Design Considerations for Open Channel and Detention Pond Design

Howard Redfearn, City of Mansfield

OverviewOverview

• Previous Experience/Criteria/Maintenance Issues

• New Criteria• Discussion

Where is Mansfield?Where is Mansfield?

DemographicsDemographics

• Currently about 60,000• Only around 50,000 when I started 5 years

ago• About 30,000 15 years ago

• Fairly diverse population• Covers 3 Counties• 2 major TxDOT highways• More than just a bedroom community

Open ChannelsOpen Channels

• One of the developers backs up to this channel

• Significant slope failures• Loose soils• No armoring on side that needed it• Drainage easement all on one property

owner• No gates in fences to allow for maintenance

– and how were they supposed to get to the other side anyway??

• In compliance with drainage criteria of the time

• No consideration of water quality impacts

• Evaluated underground box vs. gabion walls• Underground box essentially same price

• Even though underground box selected, do receive some benefit

• Decent infiltration with vegetative cover• Better temperature control• Reduced algae formation

• Definitely easier to maintain• Maintenance will fall to City

• Leaving maintenance to homeowners leads to loss of capacity

• My experience – every time• Same homeowners not maintaining –

complain about mosquitoes, fences falling over, smells, etc…

• Original design a capacity good – until upstream developed into high school

• One of first tasks when I took over drainage maintenance – stop spraying everything

• Streets supervisor used to oversee drainage maintenance

• Can’t see the pervious pavers in the channel bottom any more

• Was not designed with water quality benefit in mind – just as most cost effective solution

• Provides habitat• Slows velocities• Controls temperatures

Same song different verseSame song different verse

• Stabilization project designed to reduced sediment load downstream

• Called for millet, rye and bermuda on slopes• Vegetation looked great until scalped by

City’s contract mower• Now a weedy mess

• Still better than original state• Stable

Not all old channels are badNot all old channels are bad

• Access still an issue• Maintenance still an issue• Channel cutting still occurring

Intentional DesignIntentional Design

• “Improved” existing natural drainage• Preserved more open space than in previous

areas• Intent of design was to reduce maintenance,

provide space for migration, and prevent flooding – not water quality

• Design intent was for the channels to return to a “natural” state over time

• Still have issues• Residents don’t understand why can’t build

fences in drainage easements• Erosion controls not effective and

sedimentation occurs• Did not spec out vegetation

– Cattails, black willow, rag weed, other nuisance veg– Will event evolve into intended channel

• Residents don’t like the wildlife• Use of concrete, rip rap, gabions and open

vegetation strategic

PondsPonds

• Designed as water hazard in golf course• Opportunity for water quality• Eye sore, odor nuisance, fish killer• Currently working with course to improve

water quality in pond• Planting native aquatic plants

– Water lilies, soft stem bulrush, American water willow

– Also providing shade» Bald cypress, River Birch

BeaverBeaverBeaver

• Originally designed as dry pond• Infall elevation lower than outfall

• So developer thought it would best to use as a wet pond

• Didn’t want side slopes to be 4:1, so he built a wall

• Didn’t line it until pond started leaking• Pump for 1 original aeration fountain

constantly plugged• HOA spent $18,000 to get in this condition

Typical ProblemsTypical Problems

• Houses too close• Fences too close• Slopes too steep• Improper slope prep• Drainage easements in back yards behind

fences – 1 foot deep grassed swale• Channels migrate• Nuisances – weeds, mosquitoes, weeds• Contributing areas not caught

Mansfield’s take on regional iSWMMansfield’s take on regional iSWM

• iSWM, uSWM, They Swim, We All SWIM!!• First true adoption of iSWM in region was by

City of Fort Worth• Followed their example with local criteria• Changed some criteria related to calculations

and such, but no drastic departures• Open channel brand new for our region• Pond design took essential portions of water

quality pond design and applied to flood control ponds

• Have not adopted water quality portion of iSWM

So What is iSWM????So What is iSWM????

• iSWM – Integrated Stormwater Management• NCTCOG developed iSWM manual to

replace individual drainage criteria manuals • Includes water measures

• Supposed to focus developers on stormwater considerations earlier in process

• Attempts to preserve existing drainage ways• Standardizes drainage criteria

• Culvert calculations, Manning’s n’s, design storms, etc…

• Total document – 853 pages• Now converted to web based individual

manuals• Local Criteria Section to be developed by

local communities with additional requirements, specific deviations

• Mansfield’s Local Criteria – 38 pages – only 33 really matter

• First Chapter planning and design considerations

Hydrologic Analysis Hydrologic Analysis

• Identifies which models can be used under which circumstances

• No more ration method to determine detention pond size

• Rain data

Hydraulic Design of Streets/Closed Systems Hydraulic Design of Streets/Closed Systems

• Specifies which storms carried in what areas• Roadway types• Inlet calculations

• No existing inlet for parabolic crown streets• Local Criteria Section Only

• Specify items not included in other portion of existing manual

» Minimum pipe sizes, which inlets must have depressed openings, flows in streets and intersections

Hydraulic Design of Open Channels and Detention Ponds Hydraulic Design of Open Channels and Detention Ponds

• Max 4:1 side slopes• Min bottom width 8’• Not accepted by City until stabilized• Use appropriate plant material in bottom and

slopes• Ramp requirements for channel sections

requiring vehicle access

• “Improved Channels”• Must be composite

– Low flow » greater than 300 acre drainage area» Design capacity minimum 2% of 100 year

storm» Max depth 5’» Min width 8’» Lined with riprap, gabions or pervious pavers if

velocities exceed 5 fps» Recommend establish meanders

– Trickle Flow» Less than 300 acre drainage area» Design capacity minimum 2% of 100 year

storm» Permeable armor – gabions, mat, interlocking

blocks» Max depth 5’

• Erosion control setback• 4:1 from toe of slope to natural ground adjacent to bank

plus 15’• Include adjacent delineated wetlands or critical habitat• Other widths considered if supported by proper

engineering/environmental studies

• “Landscaping shall be installed to allow earthen channels to evolve into a more natural environment. Tree and shrub planting will be required to enhance habitat of channels by providing shade once mature plant growth has been reached. Mature plantings must be considered in setting design Manning’s n values

• Lined channels discouraged• City engineer may require geotech study, or

underground option analysis submittal before considering open concrete channel

Dry BasinsDry Basins

• Required when downstream conveyance insufficient

• Cannot exceed downstream flows unless additional capacity available

• Modified rational accepted for drainage area <200 acres for preliminary sizing only – hydrograph method must be used for final sizing – modified rational may be used for DA’s <25 acres

• Evaluate 2,10,25,100 year critical (3,6,24 hour) storm with greatest flow

• 10’ crown width on any embankment for tracked vehicle access

• Earthen embankment max slope 4:1 – other structural slopes as reviewed by City Engineer

• Calculation summary presented with plans• Emergency spillway required to ultimate

developed conditions• Design calcs for spillways• All ponds stabilized and with maintenance

plans

• Developer must check that structure does not need dam permit

• All impoundments must check with TCEQ for Water Rights Permit

Wet PondsWet Ponds

• In addition to dry basin requirements• 7’-10’ minimum depth• Appropriately aerated for size• Provisions to ensure normal water surface

elevations maintained • City water or well, unless 12 acres to every

acre foot • Minimum 10’ access width around entire

pond with max 6:1 slopes• Debris filter required for all outlets

• Design for sedimentation for minimum 5 years

• To minimize “short circuiting” outlet and inlet should be at opposite ends or baffled

• Artificial or clay liner to prevent loss through infiltration

• Can be waived if supported by geotech report• Reference iSWM wet pond design standard

for additional quidance• Safety bench, aquatic bench, etc…

• Exactly 1 3/4 projects since adopted

Questions???