STANDARD FOR OFFSITE STABILITYHOW DO I NAVIGATE THIS STANDARD ?

Detention / Infiltration Structure

Figure 21–1 Point

Discharge and

Downstream Stability Analysis

Procedure

WHERE IS THE STABILITY CONCERN FOR OUR SITE?

SAMPLE SITE ANALYSISPre-Development Peak Flows:

2 Yr. – 10.00 cfs10 Yr. – 15.00 cfs

Allowable Post-Development Peak Flows:2 Yr. – 10.00 cfs x 0.50 = 5.00 cfs

10 Yr. – 15.00 cfs x 0.75 = 11.25 cfs

SAMPLE SITE ANALYSIS CONTINUED:Post-Development Detention Basin Discharge:

Scenario No. 1 – Infiltration Basin2 Yr. – 0.00 cfs

10 Yr. – 0.00 cfs- 100 % Infiltration –

Therefore the basin must be flood routed under the following criteria:

- Infiltration has failed- - The basin is filled to the crest elevation of the

primary spillway or the emergency spillway- - A ten year storm event occurs

What happens?

INFILTRATION BASIN ANALYSIS

INFILTRATION BASIN SCENARIO 1

Must analyze as follows:Post – Development Peak Discharges with failed infiltration and the

basin filled to the crest of the Emergency

Spillway:(Flood Routed Outflows)

2 Yr. – 3.50 cfs10 Yr. – 8.75 cfsPre-Development Peak Flows:2 Yr. – 10.00 cfs10 Yr. – 15.00 cfs

Allowable Post-Development Peak Flows (With Reductions):2 Yr. – 10.00 cfs x 0.50 = 5.00

cfs10 Yr. – 15.00 cfs x 0.75 =

11.25 cfs

INFILTRATION BASIN SCENARIO NO. 2

Post – Development Peak Discharges with failed

infiltration and the basin filled to the crest of the Emergency

Spillway:(Flood Routed Flows)

2 Yr. – 12.00 cfs10 Yr. – 22.00 cfs

Must construct a stable conveyance!

Pre-Development Peak Flows:2 Yr. – 10.00 cfs10 Yr. – 15.00 cfs

Allowable Post-Development Peak Flows:

2 Yr. – 10.00 cfs x 0.50 = 5.00 cfs

10 Yr. – 15.00 cfs x 0.75 = 11.25 cfs

10 cfs for a 25 yr. storm!

STABLE OUTLETS

WHAT ARE WE TRYING TO AVOID?

WHAT ARE WE TRYING TO AVOID ?

WHAT ARE WE TRYING TO AVOID ?

TABLE 21-1Where there is no well defined channel, no sandy condition, no trees or brush to substantially concentrate the flows and it can be reasonably assumed that the flow will disperse over a broad area. The combinations of slopes and soils in table 21-1 and the following criteria are considered stable for flows of 10cfs or less for a 25 year, 24hr design storm. Table 21-1 Non-Erosive Velocities for Point Discharges Maximum Stable Slope for Point Discharges

for Various Soils Soil Type Maximum Slope (%) Sands 1.8

Sandy loam 2.0Silt loam, loam 2.5Sandy clay loam 3.5Clay loam 5.0Graded loam to gravel 8.0

Stability Criteria (in conjunction with table 21-1) i. The maximum discharge rate shall be 10 cfs or less for the twenty-five (25) year storm. ii. Multiple outlets may be utilized to reduce individual outlet flow rates to levels below the thresholds noted above. Outlets should be spaced no closer than 50 ft horizontally to avoid re-mixing of flows.iii. Flow over the outlet area shall be less than 0.5 cfs/ft. Designers shall not design excessive widths which will cause flows to concentrate. iv. Conduit outlet protection shall be provided in accordance with that Standard and may include: flat aprons, preformed scour holes, impact basins, stilling wells, plunge pools, etc. Level spreaders are not an acceptable design. v. Topography shows broad uniform outlet area where flows will not concentrate. vi. Discharge locations shall contain perennial, erosion resistant vegetation.vii. Peak discharge velocities (in the last pipe section) do not exceed 2 fps. viii. The maximum length of slope below the outlet(s) is 100 feet.

MULTIPLE BASIN OUTLETS 50 ft. minimum

STANDARD DETENTION BASIN EXAMPLE WITH A DEFINED RECEIVING WATERWAY

Pre – Development Site Peak Discharges

2 Yr – 22.00 cfs10 Yr – 31.00 cfs

Post – Development Site Peak Discharges

2 Yr – 32.00 cfs10 Yr – 44.50 cfsChannel Analysis at Point of

Discharge:2 Yr. Channel Velocity – 5.45 fps10 Yr. Channel Velocity – 7.88 fps

(Based on HEC-RAS)Allowable Channel Velocities based on Standard for Channel

Stabilization:Clay Loam Soil = 5.0 fps

MODIFY CHANNEL TO A STABLE CONDITIONHere is what the existing channel looks like.

Here is what the new channel will look like.

DOWNSTREAM STABLE CONDITION

OFFSITE STABILITY QUESTIONS ?