Infiltration Trenches Dave Briglio, P.E. MACTEC Mike Novotney Center for Watershed Protection.

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Infiltration Trenches Dave Briglio, P.E. MACTEC Mike Novotney Center for Watershed Protection

Major Components Infiltration Trench 1. 1.Diversion for WQ v 2. 2.Stilling pool and spreader 3. 3.Sedimentation channel or chamber 4. 4.Overflow weir or protective covering layer 5. 5.Infiltration trench with gravel 6. 6.Overflow & outlet Pavement edges < 5 acres drainage ≤ 2 day drawdown No hotspot application 2-4 feet to water table – close may need to do mounding computation Setbacks for groundwater protection WQ v diverted into trench

Design Steps 1. 1. Compute WQ v and if applicable Cp v 2. 2. Screen site 3. 3. Screen local criteria 4. 4. Compute Q wq 5. 5. Size diversion 6. 6. Size filtration area 7. 7. Size pretreatment area 8. 8. Size overflow 24-48 hour drawdown Fill time estimate of 2 hours is normal Porosity = 0.32 If pretreatment facility size for 25% WQ v

Gravel Trench Volume n=0.32 d=depth in feet k=percolation rate (in/hr) T = fill time = 2 hrs. A = WQ v /(nd+kT/12) = WQ v /(0.32d+k/6) = WQ v /(0.32d+k/6)

Gravel Trench Area Per Acre Percent Impervious k=1 in/hr (sandy loam), n=0.32, T=2 hrs

Erosion & Sediment Control Considerations Ensure sediment is trapped before entering filter area Installation sequencing is important – –After adjacent areas are stabilized – –Converted temporary sediment basins (Sd3) Provide pretreatment with sediment basin or filter strip

An example of Infiltration trench design Taken from Appendix D4

Calculated Volumes….

Step 1. Determine if the site conditions are appropriate

Step 2. Calculate W Qv peak discharge (Q wq ) WQ v = 8102 cf, P = 1.2 ”, Q wv = WQ v /Area = 0.74 ” CN = 95 … Ia=(1000/CN-10) … Ia/P … q u … Q wq 1. 1. Back out curve number 2. 2. Calculate unit peak discharge using SCS simplified peak figures 3. 3. Calculate peak discharge as: Q wq = 2.2 cfs

Step 3. Size the infiltration trench n=porosity d=depth in feet k=percolation rate (in/hr) T = fill time A = WQ v /(nd+kT/12) = WQ v /(0.32d+k/6) = WQ v /(0.32d+k/6) n=0.32 D= 5 feet k=1 (in/hr) T = 2 hrs. A = 8,102 (32.2x5)+(1x2/12) A = 4,586 sf Max. width = 25 ft L = 4,586/25 = 183 ft

Step 3. Size Point A flow diversion structure Q 25 = 17cfs, W Qv = 2.2 cfs 2/3 of the flow to Point A (1/3 to Point B) Point A: Q 25 = 5.7cfs, W Qv = 0.73 cfs Design orifice for low flow (0.73 cfs) Set max. head = 1.5’ Q=CA(2gh) 1/2 …C=0.6…A=0.12 sf (4”dia.) …use 6-inch pipe w/ gate valve

Step 3. Size Point A flow diversion structure Q 25 = 17cfs, W Qv = 2.2 cfs 2/3 of the flow to Point A (1/3 to Point B) Point A: Q 25 = 5.7cfs, W Qv = 0.73 cfs Design weir for 25-yr flow (5.7 – 0.73 = 5cfs) Set max. head = 1.0’ Q=CLH 3/2 …C = 3.1…L=1.6 ft

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Infiltration in Coastal GA Site Characteristic How it Influences the Use of Infiltration Practices Potential Solutions  Poorly drained soils, such as hydrologic soil group C and D soils  Infiltration practices cannot be used on development sites that have soils with infiltration rates of less than 0.5 inches per hour (e.g. hydrologic soil group C and D soils).  Use other low impact development and stormwater management practices, such as stormwater ponds (Section 8.4.1) and stormwater wetlands (Section 8.4.2) and underdrained bioretention areas (Section 8.4.3)…

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Infiltration in Coastal GA Site Characteristic How it Influences the Use of Infiltration Practices Potential Solutions  Well drained soils, such as hydrologic soil group A and B soils  Enhances the ability of infiltration practices to reduce stormwater runoff rates, volumes and pollutant loads, but may allow stormwater pollutants to reach groundwater aquifers with greater ease.  Use bioretention areas (Section 8.4.3) or filtration practices (Section 8.4.4) with liners and underdrains at hotspot facilities and in areas with groundwater recharge.  In areas w/o groundwater recharge, use infiltration practices and non- underdrained bioretention areas (Section 8.4.3)

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Infiltration in Coastal GA Site Characteristic How it Influences the Use of Infiltration Practices Potential Solutions  Flat terrain  Does not influence the use of infiltration practices on development and redevelopment sites. In fact, infiltration practices should be designed with slopes that are as close to flat as possible.  Where soils are sufficiently permeable, use infiltration practices and non- underdrained bioretention areas (Section 8.4.3), to significantly reduce stormwater runoff volumes in these areas.

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Infiltration in Coastal GA Site Characteristic How it Influences the Use of Infiltration Practices Potential Solutions  Shallow water table  May cause stormwater runoff to pond in the bottom of the infiltration practice.  Ensure at least 2 feet to the water table…  Use stormwater ponds (Section 8.4.1), stormwater wetlands (Section 8.4.2) and wet swales (Section 8.4.6)...  Maximize the use of green infrastructure…

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Infiltration in Coastal GA Site Characteristic How it Influences the Use of Infiltration Practices Potential Solutions  Tidally- influenced drainage system  Does not influence the use of infiltration practices on development and redevelopment sites.

CSS Design Credits 7.4 Better Site Planning Techniques 7.5 Better Site Design Techniques 7.6 LID Practice 8.4 General Application BMPs

CSS Design Credits Table 6.5: How Stormwater Management Practices Can Be Used to Help Satisfy the Stormwater Management Criteria Stormwater Management Practice Stormwater Runoff Reduction Water Quality Protection Aquatic Resource Protection Overbank Flood Protection Extreme Flood Protection General Application Practices Stormwater Ponds “ Credit ” : None “ Credit ” : Assume that a stormwater pond provides an 80% reduction in TSS loads, a 30% reduction in TN loads and a 70% reduction in bacteria loads. “ Credit ” : A stormwater pond can be designed to provide 24-hours of extended detention for the aquatic resource protection volume (ARP v ). “ Credit ” : A stormwater pond can be designed to attenuate the overbank peak discharge (Q p25 ) on a development site. “ Credit ” : A stormwater pond can be designed to attenuate the extreme peak discharge (Q p100 ) on a development site. Stormwater Wetlands “ Credit ” : None “ Credit ” : Assume that a stormwater wetland provides an 80% reduction in TSS loads, a 30% reduction in TN loads and a 70% reduction in bacteria loads. “ Credit ” : A stormwater wetland can be designed to provide 24-hours of extended detention for the aquatic resource protection volume (ARP v ). “ Credit ” : A stormwater wetland can be designed to attenuate the overbank peak discharge (Q p25 ) on a development site. “ Credit ” : A stormwater wetland can be designed to attenuate the extreme peak discharge (Q p100 ) on a development site. Bioretention Areas, No Underdrain “ Credit ” : Subtract 100% of the storage volume provided by a non-underdrained bioretention area from the runoff reduction volume (RR v ) conveyed through the bioretention area. “ Credit ” : Assume that a bioretention area provides an 80% reduction in TSS loads, an 80% reduction in TN loads and a 90% reduction in bacteria loads. “ Credit ” : Although uncommon, on some development sites, a bioretention area can be designed to provide 24-hours of extended detention for the aquatic resource protection volume (ARP v ). “ Credit ” : Although uncommon, on some development sites, a bioretention area can be designed to attenuate the overbank peak discharge (Q p25 ). “ Credit ” : Although uncommon, on some development sites, a bioretention area can be designed to attenuate the extreme peak discharge (Q p100 ).

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