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Hydrograph Modification Management: Importance, Techniques, and Analysis

Explore the importance of Hydrograph Modification Management and the techniques used, such as Low Impact Development controls. Learn about the technical analysis behind pre-sized IMPs and the effects of urbanization on watersheds.

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Hydrograph Modification Management: Importance, Techniques, and Analysis

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  1. Major Topics to Address • Why Hydrograph Modification Management is important • How Low Impact Development controls work • Describing the technical analysis that generated the set of pre-sized IMPs

  2. Effects of Urbanization • Impervious surfaces produce higher runoff rates, volume and duration of large flows

  3. Effects within the Watershed • Urbanization alters the watershed • Channels respond with incision and/or armoring

  4. Continuous Hydrologic Modeling Examines Full Range of Local Conditions • Sizing to one ‘design storm’ is not enough

  5. Peak Flow Frequency(Partial Duration Statistics) • Identify all HSPF storms in record and rank

  6. Flow Durations • Rank hourly outputs from HSPF model

  7. Example IMP: In-Ground Planter 18-in sandy loam

  8. Peak Flow Matching Example

  9. Duration Matching Example

  10. BMP Sizing Factor Summary Under-Drain or Infiltration: Infiltration Only:

  11. Adjusting IMP Sizing to Account for Rainfall Variability

  12. Sizing Conclusions for Implementation • IMPs in Group D soil sites are generally smaller than Group A soil BMPs • Steep side walls produce smaller sizing factors • Sizing factor may be particularly important for on-site BMPs • Swales and Bioretention basin footprint may be less important if BMPs fit into otherwise undeveloped space

  13. Questions?

  14. Contra Costa Approach to Hydrograph Modification • Encourage LID to control stormwater flows • HMP is technically rigorous and easy to apply • Assumes need to match pre-project condition

  15. BMP Gallery

  16. BMP Gallery

  17. Instructions for Computing Local Sizing Factors • Describe each DMA on the project site, including area, soil type, post-project surface type • For DMA's draining to IMPs, select an IMP and configuration (e.g. swale width, dry well depth) • Pick the appropriate sizing factor from the summary sizing factor table (see handout) • Compute the rainfall adjustment using the regression equations (see handout) • Local sizing factor = Sizing Factor x RainAdj

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