Open Channel Design and Case Studies Barry Baker June 1, 2012

My Background BA – Ambassador College BS – Civil Engineering – University of Washington Professional Engineer (Civil) – WA My Job: Consulting Engineering Firm – Gray & Osborne, Inc. Consulting Engineering Firm – Gray & Osborne, Inc. Head of GIS Group/Stormwater Group Head of GIS Group/Stormwater Group Surface Water Engineering for Small to Medium Cities Surface Water Engineering for Small to Medium Cities Planning & Design to meet Stormwater Regulations Planning & Design to meet Stormwater Regulations Stream/River Bank Restoration and Stabilization Stream/River Bank Restoration and Stabilization Sediment Transport/Management Sediment Transport/Management Levee Construction and Setback Levee/Stream Restoration Levee Construction and Setback Levee/Stream Restoration Associated permitting related to storm and surface waters Associated permitting related to storm and surface waters

Lecture Take-aways Water runs downhill (and the resultant consequences) The equations are the easy part (but you need to learn how they are determined and what each element represents)

Overview Open Channel Flow: Fluid passageway that allows part of the fluid to be exposed to the atmosphere. Pipes (not pressurized system) ChannelsControlWeirsOrifices Real World Examples

Open Channel – Primary Equations Mannings Equation(s): Orifice Discharge Weir Discharge

Mannings Equation Q = flow (cfs) n = friction value A = cross sectional area (sf) R = hydraulic radius (A/P) s = slope (ft/ft)

Mannings Equation Q = flow (cfs) n = friction value A = cross sectional area (sf) R = hydraulic radius (A/P) s = slope (ft/ft)

Mannings Equation HDPE pipe (smooth wall)0.009 Brass or glass Clean cast iron Dirty tuberculated cast iron Wood stave Concrete Smooth earth0.018 Firm gravel0.023 Corrugated metal pipe0.022 Natural channels (good condition)0.025 Natural channels (stones/weeds)0.035 Natural channels (very poor)0.060 Cobbles/boulders0.075 Estimate based on substrate Challenge is in finding n, A, and r Mannings n values

Mannings Equation Mannings n values make a big difference in flow. Assuming a trapezoidal channel, 20 ft wide at the bottom, 1H:1V side slopes, 1 ft depth of flow, and channel slope of ft/ft, the table below represents only a change in the n value nQ% of Flow % % % % % %

Mannings Equation Also difficult to find the factors of area and hydraulic radius, such as depth of flow, bottom width, and side slopes, when you have the flow rate

Open Channel – Nomographs

Sanitary Sewer Analysis Basin Flows New density of development proposed for existing sewered basin. Calculate the capacity of existing pipe Estimate flows from new development density Does the existing pipe have capacity or not If not, how much will it cost to upgrade

Pipe Capacity Downstream Rim Upstream Rim Downstream Invert Upstream Invert Length

Pipe Capacity ft

Open Channel – Primary Equations Mannings Equation(s):

Flow Estimate Calculate existing flow Calculate proposed flow Compare to existing capacity = 12.2 mgd

Map of Puyallup Study area

Flow Estimate Number of houses, apartments, businesses Number of people per dwelling Water use per person Peaking factor Infiltration & Inflow

Existing Flow Estimate Houses/connections Provided by City Planning or Public Works 1.8 to 2.9 people per dwelling 65 gallons per person per day Peaking factor ranges 2.0 to 4.5 Infiltration & Inflow 1,100 gallons per acre per day 10,700 * 2.9 * 65 * ,100 * 4,500 = 10 mdg

Future Flow Estimate Houses/connections Provided by City Planning or Public Works 1.8 to 2.9 people per dwelling 65 gallons per person per day Peaking factor ranges 2.0 to 4.5 Infiltration & Inflow 1,100 gallons per acre per day 17,100 * 2.9 * 65 * ,100 * 4,500 = 13 mdg

Proposed Flow Estimate Houses/connections Provided by City Planning or Public Works 1.8 to 2.9 people per dwelling 65 gallons per person per day Peaking factor ranges 2.0 to 4.5 Infiltration & Inflow 1,100 gallons per acre per day 25,000 * 2.9 * 65 * ,100 * 4,500 = 17 mdg

Sanitary Sewer Analysis Land Use Study AreaSanitary Sewer Comp Plan No ActionAlternative 1Alternative 2Existing2030Buildout Residential Dwellings , ,522 Population 9301,8142, ,7223,135 Commercial Square Feet 446,526871,5411,136,114 Commercial Acres Infiltration & Inflow Acres Residential Average Flow 61,008118,998165,57460,666112,960205,646 Commercial Average Flow 133,816261,186340,473126,406133,816146,950 I&I Flow (gpd) 55,785 44,33755,78576,077

Sanitary Sewer Analysis Basin Flows Study Alternative Flow Scenario Total Flow (gpd)Change from Comp Plan NorthSouthNorthSouth No Action 1 (N)376,105406,257167, (S)208,503376,105-(30,152) 3 (N&S)*197,023 (11,480)(209,234) 1 1 (N)649,935406,257441, (S)208,503649, ,677 3 (N&S)*340, ,479(65,276) 2 1 (N)839,706406,257631, (S)208,503839, ,448 3 (N&S)*441, ,67834, Year Comp Plan208,503406,257 *Changes in peaking factor based on tributary population accounts for greater total peak flow using the two smaller basins than all additional flow in one basin.

Existing ScenarioBuildoutBuildout with CIP Upstream Node Downstre am Node Pipe Dia. (in.)Slope Length (ft) Design Capacit y (mgd) Flow (mgd) Excess Capacity (mgd) Flow (mgd ) Su rch arg e (ft) Excess Capacit y (mgd) Flow (mgd) New Pipe Dia. (in.) Exces s Capac ity (mgd) CIP Project ID South Basin Flows % NW % % % % % % % % NW-4

Sanitary Sewer Analysis Basin Flows Nine pipes exceed capacity for the planned flow Project NW-4 Estimated Cost $202,00 Project NW-5 Estimated Cost $480,000 Project VT-1 Estimated Cost $3,929,000

Open Channel – Bioswale Design Stormwater NPDES Permit requires treatment of average annual storm AND provide capacity for 100-year storm Bioswale (grass lined ditch) is a prescriptive method of water quality treatment allowed by the Washington State Department of Ecology Stormwater Management Manual for Western Washington.

Open Channel – Bioswale Design Develop Hydrologic Flows Runoff from precipitation events (WWHM) Model Input to determine flows 10 acres 6.5 Dwelling units/acre Moderate slopes C Soils

Model Input Typical Lot Coverage Percent of Gross Area10 Lot Size500075%7.46 Street Frontage120018%1.79 Sidewalk Width5007%0.75 Vehicle Parking Area (#)4006%0.60 House Coverage - 35%175026%2.61 Patios, decks, hardscapes80012%1.19 Total Impervious Areas465069%6.94 Total Lot + Frontage % Total Pervious Areas (Lawn)205031%3.06 Percent Impervious69%

Open Channel – Bioswale Design Develop Hydrologic Flows = Run the model Flow Frequency - Flow(CFS) WQ On-line BMP = Year = Year = Year = Year = Year = Year = Treatment Storm Runoff = 1.43 cfs 100-year Storm Runoff = 7.40 cfs

Solve for b with simplifying assumptions (see DOE Manual) Top of swale >>y Z^2 >>1 R~y (hydraulic radius ~ depth)

Open Channel – Bioswale Design Calculate bottom width based on: Mannings “n” = 0.2 for WQ event Design depth of flow = 2” (typical mower height) Longitudinal slope = 0.02 ft/ft b= 26 ft Manual allows no greater than 10 ft Increase depth of flow to 4” b= 8.12 ft Okay

Open Channel – Bioswale Design Calculate flow velocity and residence time Calculate area of flow (trapezoid) = sf Calculate velocity = ft/s Velocity must be < 1 ft/s Okay Requires 9 minutes residence time  Length = 540 s * ft/s = 244 ft Do you have that much space?

Open Channel – Bioswale Design Check 100 year flow velocity Mannings Equation again to find depth of flow (n value will change) Calculate area of flow (trapezoid) = sf Calculate velocity = ft/s Velocity must be < 3 ft/s Okay

Open Channel – Bioswale Design Spreadsheet greatly simplifies the math. But 4” of grass and length of bioswale may not be acceptable to the client. Alternative treatment method may be needed, even if the capital cost is much higher.

Flow Splitter Design Filtration treatment requires much less real estate but has a much higher capital cost. Biowswale cost ~$2,000 Filtration Unit ~$75,000

Flow Splitter Design Filtration system has limited overflow/bypass capacity. Too much high flow will lead to re- suspension of solids and cause turbidity downstream. Solution is to split WQ treatment flow to the filtration system and by pass higher flows.

Flow Splitter Design Plan Incoming flow High flow Bypass Water Quality flow Treated Stormwater Outfall

Flow Splitter Design Section WQ Discharge Orifice

Open Channel – Primary Equations Orifice Discharge

Flow Splitter As-built Section WQ Discharge Orifice

Flow Splitter As-built Plan WQ Discharge Orifice

Flow Splitter Retrofit Plan WQ Discharge Orifice

Flow Splitter Retrofit Section WQ Discharge Orifice Sharp crested weir

Open Channel – Primary Equations Weir Discharge At 0.2 ft, the overflow will nearly match the orifice flow to the WQ filtration system. At 0.6 ft of head, the overflow will convey all the overflow up to the 100-year event

Sediment Trap Design Steep tributary basins contribute significant sediment load that settles out at the outlet of a large diameter culvert under I-90 in North Bend. Aggregation of the stream bed causes flooding of the commercial outlet mall.

Open Channel – Primary Equations Weir Discharge

Sediment Control Vault

Questions?