Urban Hydrology Characteristics and their Influence on Urban Stream Restoration Technology Presented by: Michael Clar Ecosite, Inc. James Gracie Brightwater, Inc.
BACKGROUND Many streams in urban areas have been significantly impaired and are often perceived as candidate sites for urban stream restoration Much of the technology used for stream restoration is based on hydrologic regime conditions for rural watersheds For the most part this technology has not been adapted or verified for use in urban watersheds
BACKGROUND Urban land use practices, especially the creation of impervious surfaces, can result in significant alterations to the hydrologic regime of urban watersheds These hydrologic regime alterations can exert significant influence on a number of fundamental concepts currently in use in natural channel restoration technology, including: Bankfull discharge Regional curves of bankfull discharge and channel geometry Rosgen and Schumm stream classification systems Use of reference stream characteristics
LANE S STABLE CHANNEL BALANCE SCR Page 1-14
Land Development, Sediment & Discharge Urban watersheds can have a dramatic impact on both the sediment loads and stream discharge These impacts are not being addressed in current instruction programs for urban stream restoration Failure to consider these impacts can lead to incorrect assumptions and ill-conceived restoration designs
Land Development, Sediment & Discharge (Cont d) Site clearing and mass grading operations can generate high sediment loads While many jurisdictions require sediment control practices during construction, the effectiveness of these programs is inconsistent and estimated to be in the range of 50 to 60% removal of sediment. The accelerated sediment discharge is only temporary for a given site, but the development of an entire watershed can subject a stream to these loads again and again over long periods of time. The classic example of death by a thousand blows
Land Development, Sediment & Discharge (Cont d) The construction process can also generate large increases in surface runoff from exposed and compacted soils Most sediment control devices do not provide for stormwater management Once a watershed has been urbanized the sediment supply from the watershed can be dramatically reduced and lead to discharge of of sediment free or hungry waters Restoration of streams in this state is problematic due to tendency of these streams to continue removing sediment from their channels
Post Construction Land Use and Urban Runoff The land development process can greatly alter the hydrologic regime of developed watersheds The regime alterations include: Runoff volume Peak discharge Discharge frequency and duration Base flow conditions These hydrologic modifications are superimposed on construction impacts and can accelerate channel adjustment processes that are poorly understood and often not addressed in urban stream restoration
Effect of Urbanization on Mean Annual Flood
Effect of Urbanization on Increasing Frequency of Overbank Flows
Use of BMPs to Mitigate Urban Runoff Impacts Many urban watersheds were developed before the advent of SWM programs Growing recognition that many of the SWM programs used to date, particularly peak discharge control of the 2- and 10- year storm events have not been effective in addressing stream channel degradation Primary reason for failure of the peak discharge approach is the failure to effectively address the increase in the volume of runoff
Urban Hydrology and Bankfull Discharge Natural channel design is closely tied to the concept of bankfull discharge The bankfull, dominant or effective discharge is the primary design storm for stream restoration Bankfull discharge is frequently associated with a recurrence interval of 1.5 years as determined using a flood frequency analysis
The Bankfull Discharge Concept
Urban Hydrology and Bankfull Discharge (Cont d) Generally assumed that urban runoff results in a decrease in the return period of the bankfull event such that return periods approaching 1 year could occur. Also generally accepted that the peak discharge value of the bankfull discharge will increase. However, to date, very little documentation has been provided to support these assumptions
Urban Hydrology and Bankfull Discharge (Contd) Detailed procedures for field identification of bankfull discharge conditions have been provided and hands on training for this activity is available While these procedures are generally reliable for rural watersheds, they are at best difficult to apply and interpret on urban streams
Regional Relationships Regional bankfull relationships can be used in natural channel design to obtain a first approximation of field bankfull conditions. These relationships are developed for rural watersheds with 10% or less impervious cover Have limited application to urban watersheds Some attempts are being made to develop regional curves for urban watersheds Applications limited due to variances in TIA and DCIA, and lack of gauging stations for small urban watersheds
Regional Bankfull Discharge Relationships
Regional Hydraulic Geometry Relationships (Leopold, 1994)
Urban Regional Discharge Relationships 4.0 3.5 3.0 2.5 y=(2.32)+(0.71)*x Urban R 2 =0.963 bf (cfs) Log Q 2.0 1.5 1.0 y=(1.77)+(0.84)*x Rural R 2 =0.995 0.5-1.0-0.5 0.0 0.5 1.0 1.5 2.0 Log DA (sq mi)
Urban Hydrology and Stream Classification Concepts Rosgen (19960 and Schumm (1984) have both presented channel evolution concepts that describe the various stages of channel adjustment These concepts were developed for rural watersheds They project that a channel will enlarge and downcut or widen to increase its channel capacity in response to an increase in sediment load or discharge They project that a disturbed channel will abandon its former floodplain and evolve back to a channel geometry with a new floodplain at a lower elevation that approximates the pre-disturbance channel geometry
Schumm Channel Evolution Model
Urban Hydrology and Stream Classification Concepts (Cont d) These channel evolution concepts appear questionable for urban areas that have experienced a substantial and permanent hydrologic regime alteration While abandonment of the floodplain and channel enlargement is typical, it appears questionable that the channel will return to a channel geometry that approximates the pre-development condition There is little, if any, documentation that this process actually takes place in urban watersheds A more likely scenario is that the stream will remain an F stream type. This stream type is characterized by a wide crosssection, shallow depth and fully entrenched condition
Urban Hydrology and Use of Reference Stream Conditions Natural channel design calls for use of a reference reach to establish design parameters based upon a stable stream of the same stream type in the same valley type and physiographic region. The question of whether natural channel parameters from a stable reference reach are appropriate to use for design on a restored channel in a highly urbanized watershed has not been answered satisfactorily Also, geomorphic valley types may not be applicable in urbanized watersheds due to the presence of upstream and downstream culverts and bridges which limit natural adjustment
Summary This presentation has provided a discussion of the characteristics of urban stream hydrology as they relate to the following geomorphic concepts: Sediment load and stream discharge Use of BMPs to mitigate urban runoff impacts The bankfull discharge concept Use of regional discharge and channel geometry relationships Use of stream classification concepts Use of reference stream conditions Many of these concepts were developed for rural watershed conditions and have yet to be modified and validated for use in urban streams
Conclusions There are a number of issues to be resolved in the application of the natural channel approach to stream restoration design: The concept of bankfull discharge may not have the same significance in urbanized watersheds At best it is difficult to determine when the discharge and the channel have reached a regime condition in which the discharge defines the channel It is also difficult to find stable reference reaches in urbanized watersheds and questionable to use reference reaches from rural watersheds The application of reference reach geometry and geomorphic concepts developed for rural watersheds needs to be validated with long term monitoring results The issue of clear or hungry water has not been satisfactorily addressed in urban environments