“stormwater management in the coastal plain & beyond" workshop held on december, 15, 2014...
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Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
30 acre residential subdivision development
1/4 acre lots
38% Impervious Cover (includes lots, rights of way, common areas)
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Simplifying assumptions:• Soils: HSG C
• Pre-developed land cover: – Woods; Good Condition
– CNpre = 70
• Post-Developed land cover: – 38% connected impervious
– Pervious areas – open space, good condition
– CNpost = 83
• 3.2 acres of right-of-way– 50’ right of way;
– 28’ & 36’ pavement widths with C&G
• 4.5 acres of open space
Pervious Area CN reduced from 74 to 70
CNpost reduced from 83 to 77
30% impervious;25% unconnected
18’ & 22’ open section road
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
SWPPP
Initial LDA includes ~ 15 acres of disturbance:– ESC Measures
– right-of-way grading and road construction;
– Utilities & drainage
Stormwater Management Plan for the entire land-disturbing activity.
Individual lots in new residential developments shall not be considered separate land-disturbing activities.
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Avg BMP Eff = 19.89 lb/32.19 lb= 62%
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Hydrology Summary: 30.0 acres
Pre-Dev Post-Dev(no RR)
Post-Dev(with RR)
Runoff Curve Number 70 83
Time of Concentration (Tc) 0.6 hr 0.4 hr
1-YearP = 2.71”
Runoff Depth (Q or RV) 0.58 in 1.20 in
Runoff Volume 1.38 ac-ft. 3.03 ac-ft.
Peak Discharge 10.0 cfs 34.8 cfs
Energy Balance Allowable qo 4.6 cfs
𝑞1𝑝𝑜𝑠𝑡 ≤ 𝑞1𝑝𝑟𝑒𝑃𝑟𝑒 𝑉𝑜𝑙1𝑃𝑜𝑠𝑡 𝑉𝑜𝑙1
𝐼𝐹
𝑞1𝑝𝑜𝑠𝑡 ≤ 101.38
3.030.8 = 3.6 𝑐𝑓𝑠
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Hydrology Summary: 30.0 acres
Pre-Dev Post-Dev(no RR)
Post-Dev(with RR)
Runoff Curve Number 70 83
Time of Concentration (Tc) 0.6 hr 0.4 hr
1-YearP = 2.71”
Runoff Depth (Q or RV) 0.58 in 1.20 in
Runoff Volume 1.38 ac-ft. 3.03 ac-ft.
Peak Discharge 10.0 cfs 34.8 cfs
Energy Balance Allowable qo 4.6 cfs
Energy Balance Storage 1.49 ac-ft.
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
ESC Plan & Limits of Disturbed Area
Sediment Basins
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Initial infrastructure construction should include stormwater controls as soon as practicable;
9VAC25-870-54.F (40 CFR 450.21) intended to minimize impacts prior to full implementation of quality & quantity controls;
Common Plan of Development must establish strict phasing/sequencing to ensure BMPs built at appropriate times
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Finger printing on large lot construction:
Save trees, soil, etc.
Finger-printing subdivisionconstruction:
narrow streets; shorter setbacks, etc.
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Common Areas between lots
Treatment adjacent to right-of-ways; captures lot and roadway runoff
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
On-Lot Treatment• Alternative Practice
Disconnection– Raingardens– Drywells– Cisterns
• Permeable Pavement Driveways
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
SWM Plan & BMP Construction
Full Water Quality and Quantity Control provided for roads & right-of-way;Partial Water Quality and Quantity Control provided for residential lots;
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
SWM Plan for the CPD
Performance Goals for on-lot pollutant load and Runoff Reduction
Proposed BMPs
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
3,966 ft3 Raingarden Disconnection & 31,311 ft3 Bio L1 storage
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
1.15 acres impervious cover to alternate disconnection (50 lots @ 1,000 ft2 );
7 acres of Impervious Cover & 6 acres of Managed Turf to Bioretention L1
Annual Runoff Reduction Credit = 40% x 35,277 ft3 = 14,111 ft3
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Hydrology Summary: 30.0 acres
Pre-Dev Post-Dev(no RR)
Post-Dev(with RR)
Runoff Curve Number 70 83 80
Time of Concentration (Tc) 0.6 hr 0.4 hr 0.4 hr
1-YearP = 2.71”
Runoff Depth (Q or RV) 0.58 in 1.20 in 1.03 in
Runoff Volume 1.38 ac-ft. 3.03 ac-ft. 2.57 ac-ft.
Peak Discharge 10.0 cfs 34.8 cfs 27.0 cfs
Energy Balance Allowable qo 4.6 cfs 4.6 cfs
Energy Balance Storage 1.49 ac-ft. 0.98 ac-ft
38% reduction in Energy Balance Storage requirement
Double Credit:1. Inflow to detention facility is reduced: CN 80 & 1.03” R.O. depth; and2. Allowed qo is increased by the new ratio of Volpre/Volpost in EB Equation
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
25% increase in surface storage (with 12” ponding depth)
31,311 ft3 of storage increases to 35,430 ft3
(bioretention – not Raingardens)
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Uncontrolled peak
Modeled peak using ‘Annual Reduction’ CN Adjustment
Routed peak (assumed no discharge)
Modeled peak using actual storage for CN Adjustment
How does the reduced discharge influence the EB Equation?
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Each building lot shall meet the following:
• A maximum of 3,000 ft2 of impervious cover;
• A minimum of 1,000 ft2 shall be managed with one of the following stormwater management practices:1. Raingarden (micro-bioretention) Level 1;
2. Dry Well (micro-infiltration);
3. Permeable Pavement Level 1;
4. Cistern with a minimum annual runoff reduction credit of 55%
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Soil stabilization as needed
On-Lot RaingardenPerformance Goals: • Surface Area• Depth of Soil Media• Underdrain if required• Set-back from house
Standard details based on relevant design assumptions provided with CPD SWM Plan and provided to home builder
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Individual SF Construction within a CPD
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Automatic CGP Coverage
SWPPP (or SWPPP Template)ESC Plan (Agreement in lieu of ESC plan)
SWM Plan (BMP Implementation Plan; Agreement in Lieu of SWM Plan)
PP Plan (Template)
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
• Well designed and specific ESC & PP plans• Minimization of impacts • Effective (redundant) micro-scale ESC
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Parcel: 5.0 acresLand Cover: Woods, Woods Grass CombinationHSG: C
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Parcel: 5.0 acresLand Cover: 30% Impervious (connected)18 lots
Avg lot size = 0.22 acLot 10 = 0.29 ac; 31% ICLot 3 = 0.16 ac; 56% IC
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Hydrology Summary: 5.0 acres
Pre-Dev Post-Dev(no RR)
Runoff Curve Number 68 80
Time of Concentration (Tc) 0.54 hrs 0.24 hrs
1-YearP = 3.01”
Runoff Depth (Q or RV) 0.63 in 1.26 in
Runoff Volume 0.26 ac-ft. 0.53 ac-ft.
Peak Discharge 2.1 cfs 7.8 cfs
2-yearP = 3.67”
Runoff Depth (Q or RV) 1.00 in 1.76 in
Runoff Volume 0.42 ac-ft. 0.73 ac-ft.
Peak Discharge 3.7 cfs 11.1 cfs
10-yearP = 5.65”
Runoff Depth (Q or RV) 2.35 in 3.46 in
Runoff Volume 0.98 ac-ft. 1.44 ac-ft
Peak Discharge 9.6 cfs 21.5 cfs
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Water Quality Criteria: RRM & 0.41 lb/ac/yr(9VAC25-870-63.)
Water Quantity Criteria
• Channel Protection:
– 2-yr (manmade channel)
1-yr Energy Balance (natural channel)
• Flooding
– 10-yr
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Hydrology Summary: 5.0 acres
Pre-Dev Post-Dev(no RR)
Post-Dev(with RR)
Runoff Curve Number 68 80
Time of Concentration (Tc) 0.54 hrs 0.24 hrs
1-YearP = 3.01”
Runoff Depth (Q or RV) 0.63 in 1.26 in
Runoff Volume 0.26 ac-ft. 0.53 ac-ft.
Peak Discharge 2.1 cfs 7.8 cfs
2-yearP = 3.67”
Runoff Depth (Q or RV) 1.00 in 1.76 in
Runoff Volume 0.42 ac-ft. 0.73 ac-ft.
Peak Discharge 3.7 cfs 11.1 cfs
10-yearP = 5.65”
Runoff Depth (Q or RV) 2.35 in 3.46 in
Runoff Volume 0.98 ac-ft. 1.44 ac-ft
Peak Discharge 9.6 cfs 21.5 cfs
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
RR EFF = 2.33/4.38 = 54%
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Wet Pond Level 2: 65% (coastal Plain)
Level 2: TvBMP x 1.5 = 6,967 ft3 x 1.5 = 10,450 ft3
Level 2 Design Elements:• Multi-cell
• Geometry (L:W = 3:1)
• Wetlands
• Landscaping
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Issues: • Impacts to open space
– mature vegetation– Load reduction reqmt
• No RR• EB?
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Bioretention L1: TR Eff = 55%
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Bioretention Level 1
Dry Swale Level 1
Goal – remove wet pond, ED Basin, etc, from open space.
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
DA 12 to Dry Swale
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Try strategy 6: – Bio L1
– DA 12 Dry Swale L1
– DA 1 to Grass Channel
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
DA 1 to Grass Channel w/ CA
DA’s 2 - 11 to Bio Level 1
DA 12 to Dry Swale
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
BMP Strategy Compliance
1. Entire site to Wet Pond L2 Yes
2. DA’s 2 thru 11 to Bio L1 Short by 0.53 lb/yr
3. DA’s 2 thru 11 to Bio L1; to ED L1 Short by 0.16 lb/yr
4. DA’s 2 thru 11 to Bio L1; to ED L2 Yes
5. DA’s 2 thru 11 to Bio L1;
DA 12 to Dry Swale L1Short by 0.31 lb/yr
6. DA’s 2 thru 11 to Bio L1;
DA 12 to Dry Swale L1
DA 1 to Grass Channel to Bio L1
Yes
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
TvBMP = 5,819 ft3
Or if using the currently available spreadsheet:
TvBMP = Runoff Reduction + Remaining Runoff
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Porosity & Sizing• 1’ surface ponding (𝜂 = 1.0): eff depth = 1’
• 2’ soil media (𝜂 = 0.25): eff depth = 0.5’
• 1’ Stone reservoir (𝜂 = 0.40): eff depth = 0.4’
Total Effective Depth = 1.9’
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Total Tv = 5,819 ft3
Average Bioretention Surface Area = 275 ft2
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Bottom width established to ensure:– Depth of flow: qpTv ≤ 4” (qpTv = Tv peak flow rate; cfs)
(Section 11.5.3 of Chapter 11 of the VSMH 2nd Ed.)
– Velocity of flow: qpTv ≤ 1’/s
– Max slope of 4% or check dams
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Sizing (similar to Bioretention):Surface ponding;
Porosity of media, stone reservoir
Maximum eff. Slope = 2% (check dams as needed)
Turf or landscaping
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Design: 1. Overflow and drainage system for bioretention basins;2. On-line vs off-line facilities
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Design: 1. Optimize surface ponding of Bioretention: 25% 2. Surface area of grass channel, dry swale, & optimized
bioretention basins can count as open space: • ~0.3 ac of managed turf changes to open space;• Load & RR reduction
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Picking a winner:Dry Swale: expensive with very little DA• refer to spreadsheet values – 0.55 turf, 0.07 IC;
• TvBMP = 877 cf; 0.22 lb reduction
• more than ½ that reduction chieved by switching lndcover of BMP to open space);
Grass Channel – required to convey drainage; add value as a RR practice;
If Energy Balance basin is required, slightly larger ED or Wet Pond complies;
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Runoff Reduction & Energy Balance
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Hydrology Summary: 5.0 acres
Pre-Dev Post-Dev(no RR)
Post-Dev(with RR)
Runoff Curve Number 68 80 77
Time of Concentration (Tc) 0.54 hrs 0.24 hrs 0.24 hrs
1-YearP = 3.01”
Runoff Depth (Q or RV) 0.63 in 1.26 in 1.10 in
Runoff Volume 0.26 ac-ft. 0.53 ac-ft. 0.46 ac-ft.
Peak Discharge (qi) 2.1 cfs 7.8 cfs 6.6 cfs
Energy Balance allowable Peak Out (qo)
0.8 cfs 1.0 cfs
Energy Balance Storage Requirement
0.29 ac-ft. 0.23 ac-ft
Note: RR CN Adjustment is based on ‘annual’ volume reduction credit (e.g., bio L1 = 40%).
What if: CN Adjustment based on actual storage ~ 6,800 ft3 (rather than 2,863 ft3)
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Hydrology Summary: 5.0 acres
Pre-Dev Post-Dev(no RR)
Post-Dev(with RR)
Post-Dev w/ CN Adj w/
actual Storage
Runoff Curve Number 68 80 77 73
Time of Concentration (Tc)
0.54 hrs 0.24 hrs 0.24 hrs 0.24 hrs
Runoff Depth (Q or RV) 0.63 in 1.26 in 1.10 in 0.89 in
Runoff Volume 0.26 ac-ft. 0.53 ac-ft. 0.46 ac-ft. 0.37 ac-ft.
Peak Discharge (qi) 2.1 cfs 7.8 cfs 6.6 cfs 5.1 cfs
Energy Balance allowable Peak Out (qo)
0.8 cfs 1.0 cfs 1.2 cfs
Energy Balance Storage Requirement
0.29 ac-ft. 0.23 ac-ft. 0.15 ac-ft.
Note: What if we route the bioretention & Dry Swale?
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
0
1
2
3
4
5
6
7
0 0.1 0.2 0.3 0.4
Stag
e (f
t)
Storage (ac-ft.)
Storage (ac-ft)
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
0
10
20
30
40
50
60
0 1 2 3 4 5 6 7
Dis
char
ge (
cfs)
Stage (ft)
Discharge (cfs)
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Hydrology Summary: 5.0 acres
Pre-Dev Post-Dev(no RR)
Post-Dev(with RR)
Post-Dev w/ CN Adjw/ actual Storage
Post-Dev Routed Runoff
Hydrograph
Runoff Curve Number 68 80 77 73 80
Time of Concentration (Tc)
0.54 hrs 0.24 hrs 0.24 hrs 0.24 hrs 0.24 hrs
Runoff Depth (Q or RV) 0.63 in 1.26 in 1.10 in 0.89 in 1.26 in
Runoff Volume 0.26 ac-ft. 0.53 ac-ft. 0.46 ac-ft. 0.37 ac-ft. 0.53 ac-ft.
Peak Discharge (qi) 2.1 cfs 7.8 cfs 6.6 cfs 5.1 cfs 3.3 cfs
Energy Balance allowable Peak Out (qo)
0.8 cfs 1.0 cfs 1.2 cfs 1.2 cfs
Energy Balance Storage Requirement
0.29 ac-ft. 0.23 ac-ft. 0.15 ac-ft. 0.07 ac-ft.
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Sequence of Construction;
Minimize amount of exposed soil during construction;
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Minimize sediment discharges (factors such as amount, frequency, intensity of rainfall, soil characteristics, e.g., PSD);
Preservation of topsoil (or application of soil amendments);
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Proposed Property
Lines
Site development image courtesy of Draper Aden Associates
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Disturbed Area = 3.1 Ac
Site development image courtesy of Draper Aden Associates
Environmental Site Inventory:• Soil types• Water table• Bedrock• Water features• Vegetation• Steep slopes
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Impervious Buildings, Sidewalks,
and paved areas
Pervious lawn areas
(managed turf)
Site development image courtesy of Draper Aden Associates
RR EFF = 2.62/3.91 = 67%Tv = 6,225 ft3
Site = 3.1 ac
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Rooftop
Disconnection
Grass Channel
Bioretention
Basin
Treatment Train Option:
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
1. Rooftop
Disconnection
3. Bioretention
Basin
Site development image courtesy of Draper Aden Associates
2. Grass
Channel
BMP Treatment Train
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Preservation/Conserved
Open Space Option:
The area will be identified
on the plan and must be
marked and protected
from impacts during
construction.
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Preserved Open
Space
Site development image courtesy of Draper Aden Associates
RR EFF = 0.76/4.45 = 17%Tv = 7,075 ft3
Site = 9.0 ac
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Proposed
Drugstore
Limits of
Construction
(3.0 Acres)
Proposed Small Commercial Development outparcel of bigger CPD
Site development image courtesy of Draper Aden Associates
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Environmental Site Inventory:• Soil types• Water table• Bedrock• Water features• Vegetation• Steep slopes
Site development image courtesy of Draper Aden Associates
Same site analysis
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
Permeable Pavement
Dry Swale
Treatment Trains (especially on high density sites)
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
1. PermeablePavers
2. Dry
Swale
Sequence of construction, construction inspections, and final as-built record drawings
Site development image courtesy of Draper Aden Associates
Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014
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