steeper mountain terrain to relatively flat valley terrain. This results, over time, in a fan-shaped alluvial deposit that may have several stream channels that may be activated under different flow conditions.  YouTube video: https://www.youtube.com/watch?v=ELwEjenuHps

4. What is alluvial fan flooding?

Alluvial fan flooding is common to communities in the mountains of the western United States.  Alluvial fan flooding can occur when stream flows are large enough to exceed the channel capacity of the stream, causing the stream to flow out of its bank and into previously active alluvial fan channels. Many flood mitigation techniques applied to other flood-prone areas have limited or no effectiveness on alluvial fans. Many alluvial fan communities now recognize these unique hazards, or have experienced repetitive flooding problems, and are seeking to implement flood management and mitigation plans. However, existing structures may need to rely upon flood proofing measures to reduce flood damage.

5. What is the long-term plan for the RID once recommended improvements from the 2013 Master Plan are completed?

The 2013 Master Plan recommends nearly $10 million in flood mitigation improvements.  The proposed RID funding will only be able to fund a small portion, mostly providing matching funds for Federal and State grants.  However, if grant applications are not successful, projects will be funded within the limited capacity of the proposed RID.  Therefore the need for RID funding extends for the foreseeable future.  That being said, once projects within the 2013 Master Plan are completed, the need for the RID will be reassessed.

6. Is there a plan that specifies improvement projects and their costs on an annual basis that would be funded by the district?

Revenues generated from the district will primarily be utilized as match for federal grant opportunities which leverage the funds raised by the district up to 3 to 1.  Currently the County is administering the Trap Club Project, which includes the H&H Study.  The Trap Club Project is a FEMA Hazard Mitigation Grant of $728,593 that provides Hydrology and Hydraulic analysis of the Tenmile, Sevenmile, and Silver Creek drainages.  The information gathered from the study is now being used to guide design of physical improvements of the Trap Club retention pond.  The H&H study and resulting model will be continually updated and used to guide and validate future improvements as identified in the 2013 Master plan.  The match for the Trap Club HMGP work is $182,148 and is being paid for by Lewis and Clark County.  Future grant matching funds will be paid by the proposed RID.

7. Will I be required to pay for flood insurance if the district is created?

Your status regarding flood insurance will not change.  Federal regulations require that no adverse impacts may occur as a result of an improvement.  Therefore, FEMA flood maps will not be updated to include properties that aren’t currently in the floodplain.  However, there is a possibility that a property could be removed from the mapped floodplain through a Letter of Map Amendment (LOMA) for a single property or a Letter of Map Revision (LOMR) for an entire area.  If a property is no longer within the mapped floodplain, flood insurance is no longer required.

8. Will I no longer be required to pay for flood insurance if the district is created? 

Flood insurance is only required for properties with a federally backed mortgage that are located within the FEMA Special Flood Hazard Area (SFHA), which are areas subjected to inundation during the 1% annual chance flood event (100-year) as shown on the FEMA Flood Insurance Rate Map (FIRM).

Implementation of the 2013 Master Plan projects target more frequent flood events such as the 4% annual chance event (25-year) but may also reduce flooding during the 1% annual chance event. Following implementation of these projects, their effectiveness at reducing the FEMA SFHA will be evaluated and if worthwhile, the FIRMs will be revised.

9. I have never witnessed flooding on my property, so how will flood mitigation improvements benefit me?

Several benefits may result from implementation of flood mitigation improvements including:

a.  Avoided property losses

b.  Avoided travel, business, and education interruption

c.  Avoided loss of critical infrastructure

d.  Revitalized neighborhoods

e.  Improved public spaces

f.  Enhanced public safety

g.  Enjoyed ecosystem benefits

h.  Increased competitiveness for the community for other funding opportunities.

10. I have never witnessed flooding that impacts access to my property, so how will flood mitigation improvements benefit me?

FAQ 5 provides a list of benefits of flood mitigation activities. A specific example is the availability of emergency personnel and their access to your property during a personal emergency. During a flood, emergency personnel are in high demand. Reduction of flooding in magnitude and extent frees up that personnel for other needs. Another example is increased protection of utilities. During floods, utilities may be damaged and interrupt service to your property.

11. Will the flood mitigation improvements help stop groundwater flooding my basement?

It is difficult to determine site specific benefits without a detailed investigation but in general, water that flows over the surface in a concentrated manner rather than flood and pond for an extended period of time, provides less time for infiltration of surface water into groundwater, which may reduce or prevent flooding of a basement.

12. I bought my property because it is not in a floodway or “100-year” floodplain, so why am I in the district?

The district aims to control large scale flooding beyond that of individual properties. Large scale flooding interrupts businesses and education operations, transportation corridors, utility services, and emergency management officials. 

13. How can adequate drainage be achieved if most of the culverts in my subdivision are already clogged or filled, and drainage ditches have been filled in by neighbors?

The goal of implementing the 2013 Master Plan is to increase capacity of existing drainage patterns and create new drainage ways. Lewis and Clark County annually inspects and services culverts and drainage ditches to facilitate proper drainage. If you believe someone has filled a ditch or culvert, please contact the Public Works Department.

14. Is there any relationship between flooding issues and the valley aquifer?

Without large scaled detailed ground and surface water monitoring it is not possible to confidently correlate surface water flooding issues to the valley aquifer. Implementation of the 2013 Master Plan aims to mitigate flooding that has occurred in the past and will occur in the future.  

15. Are stream gauges and/or the aquifer capacity continually monitored to detect changes?

The County does not maintain an extensive surface monitoring program in the Helena Valley. However, Public Works does monitor gages at the Mchugh and Forestvale bridge crossings of Tenmile Creek.  In addition, the Montana Bureau of Mines and Geology has done some of this research in the Helena Valley so more information can be obtained through review of those reports.