Permanent Erosion/Sediment Control Measures:

All disturbed earth surfaces steeper than 3:1 and up to 2:1 (horiz:vert) sahll be restored with rock rip-rap or other pre-approved equivalent. No new slopes shall be constructed steeper than 2:1 unless specifically waived by the Conservation District. An Earth surface on pre-existing slopes steeper than 2:1 are to be armored with rip-rap or other pre-approved equivalent. These requirements apply to ditch fore-slopes and back slopes.

In all areas of channeled flow, if the water velocity is between 4cfs and 5cfs for a 25-yr/24hr storm, the channel shall be restored with pegged sod or other pre-approved equivalent. The sod shall extend a minimum of 1′ above the channel bottom, measured vertically, or above the normal depth of flow for a 25yr/24hr storm, whichever is greatest.

In all areas of channeled flow, if the water velocity is greater than 6cfs for a 25yr/24hr storm, the channel shall be armored with rip-rap, pavement, or other pre-approved equivalent materials. The armor shall extend a minimum of 1′ above the channel bottom, measured vertically, or above the normal depth of flow for a 25yr/24hr storm, whichever is greatest.

Regardless of the velocity, all areas of channeled flow having a continuous base-flow shall be permanently stabilized with rip-rap, pavement, or other pre-approved method (bioengineering is encouraged). The rip-rap, pavement, etc, shall extend above the channel bottom to the normal depth of the base-flow. The surfaces within the channel above the normal depth of base-flow must be restored according to the velocity and normal depth requirements for a 25yr/24hr storm, whichever is greatest.

All rip-rap shall be sized such that the smallest stones will not be displaced by the water velocities resulting from a 25yr/24hr storm. The depth of the rip-rap shall be 1.5 times the smallest stone dimension or 8″ whichever is greatest. All rip-rap shall be underlain by geo-textile fabric. All rip-rap shall be entrenched such that the top of the rip-rap is to the line of the adjacent ground.

Where subsurface water movement or excavation below the water table may cause seeps, soil erosion, soil slippage, sloughing, craving or other earth movement, adequate subsurface drainage facilities and permanent surface stabilization measures shall be installed as necessary to prevent slope instability, soil erosion, and sedimentation.

The same end result of structural stability is required for an earth impoundment. The suitability of the in-place foundation soils must be analyzed; the embankment cross-section, soils, compaction, outlet structures, etc. must be engineered to prevent slope instability, piping, seepage, settlement, etc. This also applies to existing earth fills that will be subjected to an increase in the backwater elevation due to an alteration of the drainage structures or due to storm water diversions. Anti-seepage collars must be installed on all impoundment pipe outlets. On the interior surfaces of an impoundment, the permanent stabilization method, materials, plant surfaces, etc. must be carefully chosen to insure that the method is appropriate for the range of water level fluctuations and/or inundation duration and frequency of occurrence.

The existing surface cover types must also be analyzed and modified as necessary in areas that are not being disturbed but will be experiencing a change in water velocities, the range of water level fluctuations, and/or inundation duration and frequency of occurrence due to storm water diversions and/or alterations of drainage control structures. The State law requires that all drainage conveyances be designed to prevent erosive velocities, therefore, in the locations where the existing ground surface cover will be subjected to erosive water velocities as a result of this project, the use of energy dissipaters and velocity control structures will be required unless all affected surfaces protected as necessary to prevent long term erosion problems.

The plans must show detail drawings of the configuration and dimensions of all rip-rap culvert aprons, energy dissipater, spillways, and down drains. All rip-rap down drains and impoundment spillways must be engineered using the USDA “Rock Chute” design method or other appropriate engineered method.


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