Frequently Asked Questions

Keeping the stream free of debris (trash, tree branches, etc.) that accumulates around bridges and culverts increase flow capacity, reduces 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.

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.

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 could be activated under different flow conditions. YouTube video: https://www.youtube.com/watch?v=ELwEjenuHps

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.

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.

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. Additionally, a Flood Mitigation Assistance (FMA) grant application was successful, which provides funding to sequence and budget future improvement projects. Work on the FMA grant will begin in early 2020.

If you are not currently required to have flood insurance, your status 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.

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 flood mitigation 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.

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). The Lewis and Clark County Floodplain Viewer found here, https://helenamtmaps.maps.arcgis.com/apps/webappviewer/index.html?id=8da88242eb344726a5fb713a8ecfdcc5 can be used to locate your home and general relation to the SFHA.

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 the implementation of these projects, their effectiveness at reducing the FEMA SFHA will be evaluated and if worthwhile, the FIRMs will be revised.

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.

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. Flooding also causes road closures to protect the integrity of the road, therefore disrupting normal traffic.

Many factors influence groundwater elevations and patterns. Typically, spring runoff recharges the groundwater aquifer as is crosses over land and soaks in, which is then accessed by individual wells for household and irrigation use, thereby drawing down the groundwater elevation. Successive years of above-average snowpack has kept the aquifer well charged, leaving little room for additional storage when runoff occurs.

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 provides less time for infiltration of surface water into groundwater, which may reduce or prevent flooding of a basement.

Individual driveway culverts are the responsibility of each individual landowner. The goal of flood mitigation is to increase capacity of existing drainage patterns and create new drainage ways to reduce impacts from flooding. 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.

Without large-scaled detailed ground and surface water monitoring, it is not possible to confidently correlate surface water flooding issues to the valley aquifer. The aim of flood mitigation efforts is to reduce the impacts of flooding that has occurred in the past and will occur in the future.

The Lewis and Clark County Water Protection District implemented a groundwater monitoring program in 2019, which can be found here https://helenamtmaps.maps.arcgis.com/apps/MapJournal/index.html?appid=196ff5fe0a4549c3917049dc0a856c47. The monitoring is done weekly during the spring and summer. The frequency and number of wells monitored is reduced during the off-peak season. The page also has links (represented as push pins) to DNRC streamflow gages of interest.