Ongoing Research

• Skunk Works project• Started before hydromod was mainstream• Hydromod only• Engineering challenge to solve agency

processing problem for clients• Approach reflects timing – private

development driven• Idea conceived prior to agency response

Development - Watershed Changes

• Urbanization changes watershed physiography

• Increased impervious area• Connected surfaces• Stormwater conveyance utilities

• The watershed becomes a freeway for the excess surface water

Hydromodification

• Change in watershed rainfall-runoff response caused by alterations to watershed physiography

• Reduced time to peak• Higher peak flows• Increased runoff volume• More frequent flow events

Storm Response

• Basin response to a storm (hydrograph) is altered

Quantifying The Changes

• Hydromod effects are cumulative over time• Commonly expressed as long term flow

frequency diagramRelative Flow Vs Exceedance

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0 0.2 0.4 0.6 0.8 1

Exceedance (%)

Rel

ativ

e F

low

(cf

s/ac

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Existing

Proposed

The Problem

• Change to runoff pattern changes stream sediment transport and stream stability

Fix It Attempts

• MS4s in California have measures aimed at reducing hydromodification

• General measures – low impact development • Specific measures – volume standards• Smaller projects are easier – set standards• Larger projects are harder – do a study

• LID, LID, LID, LID, LID ...

Development Approach 1

Development Approach 2

Property Owner Perspective

• New MS4 requirements can be onerous• Retention takes space• LID can drive property use• LID implementation standard vague• Additional approval cycles• It is all new• No more studies please

Middle Ground

• Property owners are adjusting to new requirements

• Development plans altered mid-stream• Uncertainty likely to persist even for

properties at planning stages

• Implementations may be awkward for now

Hydrologic Wishful Thinking

• Hydromod process is complicated on a regional scale

• MS4 approaches seek to reverse or reduce physiographic changes to individual properties

• Effect on watershed scale is not certain – probably better, but still modified

Engineer’s Approach

• Solve a problem for the client

• Property development changes watershed subarea

• Runoff hydrograph is altered

• Restore original runoff hydrograph

• Use off the shelf ideas• Catalytic converter for a property

Hydrograph Plastic Surgery

• Sculpt a post development hydrograph back into the shape of the pre development hydrograph

Increased Runoff

• Permanent storage is necessary

Ponds and Wetlands

• Ponds and Wetlands provide the basic structure for hydromod control

Necessary Elements and Beneficial Features

• Retention (volume control)• Detention (shape control)

• Aesthetic feature• Structurally isolated• Water quality• Retrofit potential• Added value

Storage Configuration

• Pre Post hydrograph volume difference is stored

• Sub invert storage chops off front of hydrograph

Hydrograph Shaping Problem

• Once front part of incoming hydrograph is stored problem is reduced to reshaping

Hydrograph Reshaping – What We Know

• At any time we know …• Incoming flow (chopped post development

hydrograph) • Target outflow (pre development

hydrograph)• Volume stored• Basin geometry (WSEL)

Discretized Approach

• Discretize pre development hydrograph (target points)

• Match pre development storage and flowrate (outflow) at target points

Outlet Works Driven

Pond/Wetland Tuning

• Outlet works is adjusted to match pre development flow at each target point

• Rows of orifices added at each target WSEL to adjust total pond outflow

• Orifices not mandatory – may use weirs• Tuning approach works great provided that

pre development hydrograph is rising

Going Up Is Easy

• Pond volume (WSEL) is increasing• Pre development flow is increasing• Add more orifices

Engineering Decision Point

• Match pre development hydrograph on the way up

• Cross our fingers on the way down

• Examine cumulative process

• Normal approach

Relative Flow Vs Exceedance

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0 0.2 0.4 0.6 0.8 1

Exceedance (%)

Rel

ativ

e F

low

(cf

s/ac

)

Existing

Proposed

Normal Approach

• Regard Pond or Wetland as treatment control

• Tune pond or wetland for fundamental storm• High frequency event• Stream changing effects• Adjust to obtain desired long term results• Continuous simulation

Out of The Box Approach (Research)

• Seek to continue hydrograph sculpting on falling limb of pre development hydrograph

• Requires additional design element• Idea conceived for wetlands diversion project

Latched Outlet Works

• Use a float mechanism to activate flapgate• Open up or close off a portion of the outlet

works• Alter outlet works configuration• Contour hydrograph on falling limb• Hydrograph matching• Agency approval may take work initially

Hydrograph Matching

• Discretized approach (target points) allows the discharge from the pond or wetland to closely match pre development hydrograph

• Moot point since hydrograph is matched for the fundamental storm

• Cumulative effects, long term simulation, and tuning is still required

Numerical Process

• Unit hydrograph convolution• R1 = U1 *P1• R2 = U2*P1 + U1*P2• R3 = …

• Pond WSEL and pond geometry provide means to predict pre existing outflow for any storm

Hydrologic Undo

• Pond configuration maps incoming flow to project watershed precipitation pattern

• Pre development unit hydrograph is used to convert rain pattern to pre existing runoff which provides target outflow

• Goal is to convert post development project to pre development hydrology at the D/S end

Future World

• Goal is to match hydrographs for any storm• Eliminate hydromod• Ultra low impact development• Agency rubber stamp• Everyone happy