Hydromodification Compliancein the

Bay Area

Presented byNorman Gonsalves

Caltrans District 4

July 2010

What is HYDROMODIFICATION?The alteration of natural stream hydrology by human activity.

Why could HM be a problem?

Stream bank and stream bed erosion Sedimentation Increased pollutant delivery to receiving waters Decreased salinity in estuaries and coastal

areas

Stream bank erosion

Stream bed erosion

Sedimentation

Flooding (sedimentation)

What causes HM?

Increased runoff volume from upland areas Channel lining and armoring Channel entrainment and straightening Flow impounding

Increased runoff volume

Channel lining

Channel armoring

Channel entrainment (levee)

Who considers HM a problem?

The USEPA, but leaves regulation to states. States include Washington, California, others. California counties regulating HM include: Alameda (west) Contra Costa (all) Santa Clara (west) San Mateo (all) San Diego (all) Los Angeles (all)

What are the requirements?

For areas with HM regulations: Do not exceed pre-project flow peak flows Do no exceed pre-project flow durations, from

0.1*Q2 to Q10 Applies to projects discharging to susceptible

receiving waters Applies to projects with 1 acre or more of added

impervious surfaces Applies to Caltrans projects requiring 401

Certification

Flow-duration curves

Susceptible receiving waters

Water bodies that are NOT susceptible include:• Bays, oceans, large lakes and reservoirs;• Tidal reaches;• Channels lined, armored or piped for their

entire lengths all the way to (1) or (2) above;• Aggrading channels

Exempted projects Projects with less than 1 acre of added

impervious area Projects discharging to non-susceptible water

bodies Infill projects (> 65% impervious area) in highly

developed watersheds (< 10% vacant parcels)

Example of exempt areas

Susceptibility map.

Basic ways to meet HM requirement Infiltrate excess volume created by added

impervious areas Discharge excess volume below erosive flow

rates of receiving waters (0.1Q2 pre-project), using storage structures

Storage structures for HM management

Storage structures, including: Basins (lined or unlined), Underground storage pipes, Ditches (lined or unlined), Sand filters, Bio-retention systems

Infiltration devices for HM management

Infiltration basins, Infiltration trenches, Bio-retention systems, Unlined detention basins, Unlined ditches, Biostrips, Embankment side slopes, Other pervious surfaces

Design tools for HM management Bay Area Hydrology Model (BAHM) (proprietary,

but free version available) Hydrological Simulation Program – Fortran

(HSPF) (EPA, free) Storm Water Management Model (SWMM)

(EPA, free)

Tools compared

HSPF – powerful but hard to use. No built-in data.BAHM – based on HSPF, user-friendly, set up

specifically for HM, built-in data for 4 Bay Area counties, but unstable and buggy.

SWMM – can calculate HM devices as part of entire drainage system, fairly user-friendly, no built-in data, less capable infiltration model.

BAHM – Select Project Site

BAHM – Add A Watershed

BAHM – Add A Detention Pond

BAHM – Review Results

BAHM – Produce Report

BAHM – Pond Elements

BAHM Tank and Channel Elements

BAHM – More Devices

BAHM – Outlet Structures

SWMM – User interface

SWMM – LID + Drainage Calcs

SWMM – Drainage Profile