“Stormwater Management in the Coastal Plain & Beyond" Workshop held on December, 15, 2014 - Project Examples section

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)


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


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.


Avg BMP Eff = 19.89 lb/32.19 lb= 62%


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




Post-Dev(with RR)


Time of Concentration (Tc) 0.6 hr 0.4 hr

1-YearP = 2.71”




Energy Balance Allowable qo 4.6 cfs

Energy Balance Storage 1.49 ac-ft.


ESC Plan & Limits of Disturbed Area

Sediment Basins


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


Finger printing on large lot construction:

Save trees, soil, etc.

Finger-printing subdivisionconstruction:

narrow streets; shorter setbacks, etc.


Common Areas between lots

Treatment adjacent to right-of-ways; captures lot and roadway runoff


On-Lot Treatment• Alternative Practice

Disconnection– Raingardens– Drywells– Cisterns

• Permeable Pavement Driveways


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;


SWM Plan for the CPD

Performance Goals for on-lot pollutant load and Runoff Reduction

Proposed BMPs


3,966 ft3 Raingarden Disconnection & 31,311 ft3 Bio L1 storage


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




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


25% increase in surface storage (with 12” ponding depth)

31,311 ft3 of storage increases to 35,430 ft3

(bioretention – not Raingardens)



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?


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%


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


Individual SF Construction within a CPD


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)


• Well designed and specific ESC & PP plans• Minimization of impacts • Effective (redundant) micro-scale ESC




Parcel: 5.0 acresLand Cover: Woods, Woods Grass CombinationHSG: C


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





Time of Concentration (Tc) 0.54 hrs 0.24 hrs

1-YearP = 3.01”




2-yearP = 3.67”




10-yearP = 5.65”


Runoff Volume 0.98 ac-ft. 1.44 ac-ft



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




Post-Dev(with RR)


Time of Concentration (Tc) 0.54 hrs 0.24 hrs

1-YearP = 3.01”




2-yearP = 3.67”




10-yearP = 5.65”


Runoff Volume 0.98 ac-ft. 1.44 ac-ft



RR EFF = 2.33/4.38 = 54%


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


Issues: • Impacts to open space

– mature vegetation– Load reduction reqmt

• No RR• EB?


Bioretention L1: TR Eff = 55%






Bioretention Level 1

Dry Swale Level 1

Goal – remove wet pond, ED Basin, etc, from open space.


DA 12 to Dry Swale


Try strategy 6: – Bio L1

– DA 12 Dry Swale L1

– DA 1 to Grass Channel


DA 1 to Grass Channel w/ CA

DA’s 2 - 11 to Bio Level 1

DA 12 to Dry Swale


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



TvBMP = 5,819 ft3

Or if using the currently available spreadsheet:

TvBMP = Runoff Reduction + Remaining Runoff


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’


Total Tv = 5,819 ft3

Average Bioretention Surface Area = 275 ft2


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


Sizing (similar to Bioretention):Surface ponding;

Porosity of media, stone reservoir

Maximum eff. Slope = 2% (check dams as needed)

Turf or landscaping


Design: 1. Overflow and drainage system for bioretention basins;2. On-line vs off-line facilities


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


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;


Runoff Reduction & Energy Balance




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)




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


0.8 cfs 1.0 cfs 1.2 cfs


0.29 ac-ft. 0.23 ac-ft. 0.15 ac-ft.

Note: What if we route the bioretention & Dry Swale?


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)


0

10

20

30

40

50

60

0 1 2 3 4 5 6 7

Dis

char

ge (

cfs)

Stage (ft)

Discharge (cfs)




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


0.8 cfs 1.0 cfs 1.2 cfs 1.2 cfs


0.29 ac-ft. 0.23 ac-ft. 0.15 ac-ft. 0.07 ac-ft.


Sequence of Construction;

Minimize amount of exposed soil during construction;


Minimize sediment discharges (factors such as amount, frequency, intensity of rainfall, soil characteristics, e.g., PSD);

Preservation of topsoil (or application of soil amendments);




Proposed Property

Lines

Site development image courtesy of Draper Aden Associates


Disturbed Area = 3.1 Ac


Environmental Site Inventory:• Soil types• Water table• Bedrock• Water features• Vegetation• Steep slopes


Impervious Buildings, Sidewalks,

and paved areas

Pervious lawn areas

(managed turf)


RR EFF = 2.62/3.91 = 67%Tv = 6,225 ft3

Site = 3.1 ac


Rooftop

Disconnection

Grass Channel

Bioretention

Basin

Treatment Train Option:


1. Rooftop

Disconnection

3. Bioretention

Basin


2. Grass

Channel

BMP Treatment Train


Preservation/Conserved

Open Space Option:

The area will be identified

on the plan and must be

marked and protected

from impacts during

construction.


Preserved Open

Space


RR EFF = 0.76/4.45 = 17%Tv = 7,075 ft3

Site = 9.0 ac



Proposed

Drugstore

Limits of

Construction

(3.0 Acres)

Proposed Small Commercial Development outparcel of bigger CPD



Environmental Site Inventory:• Soil types• Water table• Bedrock• Water features• Vegetation• Steep slopes


Same site analysis


Permeable Pavement

Dry Swale

Treatment Trains (especially on high density sites)


1. PermeablePavers

2. Dry

Swale

Sequence of construction, construction inspections, and final as-built record drawings

