North Peoria Area Drainage Master Plan - Flood Control District of ...

Low Impact Structural AlternativeFor the purposes of the North Peoria ADMP, a“low-impact” development alternative isdefined as any activity within the floodwayfringe or erosion hazard zone that does notsignificantly alter the natural form and functionof the watercourse. The following standardsare proposed to quantify the definitionof “low impact”:• Minimal velocity increases.• The average 10-year velocity in thechannel or overbank should not change(± 0.0 fps).• The average 100-year velocity in thechannel or overbank should not change(increase or decrease) by more than 10percent or one-foot per second (fps),whichever is less.• Minimal water surface elevation increase.• The 10-year water surface elevationshould not change (± 0.0 ft.).• The 100-year water surface elevationshould not increase or decrease by morethan 0.1-foot.• Minimal disturbance of the main channel.• No decrease in the bankfull width of themain channel.• No excavation or deepening of the streambedin the main channel.• No removal of bank vegetation. Wherebank vegetation is temporarily disturbedby construction, it should bereplaced, monitored for health, and irrigatedif required to assure its survival.• No relocation of the low-flow channelwithin the floodplain.• No offsite impacts.35• No erosion, sedimentation, or floodimpacts to adjacent properties withoutwritten permission of affected propertyowners.• Engineering and geomorphic analysisrequired to demonstrate no long-term,short-term, or 100-year off-site impacts.• Preservation of natural landscape characterand habitat within the floodplain.Alternatives that exceed the standards listedabove are not considered low-impact alternatives.Such alternatives may be acceptablemethods of mitigating flood and erosion hazards,if properly engineered.Examples of and design guidelines for a lowimpactstructural alternative are provided inthe North Peoria ADMP, Technical Data Notebook,Attachment 3, Sedimentation Engineeringand Geomorphic Evaluation TechnicalMemorandums, Chapter 5.ChannelizationChannelization is defined as construction ofan engineered channel with bank protectionand grade control structures. Channelizationis generally known to have the followingimpacts on channel stability:• Velocity. Channelization generallyincreases channel velocities. Velocity isexponentially related to sediment transportrate and erosion potential.• Depth. Channelization can increase theflow depth by eliminating the floodplainarea available for conveyance. Increaseddepths result in greater scour depths andhigher velocities.• Discharge. Channelization eliminates thearea available for storage of floodwaterson the floodplain, resulting in decreasedattenuation and increased peak dischargesdownstream. Increased peak dischargesare directly related to increased sedimenttransport rates and erosion.