Download - Hydromodification Compliance in the Bay Area
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